INTRODUCTION. Mechanical overloading of the knee can occur during activities of daily living such as stair climbing, jogging, etc. In this finite element study we aim to investigate which parameters could detrimentally influence peri-implant bone in the tibial reconstructed knee. Bone quality and patient variables are potential factors influencing knee overloading (Zimmerman 2016). METHODS. Finite element (FE) models of post-mortem retrieved tibial specimens (n=7) from a previous study (Zimmerman 2016) were created using image segmentation (Mimics Materialise v14) of CT scan data (0.6 mm voxel resolution). Tibial tray and polyethylene inserts were recreated from CT data and measurements of the specimens (Solidworks 2015). Specimens with varying implant geometry (keel/pegged) were chosen for this study. A cohesive layer between bone and cement was included to simulate the behavior of the bone–cement interface using experimentally obtained values. The FE models predict plasticity of bone according to Keyak (2005). Models were loaded to 10 body weight (BW) and then reduced to 1 BW to mimic experimental measurements. Axial FE bone strains at 1 BW were compared with experimental Digital Image Correlation (DIC) bone strains on cut sections of the specimens. After validation of the FE models using strain data, models were rotated and translated to the coordinate system defined in Bergmann (2014). Four loading cases were chosen – walking, descending stairs, sitting down and jogging. Element strains were written to file for post-processing. The bone in all FE models was divided into regions of equal thickness (10 mm) for comparison of strains. RESULTS. Results are shown for two specimens at present. Strain-maps of the specimen cut section compare reasonably well with FE cutting-plane strains. The FE models however show some regions of high strain in certain locations which do not correspond with the experimental results (Figure 1). Plasticity predicted by the models at 10 BW is shown in Figure 2. Median bone strains for two loading cases are shown as a function of distance below the tibial tray in Figure 3. This figure shows that specimen 1 is less likely to be overloaded during jogging when compared with specimen 2. Both specimens remain below the 7300 με threshold for compressive yield. DISCUSSION. Using functioning knee replacement tibial specimens, we study which factors influence bone overloading. Validation using DIC strain measurements is challenging due to the large plasticity regions predicted by the material model used here. The present results were obtained using plasticity relationships from Keyak (2005) for the proximal femur. To further improve on our results, plasticity-bone density relationships for the proximal tibia (Keyak 1996) will be included. Proximal tibial bone has been shown to be stiffer than femoral bone (Morgan 2003). Despite these limitations, FE models provide valuable information on the risk of overloading during daily living activities. The study will be expanded to include an analysis of implant geometry, bone quality and other loading cases. For any figures or tables, please contact the authors directly

Introduction. In complex primary and revision total knee replacement (TKR) the operating surgeon may encounter proximal tibial bone defects. The correct management of such defects is fundamental to both the initial stability and long-term survival of the prosthesis. Cement or metal augments have been used to address some such type II unconstrained defects [1]. Aim. The aim of this finite element (FE) study was to analyse the comparative behaviour of cement and metal based augments and quantify the stresses within these different augments and underlying cancellous bone. Materials and methods. A three-dimensional FE model was constructed from a computer tomography (CT) scan of the proximal tibia using SIMPLEWARE v3.2 image processing software. The tibial component of a TKR was implanted with either a block or wedge-shaped augment made of either metal or cement. The model was axially loaded with a force of 3600N and testing was conducted with both evenly and eccentrically distributed loads. Results. Upon loading the FE model, the von-Mises stresses in the cancellous bone underneath the augments were found to be higher with cement based augments in comparison their metal based counterparts. This was evident with both block and wedge-shaped augments. The FE model demonstrated that compressive stresses within the metal based augments were greater than those within the cement based augments. This was evident with both block and wedge designs. Upon even loading the maximum recorded compressive stresses within the metal augments were 5 times less than the endurance limit of the material [3]. However, the maximum recorded compressive stresses within cement augments were only half the endurance limit of the material [4] and upon eccentric loading compressive stresses in excess of the endurance limit were recorded. Discussion. The FE model has demonstrated that cement based augments undergo a greater deformation when loaded and therefore transfer greater loads to the underlying cancellous bone. This is a result of the inherent flexibility of the cement based augment in comparison to the stiffer metal counterparts. The greater transference of load to cancellous bone with cement based augments may reduce the possibility of stress shielding. However, the compressive stresses within cement based augments are too close to the endurance limit of the material and with uneven loading even exceed it. This would imply that cement based augments are more prone to fatigue failure than their metal counterparts. Conclusion. This FE study supports the use of metal based augments over cement based augments in augmented and revision TKR surgery

A finite element study was carried out to compare the performance of a three-hole locking plate with angled screws to the ‘gold-standard’ four-hole hip plate. Two cases of the three-hole hip plate were examined; (a) three screws and (b) two screws (most proximal and most distal). A 3D model of the proximal femur was constructed from CT scans. A 3D CAD model of the four-hole hip plate was also created. The three-hole hip plate was then created from the four-hole implant in a way that it was possible to switch between all three models by activating/deactivating sections and/or switching material properties. A single common finite element model was generated, and a static analysis of each model variation was then performed in two steps using ABAQUS/standard. In the first, screws were pre-tensioned up to 150N. In the second, loads corresponding to stair climbing were applied. Forces in the screws, permitted to change in the second step, were examined and compared. Maximum principal stresses in the bone were also examined, with a focus on the stresses in the bone at the end of the plate in each model. The highest tensile force was in the proximal screw of the three-hole plate with three screws, followed by the most distal screw in the standard four-hole plate. This suggests that the risk of screw pull-out is highest at the proximal screw of the three-hole hip plate with three screws. A comparison of the forces in the distal screws for all cases shows that the highest tensile force was in the four-hole plate, followed by the three-hole plate with two screws. The lowest was the three-hole plate with three screws, which was in compression at full load. The maximum tensile stresses in the bone at the end of the plate were greatest for the standard four-hole hip plate, followed by the three-hole plate with two screws and then the three-hole plate with three screws. This indicates that the risk of bone fracture at the end of the plate is lowest for the three-hole hip plate with three screws. The risk of bone fracture is significantly lower for the three-hole hip plate, with either two or three screws, compared to the ‘gold-standard’ four-hole hip plate. This is partially offset by a small increase in the risk of screw pull out (in the proximal rather than the distal screw)

Background

Use of a baseplate with a smaller diameter in reverse shoulder arthroplasty has been recommended, especially in patients with a small glenoid or insufficient bony stock due to severe glenoid wear. However, effect of a smaller baseplate on stability of the glenoid component has not been evaluated. The purpose of this study was to determine whether a smaller baseplate (25 mm) is beneficial to the initial primary stability of the glenoid component compared to that with a baseplate of a commonly used size (29 mm) by finite element analysis.

Introduction

The number of total knee joint replacements has increased dramatically, from 28,000 in 2004 to over 73,000 in 2008 in the UK. This increase in procedures means that there is a need to assess the performance of an implant design in the general population. For younger, more active patients, cementless tibial fixation is an attractive alternative means of fixation and has been used for over 30 years. However, the clinical results with cementless fixation have been variable, with reports of extensive radiolucent lines, rapid early migration and aseptic loosening [1]. This study investigates the inter-patient variability of bone strain at the implant-bone interface of 130 implanted tibias over a full gait cycle.

Effectiveness and long term stability of hip resurfacing and total hip arthroplasty for osteoarthritis patients are still debated nowadays. Several clinical and biomechanical issues have to be considered, including pain relief, return to function, femoral neck fractures, impingement and prosthesis loosening. Normally, patients with hip arthroplasties are facing gait adaptation and at risk of fall. Sudden impact loading and twisting during sideway falls may lead to femoral fractures and joint failures. The purposes of this study are (i) to investigate the stress behavior of hip resurfacing and total hip arthroplasty, and (ii) to predict pattern of femoral fractures during sideway falls and twisting configurations.

Computed tomography (CT) based images of a 54-year old male were used in developing a 3D femoral model. The femur model was designed to be inhomogeneous material as defined by Hounsfield Unit of the CT images. CAD data of hip arthroplasties were imported and aligned to represent RHA and THA femur modelas shown in Fig.1. Prosthesis stem is modeled as Ti-6Al-4V material while femoral ball as Alumina properties. Meanwhile, RHA implant is assigned as Co-Cr-Mo material. Four types of loading and boundary conditions were assigned to demonstrate different falling (FC) and twisting (TC) configurations (see Fig.2). Finite element analysis combined with a damage mechanics model was then performed to predict bone fractures in both arthroplasty models. Different loading magnitudes up to 4BW were applied to extrapolate the fracture patterns.

Prediction of femoral fracture for RHA and THA femurs are discussed in corresponding to maximum principal stress and damage formation criterion. The load bearing strain was set to 3000micron, the physiological bone loading that leads to bone formation. The test strength was wet to 80% of the yield strength determined from the CT images. Different locations of fracture are predicted in each configuration due to different loading direction and boundary conditions as shown in Fig.3. For falling configurations, fractures were projected at trochanteric region for intact and RHA femur, while THA femurs experience fracture at inner proximal region of bone. Differs to twisting configurations, both arthroplasties were predicted to fracture at the distal end of femurs.

Introduction

Lateralizing the center of rotation (COR) of reverse total shoulder arthroplasty (rTSA) has the potential to increase functional outcomes of the procedure, namely adduction range of motion (ROM). However, increased torque at the bone-implant interface as a result of lateralization may provoke early implant loosening, especially in situations where two, rather than four, fixation screws are used. The aim of this study was to utilize finite element (FE) models to investigate the effects of lateralization and the number of fixation screws on micromotion and adduction ROM.

For amputated patients, direct attachment of upper leg prosthesis to the skeletal system by a percutaneous implant is an alternative solution to the traditional socket fixation. Currently available implants, the OPRA system (Integrum AB, Göteborg, Sweden) and the ISP Endo/Exo prosthesis (ESKA Implants AG, Lübeck, Germany) [1-2] allow overcoming common soft tissue problems of conventional socket fixation and provide better control of the prosthetic limb [3], higher mobility and comfort [2, 4]. However, restraining issues such as soft-tissue infections, peri-prosthetic bone fractures [3, 5–8] and considerable bone loss around the stem [9], which might lead to implant's loosening, are present. Finally, a long a residual limb is required for implant fitting.

In order to overcome the limiting biomechanical issues of the current designs, a new concept of the direct intramedullary fixation was developed. The aim was to restore the natural load transfer in the femur and allow implantations in short femur remnants (Figure 1). We hypothesize that the new design will reduce the peri-prosthetic bone failure risk and adverse bone remodeling.

Generic CT-based finite element models of an intact femoral bone and amputated bones implanted with 3 analyzed implants were created for the study. Models were loaded with two loading cases from a normal walking obtained from the experimental measurements with the OPRA device [10-11]. Periprosthetic bone failure risk was evaluated by considering the von Mises stress criterion [12-14]. Subsequently the strain adaptive bone remodeling theory was used to predict long-term changes in bone mineral density (BMD) around the implants. The bone mineral content (BMC) change was measured around implants and the results were visualized in the form of DXA scans.

The OPRA and the ISP implants induced the high stress concentration in the proximal region decreasing in the distal direction to values below physiological levels as compared with the intact bone. The stresses around the new design were more uniformly distributed along the cortex and resembled better the intact case. Consequently, the bone failure risk was reduced as compared to the OPRA and the ISP implants. The adaptive bone remodeling simulations showed high bone resorption around distal parts of the OPRA and the ISP implants in the distal end of the femur (on average −75% ISP to −78% OPRA after 60 months). The bone remodeling simulation did not reveal any bone loss around the new design, but more bone densification was seen (Figure 2). In terms of total bone mineral content (BMC) the OPRA and the ISP implants induced only a short-term bone densification in contrast to the new design, which provoked a steady increase in the BMC over the whole analyzed period (Figure 3).

In conclusion, we have seen that the new design offers much better bone maintenance and lower failure probability than the current osseointegrated trans-femoral prostheses. This positive outcome should encourage further developments of the presented concept, which in our opinion has a potential to considerably improve safety of the rehabilitation with the direct fixation implants and allow treatment of patients with short stumps.

Femoral knee implants have promising outcomes, although some high-flex designs have shown rather high loosening rates (Han et al., 2007). In uncemented implants, it is vital to limit micromotions at the implant-bone interface, to facilitate secondary fixation through bone ingrowth (kienapfel et al., 1999). Hence, it is essential to investigate how micromotions of different uncemented implants are affected by various loading conditions when a range of bone qualities as a patient-related factor is applied.

Using finite element (FE) analysis, we simulated implant-bone interface micromotions during four consecutive cycles of normal gait and squat movements. An FE model of a distal femur was generated based on calibrated CT-scans, after which Sigma® and LCS® Cruciate-Retaining Porocoat® components (DePuy Synthes, Leeds, UK) were implanted. Using a frictional contact algorithm (µ=0.95), an initial press-fit fixation was simulated, which was previously validated against experimental data. The micromotions were calculated by tracking the projection of implant nodes on the bone surface excluding overhang area. The applied loading patterns were based on discretized simulations, providing incremental loads for each activity based on implant-specific kinematics, which was derived from Orthoload database using inverse dynamics (Fitzpatrick et al., 2012). This provided the opportunity to calculate incremental micromotions, but also the resulting micromotions for each single cycle, for both activities. In addition, the percentage of implant surface area with resulting micromotions less than a defined threshold was calculated.

Regardless of the type of loading, in all simulations, the predicted micromotions were highest in the first cycle, suggesting settling of the implant during initial cycle. The Sigma®implant displayed a 30% larger area with micromotions below the threshold of 5 microns, for both loading conditions (Fig. 1A). The highest micromotions occurred at the anterior flange, regardless of type of activity or design. Squatting had a more detrimental effect on the primary stability, with smaller areas of low micromotions as compared to the gait load (Fig. 1B). Bone stiffness had a minor effect, which was more apparent for squatting (Fig. 1B).

We found acceptable low ranges of micromotions in both implant designs, although demanding activities such as squatting generated higher motions. In addition, LCS® experienced higher micromotions, probably caused by the smaller contact area at bone-implant interface compared with Sigma®. Nevertheless, the predicted micromotions were all below the clinically relevant threshold for bone ingrowth (<40 microns) (kienapfel et al., 1999). Furthermore, our simulated settling behavior stresses the necessity for simulating multiple loading cycles, rather than just a single cycle. The effect of bone stiffness was evident, but only to a limited extent. The main current limitation of our study is the utilization of an elastic material model for the bone which is probably the reason to predict a low range of micromotions. We are planning to make the material model more realistic, by including plasticity and viscoelastic bone behavior.

Introduction

Total Hip Arthroplasty (THA) devices are now increasingly subjected to a progressively greater range of kinematic and loading regimes from substantially younger and more active patients. In the interest of ensuring adequate THA solutions for all patient groups, THA polyethylene acetabular liner (PE Liner) wear representative of younger, heavier, and more active patients (referred to as HA in this study) warrants further understanding.

Previous studies have investigated HA joint related morbidity [1]. Current or past rugby players are more likely to report osteoarthritis, osteoporosis, and joint replacement than a general population.

This investigation aimed to provide a preliminary understanding of HA patient specific PE liner tribological performance during Standard Walking (SW) gait in comparison to IS0:14242-1:2014 standardized testing.

Introduction

Dual-mobility (DM) liners provide increased range of motion and stability. However, large head diameters have been associated with anterior hip pain due to impingement with surrounding soft-tissues, particularly the iliopsoas. Further, during hip extension the liner can get trapped due to anterior soft-tissue impingement that resists rotation being imparted to the liner from posterior stem-liner contact. Over time this can cause liner rim damage, leading to intra-prosthetic dislocation of the small diameter inner head. To address this, an anatomically contoured dual mobility (ACDM) liner was designed to reduce the volume of the liner below the equator that can interact with soft-tissues (Fig. 1). In this study, we utilized finite element analysis to evaluate tendon-liner contact pressure and tendon stresses with ACDM and conventional designs during hip extension, wherein the posterior edge of liner is in contact with the stem while the anterior edge is exposed to the soft-tissue.

Introduction

Although total hip arthroplasty is a very successful operation, complications such as: dislocation, aseptic loosening, and periprosthetic fracture do occur. These aspects have been studied in large populations for traditional stem designs, but not for more recent short stems. The design rationale of short stems is to preserve bone stock, without compromising stability. However, due to their smaller bone contact area, high peak stresses and areas of stress shielding could appear in the proximal femur, especially in the presence of atypical bone geometries. In order to evaluate this aspect, we quantified the stress distribution in atypical proximal femurs implanted with a commercially available calcar guided short stem.

Introduction. Lipped liners have the potential to decrease the rate of revision for instability after total hip replacement since they increase the jumping distance in the direction of the lip. However, the elevated lip also may reduce the Range of Motion and may lead to early impingement of the femoral stem on the liner. It is unclear whether the use of a lipped liner has an impact on the level of lever-out moments or the contact stresses. Therefore, the aim of the current study was to calculate these values for lipped liners and compare these results to a conventional liner geometry. Materials and Methods. 3D Finite Element studies were conducted comparing a ceramic lipped liner prototype and a ceramic conventional liner both made from BIOLOX. ®. delta. The bearing diameter was 36 mm. To apply loading, a test taper made of titanium alloy was bonded to a femoral head, also made from BIOLOX. ®. delta. Titanium was modeled with a bilinear isotropic hardening law. For the bearing contact a coefficient of friction of both 0.09 or 0.3 was assumed to model a well and poorly lubricated system. Frictionless contact was modeled between taper and liner. Pre-load was varied between 500 N and 1500 N and applied along the taper axis. While keeping pre-load constant, lever-out force was applied perpendicular to the taper axis until subluxation occurred. Liners were fixed at the taper region. Lever-out moment, equivalent plastic strain and von Mises stress of the taper, bearing contact area and contact area between taper and liner was evaluated. Results. With increasing pre-load, larger lever-out moment, equivalent plastic strain, contact area between taper and liner and bearing contact area was found for both liner designs. However, von Mises stresses were nearly constant but slightly exceeded yield strength of titanium. For all evaluated parameters almost no differences were found between the liner designs. Lever-out moments were comparable for both designs ranging from 4.5–10.5 Nm for the lipped liner and 4.4–10.2 Nm for the conventional liner. The increase of the coefficient of friction strongly affected lever-out moments, equivalent plastic strain and contact area between taper and liner. The other parameters were not affected by varying the coefficient of friction. Discussion. This study did not find significant differences in the lever-out behavior of the lipped acetabular liner compared to the conventional liner design. The inner geometry of the lipped liner is comparable to the conventional liner inner geometry. Therefore, contact area showed no significant differences and contact mechanics are identical in the current setup leading to similar results of both liner designs. For both designs small plastic deformations in the contact point of the taper were found at the contact region between liner and taper. However, the investigated mechanical parameters did not differ between the two investigated liner types. For any figures or tables, please contact authors directly

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

Metal-backed tibial components in total knee arthroplasty (TKA) currently dominate the orthopaedic market due to intra-operative flexibility afforded by modularity. Metal-backing was first used in TKA as a method to potentially improve loading distributions over the tibial plateau at the interface between the prosthesis and the supporting cancellous bone. Many studies have compared metal-backed and all-polyethylene tibial components with variable survivorship. We have found decreased clinical survivorship with all-polyethylene Anatomic Graduated Component (AGC) TKA's (Biomet, Inc, Warsaw, Indiana) compared to the non-modular metal-backed design at 10-year follow up, 68% vs 98%, respectively. Loosening or bony collapse beneath the medial plateau accounted for 74% of failures in our AGC all-polyethylene cohort. We hypothesised that all-polyethylene tibial components may lead to increased strains in the proximal tibia with the AGC-TKA design, possibly correlating to osseous overload in the medial compartment and accounting for the increased observed rates of clinical failures in the all-polyethylene group. Finite element studies and our lab studies have shown that metal-backing reduces system stresses in the PMMA bone cement, as well as in the underlying cancellous bone. Overall, in every measurement region with a statistically significant difference in shear strain, higher strain was measured in the all-polyethylene implanted tibiae compared to metal-backed components. Statistically significant increases in strain from 126 μɛ (p=0.0131) to 745 μɛ (p=0.0011) and from 40% (p=0.0010) to 587% (p=0.0054) were seen in the all-polyethylene experimental group. We believe this may correlate with the higher failure rates we have observed in the AGC all-polyethylene cohort compared to the metal-backed cohort from our institution. Other all-poly TKA designs with varied articular congruities may afford improved or equal survivorship to metal backed implants at a reduced cost

Introduction. Cementless stems are fixed to the surrounding bone by means of mechanical press-fit. Short-, mid-, and long term outcomes are good for this type of fixation despite that only a part of the stem surface is in contact with the surrounding bone. Several studies show that the contact ratio achieved after surgery between the stem and the surrounding bone ranged between 15% and 60%. Then, only a part of the stem-bone interface presents a press-fit. The rest of the stem-bone interface is only in contact or presents an interfacial gap inherent to the surgical technique. Therefore, this study aimed to investigate the difference in the primary stability of a cementless stem between a press-fit combined with contact and a press-fit combined with gap achieved after the surgery. Materials & Methods. A finite element study was carried out on a composite bone implanted with a femoral stem and subjected to physiological loading simulating stair climbing [1]. All materials were defined as isotropic homogeneous. The stem-bone interface was divided into 4 areas: the superior plasma spray, the inferior plasma spray, the polished surface of the stem in contact with the cancellous bone, and the plasma spray surface of the stem in contact with the cortical bone. Each contact area can be either in contact with a press-fit, either in contact without press-fit or can present a gap. This result in a total of 28 cases: 14 where there is a press-fit combined with contact and 14 cases where there is a press-fit combined with gap. Results. When 1 press-fit is combined with 3 contacts, the average micromotion reaches 26µm. The average micromotion increases up to 47µm (+45%) when 1 press-fit is combined with 3 gaps. The detailed results can be seen on Figure 1. When 2 press-fits are combined with 2 contacts, the average micromotion reaches 28µm. The average micromotion increases up to 36µm (+29%) when 2 press-fit are combined with 2 gaps. The detailed results can be seen on Figure 2. When 3 press-fits are combined with 1 contact, the average micromotion reaches 31µm. The average micromotion increases up to 34µm (+9%) when 3 press-fits are combined to 1 gap. The detailed results can be seen on Figure 3. Discussion. As expected, micromotion predicted when press-fit is combined with contact is lower than when press-fit is combined with gap. However, the average micromotion increases with the increase of press-fit when combined with contact (from 26µm to 31µm (+19%)). The increase of the press-fit area combined with contact lead to a slightly increase of the micromotion. Conversely, the average micromotion decreases with the increase of press-fit when combined with gap (from 47µm to 34µm (−28%)). This is due to the increase of the press-fit area, and then the gap has a lesser effect. This study also shows that the difference in the micromotion between press-fit when combined with contact and press-fit when combined with gap decreases with the increase of the press-fit

Introduction. Dual-mobility (DM) liners have increased popularity due to the range of motion and stability provided by these implants. However, larger head diameters have been associated with anterior hip pain, due to surrounding soft-tissue impingement, particularly the iliopsoas. To address this, an anatomically contoured dual mobility (ACDM) liner was designed by reducing the volume of the liner below the equator (Fig1). Previous cadaver studies have shown that the ACDM significantly reduces iliopsoas tenting and trapping of the liner compared to conventional designs. We created a finite element study based on previous cadaver testing to further analyze the effectiveness of the ACDM design in reducing soft-tissue impingement, specifically the tendon-liner contact pressure and the tendon stress. Methods. The finite element model was developed within COMSOL 4.3b. The psoas tendon was modelled as a Yeoh hyper-elastic Material, which uses 3 constants (c1-c3), density (1.73g/cm3) and a bulk modulus (26GPa)[Hirokawa,2000]. In a previous, separate study, the average stiffness of 10 psoas tendon samples (5 cadavers), were measured to be 339[N/mm] in the linear region with average width and thickness of 14mmX4mm. The 3 constants were tuned to match experimental uniaxial test data, and were 5[GPa], 0[Gpa], and 46[GPa] for c1, c2, and c3 respectively. The implant components were rigidly modeled relative to the psoas. Cadaver specific CT models were used to create the FEA geometry. The insertion points for the Psoas were digitally determined on the proximal end of the lesser trochanter, and the psoas notch on the pelvis for hip flexion angles of −15°, 0°, 15° and 30°. These insertion points determined the length of the psoas and its relative position to the femoral head in 3D. The specific liner size and position for each cadaver was determined by implant planning with the CT models. In this abstract, we only present data for 2 specimens (left/right hips) with 44mm conventional DM, and 44mm ACDM, matching specimen anatomy. A 500N tensile load was applied to the psoas tendon proximally to simulate moderate physiological loading, the average/max stresses and contact pressures between the psoas and the two liner designs were determined. Results. At all flexion angles from −15° to 30°, the ACDM had lower psoas-liner contact pressure and stress compared to the conventional liner. Both contact pressure and tendon stress decreased for both liners with increasing hip flexion. At −15° flexion angle, there was an average contact pressure difference of .51MPa between the conventional and ACDM designs, or 37% decrease in pressure when using the ACDM. The average difference in tendon stress was 67.9MPa, or a 59% decrease in stress when using the ACDM (fig2, fig3). Conclusion. This study utilized cadaver specific FEA models to evaluate interaction between the iliopsoas tendon and conventional and ACDM liners. Although this abstract presented FEA models for only four hips (two specimens), the results show a notable reduction in contact pressure and tendon stress with ACDM designs. This validates findings from previous cadaver studies, suggesting that anatomically contoured designs could reduce anterior hip pain and soft tissue impingement

Introduction. Modular hip replacement systems use Morse tapers as an interlocking mechanism to connect ball heads to femoral stems. Even though this interlocking mechanism generally performs successfully for decades, failures due to disassociation of the ball head from the stem are reported in the literature. Therefore, this failure mechanism of a possible loosening is usually evaluated in the course of the development of femoral stems. The disassembly force is a possible parameter to characterize the strength of the interlocking mechanism. Thus, the aim of the current study was to examine the impact of different taper parameters on the disassembly force of ceramic ball heads from titanium stem tapers by finite element studies. Materials and Methods. A 2D axisymmetric finite element model was developed to simulate the disassembly procedure. First ball head and taper were assembled with a force of 4 kN. Afterwards the system was unloaded to simulate the settlement. Disassembly was simulated displacement controlled until no more adhesion between ball head and taper occurred. Isotropic elastic material behavior was modelled for the ceramic ball head while elastic-plastic material behavior was modelled for the titanium taper. Different angular gaps (0.2°, 0.15°, 0.1°, 0.05°, 0°, −0.05°, −0.1°) and different taper topography parameters regarding groove depth (12, 15 µm), groove distance (210, 310 µm) and plateau width (1, 5, 10, 20 µm) were examined. Frictional contact between ball head and taper was modelled. Results. The topography of the taper (groove depth, distance and plateau width) within the investigated range had only a small impact on the disassembly force (Fig. 1) while the varying angular gaps had a large effect (Fig. 2). Decreasing disassembly forces were found for decreasing angular gaps. For the negative angular gaps (i.e. male taper angle > female taper angle) the forces increased. The same trends were found for the sliding distance (sliding along the tangential direction in the taper region), deformation of the grooves and contact stresses. Reciprocal behavior was found for the contacting area. Discussion. Surface topography seems to have only minor influence, while macro-geometry seems to have major impact on the disassembly force. Higher disassembly forces are associated with smaller contacting areas, higher contact stresses, larger deformations of the grooves and larger sliding distances. For a negative angular gap the maximum stresses of the ceramic component were found at the taper mouth. This could be disadvantageous since the wall thickness in this region of the ball heads decreases and critical hoop stresses could increase the risk of a fracture. The decrease in contacting areas due to the extreme angular gaps could promote corrosive effects since a larger taper area is exposed to fluid. Furthermore, the higher contact stresses and groove deformations could increase taper wear. Therefore, a general examination of possible influencing factors and cross effects on the in vivo performance have to be conducted during the development of femoral stems. Future studies will include a wear model and include 3D calculations to examine more realistic loading scenarios. To view tables/figures, please contact authors directly

INTRODUCTION. Reverse shoulder arthroplasty (RSA) provides an effective alternative to anatomic shoulder replacements for individuals with cuff tear arthropathy, but certain osteoarthritic glenoid deformities make it challenging to achieve sufficient long term fixation. To compensate for bone loss, increase available bone stock, and lateralize the glenohumeral joint center of rotation, bony increased offset RSA (BIO-RSA) uses a cancellous autograft for baseplate augmentation that is harvested prior to humeral head resection. The motivations for this computational study are twofold: finite element (FE) studies of BIO-RSA are absent from the literature, and guidance in the literature on screw orientations that achieve optimal fixation varies. This study computationally evaluates how screw configuration affects BIO-RSA graft micromotion relative to the implant baseplate and glenoid. METHODS. A senior shoulder specialist (GSA) selected a scapula with a Walch Type B2 deformity from patient CT scans. DICOM images were converted to a 3D model, which underwent simulated BIO-RSA with three screw configurations: 2 divergent superior & inferior locking screws with 2 convergent anterior & posterior compression screws (SILS); 2 convergent anterior & posterior locking screws and 2 superior & inferior compression screws parallel to the baseplate central peg (APLS); and 2 divergent superior & inferior locking screws and 2 divergent anterior & posterior compression screws (AD). The scapula was assigned heterogeneous bone material properties based on the DICOM images’ Hounsfield unit (HU) values, and other components were assigned homogenous properties. Models were then imported into an FE program for analysis. Anterior-posterior and superior-inferior point loads and a lateral-medial distributed load simulated physiologic loading. Micromotion data between the RSA baseplate and bone graft as well as between the bone graft and glenoid were sub-divided into four quadrants. RESULTS. In all but 1 quadrant, APLS performed the worst with the graft having an average micromotion of 347.1µm & 355.9 µm relative to the glenoid and baseplate, respectively. The SILS configuration ranked second, having 211.2 µm & 274.4 µm relative to the glenoid and baseplate. AD performed best, allowing 247.4 µm & 225.4 µm of graft micromotion relative to the glenoid and baseplate. DISCUSSION. Both APLS and SILS techniques are described in the literature for BIO-RSA fixation; however, the data indicate that AD is superior in its ability to reduce graft micromotion, and thus some revision to common practices may be necessary. While these micromotion data are larger than data in the extant RSA literature, there are several factors that account for this. First, to properly model the difference between locking and compression screws, we simulated friction between the compression screw heads and baseplate rather than a tied constraint as done in other studies, resulting in larger micromotion. Second, the trabecular bone graft is at greater risk of deforming than metallic spacers used when studying micromotion with glenosphere lateralization, increasing graft deflection magnitude. Future work will investigate the effects of various BIO-RSA variables. For any figures or tables, please contact authors directly

Introduction. Fretting corrosion of the modular taper junction in total hip arthroplasty has been studied in several finite element (FE) studies. Manufacturing tolerances can result in a mismatch between the femoral head and stem, which can influence the taper mechanics leading to possibly more wear. Using FE models the effect of these manufacturing tolerances on the amount of volumetric wear can be studied. The removal of material in the FE model was validated against experiments simulating the clinical fretting wear process, subsequently the mismatch and assembly force were varied to study the effect on the volumetric wear. Methods. An FE model was developed in which the geometry can be updated to account for material removal due to wear. In this model the geometry was updated based on Archard's Law, using contact pressures, micromotions and a wear factor, which was determined based on accelerated fretting experiments. The linear wear was calculated using H=k*p*S. Where H is the linear wear depth in mm, k is a wear factor (mm. 3. /Nmm), p is the contact pressure (MPa) and S is the sliding distance (mm). 10 million cycles were simulated using 50 virtual steps. Using this scaling and the measured volumetric wear from the experiments a wear factor of 2.7*10. −5. was applied. Based on general manufacturing tolerances the resulting mismatch in taper angles were determined to be ± 1.26°. Using this mismatch a tip fit (figure 1a) and base fit (Figure 1b) model were created. In combination with a perfect fit, meaning no mismatch, and two different assembly forces of 4 kN and 15 kN, 6 different situations were studied. Results. No mismatch proved to result in the least amount of wear after 10 million simulated cycles (Figure 2). Assembling with 15 kN instead of 4 kN reduced the total volumetric wear and the volumetric wear rate. A base fit mismatch resulted in less volumetric wear than a tip fit mismatch. The 15 kN assembled mismatch cases showed a large initial amount of material removal after which the wear rate was lower than the 4 kN assembled cases. Discussion and conclusion. The results show that a perfect fit between the head and stem results in the least amount of wear. Furthermore a larger assembly force of 15 kN resulted in less wear than a 4 kN assembly force. The tip fit mismatch showed up to 144% more wear than the perfect fit where the base fit only had an increase in volumetric wear of 12%. The relative large tolerances in this study may overestimate actual mismatch, but give good insight into the effect that manufacturing tolerances can have on the taper mechanics and volumetric wear. Since manufacturing a perfect fit is impossible it is important to use a sufficiently high assembly force, when clinically possible, in order to reduce the amount of wear and wear rate significantly. For any figures or tables, please contact the authors directly