Is pelvic incidence a constant, as everyone knows? Changes of pelvic incidence in surgically corrected adult sagittal deformity

Previous investigations have recognized the critical role of pelvic parameters in the setting of a fixed sagittal deformity. Pelvic incidence (PI) is a constant, as everyone knows. However, PI might change reciprocally because of increased shear force on the sacroiliac joint, following surgical correction of fixed lumbar lordosis (LL). The disparity in PI after surgery according to the surgical method, and its impact on final follow-up, has not been reported. This study was undertaken to analyze the disparity of PI before and after surgery, and to evaluate its impact on final sagittal alignment in surgically corrected lordosis when there is immediate postoperative normal alignment following correction of adult sagittal deformity. A prospective study of 29 subjects with adult spinal deformity (average age: 67.9 years) was conducted. At final evaluation after a minimum 2-year follow-up, normal sagittal alignment was achieved following consecutive sagittal correction. Surgical changes were measured by serial, pelvic standing, lateral, and whole spine radiographs, spinopelvic parameters measured included PI, sacral slope (SS), pelvic tilt (PT), LL, thoracic kyphosis (TK), and sagittal alignment. The mean LL was 0.2A degrees before surgery; -59.3A degrees after surgery with pedicle subtraction osteotomy (PSO) (n = 20), anterior lumbar interbody fusion (ALIF) (n = 20, 33 segments), and posterior lumbar interbody fusion (PLIF) (n = 21, 36 segments); and -57.5A degrees at last follow-up. The sagittal vertical axis was +14.8 cm before surgery, -0.7 cm after surgery, and 2.2 cm at last follow-up. The mean PI was 49.4A degrees before surgery, and increased to 55.2A degrees after surgery, 57.5A degrees at 1-year follow-up, and 58.8A degrees at last follow-up (P = 0.02). The mean disparity in PI preoperatively and at last follow-up was 11.4A degrees without sacropelvic fixation (n = 18), and 5.9A degrees with sacropelvic fixation (n = 11) (P = 0.002). Analysis revealed the disparity of PI to be significantly greater in non-sacropelvic fixation, and correlated with the follow-up period (R = 0.442, P = 0.016), but not with age, bone mineral density (BMD), number of fused segments, correction methods, corrected LL, or sagittal alignment. PI increased in all patients with surgically corrected, adult sagittal deformity, following surgical correction of fixed LL. The disparity of PI after surgery was significantly higher in non-sacropelvic fixation, and showed a significant correlation with follow-up period without influence on sagittal alignment at last follow-up.