Effects of immobilization on joints.

Stress deprivation alters the morphologic, biochemical, and biomechanical characteristics of various components of synovial joints. Prominent among the protean changes that result are proliferation of fibrofatty connective tissue within the joint space, adhesions between synovial folds, adherence of fibrofatty connective tissue to cartilage surfaces, atrophy of cartilage, "ulceration" at points of cartilage-cartilage contact, disorganization of cellular and fibrillar ligament alignment, weakening of ligament insertion sites owing to osteoclastic resorption of bone and Sharpey's fibers, regional osteoporosis of the involved extremity, increased force requirement for joint cycling, and increased ligament compliance. Reduced load-to-failure and reduced energy-absorbing capacity of the bone-ligament-bone complex progresses to about one-third that of controls. Collagen mass declines by about 10%. Collagen turnover increases with accelerated degradation and synthesis. Formation of reducible collagen crosslinks increases. Content of proteoglycan, notably hyaluronic acid, falls and water content is correspondingly reduced. An understanding of the functional implications of these extensive perturbations in the phenomenon of joint immobilization is essential for progress in orthopedics.