Calculation of correction angle required for spinal osteotomy in ankylosing spondylitis patients with thoracolumbar kyphosis: comparison of FBI method and hilus pulmonis (HP) method.

Abstract Study Design：Retrospective case–control radiographic study Purpose The purpose of this study is to compare hilus pulmonis (HP) method and full balance integrated (FBI) method in calculating the amount of correction angle needed to achieve a satisfactory sagittal balance for ankylosing spondylitis (AS) patients with thoracolumbar kyphosis, then, to explore if HP method can be safely used instead when C7 vertebra is invisible. Methods 38 AS patients (34 men, 4 women; mean age, 38.4 yr) who underwent one-level PSO for thoracolumbar kyphosis deformity were retrospectively collected. The planned osteotomy angles were calculated on preoperative full-length spinal radiographs using both methods (HP and FBI). Pre- and post-operative sagittal parameters were also assessed, including Cobb T1- S1, global tilt, sagittal vertical axis (SVA), pelvic tilt, lumbar lordosis, femoral obliquity angle (FOA). Results All radiographical parameters had significantly improved from preoperative to postoperative. The T1–S1 sagittal Cobb angles significantly improved from 43.5 ° (range, -12.1° ~84.5°) preoperatively to 9.2° (range, −15.7°~33.7°) postoperatively (P < 0.001). All radiographical parameters had significantly improved from preoperative to postoperative, including global tilt (52.3°~21.8°, P<0.001), SVA (139.4 ~ 58.1mm, P < 0.001), PT (33.6° ~16.8°, P<0.001), lumbar lordosis (1.9°~-36.3°, P<0.001) and femoral tilt (22.0° ~10.1°, P<0.001). The mean osteotomy angle was 28.2° (range 20.4°~41.9°). The average predicted correction angles with FBI method and HP method were 43.70° and 51.80°, respectively. There was significant difference between these two methods in terms of predicted correction angle (P=0.001). Conclusions In AS patients with only thoracolumbar kyphosis, HP method had greater estimated correction angle than FBI method. Since our experience confirmed that all patients achieved the best equilibrium when C7 plumb line closed to the sacral plateau according to FBI method, HP method could not be safely used instead to calculate the estimated correction angle.