Thoracolumbar Sagittal Morphology and Segmental Inclination: A Cross-sectional Cohort Study of Asymptomatic Adult Volunteers.

Study design: A cross-sectional cohort study.

Objective: To present a normative value reference of spinal segmental inclination stratified by age and pelvic incidence (PI), and to clarify the impact of segmental inclination on spinal sagittal morphology.

Summary of background data: Thoracolumbar segmental inclination has been shown to correlate with the clinical outcomes of adult spinal deformity surgery. However, there currently exists no normative value reference in a large sample of asymptomatic population.

Materials and methods: Asymptomatic adult volunteers were enrolled from the community. All volunteers underwent a standing full-spine anteroposterior and lateral radiograph. Lumbar tilt (LT) and thoracic tilt (TT) were measured to quantify the segmental inclination of the lumbar and thoracic spine. Regional curvature, global balance, and thoracolumbar apex were analyzed across different age and PI groups. The correlation between sagittal parameters and age was analyzed using Pearson correlation tests.

Results: A total of 618 volunteers were included with a mean age of 38.7 ± 17.1 years (range 18-82 yr). As age increased, the LT and TT significantly increased ( P <0.001). The LT was significantly correlated with PI (r=0.410, P <0.001), with the low PI group exhibiting a greater negative LT. The TT remained constant across different PI groups. Compared with the young and middle-aged groups, the thoracic apex and lumbar apex were located more caudally in the elderly group ( P <0.001). Subjects with a more caudal lumbar apex exhibited a greater negative LT, and those with a more caudal thoracic apex exhibited a greater positive TT.

Conclusions: The thoracic spine naturally adapts to a relatively neutral position, yet it tends to tilt forward with aging. The physiological lumbar inclination is predominantly determined by the PI value with a slight backward tilt and tends to counteract the anterior truncal inclination with advanced age. Physiological segmental inclination should be considered in spinal surgical planning.