(a) Definition of foraminal (external) lumbar disc herniation: Since foraminal lumbar discherniations (FLDH) and extraforaminal discherniations (EFLDH) mostly affect the L4, L3 and L2 nerve roots, and patients present with acute pain in the anterior thigh. Abdullah’s description of this syndrome formed the basis for the definition of (E)FLDH, which is defined as an acute neurogenic syndrome (leg pain predominant) with significant disc herniation and compression of the penetrating nerve root on CT/MRI (e.g., L4-L5 foraminal disc herniation compressing the L4 nerve root). Two types of disc herniation in the lateral zone: A, foraminal-type disc herniation; B, disc herniation into the unoccupied zone. (B) Anatomic basis of foraminal (extrinsic) lumbar disc herniation: Four basic anatomic concepts in Chapter 18 need to be repeated again: (1) anatomic segments; (2) each anatomic segment is divided into three levels; (3) penetrating nerve root (exiting nerveroot) and descending nerve root (traversing nerve root); (4) central spinal canal vs. (4) the distinction between the central canal and the lateral zone. The lateral zone can be subdivided into three parts (or three zones), centered on the arch root: the first part, or the entrance zone, is designated as the subarticular synovial zone because it is just on the deep side of the medial edge of the superior synovial process, medial to the arch root; the second part, or the middle zone, is designated as the intervertebral foraminal zone and is located below the arch root; the third part, which is also the most lateral part of the lateral zone, is designated as the extradural or exit zone. 1, the subarticular zone: The subarticular zone extends from the tip of the superior articular process to its base. Its anterior wall includes the first level of the previous spinal segment and the third level of the present spinal segment. The intervertebral foraminal zone: The intervertebral foraminal zone is also known as the intervertebral foramen, the nerve root canal, and what Wiltse calls the pedicle zone. This intermediate zone is bounded superiorly and inferiorly by the two adjacent pedicles. There is no bony “roof” (roof) over the intervertebral foramen (or nerve root canal). The “posterior cap” or posterior border of the foramen is the intertransverse ligament, and this structure can be easily opened to access the foramen. The foramen area is not actually a bony canal, but more of a bony sulcus with a soft tissue “roof”. The outer edge of the isthmus on the sagittal plane of the L2, L3, and L4 segments is in the same plane as the inner edge of the pedicle, and the foramen has no bony “roof”, whereas L5 is partially covered by the isthmus (right); B: The foraminal region is a bony groove located on the lateral aspect of the second level of the anatomic spinal segment, and is covered by soft tissue –the intertransverse process ligament. 3, extradural zone: The extradural zone (what Wiltse calls the distal outlet zone) is the most lateral of the three subdivisions of the lateral zone, named the outlet zone in Lee’s classification, and houses the peripheral nerves. 4. Exiting and TraversingNerve Roots: Foraminal and extraforaminal herniated discs are located in the same anatomic segment despite outward displacement of the fragments. Disc herniations located in the spinal canal generally compress the descending nerve roots, whereas (E)FLDH always affects the exiting nerve roots. Thus, the (E)FLDH at the first level of the L4 anatomic segment (L4-L5 disc) will compress the penetrating nerve root (L4) at that segment, the (E)FLDH at L3-L4 will compress the L3 nerve root, and the (E)FLDH at L2-L3 will compress the L2 nerve root. 5. The Furcal Nerve: Kikuchi performed an autopsy of the lumbosacral plexus during his graduate studies at Macnab in Toronto and noted that there were many branches connecting the nerve roots to each other outside the spinal canal. The largest and most constant branch was located between the 4th and 5th lumbar nerve roots, emanating laterally from the vertebral arch and traveling immediately outside the lumbar 4 and 5 roots to join the lumbar 5 nerve root, which he named the bifurcation nerve [57], while noting that it contained a large number of ankle dorsiflexion motor nerve fibers and sometimes some dorsiflexion fibers of the toes. When the L4 penetrating nerve root is compressed in or outside the foramen, the clinician may be misled that there are two nerve roots (L4 and L5) compressed by the herniated disc at the same time, when in fact only one spinal nerve is compressed before the emanating bifurcation nerve.