Representing Sub-failure Quasi-static Ligament Mechanics and Bone Kinematics in a Human Ankle Finite Element Model

Syndesmotic ankle sprains, along with lateral ankle sprains, account for about 85–90% of foot and ankle injuries. The mechanism of syndesmotic ankle sprains is not well understood because knowledge of the relationship between gross rotations of the foot and in situ ligament mechanics is limited.

The objective of this thesis was to develop a human ankle finite element model to gain insight into the relationship between gross ankle mechanics and in situ ligament mechanics. The specific goals were to improve the representation of articular cartilage and ligaments in an existing finite element model and to evaluate the bone kinematic and gross moment responses of the model against data from cadaver ankle experiments.

The model was refined and optimized to meet the stated goals. This refinement included a representation of articular cartilage with reduced gaps and of ligaments as distributions of fiber bundles. Each fiber bundle was represented by a bilinear stiffness curve with physically interpretable parameters. One of the parameters was a zero force toe region. The fiber bundle toe regions were optimized to minimize the differences between the responses of the model and those measured from cadaver ankle experiments. Evaluation of the optimized model showed that the bone orientation, bone position and gross moment responses of the model were within 2°, 2 mm and 5 Nm of the experimental data. The optimized model also provided in situ ligament stiffness curves that can be used to describe in situ ligament behavior, which is difficult to measure experimentally.

The optimized model was used to describe the effect of calcaneus dorsiflexion, eversion and external rotation on the in situ force responses of the anterior tibio-fibular ligament. The optimized model predicted that calcaneus dorsiflexion may predispose the anterior tibio-fibular ligament to injury for an external rotation input to the calcaneus. This provided insight into the relationship between gross ankle kinematics and in situ ligament mechanics.

The optimized model can be extended to include failure criteria for the fiber bundles. This can facilitate the study of the injury mechanism of syndesmotic ankle sprains, as well as injury prevention strategies.