Spinal Anatomy and its Effects on Types of Spinal Stenosis

Learning about spinal stenosis requires an understanding of spinal anatomy. The vertebral column in the spine and sacrum (at the bottom of the spine) are like a stack of blocks that serve to support the structures of the body.

Each of these bony structures has additional bony attachments that serve to help stabilize the spine and to protect the spinal cord or nerves passing downward from the brain to organs, muscles, and sensory structures of the body. Each vertebral body and its attachments and the disc between the adjacent vertebrae are known as a spinal segment.

The entire length of the spinal column has a large central canal or passage through which the spinal cord descends, and holes to each side of the canal to allow emergence of spinal nerves at each level. The spinal cord stops at the upper part of the low back, and below that the tiny, contained nerve rootlets descend loosely splayed out - like a horse's tail - and are protectively enclosed in a long sack. All central nerve structures are protected further by membranes, with a tough outer membrane called the dura (tough) mater (mother).

Spinal Anatomy and the Major Types of Spinal Stenosis

  • Foraminal stenosis. As the nerve root is about to leave the canal through a side hole (lateral foramen), a bone spur (osteophyte) that has already developed from a degenerating disc can press on that nerve root. This type of stenosis is also called lateral spinal stenosis, which is by far the most common form of spinal stenosis. 72% of cases of foraminal stenosis occur at the lowest lumbar level. With this type of lumbar stenosis, the emerging nerve root is trapped (thus comprising a major part of the sciatic nerve).
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  • Central stenosis. A choking of the central canal (called central stenosis) in the lumbar (low back) area can compress the sack containing the horse's tail (cauda equina or cauda equine) bundle of loose nerve filaments. Central spinal stenosis is more common at the second from the lowest lumbar spinal level and higher, and is largely caused by a bulging of the disc margin plus a major overgrowth or redundancy of a ligament (ligamentum flavum) that is there to help protect the dura. This overgrowth is caused by segmental instability, usually from a degenerating disc between adjacent vertebrae. The ligament arises from under the flat laminas of the vertebrae and the inside part of the facet joints (stabilizing joints located on each side at the back of the spine segments).
  • Far lateral stenosis. After the nerve has left the spinal canal it can also be compressed beyond the foramen by either a bony spur protrusion, bulging disc, or herniated disc. When this happens, it is called lateral stenosis.

These differences in spine anatomy may result in similar symptoms, which is why all forms of stenosis are typically referred to as simply spinal stenosis.

However, if spinal stenosis surgery is to be performed, the differences are very important in guiding the surgeon. That is, the bad spot(s) must be exactly known in advance to guide the approach for its proper treatment or removal.