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.