Peri-articular tubercle and abscess may result from the spread of disease from the bone or joint into the surrounding tissues, either directly or by way of the lymphatics.  A peri-articular abscess may spread in several directions, sometimes invading tendon sheaths or bursae, and finally reaching the skin surface by tortuous sinuses.

Reactive changes in the vicinity of tuberculous joints are of common occurrence, and play a considerable part in the production of what is clinically known as white swelling.  New connective tissue forms in the peri-articular fat and between muscles and tendons.  It may be tough and fibrous, or soft, vascular, and oedematous, and the peri-articular fat becomes swollen and gelatinous, constituting a layer of considerable thickness.  The fat disappears and is replaced by a mucoid effusion between the fibrous bundles of connective tissue.  This is what was formerly known as gelatinous degeneration of the synovial membrane.  In the case of the wrist the newly formed connective tissue may fix the tendons in their sheaths, interfering with the movements of the fingers.  In relation to the bones also there may be reactive changes, resulting in the formation of spicules of new bone on the periosteal surfaces and at the attachment of the capsular and other ligaments; these are only met with where pyogenic infection has been superadded.

Terminations and Sequelae.—­A natural process of cure may occur at any stage, the tuberculous tissue being replaced by scar tissue.  Recovery is apt to be attended with impairment of movement due to adhesions, ankylosis, or contracture of the peri-articular structures.  Caseous foci in the interior of the bones may become encapsulated, and a cure be thus effected, or they may be the cause of a relapse of the disease at a later date.  Interference with growth is comparatively common, and may involve only the epiphysial junctions in the immediate vicinity of the joint affected, or those of all the bones of the limb.  This is well seen in adults who have suffered from severe disease of the hip in childhood—­the entire limb, including the foot, being shorter and smaller than the corresponding parts of the opposite side.

Atrophic conditions are also met with, the bones undergoing fatty atrophy, so that in extreme cases they may be cut with a knife or be easily fractured.  These atrophic conditions are most marked in bedridden patients, and are largely due to disuse of the limb; they are recovered from if it is able to resume its functions.

#Clinical Features.#—­These vary with the different anatomical forms of the disease, and with the joint affected.

Sometimes the disease is ushered in by a febrile attack attended with pains in several joints—­described by John Duncan as tuberculous arthritic fever.  This is liable to be mistaken for rheumatic fever, from which, however, it differs in that there is no real migration from joint to joint; there is an absence of sweating and of cardiac complications; and no benefit follows the administration of salicylates.