Validation of a Predictive Equation for Postoperative Pelvic Tilt in Spinal Deformity Surgery Comparing Short and Long Fusion Cases

Introduction: Pelvic Tilt (PT) is indicative of optimal sagittal alignment in spinal deformity surgery. While the ΔPT = ΔC2 Pelvic Angle (C2PA) - ΔC2 Tilt (C2T) equation has been validated in long fusion cases, its utility in short fusions (from T10 or below to the pelvis) is unknown. This study evaluates the predictive accuracy of this equation in both short and long fusion surgeries to provide a reliable tool for preoperative planning and postoperative alignment across a broad patient population.

Methods: We retrospectively analyzed patients who underwent spinal deformity surgery. Patients were divided into short (n = 50) and long fusions (n = 67). Predicted changes in PT were calculated using the ΔC2PA - ΔC2T equation. These predicted values were compared with postoperative PT. For each group, we calculated correlation coefficients (R), coefficients of determination (R²) and mean absolute error (MAE). The degree threshold capturing 95% and 80% of patients was determined for each group.

Results: For short fusion cases, the equation demonstrated strong predictive accuracy with an R of 0.82 and R² of 0.67. The MAE was 3.38 degrees. Predicted PT was within 8° of actual PT for 95% of short fusion patients and within 4.2° for 80% of patients. For long fusion cases, the equation yielded an R of 0.90 and R² of 0.81. The MAE was 3.22 degrees. Predicted PT was within 7.7° of actual PT for 95% of long fusion patients and within 4° for 80% of patients.

Conclusion : This study validates the accuracy of the ΔC2PA - ΔC2T equation in predicting PT in both short and long fusion surgeries. While the equation demonstrated greater precision in long fusion cases, it remains a reliable tool for preoperative planning and postoperative alignment across a broad range of patients. Its high accuracy across both fusion types suggests its applicability in various contexts.