Biomechanical Evaluation of Total Ankle Replacements

Globally, 1% of the population is affected by arthritis of the foot and ankle. Total ankle replacement (TAR) was developed as an alternative to fusion to treat end-stage arthritis, however failure rates are relatively high and are often related to bony damage. The purpose of this PhD was to develop a finite element (FE) model of a TAR to examine the risk of bone failure, and how this is affected by component alignment. An experimental model of a TAR implanted into synthetic bone was first created as a means to validate an initial FE model under known conditions. Location and size of the plastic deformation were compared and good agreement was found. A FE model of the natural ankle was then created from cryosectional images obtained from the Visible Human Project®. It was analysed in the natural state and after virtual implantation with a TAR. Both the cortical stiffness and the surgical positioning of the TAR were varied to represent relevant ranges seen clinically. In the TAR models, the location of the highest stress was shifted from the region of high strength to a region of lower strength of bone. The maximum von Mises stress on the cancellous bone was primarily affected by the stiffness of cortical structure and the distance between the stem and the outer surface of the cancellous bone. In some misalignment cases, the yield stress for cancellous bone was likely to be exceeded under loads representing standing. The results indicated that the quality of the bone and the thickness of the trabecular bone surrounding the TAR stem are important factors in governing the risk of bony failure following TAR, and should be taken into account clinically. The methods developed in this thesis can now be extended to examine other TAR designs and surgical approaches.