The Vertebral Column
- 1 Functions
- 2 Vertebral Structure
- 3 Classifications of Vertebrae
- 4 Joints
- 5 Clinical Relevance: Abnormal Morphology of the Spine
The vertebral column is a vertical series of approximately 33 small bones (known as vertebrae), which are separated by intervertebral discs.
It can be separated into five different regions, with each region characterised by a different vertebral structure.
In this article, we shall look at the anatomy of the vertebral column – its structure, joints and ligaments.
The vertebral column has four main functions:
- Protection – encloses and protects the spinal cord within the spinal canal.
- Support – carries the weight of the body above the pelvis.
- Axis – forms the central axis of the body.
- Movement – has roles in both posture and movement.
All vertebrae share a basic common structure. They each consist of a vertebral body, situated anteriorly, and a posterior vertebral arch.
The vertebral body is the anterior part of the vertebrae. It is the weight-bearing component, and its size increases as the vertebral column descends (having to support increasing amounts of weight).
The superior and inferior aspects of the vertebral body are lined with hyaline cartilage. Adjacent vertebral bodies are separated by a fibrocartilginous intervertebral disc.
Clinical Relevance: Herniated Intervertebral Discs
The intervertebral disc is a fibrocartilaginous cylinder that lies between the vertebrae, joining them together. They act to permit the flexibility of the spine, and also act 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 disc; the nucleus pulposus and annulus fibrosus. The annulus fibrosus is tough and collagenous, surrounding the nucleus pulposus. The nucleus pulposus is jelly-like, and is located posteriorly.
In a herniation of the intervertebral disc, the nucleus pulposus ruptures, breaking through the annulus fibrosus. This can occur in a posterior direction, putting pressure on the spinal cord, resulting in a variety of neurological and muscular symptoms.
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 foramen. The 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, which act 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 spinous 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.
Classifications of Vertebrae
There are seven cervical vertebrae in the human body. They have three main distinguishing features:
- Bifid spinous process – the spinous process bifurcates at its distal end.
- Exceptions to this are C1 (no spinous process) and C7 (spinous process is longer and does not bifurcate).
- Transverse foramina – an opening in each transverse process, which transmit the vertebral arteries
- Triangular vertebral foramen
There are some cervical vertebrae that are unique. C1 and C2 (called the atlas and axis respectively), are specialised to allow for the movement of the head.
For more information on the cervical spine, click here.
Each thoracic vertebrae has two ‘demi facets‘ on each side of its vertebral body. These articulate with the head of the 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 intervertebral discs.
In contrast to the cervical vertebrae, the vertebral foramen is circular. For more on the thoracic spine, click here.
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 transverse foramina, costal facets, or bifid spinous processes.
However, like the cervical vertebrae, they have a triangular-shaped vertebral foramen.
Sacrum and Coccyx
The sacrum is a collection of five fused vertebrae. It is described as an inverted 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.
Separation of S1 from the sacrum is termed “lumbarisation”, while fusion of L5 to the sacrum is termed “sacralisation”. These conditions are congenital abnormalities.
Each vertebra has 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 disc. 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 Flavum: extends from vertebral lamina to lamina.
- Interspinousand Supraspinous ligaments: These join the spinous processes of adjeacent vertebra. The interspinous ligaments attach between processes, and the supraspinous ligaments attach to the tips.
- 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.