Download The “Dehydrated” Lumbar Intervertebral Disk on MR, its Anatomy

NRJDigital-TheNeuroradiologyJournal1:639-644,2011
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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.
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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
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The “Dehydrated” Lumbar Intervertebral Disk on MR, its Anatomy, Biochemistry and Biomechanics
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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.
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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]