The hip is one of the largest weight-bearing joints in the body. When it's working properly, it lets you walk, sit, bend, and turn without pain. Unlike the shoulder, the hip sacrifices degree of movement for additional stability. To keep it moving smoothly, a complex network of bones, cartilage, muscles, ligaments, and tendons must all work in harmony.
The hip is a ball-and-socket joint where the head of the femur articulates with the cuplike acetabulum of the pelvic bone. The acetabulum fits tightly around the head of the femur. The ball is normally held in the socket by very powerful ligaments that form a complete sleeve around the joint (the joint capsule). The capsule has a delicate lining (the synovium).
The head of the femur is covered with a layer of smooth cartilage which is a fairly soft, white substance. The socket is also lined with cartilage. This cartilage cushions the joint, and allows the bones to move on each other with very little friction.
An x-ray of the hip joint usually shows a "space" between the ball and the socket because the cartilage does not show up on x-rays. In the normal hip this "joint space" is approximately 1/4 inch wide and fairly even in outline.
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