Site-dependent biomechanical responses of chondrocytes in the rabbit knee joint

Biomechanical responses of chondrocytes were determined in specific locations within the superficial zone of patellar, femoral groove, femoral condyle and tibial plateau cartilages obtained from female New Zealand White rabbits. A confocal laser scanning microscope combined with a custom indentation system was utilized for experimentation. Changes in cell volumes and dimensions (i.e. cell height, width and depth) due to loading, global, local axial and transverse strains were determined for each site. Tissue composition and structure was analysed at each indentation site with digital densitometry, polarized light microscopy and Fourier transform infrared imaging spectroscopy. Patellar cells underwent greater volume decreases (compared to femoral groove cells; p<0.05) primarily due to greater decreases in cell height (p<0.05), consistent with greater levels of both global and local axial strains (p<0.05). Lateral condyle cells underwent greater volume decreases (compared to lateral plateau cells; p<0.05) primarily due to greater decreases in cell height, consistent with greater levels of tissue strains (p<0.05). Medial condyle cells underwent smaller volume decreases (compared to medial plateau cells; p<0.05) primarily due to elevated cell expansions in the depth direction, which was consistent with greater levels of minor transverse strains (p<0.05). Site-dependent differences in collagen orientation angles agreed conceptually with the observed cell dimensional changes. Chondrocyte biomechanical responses were highly site-dependent and corresponded primarily with the orientation of the collagen fibrils. The observed differences were thought to be due to the different biomechanical loading conditions at each site.