Abstract
Introduction
There is great interest in technologies to improve the accuracy and precision in placing implants for total hip arthroplasty (THA). Malik et al. (J Arthroplasty, 2010) showed that an imageless navigation system could be used to produce accurate measures of acetabular cup alignment compared to a CT-based alignment method using an imaging phantom. In this study we sought to compare the precision of an image-based navigation system with post-operative CT scans in a clinical patient cohort who received navigation-assisted THA.
Methods
Eighteen patients with 20 hips consented to this IRB-approved analysis of intra- and post-operative THA cup alignment. All patients received THA with image-assisted alignment (MAKO Surgical, Fort Lauderdale). Nominal cup placement, subject to intraoperative surgeon adjustment and approval, was 40° radiographic inclination (RI) and 20° radiographic anteversion (RA) according to the definitions of Murray (JBJS-Br, 1993). Intraoperative cup alignment was measured by collecting five points on the cup rim with an optically tracked stylus. Postoperative cup alignment was measured by registering pre- and post-operative pelvic models generated from CT scans, and determining the postoperative cup orientation relative to the pre-operative pelvis coordinate system (Figure 1). Repeated measures testing of the CT-based measurements on 10 patient scans showed precision and bias of 0.7° and 0° for radiographic inclination, and 0.6° and 0.1° for radiographic anteversion.
Results
Mean cup alignment with navigation and CT was within 1° of the nominal target values (Figure 2). There was not a significant difference in the cup inclination measure between intraoperative and CT-based measures, while the intraoperative measure of anteversion showed a 2° bias compared to the CT-based measures. Using a 5° difference between the intraoperative and postoperative measures as the definition for cup placement outliers, 2 cups (10%) were outliers for inclination and 3 cups (15%) were outliers for anteversion (Figure 3). No cup in this series was an outlier for both inclination and anteversion.
Discussion
Optical navigation to confirm cup placement appears to result in relatively few alignment outliers, with no outliers for both angles and no single difference greater than 10°. Although differences between intraoperative and CT-based measures were small, it appears acquisition of a larger number of points on the cup rim could further enhance the precision of the navigation-based surgeon feedback. Consistent with previous studies, cup alignment using optical navigation is more precise than cup alignment using traditional manual methods.
