Femoroacetabular impingement, also known as FAI or hip impingement, is a condition in which there is abnormal contact between the ball (aka the femoral head) and the rim of the socket (the acetabulum). The condition is now thought to be a major contributor to the development of arthritis of the hip joint.
Femoroacetabular impingement can be classified as:
- Cam type
- Pincer type
- Combination of cam and pincer
In cam type, the impingement occurs because of the presence of build-up of bone (a "bump" or "cam") on the front of the femoral head and neck. This build-up may be the result of an abnormality of the growth plate in the femoral head that occurs during growth. Impingement occurs when the bump rolls into the socket during movement of the joint. This ultimately can result in tearing of the labrum and injury of the cartilage on the inside of the socket.
In pincer type, the socket is too deep or has abnormal anatomy that restricts motion of the joint and causes damage to the labrum or cartilage.
In most cases, FAI involves a combination of both cam and pincer type impingement.