What are some common indications on hip MRI requisitions?

Beyond the ubiquitous “pain,” you may find “Possible AVN” or “suspected FAI” on your requisitions. Occasionally, you may see a “suspected occult fracture.”

What is FAI?

Femoroacetabular impingement (FAI) used to be a debated topic. In the recent years, it seems that FAI is becoming more widely accepted. In the simplest terms, FAI is an aberrant anatomy that results in impingement of joint structures. In case of labrum, the impingement results in tears and deterioration. Joint capsule may become thickened. In the long term, these leads to hyaline cartilage degeneration and arthritis.

What are the two types of FAI?

Classically, FAI has been divided into “Cam” or the femoral type, and “Pincer” or the acetabular type. An osseous prominence at the femoral head and neck junction (“bump”) is the “Cam,” perhaps referring to similarity with a camshaft in a car transmission. “Pincer” is the over coverage of the femur by the superior acetabulum. Normally, the anterior wall of the acetabulum is medial throughout its course to the posterior wall. If the superior portion of the anterior acetabulum becomes lateralized, an overlap in a figure-of-eight fashion becomes obvious on frontal views. In reality, it is not uncommon to find a mixed type of FAI where both “Cam” and “Pincer” deformities are present.