Lumbar disc prolapse results from herniation of soft disc material from the nucleus pulposus through a tear in the annular ligament. It most often occurs laterally but may be central, compressing the cauda equina. It is most usual in fit young people 20-30 years of age, following an awkward strain or lift. In older people, herniation is less likely since the thickness of the intervertebral disc has been lessened by dessication of the nucleus pulposus.
Degeneration of the vertebrae in middle aged people results in hypertrophy of the facet joints - between superior and inferior articular facets - and predisposes to compression of the nerve root as it exits the spinal canal through the intervertebral canal. The primary pathology may be the production of substance P and plasmin in the degenerating disc.
A congenitally narrowed spinal canal increases the susceptibility of a lesion by providing less room for the encroachment of disc material or osteophytic new bone formation.
Lateral disc herniations usually compress the nerve root exiting through the foramen below the affected level.
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