Irregular rotation and shift of computer-assisted femoral stem implantation in THA--an experimental study.

Introduction: The two available computer-assisted surgery robotic systems consist of a preoperative planning computer workstation and an industrial robot with a high speed milling device. During the computed tomography (CT) scan of the hip and the ipsilateral knee for planning the hip arthroplasty, the patient's movements are registered by a bar that is fixed at the patient's leg along its axis. Despite the companies' claim that a high accuracy of implant position can be achieved by this method, misplacements of implants are reported in the literature.

Materials And Methods: In an experimental study, a cadaver femoral bone was rotated during the CT scan strictly around this bar to simulate a rotational movement of the patient. Using the CT data, the planning of the hip stem and the following preparation of the femur by the robot was possible without detection of the patient's movements by the system. According to the system manual, the computer should stop the planning or give a warning in case of patient movement during the CT scan.

Results: The postoperative CT scan of the cadaver femoral bone revealed a rotary deviation and a shift of the stem compared with the original planning, caused by the rotation during the CT scan.

Conclusion: We propose using a second bar during the CT scan to detect these movements and thus avoiding misplacement of the implant.