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.

The correct placement of the acetabular cup is the most challenging part within hip arthroplasty. For fulfilling the biomechanical requirements the three-dimensional position of the acetabular cup must be exactly adapted to the patient’s anatomy. The amount of acetabular cup malpositioning is still too high. CAS (Computer Assisted Surgery) in hip arthroplasty offers the opportunity to have an online feed-back concerning the exact 3-D position of the cup, the surgical tools, and the patient’s pelvis. Preoperatively the surgeon plans and records with the system’s software the optimum cup position, and size. Within the operation theatre optoelectronic tools serve to the CAS-system for tracking. By using these data, the CAS-system delivers real-time optical information about the 3-D position of the patient’s pelvis, the orientation of the surgical instruments (reamer, cup positioner), and the acetabular component. This allows the surgeon to navigate by these tools and to find the exact inclination, ante-version, and depth of the cup.

From Mars until December 1999, we could perform 80 CAS-system assisted cup placements. All 80 patients (80 hips) were operated on because of severe osteoarthritis. All operations were performed by one surgeon (KB). The average increase of the operation time was 20 minutes resulting an average of 70 minutes. The average loss of blood was 630 ml. No perioperative specific complications did occur. The therapeutic regimen had not to be changed in any case. There were no cases of early hip dislocation. Other early postoperative complications did not occur either.

By postoperative CT scans we could evaluate the accuracy of the computer assisted cup placement. The deviation of the postoperative cup position from the preoperative planing was each 3–5° in average. This method is a reliable support for the surgeon to be able to implant the acetabular cup exactly in the planned position.