Quantifying Simulated Syndesmotic Malreduction Forgiveness Using a Two-Phase Stabilization System

Category: Trauma; Sports Introduction/Purpose: Prior studies have shown the potential of a rigid fixation, such as a trans-syndesmotic screw, to produce tibiofibular malreduction. Flexible implants, although capable of allowing forgiveness in these situations, may not provide sufficient initial stability for all injury patterns. The purpose of this study is to assess the malreduction forgiveness of a two-phase syndesmotic device that is designed to transition from a rigid screw construct to a flexible suture-type device upon weightbearing. Methods: Below-knee cadaveric specimens were mounted in a radiolucent frame with a static axial load of 356N under four conditions: native ankle (control), syndesmotic instability (injured ), malreduced with two-phase syndesmotic device engaged (malreduced ), and post-disengagement of the two-phase device (disengaged). A lateral approach was used to destabilize the joint. The malreduction targets for each group were 5mm anterior displacement, 5mm posterior displacement, 15° of rotation, and over-compressed (140N) states. Fixation was performed with a single implant 20mm proximal to the joint. Weightbearing Computed Tomography (WBCT) scans were obtained for each condition. The device was manually disengaged to allow for semi-constrained motion of the syndesmosis, reproducing the release that would occur when a patient initiates weightbearing. Anatomic axes of the tibia and fibula were extracted from WBCT scans to calculate tibiofibular joint position with an established orthogonal joint coordinate system representation. Results were normalized to the control states and compared. Results: A total of 42 limbs were included (anterior=10, posterior=11, rotation=12, over-compression=9). Anterior and posterior malreduction specimens demonstrated 2.50mm (SD: ±1.37mm) and 5.04mm (SD: ±2.23mm) of average malreduction in the malreduced state. The disengaged device condition resulted in an average recovery of 1.79mm (95%CI: 0.72mm, 2.85mm; p=0.0034; 72% recovery) and 1.69mm (95%CI: 0.09mm, 3.28mm; p=0.0006; 33% recovery) toward the control position of the joint, for anterior and posterior malreduction, respectively. Rotational malreduction specimens demonstrated 2.44° (SD: ±2.09°) of average absolute rotational malreduction, with 1.98° (95%CI: -0.13°, 4.09°; p=0.0707; 81% recovery) of recovery. Over-compression specimens demonstrated an average medial translation of 0.89mm (SD: ±1.10mm), and the disengaged device condition had 0.74mm (95%CI: 0.05mm, 1.51mm; p=0.0128; 82% recovery) of joint pose recovery. Conclusion: In this cadaveric study, malreduction forgiveness using a two-phase syndesmotic implant that transitions from a rigid screw to a flexible device was tested. Using the computational methods described, overall syndesmotic malreduction and specific corrective changes in fibular malpositioning following disengagement from a rigid construct were noted. The results support the implant concept on malreduction recovery in multiple planes, potentially impacting the clinical management of syndesmotic injuries. Application of this implant and our study methodology to a clinical cohort of patients with syndesmotic instability is paramount to confirm our findings.