Abstract Objective: To introduce a new geometric force-line model to assess the sagittal spinal-pelvic alignments and statistically analyze the main parameters of the model, offering a novel perspective for understanding the sagittal balance. Methods: A total of 388 healthy adults aged 18 - 35 years were recruited. To designate the mid-point of the anterior edge of the S2 vertebral body as the imaging marker point of the sagittal sacroiliac joint (SIJ). Parameters were measured included inclinations of lines and angles related to T1, S2, HA (hip axis) and Co1(the first coccygeal). Results: The S2-T1 inclination angle was 89.5 ± 2.8°, close to perpendicular (90°), and it means the spinal axis (S2-T1) of the population is approximately perpendicular to the ground. In addition, spinal axis (S2-T1) crossed T11-L1 segments, which means spinal S curve is basically distributed on both sides of the axis. Angles ∠Co1-S2-HA, ∠HA-Co1-S2, and ∠S2-HA-Co1 were approximate 60°, forming a horizontal stable equilateral triangle. The anterior edge of S2 was nearly perpendicular to the pelvic force line (∠S2, HA-S2 ≈ 90°). Conclusion: The geometric performance of the proposed model is concise, clear, and logical. It provides a new perspective on the sagittal balance. This framework might help us better understand spinal-pelvic alignments and its clinical significance. Keywords: Sagittal balance, Spinal-pelvic alignments, Lumbosacral lordosis

The authors have declared no competing interest.

This study did not receive any funding

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

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