Deformation of articular cartilage during static loading of a knee joint--experimental and finite element analysis

Confined and unconfined articular cartilage mechanics: Effect of creep duration on estimations of mechanical properties

Recent findings suggest that cartilage mechanical function may be a biomarker for early osteoarthritis (OA) pathology. Thus, the development of methodologies for in-vivo applications has expanded. However, when creep tests are performed, inconsistency in applied stress and testing duration impede meaningful comparisons. Therefore, this study investigates the impact of creep duration on cartilage mechanics through ex-vivo confined and unconfined compression experiments on healthy bovine cartilage samples (n = 20), subjected to 1 MPa stress for 5 h. A Zener model was fitted to unconfined data and a nonlinear biphasic model was fitted to confined data using durations ranging from 15 min to 5 h. Mechanical properties were compared against the full 5-h dataset to determine relative errors (RE) associated with insufficient creep duration. Based on our findings, we aim to establish a common ground for both in vivo and ex vivo environments. Both unconfined (R2 = 0.96 ± 0.02) and confined (R2 = 0.997 ± 0.003) models fitted the data well over 5 h. For confined creep tests, the aggregate modulus (HA) was 0.34 ± 0.12 MPa after 5 h and 0.27 ± 0.12 MPa after 1 h (RE ∼ 20 %), while initial permeability (k0) increased from 0.17 × 10-15 m4N-1s-1 to 0.56 × 10-15 m4N-1s-1 (RE ∼ 49 %). The Zener model estimated the initial (E1) and steady-state (E2) modulus to be 3.6 ± 0.7 MPa and 3.2 ± 0.3 MPa after 5 h, respectively. After 1 h, these values were 4.7 ± 1.0 MPa (RE ∼ 29 %) and 3.2 ± 0.3 MPa (RE ∼ 2 %). A larger RE (∼57 %) was observed for the relaxation time constant (τ) determined after 5 h (1688 ± 556 s) and 1 h (781 ± 170 s) with the Zener model. The benefit of extended creep duration diminished after 1-1.5 h for confined compression and 2 h for unconfined compression for non-rate dependent stiffness parameters (i.e., HA, E1, E2). This aligned well with the predefined equilibrium criteria of less than 0.6 μm/min, with equilibrium reached at 71 ± 23 min for confined experiments and 94 ± 25 min for unconfined experiments. In contrast, for parameters controlling the nonlinear material response (i.e., τ, k0,M), 4 h were required for unconfined compression and 1.5 h for confined compression to achieve RE ∼ 10 %. Thus, insufficient creeping duration resulted in large RE, especially for strain-dependent parameters. Therefore, it is recommended to use a clear equilibrium definition when conducting ex vivo experiments. In the context of clinically viable testing duration (i.e., 45-60 min) RE was ∼20 % for the predicted aggregate modulus and ∼10 % for the nonlinear permeability coefficient. While these errors appear substantial, they may still estimate cartilage mechanics within reasonable limits given the associated variability in healthy and OA cartilage characteristics. Therefore, within a limited timeframe, it could be possible to estimate mechanical properties using in vivo creep experiments that may serve as biomarkers for early OA.

Walking recovers cartilage compressive strain in vivo

Background

Articular cartilage is a fiber reinforced hydrated solid that serves a largely mechanical role of supporting load and enabling low friction joint articulation. Daily activities that load cartilage, lead to fluid exudation and compressive axial strain. To date, the only mechanism shown to recover this cartilage strain in vivo is unloading (e.g., lying supine). Based on recent work in cartilage explants, we hypothesized that loaded joint activity (walking) would also be capable of strain recovery in cartilage.

Methods

Eight asymptomatic young adults performed a fixed series of tasks, each of which was followed by magnetic resonance imaging to track changes in their knee cartilage thickness. The order of tasks was as follows: 1) stand for 30 ​min, 2) walk for 10 ​min, 3) stand for 30 ​min, and 4) lie supine for 50 ​min. The change in cartilage thickness was used to compute the axial cartilage strain.

Results

Standing produced an average axial strain of -5.1 ​% (compressive) in the tibiofemoral knee cartilage, while lying supine led to strain recovery. In agreement with our hypothesis, walking also led to cartilage strain recovery. Interestingly, the recovery rate during walking (0.19 ​% strain/min) was nearly 3-fold faster than lying supine (0.07 ​% strain/min).

Conclusions

This study represents the first in vivo demonstration that joint activity is capable of recovering compressive strain in cartilage. These findings indicate that joint activities such as walking may play a key role in maintaining and recovering cartilage strain, with implications for maintaining cartilage health and preventing or delaying cartilage degeneration.

Acute responses and recovery in the femoral cartilage morphology following running and cool-down protocols

This study compared the immediate effects of two common post-exercise cool-down methods to a control condition on subsequent morphologic change in femoral cartilage and vascular response in the posterior tibial artery after running. Sixteen healthy young males (23.6 ± 2.2 years, 172.8 ± 4.9 cm, 72.2 ± 7.1 kg) visited the laboratory during three separate sessions and performed 30-min of treadmill running (7.5 km/h for the initial 5-min, followed 8.5 km/h for 25-min). After running, participants experienced one of three 30-min cool-down protocols: active cool-down, cold application, or control (seated rest with their knee fully extended), in a counterbalanced order. Ultrasonographic assessments of femoral cartilage thickness (intercondylar, lateral, and medial) and posterior tibial artery blood flow were compared. To test condition effects over time, two-way analysis of variances and Tukey tests were used (p < 0.05) with Cohen's d effect sizes (ES). There was no condition by time interaction in femoral cartilage thickness (intercondylar: F30,705 = 0.91, p = 0.61; lateral: F30,705 = 1.24, p = 0.18; medial: F30,705 = 0.49, p = 0.99). Regardless of time (condition effect: F2,705 > 3.24, p < 0.04 for all tests), femoral cartilage in the cold application condition was thicker than the control condition (intercondylar: p = 0.01, ES = 0.16; lateral: p < 0.0001, ES = 0.24; medial: p = 0.04. ES = 0.16). Regardless of condition (time effect: F15,705 > 10.31, p < 0.0001 for all tests), femoral cartilage thickness was decreased after running (intercondylar: p < 0.0001, ES = 1.37; lateral: p < 0.0001, ES = 1.58; medial: p < 0.0001, ES = 0.81) and returned to baseline levels within 40-min (intercondylar: p = 0.09; lateral: p = 0.64; medial: p = 0.26). Blood flow volume was different (condition × time: F30,705 = 2.36, p < 0.0001) that running-induced blood flow volume was maintained for 30-min for the active cool-down condition (p < 0.0001, ES = 1.64), whereas it returned to baseline levels within 10-min for other conditions (cold application: p = 0.67; control: p = 0.62). Neither blood flow nor temperature had a significant impact on the recovery in femoral cartilage after running.

Change in Femoral Cartilage Cross-Sectional Area After Aerobic and Resistance Exercise

We compared the immediate response and recovery of femoral cartilage morphology following aerobic or resistance exercise to a control condition. Fifteen healthy young males (23.9 years; 170.1 cm; 69.7 kg) visited the laboratory three separate days and randomly performed one of the 30-min exercise aerobic exercises (treadmill running), resistance exercises (leg presses, back squats, and knee extensions), or seated rest as the control, each followed by the 50-min recovery. Ultrasonographic images of the femoral cartilage cross-sectional area (CSA) were obtained before and after exercise and every 5 min thereafter. To test exercise effects over time, a mixed model analysis of variance and Tukey-Kramer post-hoc tests were performed (p<0.05). The femoral cartilage CSA was different (condition×time: F34,742=4.30, p<0.0001) and the femoral cartilage CSA was decreased after the aerobic (-5.8%, p<0.0001) and the resistance (-3.4%, p=0.04) exercises compared to the pre-exercise levels. Deformed femoral cartilage CSA took 35 and 10 min to return to the pre-exercise levels after aerobic and resistance exercises (p+>+0.09), respectively. Thirty minutes of moderate exertion performing aerobic or resistance exercises immediately reduced the femoral cartilage CSA. A rest period ranging from 10 to 35 min was required for cartilage recovery after weight-bearing exercises.

Quantification of Cartilage Poroelastic Material Properties Via Analysis of Loading-Induced Cell Death

Articular cartilage (AC) is a load-bearing tissue that covers long bones in synovial joints. The biphasic/poroelastic mechanical properties of AC help it to protect joints by distributing loads, absorbing impact forces, and reducing friction. Unfortunately, alterations in these mechanical properties adversely impact cartilage function and precede joint degeneration in the form of osteoarthritis (OA). Thus, understanding what factors regulate the poroelastic mechanical properties of cartilage is of great scientific and clinical interest. Transgenic mouse models provide a valuable platform to delineate how specific genes contribute to cartilage mechanical properties. However, the poroelastic mechanical properties of murine articular cartilage are challenging to measure due to its small size (thickness ∼ 50 microns). In the current study, our objective was to test whether the poroelastic mechanical properties of murine articular cartilage can be determined based solely on time-dependent cell death measurements under constant loading conditions. We hypothesized that in murine articular cartilage subjected to constant, sub-impact loading from an incongruent surface, cell death area and tissue strain are closely correlated. We further hypothesized that the relationship between cell death area and tissue strain can be used-in combination with inverse finite element modeling-to compute poroelastic mechanical properties. To test these hypotheses, murine cartilage-on-bone explants from different anatomical locations were subjected to constant loading conditions by an incongruent surface in a custom device. Cell death area increased over time and scaled linearly with strain, which rose in magnitude over time due to poroelastic creep. Thus, we were able to infer tissue strain from cell death area measurements. Moreover, using tissue strain values inferred from cell death area measurements, we applied an inverse finite element modeling procedure to compute poroelastic material properties and acquired data consistent with previous studies. Collectively, our findings demonstrate in the key role poroelastic creep plays in mediating cell survival in mechanically loaded cartilage and verify that cell death area can be used as a surrogate measure of tissue strain that enables determination of murine cartilage mechanical properties.

In-silico study of the biomechanical effects of proximal-fibular osteotomy on knee joint contact pressure in varus-valgus misalignment

The aim of this work is to investigate in-silico the biomechanical effects of a proximal fibular osteotomy (PFO) on a knee joint with different varus/valgus deformities on the progression of knee osteoarthritis (KOA). A finite element analysis (FEA) of a human lower extremity consisting of the femoral, tibial and fibular bones and the cartilage connecting them was designed. The FEA was performed in a static standing primitive position to determine the contact pressure (CP) distribution and the location of the center of pressure (CoP). The analysis examined the relationship between these factors and the degree of deformation of the hip-knee angle in the baseline condition. The results suggested that PFO could be a simple and effective surgical treatment for patients with associated genu varum. This work also reported that a possible CP homogenization and a CoP correction can be achieved for medial varus deformities after PFO. However, it reduced its effectiveness for tibial origin valgus misalignment and worsened in cases of femoral valgus misalignment.

How effective is proximal fibular osteotomy in redistributing joint pressures? Insights from an HTO comparative in-silico study

BACKGROUND

Knee osteoarthritis (KOA) represents a widespread degenerative condition among adults that significantly affects quality of life. This study aims to elucidate the biomechanical implications of proximal fibular osteotomy (PFO), a proposed cost-effective and straightforward intervention for KOA, comparing its effects against traditional high tibial osteotomy (HTO) through in-silico analysis.

METHODS

Using medical imaging and finite element analysis (FEA), this research quantitatively evaluates the biomechanical outcomes of a simulated PFO procedure in patients with severe medial compartment genu-varum, who have undergone surgical correction with HTO. The study focused on evaluating changes in knee joint contact pressures, stress distribution, and anatomical positioning of the center of pressure (CoP). Three models are generated for each of the five patients investigated in this study, a preoperative original condition model, an in-silico PFO based on the same original condition data, and a reversed-engineered HTO in-silico model.

RESULTS

The novel contribution of this investigation is the quantitative analysis of the impact of PFO on the biomechanics of the knee joint. The results provide mechanical evidence that PFO can effectively redistribute and homogenize joint stresses, while also repositioning the CoP towards the center of the knee, similar to what is observed post HTO. The findings propose PFO as a potentially viable and simpler alternative to conventional surgical methods for managing severe KOA, specifically in patients with medial compartment genu-varum.

CONCLUSION

This research also marks the first application of FEA that may support one of the underlying biomechanical theories of PFO, providing a foundation for future clinical and in-silico studies.

Biomechanical effects of femoral prosthesis misalignment on the structure of the lateral compartment during medial unicompartmental knee arthroplasty in osteoporotic patients

BACKGROUND

This study aims to investigate the impact of varying coronal alignments of femoral prostheses on stress and strain distributions within the lateral compartment following unicompartmental knee arthroplasty (UKA) in patients with normal bone density and osteoporosis using finite element analysis. Additionally, it examines the relationship between osteoporosis and the progression of osteoarthritis in the lateral compartment postoperatively.

METHODS

UKA models were developed for both normal bone and osteoporotic conditions using a validated finite element model of the knee. Seven alignment conditions for the femoral prosthesis were simulated: 0° (neutral alignment), varus angles of 3°, 6°, and 9°, and valgus angles of 3°, 6°, and 9°, resulting in a total of 14 scenarios. Stress and strain distributions in the meniscus, tibial cartilage, and femoral cartilage of the lateral compartment were evaluated.

RESULTS

The results indicated that stress and strain in the meniscus, tibial cartilage, and femoral cartilage of the lateral compartment increased with greater varus alignment and decreased with greater valgus alignment in both normal and osteoporotic models. At equivalent alignment angles, stress and strain were consistently higher in the osteoporotic model (M2) compared to the normal bone model (M1), although the peak equivalent stress in the tibial cartilage was lower in the M2 model than in the M1 model.

CONCLUSIONS

In patients with osteoporosis undergoing fixed-bearing medial UKA, varus malalignment of the femoral prosthesis can lead to increased stress and strain in the lateral compartment's meniscus, tibial cartilage, and femoral cartilage. These findings suggest that osteoporosis may contribute to abnormal stress and strain distributions in the lateral compartment following UKA, potentially accelerating the progression of osteoarthritis in this region postoperatively.

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.

Effects of gait modifications on tissue-level knee mechanics in individuals with medial tibiofemoral osteoarthritis: A proof-of-concept study towards personalized interventions

Gait modification is a common nonsurgical approach to alter the mediolateral distribution of knee contact forces, intending to decelerate or postpone the progression of mechanically induced knee osteoarthritis (KOA). Nevertheless, the success rate of these approaches is controversial, with no studies conducted to assess alterations in tissue-level knee mechanics governing cartilage degradation response in KOA patients undertaking gait modifications. Thus, here we investigated the effect of different conventional gait conditions and modifications on tissue-level knee mechanics previously suggested as indicators of collagen network damage, cell death, and loss of proteoglycans in knee cartilage. Five participants with medial KOA were recruited and musculoskeletal finite element analyses were conducted to estimate subject-specific tissue mechanics of knee cartilages during two gait conditions (i.e., barefoot and shod) and six gait modifications (i.e., 0°, 5°, and 10° lateral wedge insoles, toe-in, toe-out, and wide stance). Based on our results, the optimal gait modification varied across the participants. Overall, toe-in, toe-out, and wide stance showed the greatest reduction in tissue mechanics within medial tibial and femoral cartilages. Gait modifications could effectually alter maximum principal stress (~20 ± 7%) and shear strain (~9 ± 4%) within the medial tibial cartilage. Nevertheless, lateral wedge insoles did not reduce joint- and tissue-level mechanics considerably. Significance: This proof-of-concept study emphasizes the importance of the personalized design of gait modifications to account for biomechanical risk factors associated with cartilage degradation.

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.

Comparison of constitutive models for meniscus and their effect on the knee joint biomechanics during gait

Mechanical behavior of meniscus can be modeled using constitutive material models of varying complexity, such as isotropic elastic or fibril reinforced poroelastic (FRPE). However, the FRPE material is complex to implement, computationally demanding in 3D geometries, and simulation is time-consuming. Hence, we aimed to quantify the most suitable and efficient constitutive model of meniscus for simulation of cartilage responses in the knee joint during walking. We showed that simpler constitutive material models can reproduce similar cartilage responses to a knee model with the FRPE meniscus, but only knee models that consider orthotropic elastic meniscus can also reproduce meniscus responses adequately.

Finite element analysis of the knee joint stress after partial meniscectomy for meniscus horizontal cleavage tears

OBJECTIVE

To establish a finite element model of meniscus horizontal cleavage and partial resection, to simulate the mechanical changes of knee joint under 4 flexion angles, and to explore what is the optimal surgical plan.

METHODS

We used Mimics Research, Geomagic Wrap, and SolidWorks computer software to reconstruct the 3D model of the knee joint, and then produced the horizontal cleavage tears model of the internal and lateral meniscus, the suture model, and the partial meniscectomy model. These models were assembled into a complete knee joint in SolidWorks software, and corresponding loads and boundary constraints were added to these models in ANSYS software to simulate the changing trend of pressure and shear force on femoral condylar cartilage, meniscus, and tibial cartilage under the flexion angles of 0°, 10°, 20°, 30° and 40° of the knee joint. At the same time, the difference of force area between medial interventricular and lateral interventricular of knee joint under four states of bending the knee was compared, to explore the different effects of different surgical methods on knee joint after horizontal meniscus tear.

RESULTS

Within the four medial meniscus injury models, the lowest peak internal pressure and shear force of the knee joint was observed in the meniscal suture model; the highest values were found in the bilateral leaflet resection model and the inferior leaflet resection model; the changes of pressure, shear force and stress area in the superior leaflet resection model were the most similar to the changes of the knee model with the meniscal suture model.

CONCLUSION

Suture repair is the best way to maintain the force relationship in the knee joint. However, resection of the superior leaflet of the meniscus is also a reliable choice when suture repair is difficult.

Musculoskeletal CT Imaging: State-of-the-Art Advancements and Future Directions

CT is one of the most widely used modalities for musculoskeletal imaging. Recent advancements in the field include the introduction of four-dimensional CT, which captures a CT image during motion; cone-beam CT, which uses flat-panel detectors to capture the lower extremities in weight-bearing mode; and dual-energy CT, which operates at two different x-ray potentials to improve the contrast resolution to facilitate the assessment of tissue material compositions such as tophaceous gout deposits and bone marrow edema. Most recently, photon-counting CT (PCCT) has been introduced. PCCT is a technique that uses photon-counting detectors to produce an image with higher spatial and contrast resolution than conventional multidetector CT systems. In addition, postprocessing techniques such as three-dimensional printing and cinematic rendering have used CT data to improve the generation of both physical and digital anatomic models. Last, advancements in the application of artificial intelligence to CT imaging have enabled the automatic evaluation of musculoskeletal pathologies. In this review, the authors discuss the current state of the above CT technologies, their respective advantages and disadvantages, and their projected future directions for various musculoskeletal applications.

Signatures of disease progression in knee osteoarthritis: insights from an integrated multi-scale modeling approach, a proof of concept

Introduction: Knee osteoarthritis (KOA) is characterized by articular cartilage degeneration. It has been widely accepted that the mechanical joint environment plays a significant role in the onset and progression of this disease. In silico models have been used to study the interplay between mechanical loading and cartilage degeneration, hereby relying mainly on two key mechanoregulatory factors indicative of collagen degradation and proteoglycans depletion. These factors are the strain in collagen fibril direction (SFD) and maximum shear strain (MSS) respectively. Methods: In this study, a multi-scale in silico modeling approach was used based on a synergy between musculoskeletal and finite element modeling to evaluate the SFD and MSS. These strains were evaluated during gait based on subject-specific gait analysis data collected at baseline (before a 2-year follow-up) for a healthy and progressive early-stage KOA subject with similar demographics. Results: The results show that both SFD and MSS factors allowed distinguishing between a healthy subject and a KOA subject, showing progression at 2 years follow-up, at the instance of peak contact force as well as during the stance phase of the gait cycle. At the peak of the stance phase, the SFD were found to be more elevated in the KOA patient with the median being 0.82% higher in the lateral and 0.4% higher in the medial compartment of the tibial cartilage compared to the healthy subject. Similarly, for the MSS, the median strains were found to be 3.6% higher in the lateral and 0.7% higher in the medial tibial compartment of the KOA patient compared to the healthy subject. Based on these intersubject SFD and MSS differences, we were additionally able to identify that the tibial compartment of the KOA subject at risk of progression. Conclusion/discussion: We confirmed the mechanoregulatory factors as potential biomarkers to discriminate patients at risk of disease progression. Future studies should evaluate the sensitivity of the mechanoregulatory factors calculated based on this multi-scale modeling workflow in larger patient and control cohorts.

Computational assessment on the impact of collagen fiber orientation in cartilages on healthy and arthritic knee kinetics/kinematics

BACKGROUND

The inhomogeneous distribution of collagen fiber in cartilage can substantially influence the knee kinematics. This becomes vital for understanding the mechanical response of soft tissues, and cartilage deterioration including osteoarthritis (OA). Though the conventional computational models consider geometrical heterogeneity along with fiber reinforcements in the cartilage model as material heterogeneity, the influence of fiber orientation on knee kinetics and kinematics is not fully explored. This work examines how the collagen fiber orientation in the cartilage affects the healthy (intact knee) and arthritic knee response over multiple gait activities like running and walking.

METHODS

A 3D finite element knee joint model is used to compute the articular cartilage response during the gait cycle. A fiber-reinforced porous hyper elastic (FRPHE) material is used to model the soft tissue. A split-line pattern is used to implement the fiber orientation in femoral and tibial cartilage. Four distinct intact cartilage models and three OA models are simulated to assess the impact of the orientation of collagen fibers in a depth wise direction. The cartilage models with fibers oriented in parallel, perpendicular, and inclined to the articular surface are investigated for multiple knee kinematics and kinetics.

FINDINGS

The comparison of models with fiber orientation parallel to articulating surface for walking and running gait has the highest elastic stress and fluid pressure compared with inclined and perpendicular fiber-oriented models. Also, the maximum contact pressure is observed to be higher in the case of intact models during the walking cycle than for OA models. In contrast, the maximum contact pressure is higher during running in OA models than in intact models. Additionally, parallel-oriented models produce higher maximum stresses and fluid pressure for walking and running gait than proximal-distal-oriented models. Interestingly, during the walking cycle, the maximum contact pressure with intact models is approximately three times higher than on OA models. In contrast, the OA models exhibit higher contact pressure during the running cycle.

INTERPRETATION

Overall, the study indicates that collagen orientation is crucial for tissue responsiveness. This investigation provides insights into the development of tailored implants.

Monitoring Knee Contact Force with Force-Sensing Insoles

Numerous applications exist for monitoring knee contact force (KCF) throughout activities of daily living. However, the ability to estimate these forces is restricted to a laboratory setting. The purposes of this study are to develop KCF metric estimation models and explore the feasibility of monitoring KCF metrics via surrogate measures derived from force-sensing insole data. Nine healthy subjects (3F, age 27 ± 5 years, mass 74.8 ± 11.8 kg, height 1.7 ± 0.08 m) walked at multiple speeds (0.8-1.6 m/s) on an instrumented treadmill. Thirteen insole force features were calculated as potential predictors of peak KCF and KCF impulse per step, estimated with musculoskeletal modeling. The error was calculated with median symmetric accuracy. Pearson product-moment correlation coefficients defined the relationship between variables. Models develop per-limb demonstrated lower prediction error than those developed per-subject (KCF impulse: 2.2% vs 3.4%; peak KCF: 3.50% vs. 6.5%, respectively). Many insole features are moderately to strongly associated with peak KCF, but not KCF impulse across the group. We present methods to directly estimate and monitor changes in KCF using instrumented insoles. Our results carry promising implications for internal tissue loads monitoring outside of a laboratory with wearable sensors.

Computed Tomography: State-of-the-Art Advancements in Musculoskeletal Imaging

ABSTRACT

Although musculoskeletal magnetic resonance imaging (MRI) plays a dominant role in characterizing abnormalities, novel computed tomography (CT) techniques have found an emerging niche in several scenarios such as trauma, gout, and the characterization of pathologic biomechanical states during motion and weight-bearing. Recent developments and advancements in the field of musculoskeletal CT include 4-dimensional, cone-beam (CB), and dual-energy (DE) CT. Four-dimensional CT has the potential to quantify biomechanical derangements of peripheral joints in different joint positions to diagnose and characterize patellofemoral instability, scapholunate ligamentous injuries, and syndesmotic injuries. Cone-beam CT provides an opportunity to image peripheral joints during weight-bearing, augmenting the diagnosis and characterization of disease processes. Emerging CBCT technologies improved spatial resolution for osseous microstructures in the quantitative analysis of osteoarthritis-related subchondral bone changes, trauma, and fracture healing. Dual-energy CT-based material decomposition visualizes and quantifies monosodium urate crystals in gout, bone marrow edema in traumatic and nontraumatic fractures, and neoplastic disease. Recently, DE techniques have been applied to CBCT, contributing to increased image quality in contrast-enhanced arthrography, bone densitometry, and bone marrow imaging. This review describes 4-dimensional CT, CBCT, and DECT advances, current logistical limitations, and prospects for each technique.

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.

An EMG-assisted muscle-force driven finite element analysis pipeline to investigate joint- and tissue-level mechanical responses in functional activities: towards a rapid assessment toolbox

Joint tissue mechanics (e.g., stress and strain) are believed to have a major involvement in the onset and progression of musculoskeletal disorders, e.g., knee osteoarthritis (KOA). Accordingly, considerable efforts have been made to develop musculoskeletal finite element (MS-FE) models to estimate highly detailed tissue mechanics that predict cartilage degeneration. However, creating such models is time-consuming and requires advanced expertise. This limits these complex, yet promising MS-FE models to research applications with few participants and makes the models impractical for clinical assessments. Also, these previously developed MS-FE models have not been used to assess activities other than gait. This study introduces and verifies a semi-automated rapid state-of-the-art MS-FE modeling and simulation toolbox incorporating an electromyography- (EMG) assisted MS model and a muscle-force driven FE model of the knee with fibril-reinforced poro(visco)elastic cartilages and menisci. To showcase the usability of the pipeline, we estimated joint- and tissue-level knee mechanics in 15 KOA individuals performing different daily activities. The pipeline was verified by comparing the estimated muscle activations and joint mechanics to existing experimental data. To determine the importance of EMG-assisted MS approach, results were compared to those from the same FE models but driven by static-optimization-based MS models. The EMG-assisted MS-FE pipeline bore a closer resemblance to experiments compared to the static-optimization-based MS-FE pipeline. Importantly, the developed pipeline showed great potential as a rapid MS-FE analysis toolbox to investigate multiscale knee mechanics during different activities of individuals with KOA. The template FE model of the study is freely available here.

Effects of solvent osmolarity and viscosity on cartilage energy dissipation under high-frequency loading

Articular cartilage is a spatially heterogeneous, dissipative biological hydrogel with a high fluid volume fraction. Although energy dissipation is important in the context of delaying cartilage damage, the dynamic behavior of articular cartilage equilibrated in media of varied osmolarity and viscosity is not widely understood. This study investigated the mechanical behaviors of cartilage when equilibrated to media of varying osmolarity and viscosity. Dynamic moduli and phase shift were measured at both low (1 Hz) and high (75-300 Hz) frequency, with cartilage samples compressed to varied offset strain levels. Increasing solution osmolarity and viscosity both independently resulted in larger energy dissipation and decreased dynamic modulus of cartilage at both low and high frequency. Mechanical property alterations induced by varying osmolarity are likely due to the change in permeability and fluid volume fraction within the tissue. The effects of solution viscosity are likely due to frictional interactions at the solid-fluid interface, affecting energy dissipation. These findings highlight the significance of interstitial fluid on the energy dissipation capabilities of the tissue, which can influence the onset of cartilage damage.

Magnetic Resonance Imaging-based biomechanical simulation of cartilage: A systematic review

MRI-based mathematical and computational modeling studies can contribute to a better understanding of the mechanisms governing cartilage's mechanical performance and cartilage disease. In addition, distinct modeling of cartilage is needed to optimize artificial cartilage production. These studies have opened up the prospect of further deepening our understanding of cartilage function. Furthermore, these studies reveal the initiation of an engineering-level approach to how cartilage disease affects material properties and cartilage function. Aimed at researchers in the field of MRI-based cartilage simulation, research articles pertinent to MRI-based cartilage modeling were identified, reviewed, and summarized systematically. Various MRI applications for cartilage modeling are highlighted, and the limitations of different constitutive models used are addressed. In addition, the clinical application of simulations and studied diseases are discussed. The paper's quality, based on the developed questionnaire, was assessed, and out of 79 reviewed papers, 34 papers were determined as high-quality. Due to the lack of the best constitutive models for various clinical conditions, researchers may consider the effect of constitutive material models on the cartilage disease simulation. In the future, research groups may incorporate various aspects of machine learning into constitutive models and MRI data extraction to further refine the study methodology. Moreover, researchers should strive for further reproducibility and rigorous model validation and verification, such as gait analysis.

Creep behavior of human knee joint determined with high-speed biplanar video-radiography and finite element simulation

Creep and relaxation of knee cartilage and meniscus have been extensively studied at the tissue level with constitutive laws well established. At the joint level, however, both experimental and model studies have been focused on either elastic or kinematic responses of the knee, where the time-dependent response is typically neglected for simplicity. The objectives of this study were to quantify the in-vivo creep behavior of human knee joints produced by the cartilaginous tissues and to use the relevant data to validate a previously proposed poromechanical model. Two participants with no history of leg injury volunteered for 3T magnetic resonance imaging (MRI) of their unloaded right knees and for biplanar video-radiography (BVR) of the same knees during standing on an instrumented treadmill for 10 min. Approximately 550 temporal data points were obtained for the in-vivo displacement of the right femur relative to the tibia of the knee. Models of the bones and soft tissues were derived from the MRI. The bone models were used to reconstruct the 3D bone kinematics measured using BVR. Ground reaction forces were simultaneously recorded for the right leg, which were used as input for the subject-specific finite element knee models. Cartilaginous tissues were modeled as fluid-saturated fibril-reinforced materials. In-vivo creep of the knee was experimentally observed for both participants, i.e., the joint displacement increased with time while the reaction forces at the foot were approximately constant. The creep displacements obtained from the finite element models compared well with the experimental data when the tissue properties were calibrated (Pearson correlation coefficient = 0.99). The results showed the capacity of the poromechanical knee model to capture the creep response of the joint. The combined experimental and model study may be used to understand the fluid-pressure load support and contact mechanics of the joint using material properties calibrated from the displacement data, which enhance the fidelity of model results.

Device for Assessing Knee Joint Dynamics During Magnetic Resonance Imaging

BACKGROUND

Knee assessment with and without load using magnetic resonance imaging (MRI) can provide information on knee joint dynamics and improve the diagnosis of knee joint diseases. Performing such studies on a routine MRI-scanner require a load-exerting device during scanning. There is a need for more studies on developing loading devices and evaluating their clinical potential.

PURPOSE

Design and develop a portable and easy-to-use axial loading device to evaluate the knee joint dynamics during the MRI study.

STUDY TYPE

Prospective study.

SUBJECTS

Nine healthy subjects.

FIELD STRENGTH/SEQUENCE

A 0.25 T standing-open MRI and 3.0 T MRI. PD-T₂ -weighted FSE, 3D-fast-spoiled-gradient-echo, FS-PD, and CartiGram sequences.

ASSESSMENT

Design and development of loading device, calibration of loads, MR safety assessment (using projectile angular displacement, torque, and temperature tests). Scoring system for ease of doing. Qualitative (by radiologist) and quantitative (using structural similarity index measure [SSIM]) image-artifact assessment. Evaluation of repeatability, comparison with various standing stances load, and loading effect on knee MR parameters (tibiofemoral bone gap [TFBG], femoral cartilage thickness [FCT], tibial cartilage thickness [TCT], femoral cartilage T₂ -value [FCT2], and tibia cartilage T₂ -value [TCT2]). The relative percentage change (RPC) in parameters due to the device load was computed.

STATISTICAL TEST

Pearson's correlation coefficient (r).

RESULTS

The developed device is conditional-MR safe (details in the manuscript and supplementary materials), 15 × 15 × 45 cm³ dimension, and <3 kg. The ease of using the device was 4.9/5. The device introduced no visible image artifacts, and SSIM of 0.9889 ± 0.0153 was observed. The TFBG intraobserver variability (absolute difference) was <0.1 mm. Interobserver variability of all regions of interest was <0.1 mm. The load exerted by the device was close to the load during standing on both legs in 0.25 T scanner with r > 0.9. Loading resulted in RPC of 1.5%-11.0%, 7.9%-8.5%, and -1.5% to 13.0% in the TFBG, FCT, and TCT, respectively. FCT2 and TCT2 were reduced in range of 1.5-2.7 msec and 0.5-2.3 msec due to load.

DATA CONCLUSION

The proposed device is conditionally MR safe, low cost (material cost < INR 6000), portable, and effective in loading the knee joint with up to 50% of body weight.

EVIDENCE LEVEL

1 TECHNICAL EFFICACY: Stage 1.

Model for in-vivo estimation of stiffness of tibiofemoral joint using MR imaging and FEM analysis

BACKGROUND

Appropriate structural and material properties are essential for finite-element-modeling (FEM). In knee FEM, structural information could extract through 3D-imaging, but the individual subject's tissue material properties are inaccessible.

PURPOSE

The current study's purpose was to develop a methodology to estimate the subject-specific stiffness of the tibiofemoral joint using finite-element-analysis (FEA) and MRI data of knee joint with and without load.

METHODS

In this study, six Magnetic Resonance Imaging (MRI) datasets were acquired from 3 healthy volunteers with axially loaded and unloaded knee joint. The strain was computed from the tibiofemoral bone gap difference (ΔmBGFT) using the knee MR images with and without load. The knee FEM study was conducted using a subject-specific knee joint 3D-model and various soft-tissue stiffness values (1 to 50 MPa) to develop subject-specific stiffness versus strain models.

RESULTS

Less than 1.02% absolute convergence error was observed during the simulation. Subject-specific combined stiffness of weight-bearing tibiofemoral soft-tissue was estimated with mean values as 2.40 ± 0.17 MPa. Intra-subject variability has been observed during the repeat scan in 3 subjects as 0.27, 0.12, and 0.15 MPa, respectively. All subject-specific stiffness-strain relationship data was fitted well with power function (R² = 0.997).

CONCLUSION

The current study proposed a generalized mathematical model and a methodology to estimate subject-specific stiffness of the tibiofemoral joint for FEM analysis. Such a method might enhance the efficacy of FEM in implant design optimization and biomechanics for subject-specific studies. Trial registration The institutional ethics committee (IEC), Indian Institute of Technology, Delhi, India, approved the study on 20th September 2017, with reference number P-019; it was a pilot study, no clinical trail registration was recommended.

Comparison of Kinematics and Contact Mechanics in Normal Knee and Total Knee Replacements: A Computational Investigation

An objective of total knee replacement (TKR) is to restore the mechanical function of a normal knee. Joint kinematics and contact mechanics performance are two of the primary indices that indicate the success of TKR devices. The aim of this study was to compare the kinematics and contact mechanics of TKR and normal knee joints. An experimentally evaluated finite-element (FE) knee model was developed and used to investigate the performance of four TKR designs (fixed cruciate-retaining (CR), mobile CR, posterior-stabilized (PS), medial pivot design (MP)) and the normal knee joint during a gait cycle. The predicted kinematic results showed that the MP design presented similar kinematics to those of the normal knee joint and did not demonstrate paradoxical motion of the femur. A considerably larger contact area and lower contact pressure were found on the normal knee joint (1315 mm², and 14.8 MPa, respectively) than on the TKRs, which was consistent with the previous in-vivo fluoroscopic investigation. The mobile CR and PS designs exhibited the smallest and greatest contact pressures of the four TKR designs, respectively. The results of the present study help to understand the kinematics and contact mechanics in the TKR during the gait cycle, and provide comprehensive information about the performance of the normal knee joint.

Influence of the Mechanical Environment on the Regeneration of Osteochondral Defects

Articular cartilage is a highly specialised connective tissue of diarthrodial joints which provides a smooth, lubricated surface for joint articulation and plays a crucial role in the transmission of loads. In vivo cartilage is subjected to mechanical stimuli that are essential for cartilage development and the maintenance of a chondrocytic phenotype. Cartilage damage caused by traumatic injuries, ageing, or degradative diseases leads to impaired loading resistance and progressive degeneration of both the articular cartilage and the underlying subchondral bone. Since the tissue has limited self-repairing capacity due its avascular nature, restoration of its mechanical properties is still a major challenge. Tissue engineering techniques have the potential to heal osteochondral defects using a combination of stem cells, growth factors, and biomaterials that could produce a biomechanically functional tissue, representative of native hyaline cartilage. However, current clinical approaches fail to repair full-thickness defects that include the underlying subchondral bone. Moreover, when tested in vivo, current tissue-engineered grafts show limited capacity to regenerate the damaged tissue due to poor integration with host cartilage and the failure to retain structural integrity after insertion, resulting in reduced mechanical function. The aim of this review is to examine the optimal characteristics of osteochondral scaffolds. Additionally, an overview on the latest biomaterials potentially able to replicate the natural mechanical environment of articular cartilage and their role in maintaining mechanical cues to drive chondrogenesis will be detailed, as well as the overall mechanical performance of grafts engineered using different technologies.

Rapid CT-based Estimation of Articular Cartilage Biomechanics in the Knee Joint Without Cartilage Segmentation

Knee osteoarthritis (OA) is a painful joint disease, causing disabilities in daily activities. However, there is no known cure for OA, and the best treatment strategy might be prevention. Finite element (FE) modeling has demonstrated potential for evaluating personalized risks for the progression of OA. Current FE modeling approaches use primarily magnetic resonance imaging (MRI) to construct personalized knee joint models. However, MRI is expensive and has lower resolution than computed tomography (CT). In this study, we extend a previously presented atlas-based FE modeling framework for automatic model generation and simulation of knee joint tissue responses using contrast agent-free CT. In this method, based on certain anatomical dimensions measured from bone surfaces, an optimal template is selected and scaled to generate a personalized FE model. We compared the simulated tissue responses of the CT-based models with those of the MRI-based models. We show that the CT-based models are capable of producing similar tensile stresses, fibril strains, and fluid pressures of knee joint cartilage compared to those of the MRI-based models. This study provides a new methodology for the analysis of knee joint and cartilage mechanics based on measurement of bone dimensions from native CT scans.

A Review on Finite Element Modeling and Simulation of the Anterior Cruciate Ligament Reconstruction

The anterior cruciate ligament (ACL) constitutes one of the most important stabilizing tissues of the knee joint whose rapture is very prevalent. ACL reconstruction (ACLR) from a graft is a surgery which yields the best outcome. Taking into account the complicated nature of this operation and the high cost of experiments, finite element (FE) simulations can become a valuable tool for evaluating the surgery in a pre-clinical setting. The present study summarizes, for the first time, the current advancement in ACLR in both clinical and computational level. It also emphasizes on the material modeling and properties of the most popular grafts as well as modeling of different surgery techniques. It can be concluded that more effort is needed to be put toward more realistic simulation of the surgery, including also the use of two bundles for graft representation, graft pretension and artificial grafts. Furthermore, muscles and synovial fluid need to be included, while patellofemoral joint is an important bone that is rarely used. More realistic models are also required for soft tissues, as most articles used isotropic linear elastic models and springs. In summary, accurate and realistic FE analysis in conjunction with multidisciplinary collaboration could contribute to ACLR improvement provided that several important aspects are carefully considered.

A comprehensive testing protocol for macro-scale mechanical characterization of knee articular cartilage with documented experimental repeatability

Articular cartilage mechanics has been extensively studied with various approaches and mechanical characterization strategies. However testing protocols can be highly varying and difficult to reproduce, particularly for specimen-specific analyses. Detailed knowledge of testing protocols is important for reliable use in concordant finite element analyses. This study presents a detailed, robust procedure for cartilage testing-with multiple regions and per sample repeatability data. Samples were taken from femur, tibia and patella of a human cadaver knee and tested in unconfined compression, confined compression and uniaxial tension. Each test was repeated three times. The testing protocols provide elastic and time dependent characterization data. Results, for example equilibrium modulus of 0.28 (0.0024) MPa for patella under unconfined compression indicate that variability is well controlled and that protocol(s) presented here can generate repeatable specimen-specific data. As per the authors' knowledge this is the first study to report in-depth uncertainty assessment of the experimental procedures for multi-region knee cartilage characterization.

Direct Validation of Human Knee-Joint Contact Mechanics Derived From Subject-Specific Finite-Element Models of the Tibiofemoral and Patellofemoral Joints

The primary aim of this study was to validate predictions of human knee-joint contact mechanics (specifically, contact pressure, contact area, and contact force) derived from finite-element models of the tibiofemoral and patellofemoral joints against corresponding measurements obtained in vitro during simulated weight-bearing activity. A secondary aim was to perform sensitivity analyses of the model calculations to identify those parameters that most significantly affect model predictions of joint contact pressure, area, and force. Joint pressures in the medial and lateral compartments of the tibiofemoral and patellofemoral joints were measured in vitro during two simulated weight-bearing activities: stair descent and squatting. Model-predicted joint contact pressure distribution maps were consistent with those obtained from experiment. Normalized root-mean-square errors between the measured and calculated contact variables were on the order of 15%. Pearson correlations between the time histories of model-predicted and measured contact variables were generally above 0.8. Mean errors in the calculated center-of-pressure locations were 3.1 mm for the tibiofemoral joint and 2.1 mm for the patellofemoral joint. Model predictions of joint contact mechanics were most sensitive to changes in the material properties and geometry of the meniscus and cartilage, particularly estimates of peak contact pressure. The validated finite element modeling framework offers a useful tool for noninvasive determination of knee-joint contact mechanics during dynamic activity under physiological loading conditions.

Constitutive modeling of menisci tissue: a critical review of analytical and numerical approaches

Menisci are fibrocartilaginous disks consisting of soft tissue with a complex biomechanical structure. They are critical determinants of the kinematics as well as the stability of the knee joint. Several studies have been carried out to formulate tissue mechanical behavior, leading to the development of a wide spectrum of constitutive laws. In addition to developing analytical tools, extensive numerical studies have been conducted on menisci modeling. This study reviews the developments of the most widely used continuum models of the meniscus mechanical properties in conjunction with emerging analytical and numerical models used to study the meniscus. The review presents relevant approaches and assumptions used to develop the models and includes discussions regarding strengths, weaknesses, and discrepancies involved in the presented models. The study presents a comprehensive coverage of relevant publications included in Compendex, EMBASE, MEDLINE, PubMed, ScienceDirect, Springer, and Scopus databases. This review aims at opening novel avenues for improving menisci modeling within the framework of constitutive modeling through highlighting the needs for further research directed toward determining key factors in gaining insight into the biomechanics of menisci which is crucial for the elaborate design of meniscal replacements.

Prediction of patellofemoral joint kinematics and contact through co-simulation of rigid body dynamics and nonlinear finite element analysis

Joint-level rigid body dynamics simulations, when coupled with tissue-level finite element analyses, can simultaneously provide movement and tissue deformation metrics to understand mechanical interactions within the joint on a multi-scale level. In this study, a co-simulation workflow of a joint-level rigid body model that predicts the relative motion as a function of the non-linear cartilage response predicted by a non-linear implicit finite element solver is presented. Predictions are compared to in-vitro measurements (The Open Knee(s) project) in terms of the mean error and level-of-agreement: pressure_error = 0.46 MPa (level-of-agreement, -0.23 - 1.1 MPa); area_error = -89 mm² (level-of-agreement, -280 - 98 mm²) and contact force_error = 93 N (level-of-agreement, 7.8 - 180 N). The automated co-simulation control algorithm enables multiscale coupling between joint and tissue-level models with real-time two-way communication as opposed to the traditional feed-forward approach of multi-scale models.

Biological and Mechanical Predictors of Meniscus Function: Basic Science to Clinical Translation

Progressive knee joint degeneration occurs following removal of a torn meniscus. However, there is significant variability in the rate of development of post-meniscectomy osteoarthritis (OA). While there is no current consensus on the risk factors for development of knee OA in patients with meniscus tears, it is likely that both biological and biomechanical factors play critical roles. In this perspective paper, we review the mechanical and the biological predictors of the response of the knee to partial meniscectomy. We review the role of patient-based studies, in vivo animal models, cadaveric models, bioreactor systems, and statistically augmented computational models for the study of meniscus function and post-meniscectomy OA, providing insight into the important interplay between biomechanical and biologic factors. We then discuss the clinical translation of these concepts for "biologic augmentation" of meniscus healing and meniscus replacement. Ultimately, collaborative studies between engineers, biologists, and clinicians is the optimal way to improve our understanding of meniscus pathology and response to injury and/or disease, and to facilitate effective translation of laboratory findings to improved treatments for our patients. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:937-945, 2020.

Clinical Contrast-Enhanced Computed Tomography With Semi-Automatic Segmentation Provides Feasible Input for Computational Models of the Knee Joint

Computational models can provide information on joint function and risk of tissue failure related to progression of osteoarthritis (OA). Currently, the joint geometries utilized in modeling are primarily obtained via manual segmentation, which is time-consuming and hence impractical for direct clinical application. The aim of this study was to evaluate the applicability of a previously developed semi-automatic method for segmenting tibial and femoral cartilage to serve as input geometry for finite element (FE) models. Knee joints from seven volunteers were first imaged using a clinical computed tomography (CT) with contrast enhancement and then segmented with semi-automatic and manual methods. In both segmentations, knee joint models with fibril-reinforced poroviscoelastic (FRPVE) properties were generated and the mechanical responses of articular cartilage were computed during physiologically relevant loading. The mean differences in the absolute values of maximum principal stress, maximum principal strain, and fibril strain between the models generated from semi-automatic and manual segmentations were <1 MPa, <0.72% and <0.40%, respectively. Furthermore, contact areas, contact forces, average pore pressures, and average maximum principal strains were not statistically different between the models (p >0.05). This semi-automatic method speeded up the segmentation process by over 90% and there were only negligible differences in the results provided by the models utilizing either manual or semi-automatic segmentations. Thus, the presented CT imaging-based segmentation method represents a novel tool for application in FE modeling in the clinic when a physician needs to evaluate knee joint function.

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.

Use of Computational Modeling to Study Joint Degeneration: A Review

Osteoarthritis (OA), a degenerative joint disease, is the most common chronic condition of the joints, which cannot be prevented effectively. Computational modeling of joint degradation allows to estimate the patient-specific progression of OA, which can aid clinicians to estimate the most suitable time window for surgical intervention in osteoarthritic patients. This paper gives an overview of the different approaches used to model different aspects of joint degeneration, thereby focusing mostly on the knee joint. The paper starts by discussing how OA affects the different components of the joint and how these are accounted for in the models. Subsequently, it discusses the different modeling approaches that can be used to answer questions related to OA etiology, progression and treatment. These models are ordered based on their underlying assumptions and technologies: musculoskeletal models, Finite Element models, (gene) regulatory models, multiscale models and data-driven models (artificial intelligence/machine learning). Finally, it is concluded that in the future, efforts should be made to integrate the different modeling techniques into a more robust computational framework that should not only be efficient to predict OA progression but also easily allow a patient’s individualized risk assessment as screening tool for use in clinical practice.

Development of a Knee Joint CT-FEM Model in Load Response of the Stance Phase During Walking Using Muscle Exertion, Motion Analysis, and Ground Reaction Force Data

Background and objectives: There are no reports on articular stress distribution during walking based on any computed tomography (CT)-finite element model (CT-FEM). This study aimed to develop a calculation model of the load response (LR) phase, the most burdensome phase on the knee, during walking using the finite element method of quantitative CT images. Materials and Methods: The right knee of a 43-year-old man who had no history of osteoarthritis or surgeries of the knee was examined. An image of the knee was obtained using CT and the extension position image was converted to the flexion angle image in the LR phase. The bone was composed of heterogeneous materials. The ligaments were made of truss elements; therefore, they do not generate strain during expansion or contraction and do not affect the reaction force or pressure. The construction of the knee joint included material properties of the ligament, cartilage, and meniscus. The extensor and flexor muscles were calculated and set as the muscle exercise tension around the knee joint. Ground reaction force was vertically applied to suppress the rotation of the knee, and the thigh was restrained. Results: An FEM was constructed using a motion analyzer, floor reaction force meter, and muscle tractive force calculation. In a normal knee, the equivalent stress and joint contact reaction force in the LR phase were distributed over a wide area on the inner upper surface of the femur and tibia. Conclusions: We developed a calculation model in the LR phase of the knee joint during walking using a CT-FEM. Methods to evaluate the heteromorphic risk, mechanisms of transformation, prevention of knee osteoarthritis, and treatment may be developed using this model.

Evidence that reduction in volume protects in situ articular chondrocytes from mechanical impact

Chondrocytes, the resident cells in articular cartilage, carry the burden of producing and maintaining the extracellular matrix (ECM). However, as these cells have a low proliferative capacity and are not readily replaced, chondrocyte death due to extreme forces may contribute to the pathogenesis of osteoarthritis (OA) after injury or may inhibit healing after osteochondral transplantation, a restorative procedure for damaged cartilage that requires a series of mechanical impacts to insert the graft. Consequently, there is a need to understand what factors influence the vulnerability of in situ chondrocytes to mechanical trauma. To this end, the objective of this study was to investigate how altering cell volume by different means (hydrostatic pressure, uniaxial load, and osmotic challenge with and without inhibition of regulatory volume decrease) affects the vulnerability of in situ chondrocytes to extreme mechanical forces. Using a custom experimental platform enabling testing of viable and intact murine cartilage-on-bone explants, we established a strong correlation between chondrocyte volume and vulnerability to impact injury wherein reduced volume was protective. Moreover, we found that the volume-perturbing interventions did not affect cartilage ECM mechanical properties, suggesting that their effects on chondrocyte vulnerability occurred at the cellular level. The findings of this study offer new avenues for novel strategies aimed at preventing chondrocyte loss during osteochondral grafting or to halting the progression of cell death after a joint destabilizing injury.

Construction and analysis of finite element model of defected articular cartilage

In order to construct a finite element model of defected articular cartilage, the mechanical behavior and degeneration of articular cartilage after injury were studied. The simplified analytical models of normal and defected articular cartilage and finite element models were established, respectively. Firstly, the analytical solution model and finite element model of hollow defect were constructed by using the elasticity theory of multi-hollow medium. Then, the analytical results of each model were calculated and programmed. The software MATLAB was used for programming calculation. Finally, a finite element solid model of defected articular cartilage was established by using human femoral joint. The solid model was analyzed and calculated by magnetic resonance imaging (MRI). The results showed that when the radius of articular cartilage defect r = 0, i.e. there was no defect in articular cartilage, the internal pore pressure of the defect cartilage was the largest, and its pore pressure value was 27 × 10 3 pa. When the depth of articular cartilage defect r = 0, i.e. there was no defect in articular cartilage, the internal pore pressure of the defect cartilage was the largest, and its pore pressure value was 27.5 × 10 3 pa, and it gradually decreased towards the outer boundary of cartilage. When the surface of femoral cartilage began to defect, with the increase of the depth of the defect (from shallow to deep), the maximum pore pressure in the defect cartilage gradually decreased, but the speed is slowly. With the increase of the defect radius, that is, the area of the defect, the maximum pore pressure in the defect cartilage gradually decreased. When there was no defect of articular cartilage, the internal pore pressure of the defect cartilage was the maximum, the value of pore pressure was 8.7 × 10 3 pa, the value of pore pressure at the contact position of femoral cartilage was the largest, and it gradually decreased towards the outer boundary of cartilage. At the same location, the pore pressure of normal cartilage was significantly higher than that of defected cartilage. With the change of defect location, the pore pressure was reduced accordingly. Moreover, when the defect position moved from the outside to the inside, the corresponding pore pressure value was decreased gradually. To sum up, the finite element model of defected articular cartilage based on porous elasticity theory has better calculation ability, which proves the validity of the finite element software, and provides a strong basis for future model establishment and clinical treatment of articular cartilage.

Low-level cyclic tibial compression attenuates early osteoarthritis progression after joint injury in mice

OBJECTIVE

Mechanical loading and joint health have a unique relationship in osteoarthritis (OA) onset and progression. Although high load levels adversely affect cartilage health, exercise that involves low to moderate load levels can alleviate OA symptoms. We sought to isolate the beneficial effects of mechanical loading using controlled in vivo cyclic tibial compression. We hypothesized that low-level cyclic compression would attenuate post-traumatic OA symptoms induced by destabilization of the medial meniscus (DMM).

METHODS

10-week-old C57Bl/6J male mice underwent DMM surgery (n = 51). After a 5-day post-operative recovery period, we applied daily cyclic tibial compression to the operated limbs at low (1.0N or 2.0N) or moderate (4.5N) magnitudes for 2 or 6 weeks. At the completion of loading, we compared cartilage and peri-articular bone features of mice that underwent DMM and loading to mice that only underwent DMM.

RESULTS

Compared to DMM alone, low-level cyclic compression for 6 weeks attenuated DMM-induced cartilage degradation (OARSI score, P = 0.008, 95% confidence interval (CI): 0.093 to 0.949). Low-level loading attenuated DMM-induced osteophyte formation after 2 weeks (osteophyte size, P = 0.033, 95% CI: 3.27-114.45 μm), and moderate loading attenuated subchondral bone sclerosis after 6 weeks (tissue mineral density (TMD), P = 0.011, 95% CI: 6.32-70.60 mg HA/ccm) compared to limbs that only underwent DMM. Finally, loading had subtle beneficial effects on cartilage cellularity and aggrecanase activity after DMM.

CONCLUSION

Low-level cyclic compression is beneficial to joint health after an injury. Therefore, the progression of early OA may be attenuated by applying well controlled, low-level loading shortly following joint trauma.

Computational evaluation of altered biomechanics related to articular cartilage lesions observed in vivo

Chondral lesions provide a potential risk factor for development of osteoarthritis. Despite the variety of in vitro studies on lesion degeneration, in vivo studies that evaluate relation between lesion characteristics and the risk for the possible progression of OA are lacking. Here, we aimed to characterize different lesions and quantify biomechanical responses experienced by surrounding cartilage tissue. We generated computational knee joint models with nine chondral injuries based on clinical in vivo arthrographic computed tomography images. Finite element models with fibril-reinforced poro(visco)elastic cartilage and menisci were constructed to simulate physiological loading. Systematically, the lesions experienced increased peak values of maximum principal strain, maximum shear strain, and minimum principal strain in the surrounding chondral tissue (p < 0.01) compared with intact tissue. Depth, volume, and area of the lesion correlated with the maximum shear strain (p < 0.05, Spearman rank correlation coefficient ρ = 0.733-0.917). Depth and volume of the lesion correlated also with the maximum principal strain (p < 0.05, ρ = 0.767, and ρ = 0.717, respectively). However, the lesion area had non-significant correlation with this strain parameter (p = 0.06, ρ = 0.65). Potentially, the introduced approach could be developed for clinical evaluation of biomechanical risks of a chondral lesion and planning an intervention. Statement of Clinical Relevance: In this study, we computationally characterized different in vivo chondral lesions and evaluated their risk of cartilage degeneration. This information is vital in decision-making for intervention in order to prevent post-traumatic osteoarthritis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Experimental mechanical strain measurement of tissues

Strain, an important biomechanical factor, occurs at different scales from molecules and cells to tissues and organs in physiological conditions. Under mechanical strain, the strength of tissues and their micro- and nanocomponents, the structure, proliferation, differentiation and apoptosis of cells and even the cytokines expressed by cells probably shift. Thus, the measurement of mechanical strain (i.e., relative displacement or deformation) is critical to understand functional changes in tissues, and to elucidate basic relationships between mechanical loading and tissue response. In the last decades, a great number of methods have been developed and applied to measure the deformations and mechanical strains in tissues comprising bone, tendon, ligament, muscle and brain as well as blood vessels. In this article, we have reviewed the mechanical strain measurement from six aspects: electro-based, light-based, ultrasound-based, magnetic resonance-based and computed tomography-based techniques, and the texture correlation-based image processing method. The review may help solving the problems of experimental and mechanical strain measurement of tissues under different measurement environments.

The effect of constitutive representations and structural constituents of ligaments on knee joint mechanics

Ligaments provide stability to the human knee joint and play an essential role in restraining motion during daily activities. Compression-tension nonlinearity is a well-known characteristic of ligaments. Moreover, simpler material representations without this feature might give reasonable results because ligaments are primarily in tension during loading. However, the biomechanical role of different constitutive representations and their fibril-reinforced poroelastic properties is unknown. A numerical knee model which considers geometric and material nonlinearities of meniscus and cartilages was applied. Five different constitutive models for the ligaments (spring, elastic, hyperelastic, porohyperelastic, and fibril-reinforced porohyperelastic (FRPHE)) were implemented. Knee joint forces for the models with elastic, hyperelastic and porohyperelastic properties showed similar behavior throughout the stance, while the model with FRPHE properties exhibited lower joint forces during the last 50% of the stance phase. The model with ligaments as springs produced the lowest joint forces at this same stance phase. The results also showed that the fibril network contributed substantially to the knee joint forces, while the nonfibrillar matrix and fluid had small effects. Our results indicate that simpler material models of ligaments with similar properties in compression and tension can be used when the loading is directed primarily along the ligament axis in tension.