Pelvic Incidence Changes Between Flexion and Extension.

Study Design: Retrospective single-center.
Objectives: To investigate changes in pelvic incidence from flexion to extension. To assess interobserver error in the measurement of pelvic incidence.
Background: Pelvic incidence (PI) has been considered a static parameter since it was originally described. But recent studies have shown that PI can change with age and after spinal procedures. Changes in PI based on position have not been investigated.
Methods: Seventy-two patients who had obtained flexion and extension radiographs of the lumbar spine were identified using strict inclusion and exclusion criteria. PI along with pelvic tilt (PT), sacral slope (SS), and lumbar lordosis were measured in both flexion and extension by two independent measurers. Variations in all parameters and interobserver measurement reliability were analyzed for the entire group.
Results: PI changed significantly from flexion to extension with a general tendency to decrease: mean (-0.94°), p <.044. However, these changes might have had opposite vectors, and exceeded | 6°| (measurement error) in 20% of cases, with a maximum of 12°. Inconsistencies in changes of SS, as opposed to PT from flexion to extension, were found to be the major factor determining changes in PI (p >.001). Obesity significantly contributed to differences in PI between flexion and extension (p = .003).
Conclusions: PI is a dynamic parameter that changes between flexion and extension. Changes in SS are the main factor involved in these changes, implicating movement through the sacroiliac joints as the cause. Obese patients have greater changes in PI from flexion to extension.
Level of Evidence: Level II.
(Copyright © 2018 Scoliosis Research Society. Published by Elsevier Inc. All rights reserved.)