The Vertebral Column

The vertebral column (also known as the backbone or the spine), is a column of approximately 33 small bones, called vertebrae. The column runs from the cranium to the apex of the coccyx, on the posterior aspect of the body. It contains and protects the spinal cord

This this article, we shall look at the functions of the vertebral column as a whole, the general anatomical structure of a vertebrae, and look at some of the clinical syndromes that can occur.


Fig 1.0 - The vertebral column viewed from the side. The five different regions are shown and labelled.

Fig 1.0 – The vertebral column viewed from the side. The five different regions are shown and labelled.

Functions

The most important functions of the vertebral column are as follows:

  • Protection: it encloses the spinal cord, shielding it from damage.
  • Support: it carries the weight of the body above the pelvis (below the pelvis, the lower limbs take over).
  • Axis: the vertebral column forms the central axis of the body.
  • Movement: it has roles in both posture and movement.

The vertebral column can be separated into five different regions. Each region is characterised by a different vertebral structure. Before looking, at individual structures, we first need to look at the general blueprint of a vertebrae.


Vertebral Structure

Although the vertebrae do have significant differences in size and shape between groups, they have the same basic structure. Each vertebrae consists of a vertebral body, situated anteriorly, and a posterior vertebral arch.

Vertebral Body

This is the anterior part of the vertebrae. It it the weight baring component, and its size increases as the vertebral column descends (having to bare increasing amounts of weight each time). The superior and inferior aspects of the vertebral body are lined with hyaline cartilage.

Due to their dense bony structure, a radiograph of the vertebral column can be used clinically to diagnose osteoporosis.

Adjacent vertebral bodies are separated by a fibrocartilginous intervertebral disk.

1.1 - The Vertebral arch and body

1.1 – The Vertebral arch and body.

Clinical Relevance: Herniated Intervertebral Disks

The intervertebral disk is a fibrocartilage cylinder that lies between the vertebrae, joining them together. They act to permit the flexibility of the spine, and as a shock absorber. In the lumbar and thoracic regions, they are wedge shaped, supporting the curvature of the spine.

There are two regions in the vertebral disk; the nucleus pulposus and annulus fibrosus. The annulus fibrosis is tough and collagenous, surrounding the nucleus pulposus. The nucleus pulposus is jelly like, and is located posteriorly.

In a herniation of the intervertebral disk, the nucelus pulposus ruptures, breaking through the outer layer. This occurs in a posterior and lateral direction, putting pressure on the spinal cord, resulting in a variety of neurological and muscular symptoms.

Fig 1.2 - Diagram of a typical intervertebral disk, and MRI of a herniated disk.

Fig 1.2 – Diagram of a typical intervertebral disk, and MRI of a herniated disk.

Vertebral Arch

The vertebral arch refers to the lateral and posterior parts of the vertebrae.

With the vertebral body, the vertebral arch forms an enclosed hole, called a vertebral foramenThe foramina of the all vertebrae line up to form the vertebral canal, which encloses the spinal cord.

The vertebral arches have a number of bony prominences, acting as attachment sites for muscles and ligaments:

  • Pedicles: There are two of these, one left and one right. They point posteriorly, meeting the flatter laminae.
  • Lamina: The bone between the transverse and spinal processes
  • Transverse processes: These extend laterally and posteriorly away from the pedicles. In the thoracic vertebrae, the transverse processes articulate with the ribs
  • Articular processes: At the junction of the lamina and the pedicles,  superior and inferior processes arise. These articulate with the articular processes of the vertebrae above and below.
  • Spinous processes: Posterior and inferior projection of bone, a site of attachment for muscles and ligaments
Fig 1.2 - A superior view of a lumbar vertebrae. The features of the vertebral arch are labelled.

Fig 1.3- A superior view of a lumbar vertebrae. The features of the vertebral arch are labelled.


Classifications of Vertebrae

Cervical Vertebrae

Fig 1.3 - Distinguishing features of a cervical vertebrae. Note the triangular shape of the vertebral foramen.

Fig 1.3 – Distinguishing features of a cervical vertebrae. Note the triangular shape of the vertebral foramen.

There are seven cervical vertebrae in the human body. They have three main distinguishing features:

  • The spinous process bifurcates into two, known as a bifid spinous process.
  • There is also a foramen (known as foramen transversarium) in the each transverse process. The the vertebral arteries pass through the holes in each vertebrae as they ascend to supply the brain.
  • The vertebral foramen in triangular in shape

There are some cervical vertebrae that are unique within themselves. C1 and C2 (called the atlas and axis respectively), are specialised to allow for the movement of the head.

The C7 vertebrae has a much longer spinous process, which does not bifurcate.


Thoracic Vertebrae

The twelve thoracic vertebrae are medium sized, and increase in size as they move down the back. Their main function is to articulate with ribs, producing the bony thorax.

Each thoracic vertebrae has two ‘demi facets‘ on each side of its vertebral body. These articulate with the head of its respective rib, and the rib inferior to it. On the transverse processes of the thoracic vertebrae there is a costal facet for articulation with its respective rib.

The spinous processes are slanted inferiorly and anteriorly. This offers increased protection to the spinal cord, preventing an object like a knife entering the spinal canal through the intervetebral discs.

In contrast to the cervical vertebrae, the vertebral foramen is circular.

Fig 1.3 - Lateral view of a thoracic vertebrae.

Fig 1.3 – Lateral view of a thoracic vertebrae.


Lumbar Vertebrae

These are the largest of the vertebrae, of which there are five. They act to support the weight of the upper body, and have various specialisations to enable them do this.

Lumbar vertebrae have very large vertebral bodies, which are kidney shaped. They lack the characteristic features of other vertebrae, with no foramen transversarium, costal facets, or bifid spinous processes.

However, like the cervical vertebral, they have a triangular shaped vertebral foramen.


Sacrum and Coccyx

Fig 1.4 - Diagram of the sacrum and coccyx, articulating with the pelvic bones

Fig 1.4 – Diagram of the sacrum and coccyx, articulating with the pelvic bones

The sacrum is a collection of five fused vertebrae. It is described as a upside down triangle, with the apex pointing inferiorly. On the lateral walls of the sacrum are facets, for articulation with the pelvis at the sacro-iliac joints.

The coccyx is a small bone, which articulates with the apex of the sacrum. It is recognised by its lack of vertebral arches. Due to the lack of vertebral arches, there is no vertebral canal, and so the coccyx does not transmit the spinal cord.


Joints

For every vertebrae, there are five articulations. The vertebral bodies indirectly articulate with each other, and the articular processes also form joints.

The vertebral body joints are cartilaginous joints, designed for weight bearing. The articular surfaces are covered by hyaline cartilage, and  are connected by a fibrocartilage intervertebral disk. There are two ligaments that strengthen these joints; the anterior and posterior longitudinal ligaments. The anterior longitudinal ligament is thick and prevents hyperextension of the vertebral column. The posterior longitudinal ligament is weaker and prevents hyperflexion.

The joints between the articular facets are called facet joints. These allow for some gliding motions between the vertebrae. They are strengthened by various ligaments:

  • Ligamentum Flava: extends from lamina to lamina.
  • Infraspinous and Supraspinous: joins the spinous processes together.
  • Intertransverse ligaments: extends between transverse processes.

Clinical Relevance: Abnormal Morphology of the Spine

There are several clinical syndromes resulting from an abnormal curvature of the spine:

Kyphosis: Excessive thoracic curvature, causing a hunchback deformity.

Lordosis: Excessive lumbar curvature, causing a swayback deformity.

Scoliosis: A lateral curvature of the spine, usually of unknown cause.

Cervical Spondylosis: A decrease in the size of the intervertebral foramina, usually due to degeneration of the joints of the spine. The smaller size of the intervertebral foramina puts pressure on the exiting nerves, causing pain.

Fig 1.5 - Deformities of the  Spine. a) - Kyphosis b) - Lordosis c) Scoliosis

Fig 1.5 – Deformities of the Spine.a) – Kyphosisb) – Lordosisc) Scoliosis

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