Biomechanical Comparison of Sustained Dynamic Compression Screws vs Static Compression Screws in Subtalar Arthrodesis: A Cadaveric Study.

Abstract

Background: Subtalar arthrodesis is a reliable procedure to address subtalar arthritis, though nonunion remains a challenge. Adequate compression is critical to optimizing fusion, but bone resorption and settling impair the ability of standard static compression (SC) screws to maintain compression. Sustained dynamic compression (SDC) screw constructs that use the pseudoelastic property of nitinol may enable more sustained compression than static compression screws.

Methods: Ten fresh talo-calcaneal cadaveric specimens were instrumented with either 2 SC screws or 2 SDC nitinol screws. Using a custom mechanical fixture, the compression force generated during and after screw insertion was measured in each group. The fixture was allowed to relax over a controlled distance and time to simulate bone resorption. The force at each relaxation point was measured, and the resorption capacity was calculated for each construct.

Results: Maximum compression at implantation was 430.2 ± 237.2 N for SC and 406.8 ± 117.7 N for SDC. After a 1-minute dwell period, compression was 299.6 ± 194.4 N for SC and 367.9 ± 108.0 N for SDC, corresponding to retention of 69.6% and 90.4% of initial compression, respectively. At 2 mm of simulated resorption, the SDC construct retained 111.1 ± 45.4 N compared with 8.3 ± 18.6 N for SC. The resorption capacity was 4.2 mm compared with 1.3 mm for the sustained dynamic compression and static constructs, respectively.

Conclusion: We did not detect a difference in initial compressive force between the SC and SDC screws. However, under identical bench conditions, the SDC construct retained more of its initial compression during the first minute and maintained substantially greater compression during simulated bone resorption than the SC construct. These findings support further study of whether sustained compression may help preserve fixation-generated preload during subtalar arthrodesis.

Clinical relevance: In a cadaveric subtalar arthrodesis model, novel sustained dynamic compression screws retained more interfragmentary compression than standard static compression screws during early relaxation and simulated bone resorption. This technology may aid surgeons in maintaining compression despite postoperative settling or resorption during subtalar arthrodesis, particularly in higher-risk patients.