A herniated disc usually is caused by wear and tear of the disc (also called disc degeneration). As we age, our vertebral discs lose some of the fluid that helps them maintain flexibility. A herniated disc also may result from injuries to the spine, which may cause tiny tears or cracks in the outer layer (annulus or capsule) of the disc. The jellylike material inside the disc (nucleus) may be forced out through the tears or cracks in the capsule, which causes the disc to bulge, break open (rupture), or break into fragments. See an illustration of a herniated disc.

Injury to the disc can occur from:

• A sudden heavy strain or increased pressure to the lower back. Sometimes a sudden twisting movement or even a sneeze will force some of the nucleus (the material inside the disc) out through the disc's outer layer (annulus or capsule).
• Activities that are done over and over again that may stress the lower back, including poor lifting habits, prolonged exposure to vibration, or sports-related injuries.