When analyzing complex biomechanical problems such as predicting the effects of orthopedic surgery, subject-specific musculoskeletal models are essential to achieve reliable predictions. The aim of this paper is to present the Twente Lower Extremity Model 2.0, a new comprehensive dataset of the musculoskeletal geometry of the lower extremity, which is based on medical imaging data and dissection performed on the right lower extremity of a fresh male cadaver. Bone, muscle and subcutaneous fat (including skin) volumes were segmented from computed tomography and magnetic resonance images scans. Inertial parameters were estimated from the image-based segmented volumes. A complete cadaver dissection was performed, in which bony landmarks, attachments sites and lines-of-action of 55 muscle actuators and 12 ligaments, bony wrapping surfaces, and joint geometry were measured. The obtained musculoskeletal geometry dataset was finally implemented in the AnyBody Modeling System (AnyBody Technology A/S, Aalborg, Denmark), resulting in a model consisting of 12 segments, 11 joints and 21 degrees of freedom, and including 166 muscle-tendon elements for each leg. The new TLEM 2.0 dataset was purposely built to be easily combined with novel image-based scaling techniques, such as bone surface morphing, muscle volume registration and muscle-tendon path identification, in order to obtain subject-specific musculoskeletal models in a quick and accurate way. The complete dataset, including CT and MRI scans and segmented volume and surfaces, is made available at http://www.utwente.nl/ctw/bw/research/projects/TLEMsafe for the biomechanical community, in order to accelerate the development and adoption of subject-specific models on large scale. TLEM 2.0 is freely shared for non-commercial use only, under acceptance of the TLEMsafe Research License Agreement.

Despite the widespread use of cement as a means of fixation of implants to bone, surprisingly little is known about the micromechanical behavior in terms of the local interfacial motion. In this work, we utilized digital image correlation techniques to quantify the micromechanics of the cement-bone interface of laboratory-prepared cemented total hip replacements subjected to nondestructive, quasistatic tensile and compressive loading. Upon loading, the majority of the displacement response localized at the contact interface region between cement and bone. The contact interface was more compliant ( p ¼ 0.0001) in tension (0.0067 AE 0.0039 mm/MPa) than compression (0.0051 AE 0.0031 mm/MPa), and substantial hysteresis occurred due to sliding contact between cement and bone. The tensile strength of the cement-bone interface was inversely proportional to the compliance of the interface and proportional to the cement/bone contact area. When loaded beyond the ultimate strength, the strain localization process continued at the contact interface between cement and bone with microcracking (damage) to both. More overall damage occurred to the cement than to the bone. The opening and closing at the contact interface from loading could serve as a conduit for submicron size particles. In addition, the cement mantle is not mechanically supported by surrounding bone as optimally as is commonly assumed. Both effects may influence the longevity of the reconstruction and could be considered in preclinical tests. ß

Patients suffering from rheumatoid arthritis typically have a poor subchondral bone quality, endangering implant fixation. Using finite element analysis (FEA) an investigation was made to find whether a press-fit acetabular implant with a polar clearance would reduce interfacial micromotions and improve fixation compared with a standard hemispherical design. In addition, the effects of interference fit, friction, and implant material were analysed. Cups were introduced into an FEA model of a human pelvis with simulated subchondral bone plasticity. The models were loaded with a loading configuration simulating two cycles of normal walking, during which contact stresses and interfacial micromotions were monitored. Subsequently, a lever-out simulation was performed to assess the fixation strength of the various cases. A flattened cup with good bone quality produced the lowest interfacial micromotions. Poor bone decreased the fixation strength regardless of the geometry of the cup. Increasing the interference fit of the flattened cup compensated for the loss of fixation strength caused by poor bone quality. In conclusion, a flattened cup did not significantly improve implant fixation over a hemispherical cup in the case of poor bone quality. However, implant fixation can be optimized by increasing interference fit and avoiding inferior frictional properties and low-stiffness implants.

The mechanical effects of varying the depth of cement penetration in the cement-bone interface was investigated using finite element analysis (FEA) and validated using companion experimental data. Two FEA models of the cement-bone interface were created from microcomputed tomography data and the penetration of cement into the bone was varied over six levels each. The FEA models, consisting of the interdigitated cement-bone constructs with friction between cement and bone, were loaded to failure in tension and in shear. The cement and bone elements had provision for crack formation due to excessive stress. The interfacial strength showed a strong relationship with the average interdigitation (r 2 =0.97 and r 2 =0.93 in tension and shear, respectively). Also, the interface strength was strongly related with the contact area (r 2 =0.98 and r 2 =0.95 in tension and shear, respectively). The FEA results compared favorably to the stiffness-strength relationships determined experimentally. Overall, the cement-bone interface was 2.5 times stronger in shear than in tension and 1.15 times stiffer in tension than in shear, independent of the average interdigitation. More cracks occurred in the cement than in the bone, independent of the average interdigitation, consistent with the experimental results. In addition, more cracks were generated in shear than in tension. In conclusion, achieving and maintaining maximal infiltration of cement into the bone to obtain large interdigitation and contact area is key to optimizing the interfacial strength.

In order to gain insight into the micro-mechanical behavior of the cement-bone interface, the effect of parametric variations of frictional, morphological and material properties on the mechanical response of the cement-bone interface were analyzed using a finite element approach. Finite element models of a cement-bone interface specimen were created from micro-computed tomography data of a physical specimen that was sectioned from an in vitro cemented total hip arthroplasty. In five models the friction coefficient was varied (μ= 0.0; 0.3; 0.7; 1.0 and 3.0), while in one model an ideally bonded interface was assumed. In two models cement interface gaps and an optimal cement penetration were simulated. Finally, the effect of bone cement stiffness variations was simulated (2.0 and 2.5 GPa, relative to the default 3.0 GPa). All models were loaded for a cycle of fully reversible tension-compression. From the simulated stress-displacement curves the interface deformation, stiffness and hysteresis were calculated. The results indicate that in the current model the mechanical properties of the cement-bone interface were caused by frictional phenomena at the shape-closed interlock rather than by adhesive properties of the cement. Our findings furthermore show that in our model maximizing cement penetration improved the micromechanical response of the cement-bone interface stiffness, while interface gaps had a detrimental effect. Relative to the frictional and morphological variations, variations in the cement stiffness had only a modest effect on the micromechanical behavior of the cement-bone interface. The current study provides information that may help to better understand the load transfer mechanisms taking place at the cement-bone interface.

High-flexion knee replacements have been developed to accommodate a large range of motion (RoM > 1208). Knee implants that allow for higher flexion may be more sensitive to femoral loosening as the knee load is relatively high during deep knee flexion, which could result in an increased failure potential at the implant-cement interface of the femoral component. A 3D finite element knee model was developed including a posterior-stabilized high-flexion knee replacement to analyze the stress state at the femoral implant-cement interface during a full squatting movement (RoM 1558). During deep flexion (RoM > 1208), tensile and shear stress concentrations were found at the implant-cement interface beneath the proximal part of the anterior flange. Particularly, the shear stresses at this interface location increased during high flexion, from a peak stress of 4.03 MPa at 908 to 6.89 MPa at 1408 of flexion. Tensile stresses were substantially lower, having a peak stress of 0.72 MPa at 1008 of flexion. Using data from earlier interface strength experiments, none of the interface beneath the anterior flange was predicted to fail in the normal flexion range (RoM 1208), whereas the prediction increased to 2.2% of the interface during deeper knee flexion. Thigh-calf contact reduced the knee forces, interface load, and failure risk beyond 140-1458 of flexion. Based on the more critical stresses at the femoral fixation site between 1208 and 1458 of flexion, we conclude that the femoral component has a higher risk of loosening at high-flexion angles. Keywords: total knee arthroplasty; high flexion; femoral loosening; finite element analysis; implant-cement interfaceThe traditional goals of total knee arthroplasty (TKA) are pain relief and restoration of normal knee function. Several clinical studies demonstrated that TKA patients receiving a standard knee replacement in general achieve maximal flexion angles limited to roughly 1208 of flexion.1 Hence, active knee patients experience limitations during activities such as squatting and kneeling.2 High-flexion TKAs have been developed to facilitate a larger post-operative range of motion (RoM > 1208). High-flexion implants are mostly based on successful standard designs with the posterior condylar geometry adapted to accommodate increased joint load occurring during deep knee flexion.In a recent follow-up study, Han et al. 3 reported a disturbingly high incidence of early femoral loosening for high-flexion TKA. They observed aseptic femoral loosening in 38% of the operated cases at a mean follow-up of 23 months. Furthermore, the occurrence of loosening was closely related to the maximal flexion angle achieved after TKA. In nearly all cases of loosening, the femoral implant-cement interface debonded, particularly beneath the anterior flange, with radiolucent lines visible on lateral radiographs. Due to this debonding process, the femoral component migrated into a position of increased flexion during deep knee bends. A similar mode of failure was earlier described by King ...

Recently, experiments were performed to determine the micromechanical behavior of the cement-bone interface under tensioncompression loading conditions. These experiments were simulated using finite element analysis (FEA) to test whether the micromechanical response of the interface could be captured in micromodels. Models were created of experimental specimens based upon microcomputed tomography data, including the complex interdigitated bone-cement morphology and simulated frictional contact at the interface. The models were subjected to a fully reversed tension-compression load, mimicking the experimental protocol. Similar to what was found experimentally, the simulated interface was stiffer in compression than in tension, and the majority of displacement was localized to the cement-bone interface. A weak correlation was found between the FEA-predicted stiffness and the stiffness found experimentally, with average errors of 8 and 30% in tension and compression, respectively. The hysteresis behavior found experimentally was partially reproduced in the simulation by including friction at the cement-bone interface. Furthermore, stress analysis suggested that cement was more at risk of fatigue failure than bone, concurring with the experimental observation that more cracks were formed in the cement than in the bone. The current study provides information that may help explain the load transfer mechanisms taking place at the cement-bone interface. The fidelity of FEA studies depends on the accuracy and completeness of the experimental and clinical data used as input for the models. Data are available on the mechanical properties and behavior of implants, bone cement, 10,11 the implant-cement interface, 12,13 and bone.14-16 Research on the cement-bone interface, however, has mainly focused on interface strength, [17][18][19][20][21][22][23] while relatively little is known about the multifaceted micromechanical behavior of the interface.The cement-bone interface consists of complex structures of cement penetrating into the cortical bone structure and filling up intertrabecular marrow spaces, thereby creating a highly variable interlock between bulk cement and bone. The cement-bone interface provides the fixation of the cement mantle in the femur. Hence, the stability of the cement mantle and the implant is directly dependent on the mechanical behavior of the cement-bone interface.Recently, experiments were performed to determine the micromechanical behavior of the cement-bone interface. 24 Small laboratory specimens containing interdigitated cement-bone interfaces were loaded in fully reversed tension-compression, while the local micromotion of the cement, bone, and the cement-bone interface was monitored. The majority of the displacement response was localized at the interface, which had a lower stiffness than that of adjacent bone and cement. The interface was more compliant in tension than in compression, and substantial hysteresis occurred. Upon failure, more cracks were found in the cement than in th...

Previously, we showed that case-specific non-linear finite element (FE) models are better at predicting the load to failure of metastatic femora than experienced clinicians. In this study we improved our FE modelling and increased the number of femora and characteristics of the lesions. We retested the robustness of the FE predictions and assessed why clinicians have difficulty in estimating the load to failure of metastatic femora. A total of 20 femora with and without artificial metastases were mechanically loaded until failure. These experiments were simulated using case-specific FE models. Six clinicians ranked the femora on load to failure and reported their ranking strategies. The experimental load to failure for intact and metastatic femora was well predicted by the FE models (R(2) = 0.90 and R(2) = 0.93, respectively). Ranking metastatic femora on load to failure was well performed by the FE models (τ = 0.87), but not by the clinicians (0.11 < τ < 0.42). Both the FE models and the clinicians allowed for the characteristics of the lesions, but only the FE models incorporated the initial bone strength, which is essential for accurately predicting the risk of fracture. Accurate prediction of the risk of fracture should be made possible for clinicians by further developing FE models.