The Dehydrated Lumbar Intervertebral Disk on MR, its anatomy, biochemistry and biomechanics Victor Haughton MD University of Wisconsin

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

Background: The terms dehydrated or dessicated disk have not been adequately defined. Used in radiologic reports, the terms suggest a process different from degeneration and perhaps less significant clinically.

Methods: Scientific reports were reviewed Anatomical, biochemical, and biomechanical properties of dark disks summarized Anatomical, biochemical, and biomechanical properties of dark disks summarized

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

Dark disks: HIZ on MR High intensity zone in annulus fibrosus High intensity zone in annulus fibrosus Minority of Dark Disks Minority of Dark Disks

Dark Disk: Discography (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. Radial tear in all cases

Radial tear in anatomic sections of cadaver disks

Radial tear: axial MR and anatomic sections

Anatomic section demonstrating granulation tissue in the radial tear

Staging of intervertebral disk degeneration (MR) Pfirrmann

Differential diagnosis: annular tears Transverse tearConcentric tear

Biochemistry of dark disk Diminished glycosaminoglycans Increased collagen content Change in Glucosulfonates Increase in aggregated proteins

Water content, GAG content diminished in dark disk

Biomechanics Dark Disk responds abnormally to forces and torques Diminished resistance to rotagtory torque Behaves as a biomechanically failed disk

Biomechanical Testing Increase torque incrementally Measure rotation of vertebrae for each torque increment Calculate stiffness Small changes for aging disks Significantly larger changes in dark disks

Rotations of dark disks and control disks measured in vivo

Dark disk: Degenerating disk Type III in Pfirrmann grading on morphology Radial tear of the annulus fibrosus Abnormal biochemically: water and GAG content diminished Abnormal biomechanically : mechanical failure of intervertebral disk Early disk degeneration

Radial tear: Sagittal section