Modified Ondra's Formula to Predict Post-Op SVA after Lumbar PSO: A Preliminary Report Verified by Simulated Post-op Images Via Surgimap Software.

Abstract

This study aimed to introduce a new mathematical formula to predict sagittal vertical axis (SVA) changes after lumbar pedicle subtraction osteotomy (PSO). This cross-sectional study included 43 patients. Lateral full spinal radiographs were exported from Picture Archiving and Communicating Systemand imported to Surgimap (NemarisInc,New York, NY, version 2.3.1.1) software for PSO simulation. Regression analysis was done on the first 16 cases to find a constant variable (K) in the proposed formula. PSOs of 15°, 20°,25°, 30°, and 35°were simulated at L3,L4, and L5 for each patient, and the simulated postoperative SVAs were compared to predict postoperative SVA. Paired T-Test, Spearman test, and analysis of variance were applied to analyze the results. The Statistical Package for the Social Sciences software (Version 24.0) was used for the statistical analysis. Postoperative SVA was different in terms of PSO level and degree, with a mean of 74.87±37.88, 66.12±38.33, and 56.95±38.53 at 15°; 61.76±36.68, 50.13±37.32, and 43.03±36.67 at 20°; 48.61±35.60, 39.13±35.35, and 43.18±31.56 at 25°; and 37.73±33.78, 42.38±28.76, and 44.81±16.85 at 30° for L3, L4, and L5, respectively (P 0.05). The mean difference between the predicted SVA using the formula and the SVA simulated using Surgimap software was 2.37, 2.09, and 0.47 mm atL3, L4, andL5 levels, respectively. The mathematical formula was highly predictive for postoperative simulated SVA values in all three vertebral levels (L3-L5) (P 0.05). Additionally, a strong relationship was found between the vertebral level and the amount of SVA correction (Spearman correlation: 0.7-0.9). The mathematical formula is an accurate predictor of postoperative SVA and is helpful in PSO surgical planning. However, future studies are recommended to verify its accuracy in a clinical setting.