Knee and ankle: human joints with different susceptibility to osteoarthritis reveal different cartilage cellularity and matrix synthesis in vitro

Clinical experience shows that symptoms and pathological changes of primary osteoarthritis (OA) are more frequent and severer in the knee than in the ankle joint. The different anatomy of both weight-bearing joints implies that biomechanical differences may contribute to their varying susceptibility to OA. This study aims at elucidating other non-biomechanical factors to explain these fundamental differences in secondary OA prevalence. Human cartilage of matched ankle and knee joints from organ donors was dissected in full-thickness slices or in layers. The DNA content for estimation of cell number was analyzed fluorometrically. Chondrocytes were cultured in organ culture or after isolation in alginate. Proteoglycan synthesis was determined by 35S incorporation, and collagen synthesis by 3H-proline incorporation. This study demonstrates that in both joints, the cell density sharply declines between newborn and young infant ages. In addition, cartilage from the ankle joint is significantly more cellular than cartilage from the knee joint. In general, ankle chondrocytes synthesize more proteoglycans (PGs) and collagens than knee chondrocytes, and deep zone chondrocytes more than superficial zone chondrocytes. The biochemical properties of chondrocytes of the ankle and knee joints differ significantly and might play an important role in the pathogenesis of OA.