Strength and Compressive Ability of Midfoot Fusion Nail vs Midfoot Fusion Bolt and Role of Subtalar Fusion

CATEGORY: Midfoot/Forefoot; Other INTRODUCTION/PURPOSE: Surgical management of midfoot Charcot arthropathy typically includes fusion through the midfoot to obtain a stable plantigrade foot. Multiple surgical strategies exist, including variable combinations of internal fixation with plates, or thin wire external fixator frames, or both. More recently, intramedullary beams and bolts have been used as an alternative means of fixation. The current understanding of midfoot fusion lacks knowledge of how much stiffness, strength and compression is provided using two different intramedullary midfoot fusion devices. The purpose of this study is to assess 3 point bending strength of midfoot fusion nails versus bolts as well as to test their compressive abilities. Additionally, we looked to assess how the addition of a subtalar fusion affects midfoot stiffness and rotational control in a cadaver model. METHODS: Bone blocks and cadaver feet were used for compression and biomechanical testing, respectively. Bone blocks were pre-drilled with an undersized pilot bit and then cut in two. A washer-type load cell measured the compressive force produced by the implants. Sixteen (8 nails and 8 bolts) compressive tests were performed. Ten matched cadaver foot specimens were prepared, and medial column fusions were performed with midfoot fusion nails or bolts (10 each). Another ten matched specimens were prepared evaluating midfoot fusion nails with or without subtalar fusions. Specimens were placed on a custom platform at 20 degrees of dorsiflexion. A linear variable differential transformer (LVDT) was used to record local displacement at the midfoot (Figure 1). Each specimen underwent 3000 cycles of compressive loading (200N - 1000N) followed by displacement- controlled loading to 9mm of total axial displacement via servo-hydraulic test frame. The Wilcoxon signed rank test (paired analysis) was used for analysis. RESULTS: The overall performance between nail vs bolt matched specimens or between nail only vs nail with subtalar fusion showed no statistical difference with regards to stiffness. The compressive force tests show the nail to be significantly stronger in all aspects of the analysis. When comparing nail vs bolt matched specimens, only the accumulated height drop at the end of cycling was significantly different (p=0.008). For the nail only VS nail with subtalar fusion, only the displacement before fatigue was significantly different (p=0.035). There was no difference among the matched pairs with regards to stiffness, displacement after fatigue or maximum force during load to failure. CONCLUSION: This study highlights important biomechanical and compressive data comparing midfoot fusions using nails vs bolts. The compressive force test of nails was superior to bolts. This could provide valuable insight when considering implants for arthrodesis. The overall comparison between matched pairs of nails vs bolts did not provide significant differences among those groups. Similarly, adding a subtalar fusion did not provide significant mechanical improvement based on this model. The clinical utility of these findings is limited by the difficulty in recreating a Charcot like scenario using cadavers.