Introduction/Purpose: Syndesmotic injuries account for a substantial proportion of ankle trauma, yet fixation remains challenging, with malreduction rates exceeding 30% in some series. Although patient-specific factors such as foot alignment and osseous morphology are increasingly recognized as determinants of ankle mechanics, their contribution to syndesmotic instability has not been systematically investigated.
Weightbearing CT (WBCT) enables precise quantification of three-dimensional ankle kinematics under physiologic load, allowing both classification of morphological variation and direct linkage to injury response. We hypothesized that tibial incisura morphology, as the principal osseous constraint of the syndesmosis, may determine not only the severity but also the pattern of fibular displacement after ligamentous injury. Identifying anatomical morphologies prone to greater or distinct displacement patterns may explain persistently high malreduction rates and guide fixation strategies.
Methods: Thirty-six fresh-frozen cadaveric pairs (22 male, 14 female) underwent WBCT imaging with 356 N axial loading. Each specimen was scanned intact and again following sectioning of the anterior inferior tibiofibular ligament (AITFL), posterior inferior tibiofibular ligament (PITFL), interosseous ligament, and distal 3 cm of interosseous membrane, by a fellowship-trained foot and ankle surgeon. The deltoid and lateral ligament complexes were preserved to isolate pure syndesmotic pathology. An automated coordinate system pipeline with point-cloud registration quantified three-dimensional fibular and talar displacements relative to the tibia, including mediolateral, anteroposterior, and superoinferior translations, as well as rotations about the same axes, following standardized bone alignment. Unsupervised clustering (PaCMAP with k-means) was applied to objectively identify distinct patterns of injury response with machine learning. Statistical shape modeling was then used to evaluate morphometric variation across the tibia, fibula, and talus, with Hotelling’s T² tests localizing sites of significant anatomical differences between identified clusters.
Results: Clustering revealed two balanced, highly separable syndesmotic injury phenotypes (n=19 vs n=17, silhouette score 0.835). While both groups demonstrated similar displacement directions, their magnitudes differed. The high-instability phenotype (Cluster C1) exhibited significantly greater posterior fibular translation (1.62 vs 0.76 mm, p<0.001) and external rotation (3.29° vs 1.42°, p=0.034) compared to the stable phenotype (Cluster C0). Notably, talar displacements remained equivalent between groups, indicating that fibular instability was the primary differentiator. Statistical shape modeling identified significant morphological differences in the high-instability cluster, specifically, a shallower posterior incisura, with average depth reduced by over 2 mm compared to stable ankles (Figure 1). To eliminate confounding factors, Foot and Ankle Offset (FAO) analysis confirmed that global hindfoot alignment and loading patterns were equivalent between clusters (p=0.7).
Conclusion: Nearly half of all ankles studied possessed an anatomical variant that predisposed them to severe mechanical instability following ligamentous disruption. The shallow posterior incisura phenotype is a previously unrecognized but logical anatomical risk factor that directly governs injury severity. Patients with shallow incisura anatomy may require enhanced surgical stabilization, modified fixation strategies tailored to their anatomical vulnerability, intensified postoperative surveillance protocols, and earlier intervention to prevent chronic complications. This phenotype may confer elevated risk for postoperative complications, recurrent instability, and ultimately post-traumatic osteoarthritis. Future studies will work towards identifying if such morphology predisposes to an increased incidence of syndesmotic disruption.
