What is a spinal fusion?

Of all the procedures that I perform, the spinal fusion is one of the most misunderstood and maligned.  Indeed, the name of this blog is a nod to the confusion surrounding the procedure.  Before we get to the specific conditions that necessitate spinal fusions and the techniques used to achieve a fusion let’s talk about what a spinal fusion is in general terms.  Throughout the article I may refer to spinal fusion by its preferred medical name, spinal arthrodesis.  The discussion will generally pertain to fusions of the lumbar spine (if you want to read about cervical spine fusions you can do so here and here.) 

Whenever the spine becomes deformed or unstable it becomes painful.  It’s not as easy as you may think to pinpoint the cause of spinal pain as often it’s multifactorial.  Generally, though, the pain is caused by either compression of a nerve by a deformed spine (causing a radiculopathy) or by the structural stress of instability or deformity (causing neck or back pain.)  The goal of a spinal arthrodesis is to promote bone growth between one or more vertebral bodies in order to correct spinal deformity and instability and thus relieve pain.  Classically this bone growth is achieved between the posterior elements or transverse processes (posterolateral arthrodesis), between the vertebral bodies within the disc space (anterior/interbody arthrodesis) or a combination of both (see figure 1).  

Posterolateral fusionInterbody fusion

Figure 1: Left, robust posterolateral fusion (arrows) with pedicle screw fixation.  Right, interbody fusion with robust bone growth between two vertebral bodies (arrow). (Source: nuvasive.com

Today, advanced forms of spinal stabilization are used to stabilize the spine to allow the arthrodesis to occur more robustly and more rapidly (you can think of spinal stabilization as internal bracing.)   In the early 1900’s, when spinal fusion was first described to treat the destructive Pott’s Disease, or spinal tuberculosis, these technologies weren’t available. In these early procedures surgeons attempted to achieve a spinal arthrodesis by simply laying down harvested bone graft over the posterior aspect of the spine (either from the lamina, the iliac crest or from the fibula in the leg) and hoping that bone would eventually grow into a robust posterolateral arthrodesis.  This typically required a long, arduous surgery followed by 6 months of bedrest in a body cast.  Just imagine that for a moment.  Patients who were lucky enough to achieve a solid bony fusion may have had a permanently deformed (and painful) spine, as methods to adequately correct spinal deformity hadn’t been developed yet.  Over time surgeons realized that the simply fusing a patient in-situ, or in its original place, wasn’t adequate and that correction of spinal deformity at the time of arthrodesis was paramount.   In the 100 years since the early spinal arthrodeses for Pott’s Disease, various metallic implants, intervertebral spacers and grafting materials have been developed to dramatically improve the outcomes of spinal arthrodesis.  Today, minimally-invasive spinal arthrodesis can be performed on an outpatient basis with a near 100% success rate (see figure 2). We’ll get into the specifics of implant and graft technologies in future posts.

Post op XLIF CT

Figure 2: Postoperative Xray showing successful minimally-invasive spinal fusion (XLIF).  A large intervertebral spacer is used to restore foraminal height (blue arrow) while percutaneous pedicle screws (red arrow) are used to stabilize the posterior aspect of the spine. 

Despite recent technological advances of spinal arthrodesis, some patients are still downright terrified when I recommend the procedure.  Everyone knows someone who “was never the same” after having a spinal fusion.  Unfortunately these fears aren’t entirely unfounded as some spinal fusions are still being done on the wrong patients for the wrong reasons.  As illustrated in table 1 the odds of success (generally defined as relief of pain and disability) of spinal arthrodesis vary based on the indication for surgery.  The clearest indication for spinal arthrodesis is acute instability or deformity caused by trauma, infection or tumor.   In these instances spinal arthrodesis is almost always associated with an excellent outcome.  In contrast, the murky, poorly defined indications of degenerative disc disease (DDD) or spondylitic back pain (back pain caused by arthritis) often are NOT good indications for spinal surgery.  The problem in these cases is that back pain is notoriously cryptic and thus it can be difficult to correlate a patient’s pain with a certain structural abnormality on an MRI or X-ray.  The surgeon then has to make an educated guess as to what is generating the pain and then target it with a spinal fusion.   It’s no surprise when this effort is unsuccessful.  I typically will NOT perform spinal fusion on patients with only DDD or spondylosis and back pain (i.e. in the absence of gross instability or deformity.)  There are exceptions to this, of course.  For example, in cases of severe DDD with disc space collapse and resultant foraminal stenosis and radiculopathy (as opposed to just back pain) an interbody fusion may be needed to restore foraminal height and indirect decompression of a compressed nerve (see figure 3).  While this isn’t a case of frank instability I consider the disc space collapse with foraminal stenosis a deformity that requires correction. We’ll talk about this and other indications for spinal arthrodesis in future posts.    

Screenshot 11 27 16 11 11 AM

Table 1: Odds of success (defined as reduction of pain and disability) of spinal fusion is dependent on the indication for surgery. 

XLIF jpg

Figure 3a: severely collapsed disc space results in foraminal stenosis and compression of exiting nerve root.  3b, interbody fusion with large intervertebral spacer results in restore foraminal height and indirect decompression of nerve root.  

I realize that this post may have generated more questions than answers.  Stay tuned for future posts about indications for spinal arthrodesis as well as advanced techniques used to achieve successful arthrodesis.  I hope to prove to you that in the properly selected patient the properly executed spinal fusion can provide life-altering improvement in a patient’s quality of life. 

Thanks for reading!

J. Alex Thomas, M.D.

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