Lumbar Intervertebral Disc

• 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