Influence of bone material behavior assumption on finite element-predicted tibia-implant micromotions in total ankle replacement

Initial tibial implant stability is important for successful long-term outcome after uncemented total ankle replacement (TAR). Tibia-implant interfacial micromotion is consequently a key variable used to evaluate implant performance using finite element analysis (FEA). Our goal was to investigate how bone material behavior assumptions influence FEA-predicted tibia-implant interfacial micromotions. Five tibia geometries and their corresponding density distributions were acquired from CT scans of TAR patients. The corresponding models were then virtually implanted with two tibial implant designs. FEA was used to simulate loadings from the stance phase of gait with line-to-line implantation. FEA predictions of peak micromotions and von Mises stress differences were compared across each patient-implant configuration, when incorporating elastic-plastic versus only linear elastic bone material behavior (5 tibias x 2 implant designs x 2 tibia material behaviors). We found that peak micromotions trended larger (up to 69 % greater) when elastic-plastic bone material behavior was incorporated, and that larger differences in peak micromotions were seen with larger differences in peak interfacial von Mises stresses between simulations incorporating elastic-plastic versus linear elastic bone material behavior (r = -0.73, p < 0.001). The larger peak micromotions when elastic-plastic bone material behavior was incorporated were strongly associated with how much interfacial bone plasticly deformed (r = 0.92, p < 0.001). These results imply that tibia-implant interfacial micromotions are underestimated when bone is assumed to behave only as a linear elastic material. Thus, the results from such FEA simulations should be interpreted with caution, as they are likely conservative in their estimates of micromotion.