Calculation of the correction angle required for spinal osteotomy in ankylosing spondylitis patients with thoracolumbar kyphosis: Validation of three predictive methods.

Study design: Retrospective cohort radiographic study PURPOSE: The purpose of this study was to verify the accuracy of three common preoperative calculation methods (the hilus pulmonis (HP) method, the full balance integrated (FBI) method and the spinofemoral angle (SFA) method) for determining the optimal osteotomy size in patients with ankylosing spondylitis with thoracolumbar or lumbar kyphosis and to evaluate their clinical significance.

Methods: Data from 62 AS patients with thoracolumbar or lumbar kyphosis who underwent one-level pedicle subtraction osteotomy (PSO) for thoracolumbar kyphosis deformity were retrospectively reviewed. The planned osteotomy angles were calculated from preoperative full-length spinal radiographs using three methods: hilus pulmonis (HP) method, full balance integrated (FBI) method, and spinofemoral angle (SFA) method. Sagittal alignment parameters were measured preoperatively and at the final follow-up using Surgimap software. Functional outcomes, including the Oswestry Disability Index (ODI), Japanese Orthopaedic Association (JOA) score, and visual analog scale (VAS) score for back pain, were recorded preoperatively and at the final follow-up. Postoperative complications were also documented. Paired t-tests were used for statistical analysis.

Results: All sagittal alignment parameters improved significantly postoperatively (all p < 0.001). Sagittal vertical axis (SVA) improved markedly from 139.4 mm to 48.1 mm, and T1 pelvic angle (T1PA) decreased from 41.34° to 12.56° (p = 0.009). In addition, spinosacral angle (SSA) increased from 86.2° to 115.4°, spinopelvic angle (SPA) improved from 127.7° to 158.2°, and pelvic tilt (PT) decreased significantly from 33.6° to 16.8°, reflecting substantial improvements in global and pelvic sagittal alignment.Among the three predictive methods, the HP method significantly overestimated the required correction angle compared with the actual postoperative value (51.8° vs. 42.2°, p < 0.05), whereas SFA (45.2°) and FBI (43.7°) predictions showed no significant difference from the actual correction achieved. Clinically, mean ODI improved from 52.6 to 15.4,JOA from 12.5 to 20.9, and VAS for back pain decreased from 6.4 to markedly lower levels at final follow-up (all p < 0.001). Patient satisfaction was excellent in 63%, moderate in 26%, and poor in 11% of cases. Postoperative complications included 3 transient thoracic nerve root injuries without symptoms, 8 dural tears (6 primarily repaired, 2 with dural patches), 1 deep wound infection requiring debridement, 2 transient lower limb paralyses (both fully recovered), 2 screw loosenings without progression, and 2 proximal junctional kyphoses managed conservatively.

Conclusions: In AS patients with isolated thoracolumbar or lumbar kyphosis, both the SFA and FBI methods showed good agreement with the actual postoperative correction angle and can serve as reliable tools for preoperative planning. The HP method consistently overestimated the required correction, which may increase the risk of overcorrection; therefore, its application should be approached with caution and further refinement is warranted.