localized herniation displacement of disc material beyond the limits of the intervertebral disc space (involving less than 50% (180°) of the disc circumference).
< 25% of the disc circumference
between 25 and 50% of the disc circumference
Presence of disc tissue "circumferentially" (50-100%) beyond the edges of the ring apophyses : not considered a form of herniation, nor are diffuse adaptive alterations of disc contour secondary to adjacent deformity as may be present in severe scoliosis or spondylolisthesis
Bulging discs may be symmetrical or assymmetrical
Protrusion: depth in any plane < base of herniation in the same plane
Extrusion: depth in any plane > base of herniation in the same plane, or no continuity exists between the disc material beyond the disc space and that within the disc space
Because posteriorly displaced disc material is often constrained by the posterior longitudinal ligament, images may portray a disc displacement as a protrusion on axial sections and an extrusion on sagittal sections, in which cases the displacement should be considered an extrusion.
Sequestration: Extrusion may be further specified as sequestration, if the displaced disc material has lost completely any continuity with the parent disc (C)
Migration: displacement of disc material away from the site of extrusion, regardless of whether sequestrated or not (B,C)
Degree of Canal Compromise: (assessed from axial section at level of most severe compromise)
<1/3 = "mild," 1/3 - 2/3 = "moderate," >2/3 = "severe." The same grading can be applied for foraminal involvement.
Anatomic "zones" and "levels" are defined using the following landmarks: medial edge of the articular facets; medial, lateral, upper, and lower borders of the pedicles; and coronal and sagittal planes at the center of the disc (Figure 16).
On the horizontal (axial) plane, these landmarks determine the boundaries of the "central zone," the "sub-articular zone," the "foraminal zone," the "extra-foraminal zone," and the "anterior zone," respectively .
On the sagittal (cranio-caudal) plane, they determine the boundaries of the "disc level," the "infra-pedicular level," the "pedicular level," and the "supra-pedicular level," respectively (Figure 18).
Hence, "central," "right central," "right sub-articular," "right foraminal," or "right extra-foraminal." For reporting of image observations of a specific disc, "right central" or "left central" should supersede use of the term "paracentral." The term "far lateral" is sometimes used synonymously with "extra-foraminal."
Grade 0 (normal): No compromise of thenerve root is seen. There is no evident contact of disk materialwith the nerve root, and the epidural fat layer between thenerve root and the disk material is preserved
Illustration: A normal epidural fat layer (black arrowheads) is visible between the nerve root (arrows) and the disk material (white arrowheads)
Grade1 (contact): There is visible contact of disk material withthe nerve root, and the normal epidural fat layer between thetwo is not evident. The nerve root has a normal position, andthere is no dorsal deviation
Illustration: No epidural fat layer is visible between the nerve root and the disk material. The nerve root is in the normal position and is not dorsally deviated.
Illustration: Dorsal deviation of the right nerve root (arrow), caused by contact with disk material (arrowheads).
Illustration: Compression of the right nerve root (arrow) between disk material (arrowheads) and the wall of the spinal canal. The nerve root appears flattened and is indistinguishable from disk material.
- Fardon DF, Milette PC; Combined Task Forces of the North American Spine Society, American Society of Spine Radiology, and American Society of Neuroradiology.
Nomenclature and classification of lumbar disc pathology. Recommendations of the Combined task Forces of the North American Spine Society, American Society of Spine Radiology, and American Society of Neuroradiology.
Spine. 2001 Mar 1;26(5):E93-E113.
Available from the ASNR website: http://www.asnr.org/spine_nomenclature/
- Pfirrmann CW, Dora C, Schmid MR, Zanetti M, Hodler J, Boos N.
MR image-based grading of lumbar nerve root compromise due to disk herniation: reliability study with surgical correlation.
Radiology. 2004 Feb;230(2):583-8.