Structure and physiology of joints and their relationship to repetitive strain injuries

Joints involved in repetitive strain injuries are diathrodial, that is, two bone ends with cartilaginous end plates sheathed by a soft envelope of synovium. The cartilaginous plates, consisting of chondrocytes, ground substance, and at least seven species of collagen, but mostly Type II, cushion the bone ends during repeated elastic compression and enable them to slide with minimal friction. The metabolic needs of the avascular cartilage are met by nutrients and waste products diffusing through the synovial fluid and into and out of the synovium and its blood vessels and lymphatics. Synovial nerves give joint position information. Fat, collagen, and glycosaminoglycans constitute the deformable synovial sheath. Synovial lining cells synthesize joint lubricants, matrix molecules, digestive enzymes, and cytokines, and participate in immunologic processes that can be reparative or degradative especially of cartilage. Heavy repeated forces applied to the upper and lower extremity joints cause degenerative changes that can be documented radiographically. Repeated light loading, such as in computer keyboard use, is evaluated inadequately with current imaging and clinical techniques. Differences in individual's response to repetitive loading may be caused by subtle differences in the interaction and initial conditions of the musculoskeletal structures, including the joint, involved in repetitive strain injuries.