When the synovial membrane is diseased, it tends to grow inwards over the articular surfaces (Fig. 122), shutting off the supra-patellar pouch and fixing the knee-cap to the femur, and diminishing the area of the articular surfaces. The ingrowth of synovial membrane may fill up the cavity of the joint, or may divide it up into compartments. Ulceration of the cartilage and caries of the articular surfaces are common accompaniments. [Illustration: FIG. 122.--Tuberculous Synovial Membrane of Knee, spreading over articular surface of femur.] The femur and tibia are affected with about equal frequency, and the nature and seat of the bone lesions are subject to wide variations. Multiple small foci may be found beneath the articular cartilage of the tibia, or along the margins of the femoral condyles--especially the medial. Caseating foci are comparatively rare, but they sometimes attain a considerable size--especially in the head of the tibia, where they may take the form of a caseous abscess. Sclerosed foci, which form sequestra, are comparatively common (Fig. 123). [Illustration: FIG. 123.--Lower End of Femur from an advanced case of Tuberculous Arthritis of the Knee. Towards the posterior aspect of the medial condyle there is a wedge-shaped sequestrum, of which the surface exposed to the joint is polished like porcelain. (Anatomical Museum, University of Edinburgh.)] #Clinical Types.#--(1) _Hydrops_ usually arises from a purely synovial lesion, but the joint may suddenly become distended with fluid when an osseous focus ruptures into the synovial cavity. It is met with chiefly in young adults. As the fluid accumulates it gradually stretches the capsule, and pushes the patella forwards, so that it floats. There is little pain or interference with function; the patient is usually able to walk, but is easily tired. The amount of fluid diminishes under rest, and increases after use of the limb. In a certain number of cases it may be possible to recognise localised thickening of the synovial membrane, or the presence of floating masses of fibrin or melon-seed bodies. This is best appreciated if the knee is alternately flexed and extended by the patient while the surgeon grasps and compresses it with both hands. If the joint is opened, fibrinous material, often in the form of melon-seed bodies, may be found lining the synovial membrane. Tuberculous hydrops is to be diagnosed from the effusion that results from repeated sprain, from the hydrops of loose body, gonorrhœa, arthritis deformans, Charcot's disease, and Brodie's abscess in the adjacent bone, and from the hæmarthrosis met with in bleeders.