Effect of Insertion Trajectory on the Compressive Performance of Intramedullary Devices for Subtalar Arthrodesis

Abstract

Category: Hindfoot; Other Introduction/Purpose: Subtalar arthrodesis is a common procedure to treat hindfoot pathologies. Prior studies have evaluated the effect of screw trajectory on the initial compression of the subtalar joint. More recently, intramedullary (IM) devices using pseudoelastic NiTiNOL, developed for subtalar arthrodesis, can apply sustained dynamic compression (SDC). Headed screws' compression is achieved via the lag effect and is dependent upon its fixation in bone. The SDC device's compression is achieved by a manual mechanism; however, its fixation is dependent upon transverse screws. However, it is unknown how the trajectory of these devices in the subtalar joint alters compression. The aim of this study is to determine how the trajectory of both static compression screws and sustained dynamic compression devices affects joint compression. Methods: Based upon previously published methods, a synthetic talus and calcaneus were mechanically isolated in a custom fixture, and the compression measured via a load cell located in parallel to the subtalar construct. Static headed screws were inserted either in a parallel configuration across the posterior facet or in a diverging configuration across the posterior and anterior facets. A single IM SDC device was inserted across the posterior facet or the anterior facet and secured in the talus and calcaneus via transverse cortical screws. The resulting compression was measured in a thermal chamber at 37°C (n=8/group). Devices were inserted following the manufacturer's recommended instructions. Data was tested for normality using the Kolmogorov-Smirnov test. A one-way ANOVA was performed with a post-hoc Tukey test (α=0.05). All values are given as mean +- standard deviation. Results: The SDC device's compression was 398+-47N in the posterior facet and 417+-31N in the anterior facet (p=0.64). The screws' compression was 267+-31N in the parallel configuration and 269+-21N in the diverging configuration (p=0.99). The single SDC device, regardless of position, had significantly greater compression than the parallel or diverging headed screws configuration (p<0.01). Conclusion: The SDC device's compression is not affected by insertion trajectory, thus allowing for surgeon preference in anatomic placement. The SDC device is able to generate more compression than the headed screws, regardless of trajectory or screw configuration.