The effect of meniscal tears and resultant partial meniscectomies on the knee contact stresses: a finite element analysis

Knee osteoarthritis (OA) is believed to result from high levels of contact stresses on the articular cartilage and meniscus after meniscal damage. This study investigated the effect of meniscal tears and partial meniscectomies on the peak compressive and shear stresses in the human knee joint. An elaborate three-dimensional finite element model of knee joint including bones, articular cartilages, menisci and main ligaments was developed from computed tomography and magnetic resonance imaging images. This model was used to model four types of meniscal tears and their resultant partial meniscectomies and analysed under an axial 1150 N load at 0° flexion. Three different conditions were compared: a healthy knee joint, a knee joint with medial meniscal tears and a knee joint following partial meniscectomies. The numerical results showed that each meniscal tear and its resultant partial meniscectomy led to an increase in the peak compressive and shear stresses on the articular cartilages and meniscus in the medial knee compartment, especially for partial meniscectomy. Among the four types of meniscal tears, the oblique tear resulted in the highest values of the peak compressive and shear stresses. For the four partial meniscectomies, longitudinal meniscectomy led to the largest increase in these two stresses. The lateral compartment was minimally affected by all the simulations. The results of this study demonstrate meniscal tear and its resultant partial meniscectomy has a positive impact on the maintenance of high levels of contact stresses, which may improve the progression of knee OA, especially for partial meniscectomy. Surgeons should adopt a prudent strategy to preserve the greatest amount of meniscus possible.     

3D finite element model of meniscectomy: changes in joint contact behavior

The goal of this study is to quantify changes in knee joint contact behavior following varying degrees of the medial partial meniscectomy. A previously validated 3D finite element model was used to simulate 11 different meniscectomies. The accompanying changes in the contact pressure on the superior surface of the menisci and tibial plateau were quantified as was the axial strain in the menisci and articular cartilage. The percentage of medial meniscus removed was linearly correlated with maximum contact pressure, mean contact pressure, and contact area. The lateral hemi-joint was minimally affected by the simulated medial meniscectomies. The location of maximum strain and location of maximum contact pressure did not change with varying degrees of partial medial meniscectomy. When 60% of the medial meniscus was removed, contact pressures increased 65% on the remaining medial meniscus and 55% on the medial tibial plateau. These data will be helpful for assessing potential complications with the surgical treatment of meniscal tears. Additionally, these data provide insight into the role of mechanical loading in the etiology of post-meniscectomy osteoarthritis.     

The low permeability of healthy meniscus and labrum limit articular cartilage consolidation and maintain fluid load support in the knee and hip

The knee meniscus and hip labrum appear to be important for joint health, but the mechanisms by which these structures perform their functions are not fully understood. The fluid phase of articular cartilage provides compressive stiffness and aids in maintaining a low friction articulation. Healthy fibrocartilage, the tissue of meniscus and labrum, has a lower fluid permeability than articular cartilage. In this study we hypothesized that an important function of the knee meniscus and the hip labrum is to augment fluid retention in the articular cartilage of a mechanically loaded joint. Axisymmetric hyperporoelastic finite element models were analyzed for an idealized knee and an idealized hip. The results indicate that the meniscus maintained fluid pressure and inhibited fluid exudation in knee articular cartilage. Similar, but smaller, effects were seen with the labrum in the hip. Increasing the fibrocartilage permeability relative to that of articular cartilage gave a consolidation rate and loss of fluid load support comparable to that predicted by meniscectomy or labrectomy. The reduced articular cartilage fluid pressure that was calculated for the joint periphery is consistent with patterns of endochondral ossification and osteophyte formation in knee and hip osteoarthritis. High articular central strains and loss of fluid load support after meniscectomy could lead to fibrillation. An intact low-permeability fibrocartilage is important for limiting fluid exudation from articular cartilage in the hip and knee. This may be an important aspect of the role of fibrocartilage in protecting these joints from osteoarthritis.     

In-vitro measurement of static pressure distribution in synovial joints--Part I: Tibial surface of the knee

In this first part of a two-part paper, the results of measurement of static pressure distribution on the tibial surface of the knee are presented. Results with intact menisci have been obtained from 18 specimens. Eight of these specimens were the subject of further measurements following medial meniscectomy. The study has been carried out at various flexion angles of the knee with the joint subjected to a compressive force, with or without an initial passive relative displacement between the joint members. The results indicate that a significant fraction of the joint compressive load is transmitted through the menisci and that total meniscectomy causes a drastic alteration in the pressure distribution on the tibial surface. Clinical implications of these results, in terms of post-meniscectomy degenerative changes and mechanism of meniscal lesions, have been discussed.     

How the stiffness of meniscal attachments and meniscal material properties affect tibio-femoral contact pressure computed using a validated finite element model of the human knee joint

In an effort to prevent degeneration of articular cartilage associated with meniscectomies, both meniscal allografts and synthetic replacements are subjects of current interest and investigation. The objectives of the current study were to (1) determine whether a transversely isotropic, linearly elastic, homogeneous material model of the meniscal tissue is necessary to achieve a normal contact pressure distribution on the tibial plateau, (2) determine which material and boundary condition (attachments) parameters affect the contact pressure distribution most strongly, and (3) set tolerances on these parameters to restore the contact pressure distribution to within a specified error. To satisfy these objectives, a finite element model of the tibio-femoral joint of a human cadaveric knee (including both menisci) was used to study the contact pressure distribution on the tibial plateau. To validate the model, the contact pressure distribution on the tibial plateau was measured experimentally in the same knee used to create the model. Within physiologically reasonable bounds on five material parameters and four attachment parameters associated with a meniscal replacement, an optimization was performed under 1200 N of compressive load on the set of nine parameters to minimize the difference between the experimental and model results. The error between the experimental and model contact variables was minimized to 5.4%. The contact pressure distribution of the tibial plateau was sensitive to the circumferential modulus, axial/radial modulus, and horn stiffness, but relatively insensitive to the remaining six parameters. Consequently, both the circumferential and axial/radial moduli are important determinants of the contact pressure distribution, and hence should be matched in the design and/or selection of meniscal replacements. In addition, during surgical implantation of a meniscal replacement, the horns should be attached with high stiffness bone plugs, and the attachments of the transverse ligament and deep medial collateral ligament should be restored to minimize changes in the contact pressure distribution, and thereby possibly prevent the degradation of articular cartilage.     

A three-dimensional finite element analysis of the combined behavior of ligaments and menisci in the healthy human knee joint

We present here a three-dimensional FE model of the healthy human knee that included the main structures of the joint: bones, all the relevant ligaments and patellar tendon, menisci and articular cartilages. Bones were considered to be rigid, articular cartilage and menisci linearly elastic, isotropic and homogeneous and ligaments hyperelastic and transversely isotropic. Initial strains on the ligaments and patellar tendon were also considered. This model was validated using experimental and numerical results obtained by other authors. Our main goal was to analyze the combined role of menisci and ligaments in load transmission and stability of the human knee. The results obtained reproduce the complex, nonuniform stress and strain fields that occur in the biological soft tissues involved and the kinematics of the human knee joint under a physiological external load.     

Partial medial meniscectomy and rotational differences at the knee during walking

Loss of meniscal function due to injury or partial meniscectomy is common and represents a significant risk factor for premature osteoarthritis. The menisci can influence the transverse plane movements (anterior–posterior (AP) translation and internal–external (IE) rotation) of the knee during walking. While walking is the most frequent activity of daily living, the kinematic differences at the knee during walking associated with the meniscal injury are not well understood. This study examined the influence of partial medial meniscectomy (PMM) on the kinematics and kinetics of the knee during the stance phase of gait by testing the differences in anterior–posterior translation, internal–external rotation, knee flexion range of movement, peak flexion/extension moments, and adduction moments between the PMM and healthy contralateral limbs. Ten patients (45±9 years old, height 1.75±0.06 m, weight 76.7±13.5 kg) who had undergone partial medial meniscectomy (33±100 months post-op) in one limb with a healthy contralateral limb were tested during normal walking. The contralateral limb was compared to a matched control group and no differences were found. The primary kinematic difference was a significantly greater external rotation (3.2°) of the tibia that existed through stance phase, with 8 of 10 subjects demonstrating the same pattern. The PMM subjects also exhibited significantly lower peak flexion and extension moments in their PMM limbs. The altered rotational position found likely results in changes of tibio-femoral contact during walking and could cause the type of degenerative changes found in the articular cartilage following meniscal injury.     

Identification of link proteins and a 116,000-Dalton matrix protein in canine meniscus

The menisci are collagen-rich, fibrocartilagenous structures which are important in protecting the articular cartilage of the knee from some of the impact of weight-bearing. Meniscal proteoglycans have been studied in several mammalian species, including the dog, but very little is known about the noncollagenous proteins of the menisci. In the present study, 4 M guanidinium chloride extracts of meniscal cartilage from normal adult mongrel dogs were studied, and several noncollagenous proteins, including the link proteins and a 116,000-Da subunit protein, which we have recently described in articular cartilage, were found in meniscal cartilage. The 116,000-Da subunit protein represents 3.8% of the total protein extracted from meniscal cartilage. The link proteins sedimented in the bottom of an associative cesium chloride density gradient, where high-buoyant-density proteoglycans sediment.     

Changes in articular cartilage mechanics with meniscectomy: A novel image-based modeling approach and comparison to patterns of OA

Meniscectomy is a significant risk factor for osteoarthritis, involving altered cell synthesis, central fibrillation, and peripheral osteophyte formation. Though changes in articular cartilage contact pressure are known, changes in tissue-level mechanical parameters within articular cartilage are not well understood. Recent imaging research has revealed the effects of meniscectomy on the time-dependent deformation of physiologically loaded articular cartilage. To determine tissue-level cartilage mechanics that underlie observed deformation, a novel finite element modeling approach using imaging data and a contacting indenter boundary condition was developed. The indenter method reproduces observed articular surface deformation and avoids assumptions about tangential stretching. Comparison of results from an indenter model with a traditional femur-tibia model verified the method, giving errors in displacement, solid and fluid stress, and strain below 1% (RMS) and 7% (max.) of the absolute maximum of the parameters of interest. Indenter finite element models using real joint image data showed increased fluid pressure, fluid exudation, loss of fluid load support, and increased tensile strains centrally on the tibial condyle after meniscectomy-patterns corresponding to clinical observations of cartilage matrix damage and fibrillation. Peripherally there was decreased consolidation, which corresponds to reduced contact and fluid pressure in this analysis. Clinically, these areas have exhibited advance of the subchondral growth front, biological destruction of the cartilage matrix, cartilage thinning, and eventual replacement of the cartilage via endochondral ossification. Characterizing the changes in cartilage mechanics with meniscectomy and correspondence with observed tissue-level effects may help elucidate the etiology of joint-level degradation seen in osteoarthritis.     

Fragmentation of decorin, biglycan, lumican and keratocan is elevated in degenerate human meniscus, knee and hip articular cartilages compared with age-matched macroscopically normal and control tissues

Introduction

The small leucine-rich proteoglycans (SLRPs) modulate tissue organization, cellular proliferation, matrix adhesion, growth factor and cytokine responses, and sterically protect the surface of collagen type I and II fibrils from proteolysis. Catabolism of SLRPs has important consequences for the integrity of articular cartilage and meniscus by interfering with their tissue homeostatic functions.

Methods

SLRPs were dissociatively extracted from articular cartilage from total knee and hip replacements, menisci from total knee replacements, macroscopically normal and fibrillated knee articular cartilage from mature age-matched donors, and normal young articular cartilage. The tissue extracts were digested with chondroitinase ABC and keratanase-I before identification of SLRP core protein species by Western blotting using antibodies to the carboxyl-termini of the SLRPs.

Results

Multiple core-protein species were detected for all of the SLRPs (except fibromodulin) in the degenerate osteoarthritic articular cartilage and menisci. Fibromodulin had markedly less fragments detected with the carboxyl-terminal antibody compared with other SLRPs. There were fewer SLRP catabolites in osteoarthritic hip than in knee articular cartilage. Fragmentation of all SLRPs in normal age-matched, nonfibrillated knee articular cartilage was less than in fibrillated articular cartilage from the same knee joint or total knee replacement articular cartilage specimens of similar age. There was little fragmentation of SLRPs in normal control knee articular cartilage. Only decorin exhibited a consistent increase in fragmentation in menisci in association with osteoarthritis. There were no fragments of decorin, biglycan, lumican, or keratocan that were unique to any tissue. A single fibromodulin fragment was detected in osteoarthritic articular cartilage but not meniscus. All SLRPs showed a modest age-related increase in fragmentation in knee articular and meniscal cartilage but not in other tissues.

Conclusion

Enhanced fragmentation of SLRPs is evident in degenerate articular cartilage and meniscus. Specific decorin and fibromodulin core protein fragments in degenerate meniscus and/or human articular cartilage may be of value as biomarkers of disease. Once the enzymes responsible for their generation have been identified, further research may identify them as therapeutic targets.     

Material and functional properties of articular cartilage and patellofemoral contact mechanics in an experimental model of osteoarthritis

The purposes of this study were to determine the in situ functional and material properties of articular cartilage in an experimental model of joint injury, and to quantify the corresponding in situ joint contact mechanics. Experiments were performed in the anterior cruciate ligament (ACL) transected knee of the cat and the corresponding, intact contralateral knee, 16 weeks following intervention. Cartilage thickness, stiffness, effective Young’s modulus, and permeability were measured and derived from six locations of the knee. The total contact area and peak pressures in the patellofemoral joint were obtained in situ using Fuji Pressensor film, and comparisons between experimental and contralateral joint were made for corresponding loading conditions. Total joint contact area and peak pressure were increased and decreased significantly (=0.01), respectively, in the experimental compared to the contralateral joint. Articular cartilage thickness and stiffness were increased and decreased significantly (=0.01), respectively, in the experimental compared to the contralateral joint in the four femoral and patellar test locations. Articular cartilage material properties (effective Young’s modulus and permeability) were the same in the ACL-transected and intact joints. These results demonstrate for the first time the effect of changes in articular cartilage properties on the load transmission across a joint. They further demonstrate a substantial change in the joint contact mechanics within 16 weeks of ACL transection. The results were corroborated by theoretical analysis of the contact mechanics in the intact and ACL-transected knee using biphasic contact analysis and direct input of cartilage properties and joint surface geometry from the experimental animals. We conclude that the joint contact mechanics in the ACL-transected cat change within 16 weeks of experimental intervention.     

Deformation of articular cartilage during static loading of a knee joint – Experimental and finite element analysis

Novel conical beam CT-scanners offer high resolution imaging of knee structures with i.a. contrast media, even under weight bearing. With this new technology, we aimed to determine cartilage strains and meniscal movement in a human knee at 0, 1, 5, and 30 min of standing and compare them to the subject-specific 3D finite element (FE) model. The FE model of the volunteer?s knee, based on the geometry obtained from magnetic resonance images, was created to simulate the creep. The effects of collagen fibril network stiffness, nonfibrillar matrix modulus, permeability and fluid flow boundary conditions on the creep response in cartilage were investigated. In the experiment, 80% of the maximum strain in cartilage developed immediately, after which the cartilage continued to deform slowly until the 30 min time point. Cartilage strains and meniscus movement obtained from the FE model matched adequately with the experimentally measured values. Reducing the fibril network stiffness increased the mean strains substantially, while the creep rate was primarily influenced by an increase in the nonfibrillar matrix modulus. Changing the initial permeability and preventing fluid flow through noncontacting surfaces had a negligible effect on cartilage strains. The present results improve understanding of the mechanisms controlling articular cartilage strains and meniscal movements in a knee joint under physiological static loading. Ultimately a validated model could be used as a noninvasive diagnostic tool to locate cartilage areas at risk for degeneration.     

Effect of meniscus replacement fixation technique on restoration of knee contact mechanics and stability

The menisci are important biomechanical components of the knee. We developed and validated a finite element model of meniscal replacement to assess the effect of surgical fixation technique on contact behavior and knee stability. The geometry of femoral and tibial articular cartilage and menisci was segmented from magnetic resonance images of a normal cadaver knee using MIMICS (Materialise, Leuven, Belgium). A finite element mesh was generated using HyperWorks (Altair Inc, Santa Ana, CA). A finite element solver (Abaqus v6.9, Simulia, Providence, RI) was used to compute contact area and stresses under axial loading and to assess stability (reaction force generated during anteroposterior translation of the femur). The natural and surgical attachments of the meniscal horns and peripheral rim were simulated using springs. After total meniscectomy, femoral contact area decreased by 26% with a concomitant increase in average contact stresses (36%) and peak contact stresses (33%). Replacing the meniscus without suturing the horns did little to restore femoral contact area. Suturing the horns increased contact area and reduced peak contact stresses. Increasing suture stiffness correlated with increased meniscal contact stresses as a greater proportion of tibiofemoral load was transferred to the meniscus. A small incremental benefit was seen of simulated bone plug fixation over the suture construct with the highest stiffness (50 N/mm). Suturing the rim did little to change contact conditions. The nominal anteroposterior stiffness reduced by 3.1 N/mm after meniscectomy. In contrast to contact area and stress, stiffness of the horn fixation sutures had a smaller effect on anteroposterior stability. On the other hand suturing the rim of the meniscus affected anteroposterior stability to a much larger degree. This model emphasizes the importance of the meniscus in knee biomechanics. Appropriate meniscal replacement fixation techniques are likely to be critical to the clinical success of meniscal replacement. While contact conditions are mainly sensitive to meniscus horn fixation, the stability of the knee under anteroposterior shear loads appeared to be more sensitive to meniscal rim fixation. This model may also be useful in predicting the effect of biomaterial mechanical properties and meniscal replacement shape on knee contact conditions.     

Application of a semi-automatic cartilage segmentation method for biomechanical modeling of the knee joint

Manual segmentation of articular cartilage from knee joint 3D magnetic resonance images (MRI) is a time consuming and laborious task. Thus, automatic methods are needed for faster and reproducible segmentations. In the present study, we developed a semi-automatic segmentation method based on radial intensity profiles to generate 3D geometries of knee joint cartilage which were then used in computational biomechanical models of the knee joint. Six healthy volunteers were imaged with a 3T MRI device and their knee cartilages were segmented both manually and semi-automatically. The values of cartilage thicknesses and volumes produced by these two methods were compared. Furthermore, the influences of possible geometrical differences on cartilage stresses and strains in the knee were evaluated with finite element modeling. The semi-automatic segmentation and 3D geometry construction of one knee joint (menisci, femoral and tibial cartilages) was approximately two times faster than with manual segmentation. Differences in cartilage thicknesses, volumes, contact pressures, stresses, and strains between segmentation methods in femoral and tibial cartilage were mostly insignificant (p > 0.05) and random, i.e. there were no systematic differences between the methods. In conclusion, the devised semi-automatic segmentation method is a quick and accurate way to determine cartilage geometries; it may become a valuable tool for biomechanical modeling applications with large patient groups.     

Can altered neuromuscular coordination restore soft tissue loading patterns in anterior cruciate ligament and menisci deficient knees during walking?

Injuries to the anterior cruciate ligament (ACL) and menisci commonly lead to early onset osteoarthritis. Treatments that can restore normative cartilage loading patterns may mitigate the risk of osteoarthritis, though it is unclear whether such a goal is achievable through conservative rehabilitation. We used musculoskeletal simulation to predict cartilage and ligament loading patterns during walking in intact, ACL deficient, menisci deficient, and ACL-menisci deficient knees. Stochastic simulations with varying coordination strategies were then used to test whether neuromuscular control could be modulated to restore normative knee mechanics in the pathologic conditions. During early stance, a 3 mm increase in anterior tibial translation was predicted in the ACL deficient knee. Mean cartilage contact pressure increased by 18% and 24% on the medial and lateral plateaus, respectively, in the menisci deficient knee. Variations in neuromuscular coordination were insufficient to restore normative cartilage contact patterns in either the ACL or menisci deficient knees. Elevated cartilage contact pressures in the pathologic knees were observed in regions where cartilage wear patterns have previously been reported. These results suggest that altered cartilage tissue loading during gait may contribute to region-specific degeneration patterns, and that varying neuromuscular coordination in isolation is unlikely to restore normative knee mechanics.     

The role of viscoelasticity of collagen fibers in articular cartilage: theory and numerical formulation

The relative importance of fluid-dependent and fluid-independent transient mechanical behavior in articular cartilage was examined for tensile and unconfined compression testing using a fibril reinforced model. The collagen matrix of articular cartilage was modeled as viscoelastic using a quasi-linear viscoelastic formulation with strain-dependent elastic modulus, while the proteoglycan matrix was considered as linearly elastic. The collagen viscoelastic properties were obtained by fitting experimental data from a tensile test. These properties were used to investigate unconfined compression testing, and the sensitivity of the properties was also explored. It was predicted that the stress relaxation observed in tensile tests was not caused by fluid pressurization at the macroscopic level. A multi-step tensile stress relaxation test could be approximated using a hereditary integral in which the elastic fibrillar modulus was taken to be a linear function of the fibrillar strain. Applying the same formulation to the radial fibers in unconfined compression, stress relaxation could not be simulated if fluid pressurization were absent. Collagen viscoelasticity was found to slightly weaken fluid pressurization in unconfined compression, and this effect was relatively more significant at moderate strain rates. Therefore, collagen viscoelasticity appears to play an import role in articular cartilage in tensile testing, while fluid pressurization dominates the transient mechanical behavior in compression. Collagen viscoelasticity plays a minor role in the mechanical response of cartilage in unconfined compression if significant fluid flow is present.     

The effect of kinematic and kinetic changes on meniscal strains during gait

The menisci play an important role in load distribution, load bearing, joint stability, lubrication, and proprioception. Partial meniscectomy has been shown to result in changes in the kinematics and kinetics at the knee during gait that can lead to progressive meniscal degeneration. This study examined changes in the strains within the menisci associated with kinematic and kinetic changes during the gait cycle. The gait changes considered were a 5 deg shift toward external rotation of the tibia with respect to the femur and an increased medial-lateral load ratio representing an increased adduction moment. A finite element model of the knee was developed and tested using a cadaveric specimen. The cadaver was placed in positions representing heel-strike and midstance of the normal gait, and magnetic resonance images were taken. Comparisons of the model predictions to boundaries digitized from images acquired in the loaded states were within the errors produced by a 1 pixel shift of either meniscus. The finite element model predicted that an increased adduction moment caused increased strains of both the anterior and posterior horns of the medial meniscus. The lateral meniscus exhibited much lower strains and had minimal changes under the various loading conditions. The external tibial rotational change resulted in a 20% decrease in the strains in the posterior medial horn and increased strains in the anterior medial horn. The results of this study suggest that the shift toward external tibial rotation seen clinically after partial medial meniscectomy is not likely to cause subsequent degenerative medial meniscal damage, but the consequence of this kinematic shift on the pathogenesis of osteoarthritis following meniscectomy requires further consideration.     

Comparative assessment of articular cartilage and synovial membrane in experimental haemarthrosis

The changes in articular cartilage and synovial membrane of the knee joints were studied in two groups of rabbits and Wistar rats with experimental haemarthrosis, electron microscopically. Hamarthrosis was produced in group 1 by a single autologous blood injection, in group 2 by intraarticular fracture of the femoral condyles. Samples were taken from the intact articular cartilage, the menisci and the infrapatellar portion of the synovial membrane 12 h to 20 days after intervention. Blood resorption occurs only in the synovial membrane. Fragmentation of erythrocytes and erythrocytophagy by synovial macrophages is documented. The different stages of intracellular digestion of erythrocyte fragments are traced down. Synovial fibroblasts do not participate in erythrocytophagy, although they disclose morphological signs of enhanced functional activity. The findings show changes in the matrix and chondrocytes within the articular cartilage and menisci, and presence of free erythrocytes and lipoprotein complexes amidst the collagen fibres of the matrix. The chondrocytes are poor in cell organelles, while the intracytoplasmic filaments, lipid droplets and glycogen granules are augmented in number. There is no evidence of erythrocytophagy by cartilage cells. On single blood injection in the joint, the ensuing changes are reversible, and the normal synovial membrane structure is restored much quicker than the articular cartilage.     

Pathways of load-induced cartilage damage causing cartilage degeneration in the knee after meniscectomy

Results of both clinical and animal studies show that meniscectomy often leads to osteoarthritic degenerative changes in articular cartilage. It is generally assumed that this process of cartilage degeneration is due to changes in mechanical loading after meniscectomy. It is, however, not known why and where this cartilage degeneration starts. Load induced cartilage damage is characterized as either type (1)--damage without disruption of the underlying bone or calcified cartilage layer--or type (2), subchondral fracture with or without damage to the overlying cartilage. We asked the question whether cartilage degeneration after meniscectomy is likely to be initiated by type (1) and/or type (2) cartilage damage. To investigate that we applied an axisymmetric biphasic finite element analysis model of the knee joint. In this model the articular cartilage layers of the tibial and the femoral condyles, the meniscus and the bone underlying the articular cartilage of the tibia plateau were included. The model was validated with data from clinical studies, in which the effects of meniscectomy on contact areas and pressures were measured. It was found that both the maximal values and the distributions of the shear stress in the articular cartilage changed after meniscectomy, and that these changes could lead to both type (1) and type (2) cartilage damage. Hence it likely that the cartilage degeneration seen after meniscectomy is initiated by both type (1) and type (2) cartilage damage.     

A finite element model of the human knee joint for the study of tibio-femoral contact

As a step towards developing a finite element model of the knee that can be used to study how the variables associated with a meniscal replacement affect tibio-femoral contact, the goals of this study were 1) to develop a geometrically accurate three-dimensional solid model of the knee joint with special attention given to the menisci and articular cartilage, 2) to determine to what extent bony deformations affect contact behavior, and 3) to determine whether constraining rotations other than flexion/extension affects the contact behavior of the joint during compressive loading. The model included both the cortical and trabecular bone of the femur and tibia, articular cartilage of the femoral condyles and tibial plateau, both the medial and lateral menisci with their horn attachments, the transverse ligament, the anterior cruciate ligament, and the medial collateral ligament. The solid models for the menisci and articular cartilage were created from surface scans provided by a noncontacting, laser-based, three-dimensional coordinate digitizing system with an root mean squared error (RMSE) of less than 8 microns. Solid models of both the tibia and femur were created from CT images, except for the most proximal surface of the tibia and most distal surface of the femur which were created with the three-dimensional coordinate digitizing system. The constitutive relation of the menisci treated the tissue as transversely isotropic and linearly elastic. Under the application of an 800 N compressive load at 0 degrees of flexion, six contact variables in each compartment (ie., medial and lateral) were computed including maximum pressure, mean pressure, contact area, total contact force, and coordinates of the center of pressure. Convergence of the finite element solution was studied using three mesh sizes ranging from an average element size of 5 mm by 5 mm to 1 mm by 1 mm. The solution was considered converged for an average element size of 2 mm by 2 mm. Using this mesh size, finite element solutions for rigid versus deformable bones indicated that none of the contact variables changed by more than 2% when the femur and tibia were treated as rigid. However, differences in contact variables as large as 19% occurred when rotations other than flexion/extension were constrained. The largest difference was in the maximum pressure. Among the principal conclusions of the study are that accurate finite element solutions of tibio-femoral contact behavior can be obtained by treating the bones as rigid. However, unrealistic constraints on rotations other than flexion/extension can result in relatively large errors in contact variables.