The Relative Contribution of Fixation Features, Activity, and Tibiofemoral Conformity on Initial Stability of Cementless Tibial Trays

Initial stability of cementless total knee replacements (TKR) is critical to implant success as excessive motion between the bone and implant prevents bony ingrowth that is critical to the long-term survivability of cementless implants. Prior studies have shown that excessive micromotion causes fibrous tissue growth instead of beneficial bony growth. There are many factors that influence initial stability including the design of the tibial tray and the tibiofemoral articulations. Understanding the impacts of these design features on micromotion between the bone and implants is crucial to improving the performance of cementless TKR. Prior studies only tested for the effect of micromotions induced in the shear direction-in plane with the surface of the implant. Much of the micromotion in tibial trays is normal to the tibial plateau but the importance of normal micromotion is unknown. In this study, a validated finite element model of an AMTI VIVO knee simulator was used to load various implant designs in 6 degrees of freedom during activities of daily living. Micromotions were estimated for two cementless TKR designs. To test the contributions of individual fixation features, additional simulations were run with certain fixation features (pegs and stem) removed and micromotions, as well as forces through the fixation features, were compared with the nominal components. The effects of tibiofemoral conformity were tested by creating custom insert models to vary the anterior and posterior conformity of the insert to the femoral component and comparing the micromotions to a medium conformity insert. Overall, tibiofemoral conformity greatly influences micromotion and the cause of that seems to be the increased femoral condyle translations increasing the moment arm around the tray. Removal of individual fixation features did not have the impact expected because friction on the plateau appeared to compensate for the missing features. Axial and shear forces through the pegs and stem are controlled by different factors. Axial forces are determined by tray design while shear forces vary with the conformity of the tibiofemoral geometry