Femoroacetabular impingement (FAI) is also known as a hip impingement. The first part of the term refers to the ball on top of the femur (thighbone) that sits in the acetabulum (hip socket); the second part means tissue in that joint is pinched, causing pain.

FAI occurs when there is an abnormal amount of bone on either the ball of the thighbone or in the socket. The extra bone can cause tears to the soft tissue, which, over time, wears down the protective tissue around the bone called cartilage. When this happens, the bones rub together, causing pain, limited function and possibly early arthritis.

FAI first appears in the early teen years through adulthood, and can be present in either hip.