Morphological Similarities of Chronic Ankle Instability and Cavus Foot Type Using Statistical Shape Modeling

Chronic ankle instability (CAI) can develop in up to 40% of patients after a first-time ankle sprain. Advanced imaging and statistical shape modeling (SSM) provide the opportunity to evaluate if subtle differences in foot and ankle morphology, such as hindfoot alignment or cavus features, may contribute to structural predispositions for recurrent instability. In this study, a 14-bone SSM was created from weight-bearing CT data (n = 80) from patients with CAI (n = 23), cavus foot type (n = 29), and rectus (n = 28) foot type. Scans for the CAI group were from pre-operative imaging of patients undergoing surgical stabilization. Meary's angle (MA), hindfoot ankle alignment (HAA), and calcaneal inclination were measured for each scan using in-house code. Principal component analysis revealed that arch height (Mode 1, 36.9% of the variance) statistically distinguished the CAI and cavus groups from the rectus group. Minimal differences were observed between the CAI and cavus groups, with only Mode 5 (4.47% variance, ${\eta }^2$  = 0.09) separating the groups and Hotelling's T² confirming minimal variation (~2% of particles). Radiographic measurements supported these findings with higher MA in CAI (12°) and cavus (17°) versus rectus (0.2°), and varus HAA in CAI (6.0°) and cavus (5.9°) compared to rectus (9.8°). Individuals with CAI demonstrated cavus-like morphology, indicating that bony alignment may be a structural contributor to recurrent instability. Clinically, these findings enhance our understanding that foot alignment plays a role in CAI. Surgical correction, when clinically appropriate, may need to address both osseous deformities and soft tissue laxity to improve stability. Level III Orthopedic Research Society.