• Weiguo Zhu, Chao Kong, Sitao Zhang, Yu Wang, Wei Wang, Xiangyao Sun, and Shibao Lu.
• Department of Orthopaedic Surgery, Capital Medical University Xuanwu Hospital, Changchun Road No. 45, Beijing, 100053, China.
• Eur Spine J. 2020 Jun 1; 29 (6): 1379-1387.

PurposeTo compare the acute behaviors of pelvic incidence (PI) between elderly adult spinal deformity (ASD) patients with severe and minor sagittal deformity based on SRS-Schwab classification and to identify the mechanism of the variability in PI after long fusion to S1.MethodsPatients aged 60 years or above with available radiographs were included. The following parameters were measured pre- and postoperatively: Thoracic kyphosis, thoracolumbar kyphosis (TLK), lumbar lordosis (LL), PI, pelvic tilt, sacral slope, sagittal vertical axis (SVA), PI-LL and T1 pelvic angle (TPA).ResultsForty-two patients were found with severe sagittal deformity were assigned to Group S, and 60 patients with minor sagittal deformity were assigned to Group M. Immediately after surgery, lumbar curve, TLK and PI-LL were obviously corrected in both groups, while LL, PI, SVA and TPA were significantly increased in Group S alone. PI was significantly increased from 42.6 ± 4.7° to 51.7 ± 6.0° in Group S (P = 0.002), but changed from 45.4 ± 10.2° to 46.3 ± 10.3° in Group M without statistical significance. Pearson correlation analysis showed changes in PI was significantly correlated with changes in SVA (r = 0.415, P = 0.011) in patients with PI increased more than 5°.ConclusionPI spontaneously increases in elderly ASD patients with severe sagittal deformity after long fusion to sacrum, while is relative invariable in those with minor sagittal deformity. Variation in PI could be considered as a secondary change compensating for the spinal sagittal malalignment under long spinal fusion in elderly patients.

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