The hip is a complex ball-and-socket joint. The socket is formed by the acetabulum, which is part of the large pelvic bone. The ball is the femoral head, which is the upper end of the femur (thighbone). It is also supported by strong ligaments and a tough joint capsule which assist with movement and stability.

Articular cartilage covers the surface of the ball and the socket. It creates a smooth surface that helps the bones glide easily across each other during movement.

The acetabulum is ringed by tough fibrous cartilage called the labrum.  The labrum is triangular in cross section (see the image at right). The labrum forms a gasket around the socket, creating a tight seal and helping to provide stability to the joint.  It is critical for the hip to function correctly that this seal creates a suction effect.  In FAI this seal may be disrupted, either due to altered movement of the hip bones, or tearing of the labrum (seal).