Microindentation of the Individual Layers of Articular Cartilage

Articular cartilage is known to have three distinct layers (superficial, middle, and deep) along its depth. Each layer has different biochemical, structural, and cellular characteristics. In the superficial layer, collagen fibrils are densely arranged along a tangential direction to the articular surface. Collagen fibrils are randomly oriented in the middle layer and vertically toward the subchondral bone in the deep layer. The superficial layer has a relatively low concentration of proteoglycans while the matrix in the middle and deep zones contains higher concentrations. The chondrocytes are flattened and most densely populated in the superficial layer, are rounded in the middle zone, and are larger, elongated, and arranged vertically between extracellular matrix in the deep layer [1, 2]. Based on the depth-related differences in the structural, biochemical, and cellular compositions, it is reasonable to assume that intrinsic mechanical properties of articular cartilage vary with depth. However, little information is available on the variation of the biomechanical properties in different layers of articular cartilage. The objectives of this study were 1) to test the applicability of the microindenter for testing cartilage at the microscopic level and 2) to obtain the intrinsic biomechanical properties of rabbit articular cartilage as a function of depth.