Accuracy and risk factors for poor cup orientation in total hip arthroplasty: a comparison between supine and lateral registration using a portable navigation system.

INTRODUCTION: Accurate acetabular cup orientation is essential in total hip arthroplasty (THA). Portable inertial navigation may improve reproducibility, but accuracy in lateral decubitus THA may depend on the pelvic registration workflow. We compared cup placement accuracy between supine and lateral registration workflows using NAVBIT (Smith & Nephew PLC) and explored factors associated with large navigation errors. MATERIALS AND METHODS: We retrospectively reviewed 110 consecutive primary cementless THAs performed in the lateral decubitus position using NAVBIT. The supine-registration workflow included 52 hips (supine pelvic registration followed by repositioning), and the lateral-registration workflow included 58 hips (registration and cup implantation in the lateral decubitus position). NAVBIT recorded radiographic inclination (RI) and radiographic anteversion (RA) intraoperatively; postoperative RI and RA were measured via computed tomography. Accuracy was defined as the absolute difference between navigation records and postoperative measurements. We compared navigation accuracy outcomes between propensity score-matched groups. Outliers were defined as an absolute error > 5° in RI or RA; factors associated with outliers were evaluated in the overall cohort. RESULTS: After propensity score matching, 46 matched pairs (n = 92) were analyzed. Postoperative cup orientation was similar between workflows (RI: 40.8°±3.4° vs. 40.3°±4.1°, p = 0.51; RA: 15.2°±3.3° vs. 14.6°±5.9°, p = 0.56). Mean absolute errors were smaller with supine registration for both RI and RA, with a significant reduction for RA (RI: 2.7°±2.3° vs. 3.4°±2.7°, p = 0.24; RA: 2.2°±1.5° vs. 4.1°±3.4°, p < 0.001). Bland-Altman analysis showed narrower limits of agreement with supine registration, particularly for RA. In the overall cohort, lateral registration increased the odds of outlier status (OR 3.11; 95% CI, 1.36-7.09; p = 0.007). CONCLUSIONS: For NAVBIT-assisted THA performed in the lateral decubitus position, standardized supine pelvic registration followed by repositioning reduced absolute navigation errors, particularly for anteversion, compared with lateral registration.