Outcomes of Optimal Distraction Forces and Frequencies in Growth Rod Surgery for Different Types of Scoliotic Curves: An In Silico and In vitro Study

Abstract

ObjectiveAnalyze the effects of the distraction forces and frequencies on multiple representative scoliotic curves and to establish a relationship between high distraction forces and screw loosening. Study DesignMultiple representative finite-element models of a juvenile scoliotic spine were used to study the effects of the magnitude and frequency of distraction on growth rods. MethodsSimulation of 6 months of growth under various distraction forces to analyze the effects of distraction forces on the biomechanics of the scoliotic spine and growth rod instrumentation; simulation of 24 months of growth under various intervals of distraction to analyze the effects of the distraction interval on the propensity for rod fracture; in vitro study to assess screw loosening after 6 months. ResultsFor all scoliotic spine model instrumented with growth rods, an optimal distraction force existed at which normal T1–S1 growth was sustained, along with minimum stresses on the rods, the lowest load at the screw-bone interface, and the least alteration in the sagittal contour. The results followed similar trends for each model, with the numerical values of optimal distraction forces in proximity for all representative scoliotic spine models. The in vitro study proved that the pullout strength of pedicle screws reduced significantly after 6 months of fatigue at higher distraction forces (in comparison with optimal distraction forces). This corroborated the finite-element findings for lower loads at the screw-bone interface with optimal distraction forces. ConclusionsThis study concludes that the optimal distraction forces exists for all types of scoliotic curves that have been instrumented with growth rods, which exhibits reduction of stresses on the rods with frequent distractions. This study also links the second most common complication, screw loosening, with high distraction forces. Therefore, optimizing the biomechanical environment of the dual growth rods could drastically reduce the biomechanical complications associated with growth rods.