Syndesmotic Screw Angle in Lateral Fibular Pre-Contoured Plates Impacts Quality of Syndesmotic Reduction: A Cadaveric Study

Category: Ankle; Trauma Introduction/Purpose: Achieving adequate reduction of ankle syndesmosis after syndesmotic injury is critical as malreduction can lead to accelerated arthritis, pain, and instability. Static fixation with syndesmotic screw is the gold standard where 1-2 syndesmosis screws are placed obliquely at a 25-30° angle from posterolateral to anteromedial, parallel to the joint line. While retrospective studies have been conducted on screw level and extreme angle changes, it is unclear how changes in screw angle impacts rates of malreduction. We sought to assess the effect of syndesmotic screw angle within lateral fibular pre-contoured plates on the quality of syndesmotic reduction. We hypothesize that greater deviations from the 30° screw angle would cause malreduction of the syndesmosis, particularly as the screw head engaged with the pre-contoured plate at sharper angles. Methods: Twelve cadaveric legs were used for this study. All legs were CT scanned pre- and post-simulated syndesmosis injury, with the uninjured serving as the controls. Six pairs of legs were split into 3 groups of 4, (20, 30, and 40 degree screw angles). Each specimen was confirmed to have no prior syndesmosis injury prior to conducting the simulated syndesmosis injury. Syndesmotic screw angle was determined by 3D-printed custom drill guides with drilling performed by a fellowship-trained trauma surgeon, tricortical screws were placed and scanned, then repeated with quad-cortical screws. Syndesmosis reduction was assessed via pre- and post-injury CT scans, we compared the following measures at 1 cm above the tibial plafond: anterior and posterior tibiofibular distance, anteroposterior fibular translation, and fibular rotation. Malreduction was defined as any significant deviation of the 4 measurements compared to pre-injury. Pre-injury K-wire tracks served as additional verification of translation or rotation. Results: In total, 24 reductions were measured and analyzed for malreduction. Measurements, grouped by screw angle, of post injury CT scan at 1 cm above the tibial plafond showed malreductions as follows: 3 of 8 at 20°, 0 of 8 at 30°, and 6 of 8 at 40°. Fischer’s exact demonstrated a significant change between angle and presence of malreduction (p = 0.009). Syndesmotic screw angle significantly impacted change in anteroposterior fibular translation with positive changes indicating posterior translation and negative, anterior translation (20°: 0.98 mm, 30°: 0.068 mm, 40°: -1.91 mm, p= 0.0094). Syndesmotic screw angle did not impact change in anterior or posterior tibiofibular distance or fibula angle. Malreduction did not differ between tri and quad cortical screws (p = 0.5). Conclusion: Our study shows evidence that deviation of syndesmotic screw angle within a pre-contoured lateral fibula plate significantly impacts malreduction rates and anteroposterior fibular translation regardless of tri- or quad-cortical fixation. While our study does not support our hypothesis that plate-screw interaction is responsible for malreduction in syndesmosis fixation, it does support that the ideal syndesmotic screw angle is approximately 30 degrees and that deviation from this angle risks anteroposterior fibular translation and malreduction.