By this name is meant fixation of a joint by union of the opposing bones by means of firm adhesions. The expression "false anchylosis" is sometimes used to designate the condition in which the joint is fixed, not by adhesion between the bones, but by rigidity and shortening of the surrounding soft parts.

For the most part anchylosis is the result of inflammations of joints, where the cartilage has been destroyed and healing has subsequently occurred. In the process of healing the inflammatory tissue on the opposing surfaces develops into connective tissue, and as the two surfaces have to a considerable extent coalesced, fibrous tissue unites them permanently. In this fibrous bond of union there are often bony plates, and occasionally the union is effected by bone itself. In the latter case the term Synostosis is applied. This, however, is a very-unusual occurrence, as even slight movement of the joint is sufficient to prevent the formation of bone. It takes place chiefly in joints which have little movement naturally, such as the sacro-iliac synchondrosis. The fixation of the joint, however, is often so firm as to resemble an actual coalescence of the bones.

The term Spondylitis deformans is given to a condition in which the vertebrae are anchylosed together. There is synostosis of the arches and articular processes, while the heads of the ribs are anchylosed to the spine. The condition is a gradually advancing one, and the back becomes rigid.