Biomechanical Analysis of Vertebral Derotation Techniques for the Surgical Correction of Thoracic Scoliosis

Study Design. Biomechanical analysis of vertebral derotation techniques for the surgical correction of thoracic scoliosis. Objective. To model and analyze vertebral derotation maneuvers biomechanically to maximize the tridimensional correction of scoliosis and minimize the implant-vertebra forces. Summary of Background Data. Vertebral derotation techniques were recently developed to improve the correction of scoliotic deformities in the transverse plane. Those techniques consist in applying a combination of moments and forces using a vertebral derotation device, cohesively linked to the thoracic apical pedicle screws, to derotate the spine and the rib cage. However, many variations of the technique exist and the correction mechanisms are not fully understood to achieve an optimal correction of scoliosis. Methods. A biomechanical model was developed to simulate the instrumentation surgery numerically of 4 Lenke type 1 patients with scoliosis, instrumented using a vertebral derotation device and vertebral derotation maneuvers as major correction technique. Then, for each case, 32 additional instrumentation surgical procedures were simulated to better understand the biomechanics of the vertebral derotation technique, varying the implant type and density, the number of derotation levels, the vertebral derotation angle and the posteriorly oriented force applied during the maneuver. Results. On average, among 32 additional simulations, there was an important variability of the resulting apical vertebral rotation (15 degrees) and the mean resultant implant-vertebra force (205 N) but little variability for the main thoracic Cobb angle (6 degrees) and the thoracic kyphosis (4 degrees). The implant type, the implant density and the vertebral derotation angle were the parameters that most influenced the correction of scoliosis. The correction in the coronal and transverse planes was improved using monoaxial pedicle screw density of 2 and a bilateral vertebral derotation maneuver on 3 levels at the apex of the thoracic curve, with an extra 15 degrees applied on the vertebral derotation device. When reducing the implant density by 50%, it was possible to reduce the mean implant-vertebra forces while keeping a good correction. Conclusion. Biomechanically, it is possible to significantly improve the correction of thoracic scoliotic deformities, particularly in the transverse plane, when using vertebral derotation maneuvers.