Intervertebral discs

At a glance

English

Intervertebral discs

Latin (TA2)

Disci intervertebrales

System

Cartilage

Region

Back

In order

From the intervertebral disc c2-c3 to the intervertebral disc l5-s1

About the intervertebral discs

The intervertebral discs are the pads of fibrocartilage that join the bodies of neighbouring vertebrae. Each forms a secondary cartilaginous joint, or symphysis, which binds the two bones firmly together while letting them tilt and twist a little on each other. Added up along the spine these small movements give the trunk and neck their range of bending and turning, and the discs also spread and soften the loads passing down the column. Together they make up roughly a quarter of the length of the spine above the sacrum. There is no disc between the skull and the atlas or between the atlas and the axis: the atlas turns round the dens of the axis at synovial joints instead. The first disc therefore lies between C2 and C3 and the last between L5 and the sacrum. Each is named after the vertebrae it separates, so the disc below the fifth cervical vertebra and above the sixth is the C5-C6 disc.

Every disc has the same plan. The outer anulus fibrosus is a ring of concentric sheets, or lamellae, of fibrocartilage. Collagen fibres run obliquely in each sheet and in the opposite direction in the next, an arrangement that resists twisting and bulging; the tougher type I collagen is concentrated towards the outside. Within the ring lies the nucleus pulposus, a soft, gel-like core derived from the embryonic notochord. It holds water by means of proteoglycans such as aggrecan, whose negatively charged sugar chains draw fluid in, and under load it presses outwards evenly against the anulus like a fluid cushion. Above and below, thin plates of hyaline cartilage, the vertebral end plates, cover the bone. In the adult the disc has almost no blood vessels of its own and is fed by diffusion through the end plates; only the outer anulus receives nerve endings.

The discs change along the column. They are thinnest in the upper thoracic region and thickest in the lumbar region, matching the load each level carries. In the cervical and lumbar regions they are thicker in front than behind, helping to create the forward curves of neck and lower back; in the thoracic region the backward curve depends more on the shape of the vertebral bodies. Cervical discs are small and lie between bodies whose raised lateral lips form small uncovertebral joints at the sides. The lumbar discs are the largest, and the L5-S1 disc is usually wedge-shaped where the spine angles onto the sacrum.

With age the nucleus loses water and proteoglycan, the disc becomes thinner and less springy, fissures appear in the anulus and the end plates thin; this contributes to the loss of height in later life. These changes come with ageing in everyone and often cause no pain. If the anulus gives way, part of the nucleus may push out, usually backwards and to one side, as a disc herniation, popularly called a slipped disc although nothing slips. The bulge can press on a spinal nerve root, causing pain, tingling, numbness or weakness in the area that root serves, such as sciatica from the lower lumbar discs. A large central herniation in the lumbar region can compress the cauda equina and needs urgent treatment. Nucleus material may also break vertically into the vertebral body, forming a Schmorl's node. Pain arising from the disc itself is described as discogenic. On radiographs the disc space between vertebral bodies reflects disc height and narrows in degeneration or in discitis, infection of the disc.