The “Dehydrated” Lumbar The “Dehydrated” Lumbar Intervertebral Disk on MR, Intervertebral Disk on MR,

its anatomy, biochemistry and its anatomy, biochemistry and biomechanicsbiomechanics

Victor Haughton MDVictor Haughton MD

University of WisconsinUniversity of Wisconsin

"dehydrated", "desiccated" or "dehydrated", "desiccated" or "dark" disks. "dark" disks.

The defining feature is The defining feature is diminished signal intensity in diminished signal intensity in the nucleus pulposus on T2 the nucleus pulposus on T2 weighted images. weighted images. Meets criteria for a stage III Meets criteria for a stage III degenerated disk in the degenerated disk in the Pfirrmann or the Thompson Pfirrmann or the Thompson scalesscalesStage III is distinguished Stage III is distinguished from stage I and II disks by from stage I and II disks by decreased T2 signal decreased T2 signal

Background:Background:

The terms “dehydrated” or “dessicated The terms “dehydrated” or “dessicated disk” have not been adequately defined.disk” have not been adequately defined.

Used in radiologic reports, the terms Used in radiologic reports, the terms suggest a process different from suggest a process different from degeneration and perhaps less significant degeneration and perhaps less significant clinically. clinically.

Methods:Methods:

Scientific reports were reviewedScientific reports were reviewed

Anatomical, biochemical, and Anatomical, biochemical, and biomechanical properties of dark disks biomechanical properties of dark disks summarized summarized

Conclusions:Conclusions:

The dehydrated or dessicated disk, The dehydrated or dessicated disk, because of its morphological, biochemical because of its morphological, biochemical and biomechanical features warrants the and biomechanical features warrants the designation of early disk degeneration. designation of early disk degeneration.

Dark disks: HIZ on MRDark disks: HIZ on MR

• High intensity High intensity zone in annulus zone in annulus fibrosusfibrosus

• Minority of Dark Minority of Dark DisksDisks

Dark Disk: Discography Dark Disk: Discography (100% sensitivity)(100% sensitivity)

Concordant pain at L4-5 on discography, No pain at L3-4 or L5-S1. Disposition: L4-5 interbody fusion; pain free following surgery

Dark Disk: Anatomic correlation.Dark Disk: Anatomic correlation.Radial tear in all casesRadial tear in all cases

Radial tear in anatomic sections of Radial tear in anatomic sections of cadaver diskscadaver disks

Radial tear: axial MR and Radial tear: axial MR and anatomic sectionsanatomic sections

Anatomic section demonstrating Anatomic section demonstrating granulation tissue in the radial teargranulation tissue in the radial tear

Staging of intervertebral disk Staging of intervertebral disk degeneration (MR)degeneration (MR)

Pfirrmann

Differential diagnosis: annular tearsDifferential diagnosis: annular tears

Transverse tear Concentric tear

Biochemistry of dark diskBiochemistry of dark disk

Diminished glycosaminoglycans Diminished glycosaminoglycans

Increased collagen contentIncreased collagen content

Change in GlucosulfonatesChange in Glucosulfonates

Increase in aggregated proteinsIncrease in aggregated proteins

Water content, GAG content Water content, GAG content diminished in dark diskdiminished in dark disk

T2 relaxation times correlated to proteoglycan content in intervertebral disk

y = 11.307x + 19.092

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6 7 8

proteoglycan content (%)T

2 r

ela

xti

on

tim

e (

ms

)

BiomechanicsBiomechanics

““Dark Disk” responds abnormally to forces Dark Disk” responds abnormally to forces and torquesand torques

Diminished resistance to rotagtory torqueDiminished resistance to rotagtory torque

Behaves as a biomechanically failed diskBehaves as a biomechanically failed disk

Biomechanical TestingBiomechanical Testing

Increase torque Increase torque incrementally incrementally Measure rotation of Measure rotation of vertebrae for each torque vertebrae for each torque incrementincrementCalculate stiffnessCalculate stiffnessSmall changes for aging Small changes for aging disksdisksSignificantly larger Significantly larger changes in dark diskschanges in dark disks

Rotations of motions of segments for flexion and extension torques

-6

-4

-2

0

2

4

6

-10 -5 0 5 10

torque (Nm)

flexi

on o

r ex

tens

ion

(deg

rees

)

normal disks

transverse tears

radial tears

collapsed disks

Poly. (normal disks)

Poly. (radial tears)

Poly. (collapsed disks)

Rotations of dark disks and Rotations of dark disks and control disks measured in vivocontrol disks measured in vivo

Rotations at levels with normal disks or radial tears

0

5

10

15

0.4 1.6 2.8 4.0 >4

axial rotation (degrees)

nu

mb

er o

f sp

ecim

ens

NormaldisksRadialtears

Dark disk: Degenerating diskDark disk: Degenerating disk

Type III in Pfirrmann grading on morphologyType III in Pfirrmann grading on morphology

Radial tear of the annulus fibrosusRadial tear of the annulus fibrosus

Abnormal biochemically: water and GAG content Abnormal biochemically: water and GAG content diminisheddiminished

Abnormal biomechanically : mechanical failure of Abnormal biomechanically : mechanical failure of intervertebral diskintervertebral disk

Early disk degenerationEarly disk degeneration

Early disk degenerationEarly disk degeneration

Radial tear: Sagittal sectionRadial tear: Sagittal section