Pelvic parameters: origin and significance.

The adoption by humans of an upright position resulted in broadening and verticalisation of the pelvis together with the appearance of characteristic spinal curves, has profoundly modified the structure of the muscles supporting the spine. In order to characterise the sagittal balance of the pelvis, it is necessary to define parameters based on notable biomechanical forces involved in the transmission of constraints. The angle of incidence was constructed to enable reproducible analysis of the anatomical characteristics of the pelvis in the sagittal plane. The angle of incidence is the algebraic sum of two complementary angles: pelvic tilt (PT) and sacral slope (SS). Since the value of incidence is fixed for any given patient, the sum of pelvic tilt and sacral slope is a constant value: when one increases, the other necessarily decreases. The position of the lumbar spine, attached to the sacral plateau, is thus affected by the pelvic tilt and by the sacral slope. Consequently, the pelvic parameters affect the entire underlying sagittal spinal profile. Global spinal balance involves harmonisation of lumbar lordosis and thoracic kyphosis taking into account the pelvic parameters.