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Nomenclature and Classification of Lumbar Disc Pathology


DISCUSSION

Acceptance and standardization occur most easily when recommendations are close to common practice. However, there are many contradictory views of common practice and some common practices are contradictory to our primary purpose, which is clear communication between those who interpret images and those who make clinical decisions.

This document defines a nomenclature that describes discs and leaves to the clinician the description of the patient. In so doing, however, this provides a nomenclature that facilitates description of surgical or endoscopic findings as well as images; and also, with the caveat that it addresses only the morphology of the disc, it facilitates communication for patients, families, employers, insurers, and legal and social authorities, and permits accumulation of more reliable data for research.

Normal: Categorization of a disc as "Normal" means the disc is fully and normally developed and free of any changes of disease, trauma, or aging. Only the morphology, and not the clinical context, is considered. In common practice, people with a variety of harmless congenital or developmental variations of discs, minor bulging of anuli, anterior and lateral marginal vertebral body osteophytes, etc. are normal people. By this nomenclature and classification, however, such individual discs are not considered "normal." Therein lies a significant difference of this method from what many would consider common practice. Some people are clinically "normal" even though they have morphologically abnormal discs.

Anular Tears/Fissures: There is general agreement about the various forms of loss of integrity of the anulus, such as radial, transverse, and concentric separations. Some, including the 1995 NASS document,15 have recommended that such lesions be termed "fissures" rather than "tears," primarily for fear that the word "tear" could be misconstrued as implying a traumatic etiology. Common practice, as documented by review of contemporary specialty journal literature12 shows preference, among authors of various disciplines, for the term "tear," and frequent synonymous use in the same articles of the terms "tear" and "fissure."

In this instance, it is unwise to recommend contrary to ingrained common usage but wise to reiterate the caveat that the term "anular tear" does not imply traumatic etiology. In the case where a single, traumatic event is clearly the source of loss of integrity of a formerly normal anulus, such as with documentation and findings of violent distraction injury, the term "rupture" of the anulus is appropriate, but use of the term "rupture" as synonymous with commonly observed tears or fissures is contraindicated. In conclusion, therefore, "anular tear" and "anular fissure" are both acceptable terms, can be used properly as synonyms, and do not imply that a significant traumatic event has occurred or that the etiology is known.

Some tears may have clinical relevance and others may be asymptomatic and inconsequential components of the aging process. Correlation of the characteristics of the tear with responses to discography and other clinically relevant observations may enable the observer to make such distinctions, but such is beyond the scope of this morphologically-based definition and classification model.

Disc Degeneration: Because there is confusion in differentiation of changes of pathologic degenerative processes in the disc from those of normal aging,8,30,38the classification category "Degenerative/Traumatic" includes all such changes, thus does not compel the observer to differentiate the pathologic from the normal consequences of aging. However, this model allows the observer with adequate data to present a more enlightening report by making such a distinction, with appropriate notation of the degree of confidence.

Perceptions of what constitutes the normal aging process of the spine have been greatly influenced by post-mortem anatomical studies involving a limited number of specimens, harvested from cadavers from different age groups, with unknown past medical histories, and the presumption of absence of lumbar symptoms.7,9,17,20,23,34 With such methods, pathologic changes are easily confused with consequences of normal aging. Resnick and Niwayama35 emphasized the differentiating features of two degenerative processes involving the intervertebral disc, which had been previously described by Schmorl and Junghanns37: "spondylosis deformans," which affects essentially the anulus fibrosus and adjacent apophyses, and "intervertebral osteochondrosis," which affects mainly the nucleus pulposus and the vertebral body end-plates, but also includes extensive fissuring (numerous tears) of the anulus fibrosus, which may be followed by atrophy (Figure 3). Although Resnick and Niwayama stated that the cause of the two entities was unknown, other scientific studies suggest that spondylosis deformans is the consequence of normal aging, whereas intervertebral osteochondrosis, sometimes also called "deteriorated disc," results from a clearly pathologic, though not necessarily symptomatic, process.32,36,37,40,41

With normal aging, fibrous tissue replaces nuclear mucoid matrix, but the disc height is preserved and the disc margins remain regular.22 Radial tears of the anulus are found only in a minority of post-mortem examinations of individuals over 40 years old,23 and therefore cannot be considered a usual consequence of aging. Slight symmetric bulging of the disc may occur in the elderly due to remodeling associated with osteoporosis.41 On conventional radiographs and CT, small amounts of gas can be detected in some elderly individuals at the anular/apophyseal enthesis, probably located in small transverse anular tears, and possibly signifying early manifestations of spondylosis deformans; 49 however, a large amount of gas in the central disc space is always pathologic and is a feature of intervertebral osteochondrosis.35 Anterior and lateral marginal vertebral body osteophytes have been found in 100% of skeletons of individuals over 40, so are consequences of normal aging, whereas posterior osteophytes have been found in only a minority of skeletons of individuals over 80, so are not inevitable consequences of aging.32 End-plate erosions with osteosclerosis and chronic reactive bone marrow changes also appear to be pathologic. Slight to moderate decrease in central disc signal intensity found on T2-weighted MR images can be a non-pathologic age-related observation but, if the result of a normal process, should be relatively uniform among all discs studied in the individual. Intervertebral osteochondrosis, or "deteriorated disc," also sometimes called "chronic discopathy," shows, on microscopic examination, total structural disorganization and general replacement of normal disc tissue by fibrosis. Radiographically, intervertebral osteochondrosis is characterized by narrowing of the intervertebral space, irregular disc contour often associated with bulging, multi-directional osteophytes often involving the central spinal canal and foramina, end-plate erosions with reactive osteosclerosis, and chronic vertebral body bone marrow changes. On T2-weighted images, the central disc signal intensity is usually markedly decreased, and at distinct variance to that seen in unaffected discs of the same individual. The distinction is made at the time of the reading and does not imply that early manifestations of a pathologic process are always distinguishable from changes of normal aging.

Herniated Disc: The needs of common practice make necessary a diagnostic term that covers the various permutations of disc material displaced beyond the intervertebral disc space. Herniated disc, herniated nucleus pulposus, ruptured disc, prolapsed disc (used non-specifically), protruded disc (used non-specifically), and bulging disc (used non-specifically) have all been used in the literature in various ways to denote imprecisely defined displacement of disc material beyond the interspace. The absence of clear understanding of the meaning of these terms and lack of definition of limits that should be placed upon an ideal general term have created a great deal of confusion in clinical practice and in attempts to make meaningful comparisons of research studies.

For the general diagnosis of displacement of disc material, the single term that is most commonly used and creates least confusion is "herniated disc." Attempts to avoid whatever confusion has been created by lack of definition of the term "herniated disc" have included the recommendation to substitute the term "disc material beyond the interspace" [DEBIT],4 but that is more awkward and runs counter to common practice. "Herniated nucleus pulposus" [HNP] is inaccurate because materials other than nucleus (cartilage, fragmented apophyseal bone, fragmented anulus) are common components of displaced disc material.6,47,48 "Rupture" casts an image of tearing apart and therefore carries more implication of traumatic etiology than "herniation," which conveys an image of displacement rather than disruption.

Though "protrusion" has been used by some authors in a non-specific general sense to signify any displacement, the term has a more commonly used specific meaning for which it is best reserved. "Prolapse," which has been used as a general term, as synonymous with the specific meaning of protrusion, or to denote inferior migration of extruded disc material, is not commonly used and is best proscribed. The term "bulging disc" has been used to mean many things and has caused a great deal of confusion, as discussed below; therefore, its use as a general term to signify disc displacement should be avoided.

By exclusion of other terms, and by reasons of simplicity and common usage, "herniated disc" is the best general term to denote displacement of disc material. The term is appropriate to denote the general diagnostic category when referring to a specific disc and to be inclusive of various types of displacement when speaking of groups of discs. The term includes discs that may properly be characterized by more specific terms, such as "protruded disc" or "extruded disc."

The term "herniated disc," as defined in this work, refers to localized displacement of nucleus, cartilage, fragmented apophyseal bone, or fragmented anular tissue beyond the intervertebral disc space (disc space, interspace). The interspace is defined, craniad and caudad, by the vertebral body end-plates and, peripherally, by the edges of the vertebral ring apophyses, exclusive of osteophytic formations. This definition was deemed more practical, especially for interpretation of imaging studies, than a pathological definition requiring identification of disc material forced out of normal position through an anular defect. Displacement of disc material, either through a fracture in the bony end-plate or in conjunction with displaced fragments of fractured walls of the vertebral body, may be described as "herniated," disc, though such description should accompany description of the fracture so as to avoid confusion with primary herniation of disc material. Displacement of disc materials from one location to another within the interspace, as with intra-anular migration of nucleus without displacement beyond the interspace, is not considered herniation.

To be considered "herniated," disc material must be displaced from its normal location and not simply represent an acquired growth beyond the edges of the apophyses, as is the case when connective tissues develop in gaps between osteophytic formations. Displacement, therefore, can only occur in association with disruption of the normal anulus or, as in the case of intravertebral herniation (Schmorl's node), a break in the vertebral body end-plate. Since details of the integrity of the anulus are often unknown, the distinction of herniation is usually made by observation of displacement of disc material beyond the edges of the ring apophyses that is "localized," meaning less than 50% (180 degrees) of the circumference of the disc. Generalized, meaning greater than 50%, displacement of disc material beyond the ring apophyses, or adaptive changes of the apophyses and/or outer anulus to adjacent abnormality, such as may occur with scoliosis or spondylolisthesis, are not herniations. The 50% cut-off line is established by way of convention to lend precision to terminology and does not demarcate etiology, relation to symptoms, or treatment indications.

The term "bulge" refers to an apparent generalized extension of disc tissues beyond the edges of the apophyses. Such bulging occurs in greater than 50% of the circumference of the disc and extends a relatively short distance, usually less than 3 mm, beyond the edges of the apophyses. "Bulge" describes a morphologic characteristic of various possible causes. Bulge is a term for an image that requires a differential diagnosis. Bulging is sometimes a normal variant (usually at L5-S1); can result from advanced disc degeneration or from vertebral body remodeling (as consequent to osteoporosis, trauma, or adjacent structural deformity); can occur with ligamentous laxity in response to loading or angular motion; can be an illusion caused by posterior central sub-ligamentous disc protrusion; or can be an illusion from volume averaging (particularly with CT axial images).

Bulging, by definition, is not a herniation. Herniation is present if there is localized displacement of disc material, and not simply outward overlapping, as is the case with some types of bulging. Application of the term "bulging" to a disc does not imply any knowledge of etiology, prognosis, need for treatment or necessarily imply the presence of symptoms.

A disc may have more than one herniation. A disc herniation may be present along with other degenerative changes, fractures or other abnormalities of adjacent bone, or other abnormalities of the disc. The term "herniated disc" does not imply any knowledge of etiology, relation to symptoms, prognosis, or need for treatment.

When data are sufficient to make the distinction, a herniated disc may be more specifically characterized as "protruded" or "extruded." These distinctions are based upon the shape of the displaced material. They do not imply knowledge of the mechanism by which the changes occurred and, thereby, differ from definitions that base the distinction upon whether and how disc material has passed through a defect in the anulus.

Protruded Discs: A disc is "protruded," if the greatest distance, in any plane, between the edges of the disc material beyond the disc space is less than the distance between the edges of the base in the same plane. The term "protrusion" is only appropriate in describing herniated disc material, as discussed above.

Protrusions may be "focal" or "broad-based." The distinction between focal and broad-based is arbitrarily set at 25% of the circumference of the disc. Protrusions with a base less than 25% (90 degrees) of the circumference of the disc are "focal." If disc material is herniated so that the protrusion encompasses 25% to 50% of the circumference of the disc, it is considered "broad-based protrusion."

Extruded Discs: The term "extruded" is consistent with the lay language meaning of material forced from one domain to another through an aperture. With reference to a disc, the test of extrusion is the judgment that, in at least one plane, any one distance between the edges of the disc material beyond the disc space is greater than the distance between the edges of the base measured in the same plane, or when no continuity exists between the disc material beyond the disc space and that within the disc space. . Extruded disc material that has no continuity with the disc of origin may be further characterized as "sequestrated". A sequestrated disc is a subtype of "extruded disc" but, by definition, can never be a "protruded disc." Disc material that is displaced away from the site of extrusion, regardless of continuity, may be called "migrated," a term which is useful for the interpretation of imaging studies because it is often impossible from images to know if continuity exists.

The use of the distinction between "protrusion" and "extrusion" is optional and some observers may prefer to use, in all cases, the more general term "herniation". Further distinctions can often be made regarding containment, continuity, volume, composition, and location of the displaced disc material.

Containment/Continuity: Herniated disc material can be "contained" or "uncontained." The test of containment is whether or not the displaced disc tissues are wholly held within intact outer anulus. A disc with a "contained" herniation would not leak into the vertebral canal fluid that has been injected into the disc. Although the posterior longitudinal ligament and/or peridural membrane may partially cover extruded disc tissues, such discs are not considered "contained" unless the outer anulus is intact. Strictly speaking, containment refers to the integrity of the outer anulus covering the disc herniation. The technical limitations of currently available non-invasive imaging modalities (CT and MR) usually preclude the distinction of a contained from an uncontained disc herniation. Discography does not allow one to distinguish a containing capsule consisting of both anular fibers and longitudinal ligament fibers from one consisting only of longitudinal ligament fibers, and essentially only allows one to separate a "leaking disc" from a "non-leaking disc".

Displaced disc fragments are sometimes characterized as "free." A "free fragment" is synonymous with a "sequestrated fragment" and not the same as "uncontained," as the latter refers only to the integrity of the outer anulus and has no inference as to the continuity of the displaced disc material with the parent disc. A fragment should be considered "free," or "sequestrated," only if there is no remaining continuity of disc material between it and the disc of origin.

The term "migrated" disc or fragment refers to displacement of disc material away from the opening in the anulus through which the material has extruded. Some migrated fragments will be sequestrated, but the term migrated refers only to position and not to continuity.

Referring to the posterior longitudinal ligament (PLL), some authors have distinguished displaced disc material as "sub-ligamentous," "extra-ligamentous," "trans-ligamentous," or "perforated." When the distinction between the outer anulus and the PLL is unclear and a fragment is under such a blended structure (sometimes called "capsule"), it has been called "sub-capsular." If the peridural membrane alone surrounds the displaced disc material, the displacement is sometimes called "sub-membranous." Such permutations of continuity, containment, and relationships to ligaments and membranes are refinements that may suit certain purposes but do not supersede the basic definition of disc herniation and the major sub-categorizations of extrusion and protrusion.

Volume and Composition of Displaced Material: A scheme to define the degree of canal compromise produced by disc displacement should be practical, objective, reasonably precise, and clinically relevant. A simple scheme that fulfills the criteria utilizes measurements taken from an axial section at the site of the most severe compromise. Canal compromise of less than one third of the canal at that section is "mild," between one and two thirds is "moderate," and over two thirds is "severe." The same grading can be applied for foraminal involvement.

Such characterizations of volume describe only the cross sectional area at one section and do not account for total volume of displaced material, proximity to, compression and distortion of neural structures, or other potentially significant features, which the observer may further detail by narrative description.

Composition of the displaced material may be characterized by such terms as "nuclear," "cartilaginous," "bony," "calcified," "ossified," "collagenous," "scarred," "desiccated," "gaseous," or "liquified."

Clinical significance related to the observation of volume and composition depends upon correlation with clinical data and cannot be inferred from morphologic data alone.

Location: Bonneville proposed a useful and simple alpha-numerical system to classify, according to location, the position of disc fragments that have migrated in the horizontal or sagittal plane.2,3 Using anatomic boundaries familiar to surgeons, Wiltse proposed another system.15,45 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 (Figure 17). 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). The method is not as precise as drawings depict because borderlines such as the medial edges of facets and the walls of the pedicles are curved, but the method is simple, practical, and in common usage.

Moving from central to right lateral in the axial (horizontal) plane, location may thus be defined as "central," "right central," "right sub-articular," "right foraminal," or "right extra-foraminal." The term "paracentral" is less precise than defining "right central" or "left central," but is useful in describing groups of discs that include both, or when speaking informally when the side is not significant. 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."

In the sagittal plane, location may be defined as "discal," "infra-pedicular," "supra-pedicular," or "pedicular." In the coronal plane, "anterior," in relationship to the disc, means ventral to the mid-coronal plane of the vertebral body.


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Introduction
Preface Introduction Recommendations Discussion (part 1)
Discussion (part 2) Glossary References Appendix

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Copyright 2001 Lippincott, Williams & Wilkins
Presented by American Society of Neuroradiology, American Society of Spine Radiology

and North American Spine Society