Abstract
Introduction/Purpose:
Despite the prevalence of subtalar arthrodesis, certain risk factors (smoking, diabetes) diminish fusion outcomes and are associated with decreased bone healing, leading to increased duration of high stresses at the bone/device interface, ultimately leading to loosening/loss of compression. Existing fixation devices using static engineering materials (i.e., titanium) falter when postoperative bone resorption/joint-settling occurs, justifying improved fixation devices that drive continuous compression in the setting of bone resorption. Devices that leverage superelastic properties of NiTiNOL can provide sustained dynamic compression in the setting of bone resorption, ideally positioning them for these patient populations. Thus, the goal of this study was to characterize the biomechanical response of static and dynamic (NiTiNOL) devices across multiple trajectories in subtalar arthrodesis in the setting of simulated bone resorption.
Methods:
Compressive forces between synthetic talus and synthetic calcaneus (Sawbones) were measured for two commercially available orthopedic fixation devices (Dynamic = 7.0mm dia. 90mm length, Static = 7.0mm dia; 90mm length, N =6/group) in a custom subtalar fixture. The devices were inserted according to manufacturer instructions (FigA) while maintaining a gap between bone segments using a removable spacer. Two devices were inserted within each group (Dynamic, Static, Static+Dynamic) in either a parallel or diverging trajectory (FigA). Following insertion of devices, the spacer was removed enabling measurement of initial compression (Transducer Techniques). Subsequently, the bones were allowed to move into apposition while continued measurements of compression were collected, enabling determination of resorption capacity (distance) prior to loss of compressive force. Data were analyzed using a two-way ANOVA with Tukey post hoc test (α=.05).
Results:
The Dynamic group exhibited significantly increased initial load as compared to the Static and Static + Dynamic groups in both the parallel and diverging trajectories (FigB, p=.031). The initial load provided by the first device in the Dynamic group was significantly increased as compared to the Static group (FigB, p=.030). The Dynamic group exhibited a significant increase in initial load upon insertion of the second device (FigB, p
Conclusion:
Generating and maintaining sufficient compression is requisite for successful subtalar arthrodesis outcomes, especially in the presence of bone resorption. These data illustrate the significant increase in both initial load and resorption capacity of the sustained dynamic compression device (Dynamic) as compared to the Static and Static + Dynamic groups. The addition of a second Dynamic device markedly increased the compression, whereas the use of static devices prevented generation of increased compressive forces. Device trajectory did not significantly impact the arthrodesis construct biomechanical performance. One dynamic screw has better biomechanical performance than two static screws.
Figure A) Illustration of guide wire placements prior to device insertion. B) Initial load generated between boney segments. C) Resorption distance prior to loss of compression.