1. The hip is not the same as the shoulder.
Embryologically, the hip is the lower extremity analogue of the shoulder, so it does share some common features. There is a labrum, which is a ring of cartilage that surrounds the acetabulum, or socket. It has a similar function as in the shoulder, in that it deepens the socket, but the relative contributions are different. In the shoulder, the labrum is essential for stability of the shoulder. A labral tear in the shoulder can predispose to repeated dislocations in younger patients. In older patients, a degenerative shoulder labral tear can be a source of pain.
In the hip, the labrum is not quite as important for stability. The bony architecture of the hip is such that the acetabulum is a deep cup and provides most of the stability. The hip is more of a "ball in a socket," whereas the shoulder is more of a "ball on a plate." The function of the labrum is to take some pressure off the articular cartilage. Studies have shown that removing the labrum increases the forces on the surrounding articular cartilage. Over time, this can lead to early arthritis.
2. Because the hip is more of a ball and socket joint, the angulation of the socket is important.
In the shoulder, if the plate (glenoid) is angled slightly forward or back, the joint is usually able to compensate. First of all, a ball on a plate can more easily adapt to a change in the position of the plate. Second of all, the shoulder blade (scapula) can move along the rib cage to, again, alter the position.
In the hip, however, a socket (acetabulum) that is angled toward the back (retroverted) can cause problems with impingement. Because it is a much deeper joint, you can imagine that the thigh bone (femur) would hit the walls of the acetabulum at some point. Usually, as we bring our thighs toward our chest (hip flexion), the bones don’t make contact. In a retroverted acetabulum, this can occur, which causes hip impingement.
3. Changes in the ball (proximal femur) can also cause hip impingement.
If the ball is malrotated posteriorly or there is a deep socket (coxa profunda), these can both contribute to hip impingement because there is less range of motion. You can imagine that if the head is not directly on the neck, then the neck might hit the socket earlier as well. This occurs in developmental dysplasia of the hip (DDH) and slipped capital femoral epiphysis (SCFE). Both of these are pediatric conditions that can lead to hip problems in early adulthood, in the third or fourth decade of life. If diagnosed early enough, there are treatments that can slow the progression of the deformity.
4. N/A
N/A
5. N/A
N/A
