Validation of a Predictive Equation for Postoperative Pelvic Tilt in Spinal Deformity Surgery

Introduction: Postoperative Pelvic Tilt (PT)is indicative of optimal sagittal balance in spinal deformity surgery. An accurate prediction of PT is critical for improving surgical planning and optimizing outcomes. The ΔPT = ΔC2PA - ΔC2T equation has been proposed to predict postoperative PT, based on geometric relationships in the sagittal plane. However, the equation’s utility in a cohort of spinal deformity patients has not been validated in other datasets. This study aims to evaluate the accuracy of this equation in predicting PT.

Methods: A retrospective study of spinal deformity patients (n = 117) was conducted. Pre- and postoperative radiographic measurements were recorded at 6 months and predicted PT was computed using the equation: ΔPT = ΔC2PA - ΔC2T. The ΔC2T was calculated under the assumption that each patient’s C2 Tilt is corrected to 0° postoperatively. The actual postoperative PT at 6 months was compared to the predicted values. Mean Absolute Error (MAE) was calculated. The correlation between predicted and actual PT was evaluated using the correlation coefficient (R) and the coefficient of determination (R²). The degree threshold necessary to predict PT for 80% of patients was determined.

Results: The ΔC2PA - ΔC2T equation demonstrated strong predictive accuracy, with a MAE of 3.29°. The change in PT was strongly correlated to ΔC2PA - ΔC2T (r = 0.875) and accounted for 77% of the variance. The equation successfully predicted postoperative PT within 4 degrees for 80% of patients.

Conclusion : Predicting postoperative PT using ΔPT = ΔC2PA - ΔC2T is validated as a reliable predictive tool in ASD surgery, demonstrating strong correlation with actual PT and acceptable predictive error. These results support the use of this equation in preoperative planning to enhance outcomes in ASD patients.