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. 

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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. 

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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. 

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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. 

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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.

Sources:

  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.

Implants in Spinal Fusion, Part I: In-situ Fusions Rarely Fused

As I was writing the previous few posts I realized that I was relying on terms such as pedicle screw and intervertebral spacer to begin the explain techniques used to achieve spinal fusion.  Before we get further into our discussion on these techniques I think it would benefit you, devoted Spinal (con)Fusion reader, if I spent a few posts discussing the various implants used during spinal fusion procedures.   It seems like in my clinic everyone knows someone who didn’t do well after getting “a whole bunch of screws and rods in their back”.  Granted, on their own these sound like medieval torture devices that no sane person would want implanted into their spine.  Hopefully by shining some light on the screws, rods and various other spinal implants used during fusion procedures, I can put prospective patients’ minds at ease if they’re considering a spinal fusion.

In order to appreciate the benefits of today’s spinal instrumentation, you must first understand how terribly inadequate early non-instrumented spinal fusions were.  Recall that we discussed that the main goal of a spinal fusion procedure is to promote bone growth across one or more spinal motion segments.  This bone growth immobilizes what is felt to be an unstable, and thus painful, part of the spine.   While spinal fusions have been done since the early 20th century, the only strategy that early spine surgeons could employ to achieve a bony fusion was to harvest autograft (bone harvested from a site within the patient such as the spinal lamina or the iliac crest) and lay it down over an exposed part of the spine that they wished to fuse.  This primitive in-situ fusion technique, first described by Albee and Hibbs in the early 1900s, was problematic for two reasons.  First, there was no good way to correct spinal deformity while promoting a bony fusion.  Thus, after a long, morbid surgery, patients were often fused with painfully deformed spines and no better than they were prior to surgery.  Second, in order for any bone to fuse together the adjacent pieces of bone have to be immobilized (think of a cast on a broken arm.)  In an in-situ fusion, with the bone graft simply laying on top of a segment of spine, the only way to immobilize a patient’s spine to promote bone growth was to keep them on bedrest, often in full body braces or casts, FOR MONTHS.  UGH!  What’s worse is that because of these inadequate forms of immobilization, in half the cases the new bone wouldn’t grow, the spinal segment wouldn’t fuse and the patient would be left with a painful condition called a non-union, or failed fusion, often requiring subsequent revision surgeries.  It’s no surprise, then, why many at the time considered early spinal fusion procedures to be painfully ineffective.     

Beginning in the mid-20th century, various forms of spinal instrumentation were developed in order to help mitigate the above limitations of early in-situ fusions.  First, spinal implants provide the necessary internal bracing that immobilizes the diseased motion segment so that robust bone growth can occur.  No more full-body casts!   Also, spinal implants, particularly the intervertebral spacers inserted into the disc space at the front of the spine, allow for correction of spinal deformity.  This deformity correction, equally important as correction of instability, restores the spine to its normal form and alignment prior to it being permanently immobilized by the new bone growth of a spinal fusion.  In short, spinal implants create the optimal conditions for new bone to grow to achieve a spinal fusion and thus correct painful spinal instability and deformity. 

In our next post I’ll dive right into the world of spinal implants with a discussion on pedicle screws and other forms of posterior instrumentation. 

Thanks for reading!

J. Alex Thomas, M.D.

 

True spinal instability is a clear indication for spinal fusion

As we illustrated in our last post there is a wide spectrum of indications for lumbar spinal fusion.   As you move along this spectrum from unstable to more stable pathology the odds of a successful outcome decrease.  At the far end of the spectrum of diagnoses, the end at which there is a lesser chance of a favorable outcome after fusion, is degenerative disc disease (DDD) and spondylosis (without instability) causing back pain.  In my opinion this is softest indication for spinal fusion.  I’m not saying that you should never perform a spinal fusion on a patient with only DDD, the patient just has to be properly vetted and they must understand that a good outcome isn’t guaranteed in these cases.  On the opposite end of the spectrum is acute spinal instability caused by trauma or some other acutely destructive process such as tumor or infection.  This is the clearest indication for a spinal fusion.  NOTE: we’ve already discussed cervical spinal fusion (ACDF) here and here so this discussion will pertain primarily to the lumbar spine.

Classically, spinal stability is defined as the spine’s ability, under normal physiological loads (“normal” obviously varies widely depending on whether you’re a bank clerk or a mixed martial arts fighter), to a) protect the neural elements (i.e. nerve roots and spinal cord), and b) avoid painful deformity.  Sounds complicated right?  It may be easier to think about what happens when the spine becomes unstable: a) it may not be able to maintain proper alignment and thus may become deformed which causes severe pain; and b) it may not be able to properly protect the spinal cord within which could cause paralysis.  So in a nutshell: a stable spine is one that is protecting you against pain and/or paralysis. 

The concept of traumatic spinal fractures is a vast one that I won’t get into too much here.  Generally, though, fractures are classified as stable or unstable (hopefully you’re starting to pick up on a theme here.)  There are many complicated grading schemes that allow spine surgeons to look at a fracture on imaging and determine if it’s unstable or not.  One classic scheme is that of Denis which divides the spine into three columns.  Stable fractures typically only involve one column of the spine. Examples of stable fractures include fractures of the spinous process (a so-called clay shoveler’s fracture, see Figure 2), compression fractures and transverse process fractures.  Stable fractures may be painful from the local trauma of the injury but they do not cause painful deformity nor do they threaten the spinal cord or nerve roots.  Thus these types of fractures may be treated conservatively such as with bracing. 

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Figure 1: Illustration from Denis’ 1983 paper discussing his three spinal columns and their involvement in traumatic injuries.

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Figure 2: Fracture of the C6 spinous process (clay shoveler’s fracture).  Source: https://radiopaedia.org/images/3175670

Generally speaking if two or more of Denis’ columns are involved in a fracture it is considered unstable (again let me reiterate that Denis’ model is quite simplistic and analyzing a fracture isn’t always as easy as looking at the spine in only 3 columns.)  When a fracture is determined to be unstable a spinal fusion may be indicated to restore stability.  If an unstable fracture is left to heal without surgery it may heal poorly resulting in a painful deformity. Worse, if a patient with an unstable fracture is allowed to get up out of bed and loads their spine the fracture may shift resulting in injury to the spinal cord and paralysis. 

Trauma isn’t the only cause of acute spinal instability.  Indeed, aggressive tumors or infections can destroy the integrity of the spine thereby causing painful spinal deformity and perhaps paralysis.  These lesions are treated in a similar manner as acute fractures depending on which part of the spinal column has been damaged. The case presentation below describes a case I had a few years ago of an elderly gentleman with severe damage to his spine caused by a staph infection. 

Generally speaking when deciding which type of spinal fusion to perform for acute spinal instability, I’ll go to where the problem is:  if the pathology primarily involves the vertebral body in front of the spine, for example, I’ll do a corpectomy to remove the fractured vertebral body.  Once the body is removed I’ll reconstruct the spine with a spacer inserted where the damaged vertebral body was, and a combination of plating or screws to provide extra stability (we’ll talk about these devices in more detail in future posts.)  The main goal of all of that surgery is to promote new bone growth across the damaged segment of the spine.  It’s this new bone growth that restores spinal stability.  

CASE PRESENTATION:

The patient is a 75yo male with methicillin-resistant staph aureus (MRSA) bacteremia (in his bloodstream) who presents with worsening mid-back pain.  Imaging reveals T11-12 discitis.  (Discitis is an infection of the intervertebral disc space that is probably the most painful condition that I see.  You can usually make the diagnosis by very gently bumping the patient’s bed when you approach the bedside; if the patient screams out in pain it’s probably discitis.  That’s how bad it is.)  The medicine doctors tried a long course of antibiotics but unfortunately his pain didn’t improve.  Repeat imaging revealed that the infection hadn’t been cleared and in fact had caused further destruction of the T11 and T12 vertebral bodies (see Figure 3.) This destruction resulted in spinal instability and kyphosis (a painful deformity in which the spine falls forward.)  

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Figure 3: CT scan illustrating T11-12 discitis resulting in severe bony destruction (red arrow) and resultant kyphotic deformity (blue arrow indicates top of spine falling forward). 

When I met this patient he looked like he had given up and wanted to die.  He’d been bedbound from his infection for weeks and now was quite debilitated.  He agreed to undergo surgery and underwent a T11 and T12 corpectomy (via a lateral approach through the chest and behind the lung) followed by reconstruction of the spine with an expandable cage and percutaneous pedicle screws (see Figure 4.)  By one month post-op the patient reported no pain and was walking without assistance.  The last time I saw him about a year after his surgery he was living a normal life at home with his family.  He looked like he’d been given a new chance at life. 

Post op T11 12 corpectomy

Figure 4: Postoperative AP (left) and lateral (right) X-rays with expandable corpectomy spacer at T11-12 (red arrow) and percutaneous pedicle screws from T9-L2 (blue arrows).

I think I’ll spend the next post or two talking about the various forms of spinal implants that we use to achieve a spinal fusion. I had planned to do this later but I think that by presenting it first it will help you better understand the various spinal fusion procedures discussed in later posts. 

 Thanks for reading!

 J. Alex Thomas, M.D.

Sources 

Denis F: The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine (Phila Pa 1976) 8:817–31, 1983.

What is Adjacent Segment Degeneration?

At least once a week a patient will say to me: “I’m not getting spinal surgery because I heard that once you have spinal surgery your spine is never the same and you’ll only need more spinal surgery!”  Well, to some extent there is some truth to this statement.  Whenever a patient undergoes spinal surgery, a well-known long-term side effect is that the level above or below the surgery can degenerate.  This is called adjacent segment degeneration or ASD.  

Why does ASD happen?  ASD can occur whenever the normal anatomy of the spine is disrupted and as a result, a segment of the spine has to handle more stress than it’s used to. This can happen after even the most minor spinal procedure but more commonly happens after fusions. (see figure 1)  Any length of spinal fusion can lead to ASD.  However, there has been some evidence to suggest that the more levels fused, the higher the risk of ASD.  This is because the longer fused segment acts as a longer lever arm and causes more stress on the disc and facet joints above or below the segment fused.  Of course, sometimes long fusion segments are mandatory (as in deformity or trauma surgery) but whenever possible, the number of levels fused should be minimized. 

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Figure 1: Image on left shows ASD at L4/5 after an open fusion at L5/S1. The L4/5 disc is now so degenerated that it allows the L4 body to slip forward in relation to L5 (a condition known as spondylolisthesis).  The image on the right is of a patient with severe ASD at L2/3 several years after she underwent an open L3-5 laminectomy.   

 There is some controversy as to what actually causes ASD.  Here’s what we know for certain: ASD is a common long-term complication of spinal fusions. If you look at the rates of ASD across the literature, the rate of symptomatic ASD (i.e. that requiring additional surgery) after anterior cervical discectomy and fusion, or ACDF, is anywhere from 9-25% (Yang et al, 2012).  In the lumbar spine, the rate of symptomatic ASD after fusion is as high as 30% (Cheh et al, 2007). Clearly any patient who undergoes a spinal fusion has to accept the risk that at some point later in their life they may need more spinal surgery.  

ASD (and this is where it gets controversial) may also be seen more commonly after traditional, open spinal surgery vs. after minimally-invasive surgery.  As you have seen in some of my previous posts, open spinal surgery can be quite destructive.  These procedures utilize long midline incisions that strip the supporting muscles and ligaments off of the spine.  I equate these muscles and ligaments to the cables of a suspension bridge: if you disrupt these structures at one level you affect the functional stability at multiple levels.  Minimally-invasive procedures in spinal surgery are still fairly new and only recently has long-term data on these procedures become available.  One recent study (Park et al, 2011) followed patients who underwent minimally-invasive lumbar fusions for an average of 36 months and only 2 out of 66 (3%) patients developed ASD.  Compare that to the 30% rate of ASD for open fusions.  Recall that minimally-invasive procedures spare the normal supporting structures of the spine and theoretically prevent the “collateral damage” that leads the ASD.  In my opinion the Park study is just the first of many studies that will prove that minimally-invasive procedures have a much lower rate of ASD when compared to open spinal procedures.