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Basic Aspects in MR Imaging of Degenerative Lumbar Disk Disease Andreas Heuck, MD1
Christian Glaser, MD1
1 Radiologisches Zentrum München (RZM), Munich Germany 2 LMU Munchen, München, Germany 3 NYU Langone Medical Center, New York, New York
Address for correspondence Andreas Heuck, MD, Radiologisches Zentrum München (RZM), Pippingerstr, 25, 81245 München, Germany (e-mail: [email protected]).
Abstract
Keywords
► ► ► ►
MRI disk degenerative spine
Degenerative disease may lead to spinal canal stenosis and long-lasting pain. It is among the leading cause of disability that may affect the ability to work. It has become more common in an increasingly aging population. MRI is the most comprehensive imaging modality and provides detailed morphologic information. A standardized terminology facilitates communication with referring physicians. Yet imaging findings need careful interpretation in conjunction with the results of clinical tests and symptoms to truly help guide therapeutic decision making. This review summarizes aspects of normal anatomy of the intervertebral disk, pathologic mechanisms, terminology, and examples of the imaging spectrum of disk degeneration and herniation.
Degenerative disease is strongly associated with back pain1 but often may be asymptomatic2 as well. Disk degeneration is associated with disk herniation and sciatica and affects the biomechanics of the spine based on interactive effects on the facet joints, spinal ligaments, and muscles, ultimately causing degeneration of the spinal unit as a whole.3,4 In the long term this can lead to spinal canal stenosis, a major cause of pain and disability in the elderly with increasing incidence due to demographic changes.5 However, the sequelae of disk degeneration are also among the leading causes of functional incapacity and a common source of chronic disability during work life.6 The role of imaging in degenerative disk disease is to provide accurate morphologic information that—together with clinical symptoms —guides therapeutic decision making. Hence understanding of pathologic and radiologic manifestations as well as a uniform classification of pathologic conditions is essential.7 This review summarizes aspects of normal anatomy of the intervertebral disk, pathologic mechanisms, terminology, and examples of the imaging spectrum of disk degeneration and herniation.
Anatomy The intervertebral articulation includes two types of joints. The symphyseal joint between the vertebral bodies consists of
Issue Theme Spine; Guest Editor, Mara Epermane, MD
the intervertebral disk and the vertebral end plates, the diskovertebral unit; it is part of the anterior column. The two synovial facet joints of the neural arch are part of the posterior column and supported by ligaments and muscle groups. Understanding the interrelationship of these components is critical, as surgical intervention is transitioning from fusion to joint replacement.
Intervertebral Disk The intervertebral disk is composed of three components: the nucleus pulposus, the inner part of the intervertebral disk, is located within the anulus fibrosus, in a slightly eccentric position toward the posterior surface of the disk. The nucleus is composed of a thin lattice of collagen fibers (type II collagen) that traverse through the gelatinous ground substance with a higher concentration of fibers in the equator of the disk. Regions of higher collagen concentration display a lower signal in T2-weighted sequences than the remainder of the disk. The ground substance of the nucleus consists of hydrophilic glycosaminoglycans and hyaluronic acid. It creates a high intrinsic pressure that allows the disk to resist compressive forces. The nucleus is avascular and its nutrition depends on diffusion. The anulus fibrosus is a complex fibrous and fibrocartilaginous structure that surrounds the nucleus pulposus. It
Copyright © 2014 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0034-1375566. ISSN 1089-7860.
This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited.
Semin Musculoskelet Radiol 2014;18:228–239.
MR Imaging of Degenerative Lumbar Disk Disease
Facet Joints The facet (or zygapophyseal) joints are the articulations of the posterior arch of the vertebrae and a part of the posterior column. They are synovial-lined joints with a fibrous capsule and connect the articular facets of adjacent vertebrae. The facets are located between the pedicle and lamina of each vertebra and form the articular pillars that provide structural stability but at the same time allow for a certain mobility to the vertebral column. The facet joints are symmetrical, supported and kept in position by the posterior ligamentous complex.
Ligaments The spinal ligaments act to stabilize the vertebral column together with several muscle groups. The anterior longitudinal ligament (ALL) is a strong band of fibers descending the anterior surface of the vertebral column and consists of three layers. The superficial layer traverses four to five vertebrae, the intermediate layer covers two to three, and the deep layer spans only one intervertebral articulation. The posterior ligamentous complex acts to hold the facet joints of adjacent vertebrae in a defined relation to each other. It comprises the
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posterior longitudinal ligament (PLL), the paired sets of ligamenta flava (connecting the laminae of adjacent vertebrae), the intertransverse ligaments (extending between the transverse processes), the interspinous ligaments (between the spinous processes), and the supraspinous ligament (connecting the very tips of the spinous processes). The PLL extends along the posterior surface of the vertebral column within the spinal canal. Its fibers are attached to the intervertebral disks and the margins of the vertebral bodies but not to the midposterior surface of the vertebral bodies, allowing vascular structures to enter and leave the medullary sinus beneath its fibers.
Age-related Changes The intervertebral disk displays age-related changes that may be distinguished from true degeneration. During normal aging the glycosaminoglycan content decreases and collagen content increases in the disk, and there is a diminished water content. This may lead to a mild loss of the normal high signal intensity in T2-weighted MR images (►Fig. 1). But because the nucleus pulposus volume remains the same with normal aging, it causes neither a reduction of intervertebral disk height (intervertebral chondrosis) nor additional alterations of the vertebral body end plates (intervertebral osteochondrosis). Pfirrmann et al8 suggest a systematic and reproducible MR imaging based classification of changes in the aging disk based on its internal structure, signal intensity, the distinction between the nucleus and anulus, and disk height. Several studies suggest that spondylosis deformans is a consequence of normal aging, since anterior and lateral marginal vertebral body osteophytes (spondylophytes) have been found in all individuals over 40 years of age.9 However, posterior osteophytes have been found in only a minority of skeletons of individuals over 80 years, and therefore are not considered inevitable consequences of aging.10
Degeneration Degeneration of the intervertebral disk and the facet joints appears to be interrelated rather than two separated processes,9 causing altered biomechanical stresses and acting on the integrity and mechanical properties of the spinal ligaments, which results in instability and degeneration of the spinal unit as a whole.3,4,11 The manner of degeneration of the various components of the spine depends on the structures involved. Collagen, ground substance, and cartilaginous and synovial structures each degenerate in a specific manner associated with characteristic pathologic and imaging findings.
Disk Degeneration The etiology of disk degeneration is multifactorial. In addition to mechanical loading, traumatic, biochemical, nutritional, and genetic factors play a role in the development of disk degeneration.5,12–16 Seminars in Musculoskeletal Radiology
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consists of 15 to 20 collagenous (type I collagen) layers that run obliquely from the edge of one vertebra to the edge of the adjacent vertebra. The direction of the fibers alternates from lamina to lamina. The outer rim contains the most densely woven fibrous laminae that insert in the cortical bone of the ring apophysis; this portion displays low signal intensity on T2- and T1-weighted images. The inner portion of the anulus is made of fibrocartilage, contains chondrocytes and some amount of ground substance, and gradually blends with the nucleus pulposus; therefore this portion has higher signal intensity similar to the nucleus in T2weighted images and thus cannot be clearly differentiated from it. Posterolaterally, the anulus is thinner and contains more vertical fibers and a greater amount of disorganized collagen bundles. This feature makes the posterolateral area the weakest part of the disk and accounts for a high rate of disk herniation. The outer fibers of the anulus fibrosus receive blood supply from adjacent vessels in the end plate and are innervated by sinuvertebral nerves arising from the dorsal root ganglia. The inner layers have neither innervation nor blood supply; like the nucleus they receive nutrition via diffusion. The cartilaginous end plate covers most of the vertebral end plate; it includes hyaline cartilage at the vertebral aspect and fibrocartilage toward the anulus fibrosus. The ring apophysis surrounds the cartilaginous end plate; it consists of dense bone that fuses to the vertebra in the second decade of life. With its numerous collagen fibers the cartilaginous end plate is firmly attached to the osseous end plate, which contains multiple perforations for vascular channels. The cartilaginous end plate displays low signal on T2-weighted images and cannot be clearly differentiated from the bony end plate. However, due to its vascularization, enhancement can be noted after intravenous administration of contrast material.
Heuck, Glaser
MR Imaging of Degenerative Lumbar Disk Disease
Heuck, Glaser
Fig. 1 (a–f) Disk aging and degeneration. (a, e) Sagittal T2-weighted turbo spin echo (TSE); (b) sagittal T1-weighted spin echo; (c, d, f) axial T2weighted TSE. (a) From L1-L2 down to L4-L5, an increasingly broad hypointense signal central in the disks can be observed, and the distinction between nucleus and anulus becomes less clear in T2-weighted sequences. (a, d)There is subtle bulging of the disk at L4-L5 in the axial and sagittal planes as opposed to the normal circumference of a healthy disk following the contour of the vertebral end plate (c); note the small focal midline protrusion (d). Hypointensity almost throughout the disk in T2-weighted sequences, here at the L5-S1 level (a), is suggestive of desiccation. Similarly, there is initial loss of signal indicative of incipient desiccation at the L4-S1 levels and disk bulging with a midline protrusion at L4-L5 (e, f). In all segments, note that there are no discernible bony end-plate changes.
Degenerative changes of the intervertebral disk include desiccation and fibrosis of the nucleus, mucinous degeneration and fissuring of the anulus, bulging of the anulus beyond the edges of the vertebral body ring apophyses with consecutive narrowing of the disk space, and defects and sclerosis of the cartilaginous end plates.
Desiccation Disk desiccation describes an apparently reduced water content of the disk that results from significant replacement of the hydrophilic glycosaminoglycans within the nucleus pulposus by collagen type I, the same collagen as in the anulus fibrosus. Desiccation is common, and its incidence increases with age (►Fig. 1). Although intriguing at first thought, disk desiccation per se does not necessarily result in a reduction of the volume of the nucleus pulposus. Therefore, it appears not to be the primary cause of disk space narrowing. This is rather a result of anular bulging, advanced degenerative changes, or vertebral end-plate bowing. Seminars in Musculoskeletal Radiology
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With progressing desiccation, the nucleus pulposus becomes gradually friable and disintegrated. Clefts appear that eventually may extend into the anulus fibrosus.
Anular Degeneration and Fissures Like other fibrous structures, the anulus fibrosus can be subject to mucinous degeneration and fissuring. Fissures (or tears) consist of separation of one or more layers of the anulus, avulsion of fibers from their vertebral body insertions, or breaches through fibers that may be radial, transverse, or concentric in orientation. Degeneration and some limited degree of fissuring is thought to be a precondition for disk bulging. Most anular fissures are asymptomatic; however, some are painful. The defects allow ingrowth of nerve endings and granulation tissue. Fissures near the dorsal root ganglion are especially likely to be painful.
Cartilaginous End-Plate Degeneration Degeneration of the cartilaginous end plates includes defects and sclerosis of the hyaline and fibrocartilage. In an attempt
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Table 1 Glossary of nomenclature and classification of disk pathology termsa • Aging disk: Disk demonstrating the features of normal aging. Spondylosis deformans possibly represents the normal aging process.
• Anterior displacement: Displacement of disk tissues beyond the disk space into the anterior zone.
• Anulus fibrosus: Multilaminated ligament surrounding the periphery of each disk space, attaching craniad and caudad to endplate cartilage and ring apophyseal bone and blending centrally with nucleus pulposus.
• Anulus rupture: Disruption of the fibers of the previously normal anulus by sudden violent injury.
• Base (of displaced disk): Cross-sectional area of disk material at the outer margin of the disk space of origin, where disk material displaced beyond the disk space is continuous with disk material within the disk space. In the craniocaudal direction, the length of the base cannot exceed, by definition, the height of the intervertebral space.
• Broad-based protrusion: Herniation of disk material extending beyond the outer edges of the vertebral body apophyses over an area > 25% (90 degrees) and < 50% (180 degrees) of the circumference of the disk.
• Bulging disk: Disk in which the contour of the outer anulus extends in the axial plane beyond the edges of the disk space, > 50% (180 degrees) of the disk circumference and usually 3 mm beyond the edges of the vertebral body apophyses.
• Central zone: Zone within the vertebral canal between sagittal planes through the medial edges of each facet.
• Chondrosis: Degenerative process of the intervertebral disk without vertebral body reactive changes. Characterized by disk space narrowing and eventually vacuum phenomenon and calcification.
• Chronic disk herniation: Disk herniation with presence of calcification, ossification, or gas accumulation within the displaced disk material.
• Contained herniation: Displaced disk tissue that is wholly within an outer perimeter of uninterrupted outer anulus.
• Continuity: Connection of displaced disk tissue by a bridge of disk tissue, however thin, to tissue within the disk of origin.
• Degenerated disk: Changes in a disk characterized by desiccation, fibrosis, and cleft formation in the nucleus, fissuring and mucinous degeneration of the anulus, defects and sclerosis of end plates, and/or osteophytes at the vertebral apophyses.
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• Anterior zone: Peridiskal zone anterior to the midcoronal plane of the vertebral body.
MR Imaging of Degenerative Lumbar Disk Disease
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• Degenerative disk disease: Clinical syndrome characterized by manifestations of disk degeneration and symptoms thought to be related to those changes.
• Desiccated disk: Disk with reduced water content, usually primarily of nuclear tissues.
• Disk: Complex structure composed of nucleus, anulus, cartilaginous end plates, and vertebral body ring apophyseal attachments of anulus.
• Disk space: Space limited craniad and caudad by the end plates of the vertebrae and peripherally by the edges of the vertebral body ring apophyses exclusive of osteophytes. Synonym: intervertebral disk space.
• Diskogenic vertebral sclerosis: Increased bone density and calcification adjacent to the end plates of the vertebrae craniad and caudad to a degenerated disk, usually a manifestation of intervertebral osteochondrosis.
• Displaced disk: Disk in which disk material is beyond the outer edges of the vertebral body ring apophyses (exclusive of osteophytes) of the craniad and caudad vertebrae, or, as in the case of intravertebral herniation, penetrated through the vertebral body end plate.
• Extraforaminal zone: Zone beyond the sagittal plane of the lateral edges of the pedicles, having no well-defined lateral border.
• Extruded disk: Herniated disk in which, in at least one plane, 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 in the same plane, or when no continuity exists between the disk material beyond the disk space and that within the disk space.
• Fissure of anulus: Separations between anular fibers, avulsion of fibers from their vertebral body insertions, or breaks through fibers that extend radially, transversely, or concentrically. Synonym: tear of annulus.
• Focal protrusion: Protrusion of disk material, the base of the displaced material is < 25% (90 degrees) of the circumference of the disk.
• Foraminal zone: Zone between planes passing through the medial and lateral edges of the pedicles.
• Herniated disk: Localized displacement of disk material beyond the normal margins of the intervertebral disk space < 50% (180 degrees) of the circumference of the disk.
• Intervertebral chondrosis: Degenerative process limited to the intervertebral disk.
• Intervertebral osteochondrosis: Degenerative process involving the nucleus pulposus, the anulus fibrosus, and the vertebral end plate.
• Intradural herniation: Disk from which displaced tissue has penetrated the dura so that it lies within the thecal sac.
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• Intravertebral herniation: Disk in which a portion of the disk is displaced through the end plate into the centrum of the vertebral body. Synonym: Schmorl’s node.
• Limbus vertebrae: Separation of a segment of rim of vertebral body ring apophysis. Note: Limbus vertebrae may result from fracture or from developmental abnormalities. Limbus vertebrae is commonly seen in patients who have had Scheuermann disease. The lesions may be called rim lesions.
• Migrated disk: Herniated disk in which a portion of extruded disk material is displaced away from the tear in the outer anulus through which it has extruded.
• Nucleus of origin: Central nuclear portion of the disk of reference, usually used to reference the disk from which tissue has been displaced. Synonyms: parent nucleus, disk of origin.
• Osteophytes: Focal hypertrophy of bone surface and/or ossification of soft tissue attachments to the bone.
• Pedicular level: Level between axial planes through the upper and lower edges of the pedicle.
• Prolapsed disk: Herniated disk in which disk tissue has protruded or extruded at the level of the disk and below into the suprapedicular level.
• Protruded disk: Herniated disk in which the greatest distance, in any plane, between the edges of the disk material beyond the disk space is less than the distance between the edges of the base in the same plane.
• Sequestrated disk: Extruded disk in which a portion of the disk tissue is displaced beyond the outer anulus and maintains no connection by disk tissue with the disk of origin.
• Sequestrum: Disk tissue that has become displaced from the disk space of origin and lacks any continuity with disk material within the disk space of origin. Synonym: disk fragment.
• Spondylosis deformans: Degenerative process of the spine involving the anulus fibrosus and characterized by anterior and lateral marginal osteophytes arising from the vertebral body apophyses; the intervertebral disk height is normal or only slightly decreased.
• Subarticular zone: Zone, within the vertebral canal, sagittally between the plane of the medial edges of the pedicles and the plane of the medial edges of the facets, and coronally between the planes of the posterior surfaces of the vertebral bodies and the under anterior surfaces of the superior facets. Synonyms: lateral recess, posterolateral zone.
• Syndesmophytes: Thin and vertically oriented bony outgrowths extending from one vertebral body to the next and representing ossification within the outer portion of the anulus fibrosus.
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• Marginal osteophyte: Osteophyte that protrudes from and beyond the outer perimeter of the vertebral end-plate apophysis.
MR Imaging of Degenerative Lumbar Disk Disease
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• Transligamentous: Displacement, usually extrusion, of disk material through the posterior longitudinal ligament. Synonym: perforated.
• Vacuum disk: Disk with imaging characteristics suggestive of gas in the center of the disk space, usually a manifestation of disk degeneration
a
Short list adapted from American Journal of Neuroradiology Web site.18
to heal, chondrocyte degeneration and granulation tissue formation may occur within the cartilaginous endplate extending into the anulus fibrosus and sometimes even into the degenerated nucleus pulposus.17 As a result of more advanced disk degeneration, alterations of the adjacent vertebral body architecture may occur. These diskogenic vertebral changes involve the vertebral end plates and the subchondral bone marrow space. Any clinical approach to degenerative disk disease, whether related to diagnosis or management, demands agreement on a terminology that is reliable and reproducible to describe the pathologic condition of a disk. In this review we use the most widely accepted terminology that was originally set up by the American Society of Neuroradiology, the American Society of Spine Radiology, and the North American Spine Society and has been adopted by many other scientific societies worldwide.7,18 ►Table 1 provides a short version of this nomenclature and classification.
Imaging of Disk Degeneration The first sign of disk degeneration on imaging is desiccation in which the disk loses its high T2 signal and the horizontal midline low signal cleft is becoming gradually inapparent (►Fig. 1).
Although anular fissures are common, only a minority is identified on MRI, and if so, it is characterized by a region of high T2 signal in the otherwise low-signal anulus (►Fig. 2). If there is ingrowth of vascularized granulation tissue into a fissure, there may be enhancement within the cleft after intravenous contrast administration. A fissure may involve all layers or only some laminae of the anulus, but this distinction is difficult on MR imaging and impossible on computed tomography (CT) if no disk extrusion is seen. Disk bulging defines a disk in which the contour of the anulus extends 3 mm beyond the edges of the vertebral body apophyses, along more than half of its circumferential length (>180 degrees). Because bulging is limited by the anulus fibrosus, it does not extend above or below the attached margins of the disk. Bulging is in part responsible for narrowing of the intervertebral disk space. It can be easily identified on CT and MR imaging by a broad, more than semicircular extension of the anulus. The degree of extension ( 3 mm), per definition, may be estimated from axial, sagittal, and coronal MR images. However, the circumferential involvement is best defined in the axial plane. A disk herniation refers to a localized displacement of intervertebral disk material beyond the normal margins of the intervertebral disk space but involving less than half the
Table 2 Intervertebral disk displacements Bulge Herniation
> 180 degrees of circumference, 3-mm displacement < 180 degrees of circumference
Protrusion
< 180 degrees of circumference, base > dome
Broad based
90–180 degrees of circumference, base > dome
Focal
< 90 degrees of circumference, base > dome
Contained
outer laminae of anulus intact
Noncontained
all laminae of anulus deficient)
Extrusion
< 180 degrees of circumference, waist < dome
Sequestration
Portion of extruded disk is displaced beyond the anulus; no connecting disk tissue with the disk of origin
Migration
Portion of extruded disk is displaced away from tear in outer anulus
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Heuck, Glaser
Fig. 2 (a–d) Disk degeneration. (a-c) Sagittal T2-weighted turbo spin echo (TSE), T1-weighted spin echo, T1-weighted fat saturation with intravenous contrast; (d) axial T2-weighted TSE. There is desiccation from level L3 to S1. (a-c) Note the subtle anular fissure at L3-L4 with nucleus material extending behind the posterior edge of L3 and strong contrast uptake indicating vascularization without a prominent change of disk shape. There is a small intraosseous disk herniation (Schmorl node) in L4 with predominantly fatty peripheral reactive changes and central granulation tissue. (d ) Small extrusions at the L4-S1 levels show displaced disk material extending beyond the level of the disk space in the sagittal and axial planes, also showing at least partial contrast enhancement.
circumference (
Basic Aspects in MR Imaging of Degenerative Lumbar Disk Disease Andreas Heuck, MD1
Christian Glaser, MD1
1 Radiologisches Zentrum München (RZM), Munich Germany 2 LMU Munchen, München, Germany 3 NYU Langone Medical Center, New York, New York
Address for correspondence Andreas Heuck, MD, Radiologisches Zentrum München (RZM), Pippingerstr, 25, 81245 München, Germany (e-mail: [email protected]).
Abstract
Keywords
► ► ► ►
MRI disk degenerative spine
Degenerative disease may lead to spinal canal stenosis and long-lasting pain. It is among the leading cause of disability that may affect the ability to work. It has become more common in an increasingly aging population. MRI is the most comprehensive imaging modality and provides detailed morphologic information. A standardized terminology facilitates communication with referring physicians. Yet imaging findings need careful interpretation in conjunction with the results of clinical tests and symptoms to truly help guide therapeutic decision making. This review summarizes aspects of normal anatomy of the intervertebral disk, pathologic mechanisms, terminology, and examples of the imaging spectrum of disk degeneration and herniation.
Degenerative disease is strongly associated with back pain1 but often may be asymptomatic2 as well. Disk degeneration is associated with disk herniation and sciatica and affects the biomechanics of the spine based on interactive effects on the facet joints, spinal ligaments, and muscles, ultimately causing degeneration of the spinal unit as a whole.3,4 In the long term this can lead to spinal canal stenosis, a major cause of pain and disability in the elderly with increasing incidence due to demographic changes.5 However, the sequelae of disk degeneration are also among the leading causes of functional incapacity and a common source of chronic disability during work life.6 The role of imaging in degenerative disk disease is to provide accurate morphologic information that—together with clinical symptoms —guides therapeutic decision making. Hence understanding of pathologic and radiologic manifestations as well as a uniform classification of pathologic conditions is essential.7 This review summarizes aspects of normal anatomy of the intervertebral disk, pathologic mechanisms, terminology, and examples of the imaging spectrum of disk degeneration and herniation.
Anatomy The intervertebral articulation includes two types of joints. The symphyseal joint between the vertebral bodies consists of
Issue Theme Spine; Guest Editor, Mara Epermane, MD
the intervertebral disk and the vertebral end plates, the diskovertebral unit; it is part of the anterior column. The two synovial facet joints of the neural arch are part of the posterior column and supported by ligaments and muscle groups. Understanding the interrelationship of these components is critical, as surgical intervention is transitioning from fusion to joint replacement.
Intervertebral Disk The intervertebral disk is composed of three components: the nucleus pulposus, the inner part of the intervertebral disk, is located within the anulus fibrosus, in a slightly eccentric position toward the posterior surface of the disk. The nucleus is composed of a thin lattice of collagen fibers (type II collagen) that traverse through the gelatinous ground substance with a higher concentration of fibers in the equator of the disk. Regions of higher collagen concentration display a lower signal in T2-weighted sequences than the remainder of the disk. The ground substance of the nucleus consists of hydrophilic glycosaminoglycans and hyaluronic acid. It creates a high intrinsic pressure that allows the disk to resist compressive forces. The nucleus is avascular and its nutrition depends on diffusion. The anulus fibrosus is a complex fibrous and fibrocartilaginous structure that surrounds the nucleus pulposus. It
Copyright © 2014 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0034-1375566. ISSN 1089-7860.
This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited.
Semin Musculoskelet Radiol 2014;18:228–239.
MR Imaging of Degenerative Lumbar Disk Disease
Facet Joints The facet (or zygapophyseal) joints are the articulations of the posterior arch of the vertebrae and a part of the posterior column. They are synovial-lined joints with a fibrous capsule and connect the articular facets of adjacent vertebrae. The facets are located between the pedicle and lamina of each vertebra and form the articular pillars that provide structural stability but at the same time allow for a certain mobility to the vertebral column. The facet joints are symmetrical, supported and kept in position by the posterior ligamentous complex.
Ligaments The spinal ligaments act to stabilize the vertebral column together with several muscle groups. The anterior longitudinal ligament (ALL) is a strong band of fibers descending the anterior surface of the vertebral column and consists of three layers. The superficial layer traverses four to five vertebrae, the intermediate layer covers two to three, and the deep layer spans only one intervertebral articulation. The posterior ligamentous complex acts to hold the facet joints of adjacent vertebrae in a defined relation to each other. It comprises the
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posterior longitudinal ligament (PLL), the paired sets of ligamenta flava (connecting the laminae of adjacent vertebrae), the intertransverse ligaments (extending between the transverse processes), the interspinous ligaments (between the spinous processes), and the supraspinous ligament (connecting the very tips of the spinous processes). The PLL extends along the posterior surface of the vertebral column within the spinal canal. Its fibers are attached to the intervertebral disks and the margins of the vertebral bodies but not to the midposterior surface of the vertebral bodies, allowing vascular structures to enter and leave the medullary sinus beneath its fibers.
Age-related Changes The intervertebral disk displays age-related changes that may be distinguished from true degeneration. During normal aging the glycosaminoglycan content decreases and collagen content increases in the disk, and there is a diminished water content. This may lead to a mild loss of the normal high signal intensity in T2-weighted MR images (►Fig. 1). But because the nucleus pulposus volume remains the same with normal aging, it causes neither a reduction of intervertebral disk height (intervertebral chondrosis) nor additional alterations of the vertebral body end plates (intervertebral osteochondrosis). Pfirrmann et al8 suggest a systematic and reproducible MR imaging based classification of changes in the aging disk based on its internal structure, signal intensity, the distinction between the nucleus and anulus, and disk height. Several studies suggest that spondylosis deformans is a consequence of normal aging, since anterior and lateral marginal vertebral body osteophytes (spondylophytes) have been found in all individuals over 40 years of age.9 However, posterior osteophytes have been found in only a minority of skeletons of individuals over 80 years, and therefore are not considered inevitable consequences of aging.10
Degeneration Degeneration of the intervertebral disk and the facet joints appears to be interrelated rather than two separated processes,9 causing altered biomechanical stresses and acting on the integrity and mechanical properties of the spinal ligaments, which results in instability and degeneration of the spinal unit as a whole.3,4,11 The manner of degeneration of the various components of the spine depends on the structures involved. Collagen, ground substance, and cartilaginous and synovial structures each degenerate in a specific manner associated with characteristic pathologic and imaging findings.
Disk Degeneration The etiology of disk degeneration is multifactorial. In addition to mechanical loading, traumatic, biochemical, nutritional, and genetic factors play a role in the development of disk degeneration.5,12–16 Seminars in Musculoskeletal Radiology
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No. 3/2014
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consists of 15 to 20 collagenous (type I collagen) layers that run obliquely from the edge of one vertebra to the edge of the adjacent vertebra. The direction of the fibers alternates from lamina to lamina. The outer rim contains the most densely woven fibrous laminae that insert in the cortical bone of the ring apophysis; this portion displays low signal intensity on T2- and T1-weighted images. The inner portion of the anulus is made of fibrocartilage, contains chondrocytes and some amount of ground substance, and gradually blends with the nucleus pulposus; therefore this portion has higher signal intensity similar to the nucleus in T2weighted images and thus cannot be clearly differentiated from it. Posterolaterally, the anulus is thinner and contains more vertical fibers and a greater amount of disorganized collagen bundles. This feature makes the posterolateral area the weakest part of the disk and accounts for a high rate of disk herniation. The outer fibers of the anulus fibrosus receive blood supply from adjacent vessels in the end plate and are innervated by sinuvertebral nerves arising from the dorsal root ganglia. The inner layers have neither innervation nor blood supply; like the nucleus they receive nutrition via diffusion. The cartilaginous end plate covers most of the vertebral end plate; it includes hyaline cartilage at the vertebral aspect and fibrocartilage toward the anulus fibrosus. The ring apophysis surrounds the cartilaginous end plate; it consists of dense bone that fuses to the vertebra in the second decade of life. With its numerous collagen fibers the cartilaginous end plate is firmly attached to the osseous end plate, which contains multiple perforations for vascular channels. The cartilaginous end plate displays low signal on T2-weighted images and cannot be clearly differentiated from the bony end plate. However, due to its vascularization, enhancement can be noted after intravenous administration of contrast material.
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MR Imaging of Degenerative Lumbar Disk Disease
Heuck, Glaser
Fig. 1 (a–f) Disk aging and degeneration. (a, e) Sagittal T2-weighted turbo spin echo (TSE); (b) sagittal T1-weighted spin echo; (c, d, f) axial T2weighted TSE. (a) From L1-L2 down to L4-L5, an increasingly broad hypointense signal central in the disks can be observed, and the distinction between nucleus and anulus becomes less clear in T2-weighted sequences. (a, d)There is subtle bulging of the disk at L4-L5 in the axial and sagittal planes as opposed to the normal circumference of a healthy disk following the contour of the vertebral end plate (c); note the small focal midline protrusion (d). Hypointensity almost throughout the disk in T2-weighted sequences, here at the L5-S1 level (a), is suggestive of desiccation. Similarly, there is initial loss of signal indicative of incipient desiccation at the L4-S1 levels and disk bulging with a midline protrusion at L4-L5 (e, f). In all segments, note that there are no discernible bony end-plate changes.
Degenerative changes of the intervertebral disk include desiccation and fibrosis of the nucleus, mucinous degeneration and fissuring of the anulus, bulging of the anulus beyond the edges of the vertebral body ring apophyses with consecutive narrowing of the disk space, and defects and sclerosis of the cartilaginous end plates.
Desiccation Disk desiccation describes an apparently reduced water content of the disk that results from significant replacement of the hydrophilic glycosaminoglycans within the nucleus pulposus by collagen type I, the same collagen as in the anulus fibrosus. Desiccation is common, and its incidence increases with age (►Fig. 1). Although intriguing at first thought, disk desiccation per se does not necessarily result in a reduction of the volume of the nucleus pulposus. Therefore, it appears not to be the primary cause of disk space narrowing. This is rather a result of anular bulging, advanced degenerative changes, or vertebral end-plate bowing. Seminars in Musculoskeletal Radiology
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With progressing desiccation, the nucleus pulposus becomes gradually friable and disintegrated. Clefts appear that eventually may extend into the anulus fibrosus.
Anular Degeneration and Fissures Like other fibrous structures, the anulus fibrosus can be subject to mucinous degeneration and fissuring. Fissures (or tears) consist of separation of one or more layers of the anulus, avulsion of fibers from their vertebral body insertions, or breaches through fibers that may be radial, transverse, or concentric in orientation. Degeneration and some limited degree of fissuring is thought to be a precondition for disk bulging. Most anular fissures are asymptomatic; however, some are painful. The defects allow ingrowth of nerve endings and granulation tissue. Fissures near the dorsal root ganglion are especially likely to be painful.
Cartilaginous End-Plate Degeneration Degeneration of the cartilaginous end plates includes defects and sclerosis of the hyaline and fibrocartilage. In an attempt
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Table 1 Glossary of nomenclature and classification of disk pathology termsa • Aging disk: Disk demonstrating the features of normal aging. Spondylosis deformans possibly represents the normal aging process.
• Anterior displacement: Displacement of disk tissues beyond the disk space into the anterior zone.
• Anulus fibrosus: Multilaminated ligament surrounding the periphery of each disk space, attaching craniad and caudad to endplate cartilage and ring apophyseal bone and blending centrally with nucleus pulposus.
• Anulus rupture: Disruption of the fibers of the previously normal anulus by sudden violent injury.
• Base (of displaced disk): Cross-sectional area of disk material at the outer margin of the disk space of origin, where disk material displaced beyond the disk space is continuous with disk material within the disk space. In the craniocaudal direction, the length of the base cannot exceed, by definition, the height of the intervertebral space.
• Broad-based protrusion: Herniation of disk material extending beyond the outer edges of the vertebral body apophyses over an area > 25% (90 degrees) and < 50% (180 degrees) of the circumference of the disk.
• Bulging disk: Disk in which the contour of the outer anulus extends in the axial plane beyond the edges of the disk space, > 50% (180 degrees) of the disk circumference and usually 3 mm beyond the edges of the vertebral body apophyses.
• Central zone: Zone within the vertebral canal between sagittal planes through the medial edges of each facet.
• Chondrosis: Degenerative process of the intervertebral disk without vertebral body reactive changes. Characterized by disk space narrowing and eventually vacuum phenomenon and calcification.
• Chronic disk herniation: Disk herniation with presence of calcification, ossification, or gas accumulation within the displaced disk material.
• Contained herniation: Displaced disk tissue that is wholly within an outer perimeter of uninterrupted outer anulus.
• Continuity: Connection of displaced disk tissue by a bridge of disk tissue, however thin, to tissue within the disk of origin.
• Degenerated disk: Changes in a disk characterized by desiccation, fibrosis, and cleft formation in the nucleus, fissuring and mucinous degeneration of the anulus, defects and sclerosis of end plates, and/or osteophytes at the vertebral apophyses.
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• Anterior zone: Peridiskal zone anterior to the midcoronal plane of the vertebral body.
MR Imaging of Degenerative Lumbar Disk Disease
Heuck, Glaser
• Degenerative disk disease: Clinical syndrome characterized by manifestations of disk degeneration and symptoms thought to be related to those changes.
• Desiccated disk: Disk with reduced water content, usually primarily of nuclear tissues.
• Disk: Complex structure composed of nucleus, anulus, cartilaginous end plates, and vertebral body ring apophyseal attachments of anulus.
• Disk space: Space limited craniad and caudad by the end plates of the vertebrae and peripherally by the edges of the vertebral body ring apophyses exclusive of osteophytes. Synonym: intervertebral disk space.
• Diskogenic vertebral sclerosis: Increased bone density and calcification adjacent to the end plates of the vertebrae craniad and caudad to a degenerated disk, usually a manifestation of intervertebral osteochondrosis.
• Displaced disk: Disk in which disk material is beyond the outer edges of the vertebral body ring apophyses (exclusive of osteophytes) of the craniad and caudad vertebrae, or, as in the case of intravertebral herniation, penetrated through the vertebral body end plate.
• Extraforaminal zone: Zone beyond the sagittal plane of the lateral edges of the pedicles, having no well-defined lateral border.
• Extruded disk: Herniated disk in which, in at least one plane, 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 in the same plane, or when no continuity exists between the disk material beyond the disk space and that within the disk space.
• Fissure of anulus: Separations between anular fibers, avulsion of fibers from their vertebral body insertions, or breaks through fibers that extend radially, transversely, or concentrically. Synonym: tear of annulus.
• Focal protrusion: Protrusion of disk material, the base of the displaced material is < 25% (90 degrees) of the circumference of the disk.
• Foraminal zone: Zone between planes passing through the medial and lateral edges of the pedicles.
• Herniated disk: Localized displacement of disk material beyond the normal margins of the intervertebral disk space < 50% (180 degrees) of the circumference of the disk.
• Intervertebral chondrosis: Degenerative process limited to the intervertebral disk.
• Intervertebral osteochondrosis: Degenerative process involving the nucleus pulposus, the anulus fibrosus, and the vertebral end plate.
• Intradural herniation: Disk from which displaced tissue has penetrated the dura so that it lies within the thecal sac.
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• Intravertebral herniation: Disk in which a portion of the disk is displaced through the end plate into the centrum of the vertebral body. Synonym: Schmorl’s node.
• Limbus vertebrae: Separation of a segment of rim of vertebral body ring apophysis. Note: Limbus vertebrae may result from fracture or from developmental abnormalities. Limbus vertebrae is commonly seen in patients who have had Scheuermann disease. The lesions may be called rim lesions.
• Migrated disk: Herniated disk in which a portion of extruded disk material is displaced away from the tear in the outer anulus through which it has extruded.
• Nucleus of origin: Central nuclear portion of the disk of reference, usually used to reference the disk from which tissue has been displaced. Synonyms: parent nucleus, disk of origin.
• Osteophytes: Focal hypertrophy of bone surface and/or ossification of soft tissue attachments to the bone.
• Pedicular level: Level between axial planes through the upper and lower edges of the pedicle.
• Prolapsed disk: Herniated disk in which disk tissue has protruded or extruded at the level of the disk and below into the suprapedicular level.
• Protruded disk: Herniated disk in which the greatest distance, in any plane, between the edges of the disk material beyond the disk space is less than the distance between the edges of the base in the same plane.
• Sequestrated disk: Extruded disk in which a portion of the disk tissue is displaced beyond the outer anulus and maintains no connection by disk tissue with the disk of origin.
• Sequestrum: Disk tissue that has become displaced from the disk space of origin and lacks any continuity with disk material within the disk space of origin. Synonym: disk fragment.
• Spondylosis deformans: Degenerative process of the spine involving the anulus fibrosus and characterized by anterior and lateral marginal osteophytes arising from the vertebral body apophyses; the intervertebral disk height is normal or only slightly decreased.
• Subarticular zone: Zone, within the vertebral canal, sagittally between the plane of the medial edges of the pedicles and the plane of the medial edges of the facets, and coronally between the planes of the posterior surfaces of the vertebral bodies and the under anterior surfaces of the superior facets. Synonyms: lateral recess, posterolateral zone.
• Syndesmophytes: Thin and vertically oriented bony outgrowths extending from one vertebral body to the next and representing ossification within the outer portion of the anulus fibrosus.
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• Marginal osteophyte: Osteophyte that protrudes from and beyond the outer perimeter of the vertebral end-plate apophysis.
MR Imaging of Degenerative Lumbar Disk Disease
Heuck, Glaser
• Transligamentous: Displacement, usually extrusion, of disk material through the posterior longitudinal ligament. Synonym: perforated.
• Vacuum disk: Disk with imaging characteristics suggestive of gas in the center of the disk space, usually a manifestation of disk degeneration
a
Short list adapted from American Journal of Neuroradiology Web site.18
to heal, chondrocyte degeneration and granulation tissue formation may occur within the cartilaginous endplate extending into the anulus fibrosus and sometimes even into the degenerated nucleus pulposus.17 As a result of more advanced disk degeneration, alterations of the adjacent vertebral body architecture may occur. These diskogenic vertebral changes involve the vertebral end plates and the subchondral bone marrow space. Any clinical approach to degenerative disk disease, whether related to diagnosis or management, demands agreement on a terminology that is reliable and reproducible to describe the pathologic condition of a disk. In this review we use the most widely accepted terminology that was originally set up by the American Society of Neuroradiology, the American Society of Spine Radiology, and the North American Spine Society and has been adopted by many other scientific societies worldwide.7,18 ►Table 1 provides a short version of this nomenclature and classification.
Imaging of Disk Degeneration The first sign of disk degeneration on imaging is desiccation in which the disk loses its high T2 signal and the horizontal midline low signal cleft is becoming gradually inapparent (►Fig. 1).
Although anular fissures are common, only a minority is identified on MRI, and if so, it is characterized by a region of high T2 signal in the otherwise low-signal anulus (►Fig. 2). If there is ingrowth of vascularized granulation tissue into a fissure, there may be enhancement within the cleft after intravenous contrast administration. A fissure may involve all layers or only some laminae of the anulus, but this distinction is difficult on MR imaging and impossible on computed tomography (CT) if no disk extrusion is seen. Disk bulging defines a disk in which the contour of the anulus extends 3 mm beyond the edges of the vertebral body apophyses, along more than half of its circumferential length (>180 degrees). Because bulging is limited by the anulus fibrosus, it does not extend above or below the attached margins of the disk. Bulging is in part responsible for narrowing of the intervertebral disk space. It can be easily identified on CT and MR imaging by a broad, more than semicircular extension of the anulus. The degree of extension ( 3 mm), per definition, may be estimated from axial, sagittal, and coronal MR images. However, the circumferential involvement is best defined in the axial plane. A disk herniation refers to a localized displacement of intervertebral disk material beyond the normal margins of the intervertebral disk space but involving less than half the
Table 2 Intervertebral disk displacements Bulge Herniation
> 180 degrees of circumference, 3-mm displacement < 180 degrees of circumference
Protrusion
< 180 degrees of circumference, base > dome
Broad based
90–180 degrees of circumference, base > dome
Focal
< 90 degrees of circumference, base > dome
Contained
outer laminae of anulus intact
Noncontained
all laminae of anulus deficient)
Extrusion
< 180 degrees of circumference, waist < dome
Sequestration
Portion of extruded disk is displaced beyond the anulus; no connecting disk tissue with the disk of origin
Migration
Portion of extruded disk is displaced away from tear in outer anulus
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Heuck, Glaser
Fig. 2 (a–d) Disk degeneration. (a-c) Sagittal T2-weighted turbo spin echo (TSE), T1-weighted spin echo, T1-weighted fat saturation with intravenous contrast; (d) axial T2-weighted TSE. There is desiccation from level L3 to S1. (a-c) Note the subtle anular fissure at L3-L4 with nucleus material extending behind the posterior edge of L3 and strong contrast uptake indicating vascularization without a prominent change of disk shape. There is a small intraosseous disk herniation (Schmorl node) in L4 with predominantly fatty peripheral reactive changes and central granulation tissue. (d ) Small extrusions at the L4-S1 levels show displaced disk material extending beyond the level of the disk space in the sagittal and axial planes, also showing at least partial contrast enhancement.
circumference (
