Correction of syndesmotic malreduction following fixation flexibilization

Abstract

Prior studies showed the potential of a rigid fixation, such as a trans-syndesmotic screw, to produce tibiofibular malreduction. Flexible implants, although capable of allowing forgiveness, may not provide sufficient stability for all injury patterns. 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. Cadaveric specimens were loaded in a frame under four conditions: native (control), syndesmotic instability (injured), malreduced with engaged syndesmotic device (malreduced), and post-disengagement (disengaged). The malreduction targets were 5 mm anterior displacement, 5 mm posterior displacement, 15° of rotation, and 140 N over-compressed. Fixation was performed with a single device 20 mm proximal to the ankle, manually disengaged to allow semi-constrained motion of the syndesmosis. Weightbearing Computed Tomography (WBCT) scans were obtained, and anatomic axes of the tibia and fibula extracted to calculate tibiofibular joint position with an established orthogonal system. A total of 42 specimens were included and allocated in the four groups. Anterior and posterior malreduction demonstrated 2.50 mm (SD: ±1.37 mm) and 5.04 mm (SD: ±2.23 mm) of average malreduction. The disengaged condition resulted in average recovery of 1.79 mm (95%CI: 0.72|2.85; p = 0.0034; 72% recovery) and 1.69 mm (95%CI: 0.09|3.28; p = 0.0006; 33% recovery) toward the control position, for anterior and posterior malreduction, respectively. Rotational malreduction demonstrated 2.44° (SD: ±2.09°) of average rotational malreduction, with 1.98° (95%CI: -0.13°|4.09°; p = 0.0707; 81%) of recovery. Over-compression specimens showed average medial translation of 0.89 mm (SD: ±1.10 mm), with 0.74 mm (95%CI: 0.05|1.51; p = 0.0128; 82%) of recovery when disengaged. The two-phase syndesmotic device was able to allow partial malreduction recovery in different scenarios after transitioning to the flexible state. The use of this implant might mitigate potential surgical tibiofibular malreductions while providing the mechanical and clinical advantages of both rigid and flexible devices. Level V. Controlled laboratory study.

Fig. 1

Study flowchart. WBCT weight-bearing computed tomography, N newtons, mm millimetres, n number of specimens.

Fig. 2

Broad view of the specimen placed inside WBCT scanner. The leg is secured in a simulated weightbearing state by the radiolucent external frame that hold the weights (36.3 kg) on top of the construct.

Fig. 3

Graphical representation of the two-phase syndesmotic device. The screw housing contains an internal suture loop with a disengaging mechanism that allows the device (rigid fixation—A) to become flexible, turning into a dynamic/flexible suture-type fixation (B). The screws were 4.2 mm in external major diameter, with an internal major diameter of 3.3 mm in the fibula and 2.8 mm in the tibia. The screws were solid and had an internal suture loop with a disengaging mechanism that allowed the screw (rigid fixation) to become flexible, turning into a dynamic/flexible suture-type fixation. Repetitive cycling (load, motion) would disengage the internal mechanism. Fluoroscopic image showing the external ball-tipped guide wire and the targeting guide for the syndesmotic device (C) and the device inserted through the fibula and tibia with the disengagement notch positioned in the syndesmotic space (D).

Fig. 4

External jigs for induced malrotation (A) and anterior/posterior displacement (B). Example of induced posterior fibular displacement (C). Example of overcompression (D) with the load-cell controlled compression (E, F) clamp (above 140 N).

Fig. 5

Examples of weightbearing CT images (axial plane) of control and injured ankles as well as induced over-compression, rotation, posterior displacement, and anterior displacement. Top row images demonstrating displacement 1 cm proximal to the ankle, middle row at the level of the device, and bottom row with the disengaged state, demonstrating partial correction of the syndesmotic malreduction.

Fig. 6

Visualization of anatomical joint coordinate system and positive translations and rotations as perceived by the fibula. Figure was created using MATLAB R2022b (Mathworks, Natick, MA. https://www.mathworks.com/).

Fig. 7

Anatomic axes configurations exemplified by each malreduction pattern and situation. In these examples, axes were compared between the control (grey) and malreduction (purple) states. Vectors and positions are represented by colored lines. (A) Anterior malreduction. (B) Posterior malreduction. (C) Malrotation. (D) Over-compression. Figure was created using MATLAB R2022b (Mathworks, Natick, MA. https://www.mathworks.com/).