Prognostic Factors of Thoracic Hypokyphosis after Posterior Spinal Fusion in Adolescent Idiopathic Scoliosis using by All Pedicle Screws Instrumentation

Introduction The use of pedicle screws has spread as a method of correction of scoliosis, by his great power correction in the coronal plane and low failure rate. The problem with pedicle screws in scoliosis correction is possible to produce hypokyphosis (thoracic kyphosis < 20 °) leading to the disappearance of the lumbar lordosis, increased junctional kyphosis in cervicothoracic union and impairment of respiratory function. Our goal is to determine the degree of postoperative kyphosis and the factors that influence the correction of such deformity in the sagittal plane. Material and Methods We present the results of a retrospective analysis of 202 patients with adolescent idiopathic scoliosis after surgery. 64 patients were selected with an average age of 13 years with two years of follow-up. Inclusion criteria were: patients undergoing adolescent idiopathic scoliosis, by the same surgical team using instrumented posterior fusion with all pedicle screws using 3 different types of instrumentation (different hardness and diameter). They were evaluated pre and postoperative radiographic parameters. We performed an univariate analysis to quantify the preoperative and postoperative kyphosis, and performed a multivariate binary logistic regression analysis of the factors influencing the postoperative kyphosis with a statistically significant association ( p < 0.05 or OR when this interval does not contain 1). The parameters considered are: demographic, kyphosis (T4-T12) and lordosis (T12-S1) pre and postoperative, Cobb angle pre and postoperative main curve, type of instrumentation and fusion levels Hypokyphosis postoperative consider if it is < 20 ° or if no improvement  > 5°. Results The mean preoperative kyphosis is 27 ° (6 ° -45 °) and postoperative is 21 ° (10 ° -40 °), decreasing an average of 6°. In the preoperative study 15 patients (23.4%) had hypokyphosis, 6 patients (9.4%) hyperkyphosis and 43 patients (67.2%) normal kyphosis. 47% of patients with preoperative hypokyphosis remained postoperative hypokyphosis, and 46.5% of patients with preoperative normal kyphosis ended in postoperative hypokyphosis. Variables showing a statistically significant effect for the occurrence of postoperative hypokyphosis: young patients, the worse correction in coronal plane, and type of instrumentation used (smaller diameter and low hardness) ( p < 0.05 OR 0.98). No statistical occurrence ratio of postoperative hypokyphosis to the number of fusion levels or cobb angle of the main curve (p > 0.05) Conclusion The pedicle screws hold prior to surgery thoracic kyphosis in 87%. The use of rigid and larger diameter bar appears important in preventing hypokyphosis factor associated with scoliosis. Minimize the risk of the presence of hypokyphosis and therefore improve postoperative sagittal profile is related to the type of instrumentation, the correction in the coronal plane and the age at which the patient is performed to the correction of the deformity. The limitations of our study are the selection and limited number of cases, intraobserver variability in measurements and the lack of clinical and functional impact of the results.