In the case of femoroacetabular impingement (FAI), the head of the femur butts up against the cartilage and pinches it. Normally, the femoral head moves smoothly inside the hip socket. The socket is just the right size to hold the head in place.

If the acetabulum is too shallow or too small, the hip can dislocate. In the case of FAI, the socket may be too deep. The rim of the cartilage hangs too far over the head. When the femur flexes and rotates, the cartilage gets pinched.

This causes deep groin pain with activities that stress hip motion. Prolonged walking is especially difficult.

The cause of the problem is under considerable debate. For a long time it was assumed that overload of the joint caused this kind of OA. But no one could identify what was causing the stress overload.

Now with better imaging studies, we know that some subtle changes in the shape of the femoral head may be the cause of FAI. Other anatomical changes in the angle of the hip may also contribute to this problem. And as mentioned, FAI can occur if the hip socket is too deep.