After experimental and preclinical evaluation (HAP Paul Award 2001) of a CT-free image guided surgical navigation system for acetabular cup placement, the system was introduced into clinical routine. The computation of the angular orientation of the cup is based on reference coordinates from the anterior pelvic plane concept. A hybrid strategy for pelvic landmark acquisition has been introduced involving percutaneous pointer-based digitisation with the non-invasive bi-planar landmark reconstruction using multiple registered fluoroscopy images.

From January 2001 to May 2002 a total of 118 consecutive patients (mean age 68 years, 82 male, 36 female, 62 left and 56 right hip joints) were operated on with the hybrid CT-free navigation system. During each operation the angular orientation of the inserted implant was recorded.

To determine the placement accuracy of the acetabular components the first 50 consecutive patients underwent a CT scan seven to ten days postoperatively to analyse the cup position related to the anterior pelvic plane. This was done blinded with commercial planning software. There was no significant learning curve observed for the use of the system.

Mean values for postoperative inclination read 43° (SD 3.0, range 37 to 49) and anteversion 19° (SD 3.9, range 10 to 28). The resulting system accuracy, i.e., the difference between intraoperatively calculated cup orientation and postoperatively measured implant position shows a maximum error of 5° for the inclination (mean 1.5°, SD 1.1) and 6° for the anteversion (mean 2.4°, SD 1.3).

An accuracy of better than 5° inclination and 6° ante-version was achieved under clinical conditions, which implies that there is no significant difference in performance from the established CT-based navigation methods. Image guided CT-free cup navigation provides a reliable solution for future THA.