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http://upload.orthobullets.com/topic/9020/images/disc.jpg
http://upload.orthobullets.com/topic/9020/images/blood supply disc.jpg
Anatomy
  • Function
    • allows spinal motion and provides stability
    • links adjacent vertebral bodies together
    • responsible for 25% of spinal column height
  • Composition 
    • annulus fibrosus
      • outer structure that encases the nucleus pulposus
      • composed of type I collagen that is obliquely orientedwater, 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
    • 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
        • survive in hypoxic conditions
  • 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 
  • 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
  • Fixation
    • attached to vertebral bodies by hyaline cartilage
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
  • 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
Pathoanatomy
  • Disc Herniation
    • 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)
  • 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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