Evaluation of Sustained Dynamic Compression vs Static Intramedullary Fixation in a Cadaveric Viscoelasticity Model

Research Type: Level 5 - Case report, Expert opinion, Personal observation Introduction/Purpose: Intramedullary fixation devices for midfoot and hindfoot arthrodesis are subject to bone settling, which can reduce interfragmentary compression over time. This phenomenon is attributed, in part, to the viscoelastic properties of bone. Sustained dynamic compression (SDC) intramedullary devices integrate a NiTiNOL element designed to compensate for bone resorption and settling, potentially compensating for the postoperative compression loss that occurs due to viscoelastic relaxation. The purpose of this study was to validate this principle by comparing the immediate and short-term (1 hour post-implantation) interfragmentary compression achieved by an SDC intramedullary device with that of a conventional static device in cadaveric bone. Methods: In this biomechanical study, paired cadaveric bone specimens were prepared to simulate medial column arthrodesis and instrumented with either a standard static intramedullary beam device (Static group) or the SDC intramedullary device (SDC group). Following device implantation, compression was measured immediately and again at 1 hour to allow for viscoelastic settling. The final compression (N) was recorded at 1 hour, and the normalized compression loss (percentage decrease in compression from immediate post-implantation to one hour subsequently) was calculated. Statistical analysis was performed using a paired two-sample t-test with significance set at p < 0.05. Results: At 1 hour, the SDC group demonstrated significantly higher final compression compared to the Static group (39% greater, mean 132.3N vs. 95.4N, p = 0.03). Furthermore, the normalized compression loss in the SDC group was significantly lower than that observed in the Static group (approximately 42.8% vs. 73.7%, p < 0.001). Conclusion: In this cadaveric viscoelasticity model, the SDC device incorporating a NiTiNOL-driven dynamic compression element provided higher clinically-relevant sustained compression when compared with a conventional static beam device. This enhanced interfragmentary compression provides for a biomechanically superior construct by compensating for viscoelastic bone relaxation and postoperative compression loss. This phenomenon, in combination with immediate dynamization of these implants (avoiding a subsequent operation), are likely the underlying cause of the significantly improved clinical outcomes (36% faster time to fusion, 90% vs 65% fusion rate in highly comorbid patients) that have been reported for these SDC devices in other studies. A) Comparison of clinically-relevant interfragmentary compression (measured 60-minutes post-implantation) in cadaver bone specimens instrumented with a standard static beam intramedullary device versus a sustained dynamic compression (SDC) device. B) Normalized compression loss illustrating the marked compression loss that occurs postoperatively with standard hardware due to bone viscoelastic relaxation.