Pelvic Position at Cup Placement Compared With Intraoperative Pelvic Motion in the Lateral Decubitus Position.

Background: It is important for surgeons to consider the pelvic position at cup placement for accurate cup placement. This position depends on both the pelvic position at set-up and the degree of intraoperative pelvic motion from set-up to cup placement. The present study aimed to characterize and compare the pelvic position at cup placement with intraoperative pelvic motion during primary total hip arthroplasty in the lateral decubitus position. This study also investigated whether the pelvic position at set-up or intraoperative pelvic motion had a greater impact on the pelvic position at cup placement. Methods: Pelvic positions were measured in 86 hips using an augmented reality-based portable hip navigation system. The pelvic reference plane was registered in the supine position, and patients were then positioned in the lateral decubitus position for surgery. Pelvic positions were recorded at set-up and at the time of cup placement. Results: Mean pitch and roll angles at cup placement were significantly different from those of intraoperative pelvic motion (P < .001, .009), with greater variations in pitch and roll angles at cup placement. Directional differences in pitch and roll were also observed. Comparison of means showed that the pelvic position at set-up was close to the pelvic position at cup placement for pitch, whereas intraoperative pelvic motion was close to the pelvic position at cup placement for roll. Conclusions: The pelvic position at cup placement differed from intraoperative pelvic motion. The pelvic position at set-up and intraoperative pelvic motion contributed differently to the pelvic position at cup placement.