Degenerative disc disease

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Degenerative disc disease (DDD) is an exceedingly common entity in the spine, encountered with increasing frequency throughout life and becoming almost universal in late adulthood to a varying degree. It is related to a combination of biomechanical stresses and genetic predisposition which alter the metabolic and structural integrity of the intervertebral disc. These changes, in turn, modify the ability of the intervertebral disc to sustain and transmit forces, and subsequently, compensate for these alterations (or not) in a variety of ways, leading to billions of dollars in health care services and lost productivity every year due to associated morbidity from these conditions.

Epidemiology

Wide variations exist in the reported prevalence of degenerative changes in the spine, likely related to differences in populations studied and variable definitions of what constitutes degenerative change. For example, is a Schmorl node a degenerative change? Additionally, because degenerative disc disease encompasses such a wide variety of individually defined entities, it is hard to make broad statements about its incidence and related symptomatology.

Vertebral osteophytes, disc height loss, disc signal intensity loss, and disc bulging increase in frequency with age in a nearly linear fashion, but to different degrees, with disc signal loss and vertebral osteophytes increasing the most with age.

Lumbar spine

Below is a list of conditions that are frequently considered degenerative in nature, and their reported incidence is given in both asymptomatic and symptomatic adult populations :

  • disc bulge
    • asymptomatic: ~45% (range 10-81%)
    • symptomatic: ~35 % (range 22-48%)
  • disc protrusion
    • asymptomatic: ~35% (range 3-63%)
    • symptomatic: ~40% (range 0-79%)
  • disc extrusion
    • asymptomatic: ~12% (range 0-24%)
    • symptomatic: ~30% (range 1-55%)
  • disc sequestration
    • asymptomatic: 0%
  • reduction in signal intensity of the disc
    • asymptomatic: ~50% (range 20-83%)
    • symptomatic:~50% (range 9-86%)
  • disc height loss
    • asymptomatic: ~30% (range 3-56%)
    • symptomatic:~35% (range 15-53%)
  • annular tears (high-intensity zones)
    • asymptomatic: 30% (range 6-56%)
    • symptomatic: 15%
  • Schmorl node
    • asymptomatic: ~15% (range 8-19%)
    • symptomatic: ~45% (range 6-79%)

Pathology

Generally, degenerative changes of the disc affect hydration and elasticity of the cartilaginous endplate, annulus fibrosus and nucleus pulposus, with changes most pronounced in the nucleus pulposus. Within the nucleus pulposus, a loss of water-holding proteoglycans occurs, and within the annulus fibrosus type II collagen increases which reduce disc hydration as well. This makes the disc friable, precipitating fissures and progressive structural degradation. Nitrogen gas may be observed filling these defects as it is pulled from surrounding extracellular fluid, creating the vacuum cleft phenomenon.