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Your hip joint is a ball-and-socket joint, formed by the ball, or femoral head, at the upper end of the thighbone, and the rounded socket, or acetabulum, in the pelvis. The bone ends of a joint are covered with a smooth, tough material called cartilage. Normal cartilage cushions the bones and allows nearly frictionless and pain-free movement. The rest of the surfaces of the joint are covered by a thin, smooth tissue lining called the synovium. The synovium produces fluid that acts as a lubricant to reduce friction and wear in the joint.