Computational investigation of the time-dependent contact behaviour of the human tibiofemoral joint under body weight

Computational modelling of articular joints with biphasic cartilage: recent advances, challenges and opportunities

Biphasic models have been widely used to simulate the time-dependent biomechanical response of soft tissues. Modelling techniques of joints with biphasic weight-bearing soft tissues have been markedly improved over the last decade, enhancing our understanding of the function, degenerative mechanism and outcomes of interventions of joints. This paper reviews the recent advances, challenges and opportunities in computational models of joints with biphasic weight-bearing soft tissues. The review begins with an introduction of the function and degeneration of joints from a biomechanical aspect. Different constitutive models of articular cartilage, in particular biphasic materials, are illustrated in the context of the study of contact mechanics in joints. Approaches, advances and major findings of biphasic models of the hip and knee are presented, followed by a discussion of the challenges awaiting to be addressed, including the convergence issue, high computational cost and inadequate validation. Finally, opportunities and clinical insights in the areas of subject-specific modeling and tissue engineering are provided and discussed.

The effect of different posterior inclinations of tibial component on tibiofemoral contact pressures after unicompartmental knee arthroplasty

BACKGROUND

Different posterior inclinations of tibial component after unicompartmental knee arthroplasty (UKA) may lead to different biomechanical characteristics of the knee joint. This finite element study was designed to investigate the tibiofemoral contact pressures after UKA with different posterior inclinations of tibial component.

METHODS

Finite element model of a healthy knee joint was constructed, and mobile-bearing (MB) UKA models with 5 different posterior inclinations (3°, 5°, 7°, 9° and 11°) of tibial components were simulated. The maximum contact pressures of tibial plateau cartilage in the lateral compartment and polyethylene insert in the medial compartment were calculated based on the ground reaction force and the angle of the knee flexion obtained by 3D motion capture system.

RESULTS

The loading ratio of medial and lateral compartments during standing stance (medial 54.49%, lateral 45.51%) and tibial anterior displacement (134 N, 3.89 mm) of healthy knee was basically consistent with previous experimental data. The maximum contact pressures of the medial meniscus and lateral tibial plateau cartilage of the healthy knee during standing stance were 2.14 MPa and 1.57 MPa, respectively. At the static standing phase, the maximum contact pressures of the polyethylene insert decreased from 17.90 to 17.29 Mpa, and the maximum contact pressures of the tibial plateau cartilage in the lateral compartment increased from 0.81 to 0.92 Mpa following an increase in the posterior inclination of the tibial component. At the first peak of ground reaction force, the maximum contact pressures of polyethylene insert increased from 22.37 to 25.16 MPa, and the maximum contact pressures of tibial plateau cartilage in the lateral compartment increased from 3.03 to 3.33 MPa, with the increase in the posterior inclination of the tibial component. At the second peak of ground reaction force, the maximum contact pressures of polyethylene insert decreased from 2.34 to 2.22 MPa with the increase in posterior inclination of tibial component.

CONCLUSION

The preoperative and postoperative finite element models of MB UKA were well established. The results showed that the maximum contact pressures of the polyethylene insert did not change significantly with the increase in the posterior inclination of the tibial prosthesis, while the maximum contact pressures of the tibial plateau cartilage of the lateral compartment increased when the posterior inclination of the tibial prosthesis was > 7°. Our results also show that the maximum contact pressures were greater with an excessive inclination angle (11°) of the tibial component, and the pressures of the tibial plateau cartilage in the lateral compartment were more concentrated on the posterior area. This study, therefore, proposes that excessive osteotomy should be avoided.

Sensitivity of simulated knee joint mechanics to selected human and bovine fibril-reinforced poroelastic material properties

Fibril-reinforced poroviscoelastic material models are considered state-of-the-art in modeling articular cartilage biomechanics. Yet, cartilage material parameters are often based on bovine tissue properties in computational knee joint models, although bovine properties are distinctly different from those of humans. Thus, we aimed to investigate how cartilage mechanical responses are affected in the knee joint model during walking when fibril-reinforced poroviscoelastic properties of cartilage are based on human data instead of bovine. We constructed a finite element knee joint model in which tibial and femoral cartilages were modeled as fibril-reinforced poroviscoelastic material using either human or bovine data. Joint loading was based on subject-specific gait data. The resulting mechanical responses of knee cartilage were compared between the knee joint models with human or bovine fibril-reinforced poroviscoelastic cartilage properties. Furthermore, we conducted a sensitivity analysis to determine which fibril-reinforced poroviscoelastic material parameters have the greatest impact on cartilage mechanical responses in the knee joint during walking. In general, bovine cartilage properties yielded greater maximum principal stresses and fluid pressures (both up to 30%) when compared to the human cartilage properties during the loading response in both femoral and tibial cartilage sites. Cartilage mechanical responses were very sensitive to the collagen fibril-related material parameter variations during walking while they were unresponsive to proteoglycan matrix or fluid flow-related material parameter variations. Taken together, human cartilage material properties should be accounted for when the goal is to compare absolute mechanical responses of knee joint cartilage as bovine material parameters lead to substantially different cartilage mechanical responses.

A novel knee joint model in FEBio with inhomogeneous fibril-reinforced biphasic cartilage simulating tissue mechanical responses during gait: data from the osteoarthritis initiative

We developed a novel knee joint model in FEBio to simulate walking. Knee cartilage was modeled using a fibril-reinforced biphasic (FRB) formulation with depth-wise collagen architecture and split-lines to account for cartilage structure. Under axial compression, the knee model with FRB cartilage yielded contact pressures, similar to reported experimental data. Furthermore, gait analysis with FRB cartilage simulated spatial and temporal trends in cartilage fluid pressures, stresses, and strains, comparable to those of a fibril-reinforced poroviscoelastic (FRPVE) material in Abaqus. This knee joint model in FEBio could be used for further studies of knee disorders using physiologically relevant loading.

Analysis and multiple index evaluation of SPO distribution of bone traction needles for pelvic fracture reduction

For robot-assisted pelvic fracture reduction, at least two bone needles need to be inserted into the ilium of the affected pelvis, and the robot clamping device is connected with the bone needles. The biomechanical properties of the pelvic musculoskeletal tissues are different with the different Spatial Position and Orientation (SPO) of the bone needles. In order to determine the optimal SPO of bone needle pairs, the constraints between the bone needles and the pelvis are analyzed, and the SPO vectors of 150 groups bone needles are obtained by the KNN-hierarchical clustering method; a batch modeling method of bone needles with different SPO is proposed. 150 finite element models of damaged pelvic musculoskeletal tissue with different SPO of bone needles are established and simulated. The stress and strain distribution homogenization of musculoskeletal tissue with bone needles as evaluation index, the simulation results of 150 models are evaluated. Results show that, the anterior superior iliac spine and the anterior inferior iliac spine are suitable regions to place bone needles in the pelvis, and the optimal distribution of the needle combination is found in this region. The overall stress and strain distribution of the damaged pelvic musculoskeletal tissue under the large reduction force is the best.

Influence of material heterogeneity on the mechanical response of articulated cartilages in a knee joint

Structurally, the articular cartilages are heterogeneous owing to nonuniform distribution and orientation of its constituents. The oversimplification of this soft tissue as a homogeneous material is generally considered in the simulation domain to estimate contact pressure along with other physical responses. Hence, there is a need for investigating knee cartilages for their actual response to external stimuli. In this article, impact of material and geometrical heterogeneity of the cartilage is resolved using well known material models. The findings are compared with conventional homogeneous models. The results indicate vital differences in contact pressure distribution and tissue deformation. Further, this study paves way for standardizing material models to extract maximum information possible for investigating knee mechanics with variable geometry and case specific parameters.

Study on circumpelvic muscle deformation and dynamic simulation of pelvic fracture reduction

For the pelvic fracture reduction, generally the fragment of the unaffected side is fixed and the affected side is moved to its correct anatomical position and orientation. During the pelvic fracture reduction, circumpelvic muscles deformation is closely related to the surgical accuracy. In this article, the biomechanical properties of musculoskeletal tissue during pelvic fracture reduction are studied. Five-parameter hyperelastic model named Mooney-Rivlin is adopted to analyze muscle's stress-strain relationship. The finite element model of the injured pelvic musculoskeletal tissue is established, and the deformation of circumpelvic main muscles is simulated. Then, the dynamic simulation of pelvic fracture reduction is performed according to the planned spatial reduction path. The results show that when the muscles are stretched the same stretch length, the strain of the gluteus medius is the largest. It is most prone to deformation under and the muscle injury is most easily to occur. During the pelvic fracture reduction, the strain of gluteus maximus is the largest, and it is most prone to deformation and injury. The traction length is the largest, and the traction force mainly comes from the gluteus maximus. This study provides reference for the robot assisted pelvic fracture reduction.

Multiscale biomechanics of the biphasic articular cartilage in the natural hip joint during routine activities

BACKGROUND AND OBJECTIVE

The investigation of the biomechanical behaviour of the articular cartilage (AC) under physiological loading is important to understand the joint function and onset of pathologies. This study aimed to develop a multiscale computational modelling approach and apply the approach to investigate the time-dependant biphasic behaviour of the AC in the natural hip joint under repetitive physiological loading over 80 cycles amongst six routine activities.

METHODS

A subject-specific musculoskeletal multibody dynamics (MBD) model was developed based on the anthropometry and motion capture data collected for a male subject. A corresponding FE model of the natural hip joint with biphasic AC was created based on the bone geometries exported from the MBD model. A multiscale computational modelling was then developed to couple the MBD model and the FE model and used to investigate the time-dependant biphasic behaviour of the AC under subject-specific physiological loading over 80 cycles amongst six routine activities.

RESULTS

The results showed that for all the activities considered, the interstitial fluid pressure in the AC supported over 80% of the loading. The maximum values of the peak contact pressure and peak fluid pressure for the whole cycle increased firstly and then remained stable over time from the 1st cycle to the 80th cycle. At these instants, the contact areas were located at the centre region of the AC. By contrast, when the contact area was located at the edge of the AC, these peak pressures were found to increase over time for some of the activities (squat, ascending stairs, descending stairs) but decrease for the other activities (normal walking, standing up, sitting down).

CONCLUSION

This study for the first time developed a multiscale computational modelling approach to couple a musculoskeletal MBD model of the body and a detailed FE model of the natural hip joint with biphasic AC, which enabled the evaluation of time-dependant biphasic behaviour of the AC under realistic physiological loading conditions. The study may have important implications in biomechanical studies of human cartilage to understand the joint function and biomechanical factors related to joint disease, and to support the development of cartilage substitution.

Study on the poroelastic behaviors of the defected articular cartilage

This article presented the possible mechanism of arthritis damaged changes in cartilage's interstitial fluid flowing behavior. Firstly, the analytical solutions for the pore fluid pressure and velocity in the idealized cartilage defect model were obtained, which are employed to validate the finite element (FE) method. Then according to the MRI data, an articular cartilage FE model was developed to study the effects of defect characteristics on its poroelastic behaviors. The results showed the interstitial fluid pressure and velocity in defected articular cartilage is diminished, moreover, this trend is even more severe as the defect radius or thickness increased. As the development of osteoarthritis goes, the fluid velocity is decreased and cause the even serious nutrients loss.

Anisotropy and inhomogeneity of permeability and fibrous network response in the pars intermedia of the human lateral meniscus

Within the human tibiofemoral joint, meniscus plays a key role due to its peculiar time-dependent mechanical characteristics, inhomogeneous structure and compositional features. To better understand the pathophysiological mechanisms underlying this essential component, it is mandatory to analyze in depth the relationship between its structure and the function it performs in the joint. Accordingly, the aim of this study was to evaluate the behavior of both solid and fluid phases of human meniscus in response to compressive loads, by integrating mechanical assessment and histological analysis. Cubic specimens were harvested from seven knee lateral menisci, specifically from anterior horn, pars intermedia and posterior horn; unconfined compressive tests were then performed according to three main loading directions (i.e., radial, circumferential and vertical). Fibril modulus, matrix modulus and hydraulic permeability of the tissue were thence estimated through a fibril-network-reinforced biphasic model. Tissue porosity and collagen fibers arrangement were assessed through histology for each region and related to the loading directions adopted during mechanical tests. Regional and strain-dependent constitutive parameters were finally proposed for the human lateral meniscus, suggesting an isotropic behavior of both the horns, and a transversely isotropic response of the pars intermedia. Furthermore, the histological findings supported the evidences highlighted by the compressive tests. Indeed, this study provided novel insights concerning the functional behavior of human menisci by integrating mechanical and histological characterizations and thus highlighting the key role of this component in knee contact mechanics and presenting fundamental information that can be used in the development of tissue-engineered substitutes. STATEMENT OF SIGNIFICANCE: This work presents an integration to the approaches currently used to model the mechanical behavior of the meniscal tissue. This study assessed in detail the regional and directional contributions of both the meniscal solid and fluid phases during compressive response, providing also complementary histological evidence. Within this updated perspective, both knee computational modeling and meniscal tissue engineering can be improved to have an effective impact on the clinical practice.

Cartilage Biomechanical Response Differs Under Physiological Biaxial Loads and Uniaxial Cyclic Compression

The main function of articular cartilage is to distribute loads and provide low friction for the opposing surfaces in synovial joints. Biphasic lubrication provided by high fluid load support due to relative motion of the contact surfaces has been widely accepted as the main lubrication mode in diarthrodial joints. However, assessment of chondrocyte response to mechanical loads typically employed nonphysiological uniaxial loads with static contact area. This study aimed to introduce a more physiologically relevant loading protocol for in vitro mechanobiological testing of cartilage explants. Finite element analysis was conducted to examine the biomechanical response of cartilage to two different loading regimes, biaxial loading, that permits migrating contact area, and unconfined uniaxial cyclic compression, traditionally used in mechanobiological experiments. Results predicted in this study showed that continuous tissue rehydration provided by relative surface motion maintained constant fluid pressure and tissue strains through the simulation. On the contrary, due to rapid tissue consolidation predicted in cyclic compression simulation, fluid pressure and transverse strain were reduced by 19% and 26%, respectively. Furthermore, relative surface motion simulation resulted in depth-dependent distribution of fluid pressure and tissue strains while unconfined uniaxial cyclic compression produced nearly uniform fluid pressure through the depth but higher at the center of the sample. Based on the results obtained from this study and since sliding contact occurs in vivo, this physiological loading mode should be considered in assessing biomechanical and mechanobiological cartilage behavior.

Development of robust finite element models of porcine tibiofemoral joints loaded under varied flexion angles and tibial freedoms

The successful development of cartilage repair treatments for the knee requires understanding of the biomechanical environment within the joint. Computational finite element models play an important role in non-invasively understanding knee mechanics, but it is important to compare model findings to experimental data. The purpose of this study was to develop a methodology for generating subject-specific finite element models of porcine tibiofemoral joints that was robust and valid over multiple different constraint scenarios. Computational model predictions of two knees were compared to experimental studies on corresponding specimens loaded under several different constraint scenarios using a custom designed experimental rig, with variations made to the femoral flexion angle and level of tibial freedom. For both in vitro specimens, changing the femoral flexion angle had a marked effect on the contact distribution observed experimentally. With the tibia fixed, the majority of the contact region shifted to the medial plateau as flexion was increased. This did not occur when the tibia was free to displace and rotate in response to applied load. These trends in contact distribution across the medial and lateral plateaus were replicated in the computational models. In an additional model with the meniscus removed, contact pressures were elevated by a similar magnitude to the increase seen when the meniscus was removed experimentally. Overall, the models were able to capture specimen-specific trends in contact distribution under a variety of different loads, providing the potential to investigate subject-specific outcomes for knee interventions.

High Tibial Osteotomy: Review of Techniques and Biomechanics

High tibial osteotomy becomes increasingly important in the treatment of cartilage damage or osteoarthritis of the medial compartment with concurrent varus deformity. HTO produces a postoperative valgus limb alignment with shifting the load-bearing axis of the lower limb laterally. However, maximizing procedural success and postoperative knee function still possess many difficulties. The key to improve the postoperative satisfaction and long-term survival is the understanding of the vital biomechanics of HTO in essence. This review article discussed the alignment principles, surgical technique, and fixation plate of HTO as well as the postoperative gait, musculoskeletal dynamics, and contact mechanics of the knee joint. We aimed to highlight the recent findings and progresses on the biomechanics of HTO. The biomechanical studies on HTO are still insufficient in the areas of gait analysis, joint kinematics, and joint contact mechanics. Combining musculoskeletal dynamics modelling and finite element analysis will help comprehensively understand in vivo patient-specific biomechanics after HTO.

Tissue material properties and computational modelling of the human tibiofemoral joint: a critical review

Understanding how structural and functional alterations of individual tissues impact on whole-joint function is challenging, particularly in humans where direct invasive experimentation is difficult. Finite element (FE) computational models produce quantitative predictions of the mechanical and physiological behaviour of multiple tissues simultaneously, thereby providing a means to study changes that occur through healthy ageing and disease such as osteoarthritis (OA). As a result, significant research investment has been placed in developing such models of the human knee. Previous work has highlighted that model predictions are highly sensitive to the various inputs used to build them, particularly the mathematical definition of material properties of biological tissues. The goal of this systematic review is two-fold. First, we provide a comprehensive summation and evaluation of existing linear elastic material property data for human tibiofemoral joint tissues, tabulating numerical values as a reference resource for future studies. Second, we review efforts to model tibiofemoral joint mechanical behaviour through FE modelling with particular focus on how studies have sourced tissue material properties. The last decade has seen a renaissance in material testing fuelled by development of a variety of new engineering techniques that allow the mechanical behaviour of both soft and hard tissues to be characterised at a spectrum of scales from nano- to bulk tissue level. As a result, there now exists an extremely broad range of published values for human tibiofemoral joint tissues. However, our systematic review highlights gaps and ambiguities that mean quantitative understanding of how tissue material properties alter with age and OA is limited. It is therefore currently challenging to construct FE models of the knee that are truly representative of a specific age or disease-state. Consequently, recent tibiofemoral joint FE models have been highly generic in terms of material properties even relying on non-human data from multiple species. We highlight this by critically evaluating current ability to quantitatively compare and model (1) young and old and (2) healthy and OA human tibiofemoral joints. We suggest that future research into both healthy and diseased knee function will benefit greatly from a subject- or cohort-specific approach in which FE models are constructed using material properties, medical imagery and loading data from cohorts with consistent demographics and/or disease states.

Ulna-humerus contact mechanics: Finite element analysis and experimental measurements using a tactile pressure sensor

Elbow articular cartilage withstands high compressive and shear forces while protecting the bone from excessive loading. Better understanding of elbow cartilage contact mechanics can provide insight into cartilage degeneration. In this study a tactile pressure sensor was used to measure the contact pressure distribution within the ulno-humeral joint of two cadaver specimens at 20° flexion angle across three different axial loads of 80 N, 110 N, and 140 N. Corresponding 3D finite element (FE) models were constructed from magnetic resonance imaging (MRI) and contact analysis was performed for each specimen with boundary and loading conditions identical to the experiment. Direct comparison between FE results and experimental measurements was conducted for the validation of the FE models and a sensitivity analysis was employed for assessing the effect of cartilage parameters on the model's outputs. The results showed a good agreement between the FE models and the experiments in terms of contact characteristics. The sensitivity analysis demonstrated that outcomes of the model, particularly peak contact pressure is more sensitive to the Poisson's ratio rather than to Young's modulus under static conditions. This result suggests that selection of Poisson's ratio is very critical for accurate prediction of contact mechanics within the ulno-humeral joint.

The effects of geometric uncertainties on computational modelling of knee biomechanics

The geometry of the articular components of the knee is an important factor in predicting joint mechanics in computational models. There are a number of uncertainties in the definition of the geometry of cartilage and meniscus, and evaluating the effects of these uncertainties is fundamental to understanding the level of reliability of the models. In this study, the sensitivity of knee mechanics to geometric uncertainties was investigated by comparing polynomial-based and image-based knee models and varying the size of meniscus. The results suggested that the geometric uncertainties in cartilage and meniscus resulting from the resolution of MRI and the accuracy of segmentation caused considerable effects on the predicted knee mechanics. Moreover, even if the mathematical geometric descriptors can be very close to the imaged-based articular surfaces, the detailed contact pressure distribution produced by the mathematical geometric descriptors was not the same as that of the image-based model. However, the trends predicted by the models based on mathematical geometric descriptors were similar to those of the imaged-based models.

Evaluating the effects of material properties of artificial meniscal implant in the human knee joint using finite element analysis

Artificial meniscal implants may replace severely injured meniscus and restore the normal functionality of the knee joint. Implant material stiffness and shape influence the longevity of implantations. This study, using 3D finite element analysis, aimed to evaluate the effects of material stiffness variations of anatomically shaped artificial meniscal implant in the knee joint. Finite element simulations were conducted on five different cases including intact knee, medial meniscectomized knee, and the knee joint with the meniscal implant with three distinct material stiffness. Cartilage contact pressures, compression stresses, shear stresses, and implant kinematics (medial-lateral and posterior-anterior displacement) were evaluated for an axial compressive load of 1150 N at full extension. Compared to the meniscectomized knee, the knee joint with the meniscal implant induced lower peak cartilage contact pressure and reduced the cartilage regions loaded with contact pressures greater than the peak cartilage contact pressure induced by the intact knee. Results of the current study also demonstrate that cartilage contact pressures and implant displacement are sensitive to the implant material stiffness. The meniscal implant with a stiffness of 11 MPa restores the intact knee contact mechanics, thereby reducing the risk of physiological damage to the articular cartilage.

Relative contribution of articular cartilage's constitutive components to load support depending on strain rate

Cartilage is considered a biphasic material in which the solid is composed of proteoglycans and collagen. In biphasic tissue, the hydraulic pressure is believed to bear most of the load under higher strain rates and its dissipation due to fluid flow determines creep and relaxation behavior. In equilibrium, hydraulic pressure is zero and load bearing is transferred to the solid matrix. The viscoelasticity of the collagen network also contributes to its time-dependent behavior, and the osmotic pressure to load bearing in equilibrium. The aim of the present study was to determine the relative contributions of hydraulic pressure, viscoelastic collagen stress, solid matrix stiffness and osmotic pressure to load carriage in cartilage under transient and equilibrium conditions. Unconfined compression experiments were simulated using a fibril-reinforced poroviscoelastic model of articular cartilage, including water, fibrillar viscoelastic collagen and non-fibrillar charged glycosaminoglycans. The relative contributions of hydraulic and osmotic pressures and stresses in the fibrillar and non-fibrillar network were evaluated in the superficial, middle and deep zone of cartilage under five different strain rates and after relaxation. Initially upon loading, the hydraulic pressure carried most of the load in all three zones. The osmotic swelling pressure carried most of the equilibrium load. In the surface zone, where the fibers were loaded in tension, the collagen network carried 20 % of the load for all strain rates. The importance of these fibers was illustrated by artificially modifying the fiber architecture, which reduced the overall stiffness of cartilage in all conditions. In conclusion, although hydraulic pressure dominates the transient behavior during cartilage loading, due to its viscoelastic nature the superficial zone collagen fibers carry a substantial part of the load under transient conditions. This becomes increasingly important with higher strain rates. The interesting and striking new insight from this study suggests that under equilibrium conditions, the swelling pressure generated by the combination of proteoglycans and collagen reinforcement accounts cartilage stiffness for more than 90 % of the loads carried by articular cartilage. This finding is different from the common thought that load is transferred from fluid to solid and is carried by the aggregate modulus of the solid. Rather, it is transformed from hydraulic to osmotic swelling pressure. These results show the importance of considering both (viscoelastic) collagen fibers as well as swelling pressure in studies of the (transient) mechanical behavior of cartilage.

Open Knee: Open Source Modeling and Simulation in Knee Biomechanics

Virtual representations of the knee joint can provide clinicians, scientists, and engineers the tools to explore mechanical functions of the knee and its tissue structures in health and disease. Modeling and simulation approaches such as finite element analysis also provide the possibility to understand the influence of surgical procedures and implants on joint stresses and tissue deformations. A large number of knee joint models are described in the biomechanics literature. However, freely accessible, customizable, and easy-to-use models are scarce. Availability of such models can accelerate clinical translation of simulations, where labor-intensive reproduction of model development steps can be avoided. Interested parties can immediately utilize readily available models for scientific discovery and clinical care. Motivated by this gap, this study aims to describe an open source and freely available finite element representation of the tibiofemoral joint, namely Open Knee, which includes the detailed anatomical representation of the joint's major tissue structures and their nonlinear mechanical properties and interactions. Three use cases illustrate customization potential of the model, its predictive capacity, and its scientific and clinical utility: prediction of joint movements during passive flexion, examining the role of meniscectomy on contact mechanics and joint movements, and understanding anterior cruciate ligament mechanics. A summary of scientific and clinically directed studies conducted by other investigators are also provided. The utilization of this open source model by groups other than its developers emphasizes the premise of model sharing as an accelerator of simulation-based medicine. Finally, the imminent need to develop next-generation knee models is noted. These are anticipated to incorporate individualized anatomy and tissue properties supported by specimen-specific joint mechanics data for evaluation, all acquired in vitro from varying age groups and pathological states.