The thickened synovial membrane is projected into the cavity of the joint, filling up its pouches and recesses, and spreading over the surface of the articular cartilage “like ivy growing on a wall.”  Wherever the synovial tissue covers the cartilage it becomes adherent to and fused with it.  The morbid process may be arrested at this stage, and fibrous adhesions form between the opposing articular surfaces, or it may progress, in which case further changes occur, resulting in destruction of the articular cartilage and exposure of the subjacent bone.

In rare instances the synovial membrane presents nodular masses or lumps, resembling the tuberculous tumours met with in the brain; they project into the cavity of the joint, are often pedunculated, and may give rise to the symptoms of loose body.  The fringes of synovial membrane may also undergo a remarkable development, like that observed in arthritis deformans, and described as arborescent lipoma.  Both these types are almost exclusively met with in the knee.

The Contents of Tuberculous Joints.—­In a large proportion of cases of synovial tuberculosis the joint is entirely filled up by the diffuse thickening of the synovial membrane.  In a small number there is an abundant serous exudate, and with this there may be a considerable formation of fibrin, covering the surface of the membrane and floating in the fluid as flakes or masses; under the influence of movement it may assume the shape of melon-seed bodies.  More rarely the joint contains pus, and the surface of the synovial membrane resembles the wall of a cold abscess.

Ulceration and Necrosis of Cartilage.—­The synovial tissue covering the cartilage causes pitting and perforation of the cartilage and makes its way through it, and often spreads widely between it and the subjacent bone; the cartilage may be detached in portions of considerable size.  It may be similarly ulcerated or detached as a result of disease in the bone.

Caries of Articular Surfaces.—­Tuberculous infiltration of the marrow in the surface cancelli breaks up the spongy framework of the bone into minute irregular fragments, so that it disintegrates or crumbles away—­caries.  When there is an absence of caseation and suppuration, the condition is called caries sicca.

The pressure of the articular surfaces against one another favours the progress of ulceration of cartilage and of articular caries.  These processes are usually more advanced in the areas most exposed to pressure—­for example, in the hip-joint, on the superior aspect of the head of the femur, and on the posterior and upper segment of the acetabulum.

The occurrence of pathological dislocation is due to softening and stretching of the ligaments which normally retain the bones in position, and to some factor causing displacement, which may be the accumulation of fluid or of granulations in the joint, the involuntary contraction of muscles, or some movement or twist of the limb.  The occurrence of dislocation is also favoured by destructive changes in the bones.