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NRJDigital-TheNeuroradiologyJournal1:639-644,2011 www.centauro.it The“Dehydrated”LumbarIntervertebralDisk onMR,itsAnatomy,Biochemistry andBiomechanics V.HAUgHToN RadiologyDepartment,WisconsinUniversityHospitals;Madison,Wisconsin,USA Key words:spinedegenerativediseases,lumbardisks,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 inradiologicreports,thetermssuggestaprocess different from and perhaps less a significantthandegeneration.Forthispresentation, scientific reports were reviewed that describe the anatomical, biochemical, and biomechanical properties of disks that have diminished signal intensity without herniation or bulging oftheannulusfibrosusandwithoutsignificant loss of height. The review revealed that the “dessicated disk” corresponds to a stage 3 degeneratingdiskinthePfirrmannclassification. It has a radial tear of the annulus fibrosus in addition to diminished signal intensity. It has diminishedproteoglycanscontentaswellasdiminishedwatercontent.Itmaycontaingranulationtissue.Thedarkdiskhasdiminishedresistancetoappliedtorques,resultingingreater rotationofthemotionsegmentwhenatorques isapplied. The dehydrated or dessicated disk, because of its morphological, biochemical and biomechanical features warrants the designation of earlydiskdegeneration. Paper presented at the XIX Symposium Neuroradiologicum, 2010. 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.Theclinicalsignificanceofdehydrateddisksmaynotbe 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 knowledgeregardingthedarkdiskseemsboth necessaryandtimely.Thepurposeofthiscommunication is to review briefly the morphologic, biochemical and biomechanical features ofthedarkdiskanditspotentialtocauselow backpainorradiculopathy. Thedefiningfeatureofthedehydrateddiskis diminishedsignalintensityinthenucleuspulposus on T2-weighted images (Figure 1). The darkdisk,withoutevidenceofherniation,protrusion, extrusion or bulging, meets the criteriaforastageIIIdegenerateddiskinthePfir639 The “Dehydrated” Lumbar Intervertebral Disk on MR, its Anatomy, Biochemistry and Biomechanics V. Haughton Figure1T2-weightedsagittalMRimageofthelumbarspine ina45-year-oldpatientillustrates“darkdisks”,indicativeof earlydegeneration,atL4/5andL5/S1. A B Figure2SagittalT2-weightedMRimage(A)ofacadaverlumbarspineandcorrelatinganatomicsection(B)illustrateadark L4/5diskthatdemonstratesnoobvious“highintensityzone”despitearadialtear(arrowsinB)intheposteriorannulusfibrosus. 640 www.centauro.it NRJDigital-TheNeuroradiologyJournal1:639-644,2011 Figure3SagittalT2-weightedMRimagedemonstratesahigh signalintensityzone(HIZ)thatcorrespondstoapresumptive radialtearintheL4/5disk.TheL5/S1diskhasaherniationof thenucleuspulposusthrougharadialtear,thisslightlyhigher signalintensitythanthedisk. rmannortheThompsonscalesforgradingdisk degeneration. Stage III is distinguished from stage I and II disks, which are normal disks, bydecreasedT2signalintensityinthenucleus pulposus1andbyconsolidationoffibroustissue inthenucleuspulposusandlossofclearannular-nucleardemarcationinanatomicsections2. A feature present in dark disks, but not alwaysevidentinMRimagesisaradialtearsof the annulus fibrosus. The radial tear may be shown by examining correlating anatomic imagesinthecaseofcadavericlumbarspines(Figure2)3orbydiscography.Discographyconsistentlyshowsinleakageofcontrastmediumfrom thenucleuspulposusintotheepiduralspacein dehydrateddisksandtypicallyelicits“concordant pain” 4-8. Radial tears may be shown less commonlybyMRimagingasa“highintensity zones”(Figure3)9oralinearregionofcontrast enhancement(Figure4)10.Inmostdarkdisks, MR fails to show the radial tear (Figure 2). Theradialtear,involvingalllayersoftheannulusfibrosus(Figure5)differsfromconcentric and transverse tears. Concentric tears, which mayalsobedemonstratedwithMR(Figure6), are accumulations of fluid between adjacent lamellae of the annulus fibrosus. Transverse tears,alsodemonstratedinMRimages(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 andnoroleprobablyinthegenesisofpain3. 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 structuretoonewithinnervation.Ifthenerve endings are nocioceptors, pain may result, whichcanbenon-specificlowbackpainorpain in the distribution of the nerve that supplies nerve endings to the granulation tissue, with theresultthatpainmaybeexperiencedbythe patientreferredtoalowerextremity,simulating the radicular pain resulting from nerve rootcompression11,12. Chemically, dark disks differ from normal disks.Diminishedwatercontentisonebutnot theonlyfeatureofthesedisks13.Thedecreased water content reflects a diminished glycosaminoglycans concentration in the disk 14,15 641 The “Dehydrated” Lumbar Intervertebral Disk on MR, its Anatomy, Biochemistry and Biomechanics A V. Haughton B Figure4SagittalT1-weightedimageofthelumbarspineinapatientwithbackpain(A)andthepost-contrastimage(B)illustratingaregionofcontrastenhancement(arrow)correspondingtopresumptiveradialtearinfiltratedwithgranulationtissue. A B Figure5AxialT2-weightedMRimage(A)demonstratinghighsignalintensityinthediskandsubadjacenttotheposteriorlongitudinalligament(arrow),probablyrepresentingaradialtear.Anaxialanatomicsectionfromacadaver(B)showingdisruptionof alllayersoftheannulusfibrosuspostero-laterally(arrows)duetoaradialtear. 642 www.centauro.it NRJDigital-TheNeuroradiologyJournal1:639-644,2011 Figure6SagittalMRimageina40-year-oldpatientdemonstratesaslimbandofhighsignalintensityintheannulusfibrosusofL4/5(arrow),illustratingthetypicalMRappearance ofaconcentrictear.ConcentrictearsareevidentintheL3/4 andL5/S1disksalso. Figure7SagittalMRimageina48-year-oldpatientdemonstrates focus of high signal intensity in the annulus fibrosus (arrowheads), illustrating the typical MR appearance of a transversetear. and other chemical changes. The terms “dehydrateddisk”or“desiccateddisk”donotconvey thecomplexityofbiochemicalchangesinthese disks. The loss of water implied by desiccation or dehydration occurs secondary to more fundamental biochemical changes, specifically the lossofglycosaminoglycans.Exceptforthediurnal change in water content, the disk does not losewaterwithoutachangeinglucosaminoglucans content. The title “early degeneration” or “stageIIIdisk”appliestothedarkdisk. Biomechanically,darkdisksdifferfromnormaldisks.Theradialtear,disruptingfibersin all layers of the annulus fibrosus, represent a biomechanical failure of the disk. The failed disk responds abnormally to forces and torsionsappliedtoit.Biomechanicalstudiesshow 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 atthelevelofadarkdiskthanatotherlevels 16 .Consequently,neuralforaminamaynarrow critically as the subject with the dark disk rotates his or her torso within a normal range ofmotions(Figure8)17-20.Theintermittent“oc643 The “Dehydrated” Lumbar Intervertebral Disk on MR, its Anatomy, Biochemistry and Biomechanics V. Haughton Figure8Sagittalanatomicsectionofacadaversubjectedtorotatorytorque,demonstratesnarrowingoftheneuralforamen, duepredominantlytobucklingoftheligamentumflavum(asterisk) and causing pressure on the spinal nerve (arrow). A radialtearintheintervertebraldiskincreasestheamountof rotation secondary to the rotatory torque and increases the riskofposition-relatedlateralspinalstenosis. cultlateralstenosis”maycauseinjurynervein theneuralforamen,apotentialmechanismfor chronicpain.Additionally,motionsofthetorso causeincreasedstressesinintervertebralligaments at levels with a failed disk, also potentiallycausingpain.Thedarkdiskhasmorpho- logic,biochemicalandbiomechanicalproperties that differentiate it from normal aging disks. The terms “early degeneration” or “Grade III degenerationinthePfirrmanscale”moreaccuratelydescribethistypeofdiskthantheterms “dessicated”or“dehydrated”. References 1 PfirrmannCW,MetzdorfA,ZanettiM,etal.Magnetic resonance classification of lumbar intervertebral disc degeneration.Spine.2001;26:1873-1878. 2 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;15:411-415. 3 Yu S, Sether LA, Ho PSP, et al. Tears of the annulus fibrosus: correlation between MR and pathologic findingsincadavers.AmJNeuroradiol.1988;9:367-370. 4 BuirskiG.Magneticresonancesignalpatternsoflumbardiscsinpatientswithlowbackpain.Aprospective study with discographic correlation. Spine. 1992; 17 (10):1199-1204. 5 VanharantaH,SachsBL,SpiveyMA,etal.TheRelationshipofPainProvocationtoLumbarDiscDeterioration as seen by CT/Discography. Spine. 1987; 12: 295-298. 6 OstiOL,FraserRD,Vernon-RobertsB.Annulartearsand degenerationoftheintervertebraldisc-preliminaryresultsofanexperimentalstudy.Spine.1990;15:762-767. 7 Horton WC, Daftari TK. Which disc as visualized by magneticresonanceimagingisactuallyasourceofpain? A correlation between magnetic resonance imaging anddiscography.Spine.1992;17(Suppl.6):S164-171. 8 LamKS,CarlinD,MulhollandRC.Lumbardischighintensity zone: the value and significance of provocativediscographyinthedeterminationofthediscogenic painsource.EurSpineJ.2000;9:36-41. 9 Aprill C, Bogduk N. High-intensity zone: a diagnostic signofpainfullumbardisconmagneticresonanceimaging.BrJRadiol.1992;65:361-369. 10 RossJS,ModicMT,MasarykTJ.TearsoftheAnulus Fibrosus: Assessment with Gd-DTPA-Enhanced MR. Imaging.AmJNeuroradiol.1989;10:1251-1254. 11 GoldieI.Granulationtissueintherupturedintervertebral disc.ActaPatholMicrobiolScand.1958;42(4):302-304. 12 Yoshida H, Fujiwara A, Tamai K, et al. Diagnosis of symptomatic disc by magnetic resonance imaging: T2-weighted and gadolinium-DTPA-enhanced T1- 644 weightedmagneticresonanceimaging.JSpinalDisord Tech.2002;15(3):193-198. 13 MarinelliNL,HaughtonVM,MuñozA,etal.T2relaxationtimesofintervertebraldisctissuecorrelatedwith water content and proteoglycan content. Spine. 2009; 34:520-524. 14 JohannessenW,AuerbachJD,WheatonAJ,etal.Assessmentofhumandiscdegenerationandproteoglycan content using T1-weighted magnetic resonance imaging.Spine.2006;31:1253-1257. 15 WeidenbaumM,FosterRJ,BestBA,etal.Correlating magneticresonanceimagingwiththebiochemicalcontentofthenormalhumanintervertebraldisc.JOrthop Res.1992;10:552-561. 16 Thompson RE, Pearcy MJ, Downing KJ, et al. Disc lesions and the mechanics of the intervertebral joint complex.Spine.2000;25:3026-3035. 17 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: 2167-2173. 18 NowickiBH,HaughtonVM,SchmidtTA,etal.Occult lumbarlateralspinalstenosisinneuralforaminasubjectedtophysiologicloading.AmJNeuroradiol.1996; 17:1605-1614. 19 Nowicki BH, Yu S, Reinartz J, et al. Effect of axial loadingonneuralforaminaandnerverootsinthelumbarspine.Radiology.1990;176:433. 20 HaughtonVM,SchmidtTA,KeeleK,etal.Flexibility oflumbarspinalmotionsegmentscorrelatedtotypeof tears in the annulus fibrosus. J Neurosurg. 2000; 92 (Suppl.1):81-86. VictorM.Haughton,MD WisconsinUniversityHospitals RadiologyDepartment 600HighlandAvenue Madison53792-3252,USA E-mail:[email protected]