How much apical derotation occurs as a result of coronal curve correction in AIS assessment using the fulcrum bending radiograph

Abstract

INTRODUCTION: Various studies have reported vertebral derotational effects with different implant constructs and surgical techniques. However none of them has considered the spontaneous coupling effect on vertebral derotation produced by correction of coronal deformity. METHOD: Patients aged between 10 to 20 years who suffered from Lenke type 1 idiopathic scoliosis with Cobb angles more than 45° and underwent posterior spinal fusion were prospectively included. Scoliosis Cobb angles and apical vertebral rotations in standing, supine and fulcrum bending positions before surgery, and in supine position after surgery, were measured on x-rays and CT scans. They were compared using paired t-test. RESULTS: Eighteen patients were included. The mean age at surgery was 15.5yr. The Cobb angles and apical vertebral rotations in standing, supine and fulcrum bending positions before surgery and in supine position after surgery were 54°,26°; 40°,19° ; 17°,10°; and 14°,8° respectively. The differences in Cobb angles and apical vertebral rotation between standing, supine and fulcrum bending positions were statistically significant. The differences in Cobb angle and apical vertebral rotation between fulcrum bending position and postoperation were statistically insignificant. DISCUSSION AND CONCLUSION: Our study demonstrated that spontaneous apical derotation occurred concomittently with correction of the coronal deformity. The amount of vertebral derotation achievable under fulcrum bending may predict the post-operative vertebral rotation correction. Apical rotation was corrected by 27% on supine and 62% on fulcrum bending positions. Such spontaneous derotation effect should be considered when assessing the effects with different implants and surgical strategies.