The role of joint architecture in the etiology of arthritis.

Focal degenerative changes occur in some joints very early in life. These changes in the articular cartilage appear to occur in the unloaded, rather than the loaded, areas of the joint. One possible cause for this pattern of degeneration is lack of use or stress in these particular areas of the joint; just as unused bone and unused muscle atrophy, so may unused cartilage. If these unloaded areas were never subjected to mechanical stress, degeneration at these sites perhaps would not be important. However, bones, including their articular ends, are in a constant state of change through the process of remodeling, which continues throughout life. Joint surfaces are not, in general, spherical, and therefore must be incongruent during most of their arc of movement. In the young person, this incongruity maintains physiologic loading and joint nutrition. Studies have shown age-related changes in the remodeling process that lead to increasing joint congruity in old age. These age-related increases in congruity may result in a redistribution of load in the joint such that there is an increased stress on formerly unloaded atrophic cartilage. Arthritis always results in a change in joint shape. It is suggested that a change in shape caused by a disturbance in the remodeling process may itself be an important contributing cause of osteoarthritis.