Softening of cartilage is the initial degenerative step of osteoarthritic cartilage by matrix degradation and corruption of interconnection of the collagen fibrillar network. The purpose of this study was to investigate the correlation of chondrocyte apoptosis, matrix degradation, and the corruption of collagen architecture in the development of severe swelling of osteoarthritic cartilage.
Twenty osteoarthritic and 7 normal femoral neck fractured cartilage samples were obtained from patients with knee osteoarthritis and normal patients with femoral neck fracture at the time of total hip joint replacement surgery. Apoptosis was verified by TUNEL (terminal deoxynucleotidyl transferase-mediated deoxyuridine 5-triphosphate nick end-labeling) staining and structural changes were observed under phase-contrast microscopy. Matrix degradation was evaluated by histochemical analysis of proteoglycans. Swelling tests were performed by immersing the cartilage slices in hypotonic solution. The results of ultrastructural study of collagen architecture of osteoarthritic cartilage performed by scanning electron microscopy before and after swelling were compared.
Matrix degradation was most prominent in the middle zone of osteoarthritic cartilage. The percentage of chondrocytes in osteoarthritic cartilage showing apoptosis ranged from 15 to 20% (average, 18%; standard deviation (SD) = 3.2%) and was correlated with the extent of structural changes and matrix degradation. The swelling strain of the osteoarthritic cartilage varied from 120 to 200% (average, 160%; SD = 40%) depending on the degree of matrix degradation and structural changes. The loss of interconnectivity of collagen fibrillar architecture was correlated with the increased swelling potential of osteoarthritic cartilage.
This study demonstrated that chondrocyte apoptosis was correlated with matrix degradation and the corruption of fibrillar architecture and that the extent of these manifestations correlated with the swelling potential of osteoarthritic cartilage. These findings also emphasize the importance of the fibrillar architecture in maintaining the mechanical properties of cartilage.