# Biomechanical Assessment of Syndesmotic and Deltoid Ligament Strain in Pronation-External Rotation Type Ankle Injuries by Musculoskeletal Computer Simulation

## TL;DR

This study uses computer simulation to show that some ankle ligaments may not be fully damaged in certain ankle injuries, challenging traditional injury models.

## Contribution

The study introduces a novel musculoskeletal computer model to simulate ankle injury mechanics and challenges traditional injury cascade theories.

## Key findings

- Posterior inferior tibiofibular and deep posterior tibiotalar ligaments show minimal strain in simulated PER injuries.
- Computer simulation reveals that not all ligaments are equally affected in PER ankle injuries.
- The model suggests some ligaments may remain intact, challenging traditional views of ligament disruption.

## Abstract

Suprasyndesmotic ankle fractures, commonly resulting from pronation-external rotation (PER) mechanisms, are traditionally associated with disruption of the syndesmotic ligaments, a medial malleolus fracture or complete deltoid ligament rupture. However, recent imaging and clinical studies suggest that key stabilizing ligaments may remain intact in certain cases, potentially affecting talocrural stability. This pilot study aims to evaluate modelled ligament tension patterns in PER injuries using a validated musculoskeletal computer simulation model.

A musculoskeletal model of the ankle joint was developed using the AnyBody Modeling System (version 7.4), incorporating detailed anatomical structures and ligament biomechanics. The PER mechanism was simulated by applying external rotation (0-50 degrees) with the foot fixed, and ligament tensional forces was recorded for the deltoid and syndesmotic complexes. Because of limitations in the computer model, a fibular fracture was not simulated. Strain patterns were analysed across simulation steps to assess the sequence and magnitude of ligament loading.

The anterior and superficial deltoid ligaments (tibionavicular, deep anterior tibiotalar, tibiospring, and tibiocalcaneal) and the anterior inferior tibiofibular ligament (AITFL) demonstrated early and substantial increases in tension. Conversely, the deep posterior tibiotalar ligament (dPTTL) and posterior inferior tibiofibular ligament (PITFL) showed minimal strain during the majority of the simulation, suggesting they may remain intact in a subset of PER injuries.

In a computer-simulated pronation-external rotation injury mechanism, the observed tensional force acting on the posterior inferior tibiofibular ligament and the deep posterior tibiotalar ligament is substantially lower compared to the remaining ligaments of the syndesmotic and deltoid complex. The study highlights the potential of a of a novel computer-based ankle/foot model as an alternative to traditional in vitro biomechanical studies.

This study finds that in a simulated pronation-external rotation injury to the ankle, key stabilizing ligaments show low tensional forces, suggesting they might be spared from complete rupture. These findings challenge traditional views of complete ligament disruption in PER injuries, and may question the injury cascade originally described by Lauge-Hansen. Additionally, it highlights the use of computer simulation as an alternative to traditional biomechanical research and offers new opportunities for hypothesis generation, improving diagnosis, and injury classification.