The spine usually consists of 7 cervical vertebrae (C1-C7), 12 thoracic vertebrae (T1-T12), 5 lumbar vertebrae (L1-L5), the 5 (fused) vertebrae of the sacral spine (S1-S5) and the 3 or 4 vertebrae of the coccygeal region (Co1-Co4). Two successive vertebrae, including the intervertebral disc and joints, together form a segment. In 10% of cases there are segmentation differences, meaning that more or fewer vertebrae are present at a certain level of the spine (see: anatomical variations in the bony spine).
A vertebra normally consists of a vertebral body (corpus vertebrae), that is levelled of at the lumbar and thoracic level both distally and proximally, and has a vertebral arch (arcus vertebrae) on the dorsal side that often has a number of bony projections: dorsal (spinous processes), lateral (transverse processes), descending (inferior articular processes) and ascending (superior articular processes). The vertebral arch can be subdivided into the lamina and pedicle. The lamina is the posterior section of the arch where the projections are located; the pedicle is the piece connecting the arch with the vertebral body. The vertebral arch and the dorsal side of the body surround a space called the intervertebral foramen. The combined intervertebral foramina of all the vertebrae form the so-called vertebral canal in which the spinal cord and parts of the extending nerve roots are located.
The first cervical vertebra (the atlas) articulates with the occipital bone (atlanto-occipital joint). The sacral bone belongs to the bony pelvic ring. It articulates with the iliac bone on both sides of the pelvis (sacro-iliac joints). An inferior articular process of one (upper) vertebra and a superior articular process of the other (lower) vertebra together form the zygapophysial or facet joint. Each segment therefore has a left and right facet joint.
Between most vertebrae, cartilaginous intervertebral discs are located; the most cranial intervertebral disc can be found between C2 and C3, the 4 most caudal discs are integrated into the sacral bone. A disc is built up from a ring-shaped anulus fibrosus (consisting of connective tissue material) containing gelatinous nuclear pulp. The nuclear pulp dries up before the 30th year of one’s life. The discs may differ in thickness depending on the location, age and stress on the spine. Depending on the location, discs may also vary in shape. The discs comprise a maximum of 25% of the total length of the spine.
The bony structures are linked with one another by ligaments. The posterior longitudinal ligament and, to a lesser degree, the anterior longitudinal ligament connect the vertebral bodies and discs. The flaval ligaments connect the ventral sides of the laminae of the vertebral arches. The interspinal ligaments are located between the various spinous processes and the supraspinal ligaments border the spinous processes on the dorsal side.
Cervically, the supraspinal ligaments are nuchal ligaments.
In this nuchal ligament, funicular and membranous parts can be distinguished. The funicular part is mainly spread between the spinous processes of the cervicothoracic transition (C5-T1) on the one hand and the posterior tubercule of atlas and external occipital protuberance on the other. The membranous part is primarily extended between the high funicular section and the other cervical spinous processes.
This part is not developed particularly well in humans, resulting in an extended freedom of movement for the spinous processes of C2 up to, and including, C5. Between the transverse processes the intertransverse ligament is stretched.
The spine forms an S-shape in the sagittal plane. The cervical and lumbar sections have a convex curvature to the ventral side (lordosis), whereas the thoracic section and the sacral bone have a convex curvature to the dorsal side (kyphosis). These curvatures may occasionally be pathological, enhanced, or flattened and therefore influence the total posture, sometimes resulting in (back or neck) symptoms.
In the following description of the vertebrae at various levels, we limit ourselves to the clinically most relevant characteristics.
