Assessing precision and accuracy of false-profile hip radiographs

Purpose: The purpose of this study was to assess the accuracy and precision of pelvic rotation in existing false-profile (FP) radiographs and to devise a method to improve accuracy and precision of FP radiographs. Methods: An imaging protocol was developed to obtain FP radiographs. Pelvic rotation was calculated using the described method for FP images obtained in the 3 months prior to and after implementation of this protocol. Student's t-test and variance ratio tests were used to determine differences in mean and variance of pelvic rotation between the 2 cohorts. Pelvic rotation calculation methodology was validated by using fluoroscopic C-arm to obtain AP and rotated images of 10 osteologic pelvises. The ratio of the distance between hip centres of each rotated image and AP image (W-P/W) was determined. Intraclass coefficient correlation (ICC) was used to verify the relationship between W-P/W and pelvic rotation. Results: Mean W-P/W was 0.47 (95% CI, 0.45-0.49). There were significant differences in mean pelvic rotation of the pre-protocol group (47.6 degrees; 95% CI, 45.6-49.5 degrees) and the post-protocol group (60.0 degrees; 95% CI, 58.7-61.3 degrees, p < 0.0001). Additionally, there was a significantly wider distribution of measurements in the pre-protocol group (SD = 7.9 degrees) compared to the post-protocol group (SD = 5.7 degrees, p = 0.0035). Conclusions: The quality of FP radiographs obtained in the clinical setting may be inconsistent. Standardising FP imaging produces more accurate images. Appropriate FP radiographs should have a distance between hip centres that is approximately 0.5 times the same distance found on an anteroposterior (AP) radiograph.