In our last post we reviewed how lumbar laminectomy is an effective treatment for central lumbar stenosis causing neurogenic claudication. Now we turn our attention to foraminal stenosis, or narrowing of the neural foramen. While lumbar central stenosis causes neurogenic claudication, lumbar foraminal stenosis causes radiculopathy, a.k.a. “a pinched nerve”, and resultant leg pain.
The neural foramen is the opening on the side of the spine through which the exiting nerve root passes. Its boundaries include the bottom of the pedicle of the vertebral body (VB) above, the back of the intervertebral disc (IVD) in front and the facet joint behind (see figure 1.) Degenerative changes of these structures can cause narrowing of the foramen, and thus pinching of the exiting nerve, in three main ways. First, as the IVD degenerates it may bulge or even herniate a piece of disc material into the foramen (a so-called far-lateral disc herniation) causing narrowing of the foramen from the front. Second, as the facet joint degenerates it can become overgrown with bone spurs causing narrowing of the foramen from behind. Finally, third, the disc space can become so degenerated that it loses height thereby causing the top and bottom of the foramen to close down on the nerve like the blades of a guillotine (see figure 2.) Usually the disc height loss occurs symmetrically and both foramina are affected. Occasionally, however, the height loss only affects one side (this often occurs after someone has had surgery on that side of the spine) resulting in a focal coronal deformity and pain in only one leg (see figure 3.) (Note: I left out one other cause of foraminal stenosis, spondylolisthesis, which will be discussed in a future post.)
Figure 1: Oblique view of lumbar spine. Note nerves exiting via the neural foramen bounded by the back of the IVD in the front, the facet joint (FJ) in the back and the pedicles (P) above and below.
Figure 2: Lateral (side-view) MRI showing nerves exiting foramen. At the L2/3 level note that the disc space (the boundaries of which are indicated by the yellow lines) is of normal height resulting in normal foraminal volume (outlined in green.) The black dot in the green circle is the exiting nerve. At the next level down, the L3/4 level, the disc space is completely collapsed causing severe narrowing (outlined in red.) Note that the exiting nerve, the black dot, is now crushed in the foramen.
Figure 3: Coronal MRI of the lumbar spine demonstrating a focal coronal deformity in which the disc at L3/4 has collapsed to the right (pink arrow.) This patient had severe right leg pain because of a pinched exiting L3 nerve. (Note that the image is reversed such that the left side of the image is actually the patient’s right side.)
The first two causes of narrowing discussed above can usually be fixed with a minor outpatient surgical procedure. For a example, for a far lateral disc herniation I can usually dock a small tubular retractor on the outside of the neural foramen. There I find the exiting nerve, move it out of the way and remove the piece of herniated or bulging disc from the foramen. In the case of an overgrown facet joint a foraminotomy is performed (usually along with a laminectomy and thus the two are sometimes referred to together as a laminoforaminotomy) to decompress the nerve from behind. The final scenario, disc space height loss from degeneration of the IVD, is a bit more complicated to fix. In this case the height loss causes circumferential narrowing of the foramen such that a simple foraminotomy usually isn’t sufficient to decompress the nerve. I could remove the entire facet joint to open up the foramen but this would destabilize the spine and thus would require a spinal fusion (a procedure called a transforaminal interbody fusion, or TLIF, which will be discuss in a later post). In my opinion the best way to fix foraminal stenosis caused by disc space height loss is to correct the underlying problem: restore disc space height. This is usually done using a large spacer which is inserted into the disc space as part of an interbody fusion. There are a variety of ways to achieve this including anterior lumbar interbody fusion (ALIF), extreme lateral interbody fusion (XLIF, which is essentially an ALIF performed via the patient’s flank), TLIF, etc. All of these procedures achieve the same goal of inserting a spacer into the disc space (note that they all end in -IF for interbody fusion) in order to restore disc space height to decompress the nerve in the foramen (see figure 4). This is a VERY important take home point here: the difference between direct decompression (i.e. a foraminotomy in which bone is drilled off of the nerve to directly decompress it) versus indirect decompression (in which a spacer in the disc space is used to restore normal disc height and alignment to indirectly decompress the nerve in the foramen.) Again, in my opinion, if it’s an option I think it’s always best to try to restore the patient’s anatomy to what it once was (i.e. with normal disc height) in order to definitively treat foraminal stenosis.
Figure 4: Image on left shows disc degeneration with loss of disc height; note the compression of the exiting nerve in the foramen. The image on the right demonstrates restoration of disc height with a large spacer inserted during an XLIF; note the indirect decompression of the exiting nerve.
Thanks for reading!
J. Alex Thomas, M.D.