Response to “Letter Regarding: Achilles Tension Mitigates Fibular Malalignment Measured in Cadaveric Studies of Syndesmotic Clamping”

Dear Editor, We are very pleased that this study has sparked interest in advancing the knowledge of how to best treat the injured tibiofibular syndesmosis, and the questions raised in this letter to the editor demonstrate that there is yet ongoing interest in the biomechanics and the clinical implications of syndesmosis repairs. We are very appreciative of the complements on the clinical relevance of this basic science study. This cadaveric study aimed to introduce the effects of Achilles tendon force on syndesmotic reduction. As this force has previously not been considered in other cadaveric studies, we felt this was a strength of the work and potentially a reason for some of the disconnect between cadaveric studies and clinical studies. It was clearly stated in the discussion on limitations that future studies are needed to investigate the quantitative and qualitative effects of ankle dorsiflexion on Achilles tendon tension and resultant syndesmotic reduction. There is conflicting literature, and conflicting opinion about the appropriate flexion angle for syndesmosis fixation, as summarized nicely in an article by Schon et al. We completely agree with the statement that it would be very interesting to determine what effect ankle flexion has on syndesmotic reduction, and based on our results we would suggest that inclusion of Achilles and other muscle forces would be important in those investigations. As described in the Methods section of the article, the value of Achilles tension applied in this work was determined empirically in a cadaveric setting by measuring the tensile force applied to the tendon when a fellowshiptrained foot and ankle surgeon determined that both the passive dorsiflexion force to the foot to achieve neutral flexion and the reduction clamp force to a fully disrupted syndesmosis were similar to what would be found/used in the operating room. The force was applied through a springscale with a separate observer reading the value on the scale when the surgeon indicated the cadaveric specimen had reached a similar appearance with a similar hand force to what was seen in the operating room. It is unclear if more accurately tensioning all the muscles around the ankle rather than simulating all passive muscle tension through this single-tension application would have a different effect on syndesmotic reduction. Further studies to investigate the effects of Achilles tendon force, position of the ankle, knee flexion, pre-existing foot deformities, and varieties of rotation ankle fracture types are strongly encouraged. We appreciate that there are a great many reported measurements to quantify the position and the rotation of the tibia within the incisura. The geometric measurements that were made in this article are based on reported clinical methods for evaluation of fibula rotation. Each method has pros and cons but only those with documented excellent interobserver and intraobserver reliability were included. In this work, the selected methods were implemented so as to automatically generate measurements based on 3D models that were generated from each acquired CT scan. So although it is challenging to identify a fibular centroid on any individual axial CT slice, a geometric centroid of a 2D shape (defined as a cross-section through a 3D model) can be readily calculated. And then changes in that centroid location can be used to calculate rotation of the fibula around the tibia, because the entire tibia model was aligned to an anatomic coordinate system. While somewhat onerous to perform on axial CT scans, this method would potentially be possible to implement in the clinic. Rather than use our 3D models to define a fibular axis, and then calculate the rotation of the fibula around that axis as a measure of fibular rotation in the incisura, we elected to automate a clinically used method of evaluating fibular rotation. Although there are some confounding factors in this technique, we felt it stronger to use clinically acceptable measures with an established history of good repeatability for this study. In future studies that we hope will follow on this work, the selection of outcome metrics should be driven by the specific study question and the context in which the outcome will be evaluated.