It is a type of intervertebral disc herniation and is distinguished from a disc protrusion in that it: in at least one plane, has a broader dome (B) than a neck (A).
Diffuse bulging of disk beyond endplates can be seen with disk degeneration, whereas herniation is defined as a localized or focal disk material displacement beyond the limits of the disk space. The disk material may be composed of ≥ 1 combinations of nucleus, annular tissue, cartilage, and fragmented apophyseal bone. Herniated disks may be further categorized into disc protrusion or disc extrusion on the basis of the morphology of the displaced material. A protrusion is used to describe disk material presenting outside the disk space when its edges outside the disk space are less than the distance between the edges of the base of the disk material extending outside the disk space. An extrusion is present when any one distance between the edges of the disk material beyond the disk space is greater than the distance between the edges of the base of the disk material beyond the disk space 1) 2).