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The spinal canal, also known as the vertebral canal, is the cavity within the vertebral column that contains the thecal sac and spinal cord. The canal consists of a series of vertebral foramina (the holes at the center of the vertebra) linked with discoligamentous structures.
The spinal canal becomes progressively narrower from its superior opening at the foramen magnum to its inferior opening at the sacral hiatus 1. The canal itself is primarily formed by the vertebral foramen of adjacent vertebrae. Allowing for variation, the spinal cord occupies the superior two-thirds of the spinal canal and terminates at approximately the middle of the L1 vertebral body 2.
The narrowest diameter of the spinal canal is at the level of C5, measuring 15 mm, while the widest is at the level of S1, measuring 17 mm. AP diameter increased from C3 to T1, then narrowed at T8, and increased until T12. AP diameter also decreases from L1 to L3, then increased to S1 4. The spinal canal is considered narrowed when the anteroposterior (AP) diameter of the cervical canal is less than 13 mm at the level of C5. Meanwhile, in the lumbar spine, the spinal canal is considered narrowed if the AP diameter is less than 15 mm 3.
The interpedicular distance of the spinal cord is maximum at C5/C6, and in the thoracic spine at T12 vertebral level, as these are the sites that accommodate limb plexuses. The interpedicular distance also increases from L1 to L5 level in the lumbar spine 3.
The canal has a typical shape depending on its level:
posterior: ligamentum flavum lining the laminae
lateral: vertebral pedicles 1
spinal cord with its associated nerve roots and vessels (see blood supply of the spinal cord)