The Role of Deep Deltoid Ligament in Ankle Fracture Stability: A Biomechanical Cadaveric Study

Abstract

Category: Ankle; Basic Sciences/Biologics; Trauma Introduction/Purpose: Supination-external rotation (SER) injuries make up 80% of all ankle fractures. SER stage 2 injuries (AITFL and Weber B) are considered stable. SER stage 3 injury includes disruption of the posterior malleolus (or PITFL). In SER stage 4 there is either medial malleolus fracture or deltoid injury. SER 4 injuries have been considered unstable, requiring surgery. The deltoid ligament is a key component of ankle stability, but clinical tests to assess deltoid injury have low specificity. This biomechanical cadaveric study specifically investigates the role of the components of the deep deltoid ligament in the stabillity of SER ankle fractures. Methods: In the first phase of the study, three specimens were utilised to standardise dissection of the deltoid ligament and creation and fixation of SER ankle fracture. In phase two, four matched pairs (8 specimens) were tested using this standardised protocol (Figure1). Specimens were sequentially tested for stability when axially loaded with a custom rig with up to 750N. Specimens were tested with: ankle intact; lateral injury (AITFL and Weber B); additional posterior injury (PITFL); additional anterior deep deltoid; additional posterior deep deltoid; lateral side ORIF. Clinical photographs and radiographs were recorded at each stage. In addition, dynamic stress radiographs were performed after sectioning the deep deltoid and following fracture fixation to assess talar tilt in eversion. Results: All specimens behaved in an identical manner when subjected to this standardised protocol. When the posterior deep deltoid ligament was intact, the ankle remained stable when loaded and showed no talar tilt on dynamic stress test. Once the posterior deep deltoid ligament was sectioned, there was demonstrable instability in all specimens. Surgical stabilisation of the lateral side using standard technique with a plate prevented talar shift but not talar tilt. In adequately stabilised ankle specimens, there was no loss of fixation on axial loading. Conclusion: This biomechanical cadaveric experiment demonstrates that under the standardised test conditions, all SER fracture ankle specimens with an intact posterior deep deltoid ligament behaved as stable injuries. The posterior portion of the deep deltoid ligament is a crucial structure in conferring stability to SER stage 4 injuries. The clinical implication of this is that when the posterior deep deltoid ligament is intact, SER fractures may be managed without surgical intervention in a plantigrade cast. We also conclude that without immobilisation, the talus may tilt in the mortise risking long-term deltoid incompetence.