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Orthop Traumatol Surg Res. 2014 Dec;100(8):843-8. doi: 10.1016/j.otsr.2014.08.003. Epub 2014 Nov 6.

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

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Clinique Toulouse-Lautrec, 2, rue Jacques-Monod, 81000 Albi, France. Electronic address:
Service de chirurgie orthopédique, hôpital Pierre-Paul Riquet, CHU de Toulouse Purpan, secteur A, 5(e) étage, place du Docteur-Baylac, TSA 40 031, 31059 Toulouse cedex 9, France.



The 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.


This specific view offers good reproducibility and effectively detects abnormal values of criteria for FAI.


Inter-observer and intra-observer reproducibility of specific radiographic criteria (αangle 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).


Inter-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 2.5mm (-4.6 to 9.4) vs. 7.6mm (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>4.6mm for the HNO.


The45°-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<5mm support a diagnosis of anterior FAI.


Level III, case-control study.


Alpha angle; Cam effect; Femoroacetabular impingement; Frog-leg view; Head-neck offset

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