Time-dependent behavior of cartilage surrounding a metal implant for full-thickness cartilage defects of various sizes: a finite element study

Recently, physiological and biomechanical studies on animal models with metal implants filling full-thickness cartilage defects have resulted in good clinical outcomes. The knowledge of the time-dependent macroscopic behavior of cartilage surrounding the metal implant is essential for understanding the joint function after treating such defects. We developed a model to investigate the in vivo time-dependent behavior of the tibiofemoral cartilages surrounding the metal implant, when the joint is subjected to an axial load for various defect sizes. Results show that time-dependent effects on cartilage behavior are significant, and can be simulated. These effects should be considered when evaluating the results from an implant. In particular, the depth into the cartilage where an implant is positioned and the mechanical sealing due to solidification of the poroelastic material need a time aspect. We found the maximal deformations, contact pressures and contact forces in the joint with time for the implant positioned in flush and sunk 0.3 mm into the cartilage. The latter position gives the better joint performance. The results after 60 s may be treated as the primary results, reflecting the effect of accumulation in the joint due to repeated short-time loadings. The wedge-shaped implant showed beneficial in providing mechanical sealing of cartilages surrounding the implant with time.     

Cartilage thickness distribution affects computational model predictions of cervical spine facet contact parameters

With motion-sparing disk replacement implants gaining popularity as an alternative to anterior cervical discectomy and fusion (ACDF) for the treatment of certain spinal degenerative disorders, recent laboratory investigations have studied the effects of disk replacement and implant design on spinal kinematics and kinetics. Particularly relevant to cervical disk replacement implant design are any postoperative changes in solid stresses or contact conditions in the articular cartilage of the posterior facets, which are hypothesized to lead to adjacent-level degeneration. Such changes are commonly investigated using finite element methods, but significant simplification of the articular geometry is generally employed. The impact of such geometric representations has not been thoroughly investigated. In order to assess the effects of different models of cartilage geometry on load transfer and contact pressures in the lower cervical spine, a finite element model was generated using cadaver-based computed tomography imagery. Mesh resolution was varied in order to establish model convergence, and cadaveric testing was undertaken to validate model predictions. The validated model was altered to include four different geometric representations of the articular cartilage. Model predictions indicate that the two most common representations of articular cartilage geometry result in significant reductions in the predictive accuracy of the models. The two anatomically based geometric models exhibited less computational artifact, and relatively minor differences between them indicate that contact condition predictions of spatially varying thickness models are robust to anatomic variations in cartilage thickness and articular curvature. The results of this work indicate that finite element modeling efforts in the lower cervical spine should include anatomically based and spatially varying articular cartilage thickness models. Failure to do so may result in loss of fidelity of model predictions relevant to investigations of physiological import.     

An articular cartilage contact model based on real surface geometry

Abnormal, excessive stresses acting on articular joint surfaces are speculated to be one of the causes for joint degeneration. However, articular surface stresses have not been studied systematically, since it is technically difficult to measure in vivo contact areas and pressures in dynamic situations. Therefore, we implemented a numerical model of articular surface contact using accurate surface geometries. The model was developed for the cat patellofemoral joint. We demonstrated that small misalignments of the patella relative to the femur change the joint contact mechanics substantially for a given external load. These results suggest that misalignment might be studied as one of the factors causing articular cartilage disorder and joint degeneration.     

Modeling of constrained articular cartilage growth in an intact knee with focal knee resurfacing metal implant

The purpose of the present study was to develop a model to simulate the articular cartilage growth in an intact knee model with a metal implant replacing a degenerated portion of the femoral cartilage. The human knee joint was approximated with a simplified axisymmetric shape of the femoral condyle along with the cartilage, meniscus and bones. Two individually growing constituents (proteoglycans and collagen) bound to solid matrix were considered in the solid phase of the cartilage. The cartilage behavior was modeled with a nonlinear biphasic porohyperelastic material model, and meniscus with a transversely isotropic linear biphasic poroelastic material model. Two criteria (permeation and shear), both driven by mechanical loading, were considered to trigger the growth in the solid constituents. Mechanical loading with sixty heavy cycles was considered to represent daily walking activity. The growth algorithm was implemented for 90 days after implantation. The results from simulations show that both cartilage layers were more stimulated near the implant which lead to more growth of the cartilage near the defect. The method developed in the present work could be a powerful technique if more accurate material data and growth laws were available.     

Effect of superficial collagen patterns and fibrillation of femoral articular cartilage on knee joint mechanics-a 3D finite element analysis

Collagen fibrils of articular cartilage have specific depth-dependent orientations and the fibrils bend in the cartilage surface to exhibit split-lines. Fibrillation of superficial collagen takes place in osteoarthritis. We aimed to investigate the effect of superficial collagen fibril patterns and collagen fibrillation of cartilage on stresses and strains within a knee joint. A 3D finite element model of a knee joint with cartilage and menisci was constructed based on magnetic resonance imaging. The fibril-reinforced poroviscoelastic material properties with depth-dependent collagen orientations and split-line patterns were included in the model. The effects of joint loading on stresses and strains in cartilage with various split-line patterns and medial collagen fibrillation were simulated under axial impact loading of 1000 N. In the model, the collagen fibrils resisted strains along the split-line directions. This increased also stresses along the split-lines. On the contrary, contact and pore pressures were not affected by split-line patterns. Simulated medial osteoarthritis increased tissue strains in both medial and lateral femoral condyles, and contact and pore pressures in the lateral femoral condyle. This study highlights the importance of the collagen fibril organization, especially that indicated by split-line patterns, for the weight-bearing properties of articular cartilage. Osteoarthritic changes of cartilage in the medial femoral condyle created a possible failure point in the lateral femoral condyle. This study provides further evidence on the importance of the collagen fibril organization for the optimal function of articular cartilage.     

Load sharing between solid and fluid phases in articular cartilage: I--Experimental determination of in situ mechanical conditions in a porcine knee.

The in situ mechanical conditions of cartilage in the articulated knee were quantified during joint loading. Six porcine knees were subjected to a 445 N compressive load while cartilage deformations and contact pressures were measured. From roentgenograms, cartilage thickness before and during loading allowed the calculation of tissue deformation on the lateral femoral condyle at different times during the loading process. Contact pressures on the articular surface were measured with miniature fiber-optic pressure transducers. Results showed that the medial side of the lateral femoral condyle had higher contact pressures, as well as deformations. To begin to correlate the pressures and resulting deformations, the intrinsic material properties of the cartilage on the lateral condyle were obtained from indentation tests. Data from four normal control specimens indicated that the aggregate modulus of the medial side was significantly higher than in other areas of the condyle. These experimental measures of the in situ mechanical conditions of articular cartilage can be combined with theoretical modeling to obtain valuable information about the relative contributions of the solid and fluid phases to supporting the applied load on the cartilage surface (see Part II).     

Influence of the stiffness of bone defect implants on the mechanical conditions at the interface--a finite element analysis with contact

The study focused on the influence of the implant material stiffness on stress distribution and micromotion at the interface of bone defect implants. We hypothesized that a low-stiffness implant with a modulus closer to that of the surrounding trabecular bone would yield a more homogeneous stress distribution and less micromotion at the interface with the bony bed. To prove this hypothesis we generated a three-dimensional, non-linear, anisotropic finite element (FE) model. The FE model corresponded to a previously developed animal model in sheep. A prismatic implant filled a standardized defect in the load-bearing area of the trabecular bone beneath the tibial plateau. The interface was described by face-to-face contact elements, which allow press fits, friction, sliding, and gapping. We assumed a physiological load condition and calculated contact pressures, shear stresses, and shear movements at the interface for two implants of different stiffness (titanium: E=110GPa; composite: E=2.2GPa). The FE model showed that the stress distribution was more homogeneous for the low-stiffness implant. The maximum pressure for the composite implant (2.1 MPa) was lower than for the titanium implant (5.6 MPa). Contrary to our hypothesis, we found more micromotion for the composite (up to 6 microm) than for the titanium implant (up to 4.5 microm). However, for both implants peak stresses and micromotion were in a range that predicts adequate conditions for the osseointegration. This was confirmed by the histological results from the animal studies.     

Effects of inserting a pressensor film into articular joints on the actual contact mechanics.

Fuji film has been widely used in studies aimed at obtaining the contact mechanics of articular joints. Once sealed for practical use in biological joints, Fuji Pressensor film has a total effective thickness of 0.30 mm, which is comparable to the cartilage thickness in the joints of many small animals. The average effective elastic modulus of Fuji film is approximately 100 MPa in compression, which is larger by a factor of 100-300 compared to that of normal articular cartilage. Therefore, inserting a Pressensor film into an articular joint will change the contact mechanics of the joint. The measurement precision of the Pressensor film has been determined systematically; however, the changes in contact mechanics associated with inserting the film into joints have not been investigated. This study was aimed at quantifying the changes in the contact mechanics associated with inserting sealed Fuji Pressensor film into joints. Spherical and cylindrical articular joint contact mechanics with and without Pressensor film and for varying degrees of surface congruency were analyzed and compared by using finite element models. The Pressensor film was taken as linearly elastic and the cartilage was assumed to be biphasic, composed of a linear elastic solid phase and an inviscid fluid phase. The present analyses showed that measurements of the joint contact pressures with Fuji Pressensor film will change the maximum true contact pressures by 10-26 percent depending on the loading, geometry of the joints, and the mechanical properties of cartilage. Considering this effect plus the measurement precision of the film (approximately 10 percent), the measured joint contact pressures in a joint may contain errors as large as 14-28 percent.     

Potential for thermal damage to articular cartilage by PMMA reconstruction of a bone cavity following tumor excision: A finite element study

Benign, giant cell tumors are often treated by intralesional excision and reconstruction with polymethylmethacrylate (PMMA) bone cement. The exothermic reaction of the in-situ polymerizing PMMA is believed to beneficially kill remaining tumor cells. However, at issue is the extent of this necrotic effect into the surrounding normal bone and the adjacent articular cartilage. Finite element analysis (ABAQUS 6.4-1) was used to determine the extent of possible thermal necrosis around prismatically shaped, PMMA implants (8–24 cc in volume), placed into a peripheral, sagittally symmetric, metaphyseal defect in the proximal tibia. Temperature/exposure time conditions indicating necrotic potential during the exotherm of the polymerizing bone cement were found in regions of the cancellous bone within 3 mm of the superior surface of the PMMA implant. If less than 3 mm of cancellous bone existed between the PMMA implant and the subchondral bone layer, regions of the subchondral bone were also exposed to thermally necrotic conditions. However, as long as there were at least 2 mm of uniform subchondral bone above the PMMA implant, the necrotic regions did not extend into the overlying articular cartilage. This was the case even when the PMMA was in direct contact with the subchondral bone. If the subchondral bone is not of sufficient thickness, or is not continuous, then care should be taken to protect the articular cartilage from thermal damage as a result of the reconstruction of the tumor cavity with PMMA bone cement.     

Articular cartilage reconstruction using xenogeneic epiphyses slices

Reconstruction of articular cartilage defects using adult osteochondral allografts is an established clinical procedure, whose principal drawback is lack of lateral integration of the grafts to the surrounding tissue. Autologous chondrocytes transplantation is a sophisticated technique requiring cell culture and a staged operation. Its main draw back is the lack of mechanical strength early on. This study was conducted in order to evaluate the possibility of using embryonal epiphyses as a cartilage reconstruction tissue. A xenogeneic human to rabbit sub-acute osteochondral defect model was designed to evaluate the possibility of allogeneic implantation in humans. The following procedures were perfomed (n = 5): transplantation of 1. live epiphyses 2. live epiphyses with autogeneic periosteum 3. de-vitalized epiphyses and 4. devitalized epiphyses with autogeneic articular chondrocytes. A fifth control group did not receive any implant. Animals in groups 1 and 2 had a viable reconstruction of the articular surface with little evidence of rejection and without pannus formation. Animals in groups 3 and 4 became severely arthritic and the graft was resorbed. Nitric oxide synthase accumulation was reduced in group 1 and 2 as compared to groups 3, 4, and 5, indicating a joint preserving function of the epiphyseal grafts. Epiphyseal grafts appear to be a feasible procedure for reconstruction of articular cartilage defects even in a xenogeneic model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Osteochondral reconstruction of a non-weight-bearing joint using a high-density porous polyethylene implant

Currently, there is no reliable reconstructive modality allowing anatomic resurfacing of traumatic digital osteochondral articular defects. The purpose of the present study is to demonstrate the utility of Medpor, a high-density porous polyethylene (HDPP) scaffold biomaterial that can (1) be readily contoured to fit any joint defect, (2) permit stable internal fixation, and (3) permit osteocyte and chondrocyte ingrowth and subsequent articular cartilage resurfacing necessary to restore joint congruity. HDPP has gained wide acceptance for use in craniofacial and skeletal reconstruction and augmentation. An avian non-weight-bearing joint model was designed to study the role of the HDPP implant in small joint reconstruction. An osteochondral defect was created with a 5-mm circular punch in the humeral articular surface of both glenohumeral joints of 32 adult White Leghorn chickens. In each animal, one defect was press-fitted with a correspondingly sized HDPP implant (HDPP implant group); the contralateral defect was filled with the original osteochondral plug (isograft group) or left unrepaired (control group). At 2 weeks, and 1, 3, and 6 months,joints from each group were harvested and evaluated. Over the 6-month study period, joints in the control group demonstrated healing with dense collagenous scar tissue leaving residual defects at the articular surfaces and significant degenerative disease of the glenohumeral joints radiographically. Joints in the isograft group demonstrated near-complete resorption with some preservation of the cartilaginous cap but overall depression of the articular surface and significant degenerative joint disease. Joints in the HDPP implant group demonstrated stable fixation by highly mineralized bony trabecular ingrowth, preservation of the articular contour of the humeral head, and no evidence of significant degenerative joint disease. These findings indicate a potential role for this high-density porous polyethylene implant in the reconstruction of small joint articular and osseous defects.     

Mechanobiology of cartilage: how do internal and external stresses affect mechanochemical transduction and elastic energy storage?

 Articular cartilage is a multilayered structure that lines the surfaces of all articulating joints. It contains cells, collagen fibrils, and proteoglycans with compositions that vary from the surface layer to the layer in contact with bone. It is composed of several zones that vary in structure, composition, and mechanical properties. In this paper we analyze the structure of the extracellular matrix found in articular cartilage in an effort to relate it to the ability of cartilage to store, transmit, and dissipate mechanical energy during locomotion. Energy storage and dissipation is related to possible mechanisms of mechanochemical transduction and to changes in cartilage structure and function that occur in osteoarthritis. In addition, we analyze how passive and active internal stresses affect mechanochemical transduction in cartilage, and how this may affect cartilage behavior in health and disease. Received: 8 February 2002 / Accepted: 9 July 2002     

Evaluation of a subject-specific finite-element model of the equine metacarpophalangeal joint under physiological load

The equine metacarpophalangeal (MCP) joint is frequently injured, especially by racehorses in training. Most injuries result from repetitive loading of the subchondral bone and articular cartilage rather than from acute events. The likelihood of injury is multi-factorial but the magnitude of mechanical loading and the number of loading cycles are believed to play an important role. Therefore, an important step in understanding injury is to determine the distribution of load across the articular surface during normal locomotion. A subject-specific finite-element model of the MCP joint was developed (including deformable cartilage, elastic ligaments, muscle forces and rigid representations of bone), evaluated against measurements obtained from cadaver experiments, and then loaded using data from gait experiments. The sensitivity of the model to force inputs, cartilage stiffness, and cartilage geometry was studied. The FE model predicted MCP joint torque and sesamoid bone flexion angles within 5% of experimental measurements. Muscle–tendon forces, joint loads and cartilage stresses all increased as locomotion speed increased from walking to trotting and finally cantering. Perturbations to muscle–tendon forces resulted in small changes in articular cartilage stresses, whereas variations in joint torque, cartilage geometry and stiffness produced much larger effects. Non-subject-specific cartilage geometry changed the magnitude and distribution of pressure and the von Mises stress markedly. The mean and peak cartilage stresses generally increased with an increase in cartilage stiffness. Areas of peak stress correlated qualitatively with sites of common injury, suggesting that further modelling work may elucidate the types of loading that precede joint injury and may assist in the development of techniques for injury mitigation.     

Finite element study of a tissue-engineered cartilage transplant in human tibiofemoral joint

Most tissue-engineered cartilage constructs are more compliant than native articular cartilage (AC) and are poorly integrated to the surrounding tissue. To investigate the effect of an implanted tissue-engineered construct (TEC) with these inferior properties on the mechanical environment of both the engineered and adjacent native tissues, a finite element study was conducted. Biphasic swelling was used to model tibial cartilage and an implanted TEC with the material properties of either native tissue or a decreased elastic modulus and fixed charged density. Creep loading was applied with a rigid impermeable indenter that represented the femur. In comparison with an intact joint, compressive strains in the transplant, surface contact stress in the adjacent native AC and load partitioning between different phases of cartilage were affected by inferior properties of TEC. Results of this study may lead to a better understanding of the complex mechanical environment of an implanted TEC.     

Finite element study of a tissue-engineered cartilage transplant in human tibiofemoral joint

Most tissue-engineered cartilage constructs are more compliant than native articular cartilage (AC) and are poorly integrated to the surrounding tissue. To investigate the effect of an implanted tissue-engineered construct (TEC) with these inferior properties on the mechanical environment of both the engineered and adjacent native tissues, a finite element study was conducted. Biphasic swelling was used to model tibial cartilage and an implanted TEC with the material properties of either native tissue or a decreased elastic modulus and fixed charged density. Creep loading was applied with a rigid impermeable indenter that represented the femur. In comparison with an intact joint, compressive strains in the transplant, surface contact stress in the adjacent native AC and load partitioning between different phases of cartilage were affected by inferior properties of TEC. Results of this study may lead to a better understanding of the complex mechanical environment of an implanted TEC.     

Effects of shear stress on articular chondrocyte metabolism

The articular cartilage of diarthrodial joints experiences a variety of stresses, strains and pressures that result from normal activities of daily living. In normal cartilage, the extracellular matrix exists as a highly organized composite of specialized macromolecules that distributes loads at the bony ends. The chondrocyte response to mechanical loading is recognized as an integral component in the maintenance of articular cartilage matrix homeostasis. With inappropriate mechanical loading of the joint, as occurs with traumatic injury, ligament instability, bony malalignment or excessive weight bearing, the cartilage exhibits manifestations characteristic of osteoarthritis. Breakdown of cartilage in osteoarthritis involves degradation of the extracellular matrix macromolecules and decreased expression of chondrocyte proteins necessary for normal joint function. Osteoarthritic cartilage often exhibits increased amounts of type I collagen and synthesis of proteoglycans characteristic of immature cartilage. The shift in cartilage phenotype in response to altered load yields a matrix that fails to support normal joint function. Mathematical modeling and experimental studies in animal models confirm an association between altered loading of diarthrotic joints and arthritic changes. Both types of studies implicate shear forces as a critical component in the destructive profile. The severity of cartilage destruction in response to altered loads appears linked to expression of biological factors influencing matrix integrity and cellular metabolism. Determining how shear stress alters chondrocyte metabolism is fundamental to understanding how to limit matrix destruction and stimulate cartilage repair and regeneration. At present, the precise biochemical and molecular mechanisms by which shear forces alter chondrocyte metabolism from a normal to a degenerative phenotype remain unclear. The results presented here address the hypothesis that articular chondrocyte metabolism is modulated by direct effects of shear forces that act on the cell through mechanotransduction processes. The purpose of this work is to develop critical knowledge regarding the basic mechanisms by which mechanical loading modulates cartilage metabolism in health and disease. This presentation will describe the effects of using fluid induced shear stress as a model system for stimulation of articular chondrocytes in vitro. The fluid induced shear stress was applied using a cone viscometer system to stimulate all the cells uniformly under conditions of minimal turbulence. The experiments were carried using high-density primary monolayer cultures of normal and osteoarthritic human and normal bovine articular chondrocytes. The analysis of the cellular response included quantification of cytokine release, matrix metalloproteinase expression and activation of intracellular signaling pathways. The data presented here show that articular chondrocytes exhibit a dose- and time-dependent response to shear stress that results in the release of soluble mediators and extracellular matrix macromolecules. The data suggest that the chondrocyte response to mechanical stimulation contributes to the maintenance of articular cartilage homeostasis in vivo.     

Importance of collagen orientation and depth-dependent fixed charge densities of cartilage on mechanical behavior of chondrocytes

The collagen network and proteoglycan matrix of articular cartilage are thought to play an important role in controlling the stresses and strains in and around chondrocytes, in regulating the biosynthesis of the solid matrix, and consequently in maintaining the health of diarthrodial joints. Understanding the detailed effects of the mechanical environment of chondrocytes on cell behavior is therefore essential for the study of the development, adaptation, and degeneration of articular cartilage. Recent progress in macroscopic models has improved our understanding of depth-dependent properties of cartilage. However, none of the previous works considered the effect of realistic collagen orientation or depth-dependent negative charges in microscopic models of chondrocyte mechanics. The aim of this study was to investigate the effects of the collagen network and fixed charge densities of cartilage on the mechanical environment of the chondrocytes in a depth-dependent manner. We developed an anisotropic, inhomogeneous, microstructural fibril-reinforced finite element model of articular cartilage for application in unconfined compression. The model consisted of the extracellular matrix and chondrocytes located in the superficial, middle, and deep zones. Chondrocytes were surrounded by a pericellular matrix and were assumed spherical prior to tissue swelling and load application. Material properties of the chondrocytes, pericellular matrix, and extracellular matrix were obtained from the literature. The loading protocol included a free swelling step followed by a stress-relaxation step. Results from traditional isotropic and transversely isotropic biphasic models were used for comparison with predictions from the current model. In the superficial zone, cell shapes changed from rounded to elliptic after free swelling. The stresses and strains as well as fluid flow in cells were greatly affected by the modulus of the collagen network. The fixed charge density of the chondrocytes, pericellular matrix, and extracellular matrix primarily affected the aspect ratios (height/width) and the solid matrix stresses of cells. The mechanical responses of the cells were strongly location and time dependent. The current model highlights that the collagen orientation and the depth-dependent negative fixed charge densities of articular cartilage have a great effect in modulating the mechanical environment in the vicinity of chondrocytes, and it provides an important improvement over earlier models in describing the possible pathways from loading of articular cartilage to the mechanical and biological responses of chondrocytes.     

Contact analysis of biphasic transversely isotropic cartilage layers and correlations with tissue failure.

Failure of articular cartilage has been investigated experimentally and theoretically, but there is only partial agreement between observed failure and predicted regions of peak stresses. Since trauma and repetitive stress are implicated in the etiopathogenesis of osteoarthritis, it is important to develop cartilage models which correctly predict sites of high stresses. Cartilage is anisotropic and inhomogeneous, though it has been difficult to incorporate these complexities into engineering analyses. The objectives of this study are to demonstrate that a transversely isotropic, biphasic model of cartilage can provide agreement between predicted regions of high stresses and observed regions of cartilage failure and that with transverse isotropy cartilage stresses are more sensitive to convexity and concavity of the surfaces than with isotropy. These objectives are achieved by solving problems of diarthrodial joint contact by the finite-element method. Results demonstrate that transversely isotropic models predict peak stresses at the cartilage surface and the cartilage-bone interface, in agreement with sites of fissures following impact loading; isotropic models predict peak stresses only at the cartilage-bone interface. Also, when convex cartilage layers contacted concave layers in this study, the highest tensile stresses occur in the convex layer for transversely isotropic models; no such differences are found with isotropic models. The significance of this study is that it establishes a threshold of modeling complexity for articular cartilage that provides good agreement with experimental observations under impact loading and that surface curvatures significantly affect stress and strain within cartilage when using a biphasic transversely isotropic model.