**1. Introduction**

Ankle fractures are relatively common musculoskeletal injuries with an average incidence of 168.7/100,000/year [1]. The posterior tibia is involved in almost half of the Weber type B or C ankle fracture dislocations [2]. In most cases, a high-energy trauma is the primary cause of a trimalleolar ankle fracture (TAF), wherein both the medial, lateral,

**Citation:** Hoekstra, H.; Vinckier, O.; Staes, F.; Berckmans, L.; Coninx, J.; Matricali, G.; Wuite, S.; Vanstraelen, E.; Deschamps, K. In Vivo Foot Segmental Motion and Coupling Analysis during Midterm Follow-Up after the Open Reduction Internal Fixation of Trimalleolar Fractures. *J. Clin. Med.* **2023**, *12*, 2772. https:// doi.org/10.3390/jcm12082772

Academic Editors: Andreas Neff and Wing Hoi Cheung

Received: 19 February 2023 Revised: 31 March 2023 Accepted: 5 April 2023 Published: 7 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and posterior malleolus are involved. The trauma-mechanism-based Lauge–Hansen classification has been used for many years in guiding treatment and predicting instability. This system is built on a comprehensive understanding of trauma mechanism and the interplay between fracture morphology and ligamentous injury. The open reduction and internal fixation of the posterior malleolus, via either a posterolateral or posteromedial approach, is most commonly applied in our center for these fractures [3]. However, there is still a lack of consensus on whether the fixation of the posterior malleolus is always necessary. Treatment strategies that take the size and displacement of the posterior fracture fragment into consideration remain an issue of debate [4–6].

TAFs and subsequent plate osteosynthesis are associated with osteoarthritis, arthrofibrosis, and fibro-adhesions (i.e., flexor hallucis longus and peroneal muscles), leading to restriction in ankle joint mobility [7–10]. Subsequently, alterations in foot joint mobility, segment coupling (kinematic relationship between adjacent foot segments), and kinetics (forces acting on the foot joints) during gait may occur as well. It has been theorized that altered or disrupted coupling mechanisms may contribute to poor functional outcome scores due to pathological joint contact forces and soft tissue stress [11,12]. Nevertheless, there is a lack of in vivo dynamic assessments on the foot which specific focus on the functional outcomes of patients with a history of TAF osteosynthesis. Previous studies that reported on foot kinematics after the operative treatment of ankle fractures consisted of heterogeneous groups and used the Oxford foot model, which does not include the midfoot as a separate segment [13,14].

Therefore, we aimed to measure the segmental foot mobility and joint coupling of patients with a history of a TAF osteosynthesis. The affected side was compared to the non-affected contralateral side, as well as with a healthy control group. We hypothesized a reduction in the affected sides' hindfoot range of motion (ROM) during the loading response and the pre-swing of the stance phase, with most distinct changes seen in the frontal plane and sagittal plane, respectively. Furthermore, we hypothesized a reduced joint coupling between hindfoot and shank, as well as between the hindfoot and the forefoot.
