Optimization of cup positioning in THA - Comparison between conventional mechanical instrumentation and computer-assisted implanted cups by using the orthopilot navigation system

Aim: The goal of the current study was to determine the accuracy of free-hand and computer-assisted cup replacement. Material and Methods: We analysed the cup positions after 50 conventional instrumented THAs and compared them with the positions of 50 navigated cups. The operations were done between October 2002 and November 2004. To determine the accuracy of the cup position, the author developed a special measurement tool. With the help of the method it was ensured that the radiograph central beam reached the pelvis in a horizontal position. The central beam was focused directly on the head. We calculated the anteversion and the inclination. Results: The variability of the cup position was significantly higher in the free-hand implanted group. The standard deviation for antetorsion in the navigation group was 5.0 and for inclination angles 2.8. In the conventionally implanted group the standard deviation was 7.4 for inclination and 6.9 for antetorsion. In particular, the anteversion cannot be exactly calculated without a navigation tool since the individual position of the patient's pelvis cannot be assessed by the surgeon without aids. Clinical long-term studies for the clarification of a higher stand-time of the computer-navigated cups are still necessary.