Biology of the aging joint.

Aging implies changes in joint components over a continuum of time that contribute later in life to an increasing frequency of clinical complaints and impairments in function and mobility. This article considers how aging appears to modify the articular cartilage, subchondral bone, muscle, the soft tissues, synovial membrane, and synovial fluid. Whether the changes in aging inevitably progress through an intermediary phase of "degenerated cartilage" to the fibrillated state of osteoarthritis is not clear.