The Effect of Screw Fixation Height on Syndesmotic Malreduction: A Cadaveric Study Utilizing Weightbearing Computed Tomography

Research Type: Level 5 - Case report, Expert opinion, Personal observation Introduction/Purpose: Syndesmotic malreduction is a well-documented problem negatively impacting outcomes with a reported incidence approaching 52%. Screw placement between 1.5 – 6 cm above the tibial plafond has been traditionally advocated. Despite this, the optimal height for screw fixation remains unclear and its potential effects on syndesmotic malreduction unknown. As more proximal placement would theoretically lead to increased syndesmotic malreduction given increased distance from the osseous constraints of the incisura, screw height placement may be a modifiable variable to improve syndesmotic reduction. This cadaveric study employed weightbearing computed tomography (WBCT) to evaluate the effects of screw fixation height on syndesmotic alignment in a simulated pronation-external rotation (PER) ankle fracture model. We hypothesized that increasing screw fixation height correlates with a higher degree of syndesmotic malreduction. Methods: Twelve knee-disarticulated, cadaveric specimens underwent simulated PER ankle fracture injuries with disruption of the deep deltoid ligament, syndesmosis and creation of a proximal fibula fracture. A 1/3 tubular plate was affixed to the distal fibula with screw fixation placed incrementally from 1.5 cm to 6.5 cm above the tibial plafond. Four fellowship-trained orthopaedic surgeons performed all trials and manually reduced the syndesmoses by direct thumb-reduction followed by fixation using a fully threaded quad-cortical metal screw under fluoroscopic guidance. WBCT imaging was performed post-fixation after each trial at each screw fixation height to assess syndesmotic alignment, screw angulation, and incisural morphology. WBCT imaging were analyzed utilizing both traditional 2-D measurements (distance, area) and advanced 3-D volumetric analyses. Results: Syndesmotic diastasis measurements varied with screw fixation height (p = 0.035 for posterior diastasis, p < 0.0001 for mid-diastasis, and p = 0.023 for anterior diastasis). Fibular rotation and translation also demonstrated significant differences depending on height (p < 0.001). Syndesmotic volume analyses showed no significant differences between fixation levels (p = 0.054) indicating no effect of screw fixation height on total volumes. However, more proximal screw fixation levels increased total volumes and resulted in distal widening supporting a “see saw” effect of more proximal fixation. While no screw fixation level fully restored pre-sectioned syndesmotic alignment, a screw placed 27mm above the tibial plafond lead to measurements closest to the intact state on both 2-D and 3-D analyses. Incisural morphology did not affect syndesmotic measurements. Conclusion: This study confirms that screw fixation height influences syndesmotic reduction with increasing distance from the tibial plafond increasing volume and altering diastasis measurements. A “see saw” effect was evident as more proximal fixation lead to distal syndesmotic widening. While no screw level perfectly restored normal syndesmotic alignment, a screw placed 27mm above the tibial plafond resulted in measurements closest to native pre-injury syndesmotic alignment and appears optimal for minimizing malreduction risks.