Deltoid and Syndesmotic Ligaments, Part 1. The Role of Injury on Biomechanical Rotational Ankle Instability.

Background: The syndesmotic and deltoid ligament (DL) complex contributes to ankle stability. However, the biomechanical role of (partial or complete) ligamentous injuries on external rotation instability remains unclear.

Purpose: To determine the contribution of syndesmosis and DL injuries to external rotation instability compared with the intact ankle.

Study Design: Controlled laboratory study.

Methods: In total, 24 cadaveric lower extremities were assigned to 3 groups with sequential dissection of the anterior inferior tibiofibular ligament (AiTFL), interosseous ligament (IOL), superficial DL (SDL), and deep DL (DDL) in different orders to evaluate their specific contribution to rotational instability. First, intact ankles were loaded up to 7.0 N·m external torque to evaluate time-zero ligament rotations at 2.5, 4.0, 5.5, and 7.0 N·m. Rotation-controlled cycling was performed consecutively (a total of 1000 cycles) for each dissection level with peak torque and stiffness analyzed at the end of each rotation level (250th cycle). The contributions of sequential dissection levels to ankle instability were calculated using peak torque changes divided by the intact state.

Results: Either syndesmotic (AiTFL+IOL) (31% to 45% across α to α) or complete deltoid dissection (SDL+DDL) (26% to 40% across α to α) decreased ankle stability the most as it resulted in lower peak torques ( < .001) and torsional stiffness ( ≤ .030, except SDL+DDL at α) compared with the intact state. Syndesmosis with individual DL dissection demonstrated a higher ( < .031) contribution of SDL to rotational ankle instability than DDL. Syndesmosis with SDL ( < .015) or complete DL dissection further significantly reduced ( < .001) external peak torque loading capability. With sequential ligament dissection, the functional behavior shifted toward a completely loose state, with the lowest resistance to rotational loading in the unstable ankle injury state with all ligaments dissected. Dissection of all ligaments reduced the ankle stability between 56% (α) and 62% (α).

Conclusion: While both a 2-ligament syndesmosis and complete DL injury significantly destabilized the ankle, additional SDL dissection was more critical than DDL dissection with concomitant syndesmosis injury and resembled the condition of a completely unstable ankle injury state.

Clinical Relevance: Knowledge of the individual and combined syndesmotic and DL injury patterns to rotational ankle stability is crucial for appropriate surgical intervention in treating unstable ankles.