Joint physiology, cartilage metabolism, and the etiology of osteoarthritis

Articular cartilage is hypocellular, avascular, aneural, and alymphatic. Nutrition derives predominantly from the synovial fluid. The cartilage matrix is hyperhydrated. Water represents 80 per cent of the total weight. The water is very important in joint lubrication and wear resistance. The dry weight consists mainly of proteoglycan and type II collagen. Osteoarthritis is not the result of a diminution in metabolic activity but is a very active catabolic process. Matrix synthesis and cell replication proceed at greater rates in damaged than in normal cartilage. Lysosomal enzymes that degrade cartilage are released. Proteoglycan content diminishes in proportion to the disease severity. Despite the heightened synthetic activity, the chondrocyte's capacity is eventually exceeded by the rate of matrix degradation and the cartilage becomes eroded. A multitude of hypotheses have been suggested to explain the etiopathogenesis of osteoarthritis. These hypotheses fall into two categories: those that point to excessive stresses imposed upon normal tissue and those that emphasize the inadequacy of the chondrocyte response. The factors that initiate the process are not fully known. Trauma, aging, joint laxity, diet, hormones, crystal deposition, bone microfractures, and immunologic factors have all been implicated.