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

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. 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

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