• Hongda Bao, Yuancheng Zhang, Shibin Shu, Tianyuan Zhang, Qi Gu, Wenting Jing, Zhen Liu, Xu Sun, Yong Qiu, and Zezhang Zhu.
• Spine Surgery, Nanjing Drum Tower Hospital, Medical School of Nanjing University, Nanjing, China.
• Spine. 2021 Jun 15; 46 (12): 796802796-802.

Study DesignA retrospective cross-sectional study.ObjectiveThis study aims to determine whether the sacroiliac (SI) joint motion correlated to pelvic incidence (PI) change from standing to supine position in patients with degenerative spinal diseases.Summary Of Background DataPI was found an unstable parameter after adolescence as the fixed nature of PI was challenged by several studies. The SI joint has been shown to have some motion, age-related degenerative changes of cartilage and SI ligaments contribute to SI joint instability.MethodsThe study contains both specimen study and radiographic study. One human specimen was acquired, on which PI was measured with different sacrum-ilium positions. In radiographic study, patients with old thoracolumbar fracture, lumbar disc herniation, stenosis, and spondylolisthesis were included. Ankylosing spondylitis (AS) patients were also included as control group. PI was measured on standing x-rays and scanogram of computed tomography images in supine position.ResultsSpecimen study result revealed that SI motion would lead to the change of PI with fixed pelvic thickness. In radiographic study, 101 patients with different etiology and 30 AS patients were included. After stratifying into different age groups, standing PI was significantly larger than supine PI in each age groups (P = 0.002, <0.001, and <0.001, respectively). In patients with degenerative diseases, PI was significantly larger on standing position than that on supine position. ΔPI showed no significant difference across etiologies. However, in AS patients, standing PI and supine PI revealed no significant difference (P = 0.528).ConclusionMobile SI joint may be the cause of increased PI in the aging spine. The dynamic change of PI is etiology-independent if the SI joint was not fused. Older patients have greater position-related change of PI.Level of Evidence: 4.Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.

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