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Category: Midfoot/Forefoot, Trauma Introduction/Purpose: Lisfranc injuries occurring between the medial cuneiform and base of the 2nd metatarsal require anatomic fixation. Suture button and screws are standard techniques for fixation, but the screw may decrease physiologic motion, whereas suture buttons may cause increased soft tissue irritation and iatrogenic cartilage damage. Potential benefits of the InternalBrace include physiologic motion, decreased iatrogenic damage, collagen ingrowth, limited bony erosion and decreased soft tissue irritation. In light of these potential benefits, no studies have investigated the biomechanical properties of the InternalBrace in a Lisfranc injury model. However, it is unknown whether there is significant difference in the biomechanical properties of the IB compared to the screw, or SB during load to failure, and cyclical loading. Methods: Three groups of sawbones were fixed together with either a 3.5 mm screw, SB, or IB, composed of a curved button, fibertape, and 4.75 mm biotenodesis screw. Sawbone constructs were held in a mechanical testing system (Model 809, MTS Systems Corp, Minneapolis MN). The first three groups of 10 were loaded in axial tension at 0.5mm/sec until failure to determine load-displacement data. Yield, stiffness, ultimate strength (US), yield energy, post-yield energy and ultimate strength energy were calculated. Three more groups of 8 constructs were loaded in-vitro at cyclical physiologic loads until displacement of 1.5 mm occurred. Constructs were first loaded for 10,000 cycles at 69 N (estimate for 50% body weight or assisted walking). Surviving specimens were loaded at 138 N (normal walk) for an additional 10,000 cycles and then 207 N (jog) for an additional 10,000 cycles. Displacement was recorded. The biomechanical properties were then compared between groups. Results: When loaded in axial tension at 0.5mm/sec until failure, the screw was found to be the stiffest construct (2,240 N/mm), while the InternalBrace (200 N/mm) was stiffer than the suture button (133 N/mm). Qualitatively, the InternalBrace was also found to hold load more consistently and for larger displacement prior to failure when compared to the suture button. Cyclic loading was performed with 10,000 cycles of 69 N, 138 N, and 207 N. The screw had the greatest resistance to fatigue. The InternalBrace maintained stiffness as well or better than the suture button, but the fatigue life was shorter than that of the suture button. Conclusion: To our knowledge, the biomechanical properties of the IB have not been compared to screw and SB for ligamentous lisfranc injuries. This study gives valuable information about the mechanical integrity of InternalBrace and supports continued use. However, further studies are warranted before making conclusions regarding early weight bearing.

Category: Sports, Biomechanics Introduction/Purpose: Lisfranc injuries are characterized by disruption between the medial cuneiform and base of the second metatarsal. Conventional interfragmentary screws decreases the amount of diastasis, however is believed to decrease the natural physiological movement of the joint compared to suture button (Tightrope, Arthrex, Inc., Naples, FL). The InternalBrace (IB, Arthrex, Inc., Naples, FL) allows physiologic movement and collagen ingrowth, while also decreasing iatrogenic bone loss. It also prevents erosion of the suture button into the medial cuneiform and prevents irritation of the tibialis anterior tendon. We hypothesized that there was no significant difference in the mechanical properties of these three constructs. Methods: Three groups of 10 sawbone models were used in this study. Two fourth generation 20 mm cylinder sawbones with open cell foam were fixed together with either a 3.5 mm conventional screw, mini Tightrope or IB with a curved button and 4.75 mm biotenodesis screw. Sawbone constructs were held in a mechanical testing system (Model 809, MTS Systems Corp, Minneapolis MN) using custom fixtures. Constructs were loaded in axial tension at 0.5mm/sec until failure. Load-displacement data were plotted for each test. Yield, stiffness, ultimate strength (US), yield energy, post-yield energy and ultimate strength energy were calculated Additionally, the load and energy to 0.5 mm, 1.0 mm and 1.5 mm of displacement were captured to relate strength at clinically relevant displacements. The residuals of an ANOVA on all mechanical testing results were not normally distributed. Therefore non-parametric comparison was used to compare fixation types (Proc NPAR1WAY, SAS 9.4, SAS Institute). Results: Compared to IB, the screw demonstrated greater stiffness, yield load and energy, and ultimate load and energy, with smaller yield, ultimate and failure displacement. When comparing the Tightrope and IB, there was no difference in stiffness (p=0.82), although the Tightrope performed greater in terms of having a larger yield load, energy and displacement, a larger ultimate strength load, energy and displacement, and a larger failure load, energy and displacement. When assessing the load at various distances of displacement, there was no significant difference between the load at 0.5 mm displacement (p=0.5, Figure 1). At greater displacement, the load was greater in the Tightrope than the IB (Figure 1). Conclusion: In this study, IB has shown proper stiffness and strength for fixation of ligamentous lisfranc injury. However, if a diastasis of >0.5 mm is evident, concerns for a clinical failure should be examined. This is the first study examining the use of an IB for treatment of a ligamentous lisfranc injury. The data supports its current clinical indications and further studies in cadaveric models are recommended.