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In the lumbar spine, the spinal canal usually has enough room for the spinal nerves. The canal is normally 17 to 18 millimeters around, slightly smaller than a penny. Spinal stenosis develops when the canal shrinks to 12 millimeters or less. When the size drops below 10 millimeters, severe symptoms of lumbar spinal stenosis occur.
There are many reasons for spinal stenosis symptoms. Some of the more common reasons are:
- Congenital stenosis (born with a small spinal canal)
- Disk herniation
- Spinal degeneration
- Spinal instability
The most-common cause of spinal stenosis is degenerative osteoarthritis of the spine, specifically the hypertrophy of the ligamentum flavum and the formation of large bone spurs next to the facet joints. This gradual process causes progressive compression of the spinal cord and neural elements. Mechanical irritation may cause inflammation, and decreased vascularity may cause decreased conduction of the nerve signals.