Assessing the accuracy factors in the determination of postoperative acetabular cup orientation using hybrid 2D-3D registration.

Single standard anteroposterior radiograph-based methods for measuring cup orientation following total hip arthroplasty (THA) are subject to substantial errors if the individual pelvic orientation with respect to X-ray plate is not taken into consideration. Previously, we proposed to use a hybrid 2D-3D registration scheme to determine the postoperative acetabular cup orientation and developed an object-oriented cross-program called "HipMatch." However, its accuracy and robustness have not been fully investigated. To assess the potential factors that may affect the accuracy and robustness of the hybrid 2D-3D registration scheme in determining the postoperative acetabular cup orientation, a comprehensive validation study using a cadaver pelvis was performed. Nine X-ray radiographs taken from different pelvic positions relative to the X-ray plate and two computed tomography volumes of the pelvis with one acquired before the cup implantation and the other acquired after the cup implantation were used in the validation study. Potential factors that may affect the accuracy and robustness of the hybrid 2D-3D registration scheme were experimentally determined. Our experimental results demonstrate that (1) the plain radiograph-based method is not accurate; (2) the hybrid 2D-3D registration scheme helps to improve the estimation accuracy; (3) the hybrid 2D-3D registration scheme can robustly and accurately estimate the cup orientation even when a big portion of the radiograph is occluded; and (4) image resolution has minor effect on the estimation accuracy. The hybrid 2D-3D registration scheme is an accurate and robust method to measure exact cup orientation in THA. It holds the promise to be a valuable tool for clinical routine usage for providing evidence-based information.