Stages in prolapse of an intervertebral disc.
a - The annulus fibrosus is torn but there has been no extrusion of nucleus pulposus.
b - Extrusion of nuclear material through the rent. The posterior longitudinal ligament is stretched but the protrusion has not reached the nerve.
c - The protrusion is larger and the nerve is stretched over it. Sometimes a fragment of the torn annulus itself protrudes backwards.