European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society
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To 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. ⋯ PI 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.