#Conditions of Impaired Mobility of Joints.#—­There are four conditions of impaired mobility in joints:  rigidity, contracture, ankylosis, and locking. Rigidity is the fixation of a joint by involuntary contraction of muscles, and is of value as a sign of disease in deep-seated joints, such as the hip.  It disappears under anaesthesia.

Contracture is the term applied when the fixation is due to permanent shortening of the soft parts around a joint—­muscles, tendons, ligaments, fasciae, or skin.  As the structures on the flexor aspect are more liable to undergo such shortening, contracture is nearly always associated with flexion.  Contracture may result from disease of the joint, or from conditions outside it—­for example, disease in one of the adjacent bones, or lesions of the nerves.

Ankylosis is the term applied when impaired mobility results from changes involving the articular surfaces.  It is frequently combined with contracture.  Three anatomical varieties of ankylosis are recognised—­(a) The fibrous, in which there are adhesions between the opposing surfaces, which may be in the form of loose isolated bands of fibrous tissue, or may bind the bones so closely together as to obliterate the cavity of the joint.  The resulting stiffness, therefore, varies from a mere restriction of the normal range of movement, up to a close union of the bones which prevents movement.  Fibrous ankylosis may follow upon injury, especially dislocation or fracture implicating a joint, or it may result from any form of arthritis. (b) Cartilaginous ankylosis implies the fusion of two apposed cartilaginous surfaces.  It is often found between the patella and the trochlear surface of the femur in tuberculous disease of the knee.  The fusion of the cartilaginous surfaces is preceded by the spreading of a vascular connective tissue, derived from the synovial membrane, over the articular cartilage.  Clinically, it is associated with absolute immobility, (c) Bony ankylosis or synostosis is an osseous union between articulating surfaces (Figs. 154 and 155).  It may follow upon fibrous or cartilaginous ankylosis, or may result from the fusion of two articular surfaces which have lost their cartilage and become covered with granulations.  In the majority of cases it is to be regarded as a reparative process, presenting analogies with the union of fracture.

[Illustration:  FIG. 154.—­Osseous Ankylosis of Femur and Tibia in position of flexion.]

The term arthritis ossificans has been applied by Joseph Griffiths to a condition in which the articular surfaces become fused without evident cause.

The occurrence of ankylosis in a joint before the skeleton has attained maturity does not appear to impair the growth in length of the bones affected; ankylosis of the temporo-maxillary joints, however, greatly impairs the growth of the mandible.  When there is arrest of growth accompanying ankylosis, it usually depends on changes in the ossifying junctions caused by the original disease.