Where do the lumbar nerve roots exit, and which root is most likely to be injured in a disc herniation?
The lumbar nerve roots exit beneath the corresponding vertebral pedicle through the respective foramen.
For example, the L5 nerve root exits beneath the L5 vertebral pedicle through the L5/S1 foramen. Since most disc herniations occur posterolaterally, the root that is compressed is actually the root that exits the foramen below the herniated disc.
So, a disc protrusion at L4/L5 will compress the L5 root, and a protrusion at L5/S1 will compress the S1 root.
Ninety-five percent of disc herniations occur at the L4/5 or L5/S1 disc spaces. Herniations at higher levels are uncommon.