Hip impingement occurs when the head of the femur (thigh bone) butts up against the acetabulum (hip socket). In the process, the labrum gets pinched. The labrum is a thin layer of cartilage around the rim of the socket.
Anatomic changes in the femoral head and neck cause impingement. If the femoral head is flattened, it changes the relationship between the head and neck of the femur as it fits into the acetabulum.
Tears in the labrum or hip fractures that don't heal can result in impingement. Childhood hip conditions such as Legg-Calvé-Perthes or slipped capital femoral epiphysis (SCFE) can lead to hip impingement.
Improved technology with MRIs have made it possible to study this problem more closely. As a result, we now know that many people who have no symptoms have femoroacetabular impingement.