Vertebral derotation in adolescent idiopathic scoliosis causes hypokyphosis of the thoracic spine

BACKGROUND The purpose of this study was to test the hypothesis that direct vertebral derotation by pedicle screws (PS) causes hypokyphosis of the thoracic spine in adolescent idiopathic scoliosis (AIS) patients, using computer simulation. METHODS Twenty AIS patients with Lenke type 1 or 2 who underwent posterior correction surgeries using PS were included in this study. Simulated corrections of each patient's scoliosis, as determined by the preoperative CT scan data, were performed on segmented 3D models of the whole spine. Two types of simulated extreme correction were performed: 1) complete coronal correction only (C method) and 2) complete coronal correction with complete derotation of vertebral bodies (C + D method). The kyphosis angle (T5-T12) and vertebral rotation angle at the apex were measured before and after the simulated corrections. RESULTS The mean kyphosis angle after the C + D method was significantly smaller than that after the C method (2.7 ± 10.0° vs. 15.0 ± 7.1°, p < 0.01). The mean preoperative apical rotation angle of 15.2 ± 5.5° was completely corrected after the C + D method (0°) and was unchanged after the C method (17.6 ± 4.2°). CONCLUSIONS In the 3D simulation study, kyphosis was reduced after complete correction of the coronal and rotational deformity, but it was maintained after the coronal-only correction. These results proved the hypothesis that the vertebral derotation obtained by PS causes hypokyphosis of the thoracic spine.