The Dehydrated Lumbar Intervertebral Disk on MR, its Anatomy, Biochemistry and Biomechanics

Transcription

1 NRJ Digital - The Neuroradiology Journal 1: , The Dehydrated Lumbar Intervertebral Disk on MR, its Anatomy, Biochemistry and Biomechanics V. HAUgHToN Radiology Department, Wisconsin University Hospitals; Madison, Wisconsin, USA Key words: spine degenerative diseases, lumbar disks, biomechanics SUMMARY MR imaging of the lumbar spine often is requested to identify the cause of back or radicular pain. Official reports of lumbar spine images tend to focus on changes in the disk margin that may cause nerve root compression. The potential role of the dark disk, in back pain has not been adequately emphasized. The purpose of this review is to discuss the dark disk that has not produced nerve root compression. On T2-weighted images, a disk that has diminished signal intensity is called a dark disk or a dehydrated disk. It corresponds to a stage III disk in the Pfirrmann or the Thompson scale. Such a disk has specific morphologic, chemical and biomechanical properties, which will be reviewed in this presentation. The goal is to suggest the clinical significance of finding a dark disk on an MR image. The terms dehydrated disk or dessicated disk have not been adequately defined. Used in radiologic reports, the terms suggest a process different from and perhaps less a significant than degeneration. For this presentation, scientific reports were reviewed that describe the anatomical, biochemical, and biomechanical properties of disks that have diminished signal intensity without herniation or bulging of the annulus fibrosus and without significant loss of height. The review revealed that the dessicated disk corresponds to a stage 3 degenerating disk in the Pfirrmann classification. It has a radial tear of the annulus fibrosus in addition to diminished signal intensity. It has diminished proteoglycans content as well as diminished water content. It may contain granulation tissue. The dark disk has diminished resistance to applied torques, resulting in greater rotation of the motion segment when a torques is applied. The dehydrated or dessicated disk, because of its morphological, biochemical and biomechanical features warrants the designation of early disk degeneration. Paper presented at the XIX Symposium Neuroradiologicum, MR imaging of the lumbar spine often reveals disks with lower than normal signal in the nucleus pulposus without decreased disk height or abnormal contours of the annulus fibrosus. These are often referred to as dehydrated, desiccated or dark disks. The clinical significance of dehydrated disks may not be generally known. Official reports emphasizing the appearance of disk margins may overlook the significance of reduced signal intensity in the disks. Bulging and protrusions may suggest more clinically significant findings than dark disks. Scientific studies in the radiologic literature on the subject of dark disks seem to be few. Therefore, a review of the current knowledge regarding the dark disk seems both necessary and timely. The purpose of this communication is to review briefly the morphologic, biochemical and biomechanical features of the dark disk and its potential to cause low back pain or radiculopathy. The defining feature of the dehydrated disk is diminished signal intensity in the nucleus pulposus on T2-weighted images (Figure 1). The dark disk, without evidence of herniation, protrusion, extrusion or bulging, meets the criteria for a stage III degenerated disk in the Pfir- 639

2 The Dehydrated Lumbar Intervertebral Disk on MR, its Anatomy, Biochemistry and Biomechanics V. Haughton Figure 1 T2-weighted sagittal MR image of the lumbar spine in a 45-year-old patient illustrates dark disks, indicative of early degeneration, at L4/5 and L5/S1. A B Figure 2 Sagittal T2-weighted MR image (A) of a cadaver lumbar spine and correlating anatomic section (B) illustrate a dark L4/5 disk that demonstrates no obvious high intensity zone despite a radial tear (arrows in B) in the posterior annulus fibrosus. 640

3 NRJ Digital - The Neuroradiology Journal 1: , 2011 Figure 3 Sagittal T2-weighted MR image demonstrates a high signal intensity zone (HIZ) that corresponds to a presumptive radial tear in the L4/5 disk. The L5/S1 disk has a herniation of the nucleus pulposus through a radial tear, this slightly higher signal intensity than the disk. rmann or the Thompson scales for grading disk degeneration. Stage III is distinguished from stage I and II disks, which are normal disks, by decreased T2 signal intensity in the nucleus pulposus 1 and by consolidation of fibrous tissue in the nucleus pulposus and loss of clear annular-nuclear demarcation in anatomic sections 2. A feature present in dark disks, but not always evident in MR images is a radial tears of the annulus fibrosus. The radial tear may be shown by examining correlating anatomic images in the case of cadaveric lumbar spines (Figure 2) 3 or by discography. Discography consistently shows in leakage of contrast medium from the nucleus pulposus into the epidural space in dehydrated disks and typically elicits concordant pain 4-8. Radial tears may be shown less commonly by MR imaging as a high intensity zones (Figure 3) 9 or a linear region of contrast enhancement (Figure 4) 10. In most dark disks, MR fails to show the radial tear (Figure 2). The radial tear, involving all layers of the annulus fibrosus (Figure 5) differs from concentric and transverse tears. Concentric tears, which may also be demonstrated with MR (Figure 6), are accumulations of fluid between adjacent lamellae of the annulus fibrosus. Transverse tears, also demonstrated in MR images (Figure 7), are focal avulsion of fibers in the annulus fibrosus from the ring apophysis. Concentric and/or transverse tears may co-exist with the radial tear, as incidental findings. These two types of tears, unlike the radial tear, have no role in the pathogenesis of disk degeneration and no role probably in the genesis of pain 3. All dark disks, having a radial tear of the annulus fibrosus, are subject to ingrowth of granulation tissue into the disk. Granulation tissue contains nerve endings, converting the intervertebral disk from a non-innervated structure to one with innervation. If the nerve endings are nocioceptors, pain may result, which can be non-specific low back pain or pain in the distribution of the nerve that supplies nerve endings to the granulation tissue, with the result that pain may be experienced by the patient referred to a lower extremity, simulating the radicular pain resulting from nerve root compression 11,12. Chemically, dark disks differ from normal disks. Diminished water content is one but not the only feature of these disks 13. The decreased water content reflects a diminished glycosaminoglycans concentration in the disk 14,15 641

4 The Dehydrated Lumbar Intervertebral Disk on MR, its Anatomy, Biochemistry and Biomechanics V. Haughton A B Figure 4 Sagittal T1-weighted image of the lumbar spine in a patient with back pain (A) and the post-contrast image (B) illustrating a region of contrast enhancement (arrow) corresponding to presumptive radial tear infiltrated with granulation tissue. A B Figure 5 Axial T2-weighted MR image (A) demonstrating high signal intensity in the disk and subadjacent to the posterior longitudinal ligament (arrow), probably representing a radial tear. An axial anatomic section from a cadaver (B) showing disruption of all layers of the annulus fibrosus postero-laterally (arrows) due to a radial tear. 642

5 NRJ Digital - The Neuroradiology Journal 1: , 2011 Figure 6 Sagittal MR image in a 40-year-old patient demonstrates a slim band of high signal intensity in the annulus fibrosus of L4/5 (arrow), illustrating the typical MR appearance of a concentric tear. Concentric tears are evident in the L3/4 and L5/S1 disks also. Figure 7 Sagittal MR image in a 48-year-old patient demonstrates focus of high signal intensity in the annulus fibrosus (arrowheads), illustrating the typical MR appearance of a transverse tear. all layers of the annulus fibrosus, represent a biomechanical failure of the disk. The failed disk responds abnormally to forces and torsions applied to it. Biomechanical studies show that dark disks have diminished stiffness, especially to axial rotatory torques, compared to normal disks. Therefore the application of a torque to the spine produces more rotation at the level of a dark disk than at other levels 16. Consequently, neural foramina may narrow critically as the subject with the dark disk rotates his or her torso within a normal range of motions (Figure 8) The intermittent ocand other chemical changes. The terms dehydrated disk or desiccated disk do not convey the complexity of biochemical changes in these disks. The loss of water implied by desiccation or dehydration occurs secondary to more fundamental biochemical changes, specifically the loss of glycosaminoglycans. Except for the diurnal change in water content, the disk does not lose water without a change in glucosaminoglucans content. The title early degeneration or stage III disk applies to the dark disk. Biomechanically, dark disks differ from normal disks. The radial tear, disrupting fibers in 643

6 The Dehydrated Lumbar Intervertebral Disk on MR, its Anatomy, Biochemistry and Biomechanics V. Haughton Figure 8 Sagittal anatomic section of a cadaver subjected to rotatory torque, demonstrates narrowing of the neural foramen, due predominantly to buckling of the ligamentum flavum (asterisk) and causing pressure on the spinal nerve (arrow). A radial tear in the intervertebral disk increases the amount of rotation secondary to the rotatory torque and increases the risk of position-related lateral spinal stenosis. cult lateral stenosis may cause injury nerve in the neural foramen, a potential mechanism for chronic pain. Additionally, motions of the torso cause increased stresses in intervertebral ligaments at levels with a failed disk, also potentially causing pain. The dark disk has morpho- logic, biochemical and biomechanical properties that differentiate it from normal aging disks. The terms early degeneration or Grade III degeneration in the Pfirrman scale more accurately describe this type of disk than the terms dessicated or dehydrated. References 1 Pfirrmann CW, Metzdorf A, Zanetti M, et al. Magnetic resonance classification of lumbar intervertebral disc degeneration. Spine. 2001; 26: Thompson JP, Pearce RH, Schechter MT, et al. Preliminary evaluation of a scheme for grading the gross morphology of the human intervertebral disc. Spine. 1990; 15: Yu S, Sether LA, Ho PSP, et al. Tears of the annulus fibrosus: correlation between MR and pathologic findings in cadavers. Am J Neuroradiol. 1988; 9: Buirski G. Magnetic resonance signal patterns of lumbar discs in patients with low back pain. A prospective study with discographic correlation. Spine. 1992; 17 (10): Vanharanta H, Sachs BL, Spivey MA, et al. The Relationship of Pain Provocation to Lumbar Disc Deterioration as seen by CT/Discography. Spine. 1987; 12: Osti OL, Fraser RD, Vernon-Roberts B. Annular tears and degeneration of the intervertebral disc - preliminary results of an experimental study. Spine. 1990; 15: Horton WC, Daftari TK. Which disc as visualized by magnetic resonance imaging is actually a source of pain? A correlation between magnetic resonance imaging and discography. Spine. 1992; 17 (Suppl. 6): S Lam KS, Carlin D, Mulholland RC. Lumbar disc highintensity zone: the value and significance of provocative discography in the determination of the discogenic pain source. Eur Spine J. 2000; 9: Aprill C, Bogduk N. High-intensity zone: a diagnostic sign of painful lumbar disc on magnetic resonance imaging. Br J Radiol. 1992; 65: Ross JS, Modic MT, Masaryk TJ. Tears of the Anulus Fibrosus: Assessment with Gd-DTPA-Enhanced MR. Imaging. Am J Neuroradiol. 1989; 10: Goldie I. Granulation tissue in the ruptured intervertebral disc. Acta Pathol Microbiol Scand. 1958; 42 (4): Yoshida H, Fujiwara A, Tamai K, et al. Diagnosis of symptomatic disc by magnetic resonance imaging: T2-weighted and gadolinium-dtpa-enhanced T1- weighted magnetic resonance imaging. J Spinal Disord Tech. 2002; 15 (3): Marinelli NL, Haughton VM, Muñoz A, et al. T2 relaxation times of intervertebral disc tissue correlated with water content and proteoglycan content. Spine. 2009; 34: Johannessen W, Auerbach JD, Wheaton AJ, et al. Assessment of human disc degeneration and proteoglycan content using T1-weighted magnetic resonance imaging. Spine. 2006; 31: Weidenbaum M, Foster RJ, Best BA, et al. Correlating magnetic resonance imaging with the biochemical content of the normal human intervertebral disc. J Orthop Res. 1992; 10: Thompson RE, Pearcy MJ, Downing KJ, et al. Disc lesions and the mechanics of the intervertebral joint complex. Spine. 2000; 25: Schmidt TA, An HS, Lim TH, et al. The Stiffness of Lumbar Spinal Motion Segments with a High-Intensity Zone in the Anulus Fibrosus. Spine. 1998; 23: Nowicki BH, Haughton VM, Schmidt TA, et al. Occult lumbar lateral spinal stenosis in neural foramina subjected to physiologic loading. Am J Neuroradiol. 1996; 17: Nowicki BH, Yu S, Reinartz J, et al. Effect of axial loading on neural foramina and nerve roots in the lumbar spine. Radiology. 1990; 176: Haughton VM, Schmidt TA, Keele K, et al. Flexibility of lumbar spinal motion segments correlated to type of tears in the annulus fibrosus. J Neurosurg. 2000; 92 (Suppl. 1): Victor M. Haughton, MD Wisconsin University Hospitals Radiology Department 600 Highland Avenue Madison , USA [email protected] 644