Purpose: To assess the range of femoral antetorsion with magnetic resonance (MR) imaging in asymptomatic volunteers and patients with different subtypes of femoroacetabular impingement (FAI) because abnormal femoral antetorsion might be a contributing factor in the development of FAI.
Materials and methods: This study was institutional review board approved; all individuals provided signed informed consent. Sixty-three asymptomatic volunteers and 63 patients with symptomatic FAI between age 20 and 50 years were matched for age and sex. They underwent standard MR imaging with two additional rapid transverse sequences over the proximal and distal femur for antetorsion measurement. Twenty volunteers underwent a second MR imaging examination in the same leg. Two readers independently measured femoral antetorsion. The time for the additional sequences was tabulated. Interobserver agreement was calculated; differences in antetorsion were assessed by using analysis of variance and the unpaired t test.
Results: Femoral antetorsion can be assessed with MR imaging in about 80 seconds, with high interobserver agreement (intraclass correlation coefficient [ICC] = 0.967) and high agreement between different MR examinations (ICC = 0.966). Women had a significantly larger antetorsion than men (P < .001 for both readers), and antetorsion of the left femur was significantly larger than that of the right femur (P = .01 for reader 1, P = .02 for reader 2). Overall, antetorsion was similar in volunteers and in patients for reader 1 (12.7° ± 10.0 [standard deviation] vs 12.6° ± 9.8, respectively; P = .9) and reader 2 (12.8° ± 10.1 vs 13.5° ± 9.8, respectively; P = .7). Femoral antetorsion was significantly higher in patients with pincer-type FAI than in those with cam-type FAI for reader 1 (18.3° ± 9.8 vs 10.0° ± 9.1, P = .02) and reader 2 (18.7° ± 10.5 vs 11.6° ± 8.8, P = .04).
Conclusion: Femoral antetorsion can be measured rapidly and with good reproducibility with MR imaging. Patients with pincer-type FAI had a significantly larger femoral antetorsion than patients with cam-type FAI.