Biomechanical Advantages of Sustained Dynamic Compression Over Static Fixation in Subtalar Arthrodesis
Authors: Gross Christopher E. , King Shannon , Brown Ryan , Filicette Taylor , Johnson James , Safranski David 
Published: December 2024

Category: Hindfoot; Basic Sciences/Biologics Introduction/Purpose: Prolonged stresses at the bone/device interface can predispose patients to complications such as loosening or loss of compression in the setting of normal bone resorption during healing. In efforts to address these hardware failures, sustained dynamic compression (SDC) devices were developed, enabling the application of continuous compression in the setting of bone resorption or postoperative joint settling. However, the biomechanical performance of these SDC devices in subtalar arthrodesis constructs has not yet been explored. Thus, this study sought to compare the compression, capacity to adapt to resorption, and torsional stiffness of subtalar arthrodesis constructs utilizing either static or dynamic fixation in three clinically relevant screw trajectories (one device, two devices in parallel trajectories, two devices in diverging trajectories). Methods: Subtalar arthrodesis procedures were simulated on synthetic talus/calcaneus replicates (Sawbones) using two commercially available orthopedic fixation devices (Dynamic = 7.0mm dia. 90mm length, Static = 7.0mm dia; 90mm length, N=6/group) while bones were held in a custom fixture maintaining a gap (to enable simulated resorption). Constructs tested included: 1) single device posteriorly placed, 2) two devices with parallel trajectories, and 3) two devices with diverging trajectories (FigA). After inserting devices according to manufacturer specifications, initial compression and simulated resorption testing were performed (by moving bones into apposition while measuring interfragmentary compression), generating initial load and resorption capacity (resorption distance prior to loss of compression). A second set of simulated arthrodesis procedures were performed on samples that subsequently underwent inversion and eversion torsion testing, generating torsional stiffness values. Data were analyzed using a two-way ANOVA with Tukey post hoc test (α=.05; N=6 replicates/group). Results: Device trajectory (i.e., parallel vs. diverging) did not significantly impact the subtalar construct initial load(p≥0.90), resorption capacity (p≥0.78), or torsional stiffness (p≥0.50, FigB,C,D). The single dynamic device group exhibited significantly increased initial load (p < 0.001, FigB) and resorption capacity (p < 0.001, FigC) as compared to the single and double static device groups. Torsional stiffness was significantly increased in constructs that utilized two static devices as compared to those that used one static (p≤0.023, FigD). Across all constructs conformations (device quantity and trajectory), the dynamic device exhibited significantly increased torsional stiffness as compared to the static device (p=0.006, FigD). Conclusion: These findings demonstrate a notable enhancement in both initial load, resorption capacity, and torsional stiffness with the sustained dynamic compression device (Dynamic) compared to the Static group. Furthermore, the use of a single dynamic device provided significantly increased initial compression, significantly increased resorption capacity, and equivalent (not significantly different) torsional stiffness as compared to the N=2 static device constructs. Notably, device trajectory (diverging vs parallel) did not significantly alter the biomechanical properties of the arthrodesis construct. Thus, surgeons should consider the use of one or more dynamic devices in either trajectory when optimum biomechanical performance is desired. Dynamic Device Provides Increased Subtalar Arthrodesis Construct Compression and Resorption Capacity. A) Representative construct images depicting parallel and diverging device trajectories. B) Initial interfragmentary compression was significantly increased in all dynamic device constructs as compared to static device constructs. C) Resorption capacity was significantly greater in the constructs utilizing dynamic devices as compared to those that used static. D) Across all trajectory/quantity groups, the constructs utilizing dynamic devices displayed significantly increased torsional stiffness as compared to those that used static devices.