A 45-degree radiographic method for measuring the neck shaft angle and anteversion of the femur: a pilot study.

PURPOSE To propose a novel method to measure the neck shaft angle and anteversion of the femur using anteroposterior and 45-degree oblique radiographs. METHODS Three human subjects were used to verify the 45-degree oblique method. The true neck shaft angle and anteversion of the femur were determined using computed tomography. The true values were compared with the values derived by the 45-degree oblique method after correcting the distortion using a formula. RESULTS With the true values based on computed tomography as references, the neck shaft angle and anteversion of the 3 subjects deviated +0.82 to +4.66 degrees and -2.14 to +2.55 degrees in lateral 45-degree oblique radiographs, respectively, whereas the corresponding values in medial 45-degree oblique radiographs were +0.98 to +5.93 degrees and -10.09 to +1.58 degrees. The lateral 45-degree oblique method resulted in smaller range of deviation and were more accurate. CONCLUSION This 45-degree oblique method is useful in surgical planning, especially for femoral derotational osteotomy in children. This new method enables easy calculation of the true neck shaft angle and anteversion, with low radiation exposure.