Does Calcaneal Fracture Gap Alter Stress Distribution Across the Subtalar Joint?

Category: Hindfoot Introduction/Purpose: Subtalar arthritis is a common consequence following calcaneal fracture, and its development is related to the severity of the fracture at the posterior facet. Previous calcaneus fracture models have demonstrated altered contact characteristics when a step-off is created in the posterior facet articular surface. A biomechanical model involving a fracture gap without step-off through the posterior facet has not been developed. In this study, a primary calcaneal fracture line is created to simulate fracture gap displacement without step-off, and to evaluate the resulting contact pressure changes across the posterior facet. Methods: The contact characteristics (peak pressure, area of contact, and centroid of pressure) of the posterior facet of the subtalar joint in six cadaveric below the knee specimens were determined. Contact pressures were measured using Tekscan pressure sensors in five foot positions (neutral, dorsiflexion, plantarflexion, inversion, and eversion) with an axial load of 50% of the cadaveric total body weight. Contact pressures were first determined in the intact foot in all positions in a modified MTS frame. A calcaneal osteotomy was made using a 1 mm osteotome from the posteromedial tuberosity to the anterolateral facet, exiting the facet to simulate a Sanders IIA fracture. The contact characteristics were measured before and after the osteotomy. The simulated fracture was fixed with two 6.4 mm steel dowels. The contact characteristics were then determined with the posterior facet anatomically reduced followed by an incremental increase in fracture gap displacement of 1, 2, and 4 mm. Results: In the intact foot, there was no significant difference in the peak pressures among the five foot positions (p=0.33). The area of contact was significantly less with the foot in inversion compared with neutral (p=0.002), dorsiflexion (p=0.010) or eversion (p=0.033). Peak pressure on the medial fragment (Table 1A) was significantly less with a 4 mm gap compared to a 1 mm gap (p=0.026), a 2 mm gap (p=0.031) or reduced (p=0.030). On the lateral fragment (Table 1B), the peak pressure was significantly increased with a 4 mm gap compared to a 1 mm gap (p=0.026) or a 2 mm gap (p=0.011). Measurements of the contact area and location of the pressure centroids did not show a significant difference on post-hoc testing. Conclusion: The forces across the posterior facet of the subtalar joint were significantly altered after a simulated calcaneal fracture involving gap displacement without step-off. A posterior facet gap of 2 mm did not significantly alter the contact characteristics of the subtalar joint in this study. Significant differences in peak pressure were demonstrated between 4 mm and 2 mm of gap displacement. The peak pressures with 4 mm of gap displacement were significantly increased on the lateral fragment of the posterior facet and decreased on the medial fragment. Changes in posterior facet contact characteristics have not been previously characterized for calcaneal fracture gap without step-off.