Common post-operative problems in shoulder arthroplasty such as glenoid loosening and joint instability can be reduced by improvements in glenoid design shape, material choice and fixation method [1]. Innovation in shoulder replacement is usually carried out by introducing incremental changes to functioning implants [2], possibly overlooking other successful design combinations. We propose an automated framework for parametric analysis of implant design in order to efficiently assess different possible glenoid configurations. Parametric variations of reference geometries of a glenoid implant were automatically generated in SolidWorks. The different implants were aligned and implanted with repeatability using Rhino. The glenoid-bone models were meshed in Abaqus, and boundary conditions and loading applied via a custom-made Python script. Finally, another MATLAB script integrated and automated the different steps, extracted and analysed the results. This study compared the influence of reference shape (keel vs. 2-pegged) and material on the von Mises stresses and tensile and compressive strains of glenoid components with bearing surface thickness and fixation feature width of 3, 4, 5 or 6 mm. A total of 96 different glenoid geometries were implanted into a bone cube (E = 300 MPa, ν = 0.3). Fixed boundary conditions were applied at the distal surface of the cube and a contact force of 1000 N was distributed between the central nodes on the bearing surface. The implants were assigned UHMWPE (E = 1 GPa, ν = 0.46), Vitamin E PE (E = 800 MPa, ν = 0.46), CFR-PEEK (E = 18 GPa, ν = 0.41) or PCU (E = 2 GPa, ν = 0.38) material properties and the bone-implant surface was tied (Figure 1). The von Mises stresses, compressive and tensile strains for the different models were extracted. The influence of design parameters in the mechanical environment of the implant could be assessed. In this particular example, the 95. th. percentile values of the tensile and compressive strains induced by modifications in reference shape could be evaluated for all the different geometries simultaneously in form of radar plots. 2-pegged geometries (green) consistently produced lower tensile and compressive strains than the keeled (blue) configurations (Figure 2). Vitamin E PE and PCU glenoids also produced lower maximum von Mises stresses values than CFR-PEEK and UHMWPE designs (Figure 3). The developed method allows for simple, direct, rapid and repeatable comparison of different design features, material choices or fixation methods by analysing how they influence the mechanical environment of the bone surrounding the implant. Such tool can provide invaluable insight in implant design optimisation by screening through multiple potential design modifications at an early design evaluation stage and highlighting the best performing combinations. Future work will introduce physiological bone geometries and loading, a wider variety of reference geometries and fixation features, and look at bone/interface strength and osteointegration predictions

The Lubinus SP2 femoral stem has a 10 year survivorship of 96%. Curiosity lies in that force-closed stem designs such as the Exeter appear to be more superior to that of the composite-beam like the Lubinus which performs best compared with all other stem types. Biomechanical comparisons of the stress distributions between native and implanted human femora with a cemented Lubinus stem simulating an everyday clinical activity were made. Rosette strain gauges were placed onto fourth generation composite cortical sawbone femora and placed within a hemipelvis rig simulating the dynamic position of the femur during single-legged stance. The femora were then implanted with the Lubinus and principal strain measurements calculated for both intact and implanted femora. These values correlate directly with stress. Statistical calculations were carried out including a two-way ANOVA and Student's unpaired t-test so as to ascertain any relationship between the intact and implanted femora strain values. There were significant decreases (p<0.05) in principal tensile and principal compressive strains upon implantation in the proximal and distal areas of the femur. However, there were insignificant changes (p>0.05) in principal tensile strains at the mid-stem and insignificant changes (p>0.05) in principal compressive strains at both the mid-stem and distal areas. This is the largest biomechanical study to be carried out on this stem and the first in the English language. Changes in principal stresses were not significant in all aspects of the femur upon implantation which appears to give some biomechanical explanation to its clinical success

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

The meniscus is comprised largely of type I collagen, as well as fibrochondrocytes and proteoglycans. In articular cartilage and intervertebral disc, proteoglycans make a significant contribution to mechanical stiffness of the tissue via negatively charged moieties which generate Donnan osmotic pressures. To date, such a role for proteoglycans in meniscal tissue has not been established. This study aimed to investigate whether meniscal proteoglycans contribute to mechanical stiffness of the tissue via electrostatic effects. Following local University Ethics Committee approval, discs of meniscal tissue two millimetres thick and of five millimetres diameter were obtained from 12 paired fresh frozen human menisci, from donors < 6 5 years of age, with no history of osteoarthritis or meniscal injury. Samples were taken from anterior, middle and posterior meniscal regions. Each disc was placed within a custom confined compression chamber, permeable at the top and bottom only and then bathed in one of three solutions − 0.14M PBS (mimics cellular environment), deionised water (negates effect of mobile ions) or 3M PBS (negates all ionic effects). The apparatus was mounted within a Bose Electroforce 3100 materials testing machine and a 0.3N preload was applied. The sample was allowed to reach equilibrium, before being subjected to a 10% ramp compressive strain followed by a 7200 second hold phase. Equal numbers of samples from each meniscus and meniscal region were tested in each solution. Resultant stress relaxation curves were fitted to a nonlinear poroviscoelastic model with strain dependent permeability using FEBio finite element modelling software. Goodness of fit (R2) was assessed using a coefficient of determination. All samples were assayed for proteoglycan content. Comparison of resultant mechanical parameters was undertaken using multivariate ANOVA with Bonferroni adjustment for multiple comparisons. 36 samples were tested. A significant difference (p < 0 .05) was observed in the value of the Young's modulus (E) between samples tested in deionised water compared to 0.14M/3M PBS, with the meniscus found to be stiffest in deionised water (E = 1.15 MPa) and least stiff in 3M PBS (E = 0.43 MPa), with the value of E in 0.14M PBS falling in between (0.68 MPa). No differences were observed in the zero strain permeability or the exponential strain dependent/stiffening coefficients. The viscoelastic coefficient and relaxation time values were not found to improve model fit and were thus held at zero. The mean R2 value was 0.78, indicating a good fit and did not differ significantly between solutions. Proteoglycan content was not found to differ with solution, but was found to be significantly increased in the middle region of both menisci. Proteoglycans make a significant electrostatic contribution to mechanical stiffness of the meniscus, increasing it by 58% in the physiological condition, and are hence integral to its function. It is important to include the influence of ionic effects when modelling meniscus, particularly where fluid flow or localised strain is modelled. From a clinical perspective, it is critical that meniscal regeneration strategies such as scaffolds or allografts attempt to preserve, or compensate for, the function of proteoglycans to ensure normal meniscal function

Strain is a robust indicator of bone failure initiation. Previous work has demonstrated the measurement of vertebral trabecular bone strain by Digital Volume Correlation (DVC) of µCT scan in both a loaded and an unloaded configuration. This project aims to improve previous strain measurement methods relying on image registration, improving resolution to resolve trabecula level strain and to improve accuracy by applying feature based registration algorithms to µCT images of vertebral trabecular bone to quantify strain. It is hypothesised that extracting reliable corresponding feature points from loaded and unloaded µCT scans can be used to produce higher resolution strain fields compared to DVC techniques. The feature based strain calculation algorithm has two steps: 1) a displacement field is calculated by finding corresponding feature points identified in both the loaded and unloaded µCT scans 2) strain fields are calculated from the displacement fields. Two methods of feature point extraction, Scale Invariant Feature Transform (SIFT) and Skeletonisation, were applied to unloaded (fixed) and loaded (moving) µCT images of a rat tail vertebra. Spatially non-uniform displacement fields were generated by automatically matching corresponding feature points in the unloaded and loaded scans. The Thin Plate Spline method and a Moving Least Squares Meshless Method were both tested for calculating strain from the displacement fields. Verification of the algorithms was performed by testing against known artificial strain/displacement fields. A uniform and a linearly varying 2% compressive strain field were applied separately to an unloaded 2D sagittal µCT slice to simulate the moving image. SIFT was unable to reliably match identified feature points leading to large errors in displacement. Skeletonisation generated a more accurate and precise displacement field. TPS was not tolerant to small displacement field errors, which resulted in inaccurate strain fields. The Meshless Methods proved much more resilient to displacement field errors. The combination of Skeletonisation with the Meshless Method resulted in best performance with an accuracy of −405µstrain and a detection limit of 1210µstrain at a strain resolution of 221.5µm. The DVC algorithm verified using the same validation test yielded a similar detection limit (1190µstrain), but with a lower accuracy for the same test (2370µstrain) for a lower resolution strain field (770µm) (Hardisty, 2009). The Skeletonisation algorithm combined with the Meshless Method calculated strain at a higher resolution, but with a similar detection limit, to that of traditional DVC methods. Future improvements to this method include the implementation of subpixel feature point identification and adapting this method of strain measurement into a 3D domain. Ultimately, a hybrid DVC/feature registration algorithm may further improve the ability to measure trabecular bone strain using µCT based image registration

Introduction. Modern hip replacements all have encapsulated the design concept of proximal modularity. The factors contributing to the increased wear and corrosion at the taper junction are trunnion geometry, surface characteristics, head size, impaction forces, and material coupling. This study maps the inferior and superior region of the trunnion and bore to provide a visual identification of the corrosion severity. The corrosion/wear generated inferiorly and superiorly at the bore and trunnion will be quantified to understand how corrosion is affected by mechanical stresses in relation to anatomical orientation. Methodology. Three neck tapers generated from bar stock containing a threaded trunnion Ti-6Al-4V and 3× 32mm femoral heads (Co-Cr-Mo) with a +4 offset manufactured by Signature Orthopaedics were used within this study. Rectangular Rozzette strain gauges (Tokyo Sokki Kenkyujo Co., Ltd.) were adhered onto the inferior and superior sections of the neck section. The tapers were fatigued in accordance to ISO 7206 at 5Hz for 5 million cycles at 37 degrees Celsius in phosphate buffered saline. The tapers were sectioned from the center of the femoral head to split both trunnion and bore into superior and inferior components. SEM imaging of all surface areas for each component, per taper (4) was done under ×100 magnification. The images were used to quantify the corrosion present across the surface area using a MATLAB based program called Histomorph. To obtain a visual observation of the variation of corrosion across the bore and trunnion the proximal, medial, and distal regions were mapped together for both the superior and inferior sections. Results. The superior region of the trunnion had a dominant tensile strain in comparison to the inferior region, which had a dominant compressive strain. Corrosion/wear of the inferior section of the trunnion was significantly higher (p<0.05) in comparison to the superior section (Figure 1). The bore had more corrosion/wear on the superior side in comparison to the inferior side however the difference was not significant. The mapping of the trunnion shows corrosion/wear along the whole length of the inferior side and dominantly at the distal region for the superior side (Figure 2 & 3). The superior section of the trunnion had higher corrosion/wear damage across the center and distal regions of the trunnion. The subdivision of the superior section reveals that the majority of the distal section contains higher wear/corrosion damage. However the central region also has sufficient corrosion/wear extending across the width of the bore. Conclusion. The corroded regions have shown that the type of stress present on the regions of the taper junction determines the severity of corrosion. The inferior section of the trunnion under compressive stress has significantly (p<0.05) higher corrosion/wear in comparison to the superior section dominated by tensile stress

The contact condition in the human knee joint must play important roles especially in dynamic loading situations where the loads transfer in the knee. In this study, the impact stress propagations through the inside of the knee joint were simulated using the three-dimensional finite element analysis (FEA). And the differences in the stress distribution were investigated between the intact knee and the total replacement condition. The finite element (FE) models of an intact human knee joint and a total replaced knee joint were constructed with high shape fidelity. The intact model included the cortical bone, cancellous bone, articular cartilage, bone marrow, and meniscus. And the total replacement knee FE model, which is consisted of the artificial femoral and tibial components were also prepared to compare the impact propagations with the intact model (Figure 1). Impact load were applied to the proximal femur of the FE models under the same conditions as those of the weight-drop experiments with the knee joint specimens. The FEA results showed that the impact stress propagated to the tibia through the knee joint for several milliseconds. The values and the time dependent change of the compressive strain on the cortical surface had good agreement with the experimental results. The compressive stress mainly propageted at the medial side, with 1.0 MPa at 1.2 milliseconds. Especially, the impact stress propagated not only in the cortical surface area which has hard material property but also in the soft cancellous bone region inside the knee joint. The mass density of the cancellous bone has similar to that of the cortical bone, and thus the role of the load bearing in the cancellous area must be much increasing under the impact condition. In the total replacement model, concentration of the impact compressive stress was observed with 2.8 MPa at the tibial region, while not under the normal intact conditions (Figure 2). Since the total replacement model is formed of different materials and the impact propagations were inhibited by the interfacial condition, such as sliding or debonding, it is considered that the contact condition between such materials have a great effect on the stress propagation

Introduction. Periprosthetic bone remodelling after Total Knee Arthroplasty (TKA) may be attributed to local changes in the mechanical strain field of the bone as a result of the stiffness mismatch between high modulus metallic implant materials and the supporting bone. This can lead to significant loss of periprosthetic bone density, which may promote implant loosening, and complicate revision surgery. A novel polyetheretherketone (PEEK) implant with a modulus similar to bone has the potential to reduce stress shielding whilst eliminating metal ion release. Numerical modelling can estimate the remodelling stimulus but rigorous validation is required for use as a predictive tool. In this study, a finite element (FE) model investigating the local biomechanical changes with different TKA materials was verified experimentally using Digital Image Correlation (DIC). DIC is increasingly used in biomechanics for strain measurement on complex, heterogeneous anisotropic material structures. Methodology. DIC was used following a previously validated technique [1] to compare bone surface strain distribution after implantation with a novel PEEK implant, to that induced by a contemporary metallic implant. Two distal Sawbone® femora models were implanted with a cemented cobalt-chromium (CoCr) and PEEK-OPTIMA® femoral component of the same size and geometry. A third, unimplanted, intact model was used as a reference. All models were subjected to standing loads on the corresponding UHMWPE tibial component, and resultant strain data was acquired in six repeated tests. An FE model of each case, using a CT-derived bone model, was solved using ANSYS software. Results and Discussion. The sensitivity of DIC strain measurements was <+130με and experimental error was +230με, or 8.5% of the peak magnitude in the region of interest. High bone strain adjacent to the CoCr implant and low bone strain in the central metaphyseal region compared to the intact case (Fig.1) indicated that stress shielding may lead to resorption, a theory corroborated by bone density scans of implanted metallic TKRs [2]. Quantitatively, wider scatter and greater deviation was observed between the intact-vs-CoCr datasets (R. 2. : 0.425, slope = 0.508). A closer agreement was shown between the intact-vs-PEEK datasets (R. 2. : 0.771, slope = 1.270) (Fig.2). These strain distributions corroborated the predictions of the FE analysis (Fig.1). High bone strain in regions close to the CoCr implant can be attributed to the high stiffness mismatch between implant and bone, where the bone is constrained to the implant with cement. High strain gradients near the stiff CoCr could potentially compromise implant fixation, leading to loosening. The compressive strains in the PEEK implanted model were similar to those in the intact case, suggesting that bone would be maintained in these regions, and high strain gradients were not observed. Conclusion. Digital image correlation and FE analysis have been successfully employed for evaluation of a novel PEEK-OPTIMA® TKA implant in comparison to a metallic implant. The polymeric implant produced a strain distribution closer to that of the intact bone, and therefore would be expected to have less of a stress shielding effect, improving long term bone preservation

BACKGROUND. In vitro tests have shown that when a force is applied to the proximal femur within the range of directions spanned during physiological activities, the direction of principal strain vary by a very narrow angle (Cristofolini et al, 2009, J. Engng. Med.). This shows that the anatomy and the distribution of inhomogeneous and anisotropic material properties of the bone tissue make the structure of the proximal femur optimized to withstand a wide range of loading directions. The increasing use of hip resurfacing is associated with early neck fractures of the implanted femur. The aim of this study was to elucidate if such fractures could be caused by a non-physiological state of stress/strain post-implantation. While the possible role of notching at the neck-implant interface has already been elucidated, it is not know whether a resurfacing implant could make the principal strain vary in magnitude and direction in a way that could compromise integrity of the proximal femur. METHODS. The aim of this study was to measure if the direction of the principal strain in the proximal femur was affected by the presence of a resurfacing prosthesis. Seven human cadaver femurs were instrumented with 12 triaxial strain gauges to measure the magnitude and alignment of principal strains in the head-neck region. Each femur was implanted with a typical resurfacing prosthesis (BHR). All femurs were tested in vitro before and after implantation with a range of loading conditions to explore the range of loading directions during daily activity (Fig. 1). FINDINGS. Comparison of the strain distribution before and after implantation showed that: . In the natural conditions the principal tensile strain was significantly larger where the cortical bone was thinner; the compressive strain was larger where the cortical bone was thicker. This should be considered when designing a resurfacing prosthesis. The strain magnitude varied greatly between loading configurations both in the intact and implanted condition: this suggests that different loading configurations must be simulated for the preclinical validation of a resurfacing prosthesis. In the natural conditions, the direction of the principal strain varied significantly between measurement locations, but varied little between loading configurations (less than 10° when the hip force spanned a 21° cone, Fig. 2). This confirms that the anatomy and the distribution of anisotropic material properties enable the proximal femur to respond adequately to the changing direction of daily loading. In the resurfaced femurs, when the force spanned the same 21° cone, the direction of principal strain at each measurement location varied by less than 10° (Fig. 3), similar to the natural condition. In the resurfaced femurs, the direction of principal strain lied within less than 10° from the direction in the natural conditions. INTERPRETATION. Our results show that resurfacing does not disturb the alignment of principal strain in the proximal femur. In other words, the most critical directions of stress/strain after implantation stay aligned with the same direction as in the intact femur, which is the direction for which the inhomogeneous and anisotropic structure of the proximal femur is optimized