Body Mass Hides the Curve: Thoracic Scoliometer Readings Vary by Body Mass Index Value.

BACKGROUND: Guidelines suggest referral for scoliosis when rib slope (scoliometer measurement, angle of trunk rotation) is ≥7 degrees. We hypothesized that overweight and obese patients would have lower scoliometer measurements compared with normal-weight and underweight patients for a given spinal curvature, causing overweight and obese patients with adolescent idiopathic scoliosis to present for treatment later and with larger curves. Our goal was to determine the association between scoliometer readings and major curve magnitudes in relation to body mass index (BMI).
METHODS: This retrospective cohort study at a tertiary referral center included 483 patients (420 girls) aged 10 to 18 years (mean age, 14±1.6 y) with thoracic adolescent idiopathic scoliosis who presented to 1 orthopaedic surgeon for initial evaluation of spinal deformity from 2010 to 2015. Records were reviewed for BMI percentile for age and sex (underweight, ≤fourth percentile; normal weight, fifth to 84th percentile; overweight, 85th to 94th percentile; obese, ≥95th percentile), patient characteristics, thoracic scoliometer measurements, and thoracic major curves.
RESULTS: Of the 483 patients, 23 were underweight, 372 were normal weight, 52 were overweight, and 36 were obese. Obese patients had a larger mean major curve (44 degrees) than normal-weight patients (34 degrees) (P=0.004). The odds of presenting with a major curve ≥20 degrees were 4.9 (95% confidence interval, 1.1-22; P=0.037) times higher for obese versus normal-weight patients. Receiver operating characteristic analysis of major curves (≥20 vs. <20 degrees) estimated the scoliometer values with the greatest sensitivity and specificity to be 8 degrees for underweight patients, 7 degrees for normal-weight patients, 6 degrees for overweight patients, and 5 degrees for obese patients.
CONCLUSIONS: Obese patients presented with larger thoracic curves versus normal-weight patients. Differences in chest-wall thickness in patients with different BMI values may alter scoliometer measurements for a given rotational deformity. Our data suggest new referral criteria for the scoliometer test based on BMI values. Specifically, obese patients should be referred at an angle of trunk rotation of 5 degrees. LEVEL OF EVIDENCE: Level II.