Variations in sagittal and coronal stem tilt and their impact on prosthetic impingement in total hip arthroplasty.

Optimization of the combined anteversion of cup and stem has been emphasized to avoid prosthetic impingement in total hip arthroplasty (THA). However, no study has focused on the impact of variations in sagittal and coronal stem tilt against the whole femur on prosthetic range of motion. The purposes of the present study were a) to quantify the anatomical variation of sagittal and coronal tilt of the proximal canal axis against the femoral retrocondylar coordinate system, that is variation of sagittal and coronal stem tilt and b) to determine their impact on the zone of impingement-free cup position using computer simulation. Preoperative computed tomography images of 477 femurs from 409 consecutive patients who underwent THA using computed tomography-based computer navigation were stored. Virtual implantation of an anatomical stem was performed on the navigation workstation. The safe zone of the cup position with regard to prosthetic impingement was determined by motion simulation in the range of sagittal and coronal stem tilt of the subjects. The sagittal and coronal stem tilt varied by 10°, which was smaller than the stem anteversion variation. However, there was about 3 times the difference in the impingement-free zone of cup position in the ranges of sagittal and coronal stem tilt. The safe zone was significantly decreased by posterior tilt and valgus tilt of the stem. Range of motion simulation revealed that the variations in sagittal or coronal stem tilt significantly influenced the safe zone of the cup. In conclusion, although the variations in sagittal and coronal stem tilt against the femoral retrocondylar coordinate system were small, their impact on prosthetic impingement was significant.