Assessment of accuracy of acetabular cup orientation in CT-free navigated total hip arthroplasty.

We have used the Orthopilot (Aesculap AG, Tuttlingen, Germany) computed tomography (CT)-free navigation system to ensure accurate and reproducible acetabular cup orientation. In this system, cup orientation is assessed with respect to bony configuration as determined by palpation of the anatomical landmarks (the bilateral anterosuperior iliac spines and the upper margin of the pubic symphysis). In this study, intraoperative cup orientation as presented by the OrthoPilot navigation system was compared with the value obtained through postoperative radiological assessment using CT Digital Imaging and Communications in Medicine (DICOM) data and Medical Image Processing, Analysis, and Visualization (MIPAV; National Institutes of Health, US Department of Health and Human Services, Bethesda, Maryland). Intra- and postoperative results obtained from 27 consecutive navigated total hip arthroplasties (THAs) were analyzed. For cup positioning, the desired inclination and anteversion angles were set within the "safe zone" proposed by Lewinnek. In the intraoperative evaluation, the mean inclination angle as determined by the navigation system was 43.5 degrees +/- 2.17 degrees (range, 39.9 degrees to 46.6 degrees ) after the final implantation. In contrast, the mean inclination angle determined by postoperative calculation using MIPAV was 44.9 +/- 3.3 degrees (range, 38.1 degrees to 55.0 degrees ). A discrepancy of >5 degrees was observed in only 1 hip. For the anteversion, the mean intra- and postoperative values were 11.1 degrees +/- 5.6 degrees (range, 0 degrees to 17.8 degrees ) and 13.5 degrees +/- 5.9 degrees (range, 5.1 degrees to 21.6 degrees ), respectively. Again, a discrepancy of >5 degrees was observed in 1 case. Mean differences between the intra- and postoperative values were 1.9 degrees +/- 1.9 degrees and 2.6 degrees +/- 1.6 degrees for inclination and anteversion, respectively. A good agreement between the intraoperative values presented by the navigation system and those in the postoperative CT evaluation was observed, and the validity of this navigation system was confirmed.