Accuracy of acetabular cup placement in navigated THA with modified registration technique in semilateral decubitus position.

BACKGROUND: The accuracy of cup placement in navigated total hip arthroplasty (THA) depends on the bony landmark registration intraoperatively. We created a semilateral decubitus position that combined the advantage of supine position for registration and lateral decubitus position for better femoral canal visualization.

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 CTwere compared to the intraoperative measurement with apaired t-test anda correlation test at a 0.05 level ofsignificance.

RESULTS: Sixty-five cases were included in the present study. The median CT abduction value was 41 degrees, range 35 degrees to 48 degrees and the median navigated abduction value was 39.8 degrees, range 37.5 degrees to 45.5 degrees. The median paired difference was 1.60, range -3.4 degrees to 7.9, this difference was significant (p<0.001). The median CTanteversion value was 150, range 8 degrees to 28 degrees and the median navigated anteversion value was 12 degrees, range 5.9 degrees to 16.5 degrees. The mean paired difference was 3.2 degrees, range -2.6 degrees to 14.9 degrees, 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.