P91. How does gravity influence the distribution of lordosis in sagittal malalignment?

Abstract

<h3>BACKGROUND CONTEXT</h3> Supine imaging reveals the contour of the spine without gravity, a key consideration in surgical planning. Comparing supine to standing radiographs may reveal differences in regional flexibility in adult spinal deformity (ASD). <h3>PURPOSE</h3> To determine differences in regional flexibility of the lumbar spine in patients with ASD. <h3>STUDY DESIGN/SETTING</h3> Retrospective cohort study. <h3>PATIENT SAMPLE</h3> Fifty-six patients with sagittal ASD (SRS-Schwab Type N, no previous fusion) and 119 asymptomatic volunteers. <h3>OUTCOME MEASURES</h3> PI-LL, L1-L4, and L4-S1 alignment. <h3>METHODS</h3> Asymptomatic volunteers underwent full length standing radiographs, ASD patients had standing and supine radiographs taken for preoperative planning. The standing alignment of the asymptomatic volunteers (50.7±17 years, PI 52±11.4°) was analyzed using linear regression in order to calculate PI-based formulas for normative, age-adjusted standing PI-LL, L4-S1, and L1-L4. These formulas were applied to the ASD cohort and compared to their standing and supine alignment. Given the potential for confounding by ASD patients with stiff lumbar spines, analyses were repeated in a cohort of 25 patients with at least 5° of flexibility (defined as the difference between supine and standing lordosis) and 5 degrees of offset from regional alignment norms. <h3>RESULTS</h3> Linear regression in the asymptomatic cohort resulted in the following PI based formulas: PI-LL= -38.3 +0.41*PI +0.21*Age, L4-S1= 45.3 -0.18*Age, L1-L4= -3 + 0.48*PI). The ASD cohort had a mean age of 65±14 years, BMI 29±7 kg/m2, standing PI-LL 14±22°, T1PA 22±11°). Eighty-seven percent of patients had a lordotic apex between the L3-L4 inferior endplates. PI-LL improved with supine positioning (mean 8.9±18.7°, p<0.001), though not enough to correct to age-matched norms (mean offset 12.2±16.9°). Compared to mean normative alignment at L1-L4 (22.1±6.2°), L1-L4 was flatter on standing (7.2±17.0°, p<0.001) and supine imaging (8.5±15.0°, p<0.001) while L4-S1 was comparable (norm 33.6±2.6 vs standing 30.9±12.9, p=0.325; vs supine 34.5±10.3, p=1.000). Subgroup analysis of subjects with L1-S1 flexibility >5° demonstrated a more dramatic effect—the standing loss of lordosis (32.6±1.7° norm vs 25.5±12.7° standing, p=0.031) at L4-S1 corrected on supine imaging (33.9±11.1°, p=1.000), while L1-L4 alignment did not (23.0±6.2° norm vs 2.2±14.4° standing, p<0.001; vs 7.3±12.9° supine, p<0.001). <h3>CONCLUSIONS</h3> When the effect of gravity is removed, the distal portion of the lumbar spine (i.e., below the apex of lordosis) corrects, suggesting that deformity is contained in the proximal lumbar region. These findings illuminate the etiology of lumbar hypolordosis and may influence selection of fusion levels. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.