Influence of posture on relationships between pelvic parameters and lumbar lordosis: Comparison of the standing, seated, and supine positions. A preliminary study.

Background: Pelvic incidence (PI) is an anatomical parameter that is considered invariable in a given individual. Although changes in posture influence the mobile lumbar spine, lumbar lordosis (LL) and the pelvis are typically evaluated only in the standing position. Thus, whether other positions commonly used during daily activities influence the relationship between LL and PI is unknown. The objective of this study was to determine whether LL and sacral slope (SS) correlated with PI, using two standardised positions, seated and supine, different from the standing position that is generally used.

Hypothesis: We are supposing that lumbar lordosis and sacral sloop are correlated to pelvic incidence whatever the posture. The goal of this study was to confirm or deny this hypothesis, using two standardize positions (sitting and lying) different that the usual standing position. LL and SS correlate with PI in the standing, seated, and supine positions.

Method: Lumbar and pelvic parameters were measured on radiographs obtained in the standing, seated, and supine positions in 15 asymptomatic adult volunteers younger than 50years of age. Mean values with their standard deviations were computed and compared across the three positions using ANOVA. Spearman's test was applied to assess correlations.

Results: PI had the same value in all three positions. The L1-S1 LL angle was 54.8±9.8° in the standing position, 15.9±14.6° in the seated position, and 50.2±9.6° in the supine position. Pelvic tilt (PT) in the same three positions was 12.1±6.3°, 37.7±10.4°, and 9.5±5.1°, respectively; and SS was 37.1±6.3°, 11.3±10.8°, and 41±7.2°, respectively. Correlations were strongest in the supine position between PI and LL (r=0.72), LL and SS (r=0.9), and PI and SS (r=0.84).

Conclusion: Whereas PI remains unchanged in a given individual, lumbar lordosis and sacral orientation show significant changes across positions used in daily life, with the greatest changes seen in the seated position. During spinal fusion surgery, adjusting LL based on IP is crucial even in patients who have limited physical activity.

Level Of Evidence: IV.