Discs are small circular cushions between the bones in the spine. The bones are called vertebrae. The discs are compressible. They act as cushions for the vertebrae. A herniated disc happens when discs in the spine bulge from their proper place. This is most common in the lower spine.

Herniated Lumbar Disc
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Herniated discs can occur when discs lose water content, become flatter, and provide less cushioning. If they become too weak, the outer part may tear. The inside part of the disc may then push through the tear. This can put pressure on the nerves next to the disc.