Intervertebral

What it is

The joint between the bodies of two adjacent vertebrae, formed by an intervertebral disc.

Each disc has two parts: a tough outer ring of fibrous layers, the anulus fibrosus, arranged in alternating diagonal directions like plywood; and a soft, gel-like centre, the nucleus pulposus.

The disc is bonded firmly to the vertebral bodies above and below through cartilage end plates. It is not a loose pad sitting between bones.

There are 23 of them, and together they make up a substantial share of the length of the spine.

What it does

Permits a small amount of movement between each pair of vertebrae, and transmits load down the column.

The key idea is that individually the movement is small; collectively it is large. No single intervertebral joint bends much. Stacked twenty-three times, the result is a spine that flexes, extends, side-bends and rotates substantially.

The disc distributes compressive load across the vertebral surface, and the gel centre lets it do so evenly regardless of the direction the load arrives from.

What loads it

Everything. Upright posture alone loads the lumbar discs continuously, and they carry more when the trunk is bent forward or holding weight away from the body.

What is commonly got wrong about it

"Slipped disc". Discs do not slip. They are firmly attached to the vertebrae above and below. A disc can bulge, or its outer ring can tear and let the inner material herniate — but nothing slides out of place, and the phrase gives a misleading picture of a loose object.

That a disc finding on a scan explains pain. This is one of the best-documented mismatches in musculoskeletal medicine. A systematic review of 33 studies covering 3,110 people with no back pain at all found disc degeneration in 37% of 20-year-olds rising to 96% of 80-year-olds, and disc bulges in 30% rising to 84%. The authors conclude that many such findings are likely part of normal ageing and unassociated with pain.¹

That discs cannot heal. Herniations frequently reduce or resolve over time without intervention.

That the spine is fragile. It carries substantial load routinely. Back pain is common, important, and outside what this site addresses — but it is not evidence of a fragile structure.

Sources

1. Brinjikji W, Luetmer PH, Comstock B, et al. Systematic literature review of imaging features of spinal degeneration in asymptomatic populations. American Journal of Neuroradiology 2015; 36(4):811–816. doi:10.3174/ajnr.A4173 PMID 25430861 33 studies, 3,110 people with no symptoms. Disc degeneration present in 37% of 20-year-olds rising to 96% of 80-year-olds; disc bulge 30% to 84%; protrusion 29% to 43%. The authors conclude many such findings are likely part of normal ageing and unassociated with pain.

Every source here was checked against PubMed rather than recalled. Where this site says something is disputed or unsupported and no source appears, that is deliberate: it means the claim is being conceded rather than cited.