This is a histological study of rat knee joints that were subjected to limitation of motion alone or were remobilized after varying periods of restricted movement. A progression of morphological changes consequent to restriction of motion was found, and it was possible to gain some indication of the degree of reversibility of these changes after varying periods of immobilization and remobilization either by a single forced manipulation, or by active movement alone, or by repeated gentle passive movement. On the basis of this study, we have arrived at the following conclusions in regard to experimental joint immobilization and remobilization in the rat:

1. Rigid immobilization is not essential to produce structural change, and the maintenance of a slight range of motion will not prevent structural changes;

2. After prolonged immobilization, contracture of both the muscles and the capsule is responsible for restriction of motion, with shortening of the muscles being primarily at fault;

3. Proliferation of intracapsular connective tissue and the formation of adhesions are primary responses to limitation of motion—to a limited degree these changes are reversible;

4. Major cartilage alterations, such as matrix fibrillation, cleft formation and ulceration, as well as their adjacent subchondral lesions, result from abnormal friction and pressure in a joint compromised by limitation of motion—these changes are not reversible;

5. In the rat, the joint changes that are caused by restriction of motion appear to be reversible if the period of immobilization does not exceed thirty days—after sixty days of immobilization all major joint alterations consequent to the restriction of motion have appeared, and further immobilization after this time only causes changes in the degree of these alterations;

6. With forced remobilization there is tearing of the connective tissue that has proliferated within the joint, often in a plane different from that of the original joint cleft; subsequently, associated with continued motion of the joint, a cellular layer with the characteristics of synovial membrane forms about the surfaces of the new cleft;

7. Three methods of remobilization—forced, active, and passive—were used in these experiments, which were equally effective in restoring of the range of motion; after thirty-five days of remobilization by any means the joints were histologically indistinguishable.