Abstract:
This study analyzes the stresses and strains within the talus and the ultra-high-molecular-weight polyethylene (UHMWPE) insert of the Agility ankle implant, specifically examining two talar component designs. Using a three-dimensional finite element method (FEM) model of the tibia, fibula, talus, and the implant, the research aimed to perform a parametric stress and strain analysis based on the design variations of the talar component. The study developed an analytical solution for two counterformal cylinders and a two-dimensional cross-sectional FEM model to compare with the three-dimensional model's results. The findings revealed that the wider, "Modified" talar component significantly decreased average stresses in the UHMWPE insert compared to the "Standard" design, bringing them below the yield limit in the sagittal plane, although peak stresses at the edges remained high. The analysis of the talus, modeled with heterogeneous elasto-plastic properties, showed that both implant designs caused localized yielding of the cancellous bone at the edges, particularly at the posterior-medial corner, which could be related to clinically observed implant tilt. The average strain in the cancellous bone was reduced by 24% with the "Modified" design. The paper also demonstrated that the behavior of the UHMWPE and the talus is sensitive to design choices, such as component width and thickness, and highlighted the limitations of simplified 2D models for implants that do not satisfy plane strain conditions.
