Regions of the vertebral column

The vertebral column initially develops as a series of 33 vertebrae. In the adult, however, this number is reduced to 24 movable vertebrae, along with the sacrum and coccyx. The column is divided into five regions: 

• The cervical region contains seven vertebrae, designated C1 to C7

• Below the cervical vertebrae are the 12 thoracic vertebrae, labeled T1 to T12

• The lumbar region contains five vertebrae, L1 to L5, located in the lower back

• The sacrum is formed by the fusion of five sacral vertebrae

• The coccyx, or tailbone, is formed by the fusion of four coccygeal vertebrae

The fusion of the sacral and coccygeal vertebrae begins after the age of 20 and is completed by middle age. 

The vertebrae of the cervical, thoracic, and lumbar regions remain distinct and are known as movable or true vertebrae, whereas the sacral and coccygeal vertebrae are fused and are referred to as fixed or false vertebrae.

A typical vertebra consists of a body, a vertebral arch, and seven processes. The body is the anterior portion and is responsible for bearing weight. It is cylindrical in shape, with rough upper and lower surfaces that provide attachment for the intervertebral discs. 

The anterior surface of the body is convex from side to side and concave from top to bottom, while the posterior surface is slightly concave. Small openings on the body allow for the passage of blood vessels.

Seven processes extend from the vertebral arch. 

• The spinous process projects posteriorly and can be felt along the midline of the back. 

• Two transverse processes project laterally and serve as attachment sites for muscles and ligaments. 

• Four articular processes, consisting of two superior and two inferior processes, project upward and downward, respectively. 

These processes form joints with adjacent vertebrae, allowing limited movement between them. The articular surfaces are covered with hyaline cartilage.

The vertebral arch forms the posterior part of the vertebra and consists of two pedicles and two laminae. The pedicles are short, thick projections that extend backward from the body and form the lateral walls of the arch. 

The laminae are broad plates that extend from the pedicles and fuse in the midline to complete the arch. 

The space enclosed by the body and the arch is the vertebral foramen, which contains the spinal cord. When the vertebrae are aligned, these foramina form the vertebral canal. 

Notches located above and below the pedicles form intervertebral foramina when adjacent vertebrae are joined, and these openings allow spinal nerves and vessels to pass.

Cervical Spine

The cervical region consists of seven vertebrae, designated C1 to C7, and forms the neck portion of the vertebral column. These vertebrae are the smallest of the movable vertebrae because they support less body weight. 

A distinguishing feature of cervical vertebrae is the presence of a foramen in each transverse process, known as the transverse foramen, which allows the passage of the vertebral artery, vein, and sympathetic nerves.

Typical cervical vertebrae have small bodies that are wider from side to side than from front to back. 

The upper surface of the body is concave transversely with raised lateral margins, while the lower surface is concave from front to back. The vertebral foramen is large and triangular, providing space for the spinal cord. 

The spinous process is short and usually bifid. The transverse processes project laterally and consist of anterior and posterior parts, with a groove between them for the spinal nerve.

Thoracic Spine

Thoracic vertebrae are intermediate in size between cervical and lumbar vertebrae and increase in size from above downward. 

They are distinguished by the presence of costal facets for articulation with the ribs. The bodies are typically heart-shaped and bear demi-facets for rib heads. The vertebral foramen is small and circular.

The spinous processes are long and directed downward, often overlapping adjacent vertebrae. The transverse processes are strong and bear facets for articulation with rib tubercles. The laminae are broad and overlap like tiles. The articular processes are oriented to limit movement.

Certain thoracic vertebrae have unique features. 

The first thoracic vertebra has a full facet for the first rib and a demi-facet for the second. The ninth and tenth vertebrae may vary in their facets. The eleventh and twelfth vertebrae resemble lumbar vertebrae, with shorter spinous processes and no transverse costal facets.

Sacrum

The sacrum is a large, triangular bone formed by the fusion of five sacral vertebrae. It is situated in the lower part of the vertebral column and forms the upper and posterior part of the pelvic cavity. 

In position, it is inserted like a wedge between the two hip bones, where it contributes to the stability of the pelvis. Its superior part, known as the base, articulates with the fifth lumbar vertebra, while its inferior part, the apex, articulates with the coccyx. 

The sacrum is curved upon itself and is placed obliquely, with its base projecting forward to form the prominent sacrovertebral angle. Its central portion is directed backward, which increases the capacity of the pelvic cavity.

The anterior surface of the sacrum, also called the pelvic surface, is concave from above downward and slightly concave from side to side. It is marked by four transverse ridges, which represent the lines of fusion between the original five sacral vertebrae. The portions of bone between these ridges correspond to the bodies of the sacral vertebrae. 

The first sacral segment is the largest and resembles a lumbar vertebra, while the segments below it progressively decrease in size and become more flattened. 

On either side of the pelvic surface are four anterior sacral foramina, which are rounded openings that decrease in size from above downward. These foramina transmit the anterior divisions of the sacral spinal nerves and allow the passage of lateral sacral arteries. 

Lateral to these foramina are shallow grooves that accommodate the anterior branches of the sacral nerves, separated by ridges that provide attachment for muscles such as the Piriformis.

The posterior surface, or dorsal surface, of the sacrum is convex and narrower than the anterior surface. Along the midline is the median sacral crest, which is formed by the fused spinous processes of the upper sacral vertebrae and appears as a series of tubercles.

On either side of this crest lies the sacral groove, which provides origin for muscles and is formed by the fused laminae. Lateral to the sacral groove is a row of tubercles representing the fused articular processes, forming the sacral articular crests. 

Further laterally are the posterior sacral foramina, which are smaller and less regular than the anterior foramina and transmit the posterior divisions of the sacral spinal nerves. 

Beyond these are additional tubercles representing the fused transverse processes, which form the lateral sacral crests and serve as attachment points for ligaments.

The lateral surfaces of the sacrum are broad above and narrow below. The upper part of each lateral surface presents an ear-shaped area known as the auricular surface, which is covered with cartilage in life and articulates with the ilium (hip bone) to form the sacroiliac joint. 

Coccyx

The coccyx is formed from four fused vertebrae, although the number may vary. It is small and triangular and articulates with the sacrum. 

The anterior surface is slightly concave, while the posterior surface is convex and marked by rudimentary processes. The first segment is the largest and resembles a vertebra, while the remaining segments decrease in size.

The coccyx provides attachment for ligaments and muscles and may bear weight when sitting.

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