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 envel ope 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 th e avascular cartilage are met by nutrients and waste products diffusin g through the synovial fluid and into and out of the synovium and its blood vessels and lymphatics. Synovial nerves give joint position info rmation. Fat, collagen, and glycosaminoglycans constitute the deformab le synovial sheath. Synovial lining cells synthesize joint lubricants, matrix molecules, digestive enzymes, and cytokines, and participate i n immunologic processes that can be reparative or degradative especial ly of cartilage. Heavy repeated forces applied to the upper and lower extremity joints cause degenerative changes that can be documented rad iographically. Repeated light loading, such as in computer keyboard us e, is evaluated inadequately with current imaging and clinical techniq ues. Differences in individual's response to repetitive loading may be caused by subtle differences in the interaction and initial condition s of the musculoskeletal structures, including the joint, involved in repetitive strain injuries.