Radiographs

The usefulness of conventional radiographs is primarily in the extradural space. Conventional radiographs are relatively inexpensive and easy to perform. They provide a convenient means of assessing alignment and gross bony integrity and can also be used for purposes of localization and evaluation of movement. They are capable of demonstrating the general changes involving various types of arthritis and disk space narrowing. According to recent appropriateness criteria, radiographs are considered sufficient for the initial evaluation of recent significant trauma, osteoporosis, or back pain in individuals older than 70 years.¹ Substitution of digital radiography for conventional film-screen techniques has provided more flexible handling of images (e.g., duplication, transmission, postprocessing), as well as expedited cycle time. This latter advantage is extremely useful in the intraoperative environment because the results are available immediately after exposure without the need for traditional processing.² The disadvantages of conventional radiographic techniques are that they are able to visualize only bony structures and are relatively insensitive to soft tissue changes within the disk, ligaments, nerves, and paraspinal tissues. Bone marrow involvement must be significant and far advanced before it is demonstrable on conventional radiographs.

Isotope Bone Scans

Isotope bone scans are most useful for evaluation of the extradural space. They are a moderately sensitive test for detecting the presence of tumor, infection, or occult fractures of the vertebrae but are rarely specific for diagnosis. The yield is very low in patients with normal radiographs and laboratory studies and highest for those with known malignancy. These scans are contraindicated in pregnancy. They are an important tool for surveying the entire skeleton for metastatic disease. To further evaluate sites positive on isotope bone scans, single-photon emission computed tomography (SPECT) and positron emission tomography (PET) are often used in conjunction with computed tomography (CT) and are capable of improving specificity and anatomic localization, particularly in patients with malignancies.

Computed Tomography

CT without contrast enhancement is most useful for evaluation of the extradural space. Depending on the window and center setting, CT can discriminate among various tissues by differences in attenuation. CT is readily available and can provide high-resolution bony detail and enough soft tissue information to accurately assess degenerative disk disease and associated changes or sequelae such as herniation, stenosis, and facet arthritis. CT is equivalent to magnetic resonance imaging (MRI) in the diagnosis of a lumbar herniated disk and stenosis but less accurate in depicting marrow changes and soft tissue structures such as paravertebral lesions and intraspinal structures or pathology. The primary disadvantages of CT are the overlap in attenuation with normal and abnormal soft tissue pathology and, as with conventional radiography, ionizing radiation. Recent technical advances in CT have led to the production of multidetector devices that can provide larger areas of coverage in shorter examination times. In addition, thinner sections can be used to allow the acquisition of isotropic three-dimensional (3D) data sets, which can be reformatted in any plane without loss of resolution. Subsequent 3D multiplanar reformatted images are particularly useful for the evaluation of bony trauma and assessment of fusion postoperatively, pseudoarthrosis, and instrumentation. CT remains an important tool and is the first alternative when MRI is contraindicated or unavailable.

CT Myelography

The introduction of contrast material into the intrathecal space provides both extradural and intradural information. Myelography is able to outline the nerve roots, spinal cord, and its coverings. Intradural and extradural disease can be identified and characterized on the basis of the character and morphology of its effect on the contrast column (Fig. 18-1). Plain-film contrast-enhanced myelography is rarely performed on its own and is almost always used in conjunction with CT. Nonionic contrast material has completely replaced its oil-based predecessor and is usually introduced into the lumbar region at L3 or L4 or cervically at C1-2. Plain-film conventional myelography may be useful for general anatomic appraisal in patients with complex deformities or when implanted hardware or electronic devices preclude the use of MRI or CT. CT myelography provides excellent delineation of intradural structures such as the spinal cord and nerve roots and their relationship to the soft tissue and bony surroundings. It is useful in distinguishing bone from soft tissue changes (e.g., osteophytes and disk herniations), which may be inseparable on plain-film myelography (Fig. 18-2). Multidetector CT of the entire spinal axis can be performed in 20 to 30 seconds. With intrathecal contrast media, subsequent multiplanar reformatted images can provide complex 3D assessment of anatomic and pathologic abnormalities with myelographic-like images. More recently, various forms of dual-energy multidetector CT have been introduced that can subtract out bone in both single slices and 3D multiplanar reformatted images (Fig. 18-3).

FIGURE 18-1 A, Diagrammatic representation of extradural (ED), intradural extramedullary (ED-EM), and intramedullary (IM) lesions of the spine. Purple, spinal cord; orange, mass; tan, subarachnoid space. B, Oblique view from a cervical myelogram demonstrating an extradural defect (arrow) from a herniated disk at the C6-7 level with cutoff of the nerve root. The diagram demonstrates the typical concave inward appearance of an extradural defect. C, Anteroposterior (AP) radiographs from a thoracic myelogram demonstrating an intradural extramedullary lesion. Contrast agent was introduced into the subarachnoid material below the lesion on the left and at C1-2 to outline the superior extent of the lesion on the right. One can see displacement of the cord silhouette to the left. D, AP views from a thoracic myelogram demonstrating an intramedullary mass at the conus. The contrast column in the subarachnoid space is splayed around the intramedullary mass. CT myelography demonstrates the enlarged conus.

FIGURE 18-2 Coronal and sagittal multiplanar reformatted CT myelographic images obtained from the axial data set. Hypertrophic degenerative changes of the vertebral body margins and uncinate processes are producing extradural indentations on the subarachnoid space.

FIGURE 18-3 Dual-energy subtraction CT myelography. Primarily acquired axial and multiplanar reformatted coronal images through the lumbar region from a CT myelographic study were obtained with a CT scanner that uses two x-ray tubes. Different exposure factors using 80 and 140 kV produce data sets on which subtraction can be performed on the basis of attenuation differences related to the exposure factors.

The disadvantages of myelography/CT myelography relate primarily to the introduction of contrast media into the subarachnoid space, use of ionizing radiation, and limited soft tissue discrimination. Myelography is an invasive procedure associated with postprocedural headaches (10% to 15%) and rare but possible complications such as nerve damage, formation of an iatrogenic epidermoid, infection, and arachnoiditis. The incidence of headaches severe enough to require treatment (e.g., epidural blood patch) is approximately 1%.

Magnetic Resonance Imaging

MRI has become the initial imaging technique for most patients with significant signs and symptoms of spinal disorders. It provides excellent contrast among different types of soft tissue. MRI without exogenous contrast material is capable of evaluating the extradural, intradural extramedullary, and intramedullary spaces. MRI is noninvasive and without the risks associated with ionizing radiation. Anatomic imaging can be performed in different planes, as well as used for volume acquisitions. It is the most sensitive modality and most specific technique for identifying abnormalities of the spinal cord, nerve roots, cerebrospinal fluid (CSF) space, soft tissues, and bone. When used with gadolinium complexed with diethylenetriaminepentaacetic acid (DTPA), MRI has even greater sensitivity for intramedullary disease, inflammatory changes, and reparative processes such as may be seen with postoperative changes and trauma. MRI is particularly efficacious for the evaluation of so-called red flag diagnoses such as osteomyelitis, neoplasia, and trauma.

Traditional clinical imaging has emphasized orthogonal T1- and T2-weighted imaging for morphologic assessment of the diskovertebral complex. These sequences also provide an evaluation of the changes in signal intensity associated with degenerative disk disease. Fast spin echo (SE) T2-weighted images have replaced conventional T2-weighted images because of their shorter acquisition times, but they provide no increased diagnostic advantage. Short tau inversion recovery (STIR) or fat-suppressed T2-weighted images have been added because they are more sensitive to marrow and soft tissue changes (Fig. 18-4). Although these standard sequences remain the mainstay of diagnostic MRI of the spine, new techniques continue to be evaluated in the hope of providing stronger correlation between imaging findings and patient symptoms. The utility of many of these techniques for the routine evaluation of degenerative disk disease remains unknown, and the number of patients in whom they have been evaluated remains small. Nevertheless, these approaches may be important for redefining the direction of spinal imaging from strictly anatomic imaging to imaging that combines more physiologic and functional information.³ Techniques that have been evaluated include assessment of spinal motion (dynamic imaging, kinetic assessment, or axial loading), diffusion imaging (water or contrast agents), MR neurography, spectroscopy, functional MRI of the spinal cord, and ultrashort–echo time imaging. Of the variety of techniques available, only MR neurography and dynamic imaging have expanded beyond the experimental phase to demonstrate specific clinical utility (albeit in niche populations).

FIGURE 18-4 MRI sequences. Sagittal T1-weighted spin echo, T2-weighted spin echo, and short tau inversion recovery (STIR) images through the lumbar spine demonstrate the different contrast obtained with different techniques. On the T1-weighted image, the vertebral body marrow has higher signal intensity than the intervertebral disk, primarily because of the fat content of the marrow space. On the T2-weighted spin echo image, the intervertebral disk has higher signal intensity than the vertebral body because of prolonged relaxation time related to both bound and unbound water species. This change in signal is probably a reflection of the health of the proteoglycans more than the total water content. The STIR sequence demonstrates decreased signal intensity of the marrow space and is the sequence that is most sensitive to marrow involvement. Note the degenerative changes at L4-5 and L5-S1. The decreased signal intensity of a degenerative disk is better noted on the T2-weighted and STIR sequences. Note the loss of the internuclear cleft as the degree of degeneration increases, as at the L5-S1 level.

Dynamic Imaging

The utility of dynamic spine MRI remains unclear, in part because of the varied methods used. Methods used to date include axial loading in the supine position by means of a harness attached to a nonmagnetic compression footplate with nylon straps that can be tightened or the use of an upright open MRI system that allows flexion-extension imaging (Fig. 18-5).^4,5 Dynamic MRI has been used to evaluate for the presence of occult herniations, which may not be visible or be less visible when the patient is supine, to measure motion between spinal segments, and to measure canal or foraminal diameter when subjected to axial loading.^3,5–8 Hiwatashi and colleagues evaluated 200 patients with clinical symptoms of spinal stenosis and found 20 with detectable differences in caliber of the dural sac on routine and axial-loaded studies.⁷ In 5 of these selected patients, all three neurosurgeons involved in the clinical evaluation changed their treatment recommendation from conservative management to decompressive surgery. Although a subset of patients may benefit from this type of evaluation, the benefit appears to be small for the added machine time and patient discomfort.

FIGURE 18-5 Sagittal T2-weighted spin echo images through the cervical spine in flexion (A) and extension (B). Note the decreased anterior-posterior canal diameter on the extension image (7 versus 4 mm) at C3-4.

Cerebrospinal Fluid

CSF flow studies with cine MRI provide cardiac-gated gradient phase-contrast images that are displayed qualitatively as CSF flow images in a closed loop kinematic format. This same data set can be displayed quantitatively on a graph or in numerical format as analysis of values of flow velocity and volume flow rate. These images demonstrate the pulsatile motion of CSF, not the bulk flow. Pulsatile flow is a result of expansion of the brain during systole and relaxation during diastole and is therefore bidirectional, with CSF flowing caudally during systole and cranially during diastole. Typically, the signal intensity of normal CSF flow in these studies is demonstrated as hyperintense during systole, where there is caudal (downward) flow, and hypointense during diastole, where there is cranial (upward) flow. These images are usually displayed in the sagittal and axial planes. In general, the degree of pulsatile flow diminishes as it proceeds caudally. Clinically, these studies are most often used for the evaluation of CSF flow in patients with Chiari I and II malformations and for the assessment of syrinx cavities (Fig. 18-6).

FIGURE 18-6 A-D, Sagittal T1-weighted, T2-weighted, and cerebrospinal fluid (CSF) flow studies of the craniovertebral junction in a patient with a Chiari I malformation. Note that the majority of CSF flow is within the basal cisterns and ventral to the cervical cord. Only a small amount of CSF flow is identified dorsally (arrow).

Artifacts and Contraindications

Instrumentation, metal used as part of surgical procedures, and implanted electronic devices represent challenges for both CT and MRI. In the case of CT, metal can create beam-hardening artifacts that obscure adjacent soft tissue and osseous structures (Fig. 18-7). Multidetector studies with isotropic voxels and software used to decrease beam-hardening artifact can often decrease the image degradation caused by implanted metal.²⁸ Different types of metal produce different types of artifacts on MRI that may make the examination uninterpretable. The term magnetic susceptibility describes the manner and amount by which a material becomes magnetized in a magnetic field. Nonferrous magnetic metals may produce local electrical currents induced by the changing field, which causes distortion of the field and artifacts. These artifacts take two main forms: geometric distortion and signal loss secondary to dephasing (Fig. 18-8). Different techniques are more susceptible to these artifacts, and gradient echo images in particular are especially sensitive to differences in magnetic susceptibility and field homogeneity. Although metals can cause artifacts that render the examination uninterpretable, certain indwelling devices may be a contraindication to the entire examination. Certain types of aneurysm clips, cardiac valves, and vascular devices can result in harm to patients during an MRI examination. Some implantable devices may cease to function properly after an MRI study, and others may be a source of heating with deleterious biologic effects and burns. Readers are referred to publications and websites that track information related to contraindications.²⁸

FIGURE 18-7 Beam-hardening artifact on axial CT secondary to pedicle screws.

FIGURE 18-8 Metallic artifacts seen on sagittal (A) and axial (B) T1-weighted images through the thoracic spine in a patient with steel rods. There is a geometric artifact with complete distortion of the image as a result of the stainless steel. Sagittal T1-weighted (C), sagittal gradient echo (D), axial gradient echo (E), and axial CT myelographic (F) images. On the sagittal T1 image, a small extradural defect is noted at the C5-6 level (white arrow). This defect appears more prominent on the gradient echo image (black arrow). On the axial gradient echo image this defect appears midline. Axial CT myelography shows postsurgical changes but no evidence of an extradural defect. These changes are secondary to a susceptibility effect produced by small shards of metal that are often noted on MRI after surgery but are too small to be seen on CT or plain films. The susceptibility artifact can be mistaken for a recurrent disk herniation or osteophyte. Sagittal T1-weighted (G), sagittal T2-weighted (H), and gradient echo (I) images of the lumbar spine in a patient with a metallic intradiscal fixation device at L4-5 and L5-S1. Note that the susceptibility artifact is more prominent on the T2-weighted image (white arrows) than on the T1-weighted sequence (black arrows). It is most prominent on the gradient echo image because of its heightened sensitivity to field inhomogeneity and susceptibility changes.

The primary disadvantages of MRI are related to its cost and safety concerns with respect to electronic devices or implants. The safety issues include heating, dislodgment, and malfunction, as well as image distortion in the presence of metal. Claustrophobia remains a significant problem that sometimes requires machines with a larger bore and sedation or general anesthesia.

Intravenous contrast agents are commonly used with both CT and MRI examinations. Both iodinated (CT) and paramagnetic substances (MRI) are cleared by the kidney and should be used with caution or not at all in patients with impaired renal function. Although this has been common knowledge with the iodinated contrast media used for CT, recent evidence has also documented various complications related to the use of paramagnetic contrast agents in patients with impaired renal function. Despite being uncommon, nephrogenic systemic fibrosis, a late serious adverse reaction to gadolinium, has been well documented.^28,29

Spinal Angiography

Spinal angiography can be performed with MRI (magnetic resonance angiography [MRA]), CT (CT angiography [CTA]), and catheter-based digital subtraction angiography with selective arterial catheterization. The latter is still regarded as the most accurate technique for the detection and pretreatment characterization of vascular malformations of the spinal cord and meninges. Although probably associated with the highest risk among procedures for spinal imaging, the complication rate has been greatly reduced with the use of microcatheters and more experienced users. Definitive treatment of vascular lesions by endovascular means is now more common, and the examination can be better directed by information from diagnostic MRA and CTA studies, which can help focus the catheter examination. The addition of MRA to standard MRI does not significantly alter the sensitivity, specificity, and accuracy of detection of spinal malformations but often provides detection of the level of a fistula or vascular malformation. CTA of the spinal axis is relatively easy to perform and is also useful in identifying the level of involvement.

Ultrasound

The use of ultrasound in the spine is limited because of difficulty penetrating bone. Its main value is intraoperative for additional characterization and localization of disease such as spinal cord tumors or in pediatric applications. Intraoperatively, high-frequency probes that improve spatial resolution are commonly used because of the ability to directly approach the region of pathology.

Thermography, Diskography, CT Diskography

Most expert panels believe that thermography is too nonspecific to be of significant value for the evaluation of spinal disorders. Diskography is a long-standing provocative technique that by nature of direct stimulation, is thought by some to be able to identify painful and concordant disks.^1,30 Diskography and CT diskography are still used, especially when other imaging modalities have failed to localize the cause of pain. The morphologic information is not as critical as reproduction of a patient’s characteristic pain. Although some data support its use, prospective well-controlled trials in support of its prognostic value remain absent from the literature.

Terminology

The term degeneration includes any or all of the following: real or apparent desiccation, fibrosis, narrowing of the disk space, diffuse bulging of the annulus beyond the disk space, extensive fissuring and mucinous degeneration of the annulus, defects and sclerosis of the end plates, and osteophytes at the vertebral apophyses. Conventional radiographs are able to identify disk space narrowing, bony changes, and malalignment. CT can further identify disk herniation and provide better morphologic characterization of various types of stenosis. MRI provides the greatest spectrum of morphologic findings. With MRI, degenerative changes are manifested as disk space narrowing; loss of T2-weighted signal intensity from the intervertebral disk; the presence of fissures, fluid, vacuum changes, and calcification within the vertebral disk; ligamentous signal changes; marrow signal changes; osteophytosis; disk herniation; malalignment; and stenosis.

Degenerative Disk Changes

The major cartilaginous joint (amphiarthrosis) of the vertebral column is the intervertebral disk. Each disk consists of an inner portion, the nucleus pulposus, surrounded by a peripheral portion, the annulus fibrosus. The nucleus pulposus is eccentrically located and is closer to the posterior surface of the intervertebral disk. With degeneration and aging, type II collagen increases outwardly in the annulus, and there is greater loss of water from the nucleus pulposus than from the annulus. This results in a loss of the hydrostatic properties of the disk, with an overall reduction in hydration in both areas to about 70%. In addition to water and collagen, the other important biochemical constituents of the intervertebral disk are the proteoglycans. The individual chemical structures of the proteoglycans are not changed with degeneration, but their relative composition is. The ratio of keratin sulfate to chondroitin sulfate increases, and there is a diminished association with collagen, which may reduce the tensile strength of the disk. The decrease in water-binding capacity of the nucleus pulposus is thought to be related to the decreased molecular weight of its nuclear proteoglycan complexes (aggregates). The disk becomes progressively more fibrous and disorganized, with the end stage represented by amorphous fibrocartilage and no clear distinction between the nucleus and annulus.^31–33 On T2-weighted images, central disk signal intensity is usually markedly decreased and at distinct variance to that seen in unaffected disks of the same individual. Work with T2-weighed SE sequences³⁴ suggests that MRI is capable of depicting changes in the nucleus pulposus and annulus fibrosus associated with degeneration and aging based on the loss of signal intensity (Fig. 18-9). In work with cadaver spines of various ages, absolute T2 measurements correlated more closely with the glycosaminoglycan concentration than with the absolute water content. Thus, the signal intensity may not be related to the total amount of water but rather the state of the water. At present, the role that specific biochemical changes (proteoglycan ratios, aggregation of complexes) plays in the altered signal intensity is not well understood. Given that the T2 signal intensity in the disk appears to track the concentration and regions with a high percentage of glycosaminoglycan more than the absolute water content, it seems likely that the health and status of the proteoglycans are major determinants of signal intensity.³⁵ It has been proposed that annular disruption is the critical factor in degeneration and that when a radial tear develops in the annulus, there is shrinkage with disorganization of the fibrous cartilage of the nucleus pulposus and replacement of the disk by dense fibrous tissue with cystic spaces (Fig. 18-10).^36–40 Annular tears, also properly called annular fissures, are separations between annular fibers, avulsion of fibers from their insertions on the vertebral body, or breaks through fibers that extend radially, transversely, or concentrically and involve one or many layers of the annular lamellae. The term tear or fissure describes the spectrum of such lesions and does not imply that the lesion was caused by trauma (see Fig. 18-1). Although it is established that annular disruption is a sequela of degeneration and is often associated with it, its role as the causal agent of disk degeneration has not been proved. MRI is the most accurate anatomic method for assessing intervertebral disk disease. The signal intensity characteristics of the disk on T2-weighted images reflect changes caused by aging or degeneration. A classification scheme for lumbar intervertebral disk degeneration has been proposed that has reasonable intraobserver and interobserver agreement.⁴¹ To date, however, there has been no correlation between disk changes on MRI and a patient’s symptoms. With loss of water and proteoglycans, the nucleus pulposus is desiccated and friable with yellow-brown discoloration. Its onion skin appearance begins to unravel, and cracks, clefts, or crevices appear within the nucleus and extend into the annulus fibrosus. Fissuring, chondrocyte generation, and formation of granulation tissue may be noted within the end plate, annulus fibrosus, and nucleus pulposus of degenerative disks and are indicative of attempts at healing.⁴⁰ Radiolucent collections (vacuum disk phenomena) representing gas, principally nitrogen, occur at sites of negative pressure produced by the abnormal spaces.⁴² The vacuum phenomenon within a degenerated disk is represented on SE images as areas of signal void (Fig. 18-11).⁴³ Although the presence of gas within the disk is usually suggestive of degenerative disease, spinal infection may (rarely) be accompanied by intradiscal or intraosseous gas.⁴⁴ As intervertebral osteochondrosis progresses, there may be calcification of the disk. Calcification has usually been described on MRI as a region of decreased or absent signal intensity. The loss of signal is attributed to a low mobile proton density, as well as, in the case of gradient echo imaging, to its sensitivity to the heterogeneous magnetic susceptibility found in calcified tissue. There is, however, variability in the signal intensity of calcium with various sequences, and the type and concentration of calcification are important factors. Hyperintense disks on T1-weighted MRI may be secondary to calcification (Fig. 18-12).⁴⁵ For concentrations of calcium particulates of up to 30% by weight, the signal intensity on standard T1-weighted images increases but then subsequently decreases.^46,47 These data probably reflect particulate calcium reducing T1 relaxation times by a surface relaxation mechanism. Hyperintensities that are affected by fat suppression techniques have also been noted within intervertebral disks and are thought to be related to ossification with lipid marrow formation in severely degenerated or fused disks.

FIGURE 18-9 Degenerative disk. There is mild disk space narrowing (arrow) and loss of signal intensity on T2-weighted images of the L4-5 disk.

FIGURE 18-10 Annular tear seen on parasagittal (A) and axial (B) T2-weighted images through the L4-5 disk. Note the high signal intensity in the outer annulus/longitudinal ligament complex, which represents the area of annular disruption (arrows).

FIGURE 18-11 A and B, Vacuum phenomenon. Note the decreased signal intensity within the L4-5 disk, which is better seen on the gradient echo image (white arrow) than on the spin echo image (black arrow).

FIGURE 18-12 Intervertebral disk calcification (arrows). Note the high signal intensity on T1-weighted (A) and T2-weighted (B) images at the L5-S1 level.

Degenerative Marrow Changes

Changes in signal intensity of the vertebral body marrow adjacent to the end plates of degenerated disks are a long recognized and common observation on MRI of the lumbar spine.^48,49 However, despite a growing body of literature on this subject, their clinical importance and relationship to symptoms remain unclear.⁵⁰

These marrow changes appear to take three main forms. Type I changes consist of decreased signal intensity on T1-weighted images and increased signal intensity on T2-weighted images (Fig. 18-13). They have been identified in approximately 4% of patients scanned for lumbar disease,⁴⁹ in approximately 8% of patients after diskectomy,⁵¹ and in 40% to 50% of chymopapain-treated disks, which may be viewed as a model of acute disk degeneration.⁵² Histopathologic sections of disks with type I changes show disruption and fissuring of the end plate and vascularized fibrous tissue within the adjacent marrow that prolongs T1 and T2 times. Enhancement of type I vertebral body marrow changes is seen with the administration of gadolinium, and at times the changes extend to involve the disk itself and are presumably related to the vascularized fibrous tissue within the adjacent marrow. Type II changes are represented by increased signal intensity on T1-weighted images and isointense or slightly hyperintense signal on T2-weighted images (Fig. 18-14). They have been identified in approximately 16% of patients on MRI. Disks with type II changes also show evidence of end plate disruption, with yellow (lipid) marrow replacement in the adjacent vertebral body resulting in a shorter T1 time. Type III changes are represented by decreased signal intensity on both T1- and T2-weighted images and correlate with extensive bony sclerosis on plain radiographs. The lack of signal in type III changes no doubt reflects the relative absence of marrow in areas of advanced sclerosis (Fig. 18-15). Unlike types III, types I and II changes show no definite correlation with sclerosis on radiography.⁵³ This is not surprising when one considers the histology; the sclerosis seen on plain radiographs is a reflection of dense woven bone within the vertebral body, whereas the MRI changes are more a reflection of the intervening marrow elements.

FIGURE 18-13 Type I degenerative marrow changes. Sagittal T1-weighted (A) and T2-weighted (B) images demonstrate decreased signal intensity of the L5 vertebral body adjacent to the degenerated disk space on T1 and increased signal intensity on T2.

FIGURE 18-14 Type II degenerative marrow change (arrows). There is increased signal intensity along the opposing margins of L5 and S1 on both T1-weighted (A) and T2-weighted (B) images.

FIGURE 18-15 Type III degenerative marrow change (arrows). There is decreased signal intensity of the opposing vertebral body margins at L4-5 on both T1-weighted (A) and T2-weighted (B) images.

Similar marrow changes have also been noted in the pedicles (Fig. 18-16). Although originally described as being associated with spondylolysis, they have also been noted in patients with degenerative facet disease and pedicle fractures.^54,55 The exact mechanism by which these marrow changes occur is not known. The association of these marrow changes with degenerative disk disease, facet changes, and pars and pedicle fractures suggest that they are a response to biomechanical stress.

FIGURE 18-16 Pedicle hyperintensity/pars interarticularis fracture. Parasagittal T1-weighted (A), T2-weighted (B), and short tau inversion recovery (STIR) (C) images demonstrate high signal intensity within the pedicle of L4, which is best appreciated on the STIR sequence (arrow). Parasagittal (D) and sagittal (E) CT multiplanar reformatted images demonstrate a subacute fracture through the pars at L4-5.

Of these three types, type I changes appear to be more fluid and variable, a reflection of ongoing underlying pathologic processes such as continuing degeneration with associated changing biomechanical stresses. Of the three types, type I is most often associated with ongoing low back symptoms.^56–60 In a longitudinal study, the incidence of new degenerative marrow changes was 6% over a 3-year period, with most being type I.⁵⁸ In a study of nonoperated patients with low back pain, Mitra and associates found that 92% of type I changes converted either wholly or partially to type II (52%), became more extensive (40%), or remained unchanged (8%).⁵⁷ There was an improvement in symptoms in patients in whom type I changes converted to type II.

Some diskography studies in patients with degenerative marrow changes have suggested that type I marrow changes are invariably associated with painful disks.^61,62 Others have failed to reproduce this association,^63,64 and thus the relationship between degenerative marrow changes and diskogenic pain remains unproved.

Multiple authors have observed a variety of inflammatory mediators in association with degenerative marrow changes. Burke and colleagues observed an increase in proinflammatory mediators such as interleukin-6, interleukin-8, and prostaglandin E₂ in the disks of patients with type I marrow changes who were undergoing fusion for low back pain.⁶⁴ Ohtori and coworkers found that the cartilaginous end plates of patients with type I marrow changes had more protein gene product (PGP) 9.5 immunoreactive nerve fibers and cells immunoreactive for tumor necrosis factor (TNF) than did patients with normal end plates.⁶⁵ PGP 9.5 immunoreactivity was seen exclusively in patients with diskogenic low back pain. TNF immunoreactivity in end plates with type I marrow changes was higher than in those with type II marrow changes. The authors concluded that type I marrow changes represent a more active inflammation mediated by proinflammatory cytokines whereas type II and type III changes are more quiescent.⁵⁰ In a study of infliximab, a monoclonal antibody against TNF-α, Korhonen and associates found that it was most effective when there were degenerative type I marrow changes at the symptomatic level.⁶⁶ Nevertheless, the relationship of degenerative marrow changes to immunobiologic and cellular response mechanisms, although probably important, remains unclear.

In a study by Toyone and colleagues, 70% of patients with type I marrow changes had segmental hypermobility versus 16% with type II changes.⁵⁶ Probably the greatest indication that these marrow changes, particularly type I, are related to biomechanical instability is based on observations after fusion. Chataigner and collaborators suggested that patients with type I marrow changes have much better outcomes with surgery than do those with isolated degenerative disk disease and normal or type II marrow changes.⁶⁷ In addition, conversion of type I marrow changes to either normal or type II was associated with higher fusion rates and better outcomes. Other studies support the contention that persistence of type I changes after fusion suggests pseudarthrosis and is associated with persistent symptoms. Conversely, conversion of type I marrow changes to either normal or type II is associated with higher fusion rates and better outcomes (Fig. 18-17).^68–70 The conclusion is that fusion produces greater stability, reduces biomechanical stress, and accelerates the course of type I marrow changes toward improvement.

FIGURE 18-17 Conversion of type I marrow changes. Preoperative (A and C) and postoperative (B and D) images of the lumbar spine in a patient after lumbar fusion demonstrate typical type I marrow changes at the L4-5 level preoperatively (arrows). Postoperatively, the type I changes have converted to type II changes. The decreased signal on T1 is converted to an increased signal representing lipid marrow conversion. The increased signal intensity on T2 has resolved.

As further support that these fluid marrow changes reflect biomechanical stress, we have seen similar marrow conversion in the pedicles of vertebral bodies associated with symptomatic pars and pedicle fractures, as well as in those with severe degenerative facet joint disease. Pedicle marrow change to a normal or type II appearance is associated with improvement in symptoms.⁷¹

Degenerative Facet and Ligamentous Changes

The superior articulating process of one vertebra is separated from the inferior articulating process of the vertebra above by a synovium-lined articulation, the zygapophyseal joint. Like all diarthrodial synovium-lined joints, the lumbar facet joints are predisposed to arthropathy with alterations of the articular cartilage (Fig. 18-18). With disk degeneration and loss of disk space height, the increased stress on the facet joints with craniocaudal subluxation results in arthrosis and osteophytosis. The superior articular facet is usually more substantially involved. Facet arthrosis can result in narrowing of the central canal, lateral recesses, and foramina and is an important component of lumbar stenosis. However, it has been proposed that facet arthrosis may occur independently and be a source of symptoms on its own.^72,73 Synovial villi may become entrapped within the joint with resulting joint effusions. The mechanism of pain may be related to nerve root compression from degenerative changes of the facets or direct irritation of pain fibers from the innervated synovial linings and joint capsule.⁷³ Osteophytosis and herniation of synovium through the facet joint capsule may result in synovial cysts, although the cause of these facet joint cysts is unclear. There is a more straightforward relationship of synovial cysts with osteoarthritis and instability of the facet joints than with degeneration of the intervertebral disk alone. In a review of patients with degenerative facet disease, synovial cysts occurred at anterior or intraspinal locations in 2.3% of patients and at posterior or extraspinal locations in 7.3% (Fig. 18-19).⁷⁴ The important ligaments of the spine include the anterior longitudinal ligament, the posterior longitudinal ligament, the paired sets of ligamenta flava (connecting the laminae of adjacent vertebrae), the intertransverse ligaments (extending between transverse processes), and the unpaired supraspinous ligament (along the tips of the spinous processes). Because these ligaments normally provide stability, any alteration in the vertebral articulations can lead to ligamentous laxity with subsequent deterioration. Loss of elastic tissue, calcification and ossification, and bone proliferation at sites of ligamentous attachment to bone are recognized manifestations of such degeneration. Excessive lordosis or extensive disk space loss in the lumbar spine leads to close approximation and contact of the spinous processes and to degeneration of intervening ligaments.^75,76 Histologically, granulomatous reaction and perivascular cellular infiltration characterize the condition.

FIGURE 18-18 Degenerative facet changes (arrows) seen on T1-weighted (A), high-resolution CT (B), and T2-weighted spin echo (C) axial images through the L4-5 facets. Note the bony osteophyte along the anterior margin of the inferior facet at L4 on the right and the degenerative facet narrowing bilaterally. The bony changes are not as obvious on the T1- and T2-weighted images, but the findings of narrowing and asymmetrical soft tissue are clearly identifiable on MRI.

FIGURE 18-19 Synovial cyst demonstrated on axial T1-weighted (A), contrast-enhanced T1-weighted (B), and T2-weighted (C) images through the facets at L4-5. There are severe bilateral degenerative facet changes with distraction and fluid. On the T1-weighted image, there is an ill-defined soft tissue mass projecting medially off the left facet (arrow). On the contrast-enhanced T1-weighted image, this soft tissue mass (arrow) is now clearly outlined by peripheral enhancement. On the T2-weighted image this mass (arrow) is demonstrated to have high signal intensity suggestive of fluid.

Instability

Segmental instability can result from degenerative changes involving the intervertebral disk, vertebral bodies, and facet joints that impair the usual pattern of spinal movement and produce motion that is irregular, excessive, or restricted. It can be translational or angular. Spondylolisthesis results when one vertebral body becomes displaced relative to the next most inferior vertebral body. The most common types are classified as degenerative, isthmic, iatrogenic, and traumatic. Degenerative spondylolisthesis is seen usually with an intact pars interarticularis, is related primarily to degenerative changes of the apophyseal joints, and is most common at the L4-5 vertebral level (Fig. 18-20). The predilection for degenerative spondylolisthesis at that level is thought to be related to the more sagittal orientation of the facet joints, which makes them increasingly prone to anterior displacement. Degenerative disk disease may predispose to or exacerbate this condition secondary to narrowing of the disk space, which can produce subsequent malalignment of the articular processes and lead to rostrocaudal subluxation.

FIGURE 18-20 Degenerative spondylolisthesis. Sagittal T1- and T2-weighted spin echo images (A and B) of the lumbar spine. Grade I spondylolisthesis of L4 on L5 (arrows) is present, as well as severe central canal stenosis and thickening of the posterior ligaments. Axial T1- and T2-weighted images (C and D) through the L4-5 level. Note the severe central canal stenosis, thickened posterior ligaments, and severe bilateral degenerative facet changes.

Herniation

Herniation refers to localized displacement of nucleus, cartilage, fragmented apophyseal bone, or fragmented annular tissue beyond the intervertebral disk space. The disk space is defined rostrally and caudally by the vertebral body end plates and peripherally by the outer edges of the vertebral ring apophyses, exclusive of osteophytic formations. The term localized contrasts with the term generalized, the latter being arbitrarily defined as greater than 50% (180 degrees) of the periphery of the disk.⁷⁷ Displacement, therefore, can occur only in association with disruption of the normal annulus or, as in the case of intravertebral herniation (Schmorl’s node), a break in the vertebral body end plate. Because details of the integrity of the annulus are often unknown, the diagnosis of herniation is usually made by observation of localized displacement of disk material beyond the edges of the ring apophyses—that is, less than 50% (180 degrees) of the circumference of the disk (Fig. 18-21). Localized displacement in the axial (horizontal) plane can be focal, or less than 25% of the disk circumference, or broad based, between 25% and 50% of the disk circumference. The presence of disk tissue circumferentially (50% to 100%) beyond the edges of the ring apophyses may be called bulging and is not considered a form of herniation. A disk may have more than one herniation. The term herniated disk does not imply any knowledge of etiology, relationship to symptoms, prognosis, or need for treatment. When data are sufficient to make the distinction, a herniated disk may be more specifically characterized as protruded or extruded. These distinctions are based on the shape of the displaced material. Protrusion is present if 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 (Fig. 18-22). Extrusion is present when, 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 (Fig. 18-23). Extrusion may be further specified as sequestration if the displaced disk material has completely lost any continuity with the parent disk. The term migration may be used to signify displacement of disk material away from the site of extrusion, regardless of whether it is sequestrated (Fig. 18-24). Herniated disks in the craniocaudal (vertical) direction through a break in the vertebral body end plate are referred to as intravertebral herniations. Nonacute Schmorl’s node intrabody herniations are common spinal abnormalities regarded as incidental observations. They have been reported in 38% to 75% of the population.^78,79 Although intrabody herniations may occur as a result of end plate weakness secondary to bone dysplasia, neoplasms, infections, or any process that weakens the end plate or the underlying bone, most intrabody herniations probably form after axial-loading trauma, with preferential extrusion of nuclear material through the vertebral end plate rather than an intact and normal annulus fibrosus. It has been suggested that asymptomatic intrabody herniations may be traceable to a specific occurrence of acute nonradiating low back pain in the patient’s history, which supports the concept that intrabody herniations (Schmorl’s nodes) occur through sites of end plate fracture. Type I vertebral body marrow changes have been described surrounding acute interbody herniations.⁸⁰

FIGURE 18-21 A, Symmetrical bulging disk. B, Broad-based herniation. C, Focal herniation. D, Protrusion and extrusion.

FIGURE 18-22 Disk protrusion seen on sagittal (A) and axial (B) T2-weighted images of the lumbar spine. On the sagittal image, the disk extends beyond the vertebral body margins, which on the axial image shows the base to be broader than the posterior extent (arrows). Sagittal multiplanar reformatted (MPR) (C) and axial (D) CT images through the lumbar spine. The sagittal MPR image demonstrates an ill-defined soft tissue mass projecting posteriorly at the L4-5 disk level. The broad-based disk protrusion is better depicted on the axial image (white arrow). Axial gradient echo MRI (E) and CT (F) at the C5-6 level. Note the broad-based right-sided disk protrusion (arrow). Sagittal (G) and axial (H) T2-weighted images of the cervical spine demonstrating disk protrusion at C5-6 and disk extrusion with an inferior fragment at C6-7. The axial image through the body of C7 demonstrates the rounded soft tissue mass centrally in the anterior epidural space.

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