Sagittal parameters of the spine: biomechanical approach

Abstract

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.

Fig. 1

Subdivision of the sagittal spinal curvatures. Cervical spine goes from C1 to C7. Thoraco-lumbar spine is limited by C7/T1 and the sacral plateau. Inflexion Point is the place where the lordosis curve turns in kyphosis. This is a variable limit between lordosis and kyphosis

Fig. 2

Geometrical construct of Lumbar Lordosis (LL) by two tangent arcs of circle. LL is limited proximally by the Inflexion Point (IP) and distally by the sacral plateau (S1P). C1 and C2 are two tangent circles on the apex (A) of LL. LL is divided in two arcs; the upper one from IP to A is located on C2, the lower between A and S1P is on C1. As complementary angles, lower arc equals SS

Fig. 3

Type 1: Regarding a low grade Sacral Slope, length of the inferior arch C1 is very small. Lordosis is mainly represented by the superior arch. Globally a short lordosis is associated with a long thoraco-lumbar kyphosis

Fig. 4

Type 2: Both arcs of circle (C1 and C2) have long radius and they approximate a straight line resulting in a flat lordosis

Fig. 5

Type 3: Sacral Slope is between 35° and 45°, increasing by the way, length and angle of the lower arch. Upper arch remains stable around 20°

Fig. 6

Type 4: Sacral Slope is over 45°, increasing dramatically length and angle of the inferior arch. There is an important risk of forward tilt of lumbar lordosis. The inflexion point may over pass the anatomical limit of L1T12

Fig. 7

In a Type 2 Lordosis (left) the area occupied by posterior elements is longer for a same number of vertebrae, than in a Type 4 lordosis (right). By this way posterior elements (facets and spinous processes) are smaller in Type 4 than in Type 2. This geometrical effect may be emphasized in small person

Fig. 8

On the left a backward Tilt of lordosis allows a bigger amount of kyphosis above with the risk of a local hyperextension. In the middle, Lordosis is well balanced there is no Tilt. On the right, forward Tilt, lordosis orientation is unbalanced

Fig. 9

Kyphosis sub division: when the lower lumbar lordosis angle (β) equals SS, the lower thoracic kyphosis angle (α2) equals the upper lumbar lordosis angle (α1). The orientation of the upper thoracic kyphosis angle is determined by C7 positioning

Fig. 10

In order to avoid errors with distance magnification C. Barrey proposed a ratio between horizontal distance from the centre of the femoral heads (FH) and C7PL (R1) and the distance from FH and the posterior edge of the sacral plate (R2). R1/R2 is negative when C7 PL is forward FH. 0 < R1/R2 < 1 when C7PL is between FH and the sacrum, and R1/R2 > 1 when C7PL is behind the sacrum

Fig. 11

ST determines extrinsically the position of C7 regarding the horizontal line. SSA represents the value of the global kyphosis of the spine. They are linked by the relation: SSA = SS + ST