Assessing the Need for Additional Syndesmotic Stabilization in Open Reduction of the Posterior Malleolus: A Biomechanical Study.

Abstract

BACKGROUND The treatment of ankle fractures involving the posterior malleolus (PM) has changed in favor of open reduction and internal fixation (ORIF), and the need for additional syndesmotic stabilization has decreased; however, there are still doubts regarding the diagnosis and treatment of residual syndesmotic instability. The aim of the present study was to evaluate the effect of fixation of the PM and to assess the need for additional stabilization methods. We hypothesized that ORIF of the PM would not sufficiently stabilize the syndesmosis and that additional syndesmotic reconstruction would restore kinematics. METHODS Eight unpaired, fresh-frozen, cadaveric lower legs were tested in a 6-degrees-of-freedom robotic arm with constant loading (200 N) in the neutral position and at 10° dorsiflexion, 15° plantar flexion, and 30° plantar flexion. The specimens were evaluated in the following order: intact state; osteotomy of the PM; transection of the anterior inferior tibiofibular ligament (AITFL) and interosseous ligament (IOL); ORIF of the PM; additional syndesmotic screw; combination of syndesmotic screw and AITFL augmentation; and AITFL augmentation. RESULTS A complete simulated rupture of the syndesmosis (PM osteotomy with AITFL and IOL transection) caused translational (6.9 mm posterior and 1.8 mm medial displacement) and rotational instability (5.5° external rotation) of the distal fibula. ORIF of the PM could eliminate this instability in the neutral ankle position, whereas sagittal and rotational instability remained in dorsiflexion and plantar flexion. The remaining instability could be eliminated with an additional procedure, without notable differences between screw and AITFL augmentation. CONCLUSIONS In our model, isolated PM osteotomy and isolated AITFL and IOL rupture (after PM refixation) only partially increased fibular motion in dorsiflexion and plantar flexion, whereas the combination of PM osteotomy and AITFL and IOL rupture resulted in an unstable syndesmosis in all planes. CLINICAL RELEVANCE In complex ankle fractures, ORIF of the PM is essential to restore syndesmotic stability; however, residual syndesmotic instability can be detected by a specific posterior shift of the fibula on stress testing. In these cases, anatomical AITFL augmentation is biomechanically equivalent to the use of a syndesmotic screw.