| Stemmed tibial implants are used in total knee arthroplasties requiring additional support due to deficient proximal bone. In recent years, these stemmed implants have also been implanted with hybrid fixation which has shown 5-20% failure rates up to six years post-operatively, but long-term results are not yet available. To better understand the implant and surgical parameters which may affect the long-term clinical outcome, both experimental and computational studies were performed to investigate initial post-operative implant motion.In-vitro testing was performed on seven matched-pair tibias with modular stemmed tibial implants with hybrid fixation. Motion of the tray, stem, and bone, were measured using an optical motion capture system. In-vitro data provided analysis of both the rigid body assumption of the stemmed implant, and assessment of relative implant-bone motion. A specimen-specific finite element (FE) model was created from pre- and post-operative computed tomography (CT) data. The model was tuned to the in-vitro data and used to investigate the effect fixation and bone density have on initial post-operative implant motion.In-vitro tray-stem motion was found in the anterior-posterior and medial-lateral directions. Implant-bone motion was greatest in the proximal-distal direction, with the tray having more relative motion than the stem tip. In addition, high variability in relative motion was measured both between and among specimen. No correlations were found between implant-bone motion and stem size, tibial tray coverage, age, gender, or bone density. Sensitivity studies on the FE model found the bone's deflection in bending was significantly affected by the definition of bone density and the empirical relationship used to define elastic modulus. The distal and proximal constraints of the FE model were found to have the largest effect on the motion of the bone, tray and stem. The tuned FE model found decreased implant-bone motion with cement fixation, compared to hybrid fixation, as well as normal bone density, compared to osteoporotic bone density. With additional tuning, the current FE model can be used to evaluate both implant and surgical parameters to provide predictions for long-term clinical results. |
