Embryology : Primary and secondary ossification points of the thoracic (a) and lumbar vertebrae (b). (1) marginal listel or epiphyseal disc, (2) medial primitive point, (3) articular secondary point, (4) lateral primitive point, (5) secondary spinal point, (6) mamillary secondary point 
Morphology of Posterior Thoraco-Lumbar Joints (TLJ)
Morphology of the Posterior Thoracic Joints
Between the articular processes or zygapophyses are the joints whose articular facet planes are covered with strongly oblique cartilage (65° on average compared to the plane of the discs). The thoracic facets appear to be aligned along a sphere whose center is located in the disc (Fig. 8) .
Masharawi performed a morphometric study on 240 normal dry vertebral columns [2, 3] (Figs. 4, 5, and 6). There is no difference in the size of the facet joints according to age, and men have slightly larger dimensions than women. The craniocaudal length and the anteroposterior width of the facets have a parabolic distribution with a progressive decrease in size of T1 (from 10.75 mm and 12.75 mm, respectively) to T8 (10.34 and 9.05 mm) and a gradual increase from T12 (11.45 and 9.83 mm). It is therefore towards the vertex of the thoracic kyphosis that the surface of the facet joints is the weakest. Facet asymmetry is almost constant at the level of the thoracic spine concerning both dimensions and orientations. The craniocaudal length of the upper left facets is greater than that of the upper right facets from T1 to T11 and that of the lower left facets is greater than that of the lower right facets in T2, and T6–T10 and the reverse in T12. The anteroposterior width of the upper right facets is greater than that of the upper left facets of T1–T6 and that of the lower left facets is greater than that of the lower right facets of T6–T8. In posterior view, comparing for each vertebra the distribution of interfacet distances in the horizontal plane and the vertical plane the posterior arches of T1 and T2 have an inverted trapezoidal aspect which becomes a rectangle from T3 to T12. There is also a right /left asymmetry of the facet transverse angle with a greater right-hand transverse angle than that of the left-hand side with respect to the lower and upper facets as well as for the longitudinal angle of the upper facets. The right thoracic facets are therefore a little more vertical and frontal than the left ones. Outside the TL junction, however, the angular values between the lower and upper facets are symmetrical.
The orientation of the facets and therefore of the articular line at the TL junction between T11 and L2 has many varieties according to which side or the other adopts the thoracic (spherical segment) or lumbar (pulley segment with sagittal slopes) (Fig. 9). Singer et al.  described these variations (Fig. 12). A sharp transition from frontal orientation to sagittal orientation was noted in 46% of cases in T12 and 33% in T11. Asymmetry >20° between the two joints of the same level is more marked at T11–T12 (21%) than at T12–L1 (9%).
The union of the facets is done by a fibrous capsule reinforced anteriorly by the ligamentum flavum and posteriorly by a posterior longitudinal ligament . A synovial lining on the inner aspect of the capsule with folds that are mistakenly taken on anatomical sections for a small intra-articular meniscus .
Morphology of the Posterior Lumbar Joints
The maintenance of joint integrity is the doubled synovial capsule, the ligamentum flavum anterior to the joint, a posterior ligament strengthening the capsule posteriorly, and remote ligaments. As in the thoracic stage, there is no intra-articular meniscus .
Participation of the Posterior Articulations in the Overall Stability of the Spine 
The factors of stability are different according to whether the spine is considered vertically along its long axis or perpendicular to it in the horizontal plane. The vertebral stability results in fact from the synergy of the factors of axial stability and horizontal stability. The posterior joints participate fully in vertebral stability.
In the vertical direction , stability finds its support in the elementary and global architecture of the spine.
The elementary architecture of each TL vertebra is composed of three vertical mini columns: that of the vertebral body in front and those of the articular processes behind. The three columns are joined by horizontal bars: the two pedicles and the laminae behind. The global architecture of the TL spine is composed of three vertical columns. The anterior column, the largest, takes on a quadrangular pyramid appearance formed by the alternation of vertebral bodies and discs to the sacral plateau. The two posterior columns arranged in a frontal plane are constituted by the succession of articular processes. This system of columns is reinforced by horizontal bars (pedicles and laminae), which at each vertebral level, solidify the columns together. This three-column structure is found in the development of the spine from the three primary points of ossification: the centrum for the large anterior column and the two points of ossification of the posterior arch for the two posterior columns. The increasing caliber, in the craniocaudal direction, of the three columns testifies to their bearing function. For the same reasons, the articular surfaces of each of the mobile vertebral segments increase in the craniocaudal direction. If we consider only the articular surfaces of the posterior columns, Masharawi  reports a decrease of these surfaces between T4 and T10 which can be explained by the presence at this level of the dorsal kyphosis submitting more vertical stress to the anterior column and to the posterior columns. Thus the posterior columns with their facets should not be considered as simple articulations orienting the segmental movements of the spine but as true load bearing formations of the spine following the positions of the spine.
In flexion, the articular processes whose facet direction opposes horizontal sliding play the role of bending abutments. The articular capsule, the ligamentum flavum, and all ligaments located behind the nucleus pulposus slow extreme vertebral flexion. The oblique articular facets planes allow a sliding which results in an angulation between two vertebrae but it is the limit of elasticity of the ligament brakes which prohibits the dislocation of the facet joints.
In forced extension, it is the ligaments situated in front of the nucleus pulposus which limit the movement, the capsulo-ligamentary elements of the articular facets do not play a primordial role. The most posterior part of the articular processes, and the spinous processes between them, come in contact and block extension.
Rotation and tilt movements are almost always combined. The inclination of the articular facets between 45° and 80° with respect to the plane of the intervertebral space imposes a simultaneous rocking and rotation movement. When a lower right articular facet rises and advances on the underlying facet, the left facet descends and recedes. Virtually all structures within the intervertebral union participate in the role of rotation and inclination braking. The stops that also limit these movements are always the joints, especially at the level of the lumbar region. In the thorax, the costovertebral joints considerably limit the lateral inclination and the rotation, although the orientation of the articular facets inscribed on a circular arc is favorable to rotation.