The Explanation of Postoperative Change of Vertebral Rotation and Rib Hump Using 3 Dimensional Finite Element Scoliosis Model / 대한척추외과학회지

ABSTRACT

STUDY DESIGN: An analytical study using a mathematical 3-D finite element model for thoracic scoliosis. OBJECTIVE: To find the important kinematics and post-operative changes of the spine and rib cage, in the corrective surgery for scoliosis, using the rod derotation method. SUMMARY OF LITERATURE REVIEW A conventional corrective surgery for scoliosis was performed, based on empirical knowledge, and an increase in the secondary postoperative change in the rib hump, and a shoulder level imbalance, were reported. However, no analytical data exists for the kinematics and optimal correction method. MATERIALS AND METHODS: A mathematical finite element model of a normal spine, including the rib cage, sternum, both clavicles and pelvis, was developed. Using geometric mapping, with standing radiographs and CT images, a 3-D FEM of scoliosis was reconstructed, after translating and rotating the 3-D FEM of a normal spine, with the amounts analyzed from 12 built-in digitized coordinate axes for each vertebral image. With this model, three elements; distraction, translation and derotation, in operative kinematics, were investigated by analyzing the Cobb angle, apical vertebrae axial rotation (AVAR) and thoracic kyphosis. A simulation of a segmental pedicle screw fixation, with rod derotation for scoliosis, was performed. The changes in the Cobb angle, kyphotic angle, AVAR and rib hump were compared after 0 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees and 90 degrees rod derotations. RESULTS: In kinematics, the vertebral rod derotation of a major curve, without rod deformation, is less influential in the correction of scoliosis, simply causing an increase in the rib hump. During the simulation, the co-action of distraction and translation, during rod insertion, has a major impact on the decrease in the Cobb angle and in the maintenance of the kyphotic angle. However, after a 30 degrees rod derotation, a decrease in the kyphosis, and increases in the rib hump and AVAR were observed. CONCLUSIONS: The distraction and translation factors were more important in operative kinematics than the rod derotation. With excessive rod derotation, the Cobb angle progressively decreased, but increases in the secondary change in the rib hump and rotation of the apical vertebrae were found.