Standing versus supine pelvic incidence in adult spinal deformity patients

Purpose Pelvic incidence (PI) is commonly used to determine sagittal alignment. Historically, PI was believed to be a fixed anatomic parameter. However, recent studies have suggested that there is positionally-dependent motion that occurs through the sacroiliac joint (SIJ) resulting in changes in PI. Methods We reviewed 100 consecutive adult spinal deformity (ASD) patients seen at our academic tertiary referral center. Two reviewers measured pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), and lumbar lordosis (LL) on standing radiographs and scout computed tomography scans (CT). Unilateral, bilateral, or absent SIJ vacuum sign (VS) was determined using CTs. Results Eighty-six patients (42 M:44 F) were included with an average age of 64.1 years and BMI of 28.8 kg/m2. Standing PI was low (< 50 degrees) in 35 patients (40.7%), average (50 degrees-60 degrees) in 22 (25.6%), and high (> 60 degrees) in 29 (33.7%). Average and high PI patients had significant PI changes of 3.0 degrees (p = 0.037) and 4.6 degrees (p = 0.005), respectively. Bilateral SIJ VS was seen in 68 patients, unilateral VS in 9, and VS was absent in 9. The average change in PI between standing and supine was 2.1 degrees in bilateral SIJ VS patients (p = 0.045), 2.2 degrees in unilateral SIJ VS (p = 0.23), and - 0.1 degrees in patients without SIJ VS (p = 0.93). The average absolute difference in PI between supine and standing was 5.5 degrees +/- 5.5 degrees (p < 0.001). Conclusion There is a change in PI from supine to standing. In patients with high PI and bilateral VS on CT, the change from supine to standing is significant, perhaps representing instability of the SIJ.