MR imaging of femoroacetabular impingement.

FAI is a common cause of OA of the hip. It can be caused by decreased offset of the femoral head and neck, resulting in abutment of the femoral neck against the acetabular rim (cam impingement) and abutment of this area into the well-constrained socket (pincer impingement)within normal range of motion [15]. This repetitive mechanical trauma to the hip joint causes mechanical wear of the labrum and articular cartilage; left untreated, it causes pain, labral tears, and chondral injuries leading to progressive OA of the hip. The identification of FAI as a cause of OA allows appropriate therapy early and thus delays or prevents end-stage arthritis.MR imaging and MR arthrography are accurate noninvasive imaging modalities able to demonstrate acetabular labral disease and adjacent cartilage damage as well as the subchondral cysts and synovial herniation pits associated with impingement. In addition, MR imaging is able to detect underlying subtle anatomic variations of the femoral head-neck junction and acetabulum associated with FAI.