Effect of Posterior Malleolus Fracture on Syndesmosis Reduction
Authors: Phisitkul Phinit , Goetz Jessica E. , Fitzpatrick Elizabeth M. , Sittapairoj Tinnart , Siddappa Vinay Hosuru , Hartog Bryan Den , Femino John 
Published: October 2016

Category: Trauma Introduction/Purpose: Syndesmotic malreduction and the presence of posterior malleolus fractures negatively influence outcomes in rotational ankle fractures. While there are no universally accepted criteria for posterior malleolar fixation, it has recently been shown that posterior malleolar fixation contributed to the stability of the syndesmosis. However, little is known if and how anatomic or non-anatomic fixation of the posterior malleolus affects syndesmotic reduction. A study analyzing syndesmotic reduction in specimens with varying in size and quality of reduction of the posterior malleolus was conducted. Methods: Nine through-knee cadaveric specimens were randomized into two groups with small (a third of the fibular notch, n=4) and large (two-third of the fibular notch, n=5) posterior malleolar fragments. A model of stage IV supination external rotation injury after fibular repair was created by sharply releasing anterior inferior tibiofibular ligament, superficial and deep deltoid ligaments, and interosseous membrane. Posterior malleolar fracture with predefined sizing was created with preservation of posterior inferior tibiofibular ligament. High resolution CT scan was obtained in each specimen at the four stages; intact, neutral- axis syndesmotic clamping only, with anatomic fixation of the posterior malleolus, and with non-anatomic fixation of the posterior malleolus using a 4.8 mm interposed spacer. Measurement of syndesmotic reduction in both anteroposterior and mediolateral planes was made automatically using a validated technique assisted by custom-developed software at 1 cm proximal to the ankle joint. Results: The presence of either a neutral-axis clamping alone or with an anatomically reduced fracture fragment caused a slight anterior shift of the fibula that was more pronounced in the smaller fragment group. Two-way ANOVA indicated no significant effects of fragment size (p=0.73) or reduction (p=0.09) on AP fibular movement. However, presence of non-anatomical fixation caused the fibula to move significantly posteriorly in the presence of a large posterior malleolar fragment (p=0.03 and p=0.01 relative to the intact and clamping only states). In the mediolateral direction, a neutral-axis clamping alone and clamping with an anatomically reduced fracture fragment both increased medial translation of the distal fibula. The non-anatomic reduction model of the posterior malleolus associated with corresponding lateral displacement of the distal fibula. Conclusion: The overall anteroposterior reduction of the syndesmosis using neutral-axis clamping was generally not affected by posterior malleolar fracture except in a non-anatomic fixation of large fragments. Mediolateral syndesmotic reduction was affected by the conditions of posterior malleolar fixation with best results in anatomic fixation but the intact state was still not replicated. Malreduction of the posterior malleolus led to a corresponding syndesmotic malreduction. When a posterior malleolar fixation is indicated in cases with syndesmotic injury, anatomic fracture fixation is paramount as it can affect syndesmotic reduction especially with larger fragments.