Impact of car seat recline angle on global spinal alignment and seat belt positioning: a prospective observational study in Japan.

Abstract

Prospective observational radiographic study. To evaluate how different car seat recline angles affect global spinal alignment and lap belt positioning in healthy adults, and to assess the influence of demographic factors on these changes. Seated posture alters global spinal alignment and may contribute to spinal symptoms, especially in individuals with adult spinal deformity (ASD) or following spine surgery. However, radiographic data on reclined seated postures and their impact on spinal alignment and restraint safety remain limited. Lateral radiographs were obtained from 100 healthy adults in standing and seated postures at 25°, 35°, and 45° recline angles. Spinal parameters (cervical lordosis, thoracic kyphosis, thoracolumbar kyphosis, lumbar lordosis, sacral slope, pelvic tilt, and T1 spinopelvic inclination) and seat belt metrics (X-value, Z-value, and lap belt angle) were measured. Multivariate regression was used to assess associations with sex, age, height, and body mass index (BMI). Reclining reduced cervical/lumbar lordosis and sacral slope, while thoracic kyphosis and pelvic tilt increased. Higher BMI was associated with a cranially shifted and more horizontal lap belt. Taller individuals showed greater T1 spinopelvic inclination and pelvic parameters. Females exhibited more pronounced thoracic kyphosis and pelvic retroversion across reclining angles. Car seat recline significantly alters spinal alignment and seat belt positioning, with changes influenced by age, height, and BMI. These findings support ergonomic improvements in seat design and may inform surgical planning for ASD.