Measuring the 3d-position of cementless hip implants using pre- and postoperative CT images

Abstract

Quantitative measurement of the location of cementless hip implants postoperatively not only explains subsequent common complications, such as impingement and loosening, but can also predict postoperative subject-specific primary stability, if combined with the finite element method. However, the metal artifacts on the postoperative computed tomography (CT) image make it difficult to estimate the threedimensional (3D) alignment, implant shape, bone geometry, and mineral density precisely. Therefore, we developed a novel, accurate measuring technique for the 3D implant position using pre- and postoperative CT images, which is based on a 3D surface registration algorithm. We can obtain a computer model of the femur from preoperative CT data combined with a stem model from computer-assisted design (CAD) data. This study assessed the accuracy of our novel technique for the 3D estimation of implant position. Five sawbones femurs and five cementless stems were used. Ceramic sphere markers were bonded to the bone and stem surface to define local coordinate systems for the femur and stem, respectively. Each femur underwent CT at slice pitches of 1, 2, and 3 mm before and after stem implantation. The relative positions of the stem and femur obtained using our method were compared with those measured using a 3D laser digitizer, used as the true value. The relative estimated position errors were within 0.3 mm in translation and 0.4° in rotation, regardless of slice pitch. From the viewpoint of pixel size of the CT image in a clinical situation, the estimation is sufficiently accurate. Therefore, our measurement technique is useful for quantifying the relative 3D position of implants to the bone geometry and for performing comparative studies among subjects.