The Epiphyseal Torsion Angle in MR Arthrography of the Hip: Diagnostic Utility in Patients with Femoroacetabular Impingement Syndrome

Objective: The purpose of the study was to evaluate whether patients with clinically suspected femoroacetabular impingement (FAI) show higher epiphyseal torsion angle values on MR arthrography than healthy volunteers. Another objective was to estimate whether patients with increased epiphyseal torsion angles show a higher incidence of hip abnormalities or a higher incidence of increased alpha angles on MR arthrography than patients with an epiphyseal torsion angle considered normal.

Materials And Methods: Sixty-eight consecutive patients with clinically suspected FAI, no history of hip surgery, and MR arthrography of the hip performed at our institution were included in the study group. The control group included 50 standard MR examinations of the hip of 25 healthy volunteers with negative findings on a clinical impingement test. Epiphyseal torsion angles and alpha angles were measured independently by three radiologists. Furthermore, MR arthrograms were retrospectively evaluated by two radiologists in consensus for hip abnormalities associated with FAI.

Results: The epiphyseal torsion angles were significantly higher in the study group than the control group for all three readers (reader 1, p = 0.001; reader 2, p = 0.003; reader 3, p = 0.045). There was no statistical correlation between increased epiphyseal torsion angles and pathologic alpha angles (readers 1-3: p = 0.199, p = 0.343, p = 0.058, respectively). Patients with increased epiphyseal torsion angles did not show a higher incidence of labral abnormalities than patients with epiphyseal torsion angles considered normal (readers 1-3: p = 0.335, p = 0.383, p = 0.676).

Conclusion: Patients with clinically suspected FAI show increased epiphyseal torsion angles compared with healthy volunteers. Values greater than 20° should be considered pathologic. A pathologic retrotorsion of the femoral epiphysis may play a role in developing hip pain related to FAI and premature osteoarthritis.