Accuracy of Acetabular Cup Implantation, as a Function of Body Mass Index and Soft-tissue Thickness, with a Mechanical Intraoperative Support Device: A Retrospective Observational Study

HipCOMPASS, a mechanical intraoperative support device used in total hip arthroplasty (THA), improves the cup-alignment accuracy. However, the alignment accuracy achieved by HipCOMPASS has not been specifically examined in obese patients. In this study, we retrospectively evaluated the relation between alignment accuracy and several obesity-related parameters in 448 consecutive patients who underwent primary THA using HipCOMPASS. We used computed tomography (CT) to measure the preoperative soft-tissue thickness of the anterior-superior iliac spine (ASIS) and pubic symphysis and the differences between preoperative and postoperative cup angle based on the cup-alignment error. We found significant correlations between the absolute value of radiographic anteversion difference and body mass index (r = 0.205), ASIS thickness (r = 0.419), and pubic symphysis thickness (r = 0.434). The absolute value of radiographic inclination difference was significantly correlated with ASIS (r = 0.257) and pubic symphysis thickness (r = 0.202). The receiver operating characteristic curve showed a pubic symphysis thickness of 37.2 mm for a ≥ 5° implantation error in both radiographic inclination and anteversion simultaneously. The cup-alignment error for HipCOMPASS was large in patients whose pubic symphysis thickness was ≥ 37.2 mm on preoperative CT. Our results indicate that methods other than HipCOMPASS, including computed tomography-based navigation systems, might be preferable in obese patients.