Sagittal parameters of the spine: biomechanical approach.

According to the anatomical segmentation, spine curves are the sacral kyphosis (sacrum), lumbar lordosis (L1 to L5), thoracic kyphosis (T1 to T12) and cervical lordosis (C1 to C7). From the morphological point of view the vertebrae of a curve are not identical: from cranial to caudal and vice versa there is a progressive anatomical modification. Both curves of the thoraco-lumbar spine may be divided at the Inflexion Point where lordosis turns into kyphosis. A geometrical construct of each curve by two tangent arcs of circle allows understanding the reciprocal changes between both curves. Lumbar Lordosis is mainly dependent on SS orientation, and the top of thoracic curve on C7 is very stable over the sacrum. Thoracic curve is dependent on lumbar lordosis orientation and C7 positioning. On a reverse effect, structural changing of thoracic kyphosis may affect the shape of the lumbar lordosis and the orientation of the pelvis.