The intervertebral disc

Why it matters

The discs are what let the column bend while carrying weight. Together they make up about 25 % of body height between the top of the pelvis and the base of the skull^[1]. A weakened disc can press on a spinal nerve and cause pain or weakness in the parts of the body that nerve supplies ^[1].

What it is

An intervertebral disc is a fibrocartilaginous pad that fills the gap between adjacent vertebral bodies ^[1]. Each disc has two parts ^[1]:

• the anulus fibrosus, the tough, fibrous outer layer, a ring firmly anchored to the outer margins of the two vertebral bodies ^[1];
• the nucleus pulposus inside it, a softer, gel-like material ^[1].

As a joint, the disc forms a wide symphysis between the bodies of adjacent vertebrae: a cartilaginous joint in which the bones are joined by fibrocartilage ^[2].

What it does

The discs strongly unite the bodies of neighbouring vertebrae and provide padding between them during weight bearing ^[1]. They are also flexible and change shape to allow the column to move ^[1]. Because they carry weight, they are thin in the cervical region and thickest in the lumbar region, which carries the most body weight ^[1].

How it works

The nucleus pulposus has a high water content that resists compression, which makes it important for weight bearing ^[1]. Its gel-like nature lets the disc change shape as one vertebra rocks on its neighbour: bending forward compresses the front of the disc and expands the back ^[1].

Normal behaviour

With increasing age the water content of the nucleus pulposus gradually declines ^[1]. The disc becomes thinner, total body height decreases somewhat, and the disc's flexibility and range of motion fall, making bending more difficult ^[1].

When it does not behave

If the back of the anulus fibrosus is weakened by injury or age, the pressure on the disc when bending forward and lifting a heavy object can push the nucleus pulposus out backwards through it: a herniated disc ^[1]. The bulge can compress a spinal nerve where it leaves through the intervertebral foramen ^[1].

• The most common sites are the L4/L5 and L5/S1 discs, which can cause sciatica, pain radiating from the lower back down the thigh into the leg ^[1].
• Similar injuries of the C5/C6 or C6/C7 discs, after forcible hyperflexion of the neck, can produce pain in the neck, shoulder and upper limb ^[1].

Common misconceptions

Check yourself

Why are the lumbar discs thicker than the cervical discs?

The lumbar region carries the most body weight, and the discs pad the vertebrae during weight bearing ^[1].

What happens to the back of a disc when you bend forward?

It expands while the front is compressed ^[1].

Read next

• The L4–L5 motion segment.
• The L4–L5 disc in the atlas.