Objective: To investigate sagittal spinal alignment from the perspective of the overall curvature of the "S" curve of the human spine, and explore the roles of pelvic incidence (PI) and maximal thoracolumbar vertebral tilt(TLmax) in the classification of the sagittal spinal aligment. Methods: The tilt of the sacral, lumbar, and thoracic vertebrae (from Co1, S5, S4... to C7) were measured. The minimal sacral vertebral tilt(Smin), maximal thoracolumbar vertebral tilt(TLmax), and minimal thoracic vertebral tilt (Tmin) were recorded. The concept of lumbosacral lordosis (LSL) was introduced, and the Ferguson method was utilized to measure sagittal spinal parameters both in anatomical segmentation (Ferguson L1-S2, Ferguson T1-T12) and functional segmentation (Ferguson LSLmax, Ferguson TKmax). The subjects were grouped based on pelvic incidence (PI) and TLmax separately, and the mean and standard deviation of each parameter were calculated. Chi-square tests were conducted for statistical analysis. Results: 1. Based on PI grouping: PI for all subjects was 45.4 ± 9.5°(21.7-86.4°). Group A consisted of 117 subjects with a mean PI of 34.7 ± 4.4°, Group B had 158 subjects with a mean PI of 45.2 ± 2.9°, and Group C included 113 subjects with a mean PI of 56.7 ± 5.8°. No statistically significant differences were found in tilt of S2, L1, T1, TLmax, Tmin, and Ferguson L1-S2, Ferguson T1-T12, and Ferguson TKmax among Groups A, B, and C. 2. Based on TLmax grouping: TLmax for all subjects was 110.5 ± 5.5° (94.4-132.0°). Group A had 91 subjects with a mean TLmax of 104.0 ± 2.3°, Group B comprised 216 subjects with a mean TLmax of 110.2 ± 2.1°, and Group C included 81 subjects with a mean TLmax of 118.6 ± 3.8°. Significant statistical differences were observed in tilt of S2, L1, T1, Smin, TLmax, Tmin, and Ferguson L1-S2, Ferguson T1-T12, Ferguson LSLmax, and Ferguson TKmax among Groups A, B, and C. Conclusion: There were no differences in the magnitude of LSL and TK among subjects with different PI, indicating that PI does not affect the overall curvature of the "S" curve in the sagittal spinal aligment. In contrast, TLmax effectively distinguishes the overall curvature of the "S" curve. Keywords: Sagittal spinal alignment; Sagittal spinal curvature; Lumbosacral lordosis; Pelvic incidence; Maximal thoracolumbar vertebral tilt.

The authors have declared no competing interest.

This study did not receive any funding

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Ethics committee of Chinese PLA General Hospital gave ethical approval (S2023-446-02) for this work.

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