Residual Thoracic Hypokyphosis After Posterior Spinal Fusion and Instrumentation in Adolescent Idiopathic Scoliosis Risk Factors and Clinical Ramifications

Study Design. A retrospective review of clinical and radiographic data from a single-center, prospectively collected scoliosis database. Objective. To assess risk factors for persistent thoracic hypokyphosis after posterior spinal fusion and instrumentation (PSFI) for adolescent idiopathic scoliosis (AIS) and to compare clinical outcomes between patients with residual thoracic hypokyphosis and those with normal thoracic kyphosis after PSFI for AIS. Summary of Background Data. AIS is characterized by thoracic hypokyphosis, which should be corrected at the time of surgical treatment. Risk factors for residual thoracic hypokyphosis and the clinical ramifications have not been studied. Methods. Radiographic and clinical assessments by using the Scoliosis Research Society-30 (SRS-30) and Spinal Appearance Questionnaire (SAQ) were done preoperatively and at 2 years. Patients were divided into 2 groups on the basis of a threshold of 20 degrees of thoracic kyphosis measured between T5 and T12 at 2-year follow-up. Results. Risk factors for being hypokyphotic at 2 years were male sex (21.69% vs. 12.21%, P = 0.084), preoperative kyphosis (11.4 degrees vs. 22.8 degrees, P < 0.0001), and smaller preoperative main thoracic coronal curves (58.4 degrees vs. 62.0 degrees, P = 0.004). A total of 71.5% of patients instrumented with 6.35-mm rods had normal thoracic kyphosis at 2 years compared with 47.0% instrumented with 5.5-mm rods (P = 0.0043). All-pedicle screw constructs remained hypokyphotic compared with hook-based constructs (P = 0.035). Logistic regression analysis demonstrated 2 parameters associated with persistent thoracic hypokyphosis at 2 years: preoperative hypokyphosis and larger rod diameter. Both groups had similar clinical results on the SRS-30 at 2-year follow-up (P > 0.05). There was a small but statistically significant correlation between sagittal Cobb angle and clinical deformity at 2 years based on the sagittal components of the SAQ. Conclusion. There are 2 risk factors that lead to thoracic hypokyphosis in AIS: preoperative hypokyphosis and use of a 5.5-mm-diameter rod. A larger-diameter rod should be considered when planning surgery for thoracic AIS, especially when there is preoperative hypokyphosis. Despite thoracic kyphosis measuring less than 20 degrees, these patients did not have decreased clinical outcomes as measured by the SRS-30 or SAQ.