J Radiol
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Man, with his erect posture, evolves in a world subject to the laws of gravity. His spine reflects these constraints. The morphology and static of human spine and biomechanical relationships between spine and pelvis are in direct relation with bipedia. ⋯ Incidence affects the sacral slope, which determines lumbar lordosis, which itself influences pelvic tilt, thoracic kyphosis, and even hip and knee position. All these landmarks can easily be studied on a sagittal radiograph. Knowledge of these functional relationships is essential to understand the origin of low back pain, sagittal imbalance and above all before surgical treatment of spine disorders especially when arthrodesis is considered.