Exploring Morphological Phenotypes in Chronic Ankle Instability
Authors: Fendler Alayna , LaTulip Scott , Requist Melissa , Lisonbee Rich , Nixon Devon , Lenz Amy 
Published: December 2024

Category: Ankle; Sports Introduction/Purpose: Patients with chronic ankle instability (CAI) often present with heterogenous soft tissue injuries of the lateral, medial, and/or syndesmotic ankle ligaments. Risk for the development of CAI may also be related, in part, to underlying bone anatomy like a subtle cavovarus (SCF) shape. However, reports on bone anatomy of CAI patients have largely been reserved to two-dimensional radiographic measurements. Evolving techniques for assessing bone morphology are gathering increasing interest using weightbearing computed topography scans (WBCT) and statistical shape modelling (SSM) that can quantify and compare anatomy across cohorts. No study to date, though, has explored the concept of ankle instability and bone shape using WBCT and SSM data. Methods: In this retrospective, comparative pilot study, 23 patients with CAI who underwent surgical repair, and 15 control subjects were identified. The CAI patients were selected if they had a pre-operative WBCT. Prior to analysis, patients with CAI were subdivided based on the surgery they underwent – either lateral only ligament reconstruction or combined medial-lateral ligament reconstruction. Pre-surgical WBCT scans of both surgical and control subjects were used to generate 3D models of foot and ankle morphology including the tibia, fibula, talus, calcaneus, navicular, cuboid, cuneiforms, and metatarsals. We developed a SSM with these 14 bones using Shapeworks to quantify morphological and alignment differences between groups. Modes of variation were quantified using principal component analyses to compare across the three groups (control, lateral only, and combined medial-lateral). To assess significance, modes were compared between groups with ANOVAs with a Holmes-Sidak correction. Results: One mode of anatomical variation demonstrated significant differences between the lateral only CAI and asymptomatic control groups (Figure 1). Within this noted variation in patients with lateral only CAI, clinical observations noted an anterior talar shift, first ray stabilization with a downward inclination of the first metatarsal (i.e. more plantarflexed first ray), external rotation of the calcaneus with increased hindfoot varus, and increased metatarsal splay. The group distributions of anatomical variation between asymptomatic controls and medial-lateral CAI were similar. Our results suggest subtle differences in pre-operative hindfoot and midfoot bone shape and alignment in patients undergoing surgical reconstruction for CAI. Conclusion: The use of WBCT to conduct a 3D analysis using SSM methods has yet to be explored in the literature. The value of a 3D analysis in a weightbearing position allows for detailed bony shape and alignment evaluation. The clinical significance of these findings hopefully can provide a foundation for further exploration into the possible morphological complexities in CAI. We hope this pilot investigation can allow for, with continued study, an enhanced understanding of which patients are at greatest risk for developing CAI as well for instability recurrence after previous surgery.