The hip is a ball and socket joint. Friction-free movement of the hip joint is possible when there is a perfect fit of the hip ball (femoral head) and socket (acetabulum). Any abnormality in either one of these structures can interfere with the movement of the hip. Femoral acetabular impingement (FAI) is a mismatch in shape between the femoral head and acetabulum.
The three main causes for femoroacetabular impingement (FAI) are: Pincer impingement, Cam impingement, and combined pincer-cam impingement. In most cases, these form during development. In some cases, this may be in part due to sports played during adolescence.
Apart from these, certain diseases can lead to FAI. These may include:
Legg-Calve-Perthes disease: Lack of sufficient blood supply to the femoral head leads to bone death, causing an abnormal shape of the femoral head. This typically occurs in childhood during growth.
Slipped capital femoral epiphysis: Weakness of the growth plate causes the femoral head to slip backwards from the thigh bone and can cause an abnormal shape in development. This typically occurs in childhood during growth.
Coxa vara: Alteration of the angle between the femoral head and the rest of the thigh bone.
How FAI damages the labrum:
The labrum of the hip, similar to that of the shoulder, is a ring of rubbery fibrocartilage around the rim of the acetabulum. The labrum seals the lubricating fluid within the hip and contributes to stability of the joint. FAI can lead to damage of the labrum and subsequent symptoms of pain or instability in the hip or groin.