Effect of artificial surface shapes and their malpositioning on the mechanics of the replaced ankle joint for possible better prosthesis designs

BACKGROUND: The clinical outcomes of total ankle replacement are limited by prosthesis component malpositioning during surgery. The goal of this study is to assess the mechanical impact of this malpositioning in a validated computer model.

METHODS: In a previously developed multi-body dynamic model of the human ankle complex three different artificial implants were designed, each one presenting a different approximation of the natural articular surfaces of the corresponding specimen. The most common implant translational and rotational malpositionings were defined and mimicked. Dynamic simulations of joint motion were run for the various surfaces and malpositionings. The same input loading conditions derived from a previous in-vitro experiment on the corresponding natural specimen were applied.

FINDINGS: From load-displacement graphs it was observed that all three artificial surfaces reproduced well physiological motion between the calcaneus and the tibia/fibula, with a maximum difference of 2°. It was found that antero-posterior translation of either the tibial or the talar component and inclination of the tibial component in the sagittal plane led to considerable increases in the range of motion. Antero-posterior and dorsiflexion of the tibial component resulted in an increased internal-external rotation by up to 3.5° and 4.0°, respectively. The corresponding increase of inversion-eversion was 5.0° and 6.5°.

INTERPRETATION: This study showed that relatively small surgical errors have great consequences in replaced joint mechanics. The present model can be used in future studies to analyse the effect of malpositioning with any specific current total ankle prosthesis.