Background: All computer-aided surgery technologies assume that the surgeon knows the best position for the implant components. However, there is indirect evidence that simple anatomical information may not be sufficient for the surgeon to decide size and position of the implant in a repeatable manner.
Method: In the present study we estimated the variability in choosing the size and position of the components of a cementless total hip replacement (THR), using template-on-radiograph as well as computed tomography (CT)-based computer-aided planning. In addition, using a computer model, we assessed the sensitivity to such variability of implant fitting, location of the joint centre, skeletal range of motion, and resting length of major hip muscles. Using templates, surgeons selected the size with variability up to 2.5 mm for the stem and up to 4 mm for the socket. A similar variability was also observed when the CT-based planning program was used.
Results: No major differences were found between surgeons. The standard deviation over repeated planning sessions of the selected position for each component, using the CT-based planning software, was found to be 3.9 mm and 8.9 degrees .
Conclusion: On the basis of the computer simulation, this variability did not affect the selected biomechanical parameters in a drastic way, although some differences were observed, especially in the lever arm of the hip muscles.