Background: The aim of this study was to propose a simple method to determine the pelvic inclination angle using anteroposterior radiographs of the hip.
Methods: The 283 subjects were randomly selected from patients who had undergone hip surgery at our institution from July 2002 to June 2004. Anteroposterior (AP) and lateral radiographs of the pelvis were obtained in the standing position. To assess the pelvic inclination angle from AP radiographs, a formula was determined by a trigonometric function. A significant correlation between the angle calculated by the formula and the measured angle (theta(0)) on lateral radiographs was confirmed. The calculated angle (theta(1)) was obtained from two measured values: the pelvic foramen height (H) on AP radiographs and the pelvic foramen distance (D) on lateral radiographs. The formula used was as follows: sin theta = H/D. The calculated angle (theta(2)) was obtained from one measured value--the pelvic foramen height on AP radiographs--because the pelvic foramen distance on lateral radiographs substituted for the average of the pelvic foramen distance investigated in 236 patients. The formula was sin theta = H/average.
Results: The correlation between the calculated angle (theta(1)) and the measured angle (theta(0)) was significant, and the correlation between calculated angle theta(1) and calculated angle theta(2) was also significant.
Conclusions: The pelvic inclination angle can be estimated by measuring the height of the pelvic foramen according to our formula. Our formula was shown to have adequate reliability and reproducibility.