In our last post we discussed pedicle screws, the most commonly used form of posterior instrumentation. Even in the recent past, pedicle screws were the only form of instrumentation used in a spinal fusion. Unfortunately, these posterior-only fusion constructs didn’t have excellent rates of fusion: nearly 30% of these cases ended up in non-unions. Today, most spinal fusion constructs utilize pedicle screws at the back of the spine and intervertebral spacers (or cages) in the front of the spine. These so-called 360-degree constructs (also referred to as interbody fusions) have dramatically decreased the rates of non-union and thus are now the standard way spinal fusions are done.
Of note, interbody fusion procedures are abbreviated with the suffix –IF. Common examples include the anterior lumbar interbody fusion (ALIF), posterior lumbar interbody fusion (PLIF) and extreme lateral interbody fusion (XLIF). We’ll highlight the differences between the various –IFs in a future post. The most commonly used interbody fusion procedure is actually the ACDF which we’ve already discussed here.
In the 1950s anterior interbody fusions were used in the treatment of Pott’s disease, or tuberculosis of the spine. These anterior fusions proved to be more effective than posterior fusions because they allowed for reconstruction of the front of the spine, which is where most of the destruction of Pott’s disease occurs. Once the infected vertebral bodies were resected (via a corpectomy), large defects in the front of the spine were reconstructed with pieces of bone fashioned into struts. These pieces of bone were either harvested from cadavers (allograft), or may have been harvested from somewhere within the patient’s body (autograft) like the fibula or iliac crest. These bone struts provided immediate support to the front of the spine and also promoted bony fusion. They were the earliest intervertebral spacers.
While long structural bone struts are important in the reconstruction of the front of the spine after corpectomy for infection, tumor or trauma, the more commonly used intervertebral spacers are smaller spacers inserted into the disc space for fusion for degenerative conditions. I will be referring to these spacers primarily for the remainder of the post. The first such anterior interbody fusion utilizing bone inserted into the disc space was reported by Dr. Burns in 1933 for the treatment of lumbar spondylolisthesis (“slipped bones”, to discussed in a future post.) Unfortunately, in this case, as with early anterior cervical fusions, often only loose shards of bone were packed into the disc space. While this may have promoted bony fusion across the disc space it certainly didn’t provide any structural support and may have allowed for further collapse of the spinal segment with resultant kyphosis of the spine (see figure 1). In the subsequent decades since Dr. Burns’ case, intervertebral spacers of various materials and shapes have been developed that are better at providing structural support and are also more readily available than spacers made of bone. Today most intervertebral spacers are made of a fancy plastic called PEEK (polyetheretherketone) and this is what I use almost exclusively when I perform interbody fusions. Titanium is also making a comeback (either comprising the entire spacer or as a coating on the surface of a PEEK spacer) as some feel it may be better at incorporating new bone in the early stages of bony fusion (see figure 2). I could spend multiple posts discussing the merits of different intervertebral spacer materials but that you would likely bore you (and me) to death. Regardless of what it’s made of, every intervertebral spacer should serve two purposes: a) correct spinal deformity, and b) promote a bony fusion.
Figure 1: Anterior cervical fusion done with only bone graft and no spacer for structural support. Note that the fused bones have collapsed somewhat and are now allowing the spine the fall forward The red line indicates the forward angle of the collapse and should, in fact, be straight (source: Riew et al, 2015).
Figure 2: Various intervertebral spacers used for lumbar interbody fusion (source: Williams et al, 2005).
First, let’s talk about the correction of spinal deformity. In my opinion spinal reconstruction should take place where the degenerative process starts: the disc space. As we’ve talked about previously, as the intervertebral disc (IVD) degenerates it dries out and collapses. Without the structural integrity of the IVD the adjacent vertebral bodies fall out of alignment and the spine then becomes deformed. Spinal deformity is a complicated topic that I won’t delve too far into at this point. All you need to know now is that a deformed spine causes both back pain and radicular (nerve) pain. The back pain is caused by the mechanical stress of the deformity. The radicular pain is caused by foraminal stenosis and the resulting nerve root compression. Recall that the neural foramen, the hole on the side of the spine where the nerve exits, is an aperture comprised of the top of the pedicle of the vertebral body below and the bottom of the pedicle of the vertebral body above. As the IVD collapses the diameter of the foramen narrows significantly and the nerve is guillotined within. An intervertebral spacer inserted into a collapsed disc space corrects deformity by acting like a wedge to a) restore alignment to relieve mechanical stress, and b) restore foraminal height to allow for indirect decompression of compressed nerves. The concept of indirect decompression is an immensely important one that I’ll get to in an upcoming post. (See figures 3 and 4.)
Figure 3: Image on left shows collapsed disc with resultant foraminal stenosis and nerve root compression. Image on right shows the result after a spacer is inserted with restored foraminal height and indirect nerve decompression.
Figure 4: Preoperative MRI showing a focal coronal deformity with unilateral foraminal stenosis and nerve root compression resulting in severe R leg pain (the red lines, indicating the vertebral endplates, should be parallel and instead are collapsed to one side.) Image on right is a postoperative anterior view Xray after a spacer (with plate) is inserted. Notice that the vertebral endplates (red lines) are now parallel after the deformity has been corrected. Of note, because it is PEEK the spacer isn’t seen well on Xray and instead the borders of the spacer are indicated by markers within.
The other important function of the intervertebral spacer is to promote bony fusion. Recall from previous posts that it’s the bone graft material that is applied between parts of the spine that promotes the new bone growth in a spinal fusion. There are many types of graft material used in spine surgery that I’ll discuss in a later post. Most modern intervertebral spacers have large chambers within their structure where bone graft is packed prior to insertion into the disc space (see figure 5). These chambers then hold the graft material in place, nicely apposed against the adjacent vertebral bodies, in order to promote bony ingrowth (see figure 6).
Figure 5: standard ALIF spacer used in lumbar fusions. Note the large graft chambers (red arrows) that are packed with bone graft material prior to insertion into the disc space. Source: Globus Medical.
Figure 6: Lateral postoperative CAT scan demonstrating robust bone growth (red arrow) across intervertebral spacer (spacer material isn’t seen in this cut of the scan.)
Lastly, I believe that bigger is better when selecting an intervertebral spacer in an interbody fusion. Not only do larger spacers provide more structural support for deformity correction but they can also carry more graft material to better promote bony fusion. THIS is why I prefer anterior procedures such as XLIF or ALIF with their large spacers rather than TLIF or PLIF with their quite puny spacers. More on that later.
Thanks for reading!
J. Alex Thomas, M.D.
1) Riew et al, 2015, Postoperative Deformity of the Cervical Spine, online access at iKnowledge: https://clinicalgate.com/postoperative-deformity-of-the-cervical-spine/.
2) Williams et al, 2005, CT Evaluation of Lumbar Interbody Fusion: Current Concepts, ANJR, 20: 2057-2066, September 2005.