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Updated: Jun 25 2024

Intervertebral Disc

Images
https://upload.orthobullets.com/topic/9020/images/disc.jpg
https://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 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
      • 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
        • 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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