Comparison of Elasticity and Load to Failure for Three Different Syndesmosis Devices: A Biomechanical Study

Abstract

Category: Basic Sciences/Biologics; Trauma Introduction/ Purpose: A syndesmosis injury can result in degenerative change and poor outcomes after an ankle fracture. This can occur in combination with an ankle fracture, with a proximal fibular fracture or with separation of the tibia and fibula without a fracture being present. Fixation of the fibula to the tibia can be rigid with a screw or flexible with a suture device. Flexible syndesmosis fixation has been shown to have superior outcomes and less hardware removals compared to fixed devices. However a more elastic device may result in opening of the syndesmosis before healing. A device with a lower failure load may fail in a heavier patient being more active. Three commercially available suture devices were therefore tested to assess their elasticity and failure load. Methods: Eight each of three commercially available syndesmosis suture devices were tested: Acumed Acu-Synch Knotless (AAK), Arthrex Tightrope (AT) and Zimmer Biomet ZipTight (ZT). An Instron testing machine was used to apply cyclical load through mounting blocks to the suture devices configured to match the distance between the tibia and fibular anchor points. The anchors were inserted based on the operative technique. Synthetic bone blocks with cortical and cancellous bone densities were used. The blocks were preloaded to 53 N with the displacement reset to zero. The specimens were cycled from 53 to 113 N at a rate of 2 Hz for 420,000 cycles and the displacement measured. Testing was discontinued if the suture, fibula button or flip button broke. If 3 mm or greater elongation occurred the specimen was considered failed. For the specimens not failing by 420K cycles load was applied until failure. The mode of failure was recorded. Results: The ZipTight (ZT) devices failed at an average of 122 +/- 185K cycles with 6 of 8 specimens failed by 3 mm. The ultimate load was 424 +/- 122 N. The Acu-Sinch Knotless (AAK) devices all completed 420 +/- 0 K cycles. The displacement 1.9 +/- 0.26 mm. The ultimate load to failure was 1018 +/- 46N. The Tightrope (AT) devices all completed 420 +/- 0 k cycles. The displacement averaged 1.97 +/- 0.16 mm. The failure load was 890 +/- 118 N. The ZT device had fewer cycles to failure than the AAK and AT devices. The ZT device had a greater degree of displacement on repeat loading compared to the AA and AT devices. Conclusion: The ZT device had a lower failure load compared to the AA and AT devices. The AA and AT devices had the same cycles to failure and the same displacement to failure. The AA device was significantly stronger than the AT device in failure mode. The AA and AT flexible syndesmosis fixation showed better mechanical properties than the ZT fixation. The AA device had a higher load to failure than the AT device. The mechanical properties could be relevant in clinical fixation with regards to failure of surgical stabilization.