Believe in Indirect Decompression!

One of the most common procedures that I book patients for is an extreme lateral interbody fusion (XLIF).  This is a minimally-invasive lumbar fusion procedure that has all of the benefits of the classic anterior lumbar interbody fusion (ALIF) without its downsides (all of the bad things that can occur by traversing someone’s abdomen to get to the spine.)   In previous posts we’ve alluded to various types of spinal deformity that can cause pain.  The one constant in all of these types of spinal deformity: stenosis.  Both XLIF and ALIF rely on an old orthopedic principle known as indirect decompression in which properly sized spacers are used to correct spinal deformity and thus correct stenosis. 

Recall from previous discussion that stenosis, or narrowing around nerve roots, typically results after years of spinal degeneration.  The resulting stenosis can be divided into two simplistic types (and these types are just the way that I think about it in my head, don’t go looking for them in textbooks!).  The first type of stenosis is structural stenosis.  Here, the basic anatomical components of the spine are generally unchanged in terms of their shape or volume; it’s just that these parts of the spine have collapsed onto nearby nerve roots thereby causing pain.  I believe that this is the most common form of stenosis by far.  One common example of structural stenosis occurs when the intervertebral disc (IVD) has degenerated and collapsed resulting in loss of foraminal height and foraminal stenosis.  Another example of structural stenosis is seen in spondylolisthesis when one vertebral body slides over the one below it causing a dynamic foraminal stenosis that worsens when the patient stands and loads their spine (more on this topic later.)  Lastly, it is my opinion that the central stenosis that causes neurogenic claudication in the elderly is a form of structural stenosis resulting from buckling of the ligamentum flavum (this is controversial as some believe that the body actually produces reduntant ligamentum flavum which would be more like the reactive stenosis discussed below.) In each of these cases, normal spinal structure and alignment has been lost resulting in stenosis and pain.

The other simplistic type of stenosis is reactive stenosis.  Here, there IS an increase in the shape or volume of a component of the spine, which results in stenosis and compression of nearby nerves. The most common example of this occurs when the facet joint degenerates and becomes larger as it becomes consumed by arthritis.  This leads to osteophyte (fancy word for bone spur) formation, which can cause nerve root compression and radiculopathy. 

To fix reactive stenosis the surgeon must perform a direct decompression procedure, a laminectomy or foraminotomy, to remove all excess bony overgrowth from around the nerves.  In fact, classically this is the way that all nerve compression (regardless of which type of stenosis is causing the compression) is relieved.  The typical neurosurgeon’s mentality (and I can say this because I’m one of them) is that the only way to know that a nerve is decompressed is to remove any overlying bone and actually see the nerve.  But what if this isn’t always necessary?  I believe that in most cases it’s NOT necessary (and trust me, it takes a huge leap of faith on the part of both the surgeon and the patient to come to this realization.)  Because most spinal stenosis is structural and not reactive, restoration of normal structure and alignment of the spine will relieve stenosis and pain without the extra time and risk of a laminectomy.

Recall that loss of normal spinal structure and alignment begins with degeneration and resulting collapse of the IVD.  So, to restore structure and alignment we go to the disc space (remember the post on spacers?)!  Indirect decompression is achieved when a properly sized spacer is inserted into a collapsed disc space to restore the height of the neural foramen (see figures 1 and 2).  This then relieves nerve root compression because the space around the nerve in the foramen is restored; I don’t have to do more work to remove bone that doesn’t need removing!  To be sure, this is a controversial topic.  I know plenty of very good spine surgeons who just don’t believe in indirect decompression and subject their patients to a concurrent laminectomy with every spinal fusion.  They’re paranoid (we surgeons are a VERY paranoid bunch, some are just more so than others) that if they don’t directly decompress the nerve and visually confirm that it’s decompressed then they may not relieve the patient’s pain.  I get it.  Like I said, it’s a leap of faith.  A laminectomy isn’t a benign procedure though.  There’s a 5-10% risk of dural tear and spinal fluid leak for starters.  Typically this is a minor complication but it can be catastrophic.  There’s also the risk of scar tissue formation around exposed nerve roots, which can lead to chronic pain after surgery.  Finally there’s just the risk of being under anesthesia for the extra time needed to perform the laminectomy.  Why would I subject my patients to these risks when I know that the indirect decompression achieved by the spacer will probably suffice?  Believe in indirect decompression!

XLIF

Figure 1: A, preoperative image showing severe collapse of the IVD resulting in foraminal height loss and nerve root compression; B, postoperative image demonstrating restoration of disc space and foraminal height after insertion of a large intervertebral spacer.

T LUMBAR L 5 S 1 SPOT 4AP 5130

Figue 2: As above in figure 1, image on left is preoperative image demonstrating severely collapsed IVD at L5/S1 with resultant severe foraminal stenosis (pink outline).  The image on the right is a postoperative image after an L5/S1 anterior lumbar interbody fusion (ALIF) with significant increase in disc space, and thus, foraminal height.  This patient’s leg pain was relieved immediately after surgery WITHOUT laminectomy. 

Of course there are times when relying solely on indirect decompression may not be appropriate.  In cases of severe reactive stenosis in which, say, a nerve root is encased in bone (see figure 3), indirect decompression probably isn’t going to work no matter how large of a spacer you put in.  Also, in cases of large concurrent disc herniations or facet cysts (a type of reactive stenosis, I suppose, which I’ll discuss in a later post) I may also be forced to do a direct decompression.  The more cases I do, though, the more I’m surprised at what I can get away with in terms of avoiding a direct decompression.  These days I’ll typically assume that indirect decompression will work but explain to the patient that there is a very small chance that indirect decompression may fail and that we may have to do a small “second stage” laminectomy later.  How small of a chance you ask?  I went back and looked at the data for every one of my lumbar fusions performed since December 2013.  Of nearly 250 patients only 8 needed reoperation for failure of indirect decompression.  That’s a 3% risk.  I’d say those are pretty good odds in favor of direct decompression. 

Reactive stenosis

Figure 3: Severe “reactive” foraminal stenosis at L4/5 and L5/S1 resulting from severe bony overgrowth around nerve within the neural foramen (red arrows).  This patient failed indirect decompression and required a minimally-invasive foraminotomy a few months after his initial surgery.

Thanks for reading!

J. Alex Thomas, M.D.

What is Lumbar Foraminal Stenosis?

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.)

Novaimage

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.

Foraminal stenosis

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.

 Coronal deformity

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.

XLIF

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.