The hip is a ball and socket joint. The socket, or acetabulum, and the ball, or femoral head, have specific features that maximize both stability and motion.
The femoral head is attached to the lower femur by the femoral neck. The transition between the head and neck is often referred to as the head-neck junction. It is important to have a femoral head that is round and much larger than the femoral neck supporting it. This difference in size (diameter) is called the head-neck ratio. The larger the ratio, or difference between the head and neck dimensions, the more motion available to the hip before the bone at the head-neck junction contacts the bone at the rim, or edge, of the acetabulum. If excess bone develops at the head-neck junction, the ratio between the head and neck is decreased and contact between the ball and socket occurs much earlier in motion. This early contact is often referred to as hip impingement. Early contact can also occur due to increased bone at the edge of the socket, a deep socket or a poorly positioned socket.
Both the acetabulum and femoral head are covered by a special tissue called cartilage. A white material that is extremely smooth, cartilage allows the bones to glide with minimal friction.
The articular cartilage has a specific thickness based on its location in the joint. When injury occurs to the cartilage, doctors describe the extent of damage based on the depth and area of involvement.
The labrum is a ring of fibrocartilage that surrounds the outside rim of the acetabulum. The exact function of the labrum in the hip is still under investigation. Specialists believe the labrum is important in stability and maintenance of joint fluid mechanics.
See hip anatomy animation
To see animation, click the link above. Enter the animation window. Click hip, then condition and the hip anatomy tab.