45°-45°-30°Frog-leg radiograph for diagnosing cam-type anterior femoroacetabular impingement: Reproducibility and thresholds

BackgroundThe many radiographic views suggested for evaluating anterior femoroacetabular impingement (FAI), due to a cam effect, are not specific for this condition and have not been proven of diagnostic value in studies, including control groups. Using a new and specific radiographic view, we evaluated the reproducibility of the main radiographic criteria for FAI, determined normal values for these criteria in a control group, and established diagnostic threshold values.HypothesisThis specific view offers good reproducibility and effectively detects abnormal values of criteria for FAI.Materials and methodsInter-observer and intra-observer reproducibility of specific radiographic criteria (α angle and modified head-neck offset [HNO]) were computed from preoperative and postoperative radiographs of 96 hips (75 patients, 61 males and 14 females) using the specific 45°-45°-30° frog-leg view (F45 view). Values in the group with FAI were compared to those in a control group of asymptomatic volunteers (100 hips, 27 males and 23 females).ResultsInter-observer and intra-observer reproducibility was very good, with intra-class correlation coefficients of 0.955and 0.987, respectively, for the α angle and of 0.895 and 0.984, respectively, for the HNO. Mean values of both parameters differed significantly between the FAI and control groups: 73.9° (53° to 96°) vs. 49.3° (35° to 69°) for the αangle, respectively; and 2.5 mm (–4.6 to 9.4) vs. 7.6 mm (1.7 to 11.8) for HNO, respectively. The normal values defined as the boundary of the 95% reference interval in the control group were < 60.2° for the α angle, and > 4.6 mm for the HNO.DiscussionThe45°-45°-30° frog-leg view is useful for diagnosing FAI due to a cam effect. This view is easy to perform, and the thresholds determined in our study assist in its interpretation: α angle values > 58° in females and > 63° in males indicate cam-type femoral geometry. In both genders, HNO values < 5 mm support a diagnosis of anterior FAI.Level of evidenceLevel III, case-control study.