What is a Joint?
A joint is formed by the ends of 2 or more bones that are connected by thick bands of tissue called ligaments. For example, the knee joint is formed by the lower leg bone, called the tibia or shin bone, and the thigh bone, called the femur. The hip joint is a ball-and-socket joint, formed by the ball, or femoral head, at the upper end of the thigh bone, and the rounded socket, or acetabulum, in the pelvis.
The ends of the bone in a joint are covered with a smooth, soft material called cartilage. Normal cartilage allows nearly frictionless 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.