A prolapsed (herniated) disc occurs when the outer fibres of the intervertebral disc (its annulus) are injured, and the soft material known as the nucleus pulposus, ruptures out of its enclosed space.
The prolapsed or ruptured disc material can enter the spinal canal, compressing the spinal cord, but more frequently the spinal nerves.
Herniated discs are most common in young and middle-aged adults. A herniation may develop suddenly, or gradually over weeks or months. Disc herniations most commonly occur in the lumbar spine, but also occur in the cervical spine.