Hello, I'm David Safransky. Today I'll be discussing the effect of insertion trajectory on the compressive performance of intramedullary devices for subtalar arthrodesis. The disclosures of the authors can be found in the final program. Subtalar arthrodesis has been widely implemented to relieve hindfoot issues after failure of conservative treatments. Procedure unit rates as high as 88% have been reported, but risk factors such as smoking, diabetes, obesity, and weight-bearing noncompliance can drastically decrease fusion outcomes. Many arthrodesis strategies utilize static compression fixation devices like screws, plates, staples, and IM nails. Unfortunately, these devices fail to maintain adequate joint compression in situations with postoperative bone resorption or joint settling. As postoperative compression loss increases nonunion rates, devices capable of applying sustained dynamic compression should theoretically approve union outcomes. While sustained dynamic compression device effectiveness has been documented clinically in TTC arthrodesis procedures, optimization of sustained dynamic compression device trajectory in subtalar arthrodesis procedures has not been studied nor compared to static compression devices. As such, this investigation's goal was to assess effects of device type and trajectory on subtalar joint compression. 32 foam subtalar joints were used in this investigation to minimize variability and compare effects of device type and trajectory on initial subtalar joint compression in a controlled environment. The sustained dynamic compression device tested was the DynaNail Mini from DJO Foot & Ankle, which utilizes an internal nitinol compression element, allowing the device to sustain compressive forces. The static compression device construct was two 6.5mm cannulated head edge screws from Fuse Synthes. All devices were 80mm long and tested in two configurations. The sustained dynamic compression device was tested across posterior or anterior subtalar facets, and static devices were tested diverging or parallel trajectories. Following manufacturer's instructions for both groups, guide wires were inserted in locations as marked by orthopedic surgeon co-author to replicate surgical approaches and provide for optimal joint fixation in each group. After placing initial guide wires, trajectories were drilled to respective pilot hole sizes per manufacturer's recommended surgical technique. With guide wires still present, the construct was secured into a custom compression measuring fixture described previously by Matsumoto and Chukpai Wong. The calcaneus and talus did not touch, ensuring all force was directed through the load cell by the devices. For static screw test, screws were inserted over their guide wires and the guide wires were removed. A minute after both screws were inserted, measured compressive load was recorded. Sustained dynamic compression tests were performed with devices at 37C to replicate body temperature as device mechanics are temperature dependent. Sustained dynamic compression devices were inserted based upon manufacturer's procedure guide. One minute after 9L element release, compressive loads were recorded. Data was checked for normality, and ANOVA was performed with TUFI's post-hoc test. Sustained dynamic compression devices had an average compression of 398N in the posterior facet and 417N in the anterior facet. The dual screws compression was 267N in the parallel configuration and 269N in the diverging configuration. Sustained dynamic compression device generated compression forces were significantly higher in both trajectories than the static screw devices in both trajectories. ANOVA did not reveal a significant effect of device insertion trajectory on joint compressive forces, with a p-value of 0.49 for sustained dynamic compression and 0.99 for the static screw. Across both insertion trajectories, the sustained dynamic compression device provided 53% greater compressive forces compared to static screws. Our data agrees with prior screw findings that trajectory does not impact overall joint compression. Our study limitations include using sawbone samples instead of cadaveric tissue. Additionally, measurements reflect global joint compression rather than local forces. However, a similar cadaveric experimental setup produced similar trends validating these experimental results. In conclusion, the data illustrates sustained dynamic compression device's ability to maintain significantly improved joint compressive forces as compared to static cannulated screws, regardless of insertion trajectory. The sustained dynamic compression device enables surgeons to place the device regardless of trajectory, thus using the most appropriate surgical outcome approach for each patient while ensuring sufficient joint compression outcomes.

insertion trajectory

compressive performance

intramedullary devices

subtalar arthrodesis

sustained dynamic compression devices