Page 365 - Atlas of Small Animal CT and MRI

3.5

Intervertebral disk disease and other

degenerative disorders

Intervertebral disk disease undergoes chondroid metaplasia, resulting in a loss of
water and hydroelasticity. This process occurs along the
Intervertebral disk degeneration entire vertebral column, with dystrophic mineralization
The normal intervertebral disk is comprised of four a common sequela. Disks of nonchondrodystrophoid
major components: the nucleus pulposus, the annulus breeds tend to undergo fibrous metaplasia, charac­
fibrosis, a transition zone, and the cartilaginous end­ terized by fibrous collagenization of the nucleus pulpo­
plates. The nucleus pulposus is located eccentrically in sus in concert with annulus fibrosis degeneration.
1–3
the disk and has a high mucoprotein and water con­ Degenerative changes in chondrodystrophoid breeds
tent. The annulus fibrosis surrounds the nucleus and occur at an earlier age (3–7 years) and in the cervical and
is composed of multilayered fibrocartilage. The thoracolumbar spine, while nonchondrodystrophic
1
transition zone is located between the mucoid nucleus degeneration occurs later (6–8 years) and preferentially
and the fibrous annulus and appears to be wider and affects the caudal cervical region and the lumbosacral
2
less distinct in chondrodystrophoid breeds. Cartilag­ junction, although thoracolumbar disease also occurs. 2
inous endplates form the cranial and caudal margins
of the disk with fibrous connections to the annulus Intervertebral disk extrusion and protrusion
and the adjacent bony endplates of the vertebrae Intervertebral disk lesions are classified as type I or type
(Figure 3.5.1). Vascular supply to the intervertebral II using a system first introduced by Hansen. Hansen’s
4,5
disk is minimal and limited to outer layers of the annu­ type I disk extrusion occurs when degenerated nucleus
1
lus. In addition, the dorsal longitudinal ligament pulposus herniates through all layers of a ruptured
courses over the ventral surface of the spinal canal, annulus fibrosis. Type I disease occurs predominately
2
and intercapital ligaments cross the intervertebral in chondrodystrophic breeds but is also seen in larger
disks from T2–T10. nonchondrodystrophic breeds. 2,6,7 Due to the altered
Deterioration of the extracellular matrix of the physical characteristics of the chondroid metaplastic
intervertebral disk leads to degeneration. As a disk nucleus pulposus, type I disk extrusion tends to be acute
degenerates, the nucleus pulposus and, to a lesser extent, and explosive. Because of the eccentric position of the
the remainder of the disk dehydrate, causing narrowing nucleus within the disk, herniation occurs dorsally into
of the disk. Nonphysiologic loading of the disk can also the vertebral canal or dorsolaterally into the interverte­
lead to annular tears and cartilaginous endplate fissures. bral foramina.
Structural changes to the disk lead to herniation or Hansen’s type II disk protrusion occurs when fibroid
extrusion. 1 degenerated disk material migrates dorsally or dorsolat­
Disk degenerative changes differ between chondro­ erally because of partial tearing or rupture of the annu­
dystrophoid and nonchondrodystrophoid breeds. lus. Because the nucleus pulposus is still contained
The nucleus pulposus of chondrodystrophoid breeds within the remaining annulus fibrosus, disk material is

Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
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