Hip osteoarthritis is an important cause of disability and pain in adults, for which hip morphology is a significant risk factor. Hip morphology can be quantified using morphology measurements on pelvic radiographs. However, deviations in patient positioning, such as pelvic rotation, can influence the accuracy of these measurements. As a first step towards correcting pelvic rotation, this research proposes a model that quantifies pelvic rotation on digitally reconstructed radiographs (DRRs) using the area ratio of the obturator foramina.
Segmentations of the pelvis and both femurs of 30 randomly selected participants of the Generation R study were included. The segmentations had been created based on Magnetic Resonance Imaging scans. For each segmentation, DRRs were created with pelvic rotations ranging from -10° to +10° at 1° intervals. Landmarks were used to approximate the areas of the obturator foramina. The right-to-left ratio of the obturator foramen areas was calculated for each radiograph. To find the relationship between this ratio and the angle of pelvic rotation, one function was fitted to all results, and another to only the results for small angles (within ±5°). Both functions were inverted to predict the angle of rotation for a given obturator foramen area ratio. The resulting all-angle and small-angle models were validated on 30 DRRs with weighted random pelvic rotation taken from 15 different participants of the Generation R study, with a 3:1 split between small and large (between ±6° and ±10°) angles of rotation.
An exponential function was found to best describe the relationship between the obturator foramen area ratio and the angle of pelvic rotation. The all-angle model had a median deviation of -0.5° [IQR: -2°, 1°] from the true angles of rotation, and estimated the angle of rotation within 2° in 73% of cases. The small-angle model had a median deviation of 0° [IQR: -2°, 2°] and was accurate within 2° in 67% of cases. When assessing accuracy only for small angles of rotation, the median deviations were -1° [IQR: -2°, 1°] and -0.5° [IQR: -2°, 1°], and the accuracy percentages were 86% and 73%, respectively.
The proposed models showed moderate performance in assessing pelvic rotation. The results of both models are quite similar, though the all-angle model was more significantly influenced by increased variance at larger angles of rotation. While further improvement of the models is necessary, these performances show that quantification of pelvic rotation based on the obturator foramen area ratio on radiographs is possible. As such, this research is a promising first step towards correcting pelvic rotation and improving hip morphology measurements.