Intervertebral Disc 

The intervertebral disc is a fibro cartilaginous disc that binds the two adjacent vertebral bodies, from the axis or second cervical vertebra to the sacrum. Morphologically, the intervertebral disc is a segmental structure as opposed to the vertebral body which is intersegmental.

Shape

The intervertebral disc shape corresponds to that of the vertebral bodies between which it is placed.

Thickness

The intervertebral disc varies in different regions of the column and in different parts of the same disc. In the cervical and lumbar regions, the intervertebral discs are thicker in front than behind, while in the thoracic region, they are of uniform thickness. The intervertebral discs are thinnest in the upper thoracic and thickest in the lumber regions.

The intervertebral disc contributes about one-fifth of the length of the vertebral column. Such contribution is greater in the cervical and lumbar regions than in the thoracic region.

Function of IVD

1. Intervertebral discs give shape to the vertebral column
2. Intervertebral discs act as a remarkable series of shock absorbers or buffers. Each intervertebral disc may be likened to a “COILED UP SPRING “.Should the confining walls be damaged the spring will bulge out at the weak area.

Structure of IVD

1. The nucleus pulposus is approximately 80% water, depending on age and time of day. Proteoglycans make up approximately 65% of dry weight and have the ability to attract water molecules because of the presence of glycosaminoglycans hence the high water content. The nucleus has both type 1 and type 2 collagen; type 2 predominates because of its ability to resist compressive loads.
2. Annulus fibrosis is approximately 60% water depending on age and time of day. Proteoglycans contribute approximately 20% and collagens fibers 50% of the dry weight. These are made up of type 1 and type 2 fibers; type 1 collagens predominate, particularly in the outer portions.

Articulations

Two main types of articulations are found in the vertebral column are; cartilaginous joints of the symphysis type between the vertebral bodies.  The joint between the vertebral bodies of the vertebral column are referred to as the interbody joints and the joints between the zygapophyseal facets are called the zygapophyseal joints.

Ligaments

Anterior longitudinal ligament

This ligament runs along the anterior and lateral surface from the sacrum to 2nd cervical vertebral. They are made up of two thick bundles of collagen fibers layers ;

— superficial layer is long and bridge several vertebra,

—Deep layer is short and run between single pair of vertebra,

Anterior longitudinal ligament  is compressed during flexion and stretched during extension .It is stronger than posterior longitudinal ligament .

Posterior longitudinal ligament 

This ligament runs along the posterior aspect of vertebral bodies from C2 to the sacrum . They are also made up of two layer;

—Superficial layer   is around all mobile segment ,

—Deep layer is extend only to adjacent vertebra,

The posterior longitudinal ligament is stretched during flexion and compressed(slack) during extension.

Interspinous ligament 

Interspinous ligament connect spinous process of adjacent vertebra. It is fibrous sheet consist of  type 1 collagen , Proteoglycans , elastic fibers. 

Function

1. Resist flexion

2. Resist end range of extension

SUPRASPINOUS LIGAMENT  

The supraspinous ligament is a strong cord-like structure,which connect the tips of the spinous process.

—FUNCTION

1 –  Resist separation of the spinous process

LIGMENTUM FLAVUM

—It is the thick elastic ligament that connects to lamina from C2 to the sacrum

INTERTRANSVERSE LIGAMENTS

The intertransverse ligament is a ligament that passes between traverse processes.  this type of ligament is stretched and compressed, during lateral bending.

Biomechanics of  vertebral column 

1. Flexion

Flexion is a method in which the vertebral column tilt anteriorly and the inferior facets slide upward on the vertebral facet below. it leads to compression of the anterior disc

2. Extension

An extension is a method in which the vertebral column tilts posteriorly and the inferior facets slide downward on the vertebral facet below. It leads to compression of the posterior disc.

3. Lateral flexion

Lateral flexion is a method in which the vertebral column tilts laterally. annulus fibrosis is compressed on the concavity and stretched on the convexity of the curve.

4. Rotation

Rotation is available in each spinal region.