A Reliable Prediction Method for the Pedicle Subtraction Osteotomy Angle in Ankylosing Spondylitis-Related Thoracolumbar Kyphosis: A Comparative Analysis with Existing Approaches.

Background: Careful preoperative planning regarding the reliable angle of pedicle subtraction osteotomy (PSO) in ankylosing spondylitis (AS) is beneficial for sagittal realignment, preventing undercorrection or overcorrection, and mitigating potential mechanical complications. The purpose of this study was to propose a novel method, using AS-specific realignment criteria and Surgimap software, for calculating a reliable PSO angle in AS patients with thoracolumbar kyphosis, and to compare its predictive accuracy with 3 other methods reported in the previous literature. Methods: Sixty-one AS patients who underwent 1-level PSO were analyzed, fulfilling the ideal sagittal balance criteria of adult spinal deformity at the final follow-up: pelvic tilt (PT) < 25°, sagittal vertical axis (SVA) < 50 mm, and -10° < pelvic incidence minus lumbar lordosis (PI-LL) < 10°. In the new method, the formula PT = (0.47 × PI - 7.5) was used to target reliable postoperative PT. Besides, AS-specific realignment criteria for surgical planning were as follows: PT < 24°, Spinosacral angle (SSA) > 108°, T1 pelvic angle (TPA) < 22°, and spinopelvic angle (SPA) > 152°. All the 4 methods were applied to calculate the theoretical angle of osteotomy (tAO) for each patient, including the trigonometric method, full balance integrated (FBI), SS-based ASKyphoplan, and Surgimap Spine plan. The difference between pre- and postoperative osteotomized vertebral angle in radiographs was defined as the actual angle of osteotomy (aAO). Precision of all predictive methods was evaluated on the basis of concordance correlation coefficient (CCC), accuracy rate, root mean squared prediction error (RMSE), and difference between tAO and aAO (bias). Results: Significant improvement in global kyphosis, LL, and SVA were noted postoperatively for all 61 patients without obvious correction loss at the final follow-up. The most accurate and precise method for the osteotomy angle calculation was the AS Surgimap plan (prediction accuracy, 72.13%; bias, -2.36%; RMSE, 8.34; CCC, 0.837), followed by FBI, the trigonometric method, and ASKyphoplan. The prediction accuracy, undercorrection rate, and overcorrection rate for the low and high PI groups were 80.00%, 5.71%, and 14.29% and 61.54%, 30.77%, and 7.69%, respectively. Conclusions: Surgimap Spine plan was relatively the most reliable and accurate approach to predict the PSO angle and achieve proper postoperative sagittal alignment for AS patients in comparison to other methods. Predetermined PT using the equation PT = (0.47 × PI - 7.5) and well-defined criteria (SSA > 108°, SPA > 152°, and TPA < 22°) could improve the predictive values.