Banking of osteochondral allografts, Part II. Preservation of Chondrocyte Viability During Long-Term Storage

One of the most important factors concerning the successful clinical outcome after transplantation of osteochondral allografts is the viability of the cartilage.The viability of cryopreserved cartilage is quite poor, 20–30% cell survival has been published. The purpose of this study was to develop a new storage method which improves the chondrocyte viability. The talus of cadaveric donors was used as a model tissue to compare human osteochondral allograft cartilage viability following cryopreservation with that remaining after prolonged refrigerated storage. Full-thickness cartilage punch biopsies had been cryopreserved, and tali were divided into two matched groups and stored in TCM for 60 days at +4 °C, either with or without regular medium replacement. The cartilage of each graft was biopsied and assayed for viability on every third day by the MTT reduction assay. During 4 °C storage, a recurring pattern of large fluctuations in apparent cartilage viability was observed in every stored graft, with or without medium replacement. These fluctuations did not appear in control specimens of either fresh or cryopreserved human skin that were assayed in parallel with the cartilage biopsies, so the viability fluctuation seems an intrinsic property of the cartilage in these conditions. Cartilage stored for 60 days at +4 °C showed significantly higher viability (35.2 ± 3.3 %) than fresh cartilage that had been cryopreserved (21.6 ± 1.8 %). This was true even when cryopreserved and thawed cartilage was subjected to a 3 day post thaw incubation under presumably favorable conditions (17.7 ± 1.6 %). These viability assay results, (reflective of intracellular metabolic activity), were corroborated by the fluorescent dye mixture SYTO-16 and propidium iodide. The data indicate that long-term stored refrigerated cartilage appears to retain a viability higher than that of cryopreserved cartilage for up to and perhaps beyond 60 days of storage. There was no viability index difference between the medium replaced and non-replaced groups. Although an exceptional result, in one individual case, more than 65% viable cells could be detected in the talar cartilage after 60 days storage at +4 °C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cartilage storage at 4 °C with regular culture medium replacement benefits chondrocyte viability of osteochondral grafts in vitro

Maintenance of articular cartilage allografts in culture media is a common method of tissue storage; however, the technical parameters of graft storage remain controversial. In this study, we examined the optimal temperature and culture medium exchange rate for the storage of osteochondral allografts in vitro. Cylindrical osteochondral grafts (n = 120), harvested from the talar joint surface of ten Boer goats, were randomly classified into four groups and stored under the following conditions: Group A1 was maintained at 4 °C in culture medium that was refreshed every 2 days; Group A2 was maintained at 4 °C in the same culture medium, without refreshing; Group B1, was maintained at 37 °C in culture medium that was refreshed every 2 days; Group B2, was maintained at 37 °C in the same culture medium, without refreshing. Chondrocyte viability in the grafts was determined by ethidium bromide/fluorescein diacetate staining on days 7, 21, and 35. Proteoglycan content was measured by Safranin-O staining. Group A1 exhibited the highest chondrocyte survival rates of 90.88 %, 88.31 % and 78.69 % on days 7, 21, and 35, respectively. Safranin O staining revealed no significant differences between groups on days 21 and 35. These results suggest that storage of osteochondral grafts at 4 °C with regular culture medium replacement should be highly suitable for clinical application.     

Rabbit trochlear model of osteochondral allograft transplantation

Allografting and autografting of osteochondral tissues is a promising strategy to treat articular cartilage lesions in damaged joints. We developed a new model of fresh osteochondral allografting using the entire rabbit trochlea. The objective of the current study was to demonstrate that this model would achieve reproducible graft-host healing and maintain normal articular cartilage histologic, immunolocalization, and biochemical characteristics after transplantation under diverse storage and transplantation conditions. New Zealand white (n = 8) and Dutch belted (n = 8) rabbits underwent a 2-stage transplantation operation using osteochondral grafts that had been stored for 2 or 4 wk. Trochlear grafts harvested from the left knee were transplanted to the right knee as either autografts or allografts. Grafts were fixed with 22-gauge steel wire or 3-0 nylon suture. Rabbits were euthanized for evaluation at 1, 2, 4, 6, and 12 wk after transplantation. All grafts that remained in vivo for at least 4 wk demonstrated 100% interface healing by microCT. Trabecular bridging was present at the host-graft interface starting at 2 wk after transplantation, with no significant difference in cartilage histology between the various groups. The combined histology scores indicated minimal evidence of osteoarthritis. Immunostaining revealed that superficial zone protein was localized at the surface of all transplants. The rabbit trochlear model met our criteria for a successful model in regard to the ease of the procedure, low rate of surgical complications, relatively large articular cartilage surface area, and amount of host-graft bone interface available for analysis.     

A finite deformation theory for cartilage and other soft hydrated connective tissues--I. Equilibrium results

The determination of valid stress-strain relations for articular cartilage under finite deformation conditions is a prerequisite for constructing models for synovial joint lubrication. Under physiological conditions of high strain rates and/or high stresses in the joint, large strains occur in cartilage. A finite deformation theory valid for describing cartilage, as well as other soft hydrated connective tissues under large loads, has been developed. This theory is based on the choice of a specific Helmholtz energy function which satisfies the generalized Coleman-Noll (GCN0) condition and the Baker-Ericksen (B-E) inequalities established in finite elasticity theory. In addition, the finite deformation biphasic theory includes the effects of strain-dependent porosity and permeability. These nonlinear effects are essential for properly describing the biomechanical behavior of articular cartilage, even when strain rates are low and strains are infinitesimal. The finite deformation theory describes the large strain behavior of cartilage observed in one-dimensional confined compression experiments at equilibrium, and it reduces to the linear biphasic theory under infinitesimal strain and slow strain rate conditions. Using this theory, we have determined the material coefficients of both human and bovine articular cartilages under large strain conditions at equilibrium. The theory compares very well with experimental results.     

Tsmu solution improves rabbit osteochondral allograft preservation and transplantation outcome

To compare the effects of Tsmu solution with vitrification on chondrocyte viability and examine histological and biomechanical properties of osteochondral allografts (OCAs) after storage, OCAs from femoral condyles of New Zealand rabbits were harvested, stored for 35 days in Tsmu solution or by in vitro vitrification, and subjected to in vivo and in vitro assays. Stored OCAs were transplanted into knee femoral condyle cartilage defects in recipient rabbits. Chondrocyte viability and histological changes of cartilage grafts were assessed in vitro. Gross assessment, chondrocyte viability, histological assessment, OCA biomechanics, and immunological markers were evaluated in vivo 6 months after transplantation. Fresh OCAs served as in vitro and in vivo controls. Chondrocyte viability and scores for cartilage surface and histological quantitative assessment were superior for Tsmu solution compared with vitrification, but inferior compared with fresh OCAs in vitro and in vivo. With the exception of interleukin 6 content, biomechanical features of samples stored in Tsmu solution were superior to vitrification, and inferior to fresh OCAs in vivo. Thus, Tsmu solution provided suitable storage that improved chondrocyte viability, intact OCA cartilage matrix architecture, and transplantation outcomes.     

Shear deformation kinematics during cartilage articulation: effect of lubrication, degeneration, and stress relaxation

During joint articulation, the biomechanical behavior of cartilage not only facilitates load-bearing and low-friction, but also provides regulatory cues to chondrocytes. Elucidation of cartilage kinematics under combined compression and shearing conditions clarifies these cues in health and disease. The objectives of this study were to elucidate the effects of lubricant, tissue degeneration, and stress relaxation duration on cartilage shear kinematics during articulation. Human osteochondral cores with normal and mildly degenerate surface structures were isolated. Paired blocks from each core were apposed, compressed, allowed to stress relax for 5 or 60 min, and shear tested with a micro-scale video microscopy system using phosphate-buffered saline (PBS) or synovial fluid as lubricant. During applied lateral motion, local and overall shear strain (Exz) of articular cartilage were determined. The applied lateral displacement at which Exz reached 50% of the peak (Deltax(1/2)) was also determined. Quantitatively, surface Exz increased at the onset of lateral motion and peaked just as surfaces detached and slid. With continued lateral motion, surface Exz was maintained. After short stress relaxation, effects of lubrication on Exz and Deltax(1/2) were not apparent. With prolonged stress relaxation, Exz and Deltax(1/2) near the articular surface increased markedly when PBS was used as lubricant. Similar patterns were observed for overall Exz and Deltax(1/2). With degeneration, surface Exz was consistently higher for all cases after the onset of lateral motion. Thus, cartilage shear kinematics is markedly affected by lubricant, cartilage degeneration, and loading duration. Changes in these factors may be involved in the pathogenesis of osteoarthritis.     

Application of the u-p finite element method to the study of articular cartilage.

The finite element method using the principle of virtual work was applied to the biphasic theory to establish a numerical routine for analyses of articular cartilage behavior. The matrix equations that resulted contained displacements of the solid matrix (mu) and true fluid pressure (p) as the unknown variables at the element nodes. Both small and large strain conditions were considered. The algorithms and computer code for the analysis of two-dimensional plane strain, plane stress, and axially symmetric cases were developed. The u-p finite element numerical procedure demonstrated excellent agreement with available closed-form and numerical solutions for the configurations of confined compression and unconfined compression under small strains, and for confined compression under large strains. The model was also used to examine the behavior of a repaired articular surface. The differences in material properties between the repair tissue and normal cartilage resulted in significant deformation gradients across the repair interface as well as increased fluid efflux from the tissue.     

Functional tissue engineering of chondral and osteochondral constructs

Due to the prevalence of osteoarthritis (OA) and damage to articular cartilage, coupled with the poor intrinsic healing capacity of this avascular connective tissue, there is a great demand for an articular cartilage substitute. As the bearing material of diarthrodial joints, articular cartilage has remarkable functional properties that have been difficult to reproduce in tissue-engineered constructs. We have previously demonstrated that by using a functional tissue engineering approach that incorporates mechanical loading into the long-term culture environment, one can enhance the development of mechanical properties in chondrocyte-seeded agarose constructs. As these gel constructs begin to achieve material properties similar to that of the native tissue, however, new challenges arise, including integration of the construct with the underlying native bone. To address this issue, we have developed a technique for producing gel constructs integrated into an underlying bony substrate. These osteochondral constructs develop cartilage-like extracellular matrix and material properties over time in free swelling culture. In this study, as a preliminary to loading such osteochondral constructs, finite element modeling (FEM) was used to predict the spatial and temporal stress, strain, and fluid flow fields within constructs subjected to dynamic deformational loading. The results of these models suggest that while chondral ("gel alone") constructs see a largely homogenous field of mechanical signals, osteochondral ("gel bone") constructs see a largely inhomogeneous distribution of mechanical signals. Such inhomogeneity in the mechanical environment may aid in the development of inhomogeneity in the engineered osteochondral constructs. Together with experimental observations, we anticipate that such modeling efforts will provide direction for our efforts aimed at the optimization of applied physical forces for the functional tissue engineering of an osteochondral articular cartilage substitute.     

The minipig model for experimental chondral and osteochondral defect repair in tissue engineering: retrospective analysis of 180 defects

Articular cartilage repair is still a challenge in orthopaedic surgery. Although many treatment options have been developed in the last decade, true regeneration of hyaline articular cartilage is yet to be accomplished. In vitro experiments are useful for evaluating cell-matrix interactions under controlled parameters. When introducing new treatment options into clinical routine, adequate animal models are capable of closing the gap between in vitro experiments and the clinical use in human beings. We developed an animal model in the G?ttingen minipig (GMP) to evaluate the healing of osteochondral or full-thickness cartilage defects. The defects were located in the middle third of the medial portion of the patellofemoral joint at both distal femurs. Chondral defects were 6.3 mm, osteochondral defects either 5.4 or 6.3 mm in diameter and 8 or 10 mm deep. In both defects the endogenous repair response showed incomplete repair tissue formation up to 12 months postoperatively. Based on its limited capability for endogenous repair of chondral and osteochondral defects, the GMP is a useful model for critical assessment of new treatment strategies in articular cartilage tissue engineering.     

Effects of refrigeration and freezing on the electromechanical and biomechanical properties of articular cartilage

In vitro electromechanical and biomechanical testing of articular cartilage provide critical information about the structure and function of this tissue. Difficulties obtaining fresh tissue and lengthy experimental testing procedures often necessitate a storage protocol, which may adversely affect the functional properties of cartilage. The effects of storage at either 4°C for periods of 6 days and 12 days, or during a single freeze-thaw cycle at -20°C were examined in young bovine cartilage. Non-destructive electromechanical measurements and unconfined compression testing on 3 mm diameter disks were used to assess cartilage properties, including the streaming potential integral (SPI), fibril modulus (Ef), matrix modulus (Em), and permeability (k). Cartilage disks were also examined histologically. Compared with controls, significant decreases in SPI (to 32.3±5.5% of control values, p<0.001), Ef (to 31.3±41.3% [corrected] of control values, p=0.046), Em (to 6.4±8.5% of control values, p<0.0001), and an increase in k (to 2676.7±2562.0% of control values, p=0.004) were observed at day 12 of refrigeration at 4°C, but no significant changes were detected at day 6. A trend toward detecting a decrease in SPI (to 94.2±6.2% of control values, p=0.083) was identified following a single freeze-thaw cycle, but no detectable changes were observed for any biomechanical parameters. All numbers are mean±95% confidence interval. These results indicate that fresh cartilage can be stored in a humid chamber at 4°C for a maximum of 6 days with no detrimental effects to cartilage electromechanical and biomechanical properties, while one freeze-thaw cycle produces minimal deterioration of biomechanical and electromechanical properties. A comparison to literature suggested that particular attention should be paid to the manner in which specimens are thawed after freezing, specifically by minimizing thawing time at higher temperatures.     

Mechanical and biochemical characterization of cartilage explants in serum-free culture

Allografts of articular cartilage are both used clinically for tissue-transplantation procedures and experimentally as model systems to study the physiological behavior of chondrocytes in their native extracellular matrix. Long-term maintenance of allograft tissue is challenging. Chemical mediators in poorly defined culture media can stimulate cells to quickly degrade their surrounding extracellular matrix. This is particularly true of juvenile cartilage which is generally more responsive to chemical stimuli than mature tissue. By carefully modulating the culture media, however, it may be possible to preserve allograft tissue over the long-term while maintaining its original mechanical and biochemical properties. In this study juvenile bovine cartilage explants (both chondral and osteochondral) were cultured in both chemically defined medium and serum-supplemented medium for up to 6 weeks. The mechanical properties and biochemical content of explants cultured in chemically defined medium were enhanced after 2 weeks in culture and thereafter remained stable with no loss of cell viability. In contrast, the mechanical properties of explants in serum-supplemented medium were degraded by ( approximately 70%) along with a concurrent loss of biochemical content (30-40% GAG). These results suggest that long-term maintenance of allografts can be extended significantly by the use of a chemically defined medium.     

Development of a preclinical natural porcine knee simulation model for the tribological assessment of osteochondral grafts in vitro

In order to pre-clinically evaluate the performance and efficacy of novel osteochondral interventions, physiological and clinically relevant whole joint simulation models, capable of reproducing the complex loading and motions experienced in the natural knee environment are required. The aim of this study was to develop a method for the assessment of tribological performance of osteochondral grafts within an in vitro whole natural joint simulation model.The study assessed the effects of osteochondral allograft implantation (existing surgical intervention for the repair of osteochondral defects) on the wear, deformation and damage of the opposing articular surfaces. Tribological performance of osteochondral grafts was compared to the natural joint (negative control), an injury model (focal cartilage defects) and stainless steel pins (positive controls). A recently developed method using an optical profiler (Alicona Infinite Focus G5, Alicona Imaging GmbH, Austria) was used to quantify and characterise the wear, deformation and damage occurring on the opposing articular surfaces. Allografts inserted flush with the cartilage surface had the lowest levels of wear, deformation and damage following the 2 h test; increased levels of wear, deformation and damage were observed when allografts and stainless steel pins were inserted proud of the articular surface. The method developed will be applied in future studies to assess the tribological performance of novel early stage osteochondral interventions prior to in vivo studies, investigate variation in surgical precision and aid in the development of stratified interventions for the patient population.     

Do changes in the mechanical properties of articular cartilage promote catabolic destruction of cartilage and osteoarthritis?

Osteoarthritis (OA) is a joint disease characterized by cartilage degeneration, a thickening of subchondral bone, and formation of marginal osteophytes. Previous mechanical characterization of cartilage in our laboratory suggests that energy storage and dissipation is reduced in osteoarthritis as the extent of fibrillation and fissure formation increases. It is not clear whether the loss of energy storage and dissipation characteristics is a result of biochemical and/or biophysical changes that occur to hyaline cartilage in joints. The purpose of this study is to present data, on the strain rate dependence of the elastic and viscous behaviors of cartilage, in order to further characterize changes that occur in the mechanical properties that are associated with OA. We have previously hypothesized that the changes seen in the mechanical properties of cartilage may be due to altered mechanochemical transduction by chondrocytes. Results of incremental tensile stress-strain tests at strain rates between 100%/min and 10,000%/min conducted on OA cartilage indicate that the slope of the elastic stress-strain curve increases with increasing strain rate, unlike the reported behavior of skin and self-assembled collagen fibers. It is suggested that the strain-rate dependence of the elastic stress-strain curve is due to the presence of large quantities of proteoglycans (PGs), which protect articular cartilage by increasing the apparent stiffness. The increased apparent stiffness of articular cartilage at high strain rates may limit the stresses borne and prolong the onset of OA. It is further hypothesized that increased compressive loading of chondrocytes in the intermediate zone of articular cartilage occurs as a result of normal wear to the superficial zone or from excessive impact loading. Once the superficial zone of articular cartilage is worn away, the tension is decreased throughout all cartilage zones leading to increased chondrocyte compressive loading and up-regulation of mechanochemical transduction processes that elaborate catabolic enzymes.     

Biomechanical properties of knee articular cartilage

Structure and properties of knee articular cartilage are adapted to stresses exposed on it during physiological activities. In this study, we describe site- and depth-dependence of the biomechanical properties of bovine knee articular cartilage. We also investigate the effects of tissue structure and composition on the biomechanical parameters as well as characterize experimentally and numerically the compression-tension nonlinearity of the cartilage matrix. In vitro mechano-optical measurements of articular cartilage in unconfined compression geometry are conducted to obtain material parameters, such as thickness, Young's and aggregate modulus or Poisson's ratio of the tissue. The experimental results revealed significant site- and depth-dependent variations in recorded parameters. After enzymatic modification of matrix collagen or proteoglycans our results show that collagen primarily controls the dynamic tissue response while proteoglycans affect more the static properties. Experimental measurements in compression and tension suggest a nonlinear compression-tension behavior of articular cartilage in the direction perpendicular to articular surface. Fibril reinforced poroelastic finite element model was used to capture the experimentally found compression-tension nonlinearity of articular cartilage.     

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.     

Mechano-regulation of stem cell differentiation and tissue regeneration in osteochondral defects

Cartilage defects that penetrate the subchondral bone can undergo spontaneous repair through the formation of a fibrous or cartilaginous tissue mediated primarily by mesenchymal stem cells from the bone marrow. This tissue is biomechanically inferior to normal articular cartilage, and is often observed to degrade over time. Whether or not biomechanical factors control the type and quality of the repair tissue, and its subsequent degradation, have yet to be elucidated. In this paper, we hypothesise a relationship between the mechanical environment of mesenchymal stem cells and their subsequent dispersal, proliferation, differentiation and death. The mechano-regulation stimulus is hypothesised to be a function of strain and fluid flow; these quantities are calculated using biphasic poroelastic finite element analysis. A finite element model of an osteochondral defect in the knee was created, and used to simulate the spontaneous repair process. The model predicts bone formation through both endochondral and direct intramembranous ossification in the base of the defect, cartilage formation in the centre of the defect and fibrous tissue formation superficially. Greater amounts of fibrous tissue formation are predicted as the size of the defect is increased. Large strains are predicted within the fibrous tissue at the articular surface, resulting in significant cell apoptosis. This result leads to the conclusion that repair tissue degradation is initiated in the fibrous tissue that forms at the articular surface. The success of the mechano-regulation model in predicting many of the cellular events that occur during osteochondral defect healing suggest that in the future it could be used as a tool for optimising scaffolds for tissue engineering.     

Ultrasonic characterization of articular cartilage

Osteoarthrosis is the most important joint disease that threatens health of the musculoskeletal system of elderly people. Today, there is a need for sensitive, quantitative diagnostic methods for successful and early diagnosis of the disorder. In the present study, we aimed at evaluating the applicability of ultrasound for quantitative assessment of cartilage structure and properties. Bovine articular cartilage was investigated both in vitro and in situ using high frequency ultrasound. Cartilage samples were also tested mechanically in vitro to reveal relationships between acoustic and mechanical parameters of the tissue. The collagen organization and proteoglycan content of cartilage samples were mapped, using quantitative polarized light microscopy and digital densitometry, respectively, to reveal their effect on the acoustic properties of tissue. The high frequency pulse-echo ultrasound (20-30 MHz) technique proved to be sensitive in detecting the degeneration of the superficial collagen-rich cartilage zone. In addition, ultrasound was found to be a potential tool for measuring cartilage thickness. When the results from biomechanical indentation measurements and ultrasound measurements of normal and enzymatically degraded articular cartilage were combined, collagen or proteoglycan degradation in the tissue could be sensitively and specifically differentiated from each other. To conclude, high frequency ultrasound is a useful tool for evaluation of the quality of superficial articular cartilage as well as for the measurement of cartilage thickness. Therefore, ultrasound appears to be a valuable supplement to the mechanical measurements of articular cartilage stiffness.     

A Comprehensive Specimen-Specific Multiscale Data Set for Anatomical and Mechanical Characterization of the Tibiofemoral Joint

Understanding of tibiofemoral joint mechanics at multiple spatial scales is essential for developing effective preventive measures and treatments for both pathology and injury management. Currently, there is a distinct lack of specimen-specific biomechanical data at multiple spatial scales, e.g., joint, tissue, and cell scales. Comprehensive multiscale data may improve the understanding of the relationship between biomechanical and anatomical markers across various scales. Furthermore, specimen-specific multiscale data for the tibiofemoral joint may assist development and validation of specimen-specific computational models that may be useful for more thorough analyses of the biomechanical behavior of the joint. This study describes an aggregation of procedures for acquisition of multiscale anatomical and biomechanical data for the tibiofemoral joint. Magnetic resonance imaging was used to acquire anatomical morphology at the joint scale. A robotic testing system was used to quantify joint level biomechanical response under various loading scenarios. Tissue level material properties were obtained from the same specimen for the femoral and tibial articular cartilage, medial and lateral menisci, anterior and posterior cruciate ligaments, and medial and lateral collateral ligaments. Histology data were also obtained for all tissue types to measure specimen-specific cell scale information, e.g., cellular distribution. This study is the first of its kind to establish a comprehensive multiscale data set for a musculoskeletal joint and the presented data collection approach can be used as a general template to guide acquisition of specimen-specific comprehensive multiscale data for musculoskeletal joints.     

Depth-dependent Compressive Equilibrium Properties of Articular Cartilage Explained by its Composition

For this study, we hypothesized that the depth-dependent compressive equilibrium properties of articular cartilage are the inherent consequence of its depth-dependent composition, and not the result of depth-dependent material properties. To test this hypothesis, our recently developed fibril-reinforced poroviscoelastic swelling model was expanded to include the influence of intra- and extra-fibrillar water content, and the influence of the solid fraction on the compressive properties of the tissue. With this model, the depth-dependent compressive equilibrium properties of articular cartilage were determined, and compared with experimental data from the literature. The typical depth-dependent behavior of articular cartilage was predicted by this model. The effective aggregate modulus was highly strain-dependent. It decreased with increasing strain for low strains, and increases with increasing strain for high strains. This effect was more pronounced with increasing distance from the articular surface. The main insight from this study is that the depth-dependent material behavior of articular cartilage can be obtained from its depth-dependent composition only. This eliminates the need for the assumption that the material properties of the different constituents themselves vary with depth. Such insights are important for understanding cartilage mechanical behavior, cartilage damage mechanisms and tissue engineering studies.     

Long-term clinical experience with fresh osteochondral allografts for articular knee defects in high demand patients

Fresh osteochondral allografts are used to repair osteoarticular defects of the knee. For post-traumatic defects recent advances in other techniques for cartilage repair and resurfacing have reduced the role of allograft tissue transplantation to defects larger than 3 cm in diameter and 1 cm in depth. A fresh osteochondral allograft that has been harvested from a donor within 24 h from death and preserved in 4°C for up to 4 days shows 100% viability of the cartilage. The avascular bone remains structurally intact and mechanically strong until it is replaced by host bone or until it is weakened or absorbed. The indications for fresh osteochondral allografts for reconstructive surgery of the articular surface of the knee do not justify the use of immunosuppressive drugs and we therefore believe that surgical vascularization of the grafts should not be carried out. This clinical approach can provide a reconstructive solution for younger higher demand patients where implants are not desirable and arthrodesis is not acceptable. A clinical follow-up study as early as 1975 showed successful early outcomes. More recently, survival analysis found 95% survival at 5 years, 71% at 10 years, and 66% at 20 years. It was learned that older patients, bipolar transplants, improper loading of the graft, and grafts for osteoarthritis and steroid-induced avascular necrosis do not lead to good long-term outcomes. We would like to describe here some of our long-term clinical experience concerning this surgery. This revised version was published online in July 2006 with corrections to the Cover Date.