The geometry of diarthrodial joints, its physiologic maintenance, and the possible significance of age-related changes in geometry-to-load distribution and the development of osteoarthritis.

Diarthrodial joints are governed by physiologic mechanisms that maintain stability and an equitable distribution of load. Modeling continues throughout life to maintain the necessary physiologic incongruity. However, in old age the system seems to go awry, and the result is an increasing congruity yielding possibly increased stability but interfering with cartilage nutrition and altering the distribution of load. The increasing maldistribution of load, with age, it is proposed, mechanically overtaxes the previously underloaded and, presumably, atrophic cartilage. Overtaxing the cartilage in turn leads to further depletion of proteoglycans, collagen fraying and eventually osteoarthritis. Thus arthritis, at least in one of its forms, appears to be inevitable because of the maldistribution of load that results from the age-related changes in joint shape possibly dictated by the joint's requirement for stability.