Comparison of initial compression force across the subtalar joint by two different screw fixation techniques.

The purpose of this study was to compare the magnitude of compressive force across the posterior facet of the subtalar joint between 2 different screw orientations in a simulated subtalar fusion model fixated with cannulated 7.3-mm screws. Eight paired fresh, frozen cadaver feet were used. Before testing, the bone mineral density of the calcanei and tali was measured with dual energy x-ray absorptiometry to ensure comparable bone mineral density. The paired cadaver calcanei and tali were noted to have less than .05 g/cm(2) difference in bone mineral density. The paired specimens were each randomly assigned to 1 of 2 methods of fixation across the subtalar joint: a plantar approach, where the screw was oriented vertically from the plantar aspect of the calcaneus, distal to the tuberosity; and a posterior approach, where the screw was oriented obliquely from the posterior calcaneal tuberosity. A load-sensing washer was placed under the cancellous screw head before insertion of the screw to measure the magnitude of the compression force across the simulated fusion site. The mean compression force for the posterior-to-anterior approach was 581 N, with a standard deviation of 242 N. The mean force for the plantar-to-superior approach was 604 N, with a standard deviation of 276 N. The compression force obtained by the 2 approaches was not significantly different (P = .74). The plantar-to-superior fixation technique represents an alternative subtalar arthrodesis technique to the posterior-to-anterior approach, potentially allowing one to revise a previously failed arthrodesis when the standard fixation techniques cannot be applied.