Biomechanical comparison of force levels in spinal instrumentation using monoaxial vs. multi degree of freedom post-loading pedicle screws
Two types of pedicle screws were compared using a biomechanical computer model of 6 adolescent idiopathic scoliosis cases. Compared to monoaxial screws with a rod derotation technique, the multi degree of freedom post-loading screws with a direct incremental segmental translation correction technique allowed the spinal deformity to be reduced with lower force levels and better load distributions at the implant-vertebra interface and the inter-vertebral elements.
The objective was to compare the correction mechanisms and forces at the bone-screw interface and inter-vertebral during scoliosis correction using two types of pedicle screws in order to test the hypothesis that multi degree of freedom post-loading (MDOF) screws with a direct incremental segmental translation (DIST) correction technique significantly reduce the loads as compared to monoaxial (MA) screws with a rod derotation technique (RDT).
A biomechanical model was developed to simulate the instrumentation of 6 adolescent idiopathic scoliosis patients, first with the MDOF screws and a DIST correction technique using data derived from the pre- and post-operative radiographs, the surgery documentation as well as the intra-operative video. Then, the instrumentation with MA screws and RDT was simulated using the same cases, instrumentation levels, screw positions, rod material and shape.
There was an average difference of 7°, 5° and 4° between the two simulated systems for the computed main thoracic Cobb angle, kyphosis, and apical axial rotation respectively. Significant difference was observed on the computed forces at the implant-vertebra interface and the inter-vertebral elements. On average, the mean, standard deviation, and maximum values of the implant-vertebra forces for MDOF screws were respectively 56%, 59%, and 59% lower than for the MA screws, while the inter-vertebral forces for the MDOF screws were respectively 31%, 37%, and 36% lower than for the MA screws. These loads were more evenly distributed for the MDOF screws.
Compared to the MA screws with RDT, the MDOF screws with a DIST technique make it possible for the spinal deformity to be reduced with lower force levels and better load distributions at the implant-vertebra interface and the inter-vertebral elements.
MDOF screws and the DIST technique have the potential for the spinal deformity to be corrected in a gradual and controlled way, with lower force levels and better force distribution. It is more likely to avoid damage on the bone-screw interface or instrumentation failure.