I have already covered the axial disc anatomy ad nauseam on my
Okay, lets move on to some other stuff. Remember, if you need a over-view of the anatomy, please visit my 'Disc Anatomy Page'.
Now, let's use some CT axial images (my own) to learn the different regions of the anterior epidural space; the regions where disc herniation occur in. If you have ever read your MRI report, some of these terms may sound familiar, for these are regions (zones) are often used by radiologists to describe the exact location of your disc herniation.
BLUE: This is the 'Central Region' and is located directly behind the disc and encompasses the anterior aspect of the thecal sac. Since the PLL (posterior longitudinal ligament) is at its thickest in this region, the disc usually herniates slightly to the left or right of this central zone. (As a side note, I would add that if you do have a centrally located disc herniation, the chances of a successful discectomy are reduced.)
PINK: This is the 'Paracentral Region' or 'Lateral Recess' and is located just outside of the Central Region. Because the PLL is not as thick in this region, disc herniations are frequently found here; in fact, this is the number one region for disc herniations to occur in. The Traversing Nerve Roots, which are the neural structures found in this zone, are frequently contacted, deviated and compressed in this zone. (Remember, an L5 disc herniation that occur into the lateral recess with compress the traversing S1 nerve root, not the exiting L5 nerve root that lives within the IVF.)
GREEN: This is the 'Intraforaminal Zone', also known as the 'Subarticular Zone', and is located within the intervertebral foramen (IVF). It is fairly rare for a disc to herniate into this region or beyond; in fact, only 5% to 10% of all disc herniation occur here or farther out. (241-244) When herniations do occur in this zone, they are often very troublesome for the patient. This is because a super-delicate neural structure called the 'Dorsal Root Ganglion' (DRG) lives in this zone. Any compression of the DRG can result in severe pain, sciatica (aka: radiculopathy) and nerve cell body (neuron) damage.
YELLOW: This is the 'extraforaminal zone' and, as the name implies, is just outside (lateral to) of the IVF. Again, it is very rare for a disc to herniate into this region, but when it does happen, it is often very troublesome for the patient and surgeon. A herniation in this zone may also irritate the 'Sympathetic Nervous System' and cause RSD like symptoms in the lower limb.
Let us now look at some CT Myelogram images (mine). The first image (left) is from a 'slice' (remember, CT and MRI images are all thin cuts through the spine in different planes) just above the L5 disc, through the inferior region of the vertebral body. Since the 'slice' is above the level of the disc, you will only see the posterior neural structures and not the disc itself.
Note the bright white 'ring apophysis' (not labeled) that out-lines the vertebral body - more so anteriorly (top of the picture).
Note that the cauda equina (thecal sac) is completely filled with 'white' contrast material (injected during my myelogram) that makes the thecal sac (dura and arachnoid mater that surrounds the cauda equina) and the 'Dural Sleeves' (which contain the spinal nerve roots) bright white. Since the contrast only fills the root sleeves up to the DRG, the DRG of L5 is not well visualized. I've placed a black line in the center of each DRG (dorsal root ganglion).
Also note the facet joints (oblique black slits - labeled) which are sandwiched between the superior articular process of the sacrum and the inferior articular process of L5.
The next CT slice (right) is a little be lower than the slice above and demonstrates the posterior of the disc quite nicely. Now you can plainly see the posterior disc has both bulged and herniated into the left traversing S1 nerve root, hence blotting it out (not white in color like the right S1 is).
I've high-lighted the posterior ring apophysis (white thick 'smile' line) to demonstrated how a diseased disc can 'bulge' outward. Anytime disc tissue is seen outside of the posterior vertebral body (ring apophysis), the disc is considered to be 'bulging'. Bulging discs are usually no greater than 2 or 3 millimeters (mm) and are 'concentric' or 'non-focal' in shape.
My disc bulge has an out pouching or eccentric component to its shape, as noted by the portion that has moved into the left 'Lateral Recess'. This out-pouching's is the herniation component to my disc lesion, and it has made my left S1 root disappear because its failed to properly fill with contrast - probably due to compression of that root at this level. (Note the white Nike swoosh just under the "S1" on the left, affected root. This is contrast material that "accidentally" leaked into the epidural space following my myelogram. This wasn't suppose to happen be created a semi-epidurogram effect.)
Here is that same image - the L5 disc level - without all the markings. Try and practice looking at the disc yourself without the aid of my boundary markers. Can you see the herniation? I bet you can now! This is considered a broad based herniation for its base it bigger than its anterior to posterior projection.
One thing to keep in mind: CT myelogram's are designed to look for 'filling-defects', and are not really the best imaging system for evaluating the human disc. The MRI is much better at visualizing the disc.
On a CT myelogram (left), if the disc herniation is large enough it will make the nerve roots turn black, or disappear (like it does to my left S1 root - only its confusing for you still see that tilted 'C' of white - which is only the outline of the black, contract-free, left S1 root). This is because the compression upon the nerve root will NOT allow the contract material to 'fill' into the nerve, hence, no bright white nerve root. That spells TROUBLE!
Note that this extrusion has completely blotted out (can't see) the right traversing S1 nerve roots (left side of image) and has pinched it against the lamina (tiny green arrow). Note the thecal sac is moderately to severely compressed by this large herniation, as noted on both the axial and sagittal images (between blue arrow and red star).
This young man (24 years) has avoid surgery and is doing fairly well, although his days of heavy work are probably over for good.Note the central canal of this young man is much smaller that that of the young man in figure 10 and 11. That just pure genetic folks; bigger central canals are much more forgiving for disc herniation, that smaller ones.
T1 vs. T2: the differences:
1) On the below MRI axial image of the L5 disc, name the structures that are numbered.
2) Name me the location of the 'disc lesion' using the correct zone.
3) Which nerve root is being displaced by this disc lesion?
4) bonus: What type of MRI image are we looking at??
5) What two structures are being contacted by this disc lesion?
GOOD LUCK! Make sure you look at the answers via the link below. I've got the case history to go along with the quiz MRI, including his lateral view, and the official MRI report.
Get the Quiz Answers 'Here'.
241) Epstein NC, et al. "Far lateral lumbar disc herniations and associated structural abnormalities: an evaluation in 60 patients of the comparative value of CT, MRI, and myelo-CT in diagnosis and management. Spine 1990; 15:534-49
242) Abdullah AF, et al. "Surgical management of extreme lateral lumbar disc herniations: review of 138 cases." Neurosurgery 1988;22:648-53
243) Kunogi J, Hasue M, "Diagnosis and operative treatment of intraforaminal and extraforaminal nerve root compression." Spine 1991;16:1312-20
244) Lejeune JP, et al. "Foraminal lumbar disc herniation: experience with 83 patients." Spine 1994;10:1905-8
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