Bony ankylosis of the joints of a finger, whether the result of injury or disease, is difficult to remedy by any operative procedure, for while it is possible to restore mobility, the new joint is apt to be flail-like.

Locking.—­A joint is said to lock when its movements are abruptly arrested by the coming together of bony outgrowths around the joint.  It is best illustrated in arthritis deformans of the hip in which new bone formed round the rim of the acetabulum mechanically arrests the excursions of the head of the femur.  The new bone, which limits the movements, is readily demonstrated in skiagrams; it may be removed by operative means.  Locking of joints is more often met with as a result of injuries, especially in fractures occurring in the region of the elbow.  In certain injuries of the semilunar menisci of the knee, also, the joint is liable to a variety of locking, which differs, however, in many respects from that described above.

#Errors of Development.#—­These include congenital dislocations and other deformities of intra-uterine origin, such as abnormal laxity of joints, absence, displacement, or defective growth of one or other of the essential constituents of a joint.  The more important of these are described along with the surgery of the Extremities.

DISEASES OF JOINTS

#Bacterial Diseases.#—­In most bacterial diseases the organisms are carried to the joint in the blood-stream, and they lodge either in the synovial membrane or in one of the bones, whence the disease subsequently spreads to the other structures of the joint.  Organisms may also be introduced through accidental wounds.  It has been shown experimentally that joints are among the most susceptible parts of the body to infection, and this would appear to be due to the viscid character of the synovial fluid, which protects organisms from bactericidal agents in the tissues and fluids.

PYOGENIC DISEASES

The commoner pyogenic diseases are the result of infection of one or other of the joint structures with staphylococci or streptococci, which may be demonstrated in the exudate in the joint and in the substance of the synovial membrane.  The mode of infection is the same as in the pyogenic diseases of bone, the metastasis occurring most frequently from the mucous membrane of the pharynx (J.  B. Murphy).  The localisation of the infection in a particular joint is determined by injury, exposure to cold, antecedent disease of the joint, or other factors, the nature of which is not always apparent.