An intervertebral disc is a fibrocartilaginous structure found between the bodies of vertebra. These discs are located between each pair of vertebra in the spine except for those at the first and second cervical level (called atlas and axis). Think of this disc as a jelly doughnut: it has an outer fibrous ring that surrounds a softer center. The outer annular rings are thinnest posterior and lateral; most degeneration occurs at this location due to the weakness of the disc wall. The disc functions as a cushion, allows for movement, and serves as a cartilaginous joint between adjacent vertebrae.
A slipped disc is a misnomer because the disc does not actually slip out of place between the vertebrae. Instead, a slipped disc occurs when the outer walls of the intervertebral disc tear. Other terms for this condition are herniated disc, prolapsed disc, or ruptured disc. When outer annular rings are torn the central gel-like nucleus pulposus can push out and place pressure on the other structures in the area. The slipped disc usually occurs at the posterior lateral wall where the annulus fibrosis is thinnest. Unfortunately this is where the nerves of the spine exit and track distally into the body. The larger the tear, and the further the nucleus pushes out into the surrounding area, the more severe the symptoms. The majority of slipped discs occur in the lumbar spine, at levels L4-L5 and L5-S1. A slipped disc in the lumbar spine can affect the sciatic nerve, creating a condition called sciatica.