InternalBrace Has Biomechanical Properties Comparable to Suture Button but Less Rigid than Screw in Ligamentous Lisfranc Model

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.