Correct Assessment of Acetabular Component Orientation in Total Hip Arthroplasty From Plane Radiographs

Background: Correct positioning of the cup is an important factor in total hip arthroplasty. Assessing its position from a plain anteroposterior pelvic radiograph is known to be hampered by systemic errors. This study focuses on developing a correction method to adjust for these potential sources of error and to eliminate them based on a 3D geometric analysis. Methods: Computed tomography scans of 113 (66 male, 47 female) pelvices were reconstructed and virtually projected onto a plain radiograph with varying rotational and translational positions. Thus cup inclination and anteversion as measured on a 2D-radiograph and in the 3D environment were correlated. Projected offset of the symphysis from themid-sacrumserved as amean tomeasure pelvic right/left-rotation. Pelvic tilt was determined from the projected height of the contour of the small pelvis. Correction formulas were verified by projecting a gimbal-mounted artificial pelvis with a cup implanted in a known position. Results: We found gender-specific formulas that correct for malrotated and off-centered radiographs. Applying these formulas cup inclination was assessed as close as 1.3 degrees (+/- 1.90 degrees) to the true 3D value and cup anteversion as close as 1 degrees (+/- 1.91 degrees) although deviations between directly measured plain values and corrected values rose up to 18 degrees. Conclusion: Inherent effects of central projection and malrotations due to pelvic tilt, pelvic rotation, and noncentered radiographs are corrected. Evaluation of radiographic inclination and anteversion of acetabular cups from plain 2D-radiographs show improved precision. Real values are approached better than 1.3 degrees when applying our correction formulas. (C) 2018 Elsevier Inc. All rights reserved.