Introduction. Non-union is debilitating, costly and affects 2–8% of intramedullary fixed fractures. Clinical data suggest that percutaneous interfragmentary screws offer a less invasive alternative to exchange nailing. This study aimed to assess their efficiency with biomechanical analyses. Materials and Methods. A tibia was prepared for finite element analysis by creating a fracture of AO classification 42A2b, prior to reaming and insertion of an intramedullary nail. A callus was modelled as granulation tissue and gait loads were applied. The model was validated against published data and with sensitivity studies. The effects of weightbearing, fracture gap and angle, percutaneous screws and exchange nailing were compared through quantification of interfragmentary motion and strain, with the latter used to gauge healing performance via mechano-regulation theory. Results. Axial interfragmentary motion increased with increasing weightbearing, however, shear decreased at 25–50% weightbearing, leading to superior healing performance. Fracture gap had minimal effect on axial motion, but larger gaps gave greater shear, compromising healing. Elevated fracture obliquity culminated in more shear and inferior healing. Exchange nailing reduced axial motion by ∼30%, but had little effect on shear. Conversely, percutaneous screws had negligible effect on axial motion, but reduced shear by ∼15%, with three screws having a similar net effect on healing as exchange nailing from 10 to 11mm. Conclusions. This study provides new insight into fracture healing biomechanics and discovered that partial weightbearing, less oblique fractures and percutaneous screws all reduce shear, enhancing healing

A common location for radius fracture is the proximal radial head. With the arm in neutral position, the fracture usually happens in the anterolateral quadrant (Lacheta et al., 2019). If traditional surgeries are not enough to induce bone stabilization and vascularization, or the fracture can be defined grade III or grade IV (Mason classification), a radial head prosthesis can be the optimal compromise between bone saving and recovering the “terrible triad”. A commercially available design of radial head prosthesis such as Antea (Adler Ortho, Milan, Italy) is characterized by flexibility in selecting the best matching size for patients and induced osteointegration thanks to the Ti-Por. ®. radial stem realized by 3D printing with laser technique (Figure 1). As demonstrated, Ti-Por. ®. push-out resistance increased 45% between 8 −12 weeks after implantation, hence confirming the ideal bone-osteointegration. Additional features of Antea are: bipolarity, modularity, TiN coating, radiolucency, hypoallergenic, 10° self-aligning. The osteointegration is of paramount importance for radius, in fact the literature is unfortunately reporting several clinical cases for which the fracture of the prosthesis happened after bone-resorption. Even if related to an uncommon activity, the combination of mechanical resistance provided by the prosthesis and the stabilization due to the osteointegration should cover also accidental movements. Based upon Lacheta et al. (2019), after axial compression-load until radii failure, all native specimens survived a compression-load of 500N, while the failure happened for a mean compression force of 2560N. The aim of this research study was to test the mechanical resistance of a radial head prosthesis obtained by 3D printing. In detail, a finite element analysis (FEA) was used to understand the mechanical resistance of the core of the prosthesis and the potential bone fracture induced in the radius with simulated bone- resorption (Figure 2a). The critical level was estimated at the height for which the thickness of the core is the minimum (Figure 2b). Considered boundary conditions:. - Full-length prosthesis plus radius out of the cement block equal to 60mm (Figure 2a);. - Bone inside the cement equal to 60mm (Figure 2b);. - Load inclined 10° epiphysiary component (Figure 2c);. - Radius with physiological or osteoporotic bone conditions;. - Load (concentrated in the sphere simulating full transmission from the articulation) of 500N or 1300N or 2560N. Figure 3 shows the results in terms of maximum stress on the core of the prosthesis and the risk of fracture (Schileo et al., 2008). According to the obtained results, the radial head prosthesis shows promising mechanical resistance despite of the simulated bone-resorption for all applied loads except for 2560N. The estimated mechanical limit for the material in use is 200MPa. The risk of fracture is in agreement with the experimental findings (Lacheta et al. (2019)), in fact bone starts to fail for the minimum reported failure load, but only for osteoporotic conditions. The presented FEA aimed at investigating the behavior of a femoral head prostheses made by 3D printing with simulated bone-resorption. The prosthesis shows to be a skilled solution even during accidental loads. For any figures or tables, please contact the authors directly

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

Introduction. A Finite Element Analysis (FEA) is often used to examine load transfer between prosthesis and canal. Ordinary, bone elements' type is defined as elastic material. But using this element type for FEA on stem load transfer, the stems will jump out and fly away when the load is removed even friction between the stem and the canal was defined. This is remarkably different from the reality. It happens because the canal elements return to the original shape without the load. But actually, the bone is impacted by the load without returning to the original shape. Meshing the trabecular bone with a collapsible element type, it can collapse and be hardened by the stem pressure. We have been using Revelation (DJO, USA) with lateral flare for the primary cases whom we can expect high proximal load transfer. We were going to shorten its length to secure proximal load. We have been using Modulus (Lima Corporate, Italy) with conical fixation for the cases we expect mid stem load transfer and neck modification. We were going to extend its length for wider load transfer area. To examine load transfer of the designs the collapsible FEA was used. Objectives. Our objectives are to examine load transfer between stems with different length and canal by collapsible FEA. Materials and Methods. CAT scan data performed before Hip Arthroplasty were used. Each case has different preferable size. Data conversion was done by Mimics® then LS Dina was used for FEA. For cortical bone and stems conventional elements were used. For trabecular bone, collapsible elements were used. Results. With lateral flare stem, more stable proximal load transfer was obtained (Fig. 1). With conical stems, contact pressure on the boundary between stem and bone was widely distributed and no local stress concentration was observed. During stem insertion, trabecular bone was gradually collapsed up to a stable end point. After removing the load, stems were slightly elevated then settled as observed in realty. Conclusions. 1. Stem modification for each purpose, load transfer we had expected was obtained. 2. FEA with Collapsible elements has shown results which are closer to the realty

Introduction. Varying degrees of posterior glenoid bone loss occurs in patients with end stage osteoarthritis and can result in increased glenoid retroversion. The excessive retroversion can affect implant stability, eccentric glenoid loading, and fixation stresses. Ultimately, the goal is to correct retroversion to restore normal biomechanics of the glenohumeral joint. The objective of this study was to identify the optimal augmented glenoid design based on finite element analysis (FEA) modeling which will provide key insights into implant loosening mechanisms and stability. Materials and Methods. Two different augmented glenoid designs, posterior wedge and posterior step- were created as a computer model by a computer aided design software (CAD). These implant CAD models were created per precise manufacturers dimensions and sizes of the augmented implant designs. These implants were virtually implanted to correct 20° glenoid retroversion and the different mechanical parameters were calculated including: the glenohumeral subluxation force, relative micromotion at the bone-cement interface the glenoid, implant and cement mantle stress levels. The FEA model was then utilized to make measurements while the simulating abduction with the different implant designs. The biomechanical response parameters were compared between the models at comparable retroversion correction. Results. The model prediction of force ratio for the augmented wedge design was 0.56 and for the augmented step design was 0.87. The step design had higher force ratio than the wedge one at similar conformity settings. Micromotion was defined as a combination of three components based on different directions. The distraction measured for the wedge design was 0.05 mm and for the step component, 0.14 mm. Both implants showed a similar pattern translation wise. The greatest difference between the two implants was from the compression standpoint, where the step component showed almost three times more movement than the wedge design implant. Overall, the step design registered greater micromotion than the wedge one during abduction physiologic loading. The level of stress generated during abduction on the glenoid vault was 1.65 MPa for the wedge design and 3.78 MPa for the step one. All stress levels were found below the determined bone failure limit for the bone and polyethylene (10–20 MPa). Concerning implant stress, the results measured on the backside of the wedge and step components were 6.62 MPa and 13.25 MPa, respectively. Both components showed high level of stress level measured on the cement mantle, which exceeded the endurance limit for cement fracture (4 MPa). Discussion. The augmented glenoid is a novel surgical implant for use in with severe glenohumeral osteoarthritis. Unlike standard glenoid prosthetics, the augmented glenoid is better suited for correcting moderate to severe retroversion. Whereas a step design might provide higher glenohumeral stability, the tradeoff is higher glenoid vault, implant and cement mantle stress levels, and micromotion, indicating higher risks of implant loosening, failure or fracture over time, leading to poorer clinical outcomes and higher revision rates

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. Materials and Methods. Nine healthy male subjects volunteered for a gait lab-based study to collect kinematics and loading profiles. Owing to limitations in subject selection, five subjects wore a weighted jacket to increase Body Mass Index ≥30 (BMI). An induced increase in Bodyweight was capped (<30%BW) to avoid significantly effecting gait [3] (mean=11%BW). Six subjects identified as HA per BMI≥30, but with anthropometric ratios indicative of lower body fat as previously detailed by the author [2] (Waist-to-hip circumference ratio and waist circumference-to-height ratio). Three subjects identified as Normal (BMI<25). Instrumented force plate loading profiles were scaled (≈270%BW) in agreement with instrumented hip force data [4]. A previously verified THA (Pinnacle® Marathon® 36×56mm, DePuy Synthes) Finite Element Analysis wear model based on Archard's law and modified time hardening model [5] was used to predict geometrical changes due to wear and deformation, respectively (Figure 1). Subject dependent kinematic and loading conditions were sampled to generate, for both legs, 19 SW simulation runs using a central composite design of response surface method. Results. HA group demonstrated comparable SW gait characteristics and Range of Motion (RoM) to the Normal group (p>0.1) (Figure 2) but statistically greater SW peak loads, PE liner wear rates, deformation, and penetration after 3Mc (Million cycles) of SW (p<0.01). HA group demonstrated comparable RoM (p>0.4) and peak loading to ISO-14242-1:2014 (p>0.1) although, up to 8° increase in flexion-extension angle was observed. The HA group demonstrated statistically greater wear rates (mean 7.5% increase) to ISO-14242-1:2014 (p<0.05) (Figure 3). No difference in PE liner deformation or penetration was observed (p>0.4). Discussion. This study detailed only a 19. th. percentile within a broader HA population (BW=91kg, n=485) [6] however, were statistically worst-case compared to a Normal group and ISO-14242-1:2014. A 95. th. percentile HA population (BW=127kg) may produce lower PE liner tribological performance than reported in this investigation and therefore, warrants further investigation. Further studies would be beneficial to determine whether the increase in PE liner wear rate for HA patients is predictable based on kinematics and loading alone, or whether influences exist in design inputs and surgical factors. Conclusion. The HA population detailed in this study (representative of a 19. th. percentile) demonstrated statistically greater SW PE liner wear rates compared to ISO-14242-1:2014. This study may have implications for the test methods considered appropriate to verify novel designs. For any figures or tables, please contact the authors directly

Introduction. Total hip replacement is an established surgical procedure done to alleviate hip pain due to joint diseases. However, this procedure is avoided in yonger patients with higher functional demands due to the potential for early failure. An ideal prosthesis will have have a high endurance against impact loading, with minimal micromotion at the bone cement interface, and a reduced risk of fatigue failure, with a favourable stress distribution pattern in the femur. We study the effect of varying the material properties and design element in a standard cemented total hip using Finite Element Analysis. Methods. A patient-specific 3D model of femur will be constructed from CT scan data, while a Summit® Cemented Hip System (DePuy Orthopedic) will be used to as a control for comparative evaluation. We vary the material stiffness of different parts of the prosthesis(see Fig.1) to formulate a design concept for a new total hip prosthesis design; and use Finite Element Method to predict the micromotion of the hip prosthesis at the bone cement interface, as well as the stress distribution in the the femur. Result. Validation of computational protocol was being done by comparing the principal maximum strain of the femoral cortex along the diaphysis, and the amount of deflection, with published literature, similarly, contact modelling validation was also done. Model 1–4 induced lower peak Von Mises stress in the cement, which takes a much lower value than any of the cement mechanical limits postulated. Therefore, the risk of cement failure is greatly reduced in Model 1–4. However, the effect of varying stiffness in different regions is not significant in terms of load transmission to the cement. Micromotion at the bone-cement interface was studied via two approaches: Peak micromotion at the bone cement interface; and the micromotion data at 12 Regions of Interest (ROI)s. Both results showed that model 2 and 3 are capable of reducing micromotion at bone-cement interface, in comparison with the Summit® Cemented Hip System. By comparing the Von Mises Stress distribution in the proximal femur; model 1 is found to result in a significantly reduced stress shielding effect, while model 2–4 are also favourable in comparison to the standard Summit® prosthesis in terms of stress distribution in the femur. Figure 2 shows the effects of the performance of model 1–4, presented as percentage difference from the Summit® prosthesis. Model 1 is unfavourable, despite its favourable stress distribution, because its peak and overall micromotion at the bone-cement interface is greatly increased. Conclusion. Model 2 and 3 have favourable design elements. They both have reduced micromotion at the bone-cement interface; and a favourable stress distribution in the femur. Further refining and testing of model 2 and 3 should done, as these models may provide information which may be useful in improving the performance of the current range of total hip replacement prostheses

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

Introduction. Varying degrees of posterior glenoid bone loss occurs in patients with end stage osteoarthritis and can result in increased glenoid retroversion. Ultimately, the goal is to correct retroversion to restore normal biomechanics of the glenohumeral joint. The goal of this study was to identify the optimal augmented glenoid design based on finite element model analysis which will provide key insights into implant loosening mechanisms and stability. Materials and Methods. Two different augmented glenoid designs, posterior wedge and posterior step- were created as a computer model by a computer aided design software (CAD). These implants were virtually implanted to correct 20° glenoid retroversion and the different mechanical parameters were calculated including: the glenohumeral contact pressure, the cement stress, the shear stress, and relative micromotions at the bone cement interface. Results. During abduction, high strain was concentrated around the peg and posterior glenoid bone. Strain was noticeably higher in stepped design (1–2%) than the wedged design (0.4–1.2%). Stepped glenoid models sustained 30% and 70% higher stresses than those experienced by the wedged glenoid implant models at two different corrections. Distractions predicted by the stepped designs were found to be at least twice as much as those by the wedged designs. Similarly, in compression values were 1.5–8 magnitudes higher in stepped designs than those of wedged designs. The wedged design, the amount of micromotion was not affected by the size of the augment (8° and 16°). Discussion. Our study showed that the wedged design experienced less stress compared to stepped design with abduction loading. Notably, the wedged design experienced less stress as the size of the wedge increased to correct a more retroverted arthritic glenoid. The step design also had the highest amount of micromotion which ultimately points to increased failures rate and decreased performace

Introduction. Augmented glenoid implants provide a new avenue to correct glenoid bone loss and can possibly reconcile current prosthetic failures and improve long-term performance. Biomechanical implant studies have suggested benefits from augmented glenoid components but limited evidence exists on optimal design of these augmented glenoid components. The aim of this study was to use integrated kinematic finite element analysis (FEA) model to evaluate the optimal augmented glenoid design based on biomechanical performance in extreme conditions for failure. Materials and Methods. Computer aided design software (CAD) models of two different commercially available augmented glenoid designs - wedge (Equinox®, Exactech, Inc.) and step (Steptech®, Depuy Synthes) were created per precise manufacturer's dimensions and sizes of the implants. Using FE modeling, these implants were virtually implanted to correct 20° of glenoid retroversion. Two glenohumeral radial mismatches (RM) (3.5/4mm and 10 mm) were evaluated for joint stability and implant fixation to simulate high risk conditions for failure. The following variables were recorded: glenohumeral force ratio, relative micromotion (distraction, translation and compression), and stress on the implant and at the cement mantle interface. Results. The wedged and step designs showed similar force ratio measurements with both RM [(wedge (3.5 mm: 0.69; 10 mm: 0.7) and step (4 mm: 0.72; 10 mm: 0.75)]. Surrogate for micromotion was a combination of distraction, translation and compression. As radial mismatch increased, both implants showed less distraction [wedge design (3.5 mm: 0.042 mm; 10mm: 0.030 mm); step design (4 mm: 0.04 mm; 10 mm: 0.027 mm)]. As radial mismatch increased, both implants showed more translation [wedge design (3.5 mm: 0.058 mm; 10mm: 0.062 mm); step design (4 mm: 0.023 mm; 10 mm: 0.063 mm)]. During compression measurements, the different designs did not follow the same pattern as their conformity setting changed. The wedge one decreased as radial mismatch increased, (at 3.5mm: 0.18 mm; at 10 mm: 0.10 mm) and the step design increased as its radial mismatch increased (at 3.5 mm: 0.19 mm; at 10 mm: 0.25 mm). Quantitatively, the step design showed higher risk of implant instability and loosening. As radial mismatch increased, the stress level on the backside of the implant increased as opposed to the stress levels on the cement mantle which decreased for both designs as the radial mismatch increased [wedged (3.5 mm: 2.9 MPa; 10mm: 2.6 MPa); step (3.5 mm: 4.4 MPa; 10 mm: 4.1 MPa)]. In this situation, the risk of loosening was higher for the step designwhich exceeded the endurance limit of the cement material (4 MPa). Discussion. Implant loosening and wear are associated with increased micromotion and high stress levels. Based on our FEA model, overall increased radial mismatch has an advantage of providing higher glenohumeral stability but not without tradeoffs, such as higher implant and cement mantle stress levels, and micromotion increasing the risk of implant loosening, failure or fracture over time, leading to poorer clinical outcomes and higher revision rates, especially when considering a step augmented glenoid design

Introduction. In total hip arthroplasty, press-fit anchorage is one of the most common fixation methods for acetabular cups and mostly ensures sufficient primary stability. Nevertheless, implants may fail due to aseptic loosening over time, especially when the surrounding bone is affected by stress-shielding. The use of acetabular cups made of isoelastic materials might help to avoid stress-shielding and osteolysis. The aim of the present numerical study was to determine whether a modular acetabular cup with a shell made of polyetheretherketone (PEEK) may be an alternative to conventional titanium shells (Ti6Al4V). For this purpose, a 3D finite element analysis was performed, in which the implantation of modular acetabular cups into an artificial bone stock using shells made of either PEEK or Ti6Al4V, was simulated with respect to stresses and deformations within the implants. Methods. The implantation of a modular cup, consisting of a shell made of PEEK or Ti6Al4V and an insert made of either ceramic or polyethylene (PE), into a bone cavity made of polyurethane foam (20 pcf), was analysed by 3D finite element simulation. A two-point clamping cavity was chosen to represent a worst-case situation in terms of shell deformation. Five materials were considered; with Ti6Al4V and ceramic being defined as linear elastic and PE and PEEK as plastic materials. The artificial bone stock was simulated as a crushable foam. Contacts were generated between the cavity and shell (μ = 0.5) and between the shell and insert (μ = 0.16). In total, the FE models consisted of 45,282 linear hexahedron elements and the implantation process was simulated in four steps: 1. Displacement driven insertion of the cup; 2. Relief of the cup; 3. Displacement driven placement of the insert; 4. Load driven insertion of the insert (maximum push-in force of 500 N). The FE model was evaluated with respect to the radial deformations of the shell and insert as well as the principal stresses in case of the ceramic inserts. The model was experimentally validated via comparison of nominal strains of the titanium shells. Results. The maximum radial deformation of the shell made of PEEK was 581 μm (insertion) and 470 μm (relief) and therefore multiple times higher compared to the Ti6Al4V shell (42 μm and 21 μm). As a result, larger deformations occurred at the PE and ceramic inserts in combination with the PEEK shell. Partially, the deformations were above an usual clearance of 100 μm. When the ceramic insert was combined with the shell made of PEEK, maximum principal stresses in the ceramic insert amounted to 30 MPa and were clearly lower than approved bending strength of the ceramic material (948 MPa). Conclusion. The examined acetabular shell made of PEEK was intensively deformed during insertion compared to the geometrically identical Ti6Al4V shell and is therefore not suitable for modular acetabular cups. In future studies it should be clarified to what extent acetabular cups with shells made of carbon fiber reinforced PEEK materials with higher stiffness lead to reduced deformations during the insertion procedure

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. Methods. The average uniaxial stiffness (350 N/mm), and average dimensions (width × thickness = 14mm × 4mm) of 10 cadaver psoas tendon samples were determined in a separate study. The iliopsoas tendon was modelled as a Yeoh hyper-elastic material, and the material constants were tuned to match the experimental uniaxial test data. Cadaver specific FEA models were created for 5 specimens (10 hips) using computed tomography (CT) scans. The implant components were modeled as being rigid relative to the iliopsoas tendon. The iliopsoas tendon was modelled as extending from its insertion point on the lesser trochanter to the psoas notch on the pelvis for hip flexion angles of −15°, 0°, 15° and 30°. Appropriately sized DM components were implanted virtually for each specimen. Once placed in its proper position, the liner was rotated about the flexion axis until it contacted the stem posteriorly to represent its orientation during hip extension (Fig. 2). A 500N tensile load was applied to the iliopsoas tendon and the average/max stresses within the tendon, and average/max contact pressures between the tendon and liner were measured. Results. At all hip flexion angles from −15° to 30°, the tendon-liner contact pressure and tendon stresses were lower with the ACDM liners compared to the conventional liner. Contact pressure and tendon stress decreased for both liner designs with increasing hip flexion angle. At −15° flexion angle, the average contact pressure was 42.3% lower (0.36Mpa), and the maximum contact pressure was 45.1% (8.5Mpa lower), with the ACDM compared to conventional liner design. Similarly, at −15° flexion angle the average vonMises pressure in the tendon was 32.5% lower (14.8Mpa), and the maximum vonMises stress in the tendon was 55.7% (159Mpa lower) with the ACDM design. (Fig 3). Discussion. This study utilized cadaver specific FEA models to evaluate interaction between the iliopsoas tendon and conventional and ACDM liners during hip extension. The results showed a notable reduction in contact pressure and tendon stress resulting from reduced volume and more soft-tissue friendly profile of the ACDM design. Thus, the ACDM design may be able to reduce undesirable soft-tissue interaction with dual mobility liners

Introduction. Post cam is useful to realize the intrinsic stability of a posterior-stabilized (PS) knee prosthesis replaced for a case with the severe degeneration. Some retrieval studies reveal the ultrahigh molecular weight polyethylene (UHMWPE) deformation or severe failure of the tibial post of PS knee. Strength of the tibial post of available design is obviously insufficient to prevent the severe deformation. The large size post might, however, shorten the range of knee motion. Therefore, minimally required size of the post should be clarified for polyethylene inserts. In the present study, we performed finite element (FE) analysis assumed the mechanical conditions of a tibial post in a PS knee and aimed to design criterion of a post of polyethylene insert of a knee prosthesis. Method. The shape of three commercially available knee prostheses, product A, B, and C was referred as PS knee prosthesis. The contour of the metallic femoral component and the UHMWPE insert were digitized by a computed tomography apparatus. Three dimensional finite elements were generated by modeling software (Simpleware, Ltd. UK) as four-node tetrahedral elements. In FE analysis, we used LS-DYNA ver.971 (Livemore Software Technology Corp. USA) as the software and Endeaver Pro-4500 (EPSON Corp. Japan) as the hardware. These bottoms of the tibial insert were fully constrained. The value of 30MPa was defined as yield stress of UHMWPE. 500N posterior load was applied to each femoral component at 10 degree hyperextension. Then, 1000N anterior load at 120 degree flexion, after tibial insert was located 10 degree internal rotation (Fig. 1). These loads were assumed to realize the two types of tibial post impingement under several kinds of knee motions. The distributed values of von Mises stress and plastic strain on the tibial post were shown as the results of the analysis. Results. At the 10 degree hyperextension, these maximum values of von Mises stress were 24.5, 3.23, 27.09MPa on anterior aspect of tibial post of the product A, B, and C, respectively (Fig. 2). These plastic strains were 0.045, 0.001, 0.064. At the 120 degree flexion, these maximum values of von Mises stress were 33.67, 4.53, 27.03MPa on posterior aspect of the product A, B, and C, respectively (Fig. 3). These plastic strains were 0.28, 0.004, 0.061. The stress of product A was higher than yield stress of UHMWPE. The strain was obviously higher than that of product B and C. Discussion. Our results showed that plastic deformation may occur in the posterior aspect of a tibial post by impingement during common exercises like climbing up, or squatting. In the femoro-tibial articulation, the true-stress decreases with increase in load because the compressive deformation can widen the contact area on the UHMWPE. The true-stress in the tibial post, however, increases with increase in load because bending and tensile deformation reduces the section area. Therefore, the design criterion including the post size must be revised the safety coefficient which realizes that the generated stress in the tibial post is sufficiently lower than the yield stress of UHMWPE

Introduction. The use of open wedge high tibial osteotomy (OWHTO) to reduce knee pain by transferring weight-bearing loads to the relatively unaffected lateral compartment in varus knees and to delay the need for a knee replacement by slowing or stopping destruction of the medial joint compartment. To maintain the stability of OWHTO, the most common type of plate was T-Plate as the locking compression plate (LCP) concept. Anterior portion of T-Plate infringe patient's soft tissue resulted in some complications, whereas anatomical L-plate does not. To evaluate the structural stability of the anatomically contoured L-plate in the present study, the effect of weight bearing after osteotomy should be reviewed in the point of the stress of the plate and screws. We hypothesize that its stress path diverge through collateral portion of tibia and the stress level in screws lowered comparing to the result of T-plate presented in existing literature. Materials and Methods. Based on the postoperative CT data were made from the reconstruction model for finite-element model. The value of Young's modulus and Poisson's ratio were 17,000MPa and 0.36 for cortical bone and 300MPa and 0.3 for cancellous bone. The anatomically contoured L-Plate system, the material of all plate systems were surgical Ti-Alloy were homogeneous and linear properties (Young's modulus = 113,000MPa, Poisson's ratio = 0.33). The screw system were the same as the material properties of the anatomically contoured L-Plate system. For finite element analysis, both the bone and screws were contacted as general condition. And the screws and plate were contacted as tie contact(Figure 1). The load conditions were applied to the top of the tibia based physiological (=1400N) and surgical loads (=200N). In this study, the compressive-bending load was applied to the two nodal points corresponding to the centers of each tibial condyle and divided into 60% and 40% to the medial and lateral sides, respectively. The physiological loads applied in the quadrant section on the proximal tibia.(Chu-An Luo, 2013). Results. Forlocking screw, it was calculated as 134%, 88.2%, 92.7%, 97.3%, 100%, 88.7%, 80.8%, 112.4% compared to cortical screw (No.05) in each of the portions (No.01∼08) (Figure 2). The maximum stress of the lateral cortex's stress concentration was shown as the anatomically contoured L-Plate 130MPa, it was lower than that reported in the literature T-plate 171MPa. Maximum stress in structurally weak the screw neck was shown in the anatomically contoured L-Plate with 198MPa, T-Plate 200 MPa was similar to that reported in the literature. Conclusion. The stress path of postoperative bone was found to be formed from at collateral direction in the study. The anatomically contoured L-Plate received a load similar to the T-Plate in the lateral cortex that reported in the literature. But it was confirmed that the maximum stress is more structurally stable enough to appear on the contrast of 76% T-Plate. Although T-Plate is structurally simple with a short moment arm, the anatomically contoured L-Plate is advantage to stress dispersion and more stable than T-Plate. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Unicompartmental knee arthroplasty (UKA) is often considered to be attractive alternate surgical technique to total knee arthroplasty (TKA) and high tibial osteotomy (HTO), in particular young patients. In addition, it is recently reported that preservation of joint line in UKA is crucial factor for positive long-term outcome, especially in revision case for UKA. However, the role of this joint line has neither been invested nor is it consciously bothered during surgical implantation. Validated finite element (FE) analysis was introduced in this study to investigate the effects of maximum contact stress on polyethylene (PE) insert and maximum compressive stress in opposite compartments for joint line in fixed-type UKA. As suggested by Weber et al., FE model for joint line was developed by means of determination of the angle between the pre-operative joint line and the reference line from lateral cortical is of the femur. Based on the method above, joint lines were modeled in −3, −2, −1, 0, +1, +2, and +3 mm cases and these seven FE models were compared and analyzed (Fig. 1). All implant components were modeled as linear elastic isotropic materials. However, the model was considered to have plastic characteristics of PE insert. FE analysis was performed using high kinematics displacement and rotation inputs, which were based on the kinematics of the natural knee. ISO standards were used for axial load and flexion (Fig. 2). The FE model was subjected to validation based on cadaveric experimental data available in the literature by Sohn et al. and from previous cadaveric tests conducted by current investigators. The maximum contact stress was found at around 43 % of the gait cycle in 0 mm case. There were no difference between ± 1 and 0 mm cases, but maximum contact stress on PE insert becomes greater in ± 3 mm cases. The maximum compressive stress of the lateral meniscus in 0 mm case occurred at 62 % of the gait cycle. There were no difference in positive joint line cases in maximum compressive stress, however maximum compressive stress of the lateral meniscus becomes greater in - 3 mm cases. This study emphasized the importance of joint line preservation after implantation of UKA. It would be critical to determine the joint line in UKA surgery in future based on the result showing that there has been no remarkable difference in stress but changed rapidly from the position beyond the joint line. In future study, it would be valuable study to compare between joint lines of fixed- and mobile-type UKA

INTRODUCTION

Component positioning of an artificial hip joint plays a key role in durability of implant. Despite the fact that a number of numerical, experimental and clinical studies have been carried out to investigate the effects of cup inclination on polyethylene wear, steep inclination has been reported to be associated with both high and low volumetric wear. Moreover, how cup anteversion affects wear and its interaction with inclination are still unclear. To address these knowledge gaps, in this study wear and contact mechanics of a hip joint under various cup positioning has been investigated by using FEA (Finite Element Analysis).

INTRODUCTION

Patellofemoral joint (PFJ) replacement is a successful treatment option for isolated patellofemoral osteoarthritis. With this approach only the involved joint compartment is replaced and the femoro-tibial joint remains intact. Minimizing periprosthetic bone loss, which may occur due to the stress shielding effect of the femoral component, is important to insure long-term outcomes. The objective of this study was to investigate, using finite element analyses, the effects of patellofemoral replacement on the expected stress distribution of the distal femur eventually leading to changes in bone density.

Long-term biological fixation and stability of uncemented acetabular implant are influenced by peri-prosthetic bone ingrowth which is known to follow the principle of mechanoregulatory tissue differentiation algorithm. A tissue differentiation is a complex set of cellular events which are largely influenced by various mechanical stimuli. Over the last decade, a number of cell-phenotype specific algorithms have been developed in order to simulate these complex cellular events during bone ingrowth. Higher bone ingrowth results in better implant fixation. It is hypothesized that these cellular events might influence the peri-prosthetic bone ingrowth and thereby implant fixation. Using a three-dimensional (3D) microscale FE model representing an implant-bone interface and a cell-phenotype specific algorithm, the objective of the study is to evaluate the influences of various cellular activities on peri-prosthetic tissue differentiation. Consequently the study aims at identifying those cellular activities that may enhance implant fixation.

The 3D microscale implant-bone interface model, comprising of Porocast Bead of BHR implant, granulation tissue and bone, was developed and meshed in ANSYS (Fig. 1b). Frictional contact (µ=0.5) was simulated at all interfaces. The displacement fields were transferred and prescribed at the top and bottom boundaries of the microscale model from a previously investigated macroscale implanted pelvis model (Fig. 1a) [4]. Periodic boundary conditions were imposed on the lateral surfaces. Linear elastic, isotropic material properties were assumed for all materials. Young's modulus and Poisson's ratios of bone and implant were mapped from the macroscale implanted pelvis [4]. A cell-phenotype specific mechanoregulatory algorithm was developed where various cellular activities and tissue formation were modeled with seven coupled differential equations [1, 2]. In order to evaluate the influence of various cellular activities, a Plackett-Burman DOE scheme was adopted. In the present study each of the cellular activity was assumed to be an independent factor. A total of 20 independent two-level factors were considered in this study which resulted in altogether 24 different combinations to be investigated. All these cellular activities were in turn assumed to be regulated by local mechanical stimulus [3]. The mechano-biological simulation was run until a convergence in tissue formation was attained.

The cell-phenotype specific algorithm predicted a progressive transformation of granulation tissue into bone, cartilage and fibrous tissue (Fig. 1c). Various cellular activities were found to influence the time to reach equilibrium in tissue differentiation and, thereby, attainment of sufficient implant fixation (Fig. 2, Table 1). Negative regression coefficients were predicted for the significant factors, differentiation rate of MSCs and bone matrix formation rate, indicating that these cellular activities favor peri-prosthetic bone ingrowth by facilitating rapid peri-prosthetic bone ingrowth. Osteoblast differentiation rate, on the contrary, was found to have the highest positive regression coefficient among the other cellular activities, indicating that an increase in this cellular activity delays the attainment of equilibrium in bone ingrowth prohibiting rapid implant fixation.

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Introduction

Glenoid loosening, still a main complication in shoulder arthroplasty, could be related to glenohumeral orientation and conformity, cementing techniques, fixation design and periprosthetic bone quality [1,2]. While past numerical analyses were conducted to understand the relative role of these factors, so far none used realistic representations of bone microstructure, which has an impact on structural bone properties [3]. This study aims at using refined microFE models including accurate cortical bone geometry and internal porosity, to evaluate the effects of fixation design, glenohumeral conformity, and bone quality on internal bone tissue and cement stresses under physiological and pathological loads.

Introduction

Revision total knee arthroplasty (TKA) has been often used with a metal block augmentation for patients with poor bone quality. However, bone defects are frequently detected in revision TKA used with metal block augmentation. This study focused on identification of a potential possibility of the bone defect occurrence through the evaluation of the strain distribution on the cortical bone of the tibia implanted revision TKA with metal block augmentation, during high deep flexion.