The rationale for tilt-adjusted acetabular cup navigation.

Background: When performing total hip arthroplasty without computer navigation, surgeons align the acetabular component with landmarks such as the plane of the operating table and the presumed position of the pelvis. In contrast, first-generation computer navigation systems rely on the pelvic anterior plane, defined by the anterior superior iliac spines and the pubic tubercle. We sought to study the effect of patient positioning on the tilt of the pelvis as measured in the pelvic anterior plane and its effect on cup alignment angle values.

Methods: In forty patients, the supine pelvic anterior plane tilt angle was measured with use of computed tomographic scans made before and after total hip arthroplasty (Group A). In thirty other patients undergoing total hip arthroplasty, preoperative supine pelvic anterior plane tilt angle was measured with a computed tomographic scan and the preoperative standing pelvic anterior plane tilt angle was measured on a lateral radiograph (Group B). From these data, we used hip navigation planning software to develop a nomogram providing tilt-adjusted cup angles that would align the cup in a target range of 40 degrees +/- 10 degrees of abduction and 15 degrees +/- 10 degrees of anteversion. A third group of ninety-eight patients (Group C) then underwent total hip arthroplasty with computer navigation with use of our nomogram to provide tilt-adjusted values for cup alignment. Postoperative computed tomography scans were made to evaluate cup alignment, and the patients were followed for at least one year.

Results: In Group A, the mean preoperative supine pelvic tilt angle (and standard deviation) was -8.9 degrees +/- 6.8 degrees (forward rotation of the pelvis) and the mean postoperative angle was -10.9 degrees +/- 7.6 degrees (p < 0.05). In Group B, the mean preoperative supine pelvic tilt angle was -10.4 degrees +/- 7.4 degrees and the mean preoperative standing pelvic tilt angle was -5.0 degrees +/- 9.4 degrees (p < 0.001). In the group of ninety-eight patients who underwent navigated total hip arthroplasty (Group C), there were no dislocations at one year of follow-up. Seventy-two patients underwent postoperative computed tomography scans; 99% of cup anteversion values and 97% of cup abduction values were in the target range.

Conclusions: For navigation systems that rely on the pelvic anterior plane, cup alignment values can be converted to familiar target values with our nomogram with good accuracy and reproducibility. The next generation of navigation systems should be able to measure the pelvic tilt for each individual patient and automatically adjust alignment values.