Rotations of a helix as a model for correction of the scoliotic spine

Study Design. A prospective study using intraoperative stereophotogrammetry to analyze helical motion of the spine during the correction of scoliosis. Objective. To determine whether derotation systems rotate the scoliotic hel ix. Summary of Background Data. Scoliosis is a complex three-dimensional deform ity that is difficult to visualize on standard radiographs. The use of ster eophotogrammetry has allowed study of the deformity in three dimensions dur ing surgical correction. Methods. Thirty-five patients with right thoracic adolescent idiopathic sco liosis were studied using stereophotogrammetry technique during surgical co rrection. Changes in vertebral unique rotations and spinal plate of maximum deformity were measured during three sequential stages of the surgery. Results. The mean preoperative and postoperative Cobb angles were 50 degree s and 19 degrees, respectively. Most rotation occurred at the top and botto m vertebrae in the curve, averaging 10 degrees each but in opposite directi ons. The apical vertebra rotated the least in the structural curve, with an average rotation of 5 degrees. Much of the rotation occurred during the de rotation maneuver with additional rotation occurring during the final distr action. The plane of maximum deformity changed from a mean of 50 degrees be fore instrumentation to 19 degrees at the end of the procedure. Conclusions. Multiple rotations of the scoliotic curve occur, and it can be shown when maximum rotations occur during surgery. Posterior derotational systems unwind or rotate the scoliotic helix and reposition the resultant s ine wave toward the sagittal plane as described by the change in the plane of maximum deformity.