Hip Impingement on MRI: Imaging Findings

Femoroacetabular impingement (FAI) has two morphological subtypes. Cam impingement arises from an aspherical femoral head-neck junction — a bony prominence that shears the acetabular labrum and cartilage during flexion. It is quantified on AP pelvis or Dunn-view radiographs by the alpha angle; an alpha angle greater than 55° is the accepted threshold for cam deformity. Pincer impingement results from focal or global acetabular over-coverage, trapping the labrum between the acetabular rim and the femoral neck. It is assessed on AP pelvis radiographs using the lateral center-edge angle (LCEA); an LCEA above 40° indicates coxa profunda or protrusio and suggests pincer morphology. Mixed FAI, combining both subtypes, is the most common presentation.

Conventional MRI and, more definitively, MR arthrogram (MRA) with intra-articular gadolinium provide soft-tissue detail unavailable on radiographs. MRA improves labral tear detection sensitivity to over 90%, delineates cartilage delamination at the chondrolabral junction, and identifies paralabral cysts. Radial reformats — oblique MRI sequences acquired perpendicular to the femoral neck at multiple clock-face positions — map the full circumference of the head-neck junction, allowing precise measurement of the alpha angle in the plane of maximum cam deformity, which is typically at the 1–2 o'clock position on radial imaging.

Conservative treatment — activity modification, physical therapy targeting hip rotator strengthening and lumbopelvic stability, and anti-inflammatory medication — is the first-line approach for 3 to 6 months. Arthroscopic or open surgical correction is considered when symptoms persist despite adequate conservative care, when MRA confirms a repairable labral tear contributing to mechanical symptoms, or when progressive chondral damage is documented. Early intervention before advanced joint space narrowing yields better outcomes, as significant osteoarthritis (Tonnis grade ≥ 2) is a relative contraindication to joint-preserving surgery.