Role of Pelvic Incidence, Thoracic Kyphosis, and Patient Factors on Sagittal Plane Correction Following Pedicle Subtraction Osteotomy

Study Design. An analysis of clinical and radiographic data of 40 consecutive patients with fixed sagittal imbalance. Objective. To determine the effect of mid lumbar pedicle subtraction osteotomy (PSO), pelvic incidence (PI), thoracic kyphosis (TK), and patient characteristics on correction obtained in patients with fixed sagittal imbalance. Summary of Background Data. PSO is commonly performed for spinal reconstruction in patients with fixed sagittal imbalance. Prior studies have not investigated the role that osteotomy location, PI, TK, and presenting patient characteristics may play in the correction obtained after PSO. Methods. Forty consecutive patients were identified who underwent PSO with minimum 2-year clinical and radiographic follow-up at a single institution. Data were analyzed before surgery and at 2 and 24 months after surgery to identify the magnitude and durability of correction and associated variables. Results. C7 plumb line improved from mean 15 cm anterior to the sacrum before surgery to 3.0 cm after surgery and 4.5 cm at 24 months (P < 0.0001); mean PSO wedge size was 32.4 degrees. Patients treated for idiopathic deformity (typically following prior Harrington rod fusions) had better maintenance of correction than patients with degenerative sagittal imbalance, although not statistically significant (P = 0.06). Fusion to the upper thoracic spine preserved correction better than fusion to the thoracolumbar junction. Sagittal plane correction, SRS outcome scores and Oswestry scores were equivalent comparing PSO's performed at L2 and L3. Using our patient data, we tested models of PI and TK to predict the lumbar lordosis needed to achieve ideal sagittal balance. The formula PI + LL + TK <= 45 degrees showed 91% sensitivity for predicting ideal sagittal balance at 24 months (P = 0.001). Conclusion. PI and TK can predict the lumbar lordosis necessary to correct sagittal imbalance in patients undergoing PSO with high sensitivity. Sagittal correction and clinical outcome scores were equivalent comparing PSO's performed at L2 and L3. Patients with degenerative sagittal imbalance and those with shorter fusions are more likely to lose correction with time.