Spine anatomy the intervertebral disc

1. Intervertebral disc
Tesfahun K.

2. Anatomy
• Function
• allows spinal motion and provides stability
• links adjacent vertebral bodies together
• responsible for 25% of spinal column height
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3. Composition

4. annulus fibrosus
• outer structure that encases the nucleus pulposus
• composed of type I collagen that is obliquely
oriented, water, and proteoglycans
• characterized by high tensile strength and its ability to
prevent intervertebral distraction
• remains flexible enough to allow for motion
• high collagen / low proteoglycan ratio (low % dry
weight of proteoglycans)
• fibroblast-like cells--- responsible for producing type I
collagen and proteoglycans
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5. nucleus pulposus
• central portion of the intervertebral disc that is surrounded by the annulus fibrosis
• composed of type II collagen, water, and proteoglycans-- approximately 88% water
• hydrophilic matrix is responsible for height of the intervertebral disc
• characterized by compressibility
• a hydrated gel due to high polysaccharide content and high water content (88%)
• proteoglycans interact with water and resist compression
• Aggrecan is a proteoglycan primarily responsible for maintaining water content of the
disc
• viscoelastic matrix distributes the forces smoothly to the annulus and the end plates
• low collagen / high proteoglycan ratio (high % dry weight of proteoglycans)
• chondrocyte-like cells--- responsible for producing type II collagen and proteoglycans
and survive in hypoxic conditions
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6. Blood Supply
• the disk is avascular with capillaries terminating at
the end plates
• nutrition reaches nucleus pulposus
through diffusion through pores in the endplates
• annulus is not porous enough to allow diffusion
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8. Innervation
• the dorsal root ganglion gives rise to the sinuvertebral
nerve which innervates the superficial fibers of annulus
• no nerve fibers extend beyond the superficial fibers
• neuropeptides thought to participate in sensory
transmission include
• substance P
• calcitonin
• VIP
• CPON
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9. Fixation
• attached to vertebral bodies by hyaline cartilage
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10. Disc biomechanics
• Disc
• viscoelastic characteristics
demonstrates creep which allows for
deformity over time
demonstrates hysteresis which allows for
energy absorption with repetitive axial compression
this property decreases with time

11. Stresses
• annulus fibrosus---highest tensile stresses
• nucleus pulposus-- highest compressive stress
• intradiscal pressure is position dependent
• pressure is lowest when lying supine
• pressure is intermediate when standing
• pressure is highest when sitting and flexed forward
with weights in the hands
• when carrying weight, the closer the object is to the
body the lower the pressure
• Stability
• following subtotal discectomy, extension is most stable
loading mode
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12. Pathoanatomy
• herniated disks are associated with a spontaneous
increase in the production of
• osteoprotegrin (OPG)
• interleukin-1 beta
receptor activator of nuclear factor-kB ligand
(RANKL)
parathyroid hormone (PTH)

13. Ctnd...
• Disc aging leads to an overall loss of water content and conversion to
fibrocartilage.
• Specifically there is a decrease in - nutritional transport
• - water content
• - absolute number of viable cells
• - proteoglycans
• - pH
• increase in - an increase keratin sulfate to chondroitin sulfate ratio
• - lactate
• - degradative enzyme activity
• - density of fibroblast-like cells ( fibroblast-like cells reside in the
annulus fibrosus only)
• no change in--- absolute quantity of collagen
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