Total ankle arthroplasties (TAAs) are mechanical devices used to replace the articular surfaces of the ankle joint in order to relieve pain for patients with osteoarthritis. Since most osteoarthritis is post-traumatic, and due to the highly variable individual foot geometry, TAAs are rarely inserted into normal geometry. This leads to serious problems with stresses and contact pressures in TAA components. This study uses finite element (FE) modeling to determine how hindfoot alignment, or how far in varus or valgus the most distal part of the calcaneus is perpendicularly from the axis of the tibia, affects the stresses and contact pressures in the articulating surfaces of two different TAA models.
To investigate the effects of foot alignment on hardware stresses after TAA, FE models were generated. Models of the mobile bearing, three component Scandinavian Total Ankle Replacement (STAR) and the fixed bearing, two component Zimmer Trabecular Metal Total Ankle (Zimmer) were generated from laser scans of the hardware and virtually implanted into 3D models of the tibia and talus. Ligaments were modeled as linear springs to impart physiologically realistic flexibility in the model. The stance phase of a walking gait cycle was applied and stresses and contact pressures at the articulation between model components were recorded for various degrees of hindfoot alignment [1].
Data analyzed shows that both models have areas of high concentrations of stress and contact pressure. The Zimmer TAA seems to favor a valgus alignment due to the lower stresses and contact pressures in valgus alignments compared to varus. Though the STAR does not generally favor one alignment over the other, it does have significantly lower stresses and contact pressures than the Zimmer. These differences may be due to the geometric congruency of the STAR versus the anatomical articulation of the Zimmer.