Standing whole-spine radiographic sagittal alignment response to backpack load in schoolchildren

Evidence linking backpack load to pediatric back pain is inconsistent, and imaging data under load are scarce. We quantified acute backpack-related changes in sagittal alignment using standing whole-spine radiographs in school children.

Sixty-five healthy schoolchildren underwent standing lateral whole-spine radiography under three within-participant conditions: unloaded, low-load (target 10-15% body weight), and high-load (approximately 50% greater than low load). Habitual exposure (usual backpack weight and commute time) was related to unloaded alignment using multivariable linear regression, focusing on sagittal vertical axis (SVA), pelvic incidence minus lumbar lordosis (PI-LL), and pelvic tilt (PT). Load-alignment associations were examined using linear mixed-effects models with measured applied load (% body weight) as a predictor. Abnormal SVA (≥ 40 mm) under loading was examined using a generalized linear mixed-effects model. Receiver operating characteristic (ROC) analysis and the area under the curve (AUC) evaluated applied load for discriminating abnormal SVA.

Habitual backpack weight and commute time were not independently associated with unloaded SVA, PI-LL, or PT. Increasing applied load was associated with higher SVA and thoracic kyphosis and with lower PT, while lumbar lordosis changed minimally. Applied load increased the odds of abnormal SVA (odds ratio 1.526 per 1% increase in applied load; p < 0.001), whereas greater grip strength was protective (odds ratio 0.752; p = 0.037). ROC analysis showed an AUC of 0.725 with an optimal cut-off of 16.0% body weight.

Backpack loading acutely shifts sagittal alignment forward in schoolchildren on whole-spine radiographs. Clinically meaningful pediatric thresholds under load warrant further study.