The contributing factors of tapered wedge stem alignment during mini-invasive total hip arthroplasty

BACKGROUND Minimally invasive surgical approaches are widely used for total hip arthroplasty (THA). However, potential problems related to a reduced visual field during surgery, such as implant malposition, neurovascular injury, and poor implant fixation, have been reported. In these situations, a shorter stem is easier to insert in the femoral canal. To evaluate the accuracy of shorter stem orientation, we focused on the accuracy of stem orientation especially in short tapered wedge stems and evaluated the contribution factors of stem malalignment during mini-invasive total hip arthroplasty. METHODS One hundred ten hips that underwent THA with a Summit stem (58 hips) (DePuy, Warsaw, IN) as straight stem and TriLock stem (52 hips) (DePuy) as tapered wedge stem were enrolled in this study. For preoperative and postoperative evaluation, a CT scan of the pelvis and knee joint was obtained and was transferred to 3D template software. We compared the alignment of preoperative plan for stem anteversion/valgus/anterior tilt angles and postoperative stem alignment, and the absolute error was defined as the surgical error. To clarify the factors contributing to the malalignment or surgical error, we evaluated postoperative stem alignment and several associated factors. Further, we compared the clinical parameters between two types of stems for analysis of the influence of stem type differences. RESULTS The mean absolute value of the alignment error (postoperative stem alignment-preoperative planning alignment) was not changed in the short tapered wedge and straight stems. Sex, age at operation, or original canal anteversion did not affect the accuracy of stem alignment. However, high body mass index (BMI) affected the accuracy of stem alignment. Clinical outcomes were not changed by the difference of stem types. CONCLUSION The postoperative alignment of short tapered wedge stem was accurate, same as the straight stem during mini-invasive THA, but we need to pay attention when using this in obese patients.