1. An injury to articular cartilage which does not involve the subchondral bone is not repaired, because of the inability of fully developed hyaline cartilage to revert to a more embryonic type of connective tissue, which is the precursor of cartilage.
2. Experimental injuries which include articular cartilage and subchondral bone are repaired by the invasion of fibrous connective tissue from the underlying bone. After several months, the connective tissue resembles fibrocartilage and may show areas of metaplasia into hyaline cartilage.
3. The formation of intra-articular adhesions is favored by the complete immobilization of injured joints. Such adhesions tend to produce degenerative changes in any articular surface to which they become attached.
4. In man, articular cartilage has no power of regeneration, and the metaplasia of connective tissue into cartilage is rarely seen. Such metaplasia is more apt to result from a severe injury, with an accompanying destruction of subchondral bone, than it is to occur following injuries which involve the cartilage alone.