Evaluation of subchondral bone mineral density associated with articular cartilage structure and integrity in healthy equine joints with different functional demands.

OBJECTIVE: To determine and correlate subchondral bone mineral density and overlying cartilage structure and tensile integrity in mature healthy equine stifle (low magnitude loading) and metacarpophalangeal (high magnitude loading) joints. ANIMALS: 8 healthy horses, 2 to 3 years of age. PROCEDURE: Osteochondral samples were acquired from the medial femoral condyle (FC) and medial trochlear ridge (TR) of the stifle joint and from the dorsal (MC3D) and palmar (MC3P) aspects of the distal medial third metacarpal condyles of the metacarpophalangeal joint. Articular cartilage surface fibrillation (evaluated via India ink staining) and tensile biomechanical properties were determined. The volumetric bone mineral density (vBMD) of the underlying subchondral plate was assessed via dual-energy x-ray absorptiometry. RESULTS: Cartilage staining (fibrillation), tensile moduli, tensile strength, and vBMD were greater in the MC3D and MC3P locations, compared with the FC and TR locations, whereas tensile strain at failure was less in MC3D and MC3P locations than FC and TR locations. Cartilage tensile moduli correlated positively with vBMD, whereas cartilage staining and tensile strain at failure correlated negatively with vBMD. CONCLUSIONS AND CLINICAL RELEVANCE: In areas of high joint loading, the subchondral bone had high vBMD and the articular cartilage surface layer had high tensile stiffness but signs of structural wear (fibrillation and low failure strain). The site-dependent variations and relationships in this study support the concept that articular cartilage and subchondral bone normally adapt to physiologic loading in a coordinated way.

Mechanical characterization of native and tissue-engineered cartilage.

Cartilage functions as a low-friction, wear-resistant, load-bearing tissue. During a normal gait cycle, one cartilage surface rolls and slides against another, all the while being loaded and unloaded. The durability of the tissue also makes for an impressive material to study. However, when cartilage is damaged or diseased, the tissue has little capacity to repair itself. The goal of cell-based repair strategies to replace damaged or diseased tissue requires that the functional biomechanical properties of normal (developing or mature), diseased, and repair cartilage be restored. This chapter addresses some of the major methods used to assess the biomechanical properties of native and tissue-engineered cartilage. First, the traditional methods of testing by compression, tension, shear, and indentation are reviewed. Next, additional methods to evaluate interfacial mechanics and lubrication are described. Thus, a variety of mechanical tests may be used to assess functional properties for normal, diseased, and tissue-engineered cartilage.

Interleukin-1alpha induction of tensile weakening associated with collagen degradation in bovine articular cartilage.

OBJECTIVE: To determine whether interleukin-1alpha (IL-1alpha) induces tensile weakening of articular cartilage that is concomitant with the loss of glycosaminoglycans (GAGs) or the subsequent degradation of the collagen network. METHODS: Explants of young adult bovine cartilage obtained from the superficial (including the articular surface), middle, and deep layers were cultured with or without IL-1alpha for 1 week or 3 weeks. Then, portions of the explants were analyzed for their tensile properties (ramp modulus, strength, and failure strain); other portions of explants and spent culture medium were analyzed for the amount of GAG and the amount of cleaved, denatured, and total collagen. RESULTS: The effect of IL-1alpha treatment on cartilage tensile properties and content was dependent on the duration of culture and the depth of the explant from the articular surface. The tensile strength and failure strain of IL-1alpha-treated samples from the superficial and middle layers were lower after 3 weeks of culture, but not after 1 week of culture. However, by 1 week of culture, IL-1alpha had already induced release of the majority of tissue GAGs into the medium, without detectable loss or degradation of collagen. In contrast, after 3 weeks of culture, IL-1alpha induced significant collagen degradation, as indicated by the amount of total, cleaved, or denatured collagen in the medium or in explants from the superficial and middle layers. CONCLUSION: IL-1alpha-induced degradation of cartilage results in tensile weakening that occurs subsequent to the depletion of GAG and concomitant with the degradation of the collagen network.

Indentation testing of human cartilage: sensitivity to articular surface degeneration.

OBJECTIVE: To determine, for clinical indentation testing of human articular cartilage, the effects of aging and degeneration on indentation stiffness and traditional indices of cartilage degeneration; the relationship between indentation stiffness and indices of degeneration; and the sensitivity and specificity of indentation stiffness to cartilage degeneration. METHODS: Osteochondral cores from femoral condyles of cadaveric human donors were harvested. Samples were distributed into experimental groups based on donor age (young [20-39 years], middle [40-59 years], and old [>/=60 years]), and a macroscopic articular surface appearance that was either normal or mildly degenerate, without deep erosion. Samples were analyzed for indentation stiffness, cartilage thickness, India ink staining (quantitated as the reflected light score), and Mankin-Shapiro histopathology score. RESULTS: Indentation stiffness, India ink staining, and the histopathology score each varied markedly between normal-sample and degenerate-sample groups but varied relatively little between normal samples obtained from different age groups. A decrease in indentation stiffness (softening) correlated with a decrease in the reflectance score and an increase in the overall histopathology score, especially the surface irregularity component of the histopathology score. Receiver operating characteristic analysis suggested that the indentation testing could accurately detect cartilage degeneration as indicated by macroscopic appearance, India ink staining, and histopathology score. CONCLUSION: The indentation stiffness of the normal to mildly degenerate samples tested in this study was sensitive to mild degeneration at the articular surface and was insensitive to changes associated with normal aging or to slight variations in cartilage thickness. This suggests that indentation testing may be a useful clinical tool for the evaluation of early-stage degenerative changes in articular cartilage.

Site- and exercise-related variation in structure and function of cartilage from equine distal metacarpal condyle.

OBJECTIVE: Determine (1) the site-associated response of articular cartilage of the equine distal metacarpal condyle to training at a young age as assessed by changes in indentation stiffness and alterations in cartilage structure and composition, and (2) relationships between indentation stiffness and indices of cartilage structure and composition. METHOD: Experimental animals (n=6) were trained on a track (increasing exercise to 1km/day by 5 months); controls (n=6) were pasture-reared. Animals were euthanized at 18 months and four osteochondral samples were harvested per metacarpal condyle from dorsal-medial, dorsal-lateral, palmar-medial, and palmar-lateral aspects. Cartilage was analyzed for India ink staining (quantified as reflectance score (RS)), short-term indentation stiffness (sphere-ended, 0.4mm diameter), thickness, and biochemical composition. RESULTS: Cartilage structural, biochemical and biomechanical properties varied markedly with site in the joint. Sites just medial and just lateral to the sagittal ridge showed signs of early degeneration, with relatively low RS, indentation stiffness, and collagen content, and relatively high water content. Effects of exercise and side (left vs right) were not detected for any measure. Overall, indentation stiffness correlated positively with RS and collagen content, and inversely with thickness and water content. CONCLUSION: Gentle exercise-imposed mechanical stimulation did not markedly affect articular cartilage function or structure. However, the marked site-associated variation suggests that biomechanical environment can initiate degenerative changes in immature cartilage during joint growth and maturation.

Induction of advanced glycation end products and alterations of the tensile properties of articular cartilage.

OBJECTIVE: To determine whether increasing advanced glycation end products (AGEs) in bovine articular cartilage to levels present in aged human cartilage modulates the tensile biomechanical properties of the tissue. METHODS: Adult bovine articular cartilage samples were incubated in a buffer solution with ribose to induce the formation of AGEs or in a control solution. Portions of cartilage samples were assayed for biochemical indices of AGEs and tested to assess their tensile biomechanical properties, including stiffness, strength, and elongation at failure. RESULTS: Ribose treatment of cartilage induced increases in tissue fluorescence, absorbance, and pentosidine content (P < 0.001 for each comparison) by amounts similar to those that occur during aging in humans. Ribose treatment of cartilage also induced an increase in dynamic modulus (60% increase) and strength (35% increase), and a decrease (25% decrease) in strain (P < 0.001 for each comparison). CONCLUSION: The concomitant increase in AGEs and alteration of tensile properties of cartilage after ribose treatment suggest that aging-associated changes in AGEs have functional consequences for this tissue. The AGE-associated increases in strength and stiffness of cartilage may be beneficial by counteracting the decreases in these properties that are associated with degeneration. Conversely, the AGE-associated decrease in failure length, or increase in brittleness, together with increased stiffness may predispose cartilage to increased stress concentration, fracture, and aging-associated biomechanical dysfunction.