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!
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.
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.
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.