9.1 Intervertebral Disc

  • There are 23 intervertebral discs in the spinal column
  • Key function is to allow a limited amount of relative motion between the bone 12 transmitting most of the compressive load in the spine
  • Heterogeneous organ consisting of three elements:
    • Nucleus pulposus
    • Annulus fibrosus
    • Cartilaginous end plate

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9.1.1 Composition

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  • Nucleus pulposus
    • a fluid-like gel, mostly of water (70 to 90%, decreasing with age)
    • randomly oriented type 2 collagen, proteoglycans (with negative charge)
    • most of the resistance to compression comes from annulus fibrosis (and some build up of pressure)

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  • Annulus fibrosus
    • alternating sheets of type I collagen similar to lamellar bone but without any mineral
    • increasing collagen the outside surface
    • alternating collagen fiber orientation about 30-35 degrees relative to the end plate

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  • Cartilaginous end plate
    • Bone and ~0.6 mm thick layer of hyaline cartilage which interfaces between the bone of the vertebral body and each of the annulus fibrosus and nucleus pulposus

9.1.2 Other facts

  • Aging degrades the nucleus pulposus, solidifying it, making it more like the annulus fibrosus
  • no blood supply to the disc
  • no nerves
  • nutrition occurs by fluid transport
    • injury affects this process

9.1.3 Mechanics

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  • Load-bearing mechanisms for a healthy disc loaded by (a) a uniaxial compressive force and (b) anterior bending.
  • Poroelastic characteristics typical of cartilage, and nonlinear elastic characteristics like tendon and ligament
  • Nucleus pulposus axis a pressurized fluid and it contained by tension in the annulus fibrosus

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  • Hydrostatic pressure developed in the nucleus pulposus
    • Negative charge brings water in osmotically, the resulting swelling resisted by annulus fibrosus (building pressure)
    • Compression causes a slight bulge in the annulus, creating tension. Some of the water is squeezed out.
    • Compressive loading do to activities of daily living thus brings nutrient

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  • In healthy tissue, bending create differential tension and compression on opposite sides of the annulus fibrosus. However, it remains in tension overall

Highly viscoelastic due to water content.

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May be modeled by a standard spring dashpot model

  • With aging, dehydration of the nucleus pulposis, more gelatinous, loss of pressure, loss of support of the end plates

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Comparison of the load-bearing mechanisms for a healthy (left) and degenerated (right) disc for uniaxial compression.

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Specimen preparation used for tensile testing of disc tissue properties.

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  • Typical creep curve for an intervertebral body-disc complex, showing the nonzero deformation that develops after complete unloading.
  • This latter feature is not typical of classical viscoelastic materials, but instead is due to water loss from the disc during loading.
  • During sleep, this height loss is regained as the disc is rehydrated.

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  • Comparison of the creep response to the same static load for a healthy vs. severely degenerated disc.
  • The healthy disc is more viscoelastic, since it takes a longer time for it to reach its equilibrium configuration.
  • It is also stiffer, because its final displacement is smaller

9.1.4 Herniated, or slipped, disc

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  • Annulus can tear or rupture (called disc prolapse, or “slipped disc”)
    • Excessive load (too much pressure), degeneration (insufficient development of pressure)
  • Neuromuscular function can be compromised if there is significant impingement of the herniated disc against the nerve roots that exit laterally from the spinal cord.
  • Also, can be severe pain!
  • No blood supply, disc damage cannot be repaired biologically, accumulation of micro trauma
  • With degeneration, decreased disc height, slack in ligaments, instability!