In the earlier stages of tuberculous arthritis, in joints with both free and contacted articular surfaces, the articular cartilage is usually destroyed most extensively along its free articular surfaces by granulations which overgrow it from the surrounding synovial membrane. In the regions of contact and pressure of articular surfaces, surface destruction by granulations does not take place and the cartilage is preserved longest. As the disease progresses, subchondral granulations develop which absorb articular cortex and attack the cartilage from beneath. In severe tuberculous arthritis there may be early necrosis and caseation of the overgrowing synovial granulations, so that there is little or no destruction of articular cartilage along its free surfaces. In this event the destruction of articular cartilage is brought about mainly by the subchondral granulations, which do not undergo early necrosis and caseation and do not present the usual histologic picture of tuberculous tissue. The bony involvement along the articular surface may be either primary or secondary. When primary, the dead bone may break down, leaving a cavity, or may persist and form a sequestrum which has articular cortex and in some instances cartilage preserved on it and is denser than the surrounding living bone which subsequently atrophies. Secondary involvement of bone along the articular surfaces may lead either to destruction of the entire articular cortex by superficial caries or to more or less extensive and deep areas of necrosis at the points of greatest pressure in the joint. This deep invasion is often bilateral and is followed by the formation of "kissing sequestra," which may be mistaken for primary foci of osseous infection.