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. It can also occur when the disc is damaged by trauma.