Femoroacetabular impingement

What is femoroacetabular impingement (FAI), and why is it important to recognize?

FAI is a result of abnormal mechanical contact between the bony prominences of the acetabulum and proximal femur during hip motion, resulting in labral and cartilage tears, and leading to premature osteoarthritis. Multiple developmental and acquired hip disorders, in which there is a mismatch between the femoral head and the acetabulum, are associated with FAI. Two recognized types of FAI related to anatomic variants are the cam (femoral cause) type and pincer (acetabular cause) type.

In the cam type, there is an aspherical femoral head with abnormal bony prominence at the lateral femoral head and neck junction (femoral “dysplastic bump”), while in the pincer type, there is focal overcoverage of the femoral head by the superolateral acetabulum.

Most patients with FAI demonstrate a combination of both types. Patients are usually young and athletic and typically present with symptoms of groin or trochanteric pain after sports activities, often preceded by awareness of limited hip mobility. Early diagnosis and treatment can prevent or delay the onset of degenerative arthritis.

How does MRI help in evaluation of patients with FAI?

Although the diagnosis of FAI is based primarily on clinical factors and radiographic evaluation, MRI or MR arthrography can be obtained for early detection of labral and cartilage degeneration and tears (often associated with paralabral cysts), and for demonstration of subtle anatomic variations not visible on radiographs.

MRI is also highly sensitive for more specific secondary signs of FAI, including subchondral edema and cysts in the superolateral acetabulum, absent or blunted labral tissue, a femoral “dysplastic bump” and femoral herniation pits (in cam type), and a calcified/ossified labrum and os acetabuli (in pincer type).

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