Fixation Stiffness with Increasing Stem Taper Angle After Revision Total Hip Arthroplasty

Hypothesis Stable femoral fixation during total hip arthroplasty (THA) is critical to ensure adequate implant performance. Although cylindrical implant stems designs generally have shown satisfactory long-term results, in revision cases or cases with poor bone quality, tapered hip replacement stems have been suggested as a better performing alternative. The degree of the taper has yet to be biomechanically tested in such revision cases. This study aims to compare the initial fixation stability with increasing tapered stem implant geometry using two diaphyseal bone loss models simulating revision THA. Methods Using the finite element (FE) method, a numerical technique commonly used to computationally approximate solutions for complex structural mechanics problems, two femoral diaphyseal models were used to simulate revision THA. The first femoral model simulated bone loss having a diaphyseal length of only 21.5-cm.The second femoral model was further sectioned to 21.0-cm. The taper angle of the distal stem of the prosthesis - the region of the implant in contact with the bone cortex – was varied from 0 to 2.5 degrees in 0.5 degree increments. Results As the taper was increased from 0 degrees to 2.5 degrees in the 21.5 cm diaphyseal model, there was a 79%, 30%, 7%, and 14% decrease in contact area, average bone stress, average implant stress and construct stiffness respectively. As the taper was increased from 0 degrees to 2.5 degrees in the 21.0 cm diaphyseal model there, was a 75%, 29%, 6%, and 20% decrease in contact area, average bone stress, average implant stress and construct stiffness respectively. Noteworthy, the stiffness started to fall drastically as the taper increased past 2 degrees in the 21.0-cm diaphyseal model. Conclusions The ideal implant should maximize the beneficial effects of a taper and minimize the detrimental effects. At the bone diaphyseal lengths studied, although all the tapered models showed satisfactory performance, the 0 degree no taper model showed the highest stiffness. If implant stress is a concern, from our results, it appears that the 1.5 degree taper minimizes implant stress without drastically decreasing stiffness.