Paper 96: The Biomechanical Role of the Deltoid Ligament on Ankle Stability: Injury, Repair and Augmentation

OBJECTIVES: The deltoid ligament is frequently ruptured in association with ankle fractures and syndesmotic injuries. Despite the frequency of this ligamentous injury, surgical repair is not the current practice due to the belief that non-weightbearing during fracture recovery allows for adequate healing. However, conservative management has shown poor outcomes both acutely and over time with the development of arthritis. The contribution of the anterior and posterior bundles of the deltoid ligament to the stability of the ankle remains unknown, as does the effect of deltoid repair versus repair with augmentation. To determine the biomechanical role of the native anterior and posterior deltoid ligament in ankle stability and to determine the efficacy of simple suture vs augmented repair of each bundle. It was hypothesized that sectioning of the anterior and posterior deltoid ligaments would significantly increase ankle laxity in eversion, external rotation and anterior translation, repairing the anterior and posterior deltoid ligaments using suture anchors would significantly reduce ankle laxity compared to the sectioned states, and augmenting the repairs with an Internal Brace would further reduce ankle laxity compared to the suture anchor repair. METHODS: Ten cadaveric ankles (50.9 mean age, male) were dissected free of soft tissue superficial to the medial ligamentous structures and mounted on a six-degrees-of-freedom robotic arm. The specimens underwent biomechanical testing in eight sequential states: 1) intact, 2) anterior deltoid cut, 3) anterior repair, 4) internal brace tibio-spring augmentation 5) internal brace tibio-calcaneal augmentation, 6) posterior deltoid cut, 7) posterior repair, 8) complete deltoid cut). Biomechanical testing consisted of three tests: 1) Anterior drawer 88 N, 2) Eversion 5 Nm, 3) External rotation 5 Nm, each run at 0° and 25° of plantarflexion under 100 N of joint compression. Anterior translation (mm) eversion (deg), external rotation (deg), were measured for each associated motion. A 1-factor random-intercepts linear mixed effect model was created for each test, and all pairwise comparisons were made between testing states, adjusting for multiple comparisons using Tukey’s method. RESULTS: The complete deltoid cut state had significantly higher anterior translation, eversion, and external rotation at 0⁰ and 25⁰ of plantar flexion compared to all other states (+8.6mm ATT p=0.01(-9), +34.1⁰ Eversion p=0.01(-9), +14.7⁰ p=0.01(-9) ER compared to native at 0⁰ plantar flexion, respectively). The anterior Deltoid cut showed a significant increase in eversion at 25⁰ of plantarflexion compared to the native state (+6.65⁰ P=0.0007). The anterior deltoid repair alone showed no significant decrease from the anterior deltoid cut, however both augmented anterior repairs showed significant decreases in eversion (-5.1⁰, p=0.023; -5.0, p=0.026, respectively). No other significant differences were observed between any states. CONCLUSIONS: The most important finding of this study was that the complete deltoid tear caused major ankle instability in anterior translation, external rotation and eversion. Therefore, deltoid repair is strongly recommended in the clinical setting of a complete tear. The anterior deltoid tear alone caused significant eversion instability at 25⁰ of plantarflexion, which was significantly reduced by the first augmented repair, the addition of a second augmentation showed no additional benefit over a single augmentation. Cutting the posterior deltoid in the setting of an augmented anterior repair showed no effect on ankle kinematics, therefore an augmented anterior repair can be seen a sufficient to stabilize a complete deltoid tear. Our findings suggest that surgeons may need to reconsider non-operative treatment on complete anterior deltoid injuries and that repair with augmentation offers superior stability.