Key takeaways
- Each disc has a tough outer ring (annulus) and a jelly-like center (nucleus).
- A bulge is a disc that has flattened and spread out evenly around its edges, usually from wear.
- A herniation or protrusion involves a tear in the outer ring that lets the center push outward.
- An extrusion is a larger herniation that extends up or down beyond the disc space.
Anyone who has had back pain has probably spent some time searching the internet for answers. Along the way you've likely come across disc bulges, disc herniations, protrusions and extrusions. Aren't they all the same thing? The answer is yes and no.
Start with the anatomy
Your lower back is made up of the vertebrae and the sacrum, with a disc between each pair of vertebrae. Discs are made of cartilage. They let your back bend forward and back, help hold the spine together, and act as cushions between the bones.
The healthier the disc, the more space there is between the vertebrae. Each vertebra-disc-vertebra unit forms a tunnel on each side where a nerve exits to supply a specific part of the body. These tunnels are normally much larger than the nerve itself, which is why a disc can wear somewhat without pinching anything.
What is a disc bulge?
Each disc has two distinct parts: a tough, fibrous outer ring called the annulus and a jelly-like center called the nucleus. Picture a plump, healthy disc acting like a pillow between two vertebrae.
As a disc degenerates, it loses some of its structure. Think of a balloon being pressed from the top and bottom: its height drops and its sides push outward. That's a disc bulge. It's usually broad and even, and it's very common with age.
What is a disc herniation or protrusion?
Like a bulge, a herniation can extend into a nerve tunnel and press on the nerve. The difference is that a herniation involves a tear. A crack forms in the outer ring, running from the edge of the disc toward the nucleus, and the jelly-like center pushes out through it, creating a focal out-pouching. This is called a disc herniation or, often, a disc protrusion.
What is a disc extrusion?
An extrusion describes a larger herniation, one big enough that the herniated material extends up or down past the disc space, covering part of the vertebra above or below.
Does the label matter?
The terms describe what the disc looks like, not how much it hurts. Plenty of people have bulges or even herniations on an MRI with no pain at all (see Is your MRI telling the whole story?). What matters is whether the disc is actually irritating a nerve, and that is determined by putting your exam, your symptoms and your imaging together.