Annulus fibrosus

The annulus fibrosus is the tough outer ring of each intervertebral disc, the cushions that sit between the vertebrae. It wraps around the gel-like centre of the disc, the nucleus pulposus, and together with thin cartilage endplates above and below, the two make up the disc. The name means “fibrous ring”, and anatomy texts often spell it anulus (Kenhub).

What is the annulus fibrosus made of?

The annulus is built from 15 to 25 concentric layers, called lamellae. Within each layer, collagen fibres run parallel to each other at roughly 60 degrees to the vertical, and the direction alternates from one layer to the next, a little like the grain in plywood. Elastic fibres between the layers may help the disc recover its shape after you bend forwards or backwards. The result is relatively stiff: it gives the disc its strength and resists compression (Kenhub).

Two features shape how the annulus ages and hurts:

• No blood supply. In adults, discs are the largest structures in the body without their own blood vessels. They absorb the nutrients they need from the surrounding circulation instead (Kenhub).
• Few nerves. A healthy disc is poorly supplied with nerves, mostly in its outer layers. Degenerated discs can become densely supplied, even in regions that normally have none, and this has been linked with pain that comes from the disc itself (García-Cosamalón et al., 2010).

Why does the annulus fibrosus matter for people who sit?

A classic experiment measured the pressure inside a healthy disc in the lumbar spine of one 45-year-old man through a full day (Wilke et al., 1999):

• relaxed standing: 0.5 MPa
• sitting without a backrest: 0.46 MPa
• relaxed, slouched sitting: 0.3 MPa
• sitting bent fully forward: 0.83 MPa
• lifting 20 kg with a rounded back: 2.3 MPa

Sitting, in other words, did not load the disc more than standing, which contradicted older measurements. The authors were cautious because it was a single person, but they concluded that constantly changing position helps fluid, and with it nutrition, flow into the disc. That is a better reason to move during the day than fear of any one posture. The guides to setting up an office chair and microbreaks cover the practical side.

What is an annular tear?

An annular tear, or fissure, is a split between or across the layers of the ring. The annulus becomes weaker with age, which raises the risk of tearing (Kenhub).

Tears are common in people without symptoms. A review of 33 studies covering 3,110 pain-free people found annular fissures in 19% of 20-year-olds and 29% of 80-year-olds. The authors concluded that many such findings are likely part of normal ageing and need to be read alongside a person’s symptoms (Brinjikji et al., 2015). In a degenerating disc, material from the nucleus can track through the outer annulus, which is thought to be one source of disc pain (Raj, 2008).

When part of the nucleus pushes out through the ring, the result is a herniated disc. A broad, even spread beyond the disc’s usual edge is called a bulging disc. The NHS also advises not smoking, because nicotine weakens disc tissue (NHS).

When to see a doctor

See a GP if back pain is not improving after a few weeks or painkillers are not helping, or if it comes with a very high temperature, unexplained weight loss, a swelling in your back, or pain that is worse at night. Call 999 or go to A&E if back pain comes with numbness around your bottom or genitals, you cannot pee or control when you pee or poo, you lose feeling in one or both legs, or it started after a serious accident (NHS). Pain that travels down the leg is covered under sciatica.