How Does Gravity Influence the Distribution of Lordosis in Patients With Sagittal Malalignment?

Abstract

Retrospective cohort study. Compare the supine vs standing radiographs of patients with adult spinal deformity against ideals defined by healthy standing alignment. 56 patients with primary sagittal ASD (SRS-Schwab Type N) and 119 asymptomatic volunteers were included. Standing alignment of asymptomatic volunteers was used to calculate PI-based formulas for normative age-adjusted standing PI-LL, L4-S1, and L1-L4. These formulas were applied to the supine and standing alignment of ASD cohort. Analyses were repeated on a cohort of 25 patients with at least 5 degrees of lumbar flexibility (difference between supine and standing lordosis). The asymptomatic cohort yielded the following PI-based formulas: PI-LL = -38.3 + .41*PI + .21*Age, L4-S1 = 45.3-.18*Age, L1-L4 = -3 + .48*PI). PI-LL improved with supine positioning (mean 8.9 ± 18.7°, <i>P</i> < .001), though not enough to correct to age-matched norms (mean offset 12.2 ± 16.9°). Compared with mean normative alignment at L1-L4 (22.1 ± 6.2°), L1-L4 was flatter on standing (7.2 ± 17.0°, <i>P</i> < .001) and supine imaging (8.5 ± 15.0°, <i>P</i> < .001). L4-S1 lordosis of subjects with L1-S1 flexibility >5° corrected on supine imaging (33.9 ± 11.1°, <i>P</i> = 1.000), but L1-L4 did not (23.0 ± 6.2° norm vs 2.2 ± 14.4° standing, <i>P</i> < .001; vs 7.3 ± 12.9° supine, <i>P</i> < .001). When the effects of gravity are removed, the distal portion of the lumbar spine (i.e., below the apex of lordosis) corrects, suggesting that structural lumbar deformity is primarily proximal.