Natural history of coronal balance after spinal fusion in adolescent idiopathic scoliosis

Following fusion surgery in patients with adolescent idiopathic scoliosis (AIS), the coronal balance undergoes some degrees of fluctuation. The frequent causes of these changes are stress relaxation of the spine and the gradual maturation of fusion mass. Health related quality of life is dependent on the final balance. Eighty five patients with AIS who underwent posterior spinal fusion surgery with different methods and in variable levels, were studied on time scales preoperatively, immediately postoperatively, 6 weeks, 3, 6, 12, 18 and 24 months after surgery. In every visit, the standing posteroanterior and lateral views were obtained. Coronal imbalance was proposed as the differences of C7 middle axis and S1 endplate middle axis. The imbalance less than 2cm was assumed as normal. Furthermore, postoperative trunk shift and lower instrumented vertebra (LIV) shift were measured as the other balance evaluation methods. Eighty five AIS patients were included, 17patients (20 %) were male and 68 (80%) female. The mean age of patients was 16.03±4.3 years (range 9-37 years).The mean follow up period was 38 months (24-60 months). Left side coronal deviation was assigned negatively and right shift of balance was assigned with positive amounts. The clinical measurements of coronal imbalance based on plumb-line were investigated in the medical records. The range of pre and post-operative imbalance varied between -40mm and +35mm. The correlation between clinical balance with radiographic trunk shift, C7 plumb-line and LIV were -0.185 (p= 0.04), 0.22 (p= 0.05) and 0.0341(p=0.38), respectively. The significant improvement of coronal balance was on the 12th months of follow up (p= 0.02). Based on our study, in AIS patients undergoing spinal fusion, the coronal balance is reached gradually to a steady state in an average of 12 months. Because of this spontaneous improvement in coronal balance over 12 months from a posterior spinal fusion surgery in AIS patients, surgical intervention is not recommended. Beyond 12 months, revision of symptomatic imbalance is justified before further consolidation of fusion mass.