Abnormal softening in articular cartilage: its relationship to the collagen framework

Articular cartilage fatigue causes frequency-dependent softening and crack extension

Soft biological polymers, such as articular cartilage, possess exceptional fracture and fatigue resistance, offering inspiration for the development of novel materials. However, we lack a detailed understanding of changes in cartilage material behavior and of crack propagation following cyclic compressive loading. We investigated the structure and mechanical behavior of cartilage as a function of loading frequency and number of cycles. Microcracks were initiated in cartilage samples using microindentation, then cracks were extended under cyclic compression. Thickness, apparent stiffness, energy dissipation, phase angle, and crack length were measured to determine the effects of cyclic loading at two frequencies (1 Hz and 5 Hz). To capture the fatigue-induced material response (thickness, stiffness, energy dissipation, and phase angle), material properties were compared between pre-and-post diagnostic tests. The findings indicate that irreversible structural damage (reduced thickness), cartilage softening (reduced apparent stiffness), and reduced energy dissipation (including phase angle) increased with an increase in the number of cycles. Higher frequency loading resulted in less reduction in energy dissipation, phase angle, and thickness change. Crack lengths, quantified through brightfield imaging, increased with number of cycles and frequency. This study sheds light on the complex response of cartilage under cyclic loading resulting in softening, structural damage, and altered dynamic behavior. The findings provide better understanding of failure mechanisms in cartilage and thus may help in diagnosis and treatment of osteoarthritis.

Composition, architecture and biomechanical properties of articular cartilage in differently loaded areas of the equine stifle

BACKGROUND

Strategies for articular cartilage repair need to take into account topographical differences in tissue composition and architecture to achieve durable functional outcome. These have not yet been investigated in the equine stifle.

OBJECTIVES

To analyse the biochemical composition and architecture of three differently loaded areas of the equine stifle. We hypothesise that site differences correlate with the biomechanical characteristics of the cartilage.

STUDY DESIGN

Ex vivo study.

METHODS

Thirty osteochondral plugs per location were harvested from the lateral trochlear ridge (LTR), the distal intertrochlear groove (DITG) and the medial femoral condyle (MFC). These underwent biochemical, biomechanical and structural analysis. A linear mixed model with location as a fixed factor and horse as a random factor was applied, followed by pair-wise comparisons of estimated means with false discovery rate correction, to test for differences between locations. Correlations between biochemical and biomechanical parameters were tested using Spearman's correlation coefficient.

RESULTS

Glycosaminoglycan content was different between all sites (estimated mean [95% confidence interval (CI)] for LTR 75.4 [64.5, 88.2], for intercondylar notch (ICN) 37.3 [31.9, 43.6], for MFC 93.7 [80.1109.6] μg/mg dry weight), as were equilibrium modulus (LTR2.20 [1.96, 2.46], ICN0.48 [0.37, 0.6], MFC1.36 [1.17, 1.56] MPa), dynamic modulus (LTR7.33 [6.54, 8.17], ICN4.38 [3.77, 5.03], MFC5.62 [4.93, 6.36] MPa) and viscosity (LTR7.49 [6.76, 8.26], ICN16.99 [15.88, 18.14], MFC8.7 [7.91,9.5]°). The two weightbearing areas (LTR and MCF) and the non-weightbearing area (ICN) differed in collagen content (LTR 139 [127, 152], ICN176[162, 191], MFC 127[115, 139] μg/mg dry weight), parallelism index and angle of collagen fibres. The strongest correlations were between proteoglycan content and equilibrium modulus (r: 0.642; p: 0.001), dynamic modulus (r: 0.554; p < 0.001) and phase shift (r: -0.675; p < 0.001), and between collagen orientation angle and equilibrium modulus (r: -0.612; p < 0.001), dynamic modulus (r: -0.424; p < 0.001) and phase shift (r: 0.609; p < 0.001).

MAIN LIMITATIONS

Only a single sample per location was analysed.

CONCLUSIONS

There were significant differences in cartilage biochemical composition, biomechanics and architecture between the three differently loaded sites. The biochemical and structural composition correlated with the mechanical characteristics. These differences need to be acknowledged by designing cartilage repair strategies.

The ultrastructure of cartilage tissue and its swelling response in relation to matrix health

This multi-length scale anatomical study explores the influence of mild cartilage structural degeneration on the tissue swelling response. While the swelling response of cartilage has been studied extensively, this is the first study to reveal and correlate tissue microstructure and ultrastructure, with the swelling induced cartilage tissue strains. Cartilage sample strips (n = 30) were obtained from the distal-lateral quadrant of thirty mildly degenerate bovine patellae and, following excision from the bone, the cartilage strips were allowed to swell freely for 2 h in solutions of physiological saline and distilled water successively. The swelling response of this group of samples were compared with that of healthy cartilage, with (n = 20) and without the surface layer (n = 20). The subsequent curling response of cartilage showed that in healthy tissue it was highly variable, and with the surface removed some samples curved in the opposite direction, while in the mildly degenerate tissue group, virtually all tissue strips curved in a consistent upward manner. A significant difference in strain was observed between healthy samples with surface layer removed and mildly degenerate samples, illustrating how excision of the surface zone from pristine cartilage is insufficient to model the swelling response of tissue which has undergone natural degenerative changes. On average, total tissue thickness increased from 940 µm (healthy) to 1079 µm (mildly degenerate), however, looking at the zonal strata, surface and transition zone thicknesses both decreased while deep zone thickness increased from healthy to mildly degenerate tissue. Morphologically, changes to the surface zone integrity were correlated with a diminished surface layer which, at the ultrastructural scale, correlated with a decreased fibrillar density. Similarly, fibrosity of the general matrix visible at the microscale was associated with a loss of later interconnectivity resulting in large, aggregated fibril bundles. The microstructural and ultrastructural investigation revealed that the key differences influencing the tissue swelling strain response was (1) the thickness and extent of disruption to the surface layer and (2) the amount of fibrillar network destructuring, highlighting the importance of the collagen and tissue matrix structure in restraining cartilage swelling.

Contrast-enhanced x-ray microscopy of articular cartilage

Osteoarthritis is a common chronic disease of joints characterized by degenerative changes of articular cartilage. An early diagnosis of osteoarthritis may be possible when imaging excised tissue for research in situ at the cellular-molecular scale. Whereas cartilage histopathology is destructive, time-consuming, and limited to 2D views, contrast-enhanced x-ray microscopy (XRM) can image articular cartilage and subchondral bone in 3D. This study evaluates articular cartilage structure ex vivo using both techniques.Osteochondral plugs were excised from non-diseased bovine knees and stained in phosphotungstic acid for 0 to 32 h. XRM imaging revealed an optimal staining time of 16 h and a saturated staining time of 24 h. Histology sections were cut and analyzed by polarized light microscopy (PLM) and second-harmonic-generation dual-photon (SHG-DP) microscopy. Histology photomicrographs were aligned with matching XRM slices and evaluated for features relevant in histopathological scoring of osteoarthritis cartilage, including the tidemark, collagen architecture and chondrocyte morphology.The cartilage collagen network and chondrocytes from the 3D contrast-enhanced XRM were correlated with the 2D histology. This technique has two distinct advantages over routine histopathology: (1) the avoidance of dehydration, demineralization, and deformation of histological sectioning, thereby preserving the intact articular cartilage and subchondral bone plate ex vivo, and (2) the ability to evaluate the entire osteochondral volume in 3D. This work explores several diagnostic features of imaging cartilage, including: visualization of the tidemark in XRM and SHG-DP microscopy, validating the morphology of chondrocytes and nuclei with XRM, SHG-DP and PLM, and correlating collagen birefringence with XRM image intensity.

Imbalanced cellular metabolism compromises cartilage homeostasis and joint function in a mouse model of mucolipidosis type III gamma

Mucolipidosis type III (MLIII) gamma is a rare inherited lysosomal storage disorder caused by mutations in GNPTG encoding the γ-subunit of GlcNAc-1-phosphotransferase, the key enzyme ensuring proper intracellular location of multiple lysosomal enzymes. Patients with MLIII gamma typically present with osteoarthritis and joint stiffness, suggesting cartilage involvement. Using Gnptg knockout (Gnptgko ) mice as a model of the human disease, we showed that missorting of a number of lysosomal enzymes is associated with intracellular accumulation of chondroitin sulfate in Gnptgko chondrocytes and their impaired differentiation, as well as with altered microstructure of the cartilage extracellular matrix (ECM). We also demonstrated distinct functional and structural properties of the Achilles tendons isolated from Gnptgko and Gnptab knock-in (Gnptabki ) mice, the latter displaying a more severe phenotype resembling mucolipidosis type II (MLII) in humans. Together with comparative analyses of joint mobility in MLII and MLIII patients, these findings provide a basis for better understanding of the molecular reasons leading to joint pathology in these patients. Our data suggest that lack of GlcNAc-1-phosphotransferase activity due to defects in the γ-subunit causes structural changes within the ECM of connective and mechanosensitive tissues, such as cartilage and tendon, and eventually results in functional joint abnormalities typically observed in MLIII gamma patients. This idea was supported by a deficit of the limb motor function in Gnptgko mice challenged on a rotarod under fatigue-associated conditions, suggesting that the impaired motor performance of Gnptgko mice was caused by fatigue and/or pain at the joint.This article has an associated First Person interview with the first author of the paper.

Embrittlement of collagen in early-stage human osteoarthritis

Articular cartilage is a remarkable material with mechanical performance that surpasses engineering standards. Collagen, the most abundant protein in cartilage, plays an important role in this performance, and also in disease. Building on observations of network-level collagen changes at the earliest stages of osteoarthritis, this study explores the physical role of the collagen fibril in the disease process. Specifically, we focus on the material properties of collagen fibrils in the cartilage surface. Ten human tibial plateaus were characterised by atomic force microscopy (AFM) and Raman spectroscopy, with histological scoring used to define disease state. Measures of tropocollagen remained stable with disease progression, yet a marked mechanical change was observed. A slight stiffening coupled with a substantial decrease in loss tangent suggests a physical embrittlement caused by increased inter-molecular interactions.

Cartilage-on-cartilage cyclic loading induces mechanical and structural damage

Cartilage breaks down during mechanically-mediated osteoarthritis (OA). While previous research has begun to elucidate mechanical, structural and cellular damage in response to cyclic loading, gaps remain in our understanding of the link between cyclic cartilage loading and OA-like mechanical damage. Thus, the aim of this study was to quantify irreversible cartilage damage in response to cyclic loading. A novel in vitro model of damage through cartilage-on-cartilage cyclic loading was established. Cartilage was loaded at 1 Hz to two different doses (10,000 or 50,000 cycles) between -6.0 ± 0.2 MPa and -10.3 ± 0.2 MPa 1st Piola-Kirchhoff stress. After loading, mechanical damage (altered mechanical properties: elastic moduli and dissipated energy) and structural damage (surface damage and specimen thickness) were quantified. Linear and tangential moduli were determined by fitting the loading portion of the stress-strain curves. Dissipated energy was calculated from the area between loading and unloading stress-strain curves. Specimen thickness was measured both before and after loading. Surface damage was assessed by staining samples with India ink, then imaging the articular surface. Cyclic loading resulted in dose-dependent decreases in linear and tangential moduli, energy dissipation, thickness, and intact area. Collectively, these results show that cartilage damage can be initiated by mechanical loading alone in vitro, suggesting that cyclic loading can cause in vivo damage. This study demonstrated that with increased number of cycles, cartilage undergoes both tissue softening and structural damage. These findings are a first step towards characterizing the cartilage response to cyclic loading, which can ultimately provide important insight for delaying the initiation and slowing the progression of OA.

Comparison between in vitro and in vivo cartilage overloading studies based on a systematic literature review

Methodological differences between in vitro and in vivo studies on cartilage overloading complicate the comparison of outcomes. The rationale of the current review was to (i) identify consistencies and inconsistencies between in vitro and in vivo studies on mechanically-induced structural damage in articular cartilage, such that variables worth interesting to further explore using either one of these approaches can be identified; and (ii) suggest how the methodologies of both approaches may be adjusted to facilitate easier comparison and therewith stimulate translation of results between in vivo and in vitro studies. This study is anticipated to enhance our understanding of the development of osteoarthritis, and to reduce the number of in vivo studies. Generally, results of in vitro and in vivo studies are not contradicting. Both show subchondral bone damage and intact cartilage above a threshold value of impact energy. At lower loading rates, excessive loads may cause cartilage fissuring, decreased cell viability, collagen network de-structuring, decreased GAG content, an overall damage increase over time, and low ability to recover. This encourages further improvement of in vitro systems, to replace, reduce, and/or refine in vivo studies. However, differences in experimental set up and analyses complicate comparison of results. Ways to bridge the gap include (i) bringing in vitro set-ups closer to in vivo, for example, by aligning loading protocols and overlapping experimental timeframes; (ii) synchronizing analytical methods; and (iii) using computational models to translate conclusions from in vitro results to the in vivo environment and vice versa. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. J Orthop Res 9999:1-11, 2018.

How a decreased fibrillar interconnectivity influences stiffness and swelling properties during early cartilage degeneration

OBJECTIVE

The functional coupling between the fibrillar network and the high-swelling proteoglycans largely determines the mechanical properties of the articular cartilage matrix. The objective of this new study was to show specifically how changes in fibrillar interconnectivity arising from early cartilage degeneration influence transverse stiffness and swelling properties at the tissue level.

DESIGN

Radial zone transverse layers of cartilage matrix were obtained from intact and mildly degenerate bovine patellae. Each layer was then subdivided to assess tensile stiffness, free-swelling response, glycosaminoglycan (GAG) content, and micro- and ultra-structural features.

RESULTS

The tensile modulus was significantly lower and the degree of swelling significantly higher for the degenerate matrix compared to the intact. Scanning electron microscopy revealed a homogeneous response to transverse strain in the intact cartilage, whereas large non-fibrillar spaces between fibril aggregates were visible in the degenerate matrix. Although there were no significant differences in GAG content it did correlate significantly with stiffness and swelling in the intact samples but not in the degenerate.

CONCLUSIONS

The lower degree of fibril network interconnectivity in the degenerate matrix led to both a decreased transverse stiffness and reduced resistance to osmotic swelling. This network 'de-structuring' also resulted in a reduced functional interaction between the fibrillar network and the proteoglycans. The study provides new insights into the role of the fibrillar network and how changes in the network arising from the degenerative cascade will influence tissue level behaviour.

The influence of early degenerative changes on the vulnerability of articular cartilage to impact-induced injury

BACKGROUND

Recently, the structural changes in a bovine model of early degeneration were validated by our research group to be analogous to that in early human osteoarthritis. The hypothesis of this study was that the structural changes associated with increasing levels of degeneration would lead to higher levels of tissue damage in response to impact induced injury.

METHODS

A total of forty bovine patellae were obtained for this study. Cartilage-on-bone samples were extracted from the distal lateral quarter, a region known to be affected by varying levels of degeneration. A single impact drop test was applied to these samples delivering 2.3J of energy. A dynamic load cell and image capture at 2000fps allowed for the calculation of the reaction stress and coefficient of restitution. The extent of tissue damage was examined from the micro to ultrastructural levels using differential interference contrast optical microscopy and scanning electron microscopy respectively.

FINDINGS

The impact mechanical properties of mildly degenerate articular cartilage were not significantly different but showed a significantly larger amount of structural damage. From comparing the mechanical and structural response of intact and mildly degenerate cartilage, to tissue showing increased macro-scale tissue degeneration, the significance of the surface layer and fibrillar scale transverse interconnectivity in effectively attenuating impact loads is demonstrated in this study.

INTERPRETATION

This study shows that even though articular cartilage can appear visibly normal under macroscopic observation, the micro-scale structural changes associated with very early stage osteoarthritis can have a significant effect on its vulnerability to impact damage.

Effect of crosslinking in cartilage-like collagen microstructures

The mechanical performance of biological tissues is underpinned by a complex and finely balanced structure. Central to this is collagen, the most abundant protein in our bodies, which plays a dominant role in the functioning of tissues, and also in disease. Based on the collagen meshwork of articular cartilage, we have developed a bottom-up spring-node model of collagen and examined the effect of fibril connectivity, implemented by crosslinking, on mechanical behaviour. Although changing individual crosslink stiffness within an order of magnitude had no significant effect on modelling predictions, the density of crosslinks in a meshwork had a substantial impact on its behaviour. Highly crosslinked meshworks maintained a 'normal' configuration under loading, with stronger resistance to deformation and improved recovery relative to sparsely crosslinked meshwork. Stress on individual fibrils, however, was higher in highly crosslinked meshworks. Meshworks with low numbers of crosslinks reconfigured to disease-like states upon deformation and recovery. The importance of collagen interconnectivity may provide insight into the role of ultrastructure and its mechanics in the initiation, and early stages, of diseases such as osteoarthritis.

Cartilage Strain Distributions Are Different Under the Same Load in the Central and Peripheral Tibial Plateau Regions

There is increasing evidence that the regional spatial variations in the biological and mechanical properties of articular cartilage are an important consideration in the pathogenesis of knee osteoarthritis (OA) following kinematic changes at the knee due to joint destabilizing events (such as an anterior cruciate ligament (ACL) injury). Thus, given the sensitivity of chondrocytes to the mechanical environment, understanding the internal mechanical strains in knee articular cartilage under macroscopic loads is an important element in understanding knee OA. The purpose of this study was to test the hypothesis that cartilage from the central and peripheral regions of the tibial plateau has different internal strain distributions under the same applied load. The internal matrix strain distribution for each specimen was measured on osteochondral blocks from the tibial plateau of mature ovine stifle joints. Each specimen was loaded cyclically for 20 min, after which the specimen was cryofixed in its deformed position and freeze fractured. The internal matrix was viewed in a scanning electron microscope (SEM) and internal strains were measured by quantifying the deformation of the collagen fiber network. The peak surface tensile strain, maximum principal strain, and maximum shear strain were compared between the regions. The results demonstrated significantly different internal mechanical strain distributions between the central and peripheral regions of tibial plateau articular cartilage under both the same applied load and same applied nominal strain. These differences in the above strain measures were due to differences in the deformation patterns of the collagen network between the central and peripheral regions. Taken together with previous studies demonstrating differences in the biochemical response of chondrocytes from the central and peripheral regions of the tibial plateau to mechanical load, the differences in collagen network deformation observed in this study help to provide a fundamental basis for understanding the association between altered knee joint kinematics and premature knee OA.

How changes in fibril-level organization correlate with the macrolevel behavior of articular cartilage

The primary structural components of articular cartilage are the zonally differentiated interconnected network of collagen fibrils and proteoglycans, the latter having the potential to bind large amounts of water. Both components exist in a coupled relationship that gives rise to its remarkable mechanical properties. The response of cartilage to compression is governed both by the degree to which the hydrated proteoglycans are constrained within this fibrillar network and the ease with which the matrix fluid can be displaced. The functional properties of cartilage are therefore closely linked to the integrity of the fibrillar network. Our current understanding of this network has been derived via studies conducted at the macro, micro, and ultrastructural levels. Of particular interest to joint researchers and clinicians are issues relating to how the network structure varies both directionally and with zonal depth, how its integrity is maintained via mechanisms of fibril interconnectivity, and how it is modified by ageing, degeneration, and trauma. Physical models have been developed to explore modes of interconnectivity. Combined micromechanical and structural studies confirm the critical role that this interconnectivity must play but detailed descriptions at the molecular level remain elusive. Current computationally based models of cartilage have in some cases implemented the fibrillar component, albeit simplistically, as a separate structure. Considering how important a role fibril network interconnectivity plays in actual tissue structure and mechanical behavior, and especially how it changes with degeneration, a major challenge facing joint tissue modellers is how to incorporate such a feature in their models.

UTILIZATION OF TWO-DIMENSIONAL FAST FOURIER TRANSFORM AND POWER SPECTRAL ANALYSIS FOR ASSESSMENT OF EARLY DEGENERATION OF ARTICULAR CARTILAGE

Degeneration of articular cartilage begins from deterioration of the collagen fibres in the superficial zone. Standard histology using 2D imaging technique is often used to determine the microstructure of collagen fibres and the physiological functions of articular cartilage. However, information of the 3D collageneous structure in the cartilage could be lost and misinterpreted in 2D observations. In contrast, confocal microscopy permits studying the 3D internal structure of bulk articular cartilage with minimal physical disturbing. Using fibre optic laser scanning confocal microscopy, a 3D histology has been previously developed to visualize the collagen matrix in the superficial zone by means of identifying the early arthritic changes in articular cartilage. In this study, we characterized the collagen orientation in the superficial zone of normal cartilage, the cartilage with surface disruption and fibrillated cartilage using Fast Fourier transforms and power spectral analysis techniques. Thus, we have established an objective method for assessing the early pathology changes in the articular cartilage.

Damage initiation and progression in the cartilage surface probed by nonlinear optical microscopy

With increasing interest in treating osteoarthritis at its earliest stages, it has become important to understand the mechanisms by which the disease progresses across a joint. Here, second harmonic generation (SHG) microscopy, coupled with a two-dimensional spring-mass network model, was used to image and investigate the collagen meshwork architecture at the cartilage surface surrounding osteoarthritic lesions. We found that minor weakening of the collagen meshwork leads to the bundling of fibrils at the surface under normal loading. This bundling appears to be an irreversible step in the degradation process, as the stress concentrations drive the progression of damage, forming larger bundles and cracks that eventually form lesions.

Articular cartilage compression: how microstructural response influences pore pressure in relation to matrix health

Our research investigated the influence of degeneration on both the pore-pressure development and microstructural response of cartilage during indentation with a flat-porous-indenter. Experiments were conducted to link the mechanical and structural responses of normal and degenerate articular cartilage. We found that from the instant of loading the degenerate matrix generated a higher peak hydrostatic excess pore pressure in a shorter period of time than the normal matrix. Following the attainment of this peak value the pore pressure in both tissue groups then gradually decayed toward zero over time, thus demonstrating a classical consolidation response. The microstructural analysis provided a unique insight into the influence of degeneration on the mechanisms of internal stress-sharing within the loaded matrix. Both disruption of the articular surface and general matrix destructuring results in an altered deformation field in both the directly loaded and nondirectly loaded regions. It is argued that the higher levels of matrix shear combined with less of the applied load being redirected into the wider cartilage continuum accounts for the elevated levels of peak hydrostatic pore pressure generated in the degenerate matrix.

Central and peripheral region tibial plateau chondrocytes respond differently to in vitro dynamic compression

OBJECTIVE

The objective of this study was to test the hypotheses that chondrocytes from distinct regions of the porcine tibial plateau: (1) display region-specific baseline gene expression, and (2) respond differently to in vitro mechanical loading.

METHODS

Articular cartilage explants were obtained from central (not covered by meniscus) and peripheral (covered by meniscus) regions of porcine tibial plateaus. For baseline gene expression analysis, samples were snap frozen. To determine the effect of mechanical loading, central and peripheral region explants were exposed to equivalent dynamic compression (0-100 kPa) and compared to site-matched free-swelling controls (FSCs). mRNA levels for type II collagen (CII), aggrecan (AGGR), matrix metalloproteinase 1 (MMP-1), MMP-3, MMP-13, A disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAM-TS4), ADAM-TS5, tissue inhibitor of metalloproteinases 1 (TIMP-1), TIMP-2, and tumor necrosis factor alpha (TNFalpha) were quantified using real time polymerase chain reaction (RT-PCR).

RESULTS

At baseline, mRNA levels for the structural proteins CII and AGGR were approximately twofold greater in the central region compared with peripheral region explants. In vitro dynamic compression strongly affected expression levels for CII, AGGR, MMP-3, and TIMP-2 relative to FSCs. Response differed significantly by region, with greater upregulation of CII, AGGR, and MMP-3 in central region explants.

CONCLUSIONS

Chondrocytes from different regions of the porcine tibial plateau express mRNA for structural proteins at different levels and respond to equivalent in vitro mechanical loading with distinctive changes in gene expression. These regional biological variations appear to be related to the local mechanical environment in the normal joint, and thus may indicate a sensitivity of the joint to conditions that alter joint loading such as anterior cruciate ligament (ACL) injury, meniscectomy, or joint instability.

Micro-anatomical response of cartilage-on-bone to compression: mechanisms of deformation within and beyond the directly loaded matrix

The biomechanical function of articular cartilage relies crucially on its integration with both the subchondral bone and the wider continuum of cartilage beyond the directly loaded contact region. This study was aimed at visualizing, at the microanatomical level, the deformation response of cartilage including that of the non-directly loaded continuum. Cartilage-on-bone samples from bovine patellae were loaded in static compression until a near-equilibrium deformation was achieved, and then chemically fixed in this deformed state. Full-depth cartilage-bone sections, incorporating the indentation profile and beyond, were studied in their fully hydrated state using differential interference contrast microscopy. Morphometric measurements of the indented profile were used in combination with a force analysis of the tangential layer to investigate the extent to which the applied force is attenuated in moving away from the directly loaded region. This study provides microscopic evidence of a structure-related response in the transitional zone of the cartilage matrix. It is manifested as an intense chevron-type shear discontinuity arising from the constraints provided by both the strain-limiting articular surface and the osteochondral attachment. The discontinuity persists well into the non-directly loaded continuum of cartilage and is proposed as a force attenuation mechanism. The structural and biomechanical analyses presented in this study emphasize the important role of the complex microanatomy of cartilage, highlighting the interconnectivity and optimal recruitment of the load-bearing elements throughout the zonally differentiated cartilage depth.

Deformation and failure of cartilage in the tensile mode

The aim of this study was to visualize, at the ultrastructural level, the deformation and failure mechanism of cartilage matrix in the tensile mode. Full-thickness dumbbell-shaped specimens were prepared from adult bovines. There were two specimen groups; in the 'parallel' group the specimen axis was parallel to the split lines defining the preferential orientation of the collagen in the articular surface, and in the 'perpendicular' group the specimen axis was perpendicular to the split lines. Specimens were placed with the articular surface uppermost and subjected to a graded series of strain within individual mini-tension devices, while observed with stereomicroscopy and confocal laser scanning microscopy. Thereafter, the changes in the ultrastructure were observed with both scanning and transmission electron microscopy. The mechanism of cartilage failure in the tensile mode comprised the following stages, whether the strain was applied parallel or perpendicular to the split line. (1) At 0% strain a fibrillar meshwork within the articular surface was predominantly orientated in the direction of the split line. (2) As strain increased, the fibrillar meshwork became more orientated in the parallel group and reorientated in the perpendicular group in the direction of the applied strain. (3) After complete reorientation of the fibrillar meshwork in the direction of the applied strain, the initial sign of failure was rupture of the fibrillar meshwork within the articular surface. (4) Subsequently, the rupture rapidly propagated into the deeper layers. Greater strains were required for fibrillar reorientation and complete rupture in the 'perpendicular group' than in the parallel group.

Causes of mechanically induced collagen damage in articular cartilage

Osteoarthritis (OA) is a multifactorial disease, associated with articular cartilage degeneration and eventually joint destruction. The phases of the disease have been described in detail, and mechanical factors play an important role in the initiation of OA, but many questions remain about its etiology. Swelling of cartilage, one of the earliest signs of damage, is proportional to the amount of collagen damage. This strongly suggests that damage to the collagen network is an early event in cartilage degeneration. The goal of this study was to determine the mechanical cause of early collagen damage in articular cartilage after mechanical overloading. Both the shear strain along the fibrils and the maximum fibril strains were evaluated as possible candidates for causing collagen damage. This evaluation was done by comparing the locations of maximum shear and tensile strains with the locations of initial collagen damage after mechanical overloading in bovine explants as found using antibodies directed against denatured type II collagen (Col2-3/4M). Collagen damage could be initiated by excessive shear strains along the collagen fibrils, and by excessive fibrils strains. The locations of collagen damage after mechanical overloading were highly dependent on the cartilage thickness, with thinner cartilage being more susceptible to damage than thicker samples.

The role of computational models in the search for the mechanical behavior and damage mechanisms of articular cartilage

Articular cartilage plays a vital role in the function of diarthrodial joints. Due to osteoarthritis degeneration of articular cartilage occurs. The initial event that triggers the pathological process of cartilage degeneration is still unknown. Cartilage damage due to osteoarthritis is believed to be mechanically induced. Hence, to investigate the initiation of osteoarthritis the stresses and strains in the cartilage must be determined. So far the most common method to accomplish that is finite element analysis. This paper provides an overview of computational descriptions developed for this purpose, and what they can be used for. Articular cartilage composition and structure are discussed in relation with degenerative changes, and how these affect mechanical properties.

A finite element formulation and program to study transient swelling and load-carriage in healthy and degenerate articular cartilage

The theory of poroelasticity is extended to include physico-chemical swelling and used to predict the transient responses of normal and degenerate articular cartilage to both chemical and mechanical loading; with emphasis on isolating the influence of the major parameters which govern its deformation. Using a new hybrid element, our mathematical relationships were implemented in a purpose-built poroelastic finite element analysis algorithm (u-pi-c program) which was used to resolve the nature of the coupling between the mechanical and chemical responses of cartilage when subjected to ionic transport across its membranous skeleton. Our results demonstrate that one of the roles of the strain-dependent matrix permeability is to limit the rate of transmission of stresses from the fluid to the collagen-proteoglycan solid skeleton in the incipient stages of loading, and that the major contribution of the swelling pressure is that of preventing any excessive deformation of the matrix.

Physical indicators of cartilage health: the relevance of compliance, thickness, swelling and fibrillar texture

This study uses a bovine patella model to compare the relative merits of on-bone compliance and thickness measurements, free-swelling behaviour, and structural imaging with differential interference contrast (DIC) light microscopy to assess the biomechanical normality of the cartilage matrix. The results demonstrate that across a spectrum of cartilage tissues from immature, mature, through to mildly degenerate, and all with intact articular surfaces, there is a consistent pattern of increased free swelling of the isolated general matrix with age and degeneration. High swelling was always associated with major structural alterations of the general matrix that were readily imaged using DIC light microscopy. Conversely, for all tissue groups, no relationship was observed between thickness vs. compliance and compliance vs. general matrix swelling. Only in the proximal aspects of the normal mature and degenerate tissues was there a correlation between thickness and general matrix swelling. Free-swelling measurements combined with fibrillar texture imaging using DIC light microscopy are therefore recommended as providing a reliable and quick method of assessing the biomechanical condition of the cartilage general matrix.

A finite element analysis methodology for representing the articular cartilage functional structure

Recognising that the unique biomechanical properties of articular cartilage are a consequence of its structure, this paper describes a finite element methodology which explicitly represents this structure using a modified overlay element model. The validity of this novel concept was then tested by using it to predict the axial curling forces generated by cartilage matrices subjected to saline solutions of known molality and concentration in a novel experimental protocol. Our results show that the finite element modelling methodology accurately represents the intrinsic biomechanical state of the cartilage matrix and can be used to predict its transient load-carriage behaviour. We conclude that this ability to represent the intrinsic swollen condition of a given cartilage matrix offers a viable avenue for numerical analysis of degenerate articular cartilage and also those matrices affected by disease.

Impact load on the triangular fibrocartilage of the wrist: a cadaver study

OBJECTIVE

The aim of this study was to study the role of the triangular fibrocartilage of the wrist in attenuating and transmitting axial force on the ulnar side of the wrist.

DESIGN

Ten biopsies from the triangular fibrocartilage of fresh cadaver wrists were subjected to repetitive axial load during 4 h under reproducible conditions. Another five biopsies were subjected to a higher load and for a longer compression time. Finally, five biopsies were compressed at a (three times) higher compression rate. The amount of force transmitted and attenuated as well as the loaded deformation was measured.

RESULTS

From the first experiment we concluded that 53% of the axial force was attenuated. More force was attenuated (61%) if the axial load was increased but still kept within the physiological boundaries. However, increasing the compression rate beyond the physiological boundaries showed that only very little force is attenuated (11.2%).

CONCLUSION

The triangular fibrocartilage of the wrist has an important force attenuating function and should not be easily resected.

Importance of the superficial tissue layer for the indentation stiffness of articular cartilage

Indentation testing is a widely used technique for nondestructive mechanical analysis of articular cartilage. Although cartilage shows an inhomogeneous, layered structure with anisotropic mechanical properties, most theoretical indentation models assume material homogeneity and isotropy. In the present study, quantitative polarized light microscopy (PLM) measurements from canine cartilage were utilized to characterize thickness and structure of the superficial, collageneous tissue layer as well as to reveal its relation to experimental indentation measurements. In addition to experimental analyses, a layered, transversely isotropic finite element (FE) model was developed and the effect of superficial (tangential) tissue layer with high elastic modulus in the direction parallel to articular surface on the indentation response was studied. The experimental indentation stiffness was positively correlated with the relative thickness of the superficial cartilage layer. Also the optical retardation, which reflects the degree of parallel organization of collagen fibrils as well as collagen content, was related to indentation stiffness. FE results indicated effective stiffening of articular cartilage under indentation due to high transverse modulus of the superficial layer. The present results suggest that indentation testing is an efficient technique for the characterization of the superficial degeneration of articular cartilage.

Functional anatomy of articular cartilage under compressive loading Quantitative aspects of global, local and zonal reactions of the collagenous network with respect to the surface integrity

OBJECTIVE

To assess the influence of local compressive loading on the arrangement of the collagenous fibers in intact articular cartilage. To quantitate the zonal deformation of intact cartilage under load. To analyse the influence of removal of the tangential zone on the load-induced changes.

MATERIALS AND METHODS

380 cylinder shaped cartilage-on-bone samples (d=7 mm) were harvested from 20 bovine femoral heads. In 120 of them the tangential zone was removed. All samples were loaded for 20 min by 0.42 MPa or 0.98 MPa. After proteoglycan extraction, fixation in 4% formalin, dehydration by increasing concentrations of acetone, critical point drying, freeze-fracturing and gold-coating the samples were analysed by scanning-electron-microscopy.

RESULTS

Fiber bulging away from the center of load occurred in an area larger than the directly loaded one and its extent increased parallel to loading (P< 0.01). Crimp was seen only under the indenter and spread with increasing load from the intermediate zone into the tangential zone and radial zone. The absolute height of tangential zone and intermediate zone together remained constant under all loading situations at the costs of the radial zone. All changes due to loading were fully reversible. Removal of the tangential zone reduced the area of bulging (P< 0.01) but markedly increased the amount of crimp. Overall radial strain was not altered, but overall superficial tangential strain was increased by up to 20% (P< 0.01) and high peaks in the local distribution of superficial tensile strain developed.

CONCLUSIONS

The collagenous architecture is a dynamic property of the articular cartilage adapting to its respective loading situation. Crimp reflects local compressive strain. Under compressive loading larger portions of cartilage than the directly loaded areas are functionally included in the process of load transmission. During this process the tangential zone and the intermediate zone form a common functional unit providing a high degree of fiber cross-linkage as a possible mechanism to increase zonal compressive stiffness. Removal of the tangential zone seems to impair distribution of a locally applied compressive load sideways and leads to a reduced cartilage volume included in the process of load transmission. An intact tangential zone contributes to prevent peaks of surface tensile strain.

Concerning the ultrastructural origin of large-scale swelling in articular cartilage

The swelling behaviour of the general matrix of both normal and abnormally softened articular cartilage was investigated in the context of its relationship to the underlying subchondral bone, the articular surface, and with respect to the primary structural directions represented in its strongly anisotropic collagenous architecture. Swelling behaviours were compared by subjecting tissue specimens under different modes of constraint to a high swelling bathing solution of distilled water and comparing structural changes imaged at the macroscopic, microscopic and ultrastructural levels of resolution. Near zero swelling was observed in the isolated normal general matrix with minimal structural change. By contrast the similarly isolated softened general matrix exhibited large-scale swelling in both the transverse and radial directions. This difference in dimensional stability was attributed to fundamentally different levels of fibril interconnectivity between the 2 matrices. A model of structural transformation is proposed to accommodate fibrillar rearrangements associated with the large-scale swelling in the radial and transverse directions in the softened general matrix.

Experimental determination of stress distributions in articular cartilage before and after sustained loading

OBJECTIVE

To test the hypotheses that stress concentrations exist within articular cartilage, and are intensified by sustained 'creep' loading.

DESIGN

Matched-pair comparison of stress distributions in cartilage, in vitro, before and after creep.

BACKGROUND

The ability of cartilage to equalize contact stresses between articulating surfaces may be compromised by undulations in the subchondral bone, and by variations in chemical composition. Furthermore, any stress concentrations within cartilage may be affected by creep loading, which-reduces its water content.

METHODS

Sixteen specimens of apparently normal cartilage-on-bone, 12 mm x 15 mm, were removed from the femoral condyles and tibial plateaux of patients undergoing total knee replacement. The cartilage was subjected to a nominal compressive stress of 2 MPa by means of a 10 mm-diameter flat impermeable indentor. During the 20 s loading period, a miniature pressure transducer, side-mounted in a 0.9 mm-diameter needle, was pulled through the cartilage in a direction parallel to the surface, while transducer output and position were sampled at 25 Hz. 'Stress profiles' were obtained with the transducer pointing in the 12 o'clock and 3 o'clock directions, and were repeated after creep loading at 1.6 MPa for 2 h.

RESULTS

Validation tests indicated that transducer output was proportional to the average compressive stress, but overestimated it by 9-15%. Stresses were greatest under the centre of the indentor, and showed local variations ('concentrations') which were increased in number and size after creep loading.

CONCLUSIONS

Measured values of compressive stress incorporate small systematic errors. Nevertheless, the results presented clearly support both hypotheses.

On the ultrastructure of softened cartilage: a possible model for structural transformation

The fibrillar architecture in the general matrix of softened cartilage has been compared with that of the normal matrix using both Nomarski light microscopy and transmission electron microscopy with combined stereoscopic reconstruction. A pseudorandom network developed from an overall radial arrangement of collagen fibrils is the most fundamental ultrastructural characteristic of the normal general matrix. This, in turn, provides an efficient entrapment system for the swelling proteoglycans. Conversely, the most distinctive feature of the softened matrix is the presence of parallel and relatively unentwined fibrils, strongly aligned in the radial direction. The presence of an optically resolvable fibrous texture in the softened cartilage matrix indicates the presence of discrete bundles of closely packed and aligned fibrils at the ultrastructural level of organisation. The general absence of such texture in the normal cartilage general matrix is consistent with the much greater degree of interconnectedness and related short-range obliquity in the fibrillar architecture, hence the importance of the term pseudorandom network. A mechanism of structural transformation is proposed based on the important property of lateral interconnectivity in the fibrils which involves both entwinement and nonentwinement based interactions. The previously reported difference in intrinsic mechanical strength between the normal and softened matrices is consistent with the transformation model proposed in this study.

Reduction in tensile strength of cartilage precedes surface damage under repeated compressive loading in vitro

An experimental protocol for the fatiguing and tensile testing of articular cartilage has been established. Samples were taken from the interpatellar groove of bovine femurs collected post-slaughter, split into two test groups and subjected to a cyclically varying compressive load of approximately 65 N for 64,800 cycles or 97,200 cycles. The cartilage was then removed from the underlying bone and two specimens, one from the indented region and one from an unindented region - the control, were taken from it and prepared for subsequent tensile testing using notched specimens. From data collected during tensile testing, a value of maximum tensile stress was calculated for each sample. The underlying bone was examined for evidence of microdamage using the basic fuchsin method. A decrease in values of maximum tensile stress (p < 0.05) for the indented sample of each paired group of samples loaded for 97,200 cycles was found. In contrast, those fatigued for only 64,800 cycles showed no such difference. Examination of the underlying bone of these specimens revealed no evidence of trabecular failure and crack formation beneath both the indented and control regions. It is postulated that the fatiguing process used in this experiment induces trauma in the cartilage causing a weakening of the interfibril connections which link collagen fibrils in the matrix, leading to a reduction in tensile strength. This weakening occurs, however, without the appearance of fibrillation on the cartilage surface or any evidence of failure in the bony structure which supports it.

Osteoarthritis. Joint anatomy, physiology, and pathobiology

Normal cartilage is a complex material consisting of a solid matrix composed primarily of collagen and proteoglycan, which is saturated with water. It is not a homogenous material. The interaction of the physical and biochemical structures of cartilage is necessary to allow the normal function of providing nearly frictionless motion, wear resistance, joint congruence, and transmission of load to subchondral bone. Chondrocytes are responsible for synthesizing and maintaining this material. Osteoarthritis occurs when there is disruption of normal cartilage structure and homeostasis. Osteoarthritis results from a complex interaction of biochemical and biomechanical factors that occur concurrently and serve to perpetuate degradative change. The progressive pathologic change that occurs in osteoarthritis has been characterized, not only for articular cartilage but also for periarticular tissues. The occurrence of mechanical and biochemical changes is well established, but the role of each in the etiopathogenesis of osteoarthritis is not rigidly defined. It is likely that there are multiple etiologies sharing common pathways of physical and chemical disruption. (see Fig. 1). The changes associated with osteoarthritis ultimately have an impact on the patient through decreased ability to use the joint or the production of pain, or both. Unfortunately, once these changes are severe enough to be recognized clinically, they are likely to be irreversible with current treatments. Nevertheless, understanding the basic mechanisms involved in the development and progression of osteoarthritis provides a basis for establishing a reasonable expectation for the patient and a rational plan for medical and surgical treatment of this condition.

Scanning electron microscopy of "fibrillated" and "malacic" human articular cartilage: technical considerations

Specimens of articular cartilage from human knees with gross evidence of malacia (dull appearance and/or softness) or fibrillation (exposed fibrous strands and/or staining with India ink) were prepared for scanning electron microscopy (SEM) and compared to cartilage from apparently intact regions. Vertical cryofractures were made through the center of each specimen, so the matrix collagen structure and its relationship to surface features could be examined. Soft, dull, malacic cartilage was characterized by the presence of numerous clefts among the collagen fibers within the most superficial region of the cartilage. In one form of this condition, these clefts did not extend through the articular surface. In a second form, usually observed where the tangential zone was normally thin or absent, the free ends of radial collagen fibers were exposed, but the deeper layers were intact. Two forms of fibrillation were also identified. The first is created by separation of the superficial lamellae which curl up from the tangential layer and form frondlike projections above the normal plane of the joint surface. In the second, deep radial fibers are exposed by vertical fissures. This second form is associated with advanced damage to the joint. The early stages of cartilage failure are characterized by debonding among the major collagen fiber tracts. This process may initiate in the deep tangential zone where the radial fibers cross into the surface. The patterns of the degenerative changes are dictated by the original architecture of the collagen matrix. The microscopic findings do not correlate adequately with conventional gross grading. SEM provides useful information about injured articular cartilage.

Expression of biglycan, decorin and fibromodulin in the hypertrophic phase of experimental osteoarthritis

This study sought to assess the relative levels of the mRNAs of the core proteins of the small proteoglycans (PGs) biglycan, decorin and fibromodulin in the hypertrophic phase of the early osteoarthritis (OA) that follows joint injury. Experimental OA was induced in eight dogs by transection of the anterior cruciate ligament. Articular cartilage was harvested from each joint, the total RNA was extracted and the concentration of DNA in the cartilage was measured. The relative levels of mRNA for biglycan, decorin and fibromodulin were assessed by northern blot analyses. An increase in cartilage mass with no increase in DNA concentration confirmed that the joints were in the hypertrophic phase that follows joint injury. The total RNA per microgram of DNA was increased 2.5 times. Compared with control cartilage, the mRNA levels in osteoarthritic cartilage, when normalized to the concentration of DNA, were increased 3.9 times for biglycan, 1.2 times for decorin and 2.4 times for fibromodulin. Because these small PGs affect collagen fibrillogenesis in vitro, their discoordinate metabolism may contribute to the abnormal collagen formation and deposition that occurs in OA and to the ultimate failure of the articular cartilage.

Alteration and recovery of the spatial orientation of the collagen network of articular cartilage in adolescent rabbits following intra-articular chymopapain injection

We have used polarized light (POL) to monitor changes in the organization of the articular cartilage collagen network and matrix proteoglycans (PGs) after intra-articular injection of chymopapain (CP). POL viewing of sirius red stained sections revealed a loss of normal birefringence suggesting an apparent collapse of the collagen network following intra-articular CP. After 21 days, knees injected with 2.0 mg CP showed no return of normal birefringence, however, normal birefringence was noted in knees injected with only 0.2 mg CP. POL viewing of toluidine blue stained sections revealed a severe loss of matrix PGs followed by PG restoration in animals injected with 0.2 mg CP. The most important inference from the data is that articular cartilage can recover from enzyme-induced alterations in the spatial collapse of its fibrillar network. This is an important finding since it has often been inferred that damage to the collagen network leads invariably to progressive articular cartilage destruction.

The biomechanics of cartilage load-carriage

This paper presents a review and critical appraisal of the more recent attempts to understand the fundamental mechanisms of load-carriage in articular cartilage. In the first section the question is addressed as to how the intrinsic strength of the matrix is developed with respect to its ultrastructure. In the second part we examine various models proposed to explain the response of the matrix to externally applied compressive forces in terms of its unique physico-chemical properties.

The chondrocyte, architect of cartilage. Biomechanics, structure, function and molecular biology of cartilage matrix macromolecules

Chondrocytes are specialised cells which produce and maintain the extracellular matrix of cartilage, a tissue that is resilient and pliant. In vivo, it has to withstand very high compressive loads, and that is explicable in terms of the physico-chemical properties of cartilage-specific macromolecules and with the movement of water and ions within the matrix. The functions of the cartilage-specific collagens, aggrecan (a hydrophilic proteoglycan) and hyaluronan are discussed within this context. The structures of cartilage collagens and proteoglycans and their genes are known and a number of informative mutations have been identified. In particular, collagen fibrillogenesis is a complex process which can be altered by mutations whose effects fit what is known about collagen molecular structural functions. In other instances, mutations have indicated new functions for particular molecular domains. As cartilage provides the template for the developing skeleton, mutations in genes for cartilage-specific proteins often produce developmental abnormalities. The search for mutations amongst such genes in heritable disorders is being actively pursued by many groups, although mutation and phenotype are not always well correlated, probably because of compensatory mechanisms. The special nature of the chondrocyte is stressed in connection with its cell involvement in osteoarthritis, the most widespread disease of diarthrodial joints.

The effects of selective matrix degradation on the short-term compressive properties of adult human articular cartilage

The effects of proteoglycan and collagen digestion on the transient response of human articular cartilage when tested in unconfined compression were determined. Small cylindrical specimens of cartilage, isolated from the femoral head of the hip joint and from the femoral condyles of the knee joint, were subjected to a suddenly applied compressive load using a test apparatus designed to yield a transient oscillatory response. From this response values of the elastic stiffness and the viscous damping coefficient were determined. Cathepsin D and cathepsin B1 were used to digest the proteoglycan in some specimens, while in other specimens leukocyte elastase was used to attack the non-helical terminal regions of the Type II tropocollagen molecules and possibly the Type IX collagen molecule and thereby disturb the integrity of the collagen mesh. The results showed that proteoglycan digestion alone reduced the viscous damping coefficient but it did not significantly alter the elastic stiffness as determined from the oscillatory response. In contrast, the action of elastase reduced both the damping coefficient and the elastic stiffness of the cartilage. The results demonstrated the role of proteoglycans in regulating fluid transport in cartilage and hence controlling the time-dependent viscous properties. The elastic stiffness was shown to be dependent on the integrity of the collagen fibre network and not on the proteoglycans.

Autologous rib perichondrial grafts in experimentally induced osteochondral lesions in the sheep-knee joint: morphological results

The purpose of the present study was to examine the fate of autologous perichondrial grafts after transplantation into cartilage lesions in weight-bearing joints. Osteochondral lesions were made in the articular surface of knee joints in 36 sheep. The defects were filled with autologous rib perichondrial grafts which were secured by either collagen sponges (12 animals) or fibrin glue (12 animals). Defects without perichondrial grafts served as controls (12 animals). Following 1 week of immobilization of the operated leg, the plaster was removed and the animals were allowed to move freely. Animals were sacrificed after 4, 8, 12 and 16 weeks. The grafts were removed and investigated histologically. In contrast to weight-bearing areas and control defects, hyaline-like cartilage formation was seen in non-weight-bearing areas after 4 weeks. This newly formed cartilage revealed strong metachromasia following staining with acidic toluidine blue and reacted positively with periodic acid-Schiff, indicating de novo synthesis of proteoglycans and glycoproteins. Scanning electron microscopy and examinations with polarized light confirmed a hyaline cartilage-like architecture for the surface area as well as for the fibre orientation of the whole graft. Enzyme histochemistry for alkaline and acid phosphatase activity showed positive reactivity only at the base of the transplants.

Histologic anatomy of the triangular fibrocartilage

The collagen arrangement of the triangular fibrocartilage complex was studied in 20 fresh cadaver wrists by means of standard and polarized light microscopy and scanning electron microscopy. The collagen fibres in the articular disk are arranged in undulating sheets oriented at oblique angles to each other. The fibers of the radioulnar ligaments are oriented longitudinally from the radial origin to the ulnar insertion. The origin of the articular disk from the radius is characterized by thick fibers 1 to 2 mm in length radiating from the radius into the articular disk. Five specimens were also injected with india ink. The radioulnar ligaments and the peripheral 15% to 20% of the articular disk are well vascularized, whereas the central 80% of the articular disk is avascular.

The effect of shear fatigue on bovine articular cartilage

The objective of this study was to investigate the effects of mechanical fatigue in the form of cyclic shear strain on articular cartilage. Three millimeter diameter full-thickness plugs were cored from the lateral aspect of bovine tibial plateaus. Sinusoidal shear strains of +/- 5, +/- 10, and +/- 15% were applied to the specimens at 100 Hz for 3 h (a total of 108 x 10(4) cycles). The mechanical shear properties of the tissue (loss and storage moduli) were determined as a function of the number of applied strain cycles. A rapid, irreversible decrease of approximately 35% of initial modulus was found to occur in both loss and storage modulus during application of the first 90,000 cycles. Further decay in the moduli was found to occur from 90 x 10(3) to 108 x 10(4) cycles, but was of considerably smaller magnitude than the initial decrease. The moduli remained relatively constant beyond application of 108 x 10(4) cycles. No consistent change in proteoglycan content was found to be associated with the fatigue process when comparing tested specimens with fresh, untested tissue, and with experimental controls. In addition, no structural defects in the mechanically altered tissue were revealed by scanning electron microscopy.

The three-dimensional 'knit' of collagen fibrils in articular cartilage

TEM stereoscopy of thick sections has been used to reconstruct the 3-dimensional relationships of collagen fibrils in the general matrix of articular cartilage in its relaxed and deformed states. As well as identifying a variety of fibril interactions involving direct physical entwinement which are assumed to provide matrix cohesion the study also highlights the functional importance of the repeatedly kinked morphology exhibited by the radial fibrils. It is suggested that these fibril kinks, in accommodating local compressive strains that approach 100%, function as macro-molecular hinges and permit the collagen elements to undergo large spatial rearrangement without risk to their structural integrity.

Experimental osteoarthritic articular cartilage is enriched in guanidine soluble type VI collagen

Experimental osteoarthritis was surgically induced in the right knee joint of dogs; the left knee served as a control. Articular cartilage was extracted with 4 M guanidinium chloride, 0.05 M sodium acetate, pH 6.0, containing proteinase inhibitors and the proteins purified by associative CsCl density gradient centrifugation. Equal quantities of protein were electrophoresed in agarose-acrylamide gradient gels and the high molecular weight type VI collagen bands detected in immunoblots with a polyclonal antiserum. Type VI collagen bands between 185 and 220 kDa were evident in the pathological specimens of dogs sacrificed 3, 5, and 7 months after surgery and were either absent or only very weakly visible in the controls. These results demonstrate that experimental osteoarthritic cartilage is enriched in 4 M guanidine-soluble type VI collagen.

Biomechanical properties of the canine knee articular cartilage as related to matrix proteoglycans and collagen

The instant, creep and equilibrium responses of canine knee articular cartilages were determined after a constant load application with an in situ indentation creep test and related to the chemical composition of the tissue. Instantly, the cartilage stiffness correlated inversely with the proportion of proteoglycans (PGs) extractable with guanidium chloride. The tibial cartilage, rich in PGs but relatively poor in collagen, showed a low resistance to instant rearrangement of the solid matrix after load application. However, the resistance of the tibial cartilage to water flow during creep deformation was similar or even higher than in the femur. The rate of creep correlated inversely with the PG content. The equilibrium modulus of the femoral cartilage (0.40 MPa), 29 per cent higher than in the tibia (0.31 MPa), was related to the content of PGs, while in the tibia the direct correlation between PGs and modulus was not observed. Our results suggest that while PGs control the fluid flow in articular cartilage, a high PG content alone does not guarantee high stiffness of the cartilage. Instead, the properties of the collagen network are suggested to control particularly the instant shape alterations of the articular cartilage under compression.

An enzymatically induced structural transformation in articular cartilage. Its significance with respect to matrix breakdown

It was demonstrated in this study that the 3-dimensional, "pseudo-random" architecture of collagen in the general matrix of normal cartilage can be transformed enzymatically into a radial array of fibril aggregates or fibers. By first degrading the proteoglycans and then attacking the collagen, it is possible to produce a collagenous structure almost identical to that observed in matrices exhibiting both nonprogressive softening and osteoarthritic changes, and in matrices subjected to dynamic overloading. This structural transformation is explained as a breakdown in the fibril interlinking system.

Indentation study of the biochemical properties of articular cartilage in the canine knee

Considering articular cartilage to be composed of an elastic organic matrix and interstitial water, compressional stiffness of canine knee (stifle) joint cartilage was determined in the form of shear moduli. Different stresses and indenters were used to reveal the suitability of the indentation measurement with a step-load. We were able to demonstrate the linearity of instant and equilibrium responses of cartilage. Using a theoretical solution for indentation of an elastic layer, the effect of the finite thickness of cartilage on the instant and equilibrium responses was eliminated. Using the intrinsic (equilibrium) shear modulus of the cartilage matrix, the stiffest cartilage was found to be located in the patellar surface of the femur. The thick, proteoglycan-rich tibial cartilage, which was uncovered by menisci, was softer than the opposing femoral cartilage. This finding casts doubt on the suggestion that the concentration of proteoglycans unambiguously determines the stiffness of cartilage, but emphasizes the importance of the collagen network also in the compressional behaviour of the cartilage.

Structural consequences of traumatizing articular cartilage

Articular cartilage-on-bone has been subjected to repeated impact loading in vitro and the associated structural changes occurring in the general matrix examined by optical and transmission electron microscopy (TEM). The study shows that repeated trauma transforms the pseudorandom arrangement of fibrils comprising the general matrix of normal articular cartilage into a structural configuration strongly aligned in the radial direction and displaying a prominent waveform or crimp. This stress-induced structural transformation can be predicted from the application of a recently developed structural model of articular cartilage. Further, this altered structure bears a close resemblance to that commonly observed in articular cartilage exhibiting both non-progressive degeneration and osteoarthritic changes.