The Big IF: an introduction to the Extreme Lateral Interbody Fusion (XLIF)

Ok up until this point we’ve discussed why a spinal fusion is performed and what makes for an ideal spinal fusion.  We’ve talked about the importance of minimally-invasive techniques to address pathology without the collateral damage of traditional open midline incisions.  We’ve also talked about the importance of large intervertebral spacers to a) achieve fusion, b) restore normal lordosis and c) to achieve indirect decompression of the neural elements.  Lastly, we’ve discussed the importance of restoration of lordosis to a) maximize the chances of a good clinical outcome and b) to prevent adjacent segment degeneration.  In my opinion, the one technique for lumbar fusion that best achieves all of the above goals is the Extreme Lateral Interbody Fusion (XLIF).  First developed in the mid 2000s, this technique allows for the placement of a very large intervertebral spacer at the front of the spine via a small, minimally-invasive incision on the patient’s flank.  

We’re going to talk about the specific steps of XLIF in the next post.  For now, I’d like to just focus on why I think that XLIF is superior to other fusion techniques.  In brief: XLIF is the procedure that allows for the largest possible intervertebral spacer to be inserted via the smallest incision (see figure 1).  There are several ways to achieve an interbody fusion of the spine (these procedures have the suffix -IF as in XLIF, ALIF, OLIF, TLIF, PLIF, etc.) These techniques can generally be thought of as either anterior (XLIF, ALIF, OLIF) or posterior (TLIF, PLIF) approaches.  Believe it or not, spine surgeons still fiercely debate which way is better.  Those in the posterior approach camp say that, through one incision on the patient’s back, they can directly decompress nerves (via laminectomy) and then insert spacers into the disc space and place pedicle screws.  Part of the debate here is whether or not a direct decompression of the nerves is even needed.  I personally believe that indirect decompression is all you need and you can spare the patient the risk and morbidity of removing bone off of compressed nerves.  (I don’t just believe this; I’ve proven it to myself with data from over 300 cases which show that indirect decompression works in greater than 98% of cases.)  This is very controversial though and some surgeons aren’t going to be satisfied until they’ve performed a complete bony decompression and have seen the nerves floating free and decompressed. 

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Figure 1: XLIF spacers have the most “bang for the buck” when comparing the size of the spacer with the length of the incision necessary to insert it.

Here’s the problem with posterior approaches though.  Remember that the disc space, where intervertebral spacers are placed, is at the front of the spine.  In order to place spacers there during posterior fusions the surgeon has to move the thecal sac (fluid-filled sac that contains the nerves) and nerve roots out of the way in order to sneak a spacer around them into the disc space.  There’s only so much space to do this so the surgeon really must compromise in terms of the size of the spacer that can be inserted.  Thus for TLIF and PLIF the surgeon must insert a very small spacer (see figure 2.)  We’ve talked about how important large intervertebral spacers are, the bigger the better in my opinion.  The small spacers inserted via TLIF and PLIF can’t contain much graft material to promote bony fusion, aren’t good at restoring lordosis (in fact, some studies show that patients who undergo TLIF and PLIF actually lose lordosis) and lastly, aren’t good at correcting lost disc space and foraminal height.  Also, because of where these small spacers sit in the disc space (against the soft bone at the center of the vertebral end plates) they often end up subsiding into the vertebral body above and below (see figure 3).  When subsidence occurs the surgeon has failed at achieving one of the main goals of the procedure: restoration of foraminal height and indirect decompression of the neural elements.  This failure can lead to recurrence of nerve compression and leg pain resulting in the need for revision surgery with larger spacers.


Figure 2: Size matters.  Look at the size of an XLIF/ALIF spacer compared to that of the much smaller TLIF or PLIF spacers.

 TLIF subsidence  Acquired foraminal stenosis

Figure 3: First image shows midline sagittal view of a TLIF spacer (red arrow) subsiding nearly 50% into the endplate of the L5 vertebral body below (the endplates are indicated by the thin yellow lines.)  Second image shows severe acquired foraminal stenosis (red arrow) that resulted from the subsidence of the TLIF spacer.

The anterior lumbar interbody fusion (ALIF), in contrast, allows the surgeon to come directly to the front of the spine to insert very large intervertebral spacers without disruption of the posterior elements of the spine. We’ve talked about the benefits of large spacers already so I won’t go into it extensively here.  In brief, though, these large spacers are much better at promoting fusion, restoring lordosis and restoring foraminal height for indirect decompression of the neural elements.  Think of it from a structural standpoint: would you want your house built on a small/narrow foundation or a large/wide one?   The downside to the ALIF, though, is that going through a patient’s abdomen isn’t a benign thing.  There are risks of ileus (when the bowels are “stunned” after manipulation during surgery and don’t move for several days.  Doesn’t sound like a big deal but it can be an awful complication) as well as injury to abdominal organs or the large blood vessels that sit in front of the spine.  Second, the ALIF is usually requires a vascular or general “approach” surgeon who assists in getting the spine surgeon to the spine to do his work.  Lastly, if the surgeon does desire to place pedicle screws or do any other work at the back of the spine he’ll have to close up the abdomen and then flip the patient from the supine position (patient laying on their back) to the prone position (patient laying on their abdomen.) 

XLIF is just a modified ALIF.  Rather than coming through the patient’s abdomen with the patient on their back, the patient is positioned on their side and the surgeon approaches the spine via a small incision on the patient’s flank (see figure 4). You get all of the benefits of a large intervertebral spacer at the front of the spine without the downsides of traditional ALIF.  With an XLIF spacer you get a huge graft window to promote robust fusion.  Also, because the XLIF spacer sits on the hard bone at the periphery of the vertebral endplates (the apophyseal ring) versus the soft bone at the center of the endplate, it resists subsidence and thus is better at correcting lordosis and foraminal height loss than the smaller TLIF and PLIF spacers (see figure 5). In my opinion there is no question that XLIF is superior to TLIF or PLIF and if I needed a lumbar fusion I’d ask for an XLIF.


Figure 4: Oblique view of ALIF and XLIF trajectories into disc space.


Figure 5: Image showing the stronger, more compact bone at the outer apophyseal ring of the vertebral endplates.  XLIF spacers resist subsidence by sitting on the apophyseal ring rather than the softer bone at the center of the endplate.  Source: A.A. White, M.M. Panjabi (Eds.), Clinical biomechanics of the spine, 2nd ed, JB Lippincott, Philadelphia, PA, 1990.

If you’ve gotten one of these posterior fusion surgeries don’t email me asking if you got the wrong procedure (you probably don’t need to hear my opinion any more clearly than it’s presented here.)   To be fair, there are lots of surgeons out there who routinely perform TLIF and PLIF and do them well.  In fact, TLIF is the most common technique for lumbar fusions in the US. These procedures are safe and can be effective.  I don’t do them for a reason though.  At our hospital we have surgeons who perform every one of these techniques so we have a diverse cohort with which to compare immediate outcomes of these various types of fusion procedures.  There’s just something about the immediate structural support and correction afforded by large intervertebral spacers that (again, in my opinion) leads to more rapid and dramatic clinical improvement in patients.  One clear difference: my XLIF patients almost always go home the morning after surgery (average length of stay for nearly 200 1- and 2-level cases since 2013 is 1.2 days) while the TLIF and PLIF patients of other surgeons stay in the hospital at least twice as long if not longer.  If you include training, I’ve been doing the XLIF procedure since 2006 and the clinical outcomes afforded by this technique still amaze me to this day.

So if XLIF is so great why doesn’t every spine surgeon do XLIF and only XLIF for their lumbar fusions?  The answer is that there are some perceived limitations of XLIF that scare some surgeons into not doing this procedure:

  1. Surgeons think that after XLIF you have to close up and then flip the patient prone to place screws (similar to the real limitation described for ALIF.)
  2. Because the iliac crest of the pelvis gets in the way, XLIF can’t be done at L5/S1 (ok, this is a real limitation, not a perceived one.)  So if a surgeon is fusing L4/5 and L5/S1 (which is required quite often actually) you’ll have reposition the patient from the lateral position to some other position to fuse L5/S1 using a different technique. 
  3. XLIF has an unacceptably high risk of nerve injury, especially when done at L4/5.

Over the next few posts I’ll discuss why these are just perceived limitations of XLIF.  In fact, XLIF is a very safe and effective way to perform a lumbar fusion, even at the L4/5 level (greater than 95% of the lumbar fusions that I do involve XLIF at the L4/5 level.)  Also, when XLIF is combined with Lateral ALIF (a minimally-invasive ALIF done with the patient on their side) at L5/S1 and single-position pedicle screw fixation (pedicle screws placed with the patient in the lateral position without flipping prone) a surgeon can perform a robust lumbar fusion from L1 to the sacrum without repositioning the patient.  This strategy of Lateral Single Position Surgery (LSPS) dramatically reduces the anesthesia time for patients, which translates into decreases risk and improves outcomes.  More on this ground-breaking concept in future posts.

Thanks for reading!

J. Alex Thomas, M.D.

Is your spine in line?

Before I talk about the types of spinal fusions that I perform I think it’s very important that we first discuss the concept of spinopelvic balance.  Until recently this was a concept that was only considered by academic spinal deformity surgeons (those spine surgeons who treat scoliosis and other complex spinal pathology.)  Over the past few years, however, data has emerged that suggests that restoration of lumbar lordosis (the normal backwards curvature of the lumbar spine) in order to maintain proper spinopelvic balance is critical even for patients who undergo one- or two-level spinal fusions.   Being sure to consider spinopelvic balance before fusing a patient’s spine will maximize their chance of a good outcome.

What is spinopelvic balance?  Basically, the spine should maintain an upright posture, with the head positioned directly over the pelvis, with minimal energy expenditure.  This notion was elegantly described by the French orthopedic surgeon Jean Dubousset who described a “cone of economy” of an upright patient (see figure 1A).  Neutral spinopelvic alignment keeps the patient at the center of the cone where he has to maintain little energy to stand upright and keep horizontal gaze.  As the spine pitches forward (for a variety of reasons described below) the patient falls to the periphery of the cone and thus has to expend more energy just to stay upright (see figure 1B).  If he falls too far to the periphery he’ll no longer be able to support himself and will need a cane or walker.  This forward pitching of the spine is referred to positive sagittal balance and is the torment of all patients with degenerated spines.  The more severe the imbalance the more disabled the patient.  This was first described in a landmark study in 2005 Glassman et al.  The authors examined full-length standing X-rays on 352 patients and found a direct, linear correlation between increasing positive sagittal balance and worsening patient disability (see figure 2).


Figure 1A: Dubousset’s cone of economy (source: Ames et al). A patient at the center of this cone of economy will have to expend minimal energy to keep their head upright and maintain horizontal gaze.  1B; the King of Pop WAY out of his cone of economy. 

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Figure 2. There is a linear correlation with increasing sagittal balance and poor clinical outcomes.  SF-12 and ODI scores are clinical outcomes (HRQOL) measures used in spine surgery to assess how well a patient is doing.  Lower SF-12 scores and higher ODI scores indicate worse outcomes (Source: Glassman et al.) 

The balance of the spine is assessed using several spinopelvic parameters measured on AP and lateral (front and side) standing X-rays of the patient that include the femoral heads (see figure 3.)  This X-ray is mandatory in my clinic for any patient who is being considered for a spinal fusion.  There are dozens of various spinopelvic parameters that can be measured for a given patient and it can quickly get overwhelming trying to keep track of all of them.  The Glassman study mentioned above used the sagittal vertical axis, SVA, to quantify positive sagittal balance.  SVA is the best measure to describe a patient’s global spinal balance as it assesses the position of the cervical spine over the sacrum (tailbone.)  The problem with SVA, in my opinion, is that it can be difficult to get full-length standing X-rays at most community imaging centers.  In another study by Schwab et al in 2013 the authors prospectively studied dozens of spinopelvic parameters in nearly 500 patients with spinal deformity.  These parameters were correlated with a variety of health-related quality of life (HRQOL) measures.  When they analyzed the data they found that three parameters matter most: 

1)   SVA: which we already discussed

2)   PI-LL mismatch: the amount of discrepancy between the pelvic incidence (PI, a fixed morphological characteristic of your pelvis.  Basically, the way your pelvis is shaped in relation to the hip joints) and the lumbar lordosis (LL, the normal curvature of the lumbar spine as mentioned above.) 

3)   Pelvic tilt (PT): a measure of the extent that the pelvis is tilting backwards to compensate for lost lumbar lordosis. 

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Figure 3: Standing lateral X-ray including femoral heads showing measurements of pelvic tilt (PT), pelvic incidence (PI), lumbar lordosis (LL), PI-LL mismatch and segmental angles.  This X-ray is mandatory for any patients in my clinic being considered for lumbar fusion. 

When the authors did even more in-depth analysis they found that PI-LL mismatch was the variable that most correlated with patient disability (patients with PI-LL mismatch of 11 degrees or greater were more likely to be severely disabled.)   That happens to be very convenient for spine surgeons.  First, both the PI and LL can be easily calculated on standing lumbar xrays that can be done at any imaging facility (full-length films not required!)  Even more important, PI-LL mismatch is the parameter that is most easily addressed with surgery.  Nearly 70% of a patient’s overall LL comes from the angulation at the L4/5 and L5/S1 disc spaces.  So if you’re trying to correct a patient’s PI-LL mismatch you can often do so by restoring LL with large, angled intervertebral spacers placed at one or both of these levels.  I know that was a lot of complicated stuff there but if you take away nothing else, know this: PI and LL should be assessed in all patients being considered for spinal fusion surgery so that PI-LL mismatch can be corrected.

Positive sagittal balance (and remember, “positive” balance is actually a bad thing) can have several causes.  First, pediatric patients can have so-called “idiopathic” scoliosis and other spinal deformities.  These are entirely unique entities and I won’t discuss them here.  In adults, acute changes in spinal structure such as tumor, trauma or infection can cause the spine to lose structural integrity and allow the spine to fall into positive sagittal balance.  Most commonly, however, progressive degeneration of the spine allows for the slow development of sagittal imbalance.   As the intervertebral discs degenerate over a patient’s lifetime, and supporting spinal muscles and ligaments weaken, the spine will lose its normal lordosis  (i.e. it will flatten out, forming a so-called “flat-back” deformity).  In severe cases the spine may even begin to kyphose, or bend forward (see figure 4).  Compounding matters, the spine can also start to buckle under the weight of the torso leading to an S-shaped coronal deformity (see figure 5.)NewImageNewImage 

Figure 4: Image on left shows a normal, healthy lumbar spine with adequate lumbar lordosis (backward curvature of the spine.  Image on right shows a severely degenerated spine with loss or lordosis resulting in “flat back’.  PI-LL mismatch in this patient is 23 degrees.

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Figure 5. AP (front view) Xray of lumbar spine and pelvis demonstrating a severe coronal deformity with a right-sided concavity.  Lumbar spine should be straight up and down on this view.

Perhaps the worst cause of sagittal imbalance is iatrogenic, when a patient is fixed into sagittal imbalance after a spinal fusion.  This is when patients really suffer.  First, we know that patients who are left with positive sagittal balance (as measured by PI-LL mismatch) after spinal fusion surgery have worse clinical outcomes.  Even more concerning, there’s also data to suggest that patients fixed into PI-LL mismatch are more likely to develop adjacent segment degeneration (ASD) after their fusion.  In a 2014 study by Rothenfluh et al, the authors reported a 10x (!) increase in the incidence of ASD when patients had PI-LL mismatch after their initial fusion.  It only makes sense that when a segment of spine is locked into an alignment that is pitched forward, the level above is going to more likely to continue to fall forward! (see figure 6)  (To tell you how much my understanding of this topic has evolved: one of the first articles I wrote on Spinal(con)Fusion, over 5 years ago now, was on ASD and no where in that article did I discuss positive sagittal balance.  I’m now convinced that fusing someone in poor sagittal alignment is the biggest contributor to increased risk of ASD after spinal fusion.)   Thus, one of the main goals of any spinal fusion surgery should be to restore lumbar lordosis to correct PI-LL mismatch.  This will maximize the chances of a good clinical outcome after surgery and may decrease the risk of ASD in the future.

NZAUSA Spinopelvic parameters and clinical outcomes pptx

Figure 6.  Sagittal MRI (side view) showing adjacent segment degeneration at L3/4 in a patient with previous fusion at L4/5.  Notice how L4 is fused into near straight alignment in relation to L5 (there should be 10-20 degrees of angulation there.)  It’s no surprise this patient fell forward above his flat fusion.  

This is really important stuff people.  Spinopelvic parameters should not be ignored.  I will take the time to do these measurements on every patient who undergoes a spinal fusion.  These days it’s easy too.  I can literally snap a picture of a standing X-ray with my iPhone or iPad and an app will basically do the spinopelvic measurements for me (see figure 7).  There’s just no excuse not to check.   If you’re considering a spinal fusion please be certain that your surgeon is taking your spinopelvic parameters into consideration.

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Figure 7.  Nuvaline Pro iPhone app used to measure postoperative spinopelvic parameters in patient who underwent a fusion at L5/S1. 

Thanks for reading! 

J. Alex Thomas, M.D.



  1. Ames CP, Smith JS, Scheer JK, Bess S, Bederman SS, Deviren V, et al.: Impact of spinopelvic alignment on decision making in deformity surgery in adults: A review. J Neurosurg Spine 16:547–64, 2012
  2. Glassman SD, Bridwell K, Dimar JR, Horton W, Berven S, Schwab F: The Impact of Positive Sagittal Balance in Adult Spinal Deformity. Spine (Phila Pa 1976) 30:2024–2029, 2005.
  3. Rothenfluh DA, Mueller DA: Pelvic incidence-lumbar lordosis mismatch predisposes to adjacent segment disease after lumbar spinal fusion. Eur Spine J 24:1251–8, 2014
  4. Schwab FJ, Blondel B, Bess S, Hostin R, Shaffrey CI, Smith JS, et al.: Radiographical Spinopelvic Parameters and Disability in the Setting of Adult Spinal Deformity. Spine (Phila Pa 1976) 38:E803–E812, 2013



What is a Spondylolisthesis?

At this point in our discussion of lumbar pathology we come to one very important topic: spondylolisthesis of the lumbar spine.  First described in the mid-1800s, spondylolisthesis literally means slipping bones.  In this painful condition there is forward slippage of one vertebral body over the body below.  If you want to get academic about it, the severity of this slippage is classified according to the Meyerding Scale:  

  • Grade I: <25% slip
  • Grade II: 25-50% slip
  • Grade III: 50-75% slip
  • Grade IV: 75-100% slip
  • Grade V: >100% slip (the vertebral body above is floating freely in front of the body below.) 

Some patients obsess about grading schemes like these but they’re really not that important for a typical patient with spondylolisthesis.  Grade I is by far the most common grade and if you’ve been told you have a spondylolisthesis this is probably what you have.  I will occasionally operate on a grade II and I can count on one hand the number of times I’ve operated on a grade III.  The higher grades really aren’t seen in adult patients.  They usually require a congenital defect in the bony anatomy of the spine and thus usually are already symptomatic in childhood (which is why, since I’m not a pediatric neurosurgeon, I don’t see these cases.)   A patient with spondylolisthesis may first present with only non-specific back pain.  As the patient ages and their spine degenerates the spondylolisthesis may become unstable and start to progress.  The slip eventually becomes severe enough that the patient develops back and leg pain and they come to see me.  This is the condition for which I most commonly book lumbar fusion procedures.  See figure 1. 

AH L4 5 spondy XLIF pptxIMG 0079

Figure 1: Sagittal MRI showing the forward slip of L4 on L5 in a typical spondylolisthesis.  The image on the right is a schematic of the same process (source: SpinePro III for iPad)

Spondylolisthesis is commonly asymptomatic (radiographic studies on normal volunteers tell us that nearly 10% of us are walking around with this condition yet have no pain.)  In my clinic, though, patients with spondylolisthesis have progressed to the point where they now have pain.  Patients with spondylolisthesis typically present with a combination of two types of pain: mechanical back pain from stress on the facet joints as well leg pain from compression of nerves.  As one vertebral body slips forward over the one below, this puts a tremendous amount of stress on the facet joints at the back of the spine (imagine how your knee would feel if was repeatedly bent outside of its normal range of motion.)  As they struggle to maintain the structural integrity of the spine they get stressed, become inflamed and arthritic and thus cause the back pain associated with spondylolisthesis.  Also, as the vertebral body slips forward, the nerves within are guillotined causing severe pain, numbness, tingling and even weakness.  The condition is especially debilitating because both nerves at the level of the slip can be compressed and injured.  First, there is compression of the exiting nerve because of foraminal stenosis caused by the slip and resultant foraminal height loss.  The traversing nerve (the nerve still within the spinal canal that will exit at the foramen below) also gets crushed in the lateral recess underneath the severely degenerated facet joint as one part of the joint slides forward in relation to the other (see figure 2).   For example a slip at L4/5 can cause compression of both the exiting L4 nerve and the traversing L5 nerve.  Both the back pain and leg pain associated with spondylolisthesis get worse as the patient stands up for even a short period of time.  When the patient is upright this loads the spine, aggravates the slippage and thus causes worsening pain. 

 Spondy foraminal stenosisAH L4 5 spondy XLIF pptx

Figure 2: On the left is a sagittal MRI showing severe foraminal stenosis associated with a spondylolisthesis at L4/5 (red arrow); contrast this with a normal foramen at the level above (green arrow).  On the right is an axial MRI of the same patient.  Note the severe lateral recess stenosis crushing the traversing nerve below the facet joint (red arrow).  Also seen is severe facet arthropathy (blue arrow) as indicated by a displaced joint and fluid within the joint.

There are several types of spondylolisthesis described in the textbooks.   The two most common types by far are degenerative spondylolisthesis (DS) and isthmic spondylolisthesis (IS).  DS, the most common form, occurs, as the name would suggest, as the spine degenerates over time.  As the intervertebral disc degenerates it no longer can properly absorb motion.  The facet joint tries to take up the slack but eventually, after enough time as a defacto shock absorber, becomes arthritic and incompetent (the same thing my wife says is happening to me.)  This incompetent facet joint can no longer maintain the structural integrity of the spine and the spine becomes unstable, allowing slippage to occur.  For reasons that aren’t entirely understood, most people with normal, age-related wear and tear of their spine do NOT develop DS.  A small number of unlucky folks are predisposed to this condition, however, perhaps because of the morphology of their facet joints or a genetic predisposition to accelerated disc and facet degeneration (degenerative conditions like this do run in families.)  DS more commonly occurs in older patients at the L4/5 level.

IS, a.k.a. lytic spondylolisthesis, occurs as a result of a fracture of the pars interarticularis, a small bridge of bone connecting the facet joint at one level to that of the level above (see figure 3).   This condition, also referred to as spondyloLYSIS, is thought to begin as an innocuous stress fracture in young athletes.   Only a small percentage of patients with a pars fracture will ever develop pain and an even smaller number will ever develop a slip.  Again, there seems to be a subset of patients who are predisposed to developing a slip in the setting of a pars fracture.  One theory is that patients with a high pelvic incidence (PI) are more likely to progress, mainly because of the force of gravity pulling the spine forward (PI is a measure of the morphology of one’s pelvis usually associated with a steep downward sloping sacrum.  More on this in a later post.) IS more commonly occurs in younger patients at the L5/S1 level (secondary to pars fractures at L5, see figure 4).  Again, as a patient, don’t get too bogged down in the details of different types of spondylolisthesis here.  If you have a spondylolisthesis it’s probably a degenerative one although it may be an isthmic one.  In the end, though, it doesn’t matter as the treatment is the same. 

Netter Lumbar Spine jpg Spondyolysis

Figure 3: Left, posterior view of lumbar spine at L4/5 level (red line indicates location of a fracture across the pars of L5.)  Image on right shows schematic of slip at L4/5 that has developed as a result of a pars fracture at L4 (yellow arrows.)

L5 S1 spondy Xray

Figure 4: Lateral standing Xray showing spondylolisthesis at L5/S1 associated with pars fracture at L5 (thin red lines).  This patient also has a high pelvic incidence with a steeply sloping sacrum (the top of which is indicated by blue line).  You can imagine how the force of gravity is contributing to the development of the slip in this patient by pulling L5 forward and downward (red arrow.)  

Even today there continues to be a great deal of controversy over the treatment of spondylolisthesis.  As I mentioned previously, spondylolisthesis and spondylolysis are commonly asymptomatic.  Interestingly, several large population studies have failed to show a strong correlation between the presence of spondyolysis/spondylolisthesis and pain, even when the slip progresses.  Patients can have this condition, a structural deformity of their spine, and be just fine. IT raises the question: should these patients even be treated at all?  Ultimately, though, once a spondylolisthesis progresses to the point where it’s now an unstable deformity of the spine, patients usually begin to seek treatment.  

When I first see a patient with a spondylolisthesis I’ll begin by offering conservative treatments such as physical therapy (PT) and epidural steroid injections.  PT will relieve pain in patients with spondylolisthesis, even when the slip is unstable and associated with stenosis.  I’ve been amazed at how some patients, with horrible looking MRIs, will do just fine with regular PT.  If nothing else, even if the patients do progress to surgery, I prefer that the patient has completed a course of PT because it strengthens them for recovery after surgery.  Only when PT and injections fail to provide lasting relief of pain do we consider surgical intervention.

As mentioned in a previous post, instability is one of the clearest indications for spinal fusion.  Indeed, patients with back pain and radiculopathy from an unstable spondylisthesis almost uniformly have excellent outcomes with the proper surgical intervention.  There is some controversy on the best surgical approach to address spondylolisthesis.  In my opinion, though, the best intervention is that which, using minimally-invasive approaches, best restores the structural alignment of the diseased level.  This intervention provides stability to relieve back pain and also, through indirect decompression, relieves nerve root compression and thus relieves leg pain.  More on the surgical treatment of spondylolisthesis in upcoming posts…


Thanks for reading!


J. Alex Thomas, M.D.

Lateral ALIF is a true single-position strategy for lumbar fusions

I’m back!  I realize there’s been quite a delay since my last post and for that I apologize.   For over a year now I’ve been busy helping to develop a new retractor system for lumbar spine surgery.  This retractor allows surgeons to perform an anterior lumbar interbody fusion (ALIF) in the lateral position, a procedure we (perhaps not so creatively) call lateral ALIF (see figure 1).  Why does that matter?  In my opinion, an ALIF is the most powerful way to fuse a segment of the lumbar spine and correct spinal deformity (hint: it’s because ALIF allows you to insert the largest spacers!). One drawback of ALIF, though, is that since it’s traditionally performed in the supine position (with the patient laying on his back), if the surgeon wishes to place posterior instrumentation he has to close the incision on the front of the patient and then reposition the patient prone to get access to the back of the spine.  This process of repositioning can add nearly an hour of time to the procedure and may also increase risk to the patient.  By keeping the patient on his side, in a single position, the surgeon can harness the power of ALIF and then immediately be ready to place posterior instrumentation, all without having to stop to reposition the patient (see figure 2).  Now, a so-called 360-degree lumbar fusion that used to take 3 hours to perform can now be done in an hour.  This is good for my OR throughput but it’s GREAT for you, the patient, who will avoid that extra time under anesthesia.  This could potentially be the one of the most important innovations in spine surgery in years. 


Figure 1: The new Lateral ALIF retractor during a recent ALIF at L5/S1.  The patient is in the lateral decubitus position with their left side up.  The patient’s head is at the left of the image.


Figure 2: Lateral ALIF at L5/S1.  This image gives you an idea of the massive increase in efficiency you get without having to reposition the patient.  Here, I’ve already placed wires for insertion of percutaneous pedicle screws while my physician assistant Jack Bagley continues to close the abdominal lateral ALIF incision.  Patient is right side up with head towards the left of the image. 

The ALIF, first described in the 1930s, is the original interbody fusion in which bone graft is inserted into the cleaned out intervertebral disc (IVD) space to promote fusion and correct spinal deformity (in modern ALIF the bone graft is carried in a spacer or cage).  Since then, many other techniques have been developed to place spacers into the disc space via a posterior approach.   These other –IF procedures, such as posterior lumbar interbody fusions (PLIFs) or transforaminal interbody fusions (TLIFs) represent early attempts at a single position strategy.  These procedures allow surgeons to perform the three standard steps of a spinal fusion: 1) neural decompression (laminectomy or discectomy), 2) interbody fusion, and 3) placement of posterior instrumentation with the patient in the prone position.  Thus, traditional spine surgeons may say “Well I’ve been doing ‘single-position’ lumbar fusions for years.”  Indeed, TLIF and PLIF are the most common way to perform lumbar fusions these days.  The problem with TLIF and PLIF, though, is that in order to place spacers from behind, one or more nerves have to be retracted out of the way to sneak the spacer into the disc space.  That means that for PLIF and TLIF the surgeon is forced to use very small spacers (see figure 3).  Because you’ve read recent Spinal (con)Fusion posts, though, you know that I believe in the power of large intervertebral spacers.  Bigger is better and thus ALIF is a much more powerful technique for spinal fusion than PLIF or TLIF.  Now, by doing the ALIF in the lateral position I can have concurrent access to the back of the patient to perform a decompression and place pedicle screws without “flipping” the patient. 

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Figure 3: side-by-side comparison of various intervertebral spacers.  Notice how much larger the XLIF/ALIF spacer  is versus the much smaller TLIF or PLIF spacers. Size matters!

As you can probably tell, I’m very excited about this new retractor and surgical technique (yes, I’m biased.)  In future posts we’ll talk more about the details of a lateral ALIF procedure.  You’ll also see how well lateral ALIF at L5/S1 compliments the extreme lateral interbody fusion (XLIF) at L4/5 and above.  Lots more about single-position lateral surgery to come! 

Thanks for reading!

J. Alex Thomas, M.D.

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!


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.

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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.

Implants in Spinal Surgery Part III: Intervertebral Spacers

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.   

Postoperative Deformity of the Cervical Spine Clinical Gate 

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).

Harms cage Google Search

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.   

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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).  

 ALIF spacer bone graft Google Search

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. 


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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:

2) Williams et al, 2005, CT Evaluation of Lumbar Interbody Fusion: Current Concepts, ANJR, 20: 2057-2066, September 2005.









Implants in Spinal Surgery Part II: pedicle screws and other (very rarely used) forms of posterior spinal instrumentation

In our last post we discussed how various types of spinal implants help stabilize the spine to promote a more robust bony fusion.  Recall that the main goal of a spinal fusion procedure is to promote bone growth which in turn will stabilize a painful unstable or deformed segment of the spine.  Historically, non-instrumented in-situ fusions had very high rates of non-union in which the spine didn’t fuse despite weeks of bedrest and bracing.  This may have required subsequent revision surgeries and often left the patient severely disabled.

In order to mitigate the problem of non-union, surgeons developed forms of posterior instrumentation (instrumentation applied to the back of the spine) to help stabilize the spine.  These implants also provided surgeons a more powerful way to restore alignment to the spine to correct spinal deformity.  The most primitive form of posterior spinal instrumentation, first described in the late 19th century, was posterior spinal wiring.  Here, various parts of the spine (usually the spinous process or lamina) were wired together for immobilization during spinal fusion.  While useful in the cervical spine (see figure 1), this technique never proved to be effective in the thoracic and lumbar spine, where larger forces prevail, and thus has largely been abandoned today. 


Figure 1: Cahill technique of posterior spinous wiring in the cervical spine.  Source: Omeis et al

In the 1950s Paul Harrington began performing spinal fusions using long rods anchored to the spine with hooks under the lamina (see figure 2).  The rod construct was periodically lengthened to slowly straighten the deformed spines of scoliosis patients. Harrington rod systems were state of the art for decades and I’ll still see patients in clinic today with these long rods in their spine.  Unfortunately, with only a few points of fixation anchoring the long, straight rod in place, these constructs were prone to failure and also predisposed patients to a painful “flat back” deformity.  (When a surgeon fuses a spinal segment without being mindful of its normal degree of curvature he may inadvertently cause more harm by creating spinal deformity.  More on this very important topic in later posts.)  Moving away from Harrington rods, surgeons began to develop new forms of segmental instrumentation in which each segment of the fused section of the spine was anchored with instrumentation (versus the long Harrington rod which spanned multiple non-instrumented segments.)  This segmental instrumentation vastly increased the strength and stability of the fusion construct and therefore further decreased the rate of non-union.  Early forms of segmental instrumentation include variations of trans-facet screws described by King in 1944 and Boucher in 1959 (the latter incorporated part of the pedicle in the screw trajectory and thus is considered by some to be the first pedicle screw.)  In 1970 Roy Camille described the precursor to today’s pedicle screw systems.  In his system he attached screws inserted via the pedicle to a multi-holed plate which would span across two spinal segments from screw to screw.  The screw-plate concept was expanded upon in the U.S. by Dr. Arthur Steffee in the early 1980s with his Steffee Plate/VSP stainless steel pedicle screw system (see figure 3).  Steffee’s company AcroMed was the subject of a 1993 ABC 20/20 expose which profiled several patients who were left disabled after Steffee’s VSP pedicle screws broke (all pedicle screws will eventually break, by the way, if the spinal segment doesn’t fuse properly.)  This prompted hundreds of pedicle screw-related lawsuits around the country in the mid 1990s (part of the issue was that the FDA never cleared the VSP screw for use specifically in the spine).  Ultimately most of the pedicle screw litigation was thrown out.  Today pedicle screws are a mainstay of treatment of a variety of thoracic and lumbar spinal pathology. 


Figure 2: AP (A) and lateral (B) postoperative Xrays demonstrating a single Harrington rod used in correction of a thoracic scoliosis.  Note in image B how flat the fused segment is. 

 Mendeley Desktop

Figure 3: Steffee plate and VSP pedicle screw system in L4-S1 fusion.  Source: Kabins et al.

Today’s pedicle screws are generally made of titanium and have polyaxial heads in which a rod is seated and locked in place using a locking cap.  The rod is used in favor of Camille’s and Steffee’s plates as it allows for easier insertion as well as contouring to correct spinal deformity (see figure 4). Screws vary in size depending on the size of the patients and the part of the spine being instrumented (i.e. smaller screws in the thoracic spine and larger screws for the lumbar spine.)  Typically in lumbar spine fusions I will use screws that are 6.5mm in diameter and 45mm long with a 4.5mm diameter rod.   Once the screws are inserted various attachments can be used to manipulate the screws to rotate, compress or distract across spinal segments in order to correct spinal deformity prior to fusion. 


Figure 4: Modern pedicle screw with polyaxial head and rod locked in place. Source: Zimmer Biomet.

As you can probably guess, a “pedicle” screw traverses part of the vertebral body called the pedicle.  This bridge of bone connects the anterior elements of the spine (i.e. the vertebral body) with the posterior elements (i.e. lamina, facets, spinous process, see figure 5).  In order to properly insert a screw into the pedicle the surgeon must access a starting point at the junction of the transvers process and the facet complex, several centimeters off of midline.  In traditional open spinal surgery via midline incisions the surgeon has to do quite a bit of destructive, bloody dissection to get a wide enough exposure to access this starting point of the pedicle.  Often, in order to gain enough laxity in the tissue to get out to the starting point, the surgeon must also expose the level above and below the level that is being fused.  This “collateral damage” of healthy levels in open spinal surgery is what sets patients up for adjacent segement degeneration and other problems later in life. 

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Figure 5: Axial (left) and lateral (right) views of lumbar spine demonstrating pedicle (outlined in red) bridging the anterior and posterior elements.

Pedicle Screw Incisions

Figure 6: Small stab incisions used to placed 4 percutaneous pedicle screws for an L4/5 spinal fusion.  The red line indicates the length of incision that would have been needed to place the same number of screws using traditional open techniques. 

In order to avoid the increased blood loss and tissue destruction of open spinal surgery, I insert pedicle screws percutaneously with tiny, minimally-invasive incisions off midline (see figure 6.) I’ll first identify the starting point of the pedicle using a fluoroscope (like an xray machine) and then will hammer in a large needle into the pedicle via the starting point.  I can identify the correct starting point not only using imaging but also with the tactile feedback of the hard bone of the pedicle starting point.  Once I’m happy with my position in the pedicle I’ll insert a long, rigid wire called a K-wire into the pedicle and will remove the needle.  The pedicle is tapped to make a pilot hole and the screw is then inserted over the wire (see figure 7.)  The nerve that exits the spine passes just below the pedicle so one potential complication of pedicle screw insertion is nerve injury resulting from improper positioning of the screw within the pedicle. In order to avoid this complication I always use electromyographic (EMG) monitoring in which the insertion needle and tap are stimulated with low-voltage electrical current.  If the screw trajectory is too close to a nerve the current will stimulate the nerve, the corresponding muscles in the leg will twitch and I’ll be alerted to the problem so that I can plan a new trajectory.  After the screws are inserted a rod is passed through the heads of the screw and locked in place.  While some surgeons will try to get away with placing only unilateral screws, I always place bilateral pedicle screws (i.e. on both sides of the spine), as this is the gold standard for maximum spinal stabilization (see figure 8.)  Finally, recall that the screws just serve as an internal brace to allow the bony fusion to occur.  Theoretically we could remove the screws in a year after the fusion has healed but we almost never do this.

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Figure 7: 4 K-wires placed prior to placement of percutaneous pedicle screws for lumbar fusion (red wire).  Note that wires and thus screws can be placed with the patient in the lateral position (in this case patient’s right side is up.) 

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Figure 8: Axial CT image (left) and schematic image (right) demonstrating bilateral pedicle screws traversing the lumbar pedicles. Source for schematic: DePuy Synthes

There are other types of posterior instrumentation that are used in the lumbar spine.  These include interspinous clamps as well as facet screws.  I don’t believe that these implants provide the same amount of stability as a bilateral pedicle screw construct and therefore I never use them.  Not discussed here are specialized screws used in posterior cervical spine fusions called lateral mass screws (named for the part of the cervical vertebral body that they enter.)  These are very similar to thoracic and lumbar pedicle screws (titanium, polyaxial heads, connected by rods passed through the heads) but are much smaller with a typical diameter of 3.5mm and a length of 12-14mm.  

In summary, pedicle screws act as an internal brace to immobilize the spine so that a more robust bony fusion may occur. These screws can be safely inserted into the spine using minimally-invasive percutaneous techniques.

Thanks for reading!

J. Alex Thomas, M.D.


  1. Hasler CC: A brief overview of 100 years of history of surgical treatment for adolescent idiopathic scoliosis. J Child Orthop 7:57–62, 2013.
  2. Omeis I, DeMattia J a, Hillard VH, Murali R, Das K: History of instrumentation for stabilization of the subaxial cervical spine. Neurosurg Focus 16:E10, 2004.
  3. Kabins MB, Weinstein JN: The History of Vertebral Screw and Pedicle Screw Fixation. Iowa Orthop J 11:127–136, 1991.