GENERAL SPINE OVERVIEW

Twenty-six vertebrae make up the vertebral column. These bones are grouped under the names cervical, thoracic, lumbar, sacral, and coccygeal according to the regions they occupy. When viewed from the side (figure 2-18), the vertebral column presents four normal curves that correspond with the different regions of the column: cervical, thoracic, lumbar, and sacral (or pelvic). Anteriorly, the cervical curve is convex, the thoracic curve is concave, the lumbar curve is convex, and the sacral curve is concave (this curve includes the coccyx). The thoracic and sacral curves are termed primary curves because they develop before birth. They are sometimes indicated as the accommodation curvatures because they tend to increase the size of the thoracic and pelvic cavities. The cervical and lumbar curves are known as secondary curves because they develop after birth. The cervical curve develops when the infant is able to hold up its head (at 3 or 4 months) and sit upright (at about 9 months). The lumbar curve develops when the child begins to walk (at 12 to 18 months). In addition to these alternate curvatures, the vertebral column normally has a slight lateral curvature when viewed from the anterior aspect. In most cases, the convexity of the lateral curvature is directed toward the right side and is associated with right-handedness. It is considered to be produced by the normal pull of the muscles.

Abnormal Curves of the Vertebral Column

A complex lateral curvature of the entire vertebral column, curves in thoracic and lumbar regions is called scoliosis. An exaggerated dorsal curvature is called kyphosis, or “humpback”. Exaggerated curvature of the lumbar region is called lordosis, or swayback.

Numbering System for Vertebrae

Region and number generally designate vertebrae. For convenience, abbreviations are used. Beginning superiorly at the first cervical vertebra, or atlas, the abbreviation is C-1; the second cervical vertebra, or axis is C-2; and numbering continues inferiorly to C-7. Because C-7 has a prominent spine, it is called vertebra prominens. In the thoracic region, the abbreviations are T-1 to T-12. The vertebrae in the lumbar region are abbreviated L-1 inferiorly to L-5.

FUNCTIONS OF THE VERTEBRAL COLUMN

The vertebral column functions as a strong pillar for the support of the trunk and the cranium, provides articular surfaces for the attachment of the ribs, and affords protection for the spinal cord and the roots of the spinal nerves. It transmits the weight of the trunk to the inferior extremities. Although forming a continuous support-bearing column, it is flexible enough to permit bending of the trunk in various directions. The vertebral canal, which follows the different curves of the column, accommodates and protects the spinal cord; it is formed by the superimposition of the vertebrae in each of which there is a vertebral foramen. Despite its flexibility, the vertebral column is sufficiently firm and strong to serve as a base for the origin of many ligaments and muscles and as a lever for the spinal muscles, which function to maintain the upright position of the trunk.

VERTEBRAL STRUCTURE

Most of the vertebrae have a similar general structure, that is, they all present certain characteristics. Thus, certain ones can be used as a pattern and are called typical vertebrae. In spite of this general similarity, the vertebrae in the different regions are so modified and present characteristics so peculiar to the region which they occupy that, when examined separately, it is possible to determine the region to which each belongs.b. Typical Vertebrae. Except for slight modifications due to position and function, all of the typical vertebrae have the same general structure. They are indicated as follows: C-3 to C-7, T-1 to T-12, and L-1 to L-5.

A typical vertebra is composed of:

1. A centrum, or body, which is the disk-like central portion.
2. The neural arch, which is made up of two roots or pedicles (small feet) and two laminae (layers of bone).
3. The vertebral foramen, an opening behind the body that is bounded laterally and posteriorly by the neural arch and anteriorly by the body. The apposition of all the vertebrae forms the vertebral canal that accommodates the spinal cord.
4. The vertebral notches, one on the superior border of each pedicle and one on the inferior border which, by the apposition of the adjacent vertebrae, form the intervertebral foramina (singular, foramen) for the transmission of the spinal nerves and vessels.

The processes consist of the following:

1. A spinous process that is directed dorsally and is formed at the junction of the two laminae.
2. Two transverse processes that project laterally and are formed at the junction of the pedicles and laminae.
3. Two superior articular (apophyseal) processes (with articular surfaces) that project upward and face dorsally.
4. Two inferior articular (apophyseal) processes (with articular surfaces) that project downward and face ventrally.

Articulation of Typical Vertebrae

A typical vertebra articulates with contiguous vertebrae as follows:

1. The superior and inferior surfaces of the body articulate with the bodies of the adjacent superior and inferior vertebrae through intervertebral disks of elastic fibrocartilage interposed between the bodies of the articular vertebrae to act as cushions.
2. The articular surfaces of the superior articular processes articulate with the articular surfaces of the inferior articular processes of the vertebra above.
3. The articular surfaces of the inferior articular processes articulate with the articular surfaces of the superior articular processes of the vertebra below.

DIFFERENTIATING CHARACTERISTICS OF TYPICAL VERTEBRAE BY REGIONS

Cervical Vertebrae. C-3 to C-7 have small oval-shaped bodies and the spinous processes of all these, except C-7, are short and bifid (cleft).C-7 has a long, thick, prominent, nonbifurcated process that is a vertebral landmark. Each of the cervical vertebrae has a hole in the transverse process called the transverse foramen.

Thoracic Vertebrae

The thoracic vertebrae, designated as T-1 to T-12 have large bodies. Their spinous process point inferiorly and all of them present facets (most also demifacets) on the lateral aspects of their bodies that articulate with the ribs.

Lumbar Vertebrae

Vertebrae designated L-1 to L-5 have very large bodies. Their spinous processes are broad and project horizontally. The superior articular process presents on its posterior margin, the mammillary process.

ATYPICAL VERTEBRAE

Atypical vertebrae are those vertebrae whose structure is highly modified by function and position. They consist of the first cervical vertebra (C-1) or atlas, the second cervical vertebra (C-2) or axis, the sacrum, and the coccyx.

The Atlas

The first cervical vertebra (C-1) is named the atlas because it supports the head. It is characterized by the absence of both body and spinous process and consists of an anterior and posterior arch, two lateral masses, and a vertebral foramen. The anterior surface of the anterior arch presents a slight projection, the anterior tubercle. The posterior surface is marked by a dental facet for articulation with the dens or odontoid process of the axis. On the superior surface are two grooves for the vertebral arteries.

The morphology of the atlas affords freedom of movement of the skull. The body of the atlas is transferred to the axis (second cervical vertebra) where it becomes the dens (odontoid process), which articulates with the dental facet (facet for odontoid) of the anterior arch of the atlas, thus making possible the rotary movements of the skull. The superior articular surfaces of the atlas are concave for reception of the condyles of the occipital bones, permitting flexion, extension, and hyperextensions of the skull.

The Axis

The second cervical vertebra (C-2) is named the axis, or epistropheus, because it forms the pivot upon which the atlas rotates when the head is turned from side to side. The axis differs primarily from a typical vertebra by the presence of a tooth like projection, called the dens or odontoid process, which rises perpendicularly from the upper surface of the body. On its anterior surface, the dens presents an oval facet for articulation with the dental facet on the anterior arch of the atlas. On its posterior surface is a shallow groove that receives the transverse ligament of this articulation.

The Sacrum

In the adult, the sacrum is a single bone formed by the fusion of the five sacral segments. It is a large wedge-shaped (triangular) bone situated in the lower part of the vertebral column and at the upper and back part of the pelvic cavity where it is wedged between the two hipbones. Its base is directed upward and its apex is directed downward. In the center, the base presents the kidney-shaped body; behind this is the superior opening of the sacral canal, which is bounded laterally by the articular processes. The sacral promontory is a prominent ridge at the upper anterior margin of the body. The body articulates with the body of L-5 to form the lumbosacral articulation. On either side of the body is a wing-like surface called the ala (wing) of the sacrum.

The dorsal surface of the sacrum is rough and convex. In the middle line, the dorsal surface displays a ridge, the median sacral crest, made up of three or four rudimentary spinous processes that are more or less fused to form the crest. There are four posterior sacral foramina, which transmit several sacral nerves.

The ventral or pelvic surface of the sacrum is smooth and concave. The anterior sacral foramina transmit some of the sacral nerves.

The lateral surface or margin of the sacrum presents in front an ear shaped surface for articulation with the articular surface of the ilium.

The Coccyx

The coccyx usually consists of three to five rudimentary coccygeal segments that fuse in adult life. From its base downward to its apex, the coccyx diminishes in size. It curves downward and forward from its articulation with the sacrum, often deviating from the median plane of the body.

VERTEBRAL LANDMARKS

Study these carefully. Note other structures at similar levels.

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