Biomechanical Comparison of Force Levels in Spinal Instrumentation Using Monoaxial versus Multi Degree of Freedom Postloading Pedicle Screws

Abstract

biomechanical analysis and simulations of correction mechanisms and force levels during scoliosis instrumentation using two types of pedicle screws and primary correction maneuvers. to biomechanically analyze implant-vertebra and inter-vertebral forces during scoliosis correction, to address the hypothesis that multi degree of freedom (MDOF) postloading screws with a direct incremental segmental translation (DIST) correction technique significantly reduce the loads as compared with monoaxial (MA) tulip-top design screws with a rod derotation technique (RDT). MA screw is widely used for spinal instrumentation. The MDOF screw was introduced as a refinement of the correction philosophy based on multiaxial screws. The kinematics of the MDOF construct is fundamentally different and offers more degrees of freedom than that of the MA construct; however, a systematic comparison of their biomechanics has not been done so far. a biomechanical model was developed to simulate the instrumentation of six scoliotic patients, first with the MDOF screws and DIST. Then, the instrumentation with MA screws and RDT was simulated using the same cases. Thirty more simulations were done to study the force-level sensitivity to small implant placement variation. there was a small 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. On average, the mean, standard deviation (SD), and maximum values of the implant-vertebra forces for MDOF screws were 56%, 59%, and 59%, respectively, lower than those for the MA screws, while the intervertebral forces for the MDOF screws were 31%, 37%, and 36% lower, respectively. Under the same set of random small implant placement changes, the mean, SD, and maximum values of implant-vertebra force magnitude changes for MDOF screws were 93%, 92%, and 95%, respectively, lower than those for MA screws. with MDOF screws and DIST, it is possible for spinal deformity to be reduced similarly as with the MA screws and RDT, but with lower forces and better load distributions, and the force level is less sensitive to implant placement variation.