Superficial Deltoid Ligament and Deep Deltoid Ligament Play Equally Important Roles in the Stability of Isolated Lateral Malleolus (OTA/AO 44-B1) Fractures: A Biomechanical Study.

Objective: To evaluate the individual contributions to stability of the superficial and deep deltoid ligaments in the setting of SER IV ankle fractures.

Methods: Nineteen total cadaveric specimens were used. SER IV injuries were created with the rupture of either the superficial (SER IV-S) (n = 9) or deep deltoid (SER IV-D) (n = 10). These were tested by applying an external rotation force (1 Nm, 2 Nm, 3 Nm, and 4 Nm). Changes in the position of the talus were recorded with a 3D motion tracker. Injury conditions were compared with a 4-step general linear model with repeated measures. Injury condition was also compared with the intact state and to each other using 2-tailed t tests.

Results: The general linear model showed that increased loading had a significant effect with axial rotation (P = 0.02) and sagittal translation (P = 0.003). SER IV-S and SER IV-D showed significantly greater instability compared with the intact state in axial rotation (1 Nm, 2 Nm, and 3 Nm). SER IV-S and SER IV-D did not significantly differ from each other.

Conclusions: SER IV fracture patterns can be unstable with isolated injury to either the superficial or deep deltoid. This challenges the notion that deep deltoid rupture is necessary. Further clinical studies would help quantify the consequences of this instability.