Biomechanical comparison of sustained dynamic compression and static screw fixation in subtalar arthrodesis: A benchtop synthetic bone study.

BACKGROUND

Subtalar arthrodesis depends on interfragmentary compression and rotational stability to promote fusion. Conventional screws can lose preload as interface resorption and construct settling occur. Sustained dynamic compression (SDC) devices are designed to maintain preload, but comparative data are limited.

PURPOSE: To compare construct-level compression, tolerance to simulated settling/resorption, and inversion rotational stiffness between a superelastic NiTiNOL sustained dynamic compression (SDC) device and a conventional static compression screw across three commonly used subtalar arthrodesis configurations.

STUDY

DESIGN

Controlled laboratory biomechanical study using a synthetic-bone (Sawbones) model.

METHODS

In a synthetic-bone model, constructs were instrumented with either an SDC device or a static screw in 1 of 3 configurations: single posterior, two parallel, or two diverging (n = 6/group/test phase). Initial compression was measured with an in-line load cell. The platens were then incrementally approximated (moved closer) to simulate settling/resorption until load reached 0 N. Separate constructs underwent calcaneal inversion to 4 Nm; rotational stiffness was calculated from 3 to 4 Nm.

RESULTS

SDC generated greater initial compression than static fixation across all configurations (p < .001). Single-device SDC (461.1 N) exceeded two-screw static constructs. Resorption capacity was 3.11 mm with single-device SDC and 3.96-4.08 mm with two-device SDC versus 0.085 mm with a single static screw and 0.86-0.92 mm with two-screw static constructs (all p < .001). Two-device constructs were rotationally stiffer than single-device constructs (p < .001), and SDC constructs were stiffer than static constructs (p = .001); peak stiffness occurred in the two-device diverging SDC configuration (1.96 Nm/deg).

CONCLUSIONS

In this synthetic-bone model, SDC fixation increased compression and improved tolerance to simulated resorption relative to static screw fixation across all 3 tested configurations. Two-device constructs provided greater inversion rotational stiffness than single-device constructs, with SDC showing a stiffness advantage. Cadaveric and clinical validation are needed before clinical extrapolation.