Herniated discs are extremely common back injuries. Herniated disc disease is due to a change in the structure of the normal disc. Most of the time, disc disease comes as a result of aging and the degeneration that occurs within the disc. Occasionally, severe trauma can cause a normal disc to herniate. Trauma may also cause an already herniated disc to worsen. They occur when the thick outer layer (annulus fibrosus) of an intervertebral spinal disc bulges or ruptures. Sometimes, the gel-like interior of the disc (nucleus pulposus) will actually break through the outer layer and leak out into the body. This is called a ruptured or extruded disc.