| The objective of this study was to analyze stresses and strains in the talus and the Ultra-High-Molecular-Polyethylene (UHMWPE) insert with two talar component designs of the Agility® ankle implant. A three-dimensional finite element method (FEM) model of the tibia, fibula, talus and the implant was developed for the study. An analytical solution of two counterformal cylinders was derived and a two-dimensional cross-sectional FEM model was included to investigate how the results of stresses in the UHMWPE correlate to the three-dimensional model.; The analysis of the three-dimensional model found stress concentrations in the UHMWPE due to edges of the talar components. The wider talar component decreased stresses in the middle of the sagittal plane below the yield limit, although, the peak stresses at the edges remained unchanged. We showed that it was possible for von Mises stress to increase while contact pressure decreased. This phenomenon was observed for different Poisson's ratios and thicknesses of the UHMWPE insert. We found that there was a qualitative discrepancy between the results from two-dimensional models and the three-dimensional model for the narrower original talar design which was directly related to the assumption of plane strain condition of the two-dimensional model.; The potential subsidence of talar component of the implant into the cancellous bone of the talus was investigated based on strain failure criteria. Results showed that the edges produced localized yielding of cancellous bone in both of the talar component designs. The yielding zone was wider in the narrower implant. A failure of cancellous bone, defined as 1.5% strain, was predicted for both of the talar components on the posterior-medial corner. The strain in this corner reached 1.7% for the wider component and 2.6% for the narrower component. The average strain fell from 0.39% for the standard talar component to 0.3% for the modified component, both below the yield strain of 0.8%. In studying the effect of flexion angle, a full support of the talar component by cortical bone was found to be beneficial. |
