For many years, the causes of femoroacetabular impingement were debated. Advances in medical imaging have shown that subtle variations in the shape of the femoral head or the orientation of the hip socket can contribute to the condition.
Normally, the femoral head moves smoothly within the socket. In individuals with FAI, structural differences cause the femur to press against the rim of cartilage surrounding the acetabulum. This repeated contact may pinch the labrum and surrounding cartilage.
In some cases, the acetabulum may be deeper than normal. The rim of cartilage may extend further over the femoral head than it should. When the hip bends or rotates inward, the labrum becomes trapped between the bones.
Over time, this repetitive pinching can cause fraying or tearing of the labrum. Degenerative changes may also occur in the articular cartilage at the site of impingement.
Changes in hip structure can also influence normal movement patterns. Individuals with FAI may develop altered hip mechanics such as excessive adduction or internal rotation. Muscle imbalances may further contribute to the problem.
Weakness in muscles responsible for hip stability, including the hip abductors, extensors, and external rotators, can increase abnormal stress within the joint. When these structural and muscular factors combine with repetitive movements, small injuries may accumulate within the hip.
In more severe cases, the labrum may partially tear or detach from the acetabular rim. A complete separation is referred to as a labral avulsion.