Sensitivity of the primary stability of a cementless hip stem to its position and orientation

Using computed tomography (CT)-based preoperative planning software, we can define with good accuracy the position of a cementless hip stem inside the host bone, but previous studies suggest that the pose the surgeon achieves during freehand surgery may differ from the planned one even by some millimeters. Advances in simulation now make it possible to predict the primary stability of the stem in a given position during the preoperative planning, but is the stability predicted for the planned pose indicative of that we can expect for the achieved pose? The aim of the present study was to verify how this prediction is affected by the differences observed between the planned and the achieved poses. Two finite element models of an implanted femur were generated, one with the stem in the planned pose, and one with the stem in the achieved pose, as defined from postoperative CT scans. When compared to experimental measurements, the model with the achieved position was clearly more accurate (0.6 vs. 12% error over measured peak micromotion); however, the predictions of induced micromotions were different between the two models for less than 13%. It is thus concluded that while the implant position does have an effect on primary stability, the estimate of micromotion we can get from the planned position remains a clinically relevant indicator.