The tensile properties of articular cartilage diminish during normal aging in osteoarthritis. In osteoarthritis, disruption of the collagen network is one of the earliest structural changes, and this has been associated with decreases in tensile modulus and strength and an increase in water content of the cartilage. However, for both normal and degenerate articular cartilage, the dependence of tensile mechanical properties on biochemical composition and depth-variation in structure remains unclear.

The first goal was to develop a video imaging technique for India ink stained articular surfaces in order to map and quantify the extent of cartilage degeneration. Application of the method to rabbit knees at nine weeks following transection of the anterior cruciate ligament demonstrated quantifiably greater amounts of ink-retention, as an indicator of surface fibrillation and cartilage erosion. The damaged areas were localized to the posteromedial and anterolateral regions of the femoral condyles and tibial plateaus, which appeared to be contacting surfaces.

While the tensile behavior of cartilage is a sensitive indicator of aging and degeneration, most studies have focused on measuring the tensile modulus or stiffness, with few studies examining Poisson's ratios. The second goal was to determine the three-dimensional equilibrium tensile behavior and strength of aged human patellar cartilage, harvested from superficial, middle and deep layers, using a new bi-planar epifluorescent video microscopy technique. The Poisson's ratios were greater (deformed more) in the superficial layers than the deeper layers, with significantly more thickness reduction than lateral contraction in each layer. In the same samples, there was no detectable depth variation in the tensile modulus or strength, although the proteoglycan content was significantly lower in the superficial layers than the deep layers.

Osteoarthritis is known to involve alteration of the collagen meshwork, as well as loss of proteoglycan. In addition, the collagen meshwork structure and proteoglycan content are known to vary with depth from the articular surface. How these factors affect the tensile behavior of cartilage is unknown. The third goal was to analyze cartilage behavior theoretically by relating the equilibrium tensile modulus and Poisson's ratios to proteoglycan-associated swelling and collagen network properties that provide restraining forces. The results describe how the measured tensile properties are dependent on both the glycosaminoglycan content and the mechanical properties of the collagen meshwork. Most of the cartilage deformation during tension was calculated to occur in the extrafibrillar space. The modulus of the collagen network approximated the modulus of the cartilage specimen, supporting the view that the collagen meshwork is largely responsible for tensile load carriage in cartilage.