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.

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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.

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