Abstract. Objectives. The aim of this study was to develop an open-source finite element model of the ankle for identification of the best clinical treatment to restore stability to the ankle after injury. Methods. The ankle geometry was defined from the Visible Human Project Female CT dataset available from the National Library of Medicine, and segmented using Dragonfly software (Object Research Systems, 2020). The finite element model was created with FEBio (University of Utah, 2021) using the dynamic nonlinear implicit solver. Linear isotropic material properties were assigned to the bones (E=7300MPa, ν=0.3, ρ=1730kg/m. 3. ) and cartilage (E=10MPa, ν=0.4, ρ=1100kg/m. 3. ). Spring elements were used to represent the ligaments and material properties were taken from Mondal et al. [1]. Lagrangian contact was defined between the cartilaginous surfaces with μ=0.003. A standing load case was modelled, assuming even distribution of load between the feet. A reaction force of 344.3N was applied to the base of the foot, a muscle force of 252.2N, and the proximal ends of the tibia and fibula were fully constrained. Results. The von Mises stresses closely matched those reported by Mondal et al. for the fibula (Present study: 1.00MPa, Mondal: 1.30MPa) and the talus (Present study: 2.20MPa, Mondal: 2.39MPa). However stresses within the tibia were underpredicted (Present study: 1.08MPa, Mondal: 5.86MPa). This was because the present study modelled a shorter tibial length because of a limitation in the CT slices available, which reduced the bending force. Conclusions. This first step in producing an open source ankle model for the orthopaedics community has shown the potential of the model to generate results comparable with those found in the literature. Future work is underway to examine the robustness of the model under different loading and explore alternative open-source CT datasets. [1] Mondal, S., & Ghosh, R. (2017). J Orthopaedics, 14(3), 329–335. . https://doi.org/10.1016/j.jor.2017.05.003

Finite element models of the musculoskeletal system have the possibility of describing the in vivo situation to a greater extent than a single in vitro experimental study ever could. However these models and the assumptions made must be validated before they can be considered truly useful. The object of this study was to validate, using digital image correlation (DIC) and strain gauging, a novel free boundary condition finite element model of the femur. The femur was treated as a complete musculoskeletal construct without specific fixed restraint acting on the bone. Spring elements with defined force-displacement relationships were used to characterize all muscles and ligaments crossing the hip and knee joints. This model was subjected to a loading condition representing single leg stance. From the developed model muscle, ligament and joint reaction forces were extracted as well as displacement and strain plots. The muscles with the most influence were selected to be represented in the simplified experimental setup. To validate the finite element model a balanced in vitro experimental set up was designed. The femur was loaded proximally through a construct representative of the pelvis and balanced distally on a construct representing the tibio-femoral joint. Muscles were represented using a cabling system with glued attachments. Strains were recorded using DIC and strain gauging. DIC is an image analysis technique that enables non-contact measurement of strains across surfaces. The resulting strain distributions were compared to the finite element model. The finite element model produced hip and knee joint reaction forces comparable to in vivo data from instrumented implants. The experimental models produced strain data from both DIC and strain gauging; these were in good agreement with the finite element models. The DIC process was also shown to be a viable method for measuring strain on the surface of the specimen. In conclusion a novel approach to finite element modeling of the femur was validated, allowing greater confidence for the model to be further developed and used in clinical settings

Objectives. In this prospective cohort study, we investigated whether patient-specific finite element (FE) models can identify patients at risk of a pathological femoral fracture resulting from metastatic bone disease, and compared these FE predictions with clinical assessments by experienced clinicians. Methods. A total of 39 patients with non-fractured femoral metastatic lesions who were irradiated for pain were included from three radiotherapy institutes. During follow-up, nine pathological fractures occurred in seven patients. Quantitative CT-based FE models were generated for all patients. Femoral failure load was calculated and compared between the fractured and non-fractured femurs. Due to inter-scanner differences, patients were analyzed separately for the three institutes. In addition, the FE-based predictions were compared with fracture risk assessments by experienced clinicians. Results. In institute 1, median failure load was significantly lower for patients who sustained a fracture than for patients with no fractures. In institutes 2 and 3, the number of patients with a fracture was too low to make a clear distinction. Fracture locations were well predicted by the FE model when compared with post-fracture radiographs. The FE model was more accurate in identifying patients with a high fracture risk compared with experienced clinicians, with a sensitivity of 89% versus 0% to 33% for clinical assessments. Specificity was 79% for the FE models versus 84% to 95% for clinical assessments. Conclusion. FE models can be a valuable tool to improve clinical fracture risk predictions in metastatic bone disease. Future work in a larger patient population should confirm the higher predictive power of FE models compared with current clinical guidelines. Cite this article: F. Eggermont, L. C. Derikx, N. Verdonschot, I. C. M. van der Geest, M. A. A. de Jong, A. Snyers, Y. M. van der Linden, E. Tanck. Can patient-specific finite element models better predict fractures in metastatic bone disease than experienced clinicians? Towards computational modelling in daily clinical practice. Bone Joint Res 2018;7:430–439. DOI: 10.1302/2046-3758.76.BJR-2017-0325.R2

Introduction: Notching of the femoral neck during preparation of the femur during hip resurfacing has been associated with an increased risk of femoral neck fracture. We aimed to evaluate this with the use of a finite element model. Methods: A three dimensional femoral model was used and molded with a femoral component constructed from the dimensions of a Birmingham Hip Resurfacing. Multiple constructs were made with the component inferiorly translated in order to cause a notch in the superior femoral neck. The component angulation was kept constant. Once constructed the model was imported into the Ansys finite element model software for analysis. Elements within the femoral model were assigned different material properties depending on cortical and cancellous bone distributions. Von Misses stresses were evaluated near the notches and compared in each of the cases. Results: In the un-notched case the maximum Von Mises stress was only 40MPa. However, with the formation of a 1mm notch the stress rose to 144MPa and in the 4 mm notch the stress increased to 423MPa. These values demonstrated that a 1mm notch increased the maximum stress by 361% while a 4mm notch increased the maximum stress by 1061%. Discussion: This study demonstrated that causing a notch in the superior femoral neck dramatically increases the stress within the femoral neck. This may result in the weakening of the femoral neck and potentially predispose it to subsequent femoral neck fracture. The data suggests that even a small notch of 1mm may be detrimental in weakening the femoral neck by dramatically increasing the stress in the superior neck. This study suggests that any femoral neck notching should be avoided during hip resurfacing

Introduction. On the basis of a proposal by Noble, the marrow cavity form can be classified into three categories: stovepipe, normal, and champagne-fluted. In the present study, three typical finite element femoral models were created using CT data based on Noble's three categories. The purpose was to identify the relationship of stress distribution of the surrounding areas between femoral bone marrow cavity form and hip stem. The results shed light on whether the distribution of the high-stress area reflects the stem design concept. In order to improve the results of THA, researchers need to consider the instability of a stem design based on the pressure zone and give feedback on future stem selection. Methods. To develop finite element models, two parts (cortical bone and stem) were constructed using four-node tetrahedral elements. The model consisted of about 40,000 elements. The material characteristics were defined by the combination of mass density, elastic coefficient, and Poisson's ratio. Concerning the analysis system, HP Z800 Workstation(HP, Japan) was used as hardware and LS-DYNA Ver. 971 (Livermore Software Technology Corporation, USA) as software. The distal end of the femur was constrained in all directions. On the basis of ISO 7206 Part 4,8 that specifies a method of endurance testing for joint prostheses, the stem was tilted 10°, and a 500 N resultant force in the area around the hip joint was applied to the head at an angle of 25° with the long axis. Automatic contact with a consideration of slip was used. Von Mises stress during a 1.0 s period after loading was analyzed, and stress distribution in the stem and its maximum value were calculated. Result. The maximum stress at marrow cavity form of normal was shown to be 72 MPa. The stress of champagne-fluted was evenly distributed from proximal to distal, and the maximum stress was 67 MPa. For stovepipe, the maximum proximal stress was shown to be 120 MPa; moreover, stress concentration was observed. Discussion. The design concept for a Zweymüller-type stem can distribute load across a wide range of cortical bone from the middle position to the distal femur. It is determined using this concept that a wide range of stress was absorbed at the middle position and distal femur in the champagne-fluted and normal cases. On the other hand, the contact pressure zone of stovepipe could not meet the expected level at the distal femur. The method of this research involves controlling the stress conditions within the stem design. At this point, it is considered possible for the stability of various stem designs to be predicted and the stability to be assessed positively. On the basis of Noble's categories, three types of finite element model were made, and stress distribution measurement and finite element analyses were performed. The results indicate that Zweymüller stem has clinical validity for securing force in the champagne-fluted and stovepipe types from the stress distribution

Introduction. The performance of total hip replacement (THR) devices can be affected by the quality of the tissues surrounding the joint or the mismatch of the component centres during hip replacement surgery. Experimental studies have shown that these factors can cause the separation of the two components during walking cycle (dynamic separation) and the contact of the femoral head with the rim of the acetabular liner (edge loading), which can lead to increased wear and shortened implant lifespan. 1. There is a need for flexible pre-clinical testing tools which allow THR devices to be assessed under these adverse conditions. In this work, a novel dynamic finite element model was developed that is able to generate dynamic separation as it occurs during the gait cycle. In addition, the ability to interrogate contact mechanics and material strain under separation conditions provides a unique means of assessing the severity of edge loading. This study demonstrates these model capabilities for a range of simulated surgical translational mismatch values, for ceramic-on-polyethylene implants. Methodology. The components of the THR were aligned and constrained as illustrated in Figure 1. CAD models of commercially available implant geometries were used (DePuy Synthes, Leeds, UK) modified for model simplicity by removing anti-rotation features. The polyethylene cup liner was given elastic-plastic behaviour. An axial load following the Paul cycle pattern (5 repetitive cycles) with maximum of 3KN and swing phase load of 0.3KN, was applied through the cup holder. The effect of translational mismatch was implemented by using a spring element connected to the cup unit on the lateral side. The spring was compressed by a fixed amount to replicate a degree of medial-lateral mismatch of the components. The instantaneous resultant force vector dictated the dynamic sliding behaviour of the cup against the head. In this study, translational medial-lateral mismatch values of 1, 2, 3 and 4mm were used and the medial-lateral dynamic separation, contact pressure maps and plastic strain were recorded. Results. The highest level of dynamic separation is achieved when the minimum axial load (during swing phase) is applied. The dynamic separation increases as the surgical translation mismatch increases (figure 2), with values over 0.5mm (radial clearance) representing cases where the head is in contact with the rim of the cup. Maximum separation occurred towards the end of the swing phase. Plots of the shape of the contact pressure at that point can be seen in Figure 3. Only the 4mm mismatch created substantial plastic deformation. Conclusion. The finite element model was able to predict medial-lateral separation as it occurred dynamically in the gait cycle, including cases where the femoral head was in contact with the rim of the cup. The increase in medial-lateral separation with increased translational mismatch was in broad agreement with existing experimental data. 2. Substantial plastic deformation was only seen in cases where the translational mismatch caused the femoral head to be in contact with the rim of the polyethylene cup

Introduction A very specific group within the 80 percent of the population that suffers from low back pain at some stage in life are young cricket fast bowlers. Amongst them a high occurrence of unilateral L4 pars interarticularis fractures exists, which shows a strong statistical correlation to the presence of a contralateral volumetric increase in the Quadratus Lumborum (QL) muscle. However, there is no clear physical link between these two phenomena. To investigate this relationship, we have combined a mathematical model of the lumbar spine muscles with a finite element model of the fourth lumbar vertebra and analysed the stresses occurring in the L4 vertebra throughout the bowling motion. Methods A mathematical model of the lumbar spine muscles has been developed previously at QUT. It contains 170 fascicles representing all major muscles in the lumbar region and allows for analysis of the forces and moments on the intervertebral joints caused by these muscles in any given posture. A Finite Element Model (FEM) of an L4 vertebra and intervertebral disc (IVD) was developed based on one created by Theo Smit and obtainable from the Internet through the BEL Repository of the Istituti Ortopedici Rizzoli, Bologna, Italy. Material properties were obtained from literature, while muscle forces, directions and attachment locations in the different postures came from the mathematical model. Six postures occurring in right-handed fast bowling were modelled to determine the differences in stresses between having symmetric and asymmetric QL muscles. The asymmetric condition consisted of a 30% increase in Physiological Cross-Sectional Area (PCSA) on the right side. In all cases it was assumed the left facet joints were ‘locked up’, to create a presumed worst-case scenario for the stress build-up in the pars. Results It was found that when using muscle activation levels from literature an enlarged right-side QL did not increase the stresses in the left pars noticeably, in fact in some cases it even slightly reduced those stresses. When only the right-side QL muscle was activated, while all other muscles only provided passive muscle force, a 30% PCSA increase of this muscle produced an increase in maximum Von Mises and principal stresses in the left-side pars from typically 30 MPa to 40 MPa but only in the postures close to upright stance. In more extreme postures where the maximum stresses in the pars are higher, the increased PCSA of the right QL only led to small stress increases from typically 125 to 129 MPa. Discussion Even in the worst-case scenario where only the right-side QL is active and the left-side facet joint is locked up, a PCSA increase of that muscle does not cause a large increase in stresses in postures where the stresses are high. Hence, this study has not demonstrated a clear physical link between asymmetric hypertrophy of QL and pars fractures. It may even suggest the hypertrophy is a response to postural overload attempting to reduce stresses in the pars. To clarify this, an improved FEM of the L3 and L4 vertebrae and IVDs, including all ligaments, is currently being developed. We believe that in the future this combination of models can be used for many more purposes where the influence of posture and musculature on the lumbar spine biomechanics needs investigation

The primary fixation of cementless hip prostheses is related to the shape of the stem. When there is a complication of loading in several directions, the mechanical fixation of a hip stem is considered to provide good primary fixation. The purpose of this study was to evaluate whether the IMC stem with its characteristic fixation method, which was developed by a group at Kitasato University, contributes to primary fixation by finite element analysis. Analysis was performed at a friction coefficient of 0.1 with automatic contact, under the restriction of the distal femoral end. The following three loading conditions were applied:. step loading of the joint resultant force in the region around the hip stem,. loading in the rotational direction, simulating torsion, and. loading of the femoral head equivalent to that during walking. Micromotion of the IMC stem along the x-, y-, and z-axes direction was calculated by simulation, and the stress distributed on the stem and femur was determined. Micromotion along the z-axis, which is a clinical problem in hip prosthesis stems, was lower in the IMC stem than in other stems reported. Micromotion of the stem along the z-axis was low, indicating a low risk of sinking. The interlocking mechanism, which is a characteristic of the IMC stem, functioned to suppress its micromotion, indicating that the locking method of this stem contributed to the stability. Since no stress concentration was detected, it was considered that there are no risks of breakage of the IMC stem and femur. It was suggested that effective fixation of the finite element model of the IMC stem can be achieved because the micromotion and stress level are appropriate for primary fixation

Worldwide, osteoporosis, causes more than 8.9 million fractures annually, resulting in an osteoporotic fracture every 3 seconds, where 1 in every 3 women and 1 in every 5 men aged over 50 will experience osteoporotic fractures at least once in their lifetime. Vertebral fractures, estimated at 1.4 million/year are among the most common fractures, posing enormous health and socioeconomic challenges to the individual and society at large. Considering that the great majority of individuals at high risk (up to 80%), who have already had at least one osteoporotic fracture, are neither identified nor treated, prediction of the risk factors for vertebral fractures can be of great value for prevention/early diagnosis. Recent studies show that finite element analysis of computed tomography (CT) scans provides noninvasive means to assess fracture risk and has the potential to be clinically implemented upon proper validation. The objective of this study was to develop a voxel-based finite element model using quantitative computed tomography (QCT) images in conjunction with in-vitro experiments to evaluate the strength of the vertebral bodies and predict the fracture risk criteria. A total of 10 vertebrae were dissected from juvenile sheep lumbar spines. The attached soft tissues and posterior elements and facet joints were completely removed, and the upper and lower vertebral bodies were polished using glass paper to provide smooth surfaces. The specimens were wrapped in phosphate buffer saline (PBS) soaked gauze, sealed in plastic bags, and stored in a refrigerator at −22°C. QCT scans of the specimens were captured using a bone density calibration phantom (QRM Co., Moehrendorf, Germany) with three 18 mm cylindrical inserts, providing 0, 100 and 200 mg HA/ccm, respectively. All the specimens, preserved hydrated in PBS solution, were mechanically tested at room temperature using a mechanical testing apparatus (Zwick/Roell, Ulm-Germany). The QCT images were then used to reconstruct the voxel-based FE model employing a custom-developed heterogeneous material mapping code. Five different equations for the correlation of the density and the elastic modulus were used to validate the efficiency of the FE model as compared to the in-vitro experiments. The results of the voxel-based FE models matched well with the in-vitro experiments, with an average error of 11.38 (±4.09)% based on the power law equation. A failure criterion was embedded in the FE models and the initiation of fracture was successfully predicted for all specimens. Further, typical kyphoplasty treatment was simulated in the 5 models to evaluate the application of the validated algorithm in the estimation of the failure patterns. Our novel voxel-based FE model can be used in future studies to predict the outcome of different types of therapeutic modalities/surgeries and estimate fracture risk including postoperative fractures

25–40% of unicompartmental knee replacement (UKR) revisions are performed for unexplained pain possibly secondary to elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on cancellous bone strain in a finite element model (FEM) of a cemented fixed bearing medial UKR, validated using previously published acoustic emission data (AE). FEMs of composite tibiae implanted with an all-polyethylene tibial component (AP) and a metal backed one (MB) were created. Polyethylene of thickness 6–10mm in 2mm increments was loaded to a medial load of 2500N. The volume of cancellous bone exposed to <−3000 (pathological overloading) and <−7000 (failure limit) minimum principal (compressive) microstrain (µ∊) and >3000 and >7000 maximum principal (tensile) microstrain was measured. Linear regression analysis showed good correlation between measured AE hits and volume of cancellous bone elements with compressive strain <−3000µ∊: correlation coefficients (R= 0.947, R2 = 0.847), standard error of the estimate (12.6 AE hits) and percentage error (12.5%) (p<0.001). AP implants displayed greater cancellous bone strains than MB implants for all strain variables at all loads. Patterns of strain differed between implants: MB concentrations at the lateral edge; AP concentrations at the keel, peg and at the region of load application. AP implants had 2.2 (10mm) to 3.2 (6mm) times the volume of cancellous bone compressively strained <−7000µ∊ than the MB implants. Altering MB polyethylene insert thickness had no effect. We advocate using caution with all-polyethylene UKR implants especially in large or active patients where loads are higher

Background. It is known that severe cases of intervertebral disc (IVD) disease may lead to the loss of natural intervertebral height, which can cause radiating pain throughout the lower back and legs. To this point, surgeons perform lumbar fusion using interbody cages, posterior instrumentation and bone graft to fuse adjacent vertebrae together, thus restoring the intervertebral height and alleviating the pain. However, this surgical procedure greatly decreases the range of motion (ROM) of the treated segment, mainly caused by high cage stiffness. Additive manufacturing can be an interesting tool to reduce the cage's elastic modulus (E), by adding porosity (P) in its design. A porous cage may lead to an improved osteointegration since there is more volume in which bone can grow. This work aims to develop a finite element model (FEM) of the L4-L5 functional spinal unit (FSU) and investigate the loss of ROM induced by solid and porous cages. Materials and Methods. The Intact-FEM of L4-L5 was created, which considered the vertebrae, IVD and ligaments with their respective material properties. 1. The model was validated by comparing its ROM with that of other studies. Moments of 10 Nm were applied on top of L4 while the bottom of L5 was fixed to simulate flexion, extension, lateral bending and axial rotation. 2. The lumbar cages, posterior instrumentation and bone graft were then modelled to create the Cage-FEMs. Titanium was chosen for the instrumentation and cages. Cages with different stiffness were considered to represent porous structures. The solid cage had the highest modulus (E. 0. =110 GPa, P. 0. =0%) whereas the porous cages were simulated by lowering the modulus (E. 1. =32.8 GPa, P. 1. =55%; E. 2. =13.9 GPa, P. 2. =76%; E. 3. =5.52 GPa, P. 3. =89%; E. 4. =0.604 GPa, P. 4. =98%), following the literature. 3. The IVD was removed in Cage-FEMs to allow the implant's insertion [Fig. 1] and the previous loading scenarios were simulated to assess the effects of cage porosity on ROM. Results. The Intact-FEM presents acceptable ROM according to experimental and numerical studies, as shown by the red line in Figure 2. After insertion, lower ROM values in Cage-FEMs are measured for each physiological movement [Fig. 3]. In addition, highly porous cages have greater ROM, especially in axial rotation. Discussion. Significant reduction of ROM is expected after cage insertion because the main goal of interbody fusion is to allow bone growth. As such, the procedure's success is highly dependent on segmental stability, which is achieved by using cages in combination with bone graft and posterior instrumentation. Furthermore, higher cage porosities seem to affect the FSU. In fact, ROM increases more as the cage modulus approaches that of the cancellous bone (E. canc-bone. =0.2 GPa. 1. ). Next step will be to assess the effects of cage design on the L4-L5 FSU mechanical behavior and stress distribution. To conclude, additive manufacturing offers promising possibilities regarding implant optimization, being able to create porous cages, thus reducing their stiffness. For any figures or tables, please contact the authors directly

Summary. A retrospective study on 98 patients shows that FE-based bone strength from CT data (using validated FE models) is a suitable candidate to discriminate fractured versus controls within a clinical cohort. Introduction. Subject-specific Finite element models (FEM) from CT data are a promising tool to non-invasively assess the bone strength and the risk of fracture of bones in vivo in individual patients. The current clinical indicators, based on the epidemiological models like the FRAX tool, give limitation estimation of the risk of femoral neck fracture and they do not account for the mechanical determinants of the fracture. Aim of the present study is to prove the better predictive accuracy of individualised computer models based a CT-FEM protocol, with the accuracy of a widely used standard of care, the FRAX risk indicator. Patients and Methods. This retrospective cohort is individually-matched case control study composed by 98 Caucasian women who were at least 5 years post menopause. The case group consisted of 49 patients who had sustained a hip fracture (36 intra-capsular and 13 extra-capsular fractures) within the previous 90 days due to low-energy trauma. The CT datasets were segmented (using the ITK-Snap software) in order to extract the periosteal bone surface. Unstructured meshes (10-node tetrahedral elements) were generated using ANSYS mesh morphing software. Each CT dataset was calibrated using the European Spine Phantom. The inhomogeneous material properties were mapped from CT datasets into the FEM with the BoneMat_V3 software. Bone strength was evaluated in quasi-axial loading conditions, for a set of 12 different configurations sampling the cone of recorded in vivo hip joint reactions, and was defined as the minimum load inducing on the femoral neck surface an elastic principal strain value greater than a limit value. Results. There were no statistically significant difference between the fracture and the control groups for age, height and weight (p<0.05). All indices of areal bone mineral density (aBMD) and the volumetric mineral density (vBMD) between fractured and controls showed on average a lower value for fractured respect of the controls, with similar mean difference (14% for aBMD and 13% for the vBMD). FEM-predicted strength differed between fractured and non-fractured on average for 20%. To evaluate its ability to identify patients at risk of hip fracture, FEM-based strength was compared to the FRAX predictor by computing for each predictor the Receiver Operating Characteristic (ROC) curve, and the Area Under the Curve (AUC). The individualised risk predictor based on FEM bone strength was found to perform significantly better (AUC=0.76) than FRAX (AUC=0.66). When the FEM-based strength indicator was combined with available clinical information in a logistic regression, the resulting predictor achieved in this retrospective study an excellent accuracy (AUC=0.82). Discussion. This study confirms that individualised, CT- FEM, when generated using to the state-of-the-art protocols, can provide a predictor of the risk of hip fracture more accurate than those based on clinical data alone. In the integrated workflow developed in the VPHOP Project (FP7-ICT-223865) CT-based risk prediction is requested only for those patients for whom the clinical decision is uncertain

We previously demonstrated that cartilaginous tissue was induced on a reamed acetabular articulation in an ovine hemiarthroplasty model with three different femoral head sizes. At maximum loading during stance phase, the acetabular peak stresses immediately after reaming could reach approximately 80 MPa under direct implant-bone contact with in-vitro measurements. We aimed to establish finite element (FE) models of the ovine hip hemiarthroplasty which examine stress distribution on the reamed acetabula by three head sizes. We hypothesized that the stress distribution did not differ between different sizes when the joint is congruent and that the peak stresses in the acetabulum immediately after reaming occurred in the dorsal acetabulum. Three two-dimensional FE models of ovine hip hemi-arthroplasty were built; each comprised a head component, 25, 28, and 32 mm in diameter, and an acetabular component. The acetabular geometry was acquired from an ovine acetabular histological section. The head was moved to partly intersect with the acetabulum representing the reaming procedure and a congruent contact was confirmed. Cortical bone and cancellous bone were modelled as linear elastic, with moduli of 20 and 1.2 GPa, respectively. Variable moduli were also assessed. The finest mesh for each model consisted of over 100,000 four-node quadrilateral elements. Loading conditions were chosen to represent peak hip joint force developed during the stance phase. Stress distribution in the acetabular area in contact with the head was plotted against the articulating arc length. The results confirmed that the stress distribution between different prosthetic head sizes in a reamed hemiarthroplasty model did not change when the joint was congruent. The peak compressive stresses occurred in the dorsal acetabulum with the 32 mm model being the highest at approximately 69 MPa, the 28 mm model at 63 MPa, and the 25 mm model at 54 MPa. An increase in the cancellous modulus and a decrease in the cortical modulus increased the peak stresses in the dorsal acetabulum. This presents an indicative study into the effect of prosthetic femoral head sizes on the stress distribution in the acetabulum. The idealized 2-D models showed reasonable agreement when compared quantitatively with the in vitro study

Iterative finite element (FE) models are used to simulate bone remodelling that takes place due to the surgical insertion of an implant or to simulate fracture healing. In such simulations element material properties are calculated after each iteration of solving the model. New material properties are calculated based on the results derived by the model during the last iteration. Once the FE model has gone through a number of such iterations it is often necessary to assess the remodelling that has taken place. The method widely used to do this is to analyse element Young's modulus plots taken at particular sections through the model. Although this method gives relevant information which is often helpful when comparing different implants, the information is rather abstract and is difficult to compare with patient data which is commonly in the form of radiographs. The authors suggest a simple technique that can be used to generate synthetic radiograph images from FE models. These images allow relatively easy comparisons of FE derived information with patient radiographs. Another clear advantage of this technique is that clinicians (who are familiar with reading radiographs) are able to understand and interpret them readily. To demonstrate the technique a three dimensional (3D) model of the proximal tibia implanted with an Oxford Unicompartmental Knee replacement was created based on CT data obtained from a cadaveric tibia. The model's initial element material properties were calculated from the same CT data set using a relationship between radiographic density and Young's modulus. The model was subject to simplified loading conditions and solved over 365 iterations representing one year of in vivo remodelling. After each iteration the element material properties were recalculated based on previously published remodelling rules. Next, synthetic anteroposterior radiographs were generated by back calculating radiographic densities from material properties of the model after 365 iterations. A 3D rectangular grid of sampling points which encapsulated the model was defined. For each of the elements in the FE model radiographic densities were back calculated based on the same relationships used to calculate material properties from radiographic densities. The radiographic density of each element was assigned to all the sampling grid points within the element. The 3D array of radiographic densities was summed in the anteroposterior direction thereby creating a 2D array of radiographic densities. This 2D array was plotted giving an image analogous to anteroposterior patient radiographs. Similar to a patient radiograph denser material appeared lighter while less dense material appeared darker. The resulting synthetic radiographs were compared to patient radiographs and found to have similar patterns of dark and light regions. The synthetic radiographs were relatively easy to produce based on the FE model results, represented FE results in a manner easily comparable to patient radiographs, and represented FE results in a clinician friendly manner

Clinical investigations show that the cervical spine presents wide inter-individual variability, where its motion patterns and load sharing strongly depend on the anatomy. The magnitude and scope of cervical diseases, including disc degeneration, stenosis, and spondylolisthesis, constitute serious health and socioeconomic challenges that continue to increase along with the world”s growing aging population. Although complex exact finite element (FE) modeling is feasible and reliable for biomechanical studies, its clinical application has been limited as it is time-consuming and constrained to the input geometry, typically based on one or few subjects. The objective of this study was twofold: first to develop a validated parametric subject-specific FE model that automatically updates the geometry of the lower cervical spine based on different individuals; and second to investigate the motion patterns and biomechanics associated with typical cervical spine diseases. Six healthy volunteers participated in this study upon informed consent. 26 parameters were identified and measured for each vertebra in the lower cervical spine from Lateral and AP radiographs in neutral, flexion and extension viewpoints in the standing position. The lower cervical FE model was developed including the typical vertebrae (C3-C7), intervertebral discs, facet joints, and ligaments using ANSYS (PA, USA). In order to validate the FE model, the bottom surface of C7 was fixed, and a 73.6N preload together with a 1.8 N.m pure moment were input into the model in both flexion and extension. The results were compared to experimental studies from literature. Disc degeneration disease (DDD) was used as an example, where the geometry of C5-C6 disc was changed in the model to simulate 3 different grades of disc degeneration (mimicking grades 1 to 3), and the resulting biomechanical responses were evaluated. The average ranges of motion (ROM) were found to be 4.84 (±0.73) degrees and 5.36 (±0.68) degrees for flexion and extension for C5-C6 functional unit, respectively, in alignment with literature. The total ROM of the model with disc generation grades 2 and 3 was found to have decreased significantly as compared to the intact model. In contrast, the axial stresses on the degenerated discs were significantly higher than the intact discs for all 3 degeneration grades. Our preliminary results show that this novel validated subject-specific FE model provides a potential valuable tool for noninvasive time and cost effective analyses of cervical spine biomechanical (kinematic and kinetic) changes associated with various diseases. The model also provides an opportunity for clinicians to use quantitative data towards subject-specific informed therapy and surgical planning. Ongoing and future work includes expanding the studied population to investigate individuals with different cervical spine afflictions

Summary. At the clinical CT image resolution level, there is no influence of the image voxel size on the derived finite element human cancellous bone models. Introduction. Computed tomography (CT)-based finite element (FE) models have been proved to provide a better prediction of vertebral strength than dual-energy x-ray absorptiometry [1]. FE models based on µCTs are able to provide the golden standard results [2], but due to the sample size restriction of the µCT and the XtremeCT machines, the clinical CT-based FE models is still the most promising tool for the in vivo prediction of vertebrae's strength. It has been found [3] that FE predicted Young's modulus of human cancellous bone increases as the image voxel size increases at the µCT resolution level [3]. However, it is still not clear whether the image voxel size in the clinical range has an impact on the predicted mechanical behavior of cancellous bone. This study is designed to answer this question. Methods. For this study, 6 thoracolumbar vertebrae (Th12) obtained from the female donors were scanned in the non-dissected cadavers under 2 different resolutions – group A: 120 kVp, 100 mAs, with a resolution of 0.29×0.29×1.3 mm. 3. ; group B: 120 kVp, 360 mAs, with a resolution of 0.18×0.18×0.6 mm. 3. A solid calibration phantom (QRM-BDC) was placed beneath the cadavers during the scans. Cuboids with the size of 12.3×12.3×14.3 mm. 3. were cropped from the center of each vertebral body. The FE model was created by converting each image voxel into hexahedron (C3D8). Inhomogeneous material property was defined for the cuboid [4], i.e. the image greyscale value were firstly calibrated into the bone mineral density (BMD), then the Young's modulus and yield stress were calculated from the BMD [5] for each element. Statistical analysis was performed to compare the FE predicted mechanical properties between the groups and the significance level was set to 95% (α=0.05). Results. The trabecular structure is more clearly mimicked in the models from group B than those from group A. The modulus (mean ± SE) in group A is 5.9% higher than that in group B (193.33 ± 31.67 MPa vs. 182.50 ± 27.07 MPa). The yield strength (mean ± SE) in group A is 6.4% higher than that in group B (0.99 ± 0.21MPa vs. 0.93 ± 0.17MPa). However, the paired t-test shows there is no significant difference of the mechanical properties in the two groups (p=0.109 for the modulus and p=0.234 for the yield strength). Discussion. This study shows that there is no influence of the voxel size on the clinical CT derived FE cancellous bone models. This finding can help choose a better, less invasive CT protocol for the patient when creating a clinical CT image based FE model. Acknowledgements. This study is financially supported by the Federal Ministry of Education and Research and the state of Hamburg, Germany

Adult Spine Deformity (ASD) is a degenerative condition of the adult spine leading to altered spine curvatures and mechanical balance. Computational approaches, like Finite Element (FE) Models have been proposed to explore the etiology or the treatment of ASD, through biomechanical simulations. However, while the personalization of the models is a cornerstone, personalized FE models are cumbersome to generate. To cover this need, we share a virtual cohort of 16807 thoracolumbar spine FE models with different spine morphologies, presented in an online user-interface platform (SpineView). To generate these models, EOS images are used, and 3D surface spine models are reconstructed. Then, a Statistical Shape Model (SSM), is built, to further adapt a FE structured mesh template for both the bone and the soft tissues of the spine, through mesh morphing. Eventually, the SSM deformation fields allow the personalization of the mean structured FE model, leading to generate FE meshes of thoracolumbar spines with different morphologies. Models can be selectively viewed and downloaded through SpineView, according to personalized user requests of specific morphologies characterized by the geometrical parameters: Pelvic Incidence; Pelvic Tilt; Sacral Slope; Lumbar Lordosis; Global Tilt; Cobb Angle; and GAP score. Data quality is assessed using visual aids, correlation analyses, heatmaps, network graphs, Anova and t-tests, and kernel density plots to compare spinopelvic parameter distributions and identify similarities and differences. Mesh quality and ranges of motion have been assessed to evaluate the quality of the FE models. This functional repository is unique to generate virtual patient cohorts in ASD.

Acknowledgements: European Commission (MSCA-TN-ETN-2020-Disc4All-955735, ERC-2021-CoG-O-Health-101044828)

INTRODUCTION. Growth-guidance constructs are an alternative to growing rods for the surgical treatment of early onset scoliosis (EOS). In growth-guidance systems, free-sliding anchors preserve longitudinal spinal growth, thereby eliminating the need for surgical lengthening procedures. Non-segmental constructs containing ultra-high molecular weight polyethylene (UHMWPE) sublaminar wires have been proposed as an improvement to the traditional Luque trolley. In such a construct, UHMWPE sublaminar wires, secured by means of a knot, serve as sliding anchors at the proximal and distal ends of a construct, while pedicle screws at the apex prevent rod migration and enable curve derotation. Ideally, a construct with the optimal UHMWPE sublaminar wire density, offering the best balance between providing adequate spinal fixation and minimizing surgical exposure, is designed preoperatively for each individual patient. In a previous study, we developed a parametric finite element (FE) model that potentially enables preoperative patient-specific planning of this type of spinal surgery. The objective of this study is to investigate if this model can capture the decrease in range of motion (ROM) after spinal fixation as measured in an experimental study. MATERIALS AND METHODS. In a previous in vitro study, the ROM of an 8-segment porcine spine was measured before and after instrumentation, using different instrumentation constructs with a sequentally decreasing number of wire fixation points. In the current study, the parametric FE model of the thoracolumbar spine was first validated relative to ROM values reported in the literature. The rods, screws, and sublaminar wires were implemented, and the model was subsequently used to replicate the in vitro tests. The experimental and simulated ROM”s for the different instrumentation conditions were compared. RESULTS. Good agreement between in vitro biomechanical tests and FE simulations was observed in terms of the decrease in ROM for the complete construct with wires at each level. The stepwise increase in total ROM with decreasing number of wires at the construct ends was less prominent in silico in comparison to in vitro. CONCLUSION. Important first steps in the implementation and validation of a growth-guidance construct for EOS patients in a patient-specific FE model of the spine have been made in this study. The parametric nature of the FE model allows for rapid personalization. Although further improvements to the model will be necessary to better distinguish between different spinal instrumentation constructs, we conclude that the model can well capture essential aspects of spinal motion and the overall effect of instrumentation

A recently developed parametric geometrical finite element model (p-FEM) was adapted to the specific hip geometric measurements of a group of patients with slipped capital femoral epiphysis (SCFE). The objective was to analyze the stress distribution in the growth plate of these patients and to evaluate differences for those patients who developed bilateral disease. Different geometric parameters were measured in the healthy proximal femur of 18 adolescents (mean age, 12,1 yr) with unilateral SCFE and in 23 adolescents matched in age without hip disease (control group). Five patients developed SCFE in the contralateral side during follow-up. Different geometric measurements were taken from hip conventional X-ray studies. The p-FEM of the proximal femur permits modifications of different geometrical parameters, therefore the X-ray measurements taken from each patient were applied to the model obtaining a subject-specific model for each case. In each model, different mechanical situations such as walking, stairs climbing and sitting were simulated by applying loads on the femoral head corresponding to each own weight. The risk for growth plate failure was estimated by the Tresca, von Misses and Rankine stresses. In summary, the models shows important differences between the stresses computed at the healthy femurs of patients with unilateral SCFE and femurs that further underwent bilateral SCFE. So, the 95% confidence interval of the percentage of volume of the growth plate subjected to stresses higher than 2MPa was almost similar for the control group and patients with unilateral SCFE. However, those patients who developed bilateral disease had statistically significant large physeal areas with more than 2.0 MPa (p< 0.005). Stresses were also strongly dependent on the geometry of the proximal femur, especially on the posterior sloping angle of the physis and the physeal sloping angle. In spite of simplifications of the developed p-FEM, this tool has been able to show the influence of femur geometry in growth plate stresses and to predict the sites where growth plate starts to fail

Finite element (FE) analysis is widely used to calculate stresses and strains within human bone in order to improve implant designs. Although validated FE models of the human femur have been created (Lengsfeld et al., 1998), no equivalent yet exists for the tibia. The aim of this study was to create such an FE model, both with and without the tibial component of a knee replacement, and to validate it against experimental Results: A set of reference axes was marked on a cleaned, fresh frozen cadaveric human tibia. Seventeen triaxial stacked strain rosettes were attached along the bone, which was then subjected to nine axial loading conditions, two four-point bending loading conditions, and a torsional loading condition using a materials testing machine (MTS 858). Deflections and strain readings were recorded. Axial loading was repeated after implantation of a knee replacement (medial tibial component, Biomet Oxford Unicompartmental Phase 3). The intact tibia was CT scanned (GE HiSpeed CT/i) and the images used to create a 3D FE mesh. The CT data was also used to map 600 transversely isotropic material properties (Rho, 1996) to individual elements. All experiments were simulated on the FE model. Measured principal strains and displacements were compared to their corresponding FE values using regression analysis.

Experimental results were repeatable (mean coefficients of variation for intact and implanted tibia, 5.3% and 3.9%). They correlated well with those of the FE analysis (R squared = 0.98, 0.97, 0.97, and 0.99 for axial (intact), axial (implanted), bending, torsional loading). For each of the load cases the intersects of the regression lines were small in comparison to the maximum measured strains (< 1.5%). While the model was more rigid than the bone under torsional loading (slope =0.92), the opposite was true for axial (slope = 1.14 (intact) 1.24 (implanted)) and bending (slope = 1.06) loads. This is probably due to a discrepancy in the material properties of the model.

Introduction

Fretting corrosion at the taper interface of modular connections can be studied using Finite Element (FE) analyses. However, the loading conditions in FE studies are often simplified, or based on generic activity patterns. Using musculoskeletal modeling, subject-specific muscle and joint forces can be calculated, which can then be applied to a FE model for wear predictions. The objective of the current study was to investigate the effect of incorporating more detailed activity patterns on fretting simulations of modular connections.

Finite element (FE) analysis is widely used to calculate stresses and strains within human bone in order to improve implant designs. Although validated FE models of the human femur have been created (Lengsfeld et al., 1998), no equivalent yet exists for the tibia. The aim of this study was to create such an FE model, both with and without the tibial component of a knee replacement, and to validate it against experimental results.

A set of reference axes was marked on a cleaned, fresh frozen cadaveric human tibia. Seventeen triaxial stacked strain rosettes were attached along the bone, which was then subjected to nine axial loading conditions, two four-point bending loading conditions, and a torsional loading condition using a materials testing machine (MTS 858). Deflections and strain readings were recorded. Axial loading was repeated after implantation of a knee replacement (medial tibial component, Biomet Oxford Unicompartmental Phase 3). The intact tibia was CT scanned (GE HiSpeed CT/i) and the images used to create a 3D FE mesh. The CT data was also used to map 600 transversely isotropic material properties (Rho, 1996) to individual elements. All experiments were simulated on the FE model. Measured principal strains and displacements were compared to their corresponding FE values using regression analysis.

Experimental results were repeatable (mean coeffi-cients of variation for intact and implanted tibia, 5.3% and 3.9%). They correlated well with those of the FE analysis (R squared = 0.98, 0.97, 0.97, and 0.99 for axial (intact), axial (implanted), bending, torsional loading). For each of the load cases the intersects of the regression lines were small in comparison to the maximum measured strains (< 1.5%). While the model was more rigid than the bone under torsional loading (slope =0.92), the opposite was true for axial (slope = 1.14 (intact) 1.24 (implanted)) and bending (slope = 1.06) loads. This is probably due to a discrepancy in the material properties of the model.

It is impossible to determine the effect of a single parameter in clinical or in-vitro knee research. There are also parameters which can not or hardly be determined. These disadvantages can be overcome with a model. The objective of this study was to create a dynamic FE model of a human knee joint after TKA which is applicable to a variety of research question.

The knee model consisted of a femur, tibia and patella, collateral ligaments and a PCL, combined with a CKS cruciate retaining total knee prosthesis. The patella was not resurfaced. An axialload of 150 N and a quadriceps-force of 81N was applied. The model was validated by the model prediction of joint laxities at different flexion-angles and the calculation of the knee kinematics during flexion-extension.

The predicted varus-valgus laxity at different flexion angles was in between 0 and 6.3 degrees. Laxity values decreased towards extension and towards 90 degrees of flexion. The AP test at 20, 30 and 90 degrees of flexion showed a anterior laxity of 3.1, 4.3 and 2 mm, respectively. The posterior laxity was 5.7 mm, but could only be determined at 90 degrees. The model predicted reasonable kinematics, which were identical for two consecutive flexion-extension movements.

The model predictions were well in agreement with reported values, which were measured experimentally. Differences could be well explained by ligament structures which were (still) omitted with in the model. This dynamic model, in which ligaments were actually modelled as bands, combined all major structures within the knee joint. It was well able to predict laxities and kinematics and turned out to be very stable, mathematically. With this model we will be able to address effects of prosthetic and surgical parameters on the stability and kinematics of the knee joint.

Introduction: The chevron osteotomy is an accepted method for the correction of mild and moderate hallux valgus and generally advocated for patients younger than the age of sixty years. In the current work the finite element analysis applied to calculate the stress (force per unit area) on different cuts in the metatarsal bone model of the first ray in the human foot.

Material and Methods: The cuts have the form of a simple angle with 90 degrees ‘modified chevron osteotomy’, 60 ‘typical chevron osteotomy’ 70, 50 and 30, openings correspondingly, and share a common corner C, which is at the centre of a circle that fits the head of the metatarsal. In order to calculate the maximum stresses on the cuts, the bone is assumed to be with a 150 angle to the floor, which is the angle that it takes during the push-off phase.

Results: The calculations show a considerable difference on the stress distribution on the differnt cuts. In particular in the ‘90 degrees cut’ the normal (to the cut) stress is much larger than the shear stress. The opposite is true for the 60 cut. Since shear stresses are the ones that cause material failure, it is predicted that the 90 cut will heal much faster than the 60 cut. The nodes along the cuts where the normal and the shear stress were calculated in different osteotomies.

Conclusion: The FEM analysis confirm our clinical results of this modified chevron osteotomy of 90 degrees. The osteotomy site is firmly secured, avoiding early displacement of the lateral fragment and give earlier fusion.

INTRODUCTION. In theory, Finite Element Analysis (FEA) is an attractive method for elucidating the mechanics of modular implant junctions, including variations in materials, designs, and modes of loading. However, the credence of any computational model can only be established through validation using experimental data. In this study we examine the validity of such a simulation validated by comparing values of interface motion predicted using FEA with values measured during experimental simulation of stair-climbing. MATERIALS and METHODS. Two finite element models (FEM) of a modular implant assembly were created for use in this study, consisting of a 36mm CoCr femoral head attached to a TiAlV rod with a 14/12 trunnion. Two head materials were modelled: CoCr alloy (118,706 10-noded tetrahedral elements), and alumina ceramic (124,710 10-noded tetrahedral elements). The quasi-static coefficients of friction (µ. s. ) of the CoCr-TiAlV and Ceramic-TiAlV interfaces were calculated from uniaxial assembly (2000N) and dis-assembly experiments performed in a mechanical testing machine (Bionix, MTS). Interface displacements during taper assembly and disassembly were measured using digital image correlation (DIC; Dantec Dynamics). The assembly process was also simulated using the computational model with the friction coefficient set to µ. s. and solved using the Siemens Nastran NX 11.0 Solver. The frictional conditions were then varied iteratively to find the value of µ providing the closest estimate to the experimental value of head displacement during assembly. To validate the FEA model, the relative motion between the head and the trunnion was measured during dynamic loading simulating stair-climbing. Each modular junction was assembled in a drop tower apparatus and then cyclically loaded from 230–4300N at 1 Hz for a total of 2,000 cycles. The applied load was oriented at 25° to the trunnion axis in the frontal plane and 10° in the sagittal plane. The displacement of the head relative to the trunnion during cyclic loading was measured by a three-camera digital image correlation (DIC) system. The same loading conditions were simulated using the FEA model using the optimal value of µ derived from the initial head assembly trials. RESULTS. For both head materials, the predicted values of axial displacement of the head on the trunnion closely approximated the measured values derived from DIC measurements, with differences of −0.17% to +6.5%, respectively. Larger differences were calculated for individual components of motion for the stair climbing activity. However, the predicted magnitude of interface motion was still within 10% of the observed values, ranging from −7% to −5%. CONCLUSIONS. Our simulations closely approximated physical testing using complex loading, coming within 7% of the target values. By generating a validated computational model of a modular junctions with varying head materials, we will be able to simulate additional activities of daily living to determine micromotion and areas of peak pressure and contact stresses generated. For any figures or tables, please contact authors directly

Summary Statement. A coupled finite element - analytical model is presented to predict and to elucidate a clinical healing scenario where bone regenerates in a critical-sized femoral defect, bounded by periosteum or a periosteum substitute implant and stabilised via an intramedullary nail. Introduction. Bone regeneration and maintenance processes are intrinsically linked to mechanical environment. However, the cellular and subcellular mechanisms of mechanically-modulated bone (re-) generation are not fully understood. Recent studies with periosteum osteoprogenitor cells exhibit their mechanosensitivity in vitro and in situ. In addtion, while a variety of growth factors are implicated in bone healing processes, bone morphogenetic protein-2 (BMP-2) is recognised to be involved in all stages of bone regeneration. Furthermore, periosteal injuries heal predominantly via endochondral ossification mechanisms. With this background in mind, the current study aims to understand the role of mechanical environment on BMP-2 production and periosteally-mediated bone regeneration. The one-stage bone transport model [1] provides a clinically relevant experimental platform on which to model the mechanobiological process of periosteum-mediated bone regeneration in a critical-sized defect. Here we develop a model framework to study the cellular-, extracellular- and mechanically-modulated process of defect infilling, governed by the mechanically-modulated production of BMP-2 by osteoprogenitor cells located in the periosteum. Methods. Material properties of the healing callus and periosteum contribute to the strain stimulus sensed by osteoprogenitor cells therein. Using a mechanical finite element model, periosteal surface strains are first predicted as a function of callus properties. Strains are then input to a mechanistic mathematical model, where mechanical regulation of BMP-2 production mediates rates of cellular proliferation, differentiation and extracellular matrix (ECM) production, to predict healing outcomes. A parametric approach enables the spatial and temporal prediction of tissue regeneration via endochondral ossification. Predictions are compared with experimental, micro-computed tomographic and histologic, measures of cartilage and mineralised bone tissue regenerates. Model Predictions in Light of Experimental Case Studies: A validated baseline model predicts defect healing via cellular egression, extracellular matrix production and endochondral ossification, using parameters optimised to mimic experimental outcome measures at initial and final stages of healing. To elucidate which predictive model paramenters result in the intrinsic differences in experimental outcomes between defects bounded by either periosteum in situ or a periosteum substitute implant, model parameters are then varied by orders of magnitude to determine which factors exert dominant influence on achievement of experimentally relevant ECM area outcomes. Considering the complete set of parameters relevant to healing, the rate of osteoprogenitor to osteoblast differentiation, as well as rates of chondrocyte and osteoblast proliferation must be reduced and ECM production by chondrocytes must be increased from baseline, to achieve healing outcomes analogous to those observed in experiments. Discussion/Conclusion. The novel model framework presented here integrates a mechanistic feedback system, based on the mechanosensitivity of periosteal osteoprogenitor cells, which allows for modeling and prediction of tissue regeneration on multiple length and time scales

Abstract

Aims. The presence of facet tropism has been correlated with an elevated susceptibility to lumbar disc pathology. Our objective was to evaluate the impact of facet tropism on chronic lumbosacral discogenic pain through the analysis of clinical data and finite element modelling (FEM). Methods. Retrospective analysis was conducted on clinical data, with a specific focus on the spinal units displaying facet tropism, utilizing FEM analysis for motion simulation. We studied 318 intervertebral levels in 156 patients who had undergone provocation discography. Significant predictors of clinical findings were identified by univariate and multivariate analyses. Loading conditions were applied in FEM simulations to mimic biomechanical effects on intervertebral discs, focusing on maximal displacement and intradiscal pressures, gauged through alterations in disc morphology and physical stress. Results. A total of 144 discs were categorized as ‘positive’ and 174 discs as ‘negative’ by the results of provocation discography. The presence of defined facet tropism (OR 3.451, 95% CI 1.944 to 6.126) and higher Adams classification (OR 2.172, 95% CI 1.523 to 3.097) were important predictive parameters for discography-‘positive’ discs. FEM simulations showcased uneven stress distribution and significant disc displacement in tropism-affected discs, where loading exacerbated stress on facets with greater angles. During varied positions, notably increased stress and displacement were observed in discs with tropism compared to those with normal facet structure. Conclusion. Our findings indicate that facet tropism can contribute to disc herniation and changes in intradiscal pressure, potentially exacerbating disc degeneration due to altered force distribution and increased mechanical stress. Cite this article: Bone Joint Res 2024;13(9):452–461

Objectives. Elevated proximal tibial bone strain may cause unexplained pain, an important cause of unicompartmental knee arthroplasty (UKA) revision. This study investigates the effect of tibial component alignment in metal-backed (MB) and all-polyethylene (AP) fixed-bearing medial UKAs on bone strain, using an experimentally validated finite element model (FEM). Methods. A previously experimentally validated FEM of a composite tibia implanted with a cemented fixed-bearing UKA (MB and AP) was used. Standard alignment (medial proximal tibial angle 90°, 6° posterior slope), coronal malalignment (3°, 5°, 10° varus; 3°, 5° valgus), and sagittal malalignment (0°, 3°, 6°, 9°, 12°) were analyzed. The primary outcome measure was the volume of compressively overstrained cancellous bone (VOCB) < -3000 µε. The secondary outcome measure was maximum von Mises stress in cortical bone (MSCB) over a medial region of interest. Results. Varus malalignment decreased VOCB but increased MSCB in both implants, more so in the AP implant. Varus malalignment of 10° reduced the VOCB by 10% and 3% in AP and MB implants but increased the MSCB by 14% and 13%, respectively. Valgus malalignment of 5° increased the VOCB by 8% and 4% in AP and MB implants, with reductions in MSCB of 7% and 10%, respectively. Sagittal malalignment displayed negligible effects. Well-aligned AP implants displayed greater VOCB than malaligned MB implants. Conclusion. All-polyethylene implants are more sensitive to coronal plane malalignments than MB implants are; varus malalignment reduced cancellous bone strain but increased anteromedial cortical bone stress. Sagittal plane malalignment has a negligible effect on bone strain. Cite this article: I. Danese, P. Pankaj, C. E. H. Scott. The effect of malalignment on proximal tibial strain in fixed-bearing unicompartmental knee arthroplasty: A comparison between metal-backed and all-polyethylene components using a validated finite element model. Bone Joint Res 2019;8:55–64. DOI: 10.1302/2046-3758.82.BJR-2018-0186.R2

Objectives. Up to 40% of unicompartmental knee arthroplasty (UKA) revisions are performed for unexplained pain which may be caused by elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on bone strain in a cemented fixed-bearing medial UKA using a finite element model (FEM) validated experimentally by digital image correlation (DIC) and acoustic emission (AE). Materials and Methods. A total of ten composite tibias implanted with all-polyethylene (AP) and metal-backed (MB) tibial components were loaded to 2500 N. Cortical strain was measured using DIC and cancellous microdamage using AE. FEMs were created and validated and polyethylene thickness varied from 6 mm to 10 mm. The volume of cancellous bone exposed to < -3000 µε (pathological loading) and < -7000 µε (yield point) minimum principal (compressive) microstrain and > 3000 µε and > 7000 µε maximum principal (tensile) microstrain was computed. Results. Experimental AE data and the FEM volume of cancellous bone with compressive strain < -3000 µε correlated strongly: R = 0.947, R. 2. = 0.847, percentage error 12.5% (p < 0.001). DIC and FEM data correlated: R = 0.838, R. 2. = 0.702, percentage error 4.5% (p < 0.001). FEM strain patterns included MB lateral edge concentrations; AP concentrations at keel, peg and at the region of load application. Cancellous strains were higher in AP implants at all loads: 2.2- (10 mm) to 3.2-times (6 mm) the volume of cancellous bone compressively strained < -7000 µε. Conclusion. AP tibial components display greater volumes of pathologically overstrained cancellous bone than MB implants of the same geometry. Increasing AP thickness does not overcome these pathological forces and comes at the cost of greater bone resection. Cite this article: C. E. H. Scott, M. J. Eaton, R. W. Nutton, F. A. Wade, S. L. Evans, P. Pankaj. Metal-backed versus all-polyethylene unicompartmental knee arthroplasty: Proximal tibial strain in an experimentally validated finite element model. Bone Joint Res 2017;6:22–30. DOI:10.1302/2046-3758.61.BJR-2016-0142.R1

Aims. There are concerns regarding nail/medullary canal mismatch and initial stability after cephalomedullary nailing in unstable pertrochanteric fractures. This study aimed to investigate the effect of an additional anteroposterior blocking screw on fixation stability in unstable pertrochanteric fracture models with a nail/medullary canal mismatch after short cephalomedullary nail (CMN) fixation. Methods. Eight finite element models (FEMs), comprising four different femoral diameters, with and without blocking screws, were constructed, and unstable intertrochanteric fractures fixed with short CMNs were reproduced in all FEMs. Micromotions of distal shaft fragment related to proximal fragment, and stress concentrations at the nail construct were measured. Results. Micromotions in FEMs without a blocking screw significantly increased as nail/medullary canal mismatch increased, but were similar between FEMs with a blocking screw regardless of mismatch. Stress concentration at the nail construct was observed at the junction of the nail body and lag screw in all FEMs, and increased as nail/medullary canal mismatch increased, regardless of blocking screws. Mean stresses over regions of interest in FEMs with a blocking screw were much lower than regions of interest in those without. Mean stresses in FEMs with a blocking screw were lower than the yield strength, yet mean stresses in FEMs without blocking screws having 8 mm and 10 mm mismatch exceeded the yield strength. All mean stresses at distal locking screws were less than the yield strength. Conclusion. Using an additional anteroposterior blocking screw may be a simple and effective method to enhance fixation stability in unstable pertrochanteric fractures with a large nail/medullary canal mismatch due to osteoporosis. Cite this article: Bone Joint Res 2022;11(3):152–161

Aims. In this study, we aimed to explore surgical variations in the Femoral Neck System (FNS) used for stable fixation of Pauwels type III femoral neck fractures. Methods. Finite element models were established with surgical variations in the distance between the implant tip and subchondral bone, the gap between the plate and lateral femoral cortex, and inferior implant positioning. The models were subjected to physiological load. Results. Under a load of single-leg stance, Pauwels type III femoral neck fractures fixed with 10 mm shorter bolts revealed a 7% increase of the interfragmentary gap. The interfragmentary sliding, compressive, and shear stress remained similar to models with bolt tips positioned close to the subchondral bone. Inferior positioning of FNS provided a similar interfragmentary distance, but with 6% increase of the interfragmentary sliding distance compared to central positioning of bolts. Inferior positioning resulted in a one-third increase in interfragmentary compressive and shear stress. A 5 mm gap placed between the diaphysis and plate provided stability comparable to standard fixation, with a 7% decrease of interfragmentary gap and sliding distance, but similar compressive and shear stress. Conclusion. Finite element analysis with FNS on Pauwels type III femoral neck fractures revealed that placement of the bolt tip close to subchondral bone provides increased stability. Inferior positioning of FNS bolt increased interfragmentary sliding distance, compressive, and shear stress. The comparable stability of the fixation model with the standard model suggests that a 5 mm gap placed between the plate and diaphysis could viably adjust the depth of the bolt. Cite this article: Bone Joint Res 2022;11(2):102–111

Aims. This study aimed to identify the effect of anatomical tibial component (ATC) design on load distribution in the periprosthetic tibial bone of Koreans using finite element analysis (FEA). Methods. 3D finite element models of 30 tibiae in Korean women were created. A symmetric tibial component (STC, NexGen LPS-Flex) and an ATC (Persona) were used in surgical simulation. We compared the FEA measurements (von Mises stress and principal strains) around the stem tip and in the medial half of the proximal tibial bone, as well as the distance from the distal stem tip to the shortest anteromedial cortical bone. Correlations between this distance and FEA measurements were then analyzed. Results. The distance from the distal stem tip to the shortest cortical bone showed no statistically significant difference between implants. However, the peak von Mises stress around the distal stem tip was higher with STC than with ATC. In the medial half of the proximal tibial bone: 1) the mean von Mises stress, maximum principal strain, and minimum principal strain were higher with ATC; 2) ATC showed a positive correlation between the distance and mean von Mises stress; 3) ATC showed a negative correlation between the distance and mean minimum principal strain; and 4) STC showed no correlation between the distance and mean measurements. Conclusion. Implant design affects the load distribution on the periprosthetic tibial bone, and ATC can be more advantageous in preventing stress-shielding than STC. However, under certain circumstances with short distances, the advantage of ATC may be offset. Cite this article: Bone Joint Res 2022;11(5):252–259

Aims. There are concerns regarding initial stability and early periprosthetic fractures in cementless hip arthroplasty using short stems. This study aimed to investigate stress on the cortical bone around the stem and micromotions between the stem and cortical bone according to femoral stem length and positioning. Methods. In total, 12 femoral finite element models (FEMs) were constructed and tested in walking and stair-climbing. Femoral stems of three different lengths and two different positions were simulated, assuming press-fit fixation within each FEM. Stress on the cortical bone and micromotions between the stem and bone were measured in each condition. Results. Stress concentration was observed on the medial and lateral interfaces between the cortical bone and stem. With neutral stem insertion, mean stress over a region of interest was greater at the medial than lateral interface regardless of stem length, which increased as the stem shortened. Mean stress increased in the varus-inserted stems compared to the stems inserted neutrally, especially at the lateral interface in contact with the stem tip. The maximum stress was observed at the lateral interface in a varus-inserted short stem. All mean stresses were greater in stair-climbing condition than walking. Each micromotion was also greater in shorter stems and varus-inserted stems, and in stair-climbing condition. Conclusion. The stem should be inserted neutrally and stair-climbing movement should be avoided in the early postoperative period, in order to preserve early stability and reduce the possibility of thigh pain, especially when using a shorter stem. Cite this article: Bone Joint Res 2021;10(4):250–258

Abstract. OBJECTIVES. Bone health deterioration is a major public health issue. General guidelines for the limitation of bone loss prescribe a healthy lifestyle and a minimum level of physical activity. However, there is no specific recommendation regarding targeted activities that can effectively maintain lumbar spine bone health. To provide a better understanding of such influencing activities, a new predictive modelling framework was developed to study bone remodelling under various loading conditions. METHODS. The approach is based on a full-body subject-specific musculoskeletal model [1] combined with structural finite element models of the lumbar vertebrae. Using activities recorded with the subject, musculoskeletal simulations provide physiological loading conditions to the finite element models which simulate bone remodelling using a strain-driven optimisation algorithm [2]. With a combination of daily living activities representative of a healthy lifestyle including locomotion activities (walking, stair ascent and descent, sitting down and standing up) and spine-focused activities involving twisting and reaching, this modelling framework generates a healthy bone architecture in the lumbar vertebrae. The influence of spine-focused tasks was studied by adapting healthy vertebrae to an altered loading scenario where only locomotion activities were performed. RESULTS. The spine-focused activities were responsible for 57% of the overall bone mechanical stimulus of the five lumbar vertebrae. Cortical bone maintenance was more influenced by these activities in the superior vertebrae than in the inferior ones, with a stimulus degradation of 74% in L1 against 24% in L5 when adapted to the altered loading scenario. Trabecular bone stimulus degradation varied between 53% and 68%. CONCLUSION. The study suggests that locomotion activities are insufficient to maintain lumbar spine bone health. When appropriate, larger spine movements should be recommended as part of the minimum daily physical activities. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Aims. Vertebrates have adapted to life on Earth and its constant gravitational field, which exerts load on the body and influences the structure and function of tissues. While the effects of microgravity on muscle and bone homeostasis are well described, with sarcopenia and osteoporosis observed in astronauts returning from space, the effects of shorter exposures to increased gravitational fields are less well characterized. We aimed to test how hypergravity affects early cartilage and skeletal development in a zebrafish model. Methods. We exposed zebrafish to 3 g and 6 g hypergravity from three to five days post-fertilization, when key events in jaw cartilage morphogenesis occur. Following this exposure, we performed immunostaining along with a range of histological stains and transmission electron microscopy (TEM) to examine cartilage morphology and structure, atomic force microscopy (AFM) and nanoindentation experiments to investigate the cartilage material properties, and finite element modelling to map the pattern of strain and stress in the skeletal rudiments. Results. We did not observe changes to larval growth, or morphology of cartilage or muscle. However, we observed altered mechanical properties of jaw cartilages, and in these regions we saw changes to chondrocyte morphology and extracellular matrix (ECM) composition. These areas also correspond to places where strain and stress distribution are predicted to be most different following hypergravity exposure. Conclusion. Our results suggest that altered mechanical loading, through hypergravity exposure, affects chondrocyte maturation and ECM components, ultimately leading to changes to cartilage structure and function. Cite this article: Bone Joint Res 2021;10(2):137–148

Introduction Endoscopic single rod anterior fusion surgery for the treatment of adolescent idiopathic scoliosis (AIS) offers the advantages of improved cosmetic results, the fusion of fewer segments and faster patient rehabilitation. The development of a patient-specific finite element model of the spine to be used to predict post-operative biomechanical outcomes of anterior AIS surgery will improve the pre-operative planning and performance of scoliosis instrumentation. This study aims to develop a methodology for validating the finite element modeling approach to scoliosis surgical planning by producing biomechanical data for movements of ovine lumbar spines both with and without anterior rod scoliosis instrumentation. Methods Ovine lumbar spine specimens were CT scanned, dissected and instrumented across four levels (L2–L5) with a generic anterior single rod and screw implant for scoliosis correction. A displacement controlled 6 degree-of-freedom robotic facility was used to perform biomechanical testing on the spine segments for rotations of ±4 degrees in flexion/extension and lateral bending, and ±3 degrees in axial rotation. The tests were repeated with the rod removed. Resistive force and moment data was recorded using a force transducer and strain gauges on the surface of the rod yielded torsion and bending moment strain data, recorded on a data logger. All data was synchronized with the robot position data and filtered using moving average methods. The stiffness of the spines for each movement was calculated in units of Nm/degree of rotation. Results As expected the results reflect the variability found in biological materials. The similarities of behaviour profiles however, support the use of this method for FE model validation. The addition of the rod caused an increase in stiffness for each movement. This increase was 17±7% and 23±10% for left and right axial rotation, 93±35% and 73±50% for left and right lateral bending, and 78±46% and 67±35% for flexion and extension respectively. Recorded strains on the rod surface did not exceed 400με. Discussion The outcomes of this study have provided an experimental method for validating behaviour predicted by finite element models of the spine fitted with anterior scoliosis instrumentation. Using the CT scans of the ovine spines along with documentation of the experimental positioning of the specimens, the testing conditions can be simulated in a finite element model and the experimental and predicted biomechanical outcomes compared. The study also offers comparative information about the relative stiffness of the spine with and without scoliosis instrumentation

Introduction: Aligning the tibial tray is a critical step in total knee arthroplasty (TKA). Malalignment, (especially in varus) has been associated with failure and revision surgery. While the link between varus malalignment and failure has been attributed to increased medial compartmental loading and generation of shear stress, quantitative biomechanical evidence to directly support this mechanism is incomplete. We therefore constructed a finite element model of knee arthroplasty to test the hypothesis that varus malalignment of the tibial tray would increase the risk of tray subsidence. Methods: Cadaver Testing: Fresh human knees (N = 4) were CT scanned and implanted with a TKA cruciate-retaining tibial tray (Triathlon CR. Stryker Orthopaedics). The specimens were subjected to ISO-recommended knee wear simulation loading for up to 100,000 cycles. Micromotion sensors were mounted between the tray and underlying bone to measure micromotion. In two of the specimens, the application of vertical load was shifted medially to generate a load distribution ratio of 55:45 (medial: lateral) to represent neutral varus-valgus alignment. In the remaining two specimens, a load distribution ratio of 75:25 was generated to represent varus alignment. Finite element analysis: qCT scans of the tested knees were segmented using MIMICS (Materialise, Belgium). Material properties of bone were spatially assigned after converting bone density to elastic modulus. A finite element model of the tibia implanted with a tibial tray was constructed (Abaqus 6.8, Simulia, Dassault Systèmes). Boundary conditions were applied to simulate experimental mounting conditions and the tray was subjected to a single load cycle representing that applied during cadaver loading. Results: The two cadaver specimens tested at 55:45 medial:lateral (M:L) force distribution survived the 100,000 cycle test, while both cadaver specimens tested at 75:25 M:L force distribution failed. The finite element model generated distinct differences in compressive strain distribution patterns in the proximal tibia. A threshold of 2000 microstrain was used for fatigue damage in bone under cyclic loading. Both specimens loaded under 75:25 M:L distribution demonstrated substantially larger cortical bone volumes in the proximal tibial cortex that were greater than this fatigue threshold. Discussion and Conclusion: We validated a finite element model of tibial loading after TKA. Local compressive strains directly correlated with subsidence and failure in cadaver testing. A significantly greater volume of proximal tibial cortical bone was compressed to a strain greater than the fatigue threshold in the varus alignment group, indicating an increased risk for fatigue damage. This model is extremely valuable in studying the effect of surgical alignment, loading, and activity on damage to proximal bone

Introduction. Aligning the tibial tray is a critical step in total knee arthroplasty (TKA). Malalignment, (especially in varus) has been associated with failure and revision surgery. While the link between varus malalignment and failure has been attributed to increased medial compartmental loading and generation of shear stress, quantitative biomechanical evidence to directly support this mechanism is incomplete. We therefore constructed and validated a finite element model of knee arthroplasty to test the hypothesis that varus malalignment of the tibial tray would increase the risk of tray subsidence. Methods. Cadaver Testing. Fresh human knees (N = 4) were CT scanned and implanted with TKA cruciate-retaining tibial tray (Triathlon CR, Stryker Orthopaedics, New Jersey). The specimens were subjected to ISO-recommended knee wear simulation loading for up to 100,000 cycles. Micromotion sensors were mounted between the tray and underlying bone to measure micromotion. In two of the specimens, the application of vertical load was shifted medially to generate a load distribution ratio of 55:45 (medial:lateral) to represent neutral varus-valgus alignment. In the remaining two specimens, a load distribution ratio of 75:25 was generated to represent varus alignment. Finite element analysis. qCT scans of the tested knees were segmented using MIMICS (Materialise, Belgium). Material properties of bone were spatially assigned after converting bone density to elastic modulus. A finite element model of the tibia implanted with a tibial tray was constructed (Abaqus 6.8, Simulia, Dassault Syst`mes). Boundary conditions were applied to simulate experimental mounting conditions and the tray was subjected to a single load cycle representing that applied during cadaver loading. Results. The two cadaver specimens tested at 55:45 medial:lateral (M:L) force distribution survived the 100,000 cycle test, while both cadaver specimens tested at 75:25 M:L force distribution failed. The finite element model generated distinct differences in compressive strain distribution patterns in the proximal tibia. A threshold of 2000 microstrain was used for fatigue damage in bone under cyclic loading. Both specimens loaded under 75:25 M:L distribution demonstrated substantially larger cortical bone volumes in the proximal tibial cortex that were greater than this fatigue threshold. Discussion & Conclusion. We validated a finite element model of tibial loading after TKA. Local compressive strains directly correlated with subsidence and failure in cadaver testing. A significantly greater volume of proximal tibial cortical bone was compressed to a strain greater than the fatigue threshold in the varus alignment group, indicating an increased risk for fatigue damage. This model is extremely valuable in studying the effect of surgical alignment, loading, and activity on damage to proximal bone. Emerging techniques that customize tibial tray placement to the individual patient's pre-arthritic alignment run counter to the traditional recommendations for coronal alignment to the mechanical axis of the knee. A method that determines the risk of bone damage in a patient-specific manner can provide the surgeon with a safe range for component alignment and may even be applicable in preoperative planning

Objectives. In total hip arthroplasty (THA), the cementless, tapered-wedge stem design contributes to achieving initial stability and providing optimal load transfer in the proximal femur. However, loading conditions on the femur following THA are also influenced by femoral structure. Therefore, we determined the effects of tapered-wedge stems on the load distribution of the femur using subject-specific finite element models of femurs with various canal shapes. Patients and Methods. We studied 20 femurs, including seven champagne flute-type femurs, five stovepipe-type femurs, and eight intermediate-type femurs, in patients who had undergone cementless THA using the Accolade TMZF stem at our institution. Subject–specific finite element (FE) models of pre- and post-operative femurs with stems were constructed and used to perform FE analyses (FEAs) to simulate single-leg stance. FEA predictions were compared with changes in bone mineral density (BMD) measured for each patient during the first post-operative year. Results. Stovepipe models implanted with large-size stems had significantly lower equivalent stress on the proximal-medial area of the femur compared with champagne-flute and intermediate models, with a significant loss of BMD in the corresponding area at one year post-operatively. Conclusions. The stovepipe femurs required a large-size stem to obtain an optimal fit of the stem. The FEA result and post-operative BMD change of the femur suggest that the combination of a large-size Accolade TMZF stem and stovepipe femur may be associated with proximal stress shielding. Cite this article: M. Oba, Y. Inaba, N. Kobayashi, H. Ike, T. Tezuka, T. Saito. Effect of femoral canal shape on mechanical stress distribution and adaptive bone remodelling around a cementless tapered-wedge stem. Bone Joint Res 2016;5:362–369. DOI: 10.1302/2046-3758.59.2000525

Introduction. The ankle cartilage has an important function in walking movements, mainly in sports; for active young people, between 20 and 30 years old, the incidence of osteochondral lesions is more frequent. They are also more frequent in men, affecting around 21,000 patients per year in USA with 6.5% of ankle injuries generating osteochondral lesions. The lesion is a result of ankle sprain and is most frequently found in the medial location, in 53% of cases. The main objective of this work was to develop an experimental and finite element models to study the effect of the ankle osteochondral lesion on the cartilage behavior. Materials and Methods. The right ankle joint was reconstructed from an axial CT scan presenting an osteochondral lesion in the medial position with 8mm diameter in size. An experimental model was developed, to analyze the strains and influence of lesion size and location similar to the patient. The experimental model includes two cartilages constructed by Polyjet™ 3D printing from rubber material (young modulus similar to cartilage) and bone structures from a rigid polymer. The cartilage was instrumented with two rosettes in the medial and lateral regions, near the osteochondral region. The fluid considered was water at room temperature and the experimental test was run at 1mm/s. The Finite element model (FE) includes all the components considered in the experimental apparatus and was assigned the material properties of bone as isotropic and linear elastic materials; and the cartilage the same properties of rubber material. The fluid was simulated as hyper-elastic one with a Mooney-Rivlin behavior, with constants c1=0.07506 and c2=0.00834MPa. The load applied was 680N in three positions, 15º extension, neutral and 10º flexion. Results. The experimental strain measured in the cartilage in the rosettes presents similar behavior in all experiments and repetitions. The maximum value observed near the osteochondral lesion was 3014(±5.6)µε in comparison with the intact condition it was 468 (±1.95)µε. The osteochondral lesion increases the strains around 6.5 times and the synovial liquid reduces the intensity of strain distribution. The numerical model presents a good correlation with the experiments (R2 0.944), but the FE model underestimates the values. Discussion and conclusion. As a first conclusion, the size of the osteochondral lesion is important for the strains developed in cartilage. The size of lesion greater than 10mm is critical for the strains concentration. The synovial fluid present an important aspect in the strains measured, it reduces the strains in the external surface of cartilage and induces an increase in the lower part. This phenomenon should be addressed in more studies to evaluate this effect

Aims. Focal knee arthroplasty is an attractive alternative to knee arthroplasty for young patients because it allows preservation of a large amount of bone for potential revisions. However, the mechanical behaviour of cartilage has not yet been investigated because it is challenging to evaluate in vivo contact areas, pressure, and deformations from metal implants. Therefore, this study aimed to determine the contact pressure in the tibiofemoral joint with a focal knee arthroplasty using a finite element model. Methods. The mechanical behaviour of the cartilage surrounding a metal implant was evaluated using finite element analysis. We modelled focal knee arthroplasty with placement flush, 0.5 mm deep, or protruding 0.5 mm with regard to the level of the surrounding cartilage. We compared contact stress and pressure for bone, implant, and cartilage under static loading conditions. Results. Contact stress on medial and lateral femoral and tibial cartilages increased and decreased, respectively, the most and the least in the protruding model compared to the intact model. The deep model exhibited the closest tibiofemoral contact stress to the intact model. In addition, the deep model demonstrated load sharing between the bone and the implant, while the protruding and flush model showed stress shielding. The data revealed that resurfacing with a focal knee arthroplasty does not cause increased contact pressure with deep implantation. However, protruding implantation leads to increased contact pressure, decreased bone stress, and biomechanical disadvantage in an in vivo application. Conclusion. These results show that it is preferable to leave an edge slightly deep rather than flush and protruding. Cite this article: Bone Joint Res 2023;12(8):497–503

Smith's fractures generally occur when falling on a flexed wrist; however, orthopedic trauma surgeons often encounter distal radius fractures with volar displacement in patients who have allegedly fallen on the palm of their hands. This study aimed to reveal both the basic and clinical pathogenesis of Smith's fracture through a step-by-step investigation. We enrolled 17 patients with Smith's fractures, of which 71% fell on the palm and only 6% on the dorsum of the hand. First, we interviewed the outpatients to determine the mechanics of the injury and the position of their arm during injury. Second, we created a three-dimensional (3D) finite element model to predict the arm's position when the Smith's fracture occurred, which finite element analysis revealed as a 30° angle between the long axis of the forearm and the ground in the sagittal plane. Third, using this predicted position, we conducted experiments on 10 fresh frozen cadavers to prove the possibility of causing a Smith's fracture by falling on the palm of the hand. The results showed Smith-type fractures in seven of 10 wrists, whereas Colles-type fractures did not occur. Finally, we analyzed stress distribution in the distal radius when a Smith's fracture occurs using the 3D finite element model. In conclusion, this study demonstrates that Smith's fractures can also occur by falling on the palm of the hand

Aims. The aim of this study was to develop a novel computational model for estimating head/stem taper mechanics during different simulated assembly conditions. Methods. Finite element models of generic cobalt-chromium (CoCr) heads on a titanium stem taper were developed and driven using dynamic assembly loads collected from clinicians. To verify contact mechanics at the taper interface, comparisons of deformed microgroove characteristics (height and width of microgrooves) were made between model estimates with those measured from five retrieved implants. Additionally, these models were used to assess the role of assembly technique—one-hit versus three-hits—on the taper interlock mechanical behaviour. Results. The model compared well to deformed microgrooves from the retrieved implants, predicting changes in microgroove height (mean 1.1 μm (0.2 to 1.3)) and width (mean 7.5 μm (1.0 to 18.5)) within the range of measured changes in height (mean 1.4 μm (0.4 to 2.3); p = 0.109) and width (mean 12.0 μm (1.5 to 25.4); p = 0.470). Consistent with benchtop studies, our model found that increasing assembly load magnitude led to increased taper engagement, contact pressure, and permanent deformation of the stem taper microgrooves. Interestingly, our model found assemblies using three hits at low loads (4 kN) led to decreased taper engagement, contact pressures and microgroove deformations throughout the stem taper compared with tapers assembled with one hit at the same magnitude. Conclusion. These findings suggest additional assembly hits at low loads lead to inferior taper interlock strength compared with one firm hit, which may be influenced by loading rate or material strain hardening. These unique models can estimate microgroove deformations representative of real contact mechanics seen on retrievals, which will enable us to better understand how both surgeon assembly techniques and implant design affect taper interlock strength. Cite this article: Bone Joint J 2020;102-B(7 Supple B):33–40

Aims. Commonly performed unicompartmental knee arthroplasty (UKA) is not designed for the lateral compartment. Additionally, the anatomical medial and lateral tibial plateaus have asymmetrical geometries, with a slightly dished medial plateau and a convex lateral plateau. Therefore, this study aims to investigate the native knee kinematics with respect to the tibial insert design corresponding to the lateral femoral component. Methods. Subject-specific finite element models were developed with tibiofemoral (TF) and patellofemoral joints for one female and four male subjects. Three different TF conformity designs were applied. Flat, convex, and conforming tibial insert designs were applied to the identical femoral component. A deep knee bend was considered as the loading condition, and the kinematic preservation in the native knee was investigated. Results. The convex design, the femoral rollback, and internal rotation were similar to those of the native knee. However, the conforming design showed a significantly decreased femoral rollback and internal rotation compared with that of the native knee (p < 0.05). The flat design showed a significant difference in the femoral rollback; however, there was no difference in the tibial internal rotation compared with that of the native knee. Conclusion. The geometry of the surface of the lateral tibial plateau determined the ability to restore the rotational kinematics of the native knee. Surgeons and implant designers should consider the geometry of the anatomical lateral tibial plateau as an important factor in the restoration of native knee kinematics after lateral UKA. Cite this article: Bone Joint Res 2019;8:593–600

Femoral nails are thought to be load sharing devices. However, the specific load sharing characteristics and associated stress concentrations have not yet been reported in the literature. The purpose of this study was to use a validated, three dimensional finite element model of a nailed femur subjected to gait loads in order to determine the resulting stresses in the femur and the nail. The results showed that load was shared between the nail and the bone throughout the gait cycle. In addition, high stress concentrations were noted in the bone around the screw holes, and dynamization was of minimal benefit. To determine the stresses in the bone and nail in a femur with a locked, retrograde, intramedullary nail. The retrograde femoral nail is a load sharing device. High stress concentrations occur in the bone around locking screw holes. When only one locking screw is used proximally and distally, stresses in the implant are excessive and may lead to failure. Dynamization was of minimal benefit. This is the first study to use a validated three dimensional finite element model to provide a detailed biomechanical analysis of stress patterns in a retrograde nailed femur under gait loads. The results can help resolve issues of stress shielding, implant removal, number of locking screws and dynamization. In the fully locked condition, loads in the femur were significantly higher than those in the nail for most of the gait cycle. Removal of locking screws to obtain dynamization only increased axial load in the femur by 17 %. However, stresses in the locking screws increased by as much as 250% when fewer than 4 screws were used. Maximum stresses in the bone were found around screw holes. A three dimensional finite element model of the femur and nail was developed. The model was validated by comparing results to a physical saw bone model instrumented with strain gages and subjected to a simple a compressive load. Once good correlation with simple loading patterns was demonstrated, gait loading patterns obtained from literature were incorporated and simulations were run for various conditions

Introduction. The process of wear and corrosion at the head-neck junction of a total hip replacement is initiated when the femoral head and stem are joined together during surgery. To date, the effects of the surface topography of the femoral head and metal stem on the contact mechanics during assembly and thus on tribology and fretting corrosion during service life of the implant are not well understood. Therefore, the objective of this study was to investigate the influence of the surface topography of the metal stem taper on contact mechanics and wear during assembly of the head-neck junction using Finite Element models. Materials and Methods. 2D axisymmetric Finite Element models were developed consisting of a simplified head-neck junction incorporating the surface topography of a threaded stem taper to investigate axial assembly with 1 kN. Subsequently, a base model and three modifications of the base model in terms of profile peak height and plateau width of the stem taper topography and femoral head taper angle were calculated. To account for the wear process during assembly a law based on the Archard equation was implemented. Femoral head was modeled as ceramic (linear-elastic), taper material was either modeled as titanium, stainless steel or cobalt-chromium (all elastic-plastic). Wear volume, contact area, taper subsidence, equivalent plastic strain, von Mises stress, engagement length and crevice width was analyzed. Results. Titanium tapers showed largest wear volume throughout all simulations, followed by stainless steel and cobalt-chromium. A larger head taper angle resulted in an increase of the wear volume for all taper materials while the increase of the plateau width resulted in a decrease of the wear volume. Taper subsidence, von Mises stress and equivalent plastic strain followed the same trends. Contact area was largest for the models with a large plateau width for all taper materials. Other taper parameters had little effect on contact area. A pure increase of the angular mismatch (AM) resulted in the strongest decrease of the engagement length, while a combined increase of the AM and plateau width showed only a moderate decrease. The smallest effect concerning the engagement length was found when a combined increase of the profile peak height and AM was simulated. Crevice width was largest for a pure increase of the AM and for a combined increase of the AM and profile peak height for all taper materials. Discussion. This study showed that depending on the surface topography and material of the stem taper, wear and taper mechanics during assembly could be affected. For the examined surface topographies wear is distinctively elevated by increasing the AM and the profile peak height due to the resulting higher mechanical loading. More parameter studies under in vivo loading and the study of other taper surface parameters like the peak-to-peak distance have to be conducted to get a deeper insight into taper mechanics and wear effects. However, this study demonstrates the importance of good manufacturing practice of components for hip replacement systems to guarantee reproducible taper mechanics. For any figures or tables, please contact authors directly

Micro level finite element models of bone have been extensively used in the literature to examine its mechanical behaviour and response to loads. Techniques used previously to create these models involved CT attenuations or images (e.g. micro-CT, MRI) of real bone samples. The computational models created using these methods could only represent the samples used in their construction and any possible variations due to factors such as anatomical site, sex, age or degree of osteopo-rosity cannot be included without additional sample collection and processing. This study considers the creation of virtual finite element models of trabecular bone, i.e. models that look like and mechanically behave like real trabecular bone, but are generated computationally. The trabecular bone is anisotropic both in terms of its micro-architecture and its mechanical properties. Considerable research shows that the key determinants of the mechanical properties of bone are related to its micro-architecture. Previous studies have correlated the apparent level mechanical properties with bone mineral density (BMD), which has also been the principal means of diagnosis of osteoporosis. However, BMD alone is not sufficient to describe bone micro-architecture or its mechanical behaviour. This study uses a novel approach that employs BMD in conjunction with micro-architectural indices such as trabecular thickness, trabecular spacing and degree of anisotropy, to generate virtual micro-architectural finite element models. The approach permits generation of several models, with suitable porous structure, for the same or different levels of osteoporosity. A series of compression and shear tests are conducted, numerically, to evaluate the apparent level orthotropic elastic properties. These tests show that models generated using identical micro-architectural parameters have similar apparent level properties, thus validating this initial bone modelling algorithm. Numerical tests also clearly illustrate that poor trabecular connectivity leads to inferior mechanical behaviour even in cases where the BMD values are relatively high. The generated virtual models have a range of applications such as understanding the fracture behaviour of osteoporotic bone and examining the interaction between bone and implants

Introduction: Contemporary surgical interventions for adolescent idiopathic scoliosis (AIS) include both anterior and posterior rod systems, in which a single or double rod construct provides curve correction and stability. This paper presents a methodology for development of patient-specific finite element methods to predict the biomechanical outcomes of scoliosis surgery pre-operatively, with the aim of optimising the performance of instrumentation constructs for anterior single rod AIS surgery. Methods: Geometry for each patient-specific finite element model is obtained from pre-operative thoracolumbar CT scans taken in the supine position using a low dose multi-slice imaging protocol. The finite element model incorporates vertebrae, intervertebral discs, and posterior processes with associated ligaments and zygapophysial joints. A custom pre-processor generates the entire model according to user-specified meshing parameters, providing rapid model generation once the geometric parameters have been extracted from each CT dataset. Material properties are currently based on published values. Simulated movements about axes corresponding to flexion/extension, left/right lateral bending, and trunk rotation are solved using the ABAQUS/Standard software, allowing assessment of predicted loads and stresses before and after addition of instrumentation. Results: The total time per patient required for model generation is currently about six hours, with manual measurement of spine geometry from the CT stack accounting for most of this time. Actual solution time for each finite element model is expected to be around four hours, making patient-specific pre-operative planning for endoscopic scoliosis surgery a feasible option at least in terms of processing time per patient. Discussion: A finite element methodology has been developed for patient-specific simulation of endoscopic scoliosis surgery. Issues to be addressed in future include prescription of patient-specific material properties, analysis of errors associated with geometry measurement from CT scans, and validation of the methodology by comparison of predicted and actual outcomes for scoliosis patients. Patient-specific simulation of scoliosis surgery has the potential to optimize surgical outcomes and reduce biomechanical complications associated with the use of endoscopic scoliosis instrumentation systems

Introduction. On the basis of a proposal by Noble, the marrow cavity form can be classified into three categories: normal, champagne-fluted and stovepipe. In the present study, three typical finite element femoral models were created using CT data based on Noble's three categories. The purpose was to identify the relationship of stress distribution of the surrounding areas between femoral bone marrow cavity form and hip stems. The results shed light on whether the distribution of the high-stress area reflects the stem design concept. In order to improve the results of THA, researchers need to consider the instability of a stem design based on the stress distributioin and give feedback on future stem selection. Methods. As analyzing object, we selected SL-PLUS and BiCONTACT stems. To develop finite element models, two parts (cortical bone and stem) were constructed using four-node tetrahedral elements. The model consisted of about 60,000 elements. The material characteristics were defined by the combination of mass density, elastic coefficient, and Poisson's ratio. Concerning the analysis system, HP Z800 Workstation was used as hardware and LS-DYNA Ver. 971 as software. The distal end of the femur was constrained in all directions. On the basis of ISO 7206 Part 4,8 that specifies a method of endurance testing for joint prostheses, the stem was tilted 10°, and a 1500 N resultant force in the area around the hip joint was applied to the head at an angle of 25° with the long axis. Automatic contact with a consideration of slip was used. Result. The maximum stress on femur implanted a SL-PLUS with marrow cavity form of normal, champagne-fluted and stovepipe were shown to be 90MPa, 90MPa and 45MPa. The maximum stress on a BiCONTACT with marrow cavity form of normal, champagne-fluted and stovepipe were shown to be 45MPa, 90MPa and 15MPa. Discussion. The design concept for aZweymüller-type stem can distribute load across a wide range of cortical bone from the middle position to the distal femur. It is determined using this concept that a wide range of stress was absorbed at the middle position and distal femur in the champagne-fluted and normal cases. On the other hand, the contact pressure zone of stovepipe could not meet the expected level at the distal femur. The method of this research involves controlling the stress conditions within the stem design. At this point, it is considered possible for the stability of various stem designs to be predicted and the stability to be assessed positively. On the basis of Noble's categories, three types of finite element model were made, and stress distribution measurement and finite element analyses were performed. The results indicate that Zweymüller stem has clinical validity for securing force in the champagne-fluted and stovepipe types from the stress distribution

Introduction. Density-modulus relationships are often used to map the mechanical properties of bone based on CT- intensity in finite element models (FEMs). Although these relationships are thought to be site-specific, relationships developed for alternative anatomic locations are often used regardless of bone being modeled. Six relationships are commonly used in finite element studies of the shoulder; however, the accuracy of these relationships have yet to be compared. This study compares each of these six relationships ability to predict apparent strain energy density (SED. app. ) in trabecular bone cores from the glenoid. Methods. Quantitative-CT (QCT) (0.625 mm isotropic voxels), and µ-CT scans (0.032 mm isotropic voxels) were obtained for fourteen cadaveric scapulae (7 male, 7 female). Micro finite element models (µ-FEMs) were created from 98 virtual ‘cores’ using direct conversion to hexahedral elements. Two µ-FEM cases were considered: homogeneous tissue modulus of 20 GPa, and heterogeneous tissue modulus scaled by CT intensity of the µ-CT images (196 models). Each µ- FEM model was compressively loaded to 0.5% apparent strain and apparent strain energy density (SED. app. ) was calculated. Additionally, each of the six density-modulus relationships were used to map heterogeneous material properties to co- registered QCT-derived models (588 models in total). The loading and boundary conditions were replicated in the QCT-FEMs and the SED. app. was calculated and compared to the µ-FEM SED. app. To account for more samples than donors, restricted maximum likelihood estimation (REML) linear regression compared µ-FEM SED. app. and QCT-FEM SED. app. for each relationship. Results. When considering comparisons between QCT-FEMs and µ-FEMs with a homogeneous tissue modulus, near absolute statistical agreement (Y=X) was observed between the µ-FEMs and the QCT-FEMs using the Morgan et al. (2003) pooled relationship. Not surprisingly, due to the similarity between the two relationships, the Gupta & Dan (2004) and Carter and Hayes (1977) models showed near identical REML linear regression fit parameters. All relationships other than the Morgan et al. (2003) pooled relationship, greatly underestimated the µ-FEM apparent strain energy density (SED. app. ) when considering a homogeneous tissue modulus in the µ-FEMs. The same result with the pooled relationship did not hold true when heterogeneous tissue modulus was considered in the µ-FEMs. The Büchler et al., (2002) relationship most accurately predicted the SED. app. for this comparison. Interestingly, the Gupta & Dan (2004) and Carter and Hayes (1977) relationships again showed near identical REML linear regression fit parameters. DISCUSSION. This study compared six common density-modulus relationships used to map mechanical properties of bone in shoulder FE studies. It was found that when considering a homogeneous tissue modulus for µ-FEMs, relationships pooled from alternative anatomic locations may accurately predict the mechanical properties of glenoid trabecular bone. However, when considering a heterogeneous tissue modulus, this did not hold true. Further studies to determine if these relationships can be translated to whole bones may provide insight into the predictive capabilities of using pooled density-modulus equations in the mapping of mechanical properties in future FEMs of the shoulder

Aims. The aim of this study was to determine the risk of tibial eminence avulsion intraoperatively for bi-unicondylar knee arthroplasty (Bi-UKA), with consideration of the effect of implant positioning, overstuffing, and sex, compared to the risk for isolated medial unicondylar knee arthroplasty (UKA-M) and bicruciate-retaining total knee arthroplasty (BCR-TKA). Methods. Two experimentally validated finite element models of tibia were implanted with UKA-M, Bi-UKA, and BCR-TKA. Intraoperative loads were applied through the condyles, anterior cruciate ligament (ACL), medial collateral ligament (MCL), and lateral collateral ligament (LCL), and the risk of fracture (ROF) was evaluated in the spine as the ratio of the 95. th. percentile maximum principal elastic strains over the tensile yield strain of proximal tibial bone. Results. Peak tensile strains occurred on the anterior portion of the medial sagittal cut in all simulations. Lateral translation of the medial implant in Bi-UKA had the largest increase in ROF of any of the implant positions (43%). Overstuffing the joint by 2 mm had a much larger effect, resulting in a six-fold increase in ROF. Bi-UKA had ~10% increased ROF compared to UKA-M for both the male and female models, although the smaller, less dense female model had a 1.4 times greater ROF compared to the male model. Removal of anterior bone akin to BCR-TKA doubled ROF compared to Bi-UKA. Conclusion. Tibial eminence avulsion fracture has a similar risk associated with Bi-UKA to UKA-M. The risk is higher for smaller and less dense tibiae. To minimize risk, it is most important to avoid overstuffing the joint, followed by correctly positioning the medial implant, taking care not to narrow the bone island anteriorly. Cite this article: Bone Joint Res 2022;11(8):575–584

Objectives. The aim of this study was to compare the biomechanical stability and clinical outcome of external fixator combined with limited internal fixation (EFLIF) and open reduction and internal fixation (ORIF) in treating Sanders type 2 calcaneal fractures. Methods. Two types of fixation systems were selected for finite element analysis and a dual cohort study. Two fixation systems were simulated to fix the fracture in a finite element model. The relative displacement and stress distribution were analysed and compared. A total of 71 consecutive patients with closed Sanders type 2 calcaneal fractures were enrolled and divided into two groups according to the treatment to which they chose: the EFLIF group and the ORIF group. The radiological and clinical outcomes were evaluated and compared. Results. The relative displacement of the EFLIF was less than that of the plate (0.1363 mm to 0.1808 mm). The highest von Mises stress value on the plate was 33% higher than that on the EFLIF. A normal restoration of the Böhler angle was achieved in both groups. No significant difference was found in the clinical outcome on the American Orthopedic Foot and Ankle Society Ankle Hindfoot Scale, or on the Visual Analogue Scale between the two groups (p > 0.05). Wound complications were more common in those who were treated with ORIF (p = 0.028). Conclusions. Both EFLIF and ORIF systems were tested to 160 N without failure, showing the new construct to be mechanically safe to use. Both EFLIF and ORIF could be effective in treating Sanders type 2 calcaneal fractures. The EFLIF may be superior to ORIF in achieving biomechanical stability and less blood loss, shorter surgical time and hospital stay, and fewer wound complications. Cite this article: M. Pan, L. Chai, F. Xue, L. Ding, G. Tang, B. Lv. Comparisons of external fixator combined with limited internal fixation and open reduction and internal fixation for Sanders type 2 calcaneal fractures: Finite element analysis and clinical outcome. Bone Joint Res 2017;6:433–438. DOI: 10.1302/2046-3758.67.2000640

Intervertebral discs (IVD) provide flexibility to the back and ensure functional distributions of the spinal loads. They are avascular, and internal diffusion-dependent metabolic transport is vital to supply nutrients to disc cells1, but interactions with personalized IVD shapes and mechanics remain poorly explored. Poromechanical finite element models of seven personalized lumbar IVD geometries, with mean heights ranging from 8 to 16 mm were coupled with a reactive oxygen, glucose and lactate transport model linked with tissue deformations and osmosis . In previous studies, reduced formulations of the divergence of the solute flux (∇ .J = ∇ . (D∇ C) = ∇ D. ∇ C +D∇ 2C) ignored the dependence of the diffusion on the deformation gradients, ∇ D. ∇C. We simulated this phenomenon to explore its significance in mechano-metabolic -transport couplings, in the different geometries, over 24h of simulated rest (8h) and physical activity (16h). ∇ D. ∇ C affected the daily variations of glucose concentrations in IVD thinner than 12 mm but with neglectable variation ranges, while not considering ∇ D. ∇ C in taller discs only slightly overestimated the glucose concentration. Most importantly, tall IVD had nearly 60% less glucose than thin IVD, with local drops below the concentration of 0.5 mM, considered to be critical for disc cells3, in the anterior nucleus pulposus. On the one hand, previous reduced formulations for mechanometabolic-transport models of the IVD seem acceptable, even for patient-specific modelling. On the other hand, tall IVD might suffer from unfortunate combinations of deformation-dependent solute diffusion and large diffusion distances, which may favor early. Acknowledgements: Catalan Government and European Commission (2020 BP 00282; ERC-2021-CoG-O-Health-101044828)

Virtual mechanical testing is a method for measuring bone healing using finite element models built from computed tomography (CT) scans. Previously, we validated a dual-zone material model for ovine fracture callus that differentiates between mineralized woven bone and soft tissue based on radiodensity. 1. The objective of this study was to translate the dual-zone material model from sheep to two important clinical scenarios: human tibial fractures in early-stage healing and late-stage nonunions. CT scans for N = 19 tibial shaft fractures were obtained prospectively at 12 weeks post-op. A second group of N = 33 tibial nonunions with CT scans were retrospectively identified. The modeling techniques were based on our published method. 2. The dual-zone material model was implemented for humans by performing a cutoff sweep for both the 12-week and nonunion groups. Virtual torsional rigidity (VTR) was calculated as VTR = ML/φ [N-m. 2. /°], where M is the moment reaction, L is the diaphyseal segment length, and φ is the angle of twist. As the soft tissue cutoff was increased, the rigidity of the clinical fractures decreased and soft tissue located within the fracture gaps produced higher strains that are not predicted without the dual zone approach. The structural integrity of the nonunions varied, ranging from very low rigidities in atrophic cases to very high rigidities in highly calcified hypertrophic cases, even with dual-zone material modeling. Human fracture calluses are heterogeneous, comprising of woven bone and interstitial soft tissue. Use of a dual-zone callus material model may be instrumental in identifying delayed unions during early healing when callus formation is minimal and/or predominantly fibrous with little mineralization. ACKNOWLEDGEMENTS:. This work was supported by the National Science Foundation (NSF) grant CMMI-1943287

Variations in component positioning of total hip replacements can lead to edge loading of the liner, and potentially affect device longevity. These effects are evaluated using ISO 14242:4 edge loading test results in a dynamic system. Mediolateral translation of one of the components during testing is caused by a compressed spring, and therefore the kinematics will depend on the spring stiffness and damping coefficient, and the mass of the translating component and fixture. This study aims to describe the sensitivity of the liner plastic strain to these variables, to better understand how tests using different simulator designs might produce different amounts of liner rim deformation. A dynamic explicit deformable finite element model with 36mm Pinnacle metal-on-polyethylene bearing geometry (DePuy Synthes, Leeds, UK) was used with material properties for conventional UHMWPE. Setup was 65° clinical inclination, 4mm mismatch, 70N swing phase load, and 100N/mm spring. Fixture mass was varied from 0.5-5kg, spring damping coefficient was varied from 0-2Ns/mm. They were changed independently, and in combination. Maximum separation values were relatively insensitive to changes in the mass, damping coefficient, or both. The sensitivity of peak plastic strain, to this range of inputs, was similar to changing the swing phase load from 70N to approximately 150N – 200N. Increasing the fixture mass and/or damping coefficient increased the peak plastic strain, with values from 0.15-0.19. Liner plastic deformation was sensitive to the spring damping and fixture mass, which may explain some of the differences in fatigue and deformation results in UHMWPE liners tested on different machines or with modified fixtures. These values should be described when reporting the results of ISO14242:4 testing. Acknowledgements. Funded by EPSRC grant EP/N02480X/1; CAD supplied by DePuy Synthes

Occlusal loading and muscle forces during mastication aids in assessment of dental restorations and implants and jaw implant design; however, three-dimensional bite forces cannot be measured with conventional transducers, which obstruct the native occlusion. The aim of this study was to combine accurate jaw kinematics measurements, together with subject-specific computational modelling, to estimate subject-specific occlusal loading and muscle forces during mastication. Motion experiments were performed on one male participant (age: 39yrs, weight: 82kg) with healthy dentition. Two low-profile magnetic sensors were fixed to the participant's teeth and the two dental arches digitised using an intra-oral scanner. The participant performed ten continuous of chewing on a polyurethane rubber sample of known material properties, followed by maximal compression (clenching). This was repeated at the molars, premolars of both the left and right sides, and central incisors. Jaw motion was simultaneously recorded from the sensors, and finite element modelling used to estimate bite force. Specifically, simulations of chewing and biting were performed by driving the model using the measured kinematics, and bite force magnitude and direction quantified. Muscle forces were then evaluated using a rigid-body musculoskeletal model of the patient's jaw. The first molars generated the largest bite forces during chewing (left: 309 N, right: 311 N) and maximum-force biting (left: 496 N, right: 495 N). The incisors generated the smallest bite forces during chewing (75 N) and maximum-force biting (114 N). The anterior temporalis and superficial masseter muscles had the largest contribution to maximum bite force, followed by the posterior temporalis and medial pterygoid muscles. This study presents a new method for estimating dynamic occlusal loading and muscle forces during mastication. These techniques provide new knowledge of jaw biomechanics, including muscle and occlusal loading, which will be useful in surgical planning and jaw implant design

The objective of this study was to use patient-specific finite element modeling to measure the 3D interfragmentary strain environment in clinically realistic fractures. The hypothesis was that in the early post-operative period, the tissues in and around the fracture gap can tolerate a state of strain in excess of 10%, the classical limit proposed in the Perren strain theory. Eight patients (6 males, 2 females; ages 22–95 years) with distal femur fractures (OTA/AO 33-A/B/C) treated in a Level I trauma center were retrospectively identified. All were treated with lateral bridge plating. Preoperative computed tomography scans and post-operative X-rays were used to create the reduced fracture models. Patient-specific materials properties and loading conditions (20%, 60%, and 100% body weight (BW)) were applied following our published method.[1]. Elements with von Mises strains >10% are shown in the 100% BW loading condition. For all three loading scenarios, as the bridge span increased, so did the maximum von Mises strain within the strain visualization region. The average gap closing (Perren) strain (mean ± SD) for all patient-specific models at each body weight (20%, 60%, and 100%) was 8.6% ± 3.9%, 25.8% ± 33.9%, and 39.3% ± 33.9%, while the corresponding max von Mises strains were 42.0% ± 29%, 110.7% ± 32.7%, and 168.4% ± 31.9%. Strains in and around the fracture gap stayed in the 2–10% range only for the lowest load application level (20% BW). Moderate loading of 60% BW and above caused gap strains that far exceeded the upper limit of the classical strain rule (<10% strain for bone healing). Since all of the included patients achieved successful unions, these findings suggest that healing of distal femur fractures may be robust to localized strains greater than 10%

Introduction: The mechanobiology and response of bone formation to strain under physiological loading is well established, however investigation into exceedingly soft scaffolds relative to cancellous bone is limited. In this study we designed and 3D printed mechanically-optimised low-stiffness implants, targeting specific strain ranges inducing bone formation and assessed their biological performance in a pre-clinical in vivo load-bearing tibial tuberosity advancement (TTA) model. The TTA model provides an attractive pre-clinical framework to investigate implant osseointegration within an uneven loading environment due to the dominating patellar tendon force. A knee finite element model from ovine CT data was developed to determine physiological target strains from simulated TTA surgery. We 3D printed low-stiffness Ti wedge osteotomy implants with homogeneous stiffness of 0.8 GPa (Ti1), 0.6 GPa (Ti2) and a locally-optimised design with a 0.3 GPa cortex and soft 0.1 GPa core (Ti3), for implantation in a 12-week ovine tibial advancement osteotomy (9mm). We quantitatively assessed bone fusion, bone area, mineral apposition rate and bone formation rate. Optimised Ti3 implants exhibited evenly high strains throughout, despite uneven wedge osteotomy loading. We demonstrated that higher strains above 3.75%, led to greater bone formation. Histomorphometry showed uniform bone ingrowthin optimised Ti3 compared to homogeneous designs (Ti1 and Ti2), and greater bone-implant contact. The greatest bone formation scores were seen in Ti3, followed by Ti2 and Ti1. Results from our study indicate lower stiffness and higher strain ranges than normally achieved in Ti scaffolds stimulate early bone formation. By accounting for loading environments through rational design, implants can be optimised to improve uniform osseointegration. Design and 3D printing of exceedingly soft titanium orthopaedic implants enhance strain induced bone formation and have significant importance in future implant design for knee, hip arthroplasty and treatment of large load-bearing bone defects

It is known that the gait dynamics of elderly substantially differs from that of young people. However, it has not been well studied how this age-related gait dynamics affects the knee biomechanics, e.g., cartilage mechanical response. In this study, we investigated how aging affects knee biomechanics in a female population using subject-specific computational models. Two female subjects (ages of 23 and 69) with no musculoskeletal disorders were recruited. Korea National Institute for Bioethics Policy Review Board approved the study. Participants walked at a self-selected speed (SWS), 110% of SWS, and 120% of SWS on 10 m flat ground. Three-dimensional marker trajectories and ground reaction forces (Motion Analysis, USA), and lower limbs’ muscle activities were measured (EMG, Noraxon USA). Knee cartilage and menisci geometries were obtained from subjects’ magnetic resonance images (3T, GE Health Care). An EMG-assisted musculoskeletal finite element modeling workflow was used to estimate knee cartilage tissue mechanics in walking trials. Knee cartilage and menisci were modeled using a transversely isotropic poroviscoelastic material model. Walking speed in SWS, 110%, and 120% of SWS were 1.38 m/s, 1.51 m/s, and 1.65 m/s for the young, and 1.21 m/s, 1.34 m/s and 1.46 m/s for the elderly, respectively. The maximum tensile stress in the elderly tibial cartilage was ~25%, ~33%, and ~32% lower than the young at SWS, 110%, and 120% of SWS, respectively. These preliminary results suggest that the cartilage in the elderly may not have enough stimulation even at 20% increases in walking speed, which may be one reason for tissue degeneration. To enhance these findings, further study with more subjects and different genders will investigate how age-related gait dynamics affects knee biomechanics. Acknowledgments: Australian NHMRC Ideas Grant (APP2001734), KITECH (JE220006)

3-D finite element model of a resurfaced femoral head was composed. Five configurations of cement layer were analyzed and the transient heat transfer analysis during cement polymerization was performed. Peak temperature at the bone-cement interface temperature was lower than 40 oC when there was no or 1.5 mm cement penetration but reached 54 oC and 74 oC with 6 mm penetration and 6 mm penetration plus a cement –filled cyst of 1 cm3, respectively. With deep cement penetration, and a large cement-filled cyst, the peak temperatures exceeded bone thermal osteonecrosis at 55 oC. To evaluate using a finite element analysis model, the possibility of bone thermal necrosis secondary to cement in resurfacing arthroplasty of the hip. With deep cement penetration, and the presence of a large cement-filled cyst, the peak temperatures were in the range of bone thermal osteonecrosis 55 oC. Cementing technique in resurfacing arthroplasty should strive to strike a balance between fixation and avoiding bone thermal necrosis by excessive cement penetration. This information could explain why femoral head cysts > 1cm are a risk factor for femoral loosening after resurfacing arthroplasty and excessive cement penetration could lead to femoral neck fracture. 3-D finite element model of a hemispherical resurfaced femoral head was composed of a metal shell with a diameter of 46 mm. Five configurations of cement layer were analyzed a) no penetration into the bone, b) 1.5 mm penetration, c) 6 mm penetration, d) 6 mm penetration and a 1 cm3 cement filled cyst, and e) 6 mm penetration and 2 cm3 cement-filled cyst. The transient heat transfer analysis during cement polymerization was performed in a series of time steps. The temperature within the bone and cement was lower than 40 oC when there was no or 1.5 mm cement penetration into the femoral head. In contrast, the peak temperature at the bone-cement interface reached 54 oC and 74 oC and 63 oC with 6 mm penetration and 6 mm penetration plus a cement –filled cyst of 1 cm3, respectively

Objectives. Malalignment of the tibial component could influence the long-term survival of a total knee arthroplasty (TKA). The object of this study was to investigate the biomechanical effect of varus and valgus malalignment on the tibial component under stance-phase gait cycle loading conditions. Methods. Validated finite element models for varus and valgus malalignment by 3° and 5° were developed to evaluate the effect of malalignment on the tibial component in TKA. Maximum contact stress and contact area on a polyethylene insert, maximum contact stress on patellar button and the collateral ligament force were investigated. Results. There was greater total contact stress in the varus alignment than in the valgus, with more marked difference on the medial side. An increase in ligament force was clearly demonstrated, especially in the valgus alignment and force exerted on the medial collateral ligament also increased. Conclusion. These results highlight the importance of accurate surgical reconstruction of the coronal tibial alignment of the knee joint. Varus and valgus alignments will influence wear and ligament stability, respectively in TKA. Cite this article: D-S. Suh, K-T. Kang, J. Son, O-R. Kwon, C. Baek, Y-G. Koh. Computational study on the effect of malalignment of the tibial component on the biomechanics of total knee arthroplasty: A Finite Element Analysis. Bone Joint Res 2017;6:623–630. DOI: 10.1302/2046-3758.611.BJR-2016-0088.R2

Introduction. Geometric variations of the hip joint can give rise to repeated abnormal contact between the femur and acetabular rim, resulting in cartilage and labrum damage. Population-based geometric parameterisation can facilitate the flexible and automated in silico generation of a range of clinically relevant hip geometries, allowing the position and size of cams to be defined precisely in three dimensions. This is advantageous compared to alpha angles, which are unreliable for stratifying populations by cam type. Alpha angles provide an indication of cam size in a single two-dimensional view, and high alpha angles have been observed in asymptomatic individuals. Parametric geometries can be developed into finite element models to assess the potential effects of morphological variations in bone on soft tissue strains. The aim of this study was to demonstrate the capabilities of our parameterisation research tool by assessing impingement severity resulting from a range of parametrically varied femoral and acetabular geometries. Methods. Custom made MATLAB (MathWorks) and Python codes. [1]. were used to generate bone surfaces, which were developed into finite element models in Abaqus (SIMULIA). Parametric femoral surfaces were defined by a spherical proximal head and ellipse sections through the neck/cam region. This method produced surfaces that were well fitted to bone geometry segmented from CT scans of cam patients and capable of producing trends in results similar to those found using segmented models. A simplified spherical geometry, including the labrum and acetabular cartilage, represented the acetabulum. Femoral parameters were adjusted to define relevant variations in cam size and position. Two radii (small and large cams) and two positions (anterior and superior cams) were defined resulting in four models. Alpha angles of these parametric femurs were measured in an anterior-posterior view and a cross-table lateral view using ImageJ (NIH). A further model was developed using a femur with a medium cam size and position, and the level of acetabular coverage and labrum length were varied. Bones were modelled as rigid bodies and soft tissues were modelled as transversely isotropic linearly elastic materials. With the acetabulum fully constrained in all cases, the femurs were constrained in translation and rotated to simulate flexion followed by internal rotation to cause impingement against the labrum. Results and Discussion. Models generated using the parametric approach showed that potential for tissue damage, indicated through local strain, was not predicted by measured alpha angle, but resulted from cam extent and position as defined by the ellipses. When variations were made to the acetabular rim, an increase in bone coverage had the greatest effect on impingement severity, indicated by strain in the cartilage labral-junction. An increase in labral length increased labral displacement, but had less effect on cartilage-labral strain. Patient specific models currently require full image segmentation, but there is potential to further develop these parametric methods to assess likely impingement severity based on a series of measures of the neck and acetabulum when three-dimensional imaging of patients is available

Even minor lesions in articular cartilage (AC) can cause underlying bone damage creating an osteochondral (OC) defect. OC defects can cause pain, impaired mobility and can develop to osteoarthritis (OA). OA is a disease that affects nearly 10% of the population worldwide. [1]. , and represents a significant economic burden to patients and society. [2]. While significant progress has been made in this field, realising an efficacious therapeutic option for unresolved OA remains elusive and is considered one of the greatest challenges in the field of orthopaedic regenerative medicine. [3]. Therefore, there is a societal need to develop new strategies for AC regeneration. In recent years there has been increased interest in the use of tissue-specific aligned porous freeze-dried extracellular matrix (ECM) scaffolds as an off-the-shelf approach for AC repair, as they allow for cell infiltration, provide biological cues to direct target-tissue repair and permit aligned tissue deposition, desired in AC repair. [4]. However, most ECM-scaffolds lack the appropriate mechanical properties to withstand the loads passing through the joint. [5]. One solution to this problem is to reinforce the ECM with a stiffer framework made of synthetic materials, such as polylactic acid (PLA). [6]. Such framework can be 3D printed to produce anatomically accurate implants. [7]. , attractive in personalized medicine. However, typical 3D prints are static, their design is not optimized for soft-hard interfaces (OC interface), and they may not adapt to the cyclic loading passing through our joints, thus risking implant failure. To tackle this limitation, more compliant or dynamic designs can be printed, such as coil-shaped structures. [8]. Thus, in this study we use finite element modelling to create different designs that mimic the mechanical properties of AC and prototype them in PLA, using polyvinyl alcohol as support. The optimal design will be combined with an ECM scaffold containing a tailored microarchitecture mimicking aspects of native AC. Acknowledgments: This project has received funding from the European Union's Horizon Europe research and innovation MSCA PF programme under grant agreement No. 101110000

Femoral head collapse is a possible complication after surgical treatment of femoral neck fractures. The purpose of this study was to examine whether implantation of a Sliding Hip Screw (SHS) or an X-Bolt could increase the risk of femoral head collapse. Similar to traditional hip screws, the X-Bolt is implanted through the femoral neck; however, it uses an expanding cross-shape to improve rotational stability. The risk of collapse was investigated alongside patient factors, such as osteonecrosis. This numerical study assessed the risk of femoral head collapse using linear eigenvalue buckling (an established method [1]), and also from the maximum von Mises stress within the cortical bone. The femoral head was loaded using the pressures reported by Yoshida et al. for a patient sitting down (reported to put the femoral head at greatest risk of collapse [2]), with a peak pressure of 9.4 MPa and an average pressure of 1.59 MPa. The femur was fixed in all degrees of freedom at a plane through the femoral neck. The X-Bolt and SHS were implanted in accordance with the operative techniques. The femoral head and implants were meshed with quadratic tetrahedral elements, and cortical bone was meshed with triangular thin shell elements. A converged mesh seeding density of 1.2 mm was used. All models were create and solved using ABAQUS finite element software (version 6.12, Simulia, Dassault Systèmes, France). The influence of implant type and presence was examined alongside a variety of patient factors:. Osteonecrosis, modelled as a cone of bone of varying angle, and varying modulus values. Cortical thinning. Reduced cortical modulus. Femoral head size. Twenty-two finite element models were run for each implant condition (intact; implanted with the X-Bolt; implanted with a SHS), resulting in a total of 66 models. The finite element models were validated using experimental tests performed on five 4. th. generation composite Sawbones femurs (Malmö, Sweden), and verified against previously published results [1]. No significant difference was found between the X-Bolt and the SHS, for either critical buckling pressure (p=0.964), or the maximum von Mises stress (p=0.274), indicating no difference in the risk of femoral head collapse. The maximum von Mises stress (and therefore the risk of collapse) within the cortical bone was significantly higher for the intact femoral head compared to both implants (X-Bolt: p=0.048, SHS: p=0.002). Of the factors examined, necrosis of the femoral head caused the greatest increase in risk. The study by Volokh et al. [1] concluded that deterioration of the cancellous bone underneath the cortical shell can greatly increase the risk of femoral head collapse, and the results of the present study support this finding. Interestingly the presence of either an X-Bolt or SHS implant appeared to reduce the risk of femoral head collapse

Polyethylene wear of joint replacements can cause severe clinical complications, including; osteolysis, implant loosening, inflammation and pain. Wear simulator testing is often used to assess new designs, but it is expensive and time consuming. It is possible to predict the volume of polyethylene implant wear from finite element models using a modification of Archard's classic wear law [1–2]. Typically, linear elastic isotropic, or elasto-plastic material models are used to represent the polyethylene. The purpose of this study was to investigate whether use of a viscoelastic material model would significantly alter the predicted volumetric wear of a mobile-bearing unicompartmental knee replacement. Tensile creep-recovery experiments were performed to characterise the creep and relaxation behaviour of the polyethylene (moulded GUR 4150 samples machined to 180×20×1 mm). Samples were loaded to 3 MPa stress in 4 minutes, and then held for 6 hours, the tensile stress was removed and samples were left to relax for 6 hours. The mechanical test data was used fit to a validated three–dimensional fractional Maxwell viscoelastic constitutive material model [3]. An explicit finite element model of a mobile–bearing unicompartmental knee replacement was created, which has been described previously [4]. The medial knee replacement was loaded to 1200 N over a period of 0.2 s. The bearing was meshed using quadratic tetrahedral elements (1.5 mm seeding size based on results of a mesh convergence study), and the femoral component was represented as an analytical rigid body. Wear predictions were made from the contact stress and sliding distance using Archard's law, as has been described in the literature [1–2]. A wear factor of 5.24×10. −11. was used based upon the work by Netter et al. [2]. All models were created and solved using ABAQUS finite element software (version 6.14, Simulia, Dassault Systemes). The fractional viscoelastic material model predicted almost twice as much wear (0.119 mm. 3. /million cycles) compared to the elasto-plastic model (0.069 mm. 3. /million cycles). The higher wear prediction was due to both an increased sliding distance and higher contact pressures in the viscoelastic model. These preliminary findings indicate the simplified elasto-plastic polyethylene material representation can underestimate wear predictions from numerical simulations. Polyethylene is known to be a viscoelastic material which undergoes creep clinically, and it is not surprising that it is necessary to represent that viscoelastic behaviour to accurately predict implant wear. However, it does increase the complexity and run time of such computational studies, which may be prohibitive

Although 3D-printed porous dental implants may possess improved osseointegration potential, they must exhibit appropriate fatigue strength. Finite element analysis (FEA) has the potential to predict the fatigue life of implants and accelerate their development. This work aimed at developing and validating an FEA-based tool to predict the fatigue behavior of porous dental implants. Test samples mimicking dental implants were designed as 4.5 mm-diameter cylinders with a fully porous section around bone level. Three porosity levels (50%, 60% and 70%) and two unit cell types (Schwarz Primitive (SP) and Schwarz W (SW)) were combined to generate six designs that were split between calibration (60SP, 70SP, 60SW, 70SW) and validation (50SP, 50SW) sets. Twenty-eight samples per design were additively manufactured from titanium powder (Ti6Al4V). The samples were tested under bending compression loading (ISO 14801) monotonically (N=4/design) to determine ultimate load (F. ult. ) (Instron 5866) and cyclically at six load levels between 50% and 10% of F. ult. (N=4/design/load level) (DYNA5dent). Failure force results were fitted to F/F. ult. = a(N. f. ). b. (Eq1) with N. f. being the number of cycles to failure, to identify parameters a and b. The endurance limit (F. e. ) was evaluated at N. f. = 5M cycles. Finite element models were built to predict the yield load (F. yield. ) of each design. Combining a linear correlation between FEA-based F. yield. and experimental F. ult. with equation Eq1 enabled FEA-based prediction of F. e. . For all designs, F. e. was comprised between 10% (all four samples surviving) and 15% (at least one failure) of F. ult. The FEA-based tool predicted F. e. values of 11.7% and 12.0% of F. ult. for the validation sets of 50SP and 50SW, respectively. Thus, the developed FEA-based workflow could accurately predict endurance limit for different implant designs and therefore could be used in future to aid the development of novel porous implants. Acknowledgements: This study was funded by EU's Horizon 2020 grant No. 953128 (I-SMarD). We gratefully acknowledge the expert advice of Prof. Philippe Zysset

Distal radius fractures are the most common osteoporotic fractures among women. The treatment of these fractures has been shifting from a traditional non-operative approach to surgery, using volar locking plate (VLP) technology. Surgery, however, is not without risk, complications including failure to restore an anatomic reduction, fracture re-displacement, and tendon rupture. The VLP implant is also marked by bone loss due to stress-shielding related to its high stiffness relative to adjacent bone. Recently, a novel internal, composite-based implant, with a stiffness less than the VLP, was designed to eradicate the shortcomings associated with the VLP implant. It is unclear, however, what effect this less-stiff implant will have upon adjacent bone density distributions long-term. The objective of this study was to evaluate the long-term effects of the two implants (the novel surgical implant and the gold-standard VLP) by using subject-specific finite element (FE) models integrated with an adaptive bone formation/resorption algorithm. Specimen: One fresh-frozen human forearm specimen (female, age = 84 years old) was imaged using CT and was used to create a subject-specific FE model of the radius. Finite element modeling: In order to simulate a clinically relevant (unstable) fracture of the distal radius, a wedge of bone was removed from the model, which was approximately 10 mm wide and centered 20 mm proximal to the tip of the radial styloid. Bone remodeling algorithm: A strain-energy density (SED) based bone remodeling theory was used to account for bone remodeling. With this approach, bone density decreased linearly when SED per bone density was less than 67.5 µJ/g and increased when it was more than 232.5 µJ/g. When it was in the lazy zone (67.5 to 232.5 µJ/g), no changes in density occurred. Boundary conditions: A 180 N quasi-static force representing the scaphoid, and a 120 N quasi-static force representing the lunate was applied to the radius. The midshaft of the radius was constrained. FE outcomes: To examine the effects of stress shielding associated with each implant, the long-term changes of bone density within proximal transverse cross-sections of radius were inspected. The regional density analysis focused on three transverse cross-sections. The transverse cross-sections were positioned proximal to the subchondral plate, and were distanced 50 (cross-section A), 57 (cross-section B), and 64 mm (cross-section C) from the subchondral endplate. For both implants in all three cross-sections, cortical bone was reserved completely at the volar side. On the dorsal side, the cortical bone was completely resorbed in the VLP model. In all cross-sections, the averaged resultant density was higher for the “novel implant”. The difference ranged from 33% (cross-section A) to 36% (cross-section C) in favor of the “novel implant”. On average, the density values of the novel implant were 34% higher in transverse cross-sections (A, B, and C). This study showed that the novel implant offered higher density distributions compared to the VLP, which suggests that the novel implant may be superior to the VLP in terms of avoiding stress shielding

The advantages of unicompartmental knee arthroplasty (UKA) include its bone preserving nature, lower relative cost and superior functional results. Some temporary pain has been reported clinically following this procedure. Could this be related to bone remodeling? A validated bone remodeling algorithm may have the answers…. A 3D geometry of an intact human cadaveric tibia was generated using CT images. An all poly unicompartmental implant geometry was positioned in an inlay and onlay configuration on the tibia and the post-operative models created. An adaptive bone remodeling algorithm was used with finite element modeling to predict the bone remodeling behavior surrounding the implant in both scenarios. Virtual DEXA images were generated from the model and bone mineral density (BMD) was measured in regions of interest in the AP and ML planes. BMD results were compared to clinical results. The bone remodelling algorithm predicted BMD growth in the proximal anterior regions of the tibia, with an inward tendency for both inlay and onlay models. Looking in the AP plane, a maximum of up to 7% BMD growth was predicted and in the ML plane this was as high as 16%. Minimal BMD loss was observed, which suggests minimal disturbance to the natural bone growth following UKA. Positron emission tomography (PET) scans showed active hot spots in the antero- medial regions of the tibia. These results were consistent with the finite element modeling results. Bone remodeling behavior was found to be sensitive to sizing and positioning of the implant. The adaptive bone remodeling algorithm predicted minimal BMD loss and some BMD growth in the anterior region of the tibia following UKA. This is consistent with patient complaint and PET scans

Objectives. Patient-specific (PS) implantation surgical technology has been introduced in recent years and a gradual increase in the associated number of surgical cases has been observed. PS technology uses a patient’s own geometry in designing a medical device to provide minimal bone resection with improvement in the prosthetic bone coverage. However, whether PS unicompartmental knee arthroplasty (UKA) provides a better biomechanical effect than standard off-the-shelf prostheses for UKA has not yet been determined, and still remains controversial in both biomechanical and clinical fields. Therefore, the aim of this study was to compare the biomechanical effect between PS and standard off-the-shelf prostheses for UKA. Methods. The contact stresses on the polyethylene (PE) insert, articular cartilage and lateral meniscus were evaluated in PS and standard off-the-shelf prostheses for UKA using a validated finite element model. Gait cycle loading was applied to evaluate the biomechanical effect in the PS and standard UKAs. Results. The contact stresses on the PE insert were similar for both the PS and standard UKAs. Compared with the standard UKA, the PS UKA did not show any biomechanical effect on the medial PE insert. However, the contact stresses on the articular cartilage and the meniscus in the lateral compartment following the PS UKA exhibited closer values to the healthy knee joint compared with the standard UKA. Conclusion. The PS UKA provided mechanics closer to those of the normal knee joint. The decreased contact stress on the opposite compartment may reduce the overall risk of progressive osteoarthritis. Cite this article: K-T. Kang, J. Son, D-S. Suh, S. K. Kwon, O-R. Kwon, Y-G. Koh. Patient-specific medial unicompartmental knee arthroplasty has a greater protective effect on articular cartilage in the lateral compartment: A Finite Element Analysis. Bone Joint Res 2018;7:20–27. DOI: 10.1302/2046-3758.71.BJR-2017-0115.R2

Revision surgeries for orthopaedic infections are done in two stages – one surgery to implant an antibiotic spacer to clear the infection and another to install a permanent implant. A permanent porous implant, that can be loaded with antibiotics and allow for single-stage revision surgery, will benefit patients and save healthcare resources. Gyroid structures can be constructed with high porosity, without stress concentrations that can develop in other period porous structures [1] [2]. The purpose of this research is to compare the resulting bone and prosthesis stress distributions when porous versus solid stems are implanted into three proximal humeri with varying bone densities, using finite element models (FEM). Porous humeral stems were constructed in a gyroid structure at porosities of 60%, 70%, and 80% using computer-aided design (CAD) software. These CAD models were analyzed using FEM (Abaqus) to look at the stress distributions within the proximal humerus and the stem components with loads and boundary conditions representing the arm actively maintained at 120˚ of flexion. The stem was assumed to be made of titanium (Ti6Al4V). Three different bone densities were investigated, representing a healthy, an osteopenic, and an osteoporotic humerus, with an average bone shape created using a statistical shape and density model (SSDM) based on 75 cadaveric shoulders (57 males and 18 females, 73 12 years) [3]. The Young's moduli (E) of the cortical and trabecular bones were defined on an element-by-element basis, with a minimum allowable E of 15 MPa. The Von Mises stress distributions in the bone and the stems were compared between different stem scenarios for each bone density model. A preliminary analysis shows an increase in stress values at the proximal-lateral region of the humerus when using the porous stems compared to the solid stem, which becomes more prominent as bone density decreases. With the exception of a few mesh dependent singularities, all three porous stems show stress distributions below the fatigue strength of Ti-6Al-4V (410 MPa) for this loading scenario when employed in the osteopenic and osteoporotic humeri [4]. The 80% porosity stem had a single strut exceeding the fatigue strength when employed in the healthy bone. The results of this study indicate that the more compliant nature of the porous stem geometries may allow for better load transmission through the proximal humeral bone, better matching the stress distributions of the intact bone and possibly mitigating stress-shielding effects. Importantly, this study also indicates that these porous stems have adequate strength for long-term use, as none were predicted to have catastrophic failure under the physiologically-relevant loads. Although these results are limited to a single boney geometry, it is based on the average shape of 75 shoulders and different bone densities are considered. Future work could leverage the shape model for probabilistic models that could explore the effect of stem porosity across a broader population. The development of these models are instrumental in determining if these structures are a viable solution to combatting orthopaedic implant infections

Purpose: The purpose of this study was to determine how to minimize intramedullary femur pressures, and therefore the risk of fat embolus syndrome, during surgical procedures which require preparation and instrumentation of the femoral canal. Methods: To study intramedullary femur pressures and experimental model and a finite element model were developed. The experimental model ustilized a bone analogue which consisted of a porous plastic cylinder, having similar porosity and pore size to human femoral bone, with bone marrow being represented by a paraffin wax/petroleum jelly mixture. The finite element model consisted of a three dimensional analysis of a cylinder filled with bone marrow with a reamer advancing through it. Variables such as speed of insertion, fluid viscosity and relative diameters of the instrument and the inner diameter of the simulated bone were varied to see how they affected pressures. Results: The intramedullary pressures increased with increasing speed of instrument insertion, increasing marrow viscosity, and increased diameter of the instrument relative to the inner diameter of the bone. Experimental and finite element results were in reasonable agreement. Conclusions: We concluded that slower instrument insertion rates and a greater ratio of bone inner diameter to instrument diameter may minimize the intramedullary pressures and therefore minimize the risk of fat embolus syndrome. In addition, two novel techniques to analyze intramedullary femur pressures have been developed. Funding: Education Grant. Funding Parties: NSERC

Introduction. Relative motion at the modular head-neck junction of hip prostheses can lead to severe surface damage through mechanically-assisted corrosion. One factor affecting the mechanical performance of modular junctions is the frictional resistance of the mating surfaces to relative motion. Low friction increasing forces normal to the head-neck interface, leading to a lower threshold for slipping during weight-bearing. Conversely, a high friction coefficient is expected to limit interface stresses but may also allow uncoupling of the interface in service. This study was performed to examine this trade-off using finite element models of the modular head-neck junction. Methods. A finite element model (FEM) of the trunnion/ head assembly of a total hip prosthesis was initially created and experimentally validated. CAD models of a stem trunnion (taper size: 12/14mm) and a prosthetic femoral head (diameter: 28mm) were discretized into elements for finite element analysis (FEA). The trunnion (Ti6Al4V) was modelled with a hexahedral mesh (33,648 elements) and the femoral head (CoCrMo) with a tetrahedral mesh (51,182 elements). A friction-based sliding contact interface was defined between the mating surfaces. The model was loaded in 2 stages: (i) an assembly load of 4000N applied along the trunnion axis, and (ii) 500N applied along the trunnion axis in combination with a torque of 10Nm. A linear static solution was set up using Siemens NX-Nastran solver. Multiple simulations were executed by modulating the frictional coefficient at the taper-bore interface from 0.05 to 0.15 in increments of 0.01, the coefficient of 0.1 serving as the control case (Swaminathan and Gilbert, 2012). Results. The vertical and tangential displacements of the nodes on the taper of the trunnion relative to the femoral head demonstrated a strong inverse dependence upon the coefficient of friction at the interface (Fig. 1). A similar trend was observed with respect to the peak interface pressure (Fig. 2). The peak von Mises stress, however, increases with increasing coefficient of friction (Fig. 2). A Fisher's R to Z correlation test was performed on each output variable to determine its correlation with coefficient of friction. The coefficient of friction correlated significantly (p<0.0001) with both tangential displacement (r = −0.990) and vertical displacement (r = −0.974). Peak von Mises stress (r = 0.995) and peak contact pressure (r = −0.984) were also found to be significantly (p<0.0001) correlated to the coefficient of friction. Discussion. A higher coefficient of friction at the taper-bore interface led to lower contact pressure and sliding at the modular junction. However, higher coefficients of friction also led to increased von Mises stresses within the bore and the trunnion increasing the risk of yielding and fatigue failure. The current results strongly indicate that factors affecting the frictional coefficient at the interface likely influence the occurrence of and severity of mechanically-assisted corrosion in THA. Significance. The results from this study will help us set tolerances for the interlocking mechanism, identifying the minimum frictional coefficient required to obtain stable implant mechanics

Objectives: The purpose of this biomechanical study was to compare the mechanical properties of locked nails and screws made from either stainless steel or titanium alloy. Methods: The specially designed locked nails and screws with the same structures were made from either stainless steel or titanium alloy. The structural factors investigated included inner diameter and root radius for locking screws and outer diameter and nail hole size for locked nails. The mechanical properties investigated included bending stiffness, strength, and fatigue life. Finite element models were used to simulate the mechanical tests and compute the stress concentration factors. Results: Increasing the root radius and the inner diameter could effectively increase the fatigue strength of the locking screws. Fatigue strength increased more in titanium than in stainless steel screws, especially when the inner diameter was increased. In contrast, the titanium locked nails were much weaker than their stainless steel counterparts. Finite element models could closely predict the results of the biomechanical tests with a correlation coefficient that ranged from −0.58 to −0.84 for screws and was −0.98 for nails. The stress concentration factors ranged from 1 to 1.81 for screws and from 3.06 to 4.17 for nails. Conclusions: With larger root radius and inner diameter, titanium locking screws could provide much stronger fatigue strength than stainless steel counterparts. However, titanium locked nails might lose their advantages of superior mechanical strength because of high notch sensitivity and this limitation should be a critical concern clinically. Finite element analyses could be reliably used in research and development of locked nails and locking screws

Abstract. Objectives. Current therapies for osteoporosis are limited to generalised antiresorptive or anabolic interventions, which do not target specific regions to improve skeletal health. Moreover, the adaptive changes of separate and combined pharmacological and biomechanical treatments in the ovariectomised (OVX) mouse tibia has not been studied yet. Therefore, this study combines micro- computed tomography (micro-CT) imaging and computational modelling to evaluate the efficacies of treatments in reducing bone loss. Methodology. In vivo micro-CT (10.4µm/voxel) images of the right tibiae of N=18 female OVX C57BL/6 mice were acquired at weeks 14, 16, 18, 20 and 22 of age for 3 groups: mechanical loading (ML), parathyroid hormone (PTH) or combined therapies (PTHML). All mice received either injection of PTH (100μg/kg/day, 5days/week) or vehicle from week 18. The right tibiae were mechanically loaded in vivo at week 19 and 21 with a 12N peak load, 40 cycles/day and 3 days/week. Bone adaptation was quantified through spatial changes in bone mineral density (BMD) and strain distribution was obtained from micro-CT-based finite element models. Results. Densitometric parameters improved for all treatment between week 18–20 (10–21%), with the strongest benefits due to loading in the proximal regions (16–35%). At week 22, PTHML treatment induced 23–76% higher bone apposition in the proximal tibia than either monotherapy. Compared to the OVX control, all treatments reduced periosteal resorption at weeks 18–20 and 20–22 (20–87%). However, resorption in weeks 20–22 were 29–55% higher than weeks 18–20, increasing the strain in the proximal tibia. Synergistic effects of PTH and ML were observed on the periosteal surface of proximal tibia, but additive effects were seen predominately on the distal and lateral tibia. Conclusions. ML had a more dominant effect in improving bone health. PTH enhances bone's osteogenic response to ML additively and synergistically in a site- and time-dependent manner

Objective: A patient-specific finite element model of the spine is being developed to aid the surgeon in the diagnosis and clinical management of spinal conditions. 1. To validate the application of the computer model, a laboratory validation spine is being developed. This study is concerned with the development and basic characteristics of the intervertebral disc component of the laboratory spine. Method: The external profile of the laboratory disc was determined from CT images of a cadaveric spine. A two-part silicon rubber was used to form the annulus part of the disc. Prior to sealing it was possible to fill the cavity with an appropriate medium (such as grease or oil) to represent the nucleus pulposus with the further option of applying external pressurisation through a small pressure inlet in the wall of the disc. The laboratory disc was then tested in denucleated form, and grease-filled with initial intradiscal pressures of 0, 0.1, 0.2 and 0.3 MPa. A finite element model of the disc was also developed and used to investigate the characteristics of the laboratory disc. Results: The agreement between the finite element results and experimental test results was excellent and the compressive and flexural load-deflection characteristics of both intact and denucleated laboratory discs were found to lie within the range of values reported in the literature for cadaveric discs. Disc bulge characteristics of the intact and denucleated silicon discs were also similar to that observed with natural discs in vitro. Conclusions: An artificial disc for a laboratory validation spine has been developed and shown to have representative characteristic properties in compression loading. The disc is now being modelled and tested in torsion

Introduction: We aimed to examine the effect of neck notching during hip resurfacing on the strength of the proximal femur. Methods: Third generation composite femurs that have been shown to replicate the biomechanical properties of human bone were utilised. Imageless computer navigation was used to position the initial guide wire during head preparation. Six specimens were prepared without a superior notch being made in the neck of the femur, six were prepared in an inferiorly translated position to cause a 2mm notch in the superior femoral neck and six were prepared with a 5mm notch. All specimens had radiographs taken to ensure that the stem shaft angle was kept constant. The specimens were then loaded to failure in the axial direction with an Instron mechanical tester. A three dimensional femoral finite element model was constructed and molded with a femoral component constructed from the dimensions of a Birmingham Hip Resurfacing. The model was created with a superior femoral neck notch of increasing depths. Results: The 2mm notched group (mean load to failure 4034N) were significantly weaker than the un-notched group (mean load to failure 5302N) when tested to failure (p=0.017). The 5mm notched group (mean load to failure 3121N) were also significantly weaker than the un-notched group (p=0.0003) and the 2mm notched group (p=0.046). All fractures initiated at the superior aspect of the neck, at the component bone interface. The finite element model revealed increasing Von Mises stresses with increasing notch depth. Discussion: A superior notch of 2mm in the femoral neck weakens the proximal femur by 24% and a 5mm notch weakens it by 41%. This study provides biomechanical evidence that notching of the femoral neck may lead to an increased risk of femoral neck fracture following hip resurfacing due to increasing stresses in the region of the notch

Load-controlled knee simulators, representing the passive constraints and joint loads observed in the natural knee, have been developed to assess device-dependent kinematics and wear damage of total knee replacements (TKR) in a controlled mechanical environment. Using a finite element model (FEM) to represent the simulator, our objective in this study was to quantify the variations in kinematics, contact stresses, and contact areas that occur with variations in the ‘soft-tissue’ spring stiffness and coefficient of friction for a conforming knee design. A finite element model was created of the Insall-Burstein Posterior-Stabilized II knee system. The model conditions corresponded with the International Standards Organisation (ISO) test protocol #14243-1 and consisted of the prescribed flexion angle, the axial compressive load, the anterior-posterior (AP) force, the internal-external (IE) moment, and linear springs mounted to provide AP and IE restraints. This setup has been validated as a reasonable equivalent system for this design in the Instron-Stanmore knee simulator. The linear spring constant was set at 7.24 N/mm and the coefficient of friction was 0.01; both values were then varied by an order of magnitude. The implant kinematics and the maximum contact stress and areas of contact over the loading cycle were determined. Varying the spring constant by a factor of two changed the AP motions and IE rotations of the tibial insert by about 20%. The maximum contact stresses, occurring during peak loads and moments, varied by 40%, while the area of contact over the full cycle changed by 30%. Changing the coefficient of friction had little effect upon the dependent variables. Wear is a function of both stresses and kinematics. This study indicates that stresses in this design are more sensitive than kinematics to changes in ‘soft-tissue’ stiffness. Therefore, both must be considered to determine wear potential

Design phase evaluation of potential implant designs requires verified computational and experimental models. Computational models are important where parametric evaluation of geometric features experimentally is both cost and time-prohibitive due to the need to manufacture complex parts, and provide information not easily measured experimentally, such as internal stresses/strains in the implant or bone. However, before implementation into the design process, a thorough verification/validation is required. In this study, a finite element model of the Kansas knee simulator (KKS) was developed and a systematic verification of predicted joint kinematics was performed by comparison with experimental measurements, including evaluating the patellofemoral joint first in isolation, followed by whole joint kinematic comparisons. Four unmatched, healthy cadaver knees (average age 63 yrs) were mounted in the KKS to reproduce a simulated gait and deep knee bend activity in their natural and implanted states. Finite element models of the KKS assembly and the four cadaver specimens in their natural and implanted states were created. Isolated patellofem-oral kinematics were initially verified during simulated deep knee bend. Average RMS differences between predicted and experimental natural patellar kinematics were less than 3.1° and 1.7 mm for rotations and translations, respectively, while differences in implanted kinematics were less than 2.1° and 1.6 mm between 10 and 110° femoral flexion. Similar agreement was found with the subsequent whole joint simulations. Deep knee bend tibiofemoral internal-external (IE) and varus-valgus (VV) rotations had average RMS differences from experimental measurements of 1.5 ± 0.4° and 0.9 ± 0.5°, respectively. Anterior-posterior (AP), inferior-superior (IS) and medial-lateral translations matched within 1.8 ±0.8 mm, 1.2 ±0.7 mm, and 0.6 ±0.1 mm, respectively. The experimental and verified computational tools can be used in harmony for pre-clinical assessment of implant designs; the computational model allows rapid screening of implant geometry or alignment issues and provides additional insight into joint mechanics such as implant stresses or bone strains, while the experimental simulator can subsequently be utilized to assess in cadavera only the most promising designs or features identified

The importance of mechanism of injury was initially introduced by Holdsworth who made the supposition that all fractures are created when the spine is subject to one of 5 types of violence. It has been our experience that similar injury mechanisms can lead to variable fracture patterns. Alternatively, different injury mechanisms can lead to the same fracture pattern. Purpose: To evaluate the variation in fracture patterns when a single and uniform force vector is applied to the spine with variable degrees of spinal flexion. Finite element modeling was used for this analysis. Methods: Three different finite element models were created to represent each accident situation. The straight spine was modeled as a simple column with alternating vertebrae and disc segment. The moderately flexed and significantly flexed spines were modeled as curved cylinders sectioned into vertebrae and discs, then bent around a solid cylinder representing the abdomen. A 1000 N compressive load was applied vertically to the top of the spine. The model was restrained along all bottom surfaces, and the interface between the spine and abdomen sections was defined as frictionless. The model is fixed at the lower end and the area of greatest interest is the transition zone from the most rigid to the less rigid portion. Although no specific area of the spine is intended for purposes of the model, this composition is much like the thoracolumbar junction – the location of the majority of spinal injuries. Results: The straight spine showed pure compression throughout the length of the spine, while the moderately curved spine showed the posterior elements of the region of interest in tension and the anterior elements in compression. The significantly curved spine was found to be in tension in both posterior and anterior elements. Conclusion: In a situation where the patient is sitting upright with a straight spine, a compressive load will cause a burst fracture. When the patient is partially bent over, such as with a shoulder seat belt, a flexion distraction injury will occur with the posterior aspect of the spine failing in tension and the anterior in compression. When the patient is fully bent over, such as with a laponly seat belt, a purely distractive fracture can occur

Notching of the anterior femoral cortex during total knee arthroplasty is thought to be a possible risk factor for subsequent periprosthetic femoral fracture. Understanding the stress pattern caused by notching may help the orthopedic surgeon reduce the risk of fracture. A validated, three dimensional, finite element model of the femur using gait loads has been used to analyze the stress concentrations caused by anterior femoral cortex notching. Three factors that increase these stresses were identified. The notch depth, radius of curvature, and its proximity to the end of the femoral prosthesis influence the state of stress in the surrounding bone. The purpose of this study was to characterize the stress concentration caused by anterior femoral notching during total knee replacement (TKR) in order to determine when a patient is at risk for a periprosthetic fracture of the femur. We concluded that notches greater than 3 mm with sharp corners located directly at the proximal end of the femoral implant produced the highest stress concentrations and may lead to a significant risk of periprosthetic femur fracture. One complication that can occur during TKR is notching of the anterior femoral cortex which results in a stress concentration. It is important to characterize this stress riser in order to determine when a stemmed femoral component should be used to minimize the risk of fracture. Three factors that affected the stress concentration were identified. First, increasing the notch depth lead to significant increased stress concentrations. When the depth was greater than 3 mm, local stresses increased markedly. Second, the radius of curvature was found to be inversely related to stress concentration. As the radius decreased, the local stress increased. Third, the proximity of the notch to the prostheses affected the stress concentration. Notches that were 1 mm proximal to the implant resulted in much larger stresses than those that were 10 mm away. A validated, three dimensional finite element model of a femur subjected to a gait loading pattern was used to characterize the stress concentration caused by anterior femoral notching. The results compared well to previous work reported in the literature

Notching of the anterior femoral cortex during total knee arthroplasty is thought to be a possible risk factor for subsequent periprosthetic femoral fracture. Understanding the stress pattern caused by notching may help the orthopedic surgeon reduce the risk of fracture. A validated, three dimensional, finite element model of the femur using gait loads has been used to analyze the stress concentrations caused by anterior femoral cortex notching. Three factors that increase these stresses were identified. The notch depth, radius of curvature, and its proximity to the end of the femoral prosthesis influence the state of stress in the surrounding bone. The purpose of this study was to characterize the stress concentration caused by anterior femoral notching during total knee replacement (TKR) in order to determine when a patient is at risk for a periprosthetic fracture of the femur. We concluded that notches greater than 3 mm with sharp corners located directly at the proximal end of the femoral implant produced the highest stress concentrations and may lead to a significant risk of periprosthetic femur fracture. One complication that can occur during TKR is notching of the anterior femoral cortex which results in a stress concentration. It is important to characterize this stress riser in order to determine when a stemmed femoral component should be used to minimize the risk of fracture. Three factors that affected the stress concentration were identified. First, increasing the notch depth lead to significant increased stress concentrations. When the depth was greater than 3 mm, local stresses increased markedly. Second, the radius of curvature was found to be inversely related to stress concentration. As the radius decreased, the local stress increased. Third, the proximity of the notch to the prostheses affected the stress concentration. Notches that were 1 mm proximal to the implant resulted in much larger stresses than those that were 10 mm away. A validated, three dimensional finite element model of a femur subjected to a gait loading pattern was used to characterize the stress concentration caused by anterior femoral notching. The results compared well to previous work reported in the literature

Aim: To investigate, quantify and model the influence of three biomechanical factors on the severity of mechanically induced nuclear disruption in healthy bovine, caudal, intervertebral discs. Method: A preliminary study was conducted with a fully divided annular wall to investigate the cohesive nature of the isolated nucleus and its tendency to form clefts when loaded. A second more clinically relevant model using whole bovine discs was then conducted to investigate whether significant clefts could be induced in healthy discs by controlling flexion, hydration and rate of compressive loading.. A finite element model of the bovine caudal disc was constructed to predict the complex stress conditions that exist within the disc. Results: We found that high degrees of flexion and hydration were significant risk factors in nuclear disruption (P < 0.005), while the rate of loading showed no significant effect (P = 0.37). The intact disc study also showed that flexion and hydration are significant risk factors (P < 0.002). In contrast with the preliminary study, the rate of loading was also shown to be mildly significant (P < 0.1). The finite element model predicted relatively high concentrations of stress and strain energy density within the nucleus. This is consistent with the experimental observations of cleft formation. Conclusions: While it is well established that dehydration of the nucleus is a symptom of degeneration this study suggested that the healthy nucleus, when maximally hydrated, is more susceptible to nuclear disruption when loaded. This supports the hypothesis that the histologically abnormal and degenerate nuclear material removed at surgery, may well have attained this state as a result of biomechanical and biochemical changes occurring in the disc following rather than preceding a prolapse. This study further defined the rôle of trauma in disc injury and prolapse of the normal disc

Introduction: Pre-operative coronal curve flexibility assessment is of key importance in the surgical planning process for scoliosis correction. The fulcrum bending radiograph is one flexibility assessment technique which has been shown to be highly predictive of potential curve correction using posterior surgery, however little is known about the extent to which soft tissue structures govern spinal flexibility. The aim of this study was to explore how the mechanical properties of spinal ligaments and intervertebral discs affect coronal curve flexibility in the fulcrum bending test. To this end a biomechanical analysis of a scoliotic thoracolumbar spine and ribcage was carried out using a three dimensional finite element model. Methods: CT-derived spinal anatomy for a 14 year old female adolescent idiopathic scoliosis patient was used to develop the 3D finite element model. Physiological loading conditions representing the gravitational body weight forces acting on the spine when the patient lies on their side over the fulcrum bolster were simulated. Initial mechanical properties for the spinal soft tissues were derived from existing literature. In six separate analyses, the disc collagen fibre and ligament stiffness values were reduced by 10%, 25% and 40% respectively, and the effects of reduced tissue stiffness on fulcrum flexibility were assessed by comparison with the initial model. Finally, the effect of discectomy on fulcrum flexibility was simulated for thoracic levels T5 to T12. Results: Reducing disc collagen fibre stiffness resulted in a greater change in segmental rotations in the fulcrum bending test than reducing ligament stiffness. However, reductions of up to 40% in disc collagen fibre stiffness and ligament stiffness produced no clinically measurable increase in fulcrum flexibility (increase of 1.2%). By contrast, following removal of the discs, the simulated fulcrum flexibility increased by more than 80% compared to the initial case. Discussion: Disc collagen fibre and ligament stiffness both have minimal influence on scoliotic curve flexibility. However, discectomy simulation shows that the intervertebral discs are of critical importance in determining spinal flexibility

Mechanical wear and corrosion lead to the release of metal particulate debris and subsequent release of metal ions at the trunnion-taper surface. In order to quantify the amount of volume loss to ultimate locations in the surrounding joint space, finite element analysis of the modular head-stem junction is being carried out. The key purpose being to determine a set of optimum design changes that offer the least material loss at the taper-trunnion junction using optimization algorithms such as the gradient based local search (Sequential Quadratic Programming–SQP) and global search (Non-Dominated Sorting Genetic Algorithm-II–NSGA-II). In a broader sense, the principal goal is to work toward the minimization of wear debris produced in the hip joint, thereby resulting in a longer prosthetic lifetime. A numerical approach that simulates wear in modular hip prostheses with due consideration to the taper-trunnion junction on metal-on-metal contacts is proposed. A quasi-static analysis is performed considering realistic loading stages in the gait cycle, and nonlinear contact analysis is to be employed. The technique incorporates a measured wear rate as an input to the finite element model. The simulation of wear is performed by progressively changing nodal coordinates to simulate the wear loss that occurs during surface interaction. The geometry of the worn surface is updated under gait loading. With a given geometry and gait loading, the linear and volumetric wear increases with the number of gait cycles. The continuous wear propagation is discretized and an approximation scheme known as surrogates is to be developed using Artificial Neural Networks (ANN) to reduce the expensive computational simulations during optimization. The model is employed in the optimization schemes coded in MATLAB and linked to the finite element model developed in ANSYS batch mode. The objective function of the optimization problem is to minimize the volumetric wear at taper-trunnion interface under some constraints. By minimizing the volumetric wear, the chance of failure of modular hip implants is also minimized. The FE model developed to reproduce fretting wear is validated through in vitro wear simulations. The important taper design variables considered to have impact on the fretting corrosion performance include; medial-lateral offset, neck length, taper head diameter, trunnion length and diameter, included angle for the head/neck tapers, angle of mismatch or variation in taper trunnion angle, etc. It is expected from clinical outcomes that increased offset and large taper diameter has serious implications in the fretting corrosion behavior primarily because these variables control the bending stresses and strains along the length of the taper. During cyclic loading of the taper, the higher the strain range, the higher will be the relative micromotion at the point of engagement between the stem and head tapers. This research is carried out with the objective to optimize the effects of these geometrical factors at the mating taper interfaces. The developed models have great potentiality for accurate assessment of wear in a range of metal-on-metal (MoM) hip prostheses at the femoral head taper-trunnion junction while substantially reducing the wear and failure rate of prostheses

Spinal metastatic disease can result in burst fracture and neurologic compromise. This study aims to examine the effects of tumour location, shape and surface texture on burst fracture risk in the metastatic spine using a parametric poroelastic finite element model. Tumours were found to be most hazardous in the posterior region of the vertebral body, whereas the multiple tumour scenarios reduced risk. Tumour shape may affect the mechanism of burst fracture. Serrated and smooth outer tumour surfaces yielded similar trends. These results can be used to improve guidelines for burst fracture risk assessment in patients with spinal metastases. This study aims to examine the effects of tumour location, shape and surface texture on burst fracture risk in the metastatic spine. Both tumour location and shape are important factors in assessing the risk of burst fracture in the meta-static spine. Improving risk prediction may reduce burst fracture in patients with spinal metastases. Vertebral bulge increased over 30% when the tumour was moved posteriorly. Conversely, for the multi-tumour scenarios, vertebral bulge and axial displacement decreased by 41% and 35% in comparison to a single central tumour. Anterior and lateral movement demonstrated only small effects. Vertebral bulge increased proportionally to mediolateral tumour length and axial displacement increased proportionally to superior-inferior tumour length. Similar trends were seen with smoothed and serrated tumour surfaces. Using a parametric poroelastic finite element model of a metastaticaly involved T7 spinal motion segment, fourteen single and two multi-tumour scenarios were analyzed, each comprising approximately 24% tumour volume. Ellispoidal tumours were positioned in central, anterior, posterior and lateral locations. Tumour shape was altered by adjusting tumour radii for a centrally located tumour. Tumours were modeled using smoothed and serrated outer surface configurations. Burst fracture risk was assessed by measuring maximum vertebral bulge and axial displacement under load. Tumours were found to be most hazardous in the posterior region of the vertebral body, whereas the multi-tumour scenarios reduced risk. Modeling of tumour surface texture did not impact shape or location effects. Tumour shape may affect the mechanism of burst fracture. Funding: This study was supported by the National Science and Engineering Research Council

Purpose: Stability of thoracic vertebrae affected by metastatic disease has been shown to be dependent on tumour size and bone density, but additional structural and geometric factors may also play a role in burst fracture risk assessment. The objective of this study was to use parametric finite element modeling to determine the effects of vertebral level, geometry, and metastatic compromise to the cortical shell on the risk of burst fracture initiation in the thoracic spine. Methods: An experimentally validated parametric biphasic finite element model of a metastatically involved spinal motion segment was analysed with scenarios representing motion segments from T2-T4 through T10-T12. Variations in vertebral geometry, kyphotic angulation and endplate angulation were evaluated. Additionally, four scenarios with transcortical breach of the tumour were compared to a central tumour scenario to determine the effect of cortical destruction. Vertebral bulge (VB), load induced canal narrowing (LICN), and posterior wall tensile hoop strain (PWTHS) were utilised as the main outcome parameters to assess burst fracture risk. Results: Burst fracture risk outcome parameters were largest in upper vertebrae, decreasing inferiorly at each subsequent level, with T11 exhibiting a 35.5% decrease in VB relative to T3, despite greater applied loads. An increase in endplate angles led to a 6.59% decrease in VB and a 2.38% decrease in LICN. A 5° increase in kyphotic angle further decreased VB and LICN by 7.29% and 4.34% respectively. Transcortical tumour scenarios led to an average decrease in PWTHS of 25.8%. Conclusions: Patients affected by spinal metastases in upper thoracic vertebrae may be at greater risk of burst fracture. Decreased burst fracture risk with greater thoracic kyphotic angulation may be due to a change in loading direction for curved segments, reducing the amount of pure axial load applied. Decreased tensile hoop strains are generated during loading of transcortical tumours. This may be attributed to large deformation of tumour tissue through the breach in the cortical shell, reducing the potential for burst fracture. Improved burst fracture risk assessment in the thoracic spine may motivate more informed clinical decision-making. Funding: Other Education Grant. Funding Parties: Natural Sciences and Engineering Research Council

Computer navigation for total knee arthroplasty (TKA) has been increasingly used because it improves the accuracy of implant placement. However, some clinical cases have reported complications caused from pin holes during the computer navigated surgery. The objective of this study is to analyse the femoral fracture risk cause by the pin hole in the computer navigated TKA by using finite element analysis. Three dimensional finite element model of the human femur was developed from CT images. A parametric investigation was conducted to analyse the femoral fracture risk for the following parameters: hole sizes (3, 4, and 5 mm) and hole position (70, 100, and 130 mm above the distal end). Four different penetrations (unicortical, bicortical, half-bicortical, and transcortical) methods in tubular bone were considered in each model, where the half-bicortical penetration was defined that the pin hole was located between the holes of bicortical and transcortical penetrations. The finite element model was rigidly fixed to a distance of 25 mm above the distal end. The vertical load of 1500 N and the torsional load of 12 Nm were applied to the femoral head. The maximum von-Mises stress, which was chosen as the fracture risk factor, was then investigated around pin hole. The maximum von-Mises stress around the pin hole was the highest in the transcortical penetration for different hole sizes: 7.8~8.5, 15.7~16.2, 15.5~16.8, and 25.5~45.3 MPa under the vertical load, and 9.6~10.5, 9.7~11.0, 8.8~10.2, and 14.2~33.8 MPa under the torsional load in unicortical, bicortical, half-bicortical, and transcortical penetrations, respectively. For the different hole position, the maximum von-Mises stress around the pin hole was: 6.0~7.8, 15.7~24.7, 16.3~19.6, and 12.2~22.4 MPa under the vertical load, and 9.6~10.7, 9.7~11.5, 8.7~9.8, and 12.2~16.6 MPa under the torsional load in unicortical, bicortical, half-bicortical, and transcortical penetrations, respectively. For the pin hole size, the maximum stress increased only in the transcortical penetration regardless of the loads as the pin hole size increased. However, there was little meaningful difference between the hole positions for each penetration method. The results of this study suggested that it would be beneficial to avoid using the transcortical penetration and large size of pin with respect to reduction of femoral fracture risk since the high stress may cause the femoral fracture

Spinal fusion has been used as the gold standard to treat some spinal disorders such as degenerative disc or disc herniation of the cervical spine. However, some clinical complications have been reported caused by high stiffness of spinal fusion. Recently, total disc arthroplasty using motion preservation devices such as artificial discs (ADs) have been proposed as an alternative treatment technique. In current study, we analysed biomechanical influences including inter-segmental motion, facet joint forces, and ligament stresses of two different clinical available ADs and compared with those of intact cervical spine in various loading conditions using finite element analysis. A three dimensional finite element model was developed for C2-C7 spinal motion segment based on CT images and previous anatomical literatures. The finite element models for two different types of ADs, semi-constraint (Prodisc-C. ®. , Synthes, U.S.A) and un-constraint (Mobi-C. ®. , LDR Spine, U.S.A), were developed. Each AD was inserted at C6–C7 segments. Superior and inferior plates of ADs were fixed on inferior plane of C6 and superior plane of C7 vertebrae, respectively. Based on the conventional surgical techniques, anterior longitudinal ligaments and some parts of intervertebral disc in C6–C7 motion segment were removed to insert ADs. Inferior plane of C7 vertebra was constrained in all directions and 1Nm of flexion, extension, lateral bending and torsion were applied on superior plane of C2 vertebra with 50N of compressive load along follower load direction. Rotation angle in flexion of C5–C6 segment in cases of semi-constraint and un-constraint AD was 3.3° and 3.7°, respectively. Both values were greater than that in case of the intact cervical spine by 18% and 32%, respectively. Rotation angle in extension, lateral bending and torsion were greater than intact model by 45%, 26% and 43% for the case of semi-constraint AD and 55%, 35%, 100% for the case of un-constraint one, respectively. In extension, facet joint forces were about two times higher than intact model in cases of semi-constraint and un-constraint AD. Also in flexion, on average, ligament stresses in cases of semi-constraint and un-constraint AD were higher than intact model by 66% and 116%, respectively. The results of this study showed that ADs were useful to generate inter-segmental motion at surgical level. And the un-constraint type of AD had higher mobility than semi-constraint one. However, high mobility of ADs would lead not only higher facet joint forces but also ligament stresses than intact cervical spine. Therefore, more careful care must be taken to choose surgical method of total disc arthroplasty

Recently, it has been reported that the posterior stabilised implant clinically used for the total knee replacement (TKR) may have a risk of failures caused by pressure and stress concentrated on the tibial post. Malalignment of the implant or variable loading applied to the implant are one of the major causes of the failure in posteriori stabilised TKR. The purpose of this study is to biomechanically analyse the effect of implant malalignment on the failure risk of the implant in posteriori stabilised TKR by estimating von-Mises stress on the implant. Finite element models of a knee joint and a posteriori stabilised implant were developed from 1mm slices of CT images and 3D CAD software, respectively. The posterior stabilised implant consists of a femoral component, a tibial post, and a tibial tray. The finite element models of TKR for the neutral alignment case as well as the different malalignment cases (3° and 5° of valgus and varus angulations, 2° and 4° of anterior and posterior tilts, and 3° of external rotation) were developed. Then, the von-Mises stress, which is which was chosen as the fracture risk parameter, acting on the implant were analysed by using CAE software. Loading condition at the 40% of one whole gait cycle such as 2000N of compressive load, 25N of anterior-posterior load, and 6.5Nm of torque was applied to the TKR models. The maximum von-Mises stresses were concentrated on the anterior region of the tibial post regardless of the oblique loadings. In the rotationally additional loading (3° of external rotation), excessive stresses occurred in the anterior medial and posterior lateral areas. The maximum stress was 18.3MPa in neutral position. The maximum stress increased by 10% in anterior tilt 2°, 15% in anterior tilt 4°, 25% in posterior tilt 2°, 54% in posterior tilt 4°, 116% in varus 3°, 262% in varus 5°, 318% in valgus 3°, 389% in valgus 5°, 6% in external rotation 3° compared with that in the neutral position case. In addition, 32.0MPa of maximum stress occurred on the posterior lateral area of the base component in rotationally additional loading. The results showed that the implant malalignment could accelerate the stress concentration on the anterior region of the tibial post as in the result of clinical study. In the case of additional rotation, high stress concentration on the anterior medial and posterior lateral areas as well as on the tibial base surface could generate wear or fracture of tibial post. From the additional rotation case, we can expect that higher conformity implant will generate higher stress concentrations than lower conformity implant even though we did not compare the effect of conformity ratio on the stress concentration in the tibial polyethylene component. This study showed that careful consideration of the implant malalignment would be necessary to improve the clinical outcome in the posteriori stabilised TKR

Introduction:. The mechanical stresses and strains surrounding orthopaedic implants can influence bone resorption and formation, micro-fracture, and consequently implant fixation or loosening. Experimental measurement of these internal parameters is generally not feasible. Computational predictions by finite element modeling are promising, but until recently have been limited to assuming the surrounding cancellous bone as a continuous volume, without modeling individual trabeculae. A recent study demonstrated errors in bone-implant stiffness exceeding 100% when using this continuum assumption [1]. Conversely, recently micro-finite element computer models have been built from high resolution imaging of trabecular bone. In the present study we developed such models of central pegs cemented into cadaveric glenoids. We hypothesized that additional applied cement would lead to stronger implant fixation, but less physiologic strains in the trabeculae. Methods:. Two cadaveric specimens were implanted, with the applied cement volume in the Specimen 2 approximately double that of Specimen 1. The specimens were imaged by micro-computed tomography (vivaCT 40, Scanco, Switzerland) with a resolution of 12 microns. Images were filtered and resampled, then imported in Mimics (Materialise, Belgium) for semi-automated segmentation and 3D reconstruction based on our laboratory's published methods. Finite element models containing 1.7 to 1.8 million elements having sides of 0.1 mm were generated by a direct image voxel-to-element approach [2] (Fig. 1). The material properties of cement and bone were assumed linear elastic (bone: E = 3.5 GPa, cement: E = 3.0 GPa, and implant (UHMWPE): E = 1.3 GPa), and interfaces were assumed fully bonded. All outer walls of the bone were fixed, and a downward force of 250 N was applied to the implant peg. Simulations were run using Abaqus (Simulia, Pawtucket RI) on a 32-core, 1 TB-memory server at PSU's High Performance Computing Systems. Results:. Specimen 1 had 254 mm. 3. cement measured in the model, whereas Specimen 2 had 535 mm. 3. Strain energy density was less for Specimen 2 for bone underneath the implant, but similar between specimens for bone around the implant sides (Figs 2 and 3), providing initial indication of complex effects of cement volume on peri-implant strains. In Specimen 2 a slightly larger volume of cement (8.6 vs. 6.8 mm. 3. ) was exposed to von Mises stresses exceeding 10 MPa. Discussion:. This study is novel in its prediction of stresses and strains down to the level of individual glenoid trabeculae surrounding a cemented implant. In this pilot investigation we found that bone embedded in the cement mantle is subject to low strains, whereas the bone immediately surrounding the cement mantle is subject to abnormally high strains, with both cement technique and trabecular architecture likely influencing results. The study is limited by the lack of application of more complex loads and boundary conditions. Future work includes modeling of additional specimens and statistical analyses, and investigation of the roles of cement stiffness and peg design in dictating peri-implant bone strains

Introduction. Modification in joint loading, and specifically shear stress, is found to be an important mechanical factor in the development of osteoarthritis (OA). Cartilage shear stresses can be investigated using finite element (FE) modelling, where typically in vivo joint loading as measured by an instrumented hip prosthesis is used as boundary condition. However, subject-specific gait characteristics substantially affect joint loading. The goal of this study is to investigate the effect of subject-specific joint loading as calculated using a subject-specific musculoskeletal model and integrated motion capture data on acetabular shear stress. Methods. Three healthy control subjects walked at self-selected speed while measuring marker trajectories (Vicon, Oxford Metrics, UK) and force data (two AMTI force platforms; Watertown, MA). A subject-specific MRI-based musculoskeletal model consisting of 14 segments, 19 degrees of freedom and 88 musculotendon actuators, and including wrapping surfaces around the hip joint, was used. All analyses were performed in OpenSim 3.1. The model was scaled to the dimensions of each subject using the marker positions of a static pose. A kalman smoother procedure was used to calculate joint angles. Muscle forces were calculated using static optimization, minimizing the sum of squared muscle activations, and hip contact forces (HCF) were calculated and normalized to body weight (BW). To calculate shear stress, HCFs and joint angles calculated during the stance phase of gait were imposed to a hip finite element model (hip_n10rb) using FFEbio 2.5. In the model, femoral and acetabular cartilage were represented using the Mooney-Rivlin formulation (c1=6.817, bulk modulus=1358.86) and the pelvis and femur bones as rigid bodies. Peak HCF as well as maximal acetabular shear stress, magnitude and location, and the HCF at the time of maximal shear stress were compared between subjects. Results. Maximal shear stress was lower for S3 compared to S1 and S2 (9.14, 9.48 and 7.14 MPa for S1, S2 and S3 respectively). Nevertheless, HCF at the time instance of peak stress as well as peak HCF were highest for S3 (S1: 2.40/4.54 BW, S2: 2.97/4.78 BW and S3: 3.13/6.46 BW respectively). Maximal shear stress also occurred earlier in the stance phase for S3 compared to S1 and S2 (31, 26 and 11% of the stance phase for S1, S2 and S3 respectively). In addition, the location of the peak maximal shear stress was found to be more superior for S3. Discussion. Subject-specific loading patterns clearly influence the calculated maximal shear stress in the acetabular cartilage, affecting both the magnitude and the location of the stress. In addition, higher shear stresses are not coinciding with higher HCFs. This finding highlights the need of subject-specific rather than generic loading patterns when assessing cartilage shear stresses and associated risk in OA development in individual patients

Accurate in vivo knee joint contact forces are required for joint simulator protocols and finite element models during the development and testing of total knee replacements (Varadarajan et al., 2008.) More accurate knowledge of knee joint contact forces during high flexion activities may lead to safer high flexion implant designs, better understanding of wear mechanisms, and prevention of complications such as aseptic loosening (Komistek et al., 2005.) High flexion is essential for lifestyle and cultural activities in the developing world, as well as in Western cultures, including ground-level tasks and chores, prayer, leisure, and toileting (Hemmerich et al., 2006.) In vivo tibial loads have been reported while kneeling; but only while the subject was at rest in the kneeling position (Zhao et al., 2007), meaning that the loads were submaximal due to muscle relaxation and thigh-calf contact support. The objective of this study was to report the in vivo loads experienced during high flexion activities and to determine how closely the measured axial joint contact forces can be estimated using a simple, non-invasive model. It provides unique data to better interpret non-invasively determined joint-contact forces, as well as directly measured tiobiofemoral joint contact force data for two subjects. Two subjects with instrumented tibial implants performed kneeling and deep knee bend activities. Two sets of trials were carried out for each activity. During the first set, an electromagnetic tracking system and two force plates were used to record lower limb kinematics and ground reaction forces under the foot and under the knee when it was on the ground. In the second set, three-dimensional joint contact forces were directly measured in vivo via instrumented tibial implants (Heinlein et al., 2007.) The measured axial joint contact forces were compared to estimates from a non-invasive joint contact force model (Smith et al., 2008.). The maximum mean axial forces measured during the deep knee bend were 24.2 N/kg at 78.2° flexion (subject A) and 31.1 N/kg at 63.5° flexion (subject B) during the deep knee bend (Figure 1.) During the kneeling activity, the maximum mean axial force measured was 29.8 N/kg at 86.8° flexion (subject B.) While the general shapes of the model-estimated curves were similar to the directly measured curves, the axial joint contact force model underestimated the measured contact forces by 7.0 N/kg on average (Figure 2.) The most likely contributor to this underestimation is the lack of co-contraction in the model. The study protocol was limited in that data could not be simultaneously collected due to electromagnetic interference between the motion tracking system and the inductively powered instrumented tibial component. Because skin-mounted markers were used, kinematics may be affected by skin motion artefacts. Despite these limitations, this study presents valuable information that will advance the development of high flexion total knee replacements. The study provides in vivo measurements and non-invasive estimates of joint contact forces during high flexion activities that can be used for joint simulator protocols and finite element modeling

Even though spinal fusion has been used as one of the common surgical techniques for degenerative lumbar pathologies, high stiffness in the fusion segment could generate clinical complications in the adjacent spinal segment. To avoid these limitations of fusion, the artificial discs have recently used to preserve the motion of the treated segment in lumbar spine surgery. However, there have been lacks of biomechanical information of the artificial discs to explain current clinical controversies such as long-term results of implant wear and excessive facet contact forces. In this study, we investigated the biomechanical performance for three artificial discs in the lumbar spinal segments by finite element analysis. A three-dimensional finite element model of five spinal motion segments, from L1 to S, in intact lumbar spine was reconstructed from CT images. Finite element models of three artificial discs, semi-constrained and metal on polyethylene core type (ProDisc. ®. II, Spine Solutions Inc., USA; Type I), semi-constrained and metal on metal type (MaverickTM, Medtronic Sofamor Danek Inc., USA; Type II), and un-constrained and metal on polyethylene core type (SB ChariteTM III, Dupuy Spine Inc., Switzerland; Type III) were developed. Each artificial disc was inserted at L4–L5 segment, respectively. Upper and lower plates of artificial discs were attached on the L4 and L5 vertebrae. Some parts of ligaments and intervertebral disc in L4–L5 motion segment were removed to insert artificial discs. Nonlinear contact conditions were applied on facet joints in lumbar spine model and artificial discs. Bottom of sacrum was fixed on the ground and 5Nm of flexion and extension moments were applied on the superior plate of L1 with 400N of compressive load along follower load direction. In extension, all three artificial disc models showed higher rotation ratio at the surgical levels, but lower rotations at the adjacent levels than those in the intact model. There was no big difference of the intersegmental rotations among the artificial disc models. For the comparison of the peak von-Mises stresses on the polyethylene core in flexion, 52.3 MPa in type I implant was higher than 20.1 MPa in Type III implant while the peak von-Mises stresses were similar, 25.3 MPa and 26.5 MPa in Type I and III, respectively in extension. The facet contact forces at the surgical level for the artificial disc models showed 140 to 160 N in extension whereas the facet contact force in the intact model was 60 N. From the results of this study, we could investigate the biomechanical characteristics of three different artificial disc models. The relative rotation at the surgical level would be increases at the early outcome after total disk replacement. The semi-constrained type artificial disc could generate higher wear risk of the implant than unconstrained type. Also all types of artificial disc model have higher risk of facet joint arthrosis, and especially in the semi-constrained and metal on metal type. The results of the present study suggested that more careful care must be taken to choose surgical technique of total disc replacement surgery

Reversed shoulder prostheses are increasingly being used for the treatment of glenohumeral arthropathy associated with a deficient rotator cuff. These non-anatomical implants attempt to balance the joint forces by means of a semi-constrained articular surface and a medialised centre of rotation. A finite element model was used to compare a reversed prosthesis with an anatomical implant. Active abduction was simulated from 0° to 150° of elevation. With the anatomical prosthesis, the joint force almost reached the equivalence of body weight. The joint force was half this for the reversed prosthesis. The direction of force was much more vertically aligned for the reverse prosthesis, in the first 90° of abduction. With the reversed prosthesis, abduction was possible without rotator cuff muscles and required 20% less deltoid force to achieve it. This force analysis confirms the potential mechanical advantage of reversed prostheses when rotator cuff muscles are deficient

Introduction. Successful designs of total hip replacement need to be robust to surgery-related variability. Until recently, only simple parametric studies have explored the influence of surgical variability [1]. This study presents a systematic method for quantifying the effect of variability in positioning on the primary stability of femoral stems using finite element (FE) models. Methods. Patient specific finite element models were generated of two femurs, one male and one female. An automated algorithm positioned and sized a Corail stem (DePuy Synthes, Warsaw) into each of the femurs to achieve maximum fill of the medullary canal without breaching into the cortical bone boundaries.. Peak joint contact and muscle forces associated with level gait were applied[2] and scaled to the body mass of each subject, whilst the distal femur was rigidly constrained. The space prone to surgical variation was defined by the “gap” between the stem and the inner boundary of the cortical bone. The anterior/posterior and the varus/valgus alignment of the stem within this “gap” was controlled by varying the location of the points defining the shaft axis. The points were taken at 20% and 80% of the stem length (Figure 1). The anteversion angle as well as the vertical and the medial position of the stem were controlled by changing the location of the head centre within the femoral head radius. The location of these points was varied using Latin Hypercube sampling to generate 200 models per femur, each with a unique stem position. The risk of failure was evaluated based on stem micromotion, equivalent strains, and percentage of the bone-prosthesis contact area experiencing more than 7000 µstrains [3]. Results. The range of positions covered in this study adhered to the anatomy of the subjects (Table 1) and none of the stem positions breached into the cortical bone of the femur. The 90th percentile peri-prosthetic strains were between 1770 – 4792 µstrains for the male subject, and 2710 – 11260µstrains for the female subject. The 90th percentile micromotion was between (15.6 – 47) µm for the male subject, and (42.4 – 102.4) µm for the female subject. The percentage of the contact area experiencing more than 7000 µstrains was between (0% – 0.33%) for the male subject, and (0% – 12%) for the female subject. Discussion. A systematic method for studying the effect of surgical-related variation on primary stability was presented its applicability demonstrated on two femurs. The study found that variation in stem position may result in large variation (up to 1.5 times the baseline position) in strains and micromotions. The magnitude Up to three times the magnitudes for the ideal stem position. This method can be applied to larger samples to understand the influence of different alignment parameters on the primary stability of femoral stems

Osteosynthesis of high-energy metaphyseal proximal tibia fractures is still challenging, especially in patients with severe soft tissue injuries and/or short stature. Although the use of external fixators is the traditional treatment of choice for open comminuted fractures, patients' acceptance is low due to the high profile and therefore the physical burden of the devices. Recently, clinical case reports have shown that supercutaneous locked plating used as definite external fixation could be an efficient alternative. Therefore, the aim of this study was to evaluate the effect of implant configuration on stability and interfragmentary motions of unstable proximal tibia fractures fixed by means of externalized locked plating. Based on a right tibia CT scan of a 48 years-old male donor, a finite element model of an unstable proximal tibia fracture was developed to compare the stability of one internal and two different externalized plate fixations. A 2-cm osteotomy gap, located 5 cm distally to the articular surface and replicating an AO/OTA 41-C2.2 fracture, was virtually fixed with a medial stainless steel LISS-DF plate. Three implant configurations (IC) with different plate elevations were modelled and virtually tested biomechanically: IC-1 with 2-mm elevation (internal locked plate fixation), IC-2 with 22-mm elevation (externalized locked plate fixation with thin soft tissue simulation) and IC-3 with 32-mm elevation (externalized locked plate fixation with thick soft tissue simulation). Axial loads of 25 kg (partial weightbearing) and 80 kg (full weightbearing) were applied to the proximal tibia end and distributed at a ratio of 80%/20% on the medial/lateral condyles. A hinge joint was simulated at the distal end of the tibia. Parameters of interest were construct stiffness, as well as interfragmentary motion and longitudinal strain at the most lateral aspect of the fracture. Construct stiffness was 655 N/mm (IC-1), 197 N/mm (IC-2) and 128 N/mm (IC-3). Interfragmentary motions under partial weightbearing were 0.31 mm (IC-1), 1.09 mm (IC-2) and 1.74 mm (IC-3), whereas under full weightbearing they were 0.97 mm (IC-1), 3.50 mm (IC-2) and 5.56 mm (IC-3). The corresponding longitudinal strains at the fracture site under partial weightbearing were 1.55% (IC-1), 5.45% (IC-2) and 8.70% (IC-3). From virtual biomechanics point of view, externalized locked plating of unstable proximal tibia fractures with simulated thin and thick soft tissue environment seems to ensure favorable conditions for callus formation with longitudinal strains at the fracture site not exceeding 10%, thus providing appropriate relative stability for secondary bone healing under partial weightbearing during the early postoperative phase

Introduction. Varus alignment in total knee replacement (TKR) results in a larger portion of the joint load carried by the medial compartment. [1]. Increased burden on the medial compartment could negatively impact the implant fixation, especially for cementless TKR that requires bone ingrowth. Our aim was to quantify the effect varus alignment on the bone-implant interaction of cementless tibial baseplates. To this end, we evaluated the bone-implant micromotion and the amount of bone at risk of failure. [2,3]. Methods. Finite element models (Fig.1) were developed from pre-operative CT scans of the tibiae of 11 female patients with osteoarthritis (age: 58–77 years). We sought to compare two loading conditions from Smith et al.;. [1]. these corresponded to a mechanically aligned knee and a knee with 4° of varus. Consequently, we virtually implanted each model with a two-peg cementless baseplate following two tibial alignment strategies: mechanical alignment (i.e., perpendicular to the tibial mechanical axis) and 2° tibial varus alignment (the femoral resection accounts for additional 2° varus). The baseplate was modeled as solid titanium (E=114.3 GPa; v=0.33). The pegs and a 1.2 mm layer on the bone-contact surface were modeled as 3D-printed porous titanium (E=1.1 GPa; v=0.3). Bone material properties were non-homogeneous, determined from the CT scans using relationships specific to the proximal tibia. [2,4]. The bone-implant interface was modelled as frictional with friction coefficients for solid and porous titanium of 0.6 and 1.1, respectively. The tibia was fixed 77 mm distal to the resection. For mechanical alignment, instrumented TKR loads previously measured in vivo. [5]. were applied to the top of the baseplate throughout level gait in 2% intervals (Fig.1a). For varus alignment, the varus/valgus moment was modified to match the ratio of medial-lateral force distribution from Smith et al. [1]. (Fig.1b). Results. For both alignments and all bones, the largest micromotion and amount of bone at risk of failure occurred during mid stance, at 16% of gait (Figs.2,3). Peak micromotion, located at the antero-lateral edge of the baseplate, was 153±32 µm and 273±48 µm for mechanical and varus alignment, respectively. The area of the baseplate with micromotion above 40 µm (the threshold for bone ingrowth. [3]. ) was 28±5% and 41±4% for mechanical and varus alignment, respectively. The amount of bone at risk of failure at the bone-implant interface was 0.5±0.3% and 0.8±0.3% for the mechanical and varus alignment, respectively. Discussion. The peak micromotion and the baseplate area with micromotion above 40 µm increased with varus alignment compared to mechanical alignment. Furthermore, the amount of bone at risk of failure, although small for both alignments, was greater for varus alignment. These results suggest that varus alignment, consisting of a combination of femoral and tibial alignment, may negatively impact bone ingrowth and increase the risk of bone failure for cementless tibial baseplates of this TKR design

Taper corrosion and fretting have been associated with oxide layer abrasion and fluid ingress that contributes to adverse local tissue reactions with potential failure of the hip joint replacement. [1,2]. Both mechanisms are considered to be affected by the precise nature of the taper design. [3]. Indeed relative motion at the taper interface that causes fretting damage and wear effects, such as pistoning and rocking, have been described following analysis of implants at retrieval. [4,5]. However, there is much less reported about the mechanisms that allow the fluid ingress/egress at the taper interface which would drive corrosion. Thus the aim of the present study was to investigate the effect of trunnion design on the gap opening and taper relative motions under different load scenarios and taper designs. A 3-D finite element model of a 40mm CoCr modular femoral head and a Ti6Al4V trunnion was established in Abaqus CAE/2018. Femoral head and trunnion geometries were meshed with an element (C3D8) size of 0.17mm. Tapers were assembled by simulating a range of impact forces (AF); taper interface behaviour was evaluated under physiological forces and frictional moments simulated during walking activity. [6]. , assuming different coefficients of friction (CF), Figure 1. The output involved the total and normal relative motion of the surfaces at the taper interface. The model predicted for a taper mismatch of 0.36° which, when combined with an assembly force of 2kN, generated the largest taper gap opening (59.2mm) during walking, Figure 2. In all trunnion designs the largest normal relative motion coincided with heel strike in the gait cycle (0–5%). The taper gap and normal relative motions were related to the initial taper lock area. Furthermore, the direction of the total motion was different in all three taper mismatches, with a shift in the direction towards the normal of the surface as the taper mismatch increased, Figure 3. By contrast, the direction of the normal relative motions did not change with different trunnion designs. Contact patterns were asymmetrical and contact areas varied throughout the walking activity; contact pressure and the largest taper gap were located on the same side of the taper, suggesting toggling of the trunnion. The relationship between taper gap opening and initial taper lock contact area suggests that the taper contact area functions as a fulcrum in a lever mechanism. Large taper mismatches create larger relative motions that will not only create more wear and fretting damage but also larger normal relative motions. This may allow fluid ingress into the taper interface and/or the egress of fluid along with any metal wear particles into the body. This increased understanding of the taper motion will result in improved designs and ultimately taper performance. For any figures or tables, please contact the authors directly

Finite element (FE) modelling has been widely used to create and assess musculoskeletal models. However to achieve a high degree of resolution in describing the structure, significant computational power and time are required. The objective of this study was to introduce a complimentary approach to FE modelling using structural beam theory. This requires far less computational power and models can be analyzed in a fraction of a second, offering quick, intuitive results for engineers and surgeons. Beam theory was first introduced as a method for analyzing the stresses in long bones in 1917. It was used as the de facto method for several decades. The introduction of FE modelling offered great advances; beam theory calculations were considered laborious and less accurate. However with the advances in computational power so too comes the ability to create modern automated beam theory models. A study was conducted using the commercially available general structural analysis software Oasys GSA. A synthetic biomechanical femur was CT scanned and the solid model constructed. This model was sectioned into approximately seventy sections in the regions of the shaft and condyles, thirty in the neck and thirty in the head. Line plots of the shape of each of the sections, for both cortical and trabecular parts, were then imported into Oasys GSA. The centroid, area, second moments of area and torsion constant were calculated for each section. The sections were plotted at the position of the cortical centroid and parallel axis theorem was used to plot the trabecular section in the same position. A force representing the hip joint reaction force was applied to a node corresponding to the centre of the femoral head. Muscular forces were applied to stiff radial elements according to those active at the point of peak joint contact force during gait. Oasys GSA produced instant results showing moment and deflection characteristics of the femur. This data was then used to predict strain plots, which were directly compared to FE results. Initial results compare favourably. This study has demonstrated an updated fast, efficient and intuitive alternative to finite element modelling