Objective: To evaluate the accuracy of cup placement within the “safe zone” and of imageless navigation measurement by comparing the intraoperative values of acetabular cup abduction and anteversion to postoperative CT values.

Material and Method: All cases were performed in semilateral decubitus position with OrthoPilot THA plus 3.2 (cup only) software (Aesculap AG). Postoperatively, a multislice computed tomographic (CT) scan was obtained at two months or later for abduction and anteversion angle measurement. The abduction and anteversion angle measured on postoperative CT were compared to the intraoperative measurement with a paired t-test and a correlation test at a 0.05 level of significance.

Results: Sixty-five cases were included in the present study. The median CT abduction value was 41°, range 35° to 48° and the median navigated abduction value was 39.8°, range 37.5° to 45.5°. The median paired difference was 1.6°, range -3.4° to 7.9°, this difference was significant (p<0.001). The median CT anteversion value was 15°, range 8° to 28° and the median navigated anteversion value was 12°, range 5.9° to 16.5°. The mean paired difference was 3.2°, range -2.6° to 14.9°, this difference was significant (p<0.0001).

Conclusion: Navigated THA with modified registration technique in semilateral decubitus position offered a more precise cup position as in supine and lateral decubitus position. It combined the advantage of supine position for registration and lateral decubitus position for better femoral canal visualization. Our modified registration technique by compressing the soft tissue above pubic symphysis (PS) to the abdomen and registering at the antero-superior-pubic-symphysis (ASPS) help us to increase the accuracy of anteversion angle.

Keywords: Hip arthroplasty, Imageless navigation