Drivers of initial stability in cementless TKA: Isolating effects of tibiofemoral conformity and fixation features

The initial fixation of cementless tibial trays after total knee arthroplasty is critical to ensure bony ingrowth and long-term fixation. Various fixed-bearing implant designs that utilize different fixation features, surface coatings, and bony preparations to facilitate this initial stability are currently used clinically. However, the role of tibiofemoral conformity and the effect of different tray fixation features on initial stability are still unclear. This study assessed the implant stability of two TKA designs during a series of simulated daily activities including experimental testing and corresponding computational models. Tray-bone interface micromotions and the porous area ideal for bone ingrowth were investigated computationally and compared between the two designs. The isolated effect of femoral-insert conformity and fixation features on the micromotion was examined separately by virtually exchanging design features. The peak interface micromotions predicted were at least 47% different for the two designs, which was a combined result of different femoral-insert conformity (contributed 79% of the micromotion difference) and fixation features (21%). A more posterior femoral-insert contact due to lower tibiofemoral conformity in a force-controlled simulation significantly increased the micromotion and reduced the surface area ideal for bone ingrowth. The maximum difference in peak micromotions caused by only changing the fixation features was up to 33%. Overall, the moment arm from the insert articular contact point to the anterolateral tray perimeter was the primary factor correlated to peak and average micromotion. Our results indicated that tray-bone micromotion could be minimized by centralizing the load transfer and optimizing the fixation features.