Can We Improve Pedicle Screw Stimulation in Spine Surgery?

Evaluation of pedicle screw placement with triggered EMG is probably the most commonly-used test in all of neuromonitoring. Unfortunately, due to lack of standardization, this robust test fails to detect medial pedicle cortex breach more often than it should. As a community of neurophysiologists, we can standardize our methods and improve patient outcomes but we need to work together. This post will take a look at common misconceptions, methodological concerns and solutions for performing the most accurate tests of pedicle screw placement.

Doesn’t everyone test screws in the same way…?

Even though the ASNM has a position paper that covers the subject in part (Leppanen, 2004), there is a serious problem with lack of standardization in our field, and I guess it’s some sort of secret. I mean, how often do you hear people talk about lack of standardization in pedicle screw testing? Almost never, right? Everyone knows that there are no “standards” for motor evoked potentials…you see it everywhere in the literature. But, until recently, lack of standardization in pedicle screw testing was a silent problem. Just this year, an awesome book chapter was published to address this very problem (Schwartz, Bhalodia and Vaccaro, 2014). The authors report that stimulation parameters, screw types, anesthesia, and other factors vary widely between labs. Adding to this problem, most groups fail to fully report their methods when they publish their results. How are we supposed to replicate people’s results if we don’t have all the methods? Jeez. Anyway, most of what I’ve written below, I thought it was all common knowledge, but I’ve been around this field long enough to know that’s simply not true. If there were a such thing as “common knowledge” in our field, I probably wouldn’t write this blog. Anyway, I hope that what I’ve written below is informative and I hope it helps to improve your outcomes. The text below assumes that you already know at least a little about pedicle screw testing.

But the surgeon made me do it that way…

Before we get into this, let’s talk about the elephant in the room. The surgeon can be the most difficult obstacle that you’ll confront in your endeavor to perform an accurate test of pedicle screw placement. Obviously, this isn’t always the case, but we know that there are some surgeons that want things done their way without question. So, even though he/she invited you to perform the test, the surgeon may refuse to perform it properly, even forcing you to use substandard methods. For example, the surgeon may say, “Just put it (intensity) on 8 mA and don’t say word unless you see a response.” In the sections that follow, you’ll see why this approach is so bad, but let’s tackle this problem first.

So, why would the surgeon invite you to the OR just to perform a test incorrectly? Lots of reasons…obviously there is a medico-legal reason for performing the test, but maybe they don’t trust the test, maybe they don’t care, or maybe they don’t know any better. I certainly can’t presume to know, but whatever the reason, what are you supposed to do? If you readily comply, then you are committing malpractice. If the case goes to court, you will be found at fault for not telling the surgeon that his/her methods are flawed. After all, you are the expert in neurophysiology, not the surgeon. However, if you argue with the surgeon, then he/she is likely to yell at you and make you feel belittled in front of the whole OR. Your day will be hell. I know it’s frustrating.

Here’s my take on the subject: I’m not there to please the surgeon. Rather, I’m there to advocate for the patient. I’m a neurophysiologist, I monitor the nervous system, I’m very good at what I do, and no one is going to compromise that. I don’t argue with the surgeon. Instead, I initiate an educated conversation with the surgeon, and I do it before the procedure (not at the time of testing). In my mind, we are establishing a plan for patient care. If he/she wants to turn it into an argument, fine. I don’t yell..I just communicate my points. If the surgeon still won’t comply with my approach, then I do what the surgeon asks; HOWEVER, I document everything that I said in the medical record, and I document what the surgeon said. If the case goes to court, then it will be shown that I did my due diligence.

Now that we’ve cleared the air and acknowledged our biggest obstacle, let’s talk about some common misconceptions in pedicle screw testing.

Common Myths:

Myth #1: The EMG test will uncover any pedicle breach.

The test is designed to evaluate the medial pedicle. Surgeons sometimes come to me confused. They say, “The screw was lateralized on CT, but the EMG test was fine, right?” I always have to remind them that the test is not designed to detect lateral breach, only medial breach.

Myth #2: No CMAP up to X mA means the screw placement is good.

Absence of CMAP means absolutely nothing at all. You must increase the intensity of the stimulation until you get a CMAP on every single screw. It not only tells you that the stim/rec system is working, it tells you something important about the nerve…that it’s intact. See below.

Myth #3: All nerve roots will depolarize with about 3.0 mA of current.

This is sooooo important. Keep in mind that the pedicle screw EMG test is based on the premise that the nerve roots are healthy. It usually takes less than 3.0 mA of current to evoke a CMAP when you stimulate a functionally intact nerve root; however, chronically-compressed nerve roots may have much higher stimulation thresholds (Holland et al, 1998). Add metabolic disorders such as diabetes to the mix, and the thresholds can get higher (Toleikis et al., 2000). Under these circumstances, direct electrical stimulation of nerve roots have CMAP thresholds reported between 18-27 mA (ibid, also Schwartz & Devlin, 2007; Schwartz et al., 2003). I have personally observed this, too, on at least two occasions where I delivered over 20 mA to an exposed nerve root before it depolarized and produced a CMAP. This should make you think about the threshold criteria that you use for breach alert. We’ll come back to this.

Myth #4: You can put your return electrode anywhere on the patient.

A poorly-placed return electrode can cause excessive stimulation artifact and increase the possibility of current shunting. When stimulating lumbar pedicle screws, your return electrode should be in the wound, contralateral and caudal to the screw you are testing. When stimulating thoracic screws, your return electrode should be in the wound and directly contralateral to the screw you are testing. That’s right…you have to move the return electrode for every single screw that you test.

Myth #5: You can put your signal ground anywhere on the patient.

If you put your signal ground in the same muscle from which you are recording, your signals will be dampened or eliminated. The signal ground acts as a common mode reference for your recording electrodes. So, it should see the same “noise” as your recording electrodes, but not the same “signal”. If you are recording CMAPs in response to triggered EMG (or MEPs for that matter), you should put your signal ground in a muscle from which you are not recording. This ensures that the noise is rejected, but the signal is maintained.

Myth #6: The surgeon knows exactly what to do, and is actually doing it.

Don’t make this assumption. Keep your eye on the surgeon. Make sure the wound is suctioned out, that the surgeon is stimulating the shaft of the screw, and that the return electrode is properly placed.

Myth #7: You don’t need to do a train-of-four test.

You should do a TOF test whenever you are testing EMG or MEPs. At its heart, the TOF test gives you documentation that there is sufficient conduction across the neuromuscular junction for reliable monitoring of EMG and MEP. In other words, it tells you whether or not your other test results are even valid. You should know that clearance of neuromuscular blockade is not temporally equal around the quadrants of the body. Schwartz et al (2014) showed that the feet can lag behind the hand by as much as 14 minutes. So, you should do your TOF test from the limbs of importance (legs for thoraco-lumbar cases, hands for cervical cases). It is recommended that you should perform all pedicle screw EMG tests when your TOF ratio is > 70%. Just divide the amplitude of twitch 4 by the amplitude of twitch 1, and then multiply by 100.

Myth #8: Triggered EMG is good enough for testing medial breach.

I once heard a story about a medial breach that was missed with tEMG. The screw tested in the 60 mA range (much higher than all the other screws which tested in the 40s). On CT, the screw was medialized and the patient woke up with severe pain and weakness. How did it test so high? Easy…the screw went through the nerve. So, let me pose the question – How else could we have detected that breach, that iatrogenic injury? Say it with me…. MULTI… MODALITY… MONITORING. The nay-sayers will disagree, but I’m telling you, with combined use of MEPs, SSEP and EMG, the nerve root compromise would have been detected. 

What stimulation parameters should I use…?

Frequency:

Many neurophysiologists choose a stimulation frequency that will yield rapid responses for the benefit of the surgeon. For legal reasons, you must be able to view and store every single trace, regardless of whether or not a CMAP is present. Also, you need to be able to show which stimulating intensities evoked CMAPs and which did not. So, if you are stimulating at 5 Hz, you should be saving 5 traces/second and the current intensity associated with each trace. Not all commercially-available systems do this, which sucks, but we’re working to fix it. Also, you don’t want to stimulate as such a high frequency that the nerve is forced into absolute refractory and won’t depolarize. Fortunately, this is not so much of a concern. To be safe, I personally I use 3.1 Hz. It gives the nerve plenty of time to recover, it is plenty fast for the surgeons, and I can record the results of every stimulus pulse delivered.

Pulse Duration:

Also called Pulse Width. No one pays much attention to this parameter, but it is soooo important. A short pulse duration (e.g., 50 µsec) requires much higher current for nerve depolarization than a long duration (e.g., 200 µsec). So, for example, if a you stimulate a pedicle screw with a 200 µsec duration stimulus, maybe you get a CMAP with 10 mA intensity. Change the duration t0 100 µsec, and it could take 15 mA to evoke a CMAP. So, it’s not just the intensity that’s important – you have to understand all stimulus parameters. Make sense? This is really important to understand no matter what you are stimulating. If you want to learn more about the basic math behind this principle, go here. For direct nerve stimulation, I recommend that you never go higher than 50-100 µsec. For pedicle screw stimulation, I recommend a pulse width of exactly 200 µsec.

Intensity:

Start at 0.00 mA and increase the intensity until you get a CMAP. Keep in mind all of the information that I mentioned under the Misconceptions section. Also, remember that healthy bone absorbs much more current than diseased bone. I have seen on many, many occasions where all screws tested above 40 or 50 mA. Increase the intensity until you get a CMAP. If you don’t get something by 100 mA (max output on most machines), something is technically wrong or the screw is lateralized. You should be able to safely and accurately stimulate a pedicle screw and obtain results in 10 seconds per screw.

Criteria for Alert:

As I noted above, you need to look at the whole patient presentation before you interpret the results of pedicle screw stimulation tests. Remember that chronic nerve compression and diabetes can significantly elevate the threshold. Also, bone density can alter the threshold. And don’t forget to check your train-of-four! Finally, look at the results of all screw tests. Did one particular screw test at a much different threshold than all of the others? This is an outlier and should be further investigated. In some cases, you may need to directly stimulate the nerve root in order to determine the depolarization threshold and then extrapolate the criterion for determining medial breach of the pedicle.

Using a pulse width of 200 µsec, Schwartz et al (2014) make the following interpretations:

CMAP Threshold and Interpretation:

< 5 mA

  • High probability of medial pedicle wall violation.
  • Frank screw penetration in proximity to dura and adjacent nerve root

5-6 mA

  • Moderate probability of medial pedicle wall violation.
  • Screw penetration may not be sufficiently deep to place dura/nerve root “at-risk”. Compare CMAP thresholds to the contralateral side and ipsilateral adjacent levels to determine outliers.

6-7 mA

  • Low probability of medial pedicle wall violation.
  • On average, safe to leave in place. Compare CMAP thresholds to the contralateral side and ipsilateral adjacent levels to determine outliers.

≥ 7 mA

  • Extremely low probability of medial pedicle wall violation.
  • On average, suggests intact cortical bone. Best practice is to compare CMAP thresholds to the contralateral side and ipsilateral adjacent levels to determine outliers.

Conclusion:

Questions, comments, thoughts, suggestions, opinions, etc…..? Feel free to leave a comment below!!

References:

  1. Devlin VJ, Schwartz DM. Intraoperative neurophysiologic monitoring during spinal surgery. J Am Acad Orthop Surg. 2007 Sep;15(9):549-60. Review.
  2. Holland NR, Lukaczyk TA, Riley LH 3rd, Kostuik JP. Higher electrical stimulus intensities are required to activate chronically compressed nerve roots. Implications for intraoperative electromyographic pedicle screw testing. Spine (Phila Pa 1976). 1998 Jan 15;23(2):224-7.
  3. Leppanen RE. Intraoperative monitoring of segmental spinal nerve root function with free-run and electrically-triggered electromyography and spinal cord function with reflexes and F-responses. A position statement by the American Society of Neurophysiological Monitoring. J Clin Monit Comput. 2005 Dec;19(6):437-61.
  4. Schwartz DM, Bhalodia VM & Vaccaro AR (2014). Neurophysiologic detection of medial pedicle wall violation in the lumbar and thoracic spine. In CM Loftus, J Biller & EM Baron (Eds.), Intraoperative Neuromonitoring (pp. 425-437). New York, NY: McGraw-Hill.
  5. Schwartz DM, Wierzbowski L, Fan D. Sestokas A. (2003). Surgical Neurophysiologic Monitoring. In. A Vaccaro, R Betz & S Zeidman (Eds.), Principles and Practices of Spine Surgery (pp. 115-126). Philadelphia, PA: Mosby.
  6. Toleikis, JR, Skelly, JP, Carlvin,AO, Toleikis,SC, Bernard, TN, Burkus, JK, Burr, ME, Dorchak, JD, Goldman, MS and Walsh, TR (2000) The usefulness of electrical stimulation for assessing pedicle screw placements. J. Spinal Disord., 13: 283–289.

Note:

The use of automated or semi-automated EMG monitoring devices is dangerous and not recommended. Electrical stimulation of nervous system tissue and recording of EMG activity should be performed by a professional neurophysiologist.

33 thoughts on “Can We Improve Pedicle Screw Stimulation in Spine Surgery?

  • Reply sergio December 8, 2014 at 08:54

    Excellent Richard, very very clear and useful for the daily practice. Thank you.

  • Reply Andrew Warrington December 8, 2014 at 17:30

    Nice article, I would like to hear your thoughts on the increase popularity of navigated screw placement and the noted decrease in the need for stEMG. I have personally seen a decrease in screw testing were Oarm/Nav has been used. If imaging confirms placement does that negate testing? I wonder how many navigated screws looked good on Oarm scan but stEMG showed potential breach thresholds (hairline fractures?). I feel this rudementary test is viewed exactly as rudementary by surgeons who assume technology has trumped it.

    • Reply Richard Vogel December 9, 2014 at 19:04

      Hi Andrew, thanks for your thoughtful comment. You know, one thing that I’ve found in recent time is that most neuro/ortho/ENT surgeons in the US are fairly mistrusting and dismissive of neuromonitoring. I was really shocked to see how often major spine and brain surgeries go without neuromonitoring, and at some of the top hospitals in the country. When queried, some surgeons say they don’t “believe” in neuromonitoring, and some even refer to it as voodoo. Other surgeons use it rather sparingly (only EMG in cervical spine cases being fairly common). I still wrestle with these observations. Indeed, until now, I never really considered that many surgeons in my geographic region “over-monitor” in the grand scheme of things across the country. But, with the overwhelming lack of empirical evidence for use of neuromonitoring, I suppose it’s no wonder that so many surgeons choose not to monitor at all. Anyway, I’m getting off topic. If I can return to your original point…. Here’s my opinion – I think the Oarm/Nav is good for short term. It demonstrates that the screw is in the pedicle, and probably not breached significantly. Since some studies demonstrate that a screw can breach 1-2 millimeters and not result in post-op pain, Oarm is good for immediate outcomes. It may be my lack of knowledge, but I don’t know what this means for the long-term. Will a minor breach migrate and break down over time? Will the patient likely need another surgery down the road as a result? I guess the importance of triggered EMG is detecting the minor breaches (that don’t cause pain, but may break down over time). In this case, there are arguments for both Oarm/Nav and t-EMG. What we really need is a good, well-controlled, long-term outcomes study looking at both triggered EMG and Oarm/Nav. But that the story of neuromonitoring, isn’t it?

      • Reply J.F. Kim July 20, 2016 at 12:57

        RE: Screw placement via navigation (O-Arm): Hi, first and foremost, GREAT article Richard! I wanted to comment on the post left by Andrew Warrington regarding the efficacy of using O-arm while placing pedicle screws. Yes, as Richard Vogel stated, it does provide immediate feedback, but at times the navigation is inaccurate (in my case it was presumably due to user error). There is a surgeon that I monitor for whom uses O-arm navigation for placement of pedicle screws, in addition to that this surgeon also stimulates the pedicle (triggered EMG). There have been numerous occasions where the navigation was “off,”as it wasn’t calibrated correctly at the start of surgery, which resulted in an inaccurate points of navigation by as much as 5mm.. In my experience I think stimulating the pedicles is optimal versus using the O-arm. In my opinion, navigation is not precise (yet).

        • Reply Richard Vogel August 1, 2016 at 10:53

          Thanks for the comment. The only other thing that I would add to the discussion is that T-EMG is not designed to detect inferior, lateral and superior pedicle breaches, and that where O-arm navigation out-shines IONM. I think they both have their limitations, but they compliment each other well.

    • Reply Richard Vogel December 9, 2014 at 20:16

      Another reply to Andrew…obviously I’m not mentioning lots of important factors to consider…. cost of IONM v. Oarm/NAV is on example. Also, my answer probably focused more on the perspective of lumbar fusion. This is one of those topics that is difficult to cover and keep short.

  • Reply Lawrence Wierzbowski December 9, 2014 at 22:58

    In reading up on O Arm….. I thought the article listed below by Santo’s might interest all in this thread. Although the O-arm is fantastic technology it “”Looks like the so called gold standard might be a bit overblown or tarnished concerning pedicle screw assessment”” An O-Arm is hugely expensive.

    “The Accuracy of Intraoperative O-arm Images for the Assessment of Pedicle Screw Postion”

    Now in this cadaver study….. **Intentional Breaches in the pedicles were made in the cadaver spine** and then O –Arm recordings were done and “Interpreted” by 3 separate fully qualified “Readers” who evaluated all the screws and looked for pedicle wall violations.

    A total of 416 screws were inserted with computer-aided navigation transpedicularly… The screw positions were intentionally randomized into “In” (fully contained within the pedicle), “Out laterally,” or “Out medially.”

    Among the pedicle screws placed in the cadavers,
    • 187 (45%) were correctly placed within the confines of the pedicle,
    • 108 (26%) were Intentionally medial breached,
    • and 121 (29%) were lateral breached
    • (all were confirmed based on later open dissection.)

    Now bring in the O-arm and then have the professional readers evaluate

    The overall readers’ assessment accuracy, sensitivity, specificity of the O-arm images for the 3 observers
    • The overall accuracy rate for the 3 observers was 70%.
    • Mean sensitivity, or the likelihood of detecting a breach, was 69%, with a range of 62% to 77 %
    for the 3 observers.
    • Mean specificity, or the likelihood of identifying a nonbreached pedicle, was 71%, with a range
    of 64% to 75%.

    All things said and done, a million dollar O-Arm is very good but with only a overall accuracy of 70% and if my spine was on an OR table….I still want my spEMG and trEMG!

    • Reply Richard Vogel December 12, 2014 at 10:38

      Hi Larry,

      Thanks for posting all of this important information. I’ll probably address comparisons between IOM and imaging for pedicle screw placement in a future post, but you’ve really done the topic justice here, and I certainly appreciate that. Cheers, Rich

  • Reply John N. Gardi December 10, 2014 at 05:06

    No one really touched on the concept advanced recently by Calancie and Donahue of using a multi-pulse stimulus to better define possible medial breaches. Also, the more time consuming technique of testing the hole before screw insertion and testing still deserves more attention, especially in the lumbosacral spine.

    • Reply Richard Vogel December 12, 2014 at 10:35

      Hi John, Thanks for your comment. I considered touching on this topic, but I decided that it might be too much for this particular post. Clearly there are many different variables to consider in the context of pedicle screw testing. I will probably write a few more posts on this topic in the coming months.

  • Reply Andrew December 10, 2014 at 08:31

    a stimulating discussion (no pun intended…well maybe…). A very interesting article Larry, those numbers say a lot. With reference to the Calancie work on train stim frow screw breach evaluation, this was a hot topic at ISIN in turkey whilst I was there. Interested to see what comes out of it.

  • Reply Bruce Katuna December 10, 2014 at 13:00

    Good article. Do you by any chance have a citation for your claim of hypertension increasing the stimulation threshold? There’s actually evidence in the literature that hypertension is protective of developing peripheral neuropathy. Thanks in advance.

    • Reply Richard Vogel December 12, 2014 at 10:41

      Hi Bruce. I went back to review my primary source, and learned that I misrepresented what they stated. The authors stated that metabolic disorders, such as diabetes, can elevate screw/nerve stimulation thresholds. So, I went back to my post above and corrected what I wrote. Also, I added the primary reference for diabetes at the bottom (Toleikis et al, 2000). Thanks very much for pointing this out. Rich

      • Reply Bruce Katuna December 12, 2014 at 14:41

        Richard, thanks so much for following up on this – I really appreciate it.

        • Reply Richard Vogel December 14, 2014 at 22:04

          You are quite welcome. Thanks for keeping me honest. 🙂

  • Reply Ellen Snider December 10, 2014 at 14:16

    Great article, particularly in stressing the importance of our role in controlling for the variables to optimize reliability of the results. I would add to the importance of using a train of four measure in ALL cases – even in those cases for which only a depolarizing muscle relaxant is used at induction, and no non-depolarizing muscle relaxants are administered. Although rare, I have documented two cases in my own experience of prolonged muscle relaxation with succinylcholine persisting throughout the surgical procedure.

    John Gardi commented on the use of pulse-train stimulation for thoracic pedicle screws (Calancie et al, J Neurosurg:Spine, April 2014). The pulse-train pedicle track stimulation method using a 15mA alarm criteria may be effective as a “screening tool” as it reports high sensitivity for detection of medial breaches: “100% of the pedicle tracks with thresholds < 15mA demonstrated clinically significant medial breaches (>2mm) on post op CT scan.” , but a rather high false positive rate >25%.

    A special note of caution that their findings were NOT threshold measures taken from pedicle screw stimulation as discussed in your article; rather from direct stimulation of the pedicle track using a ball tip probe. In fact, comparatively, 1/3 of their 32 confirmed medial breaches on post op CT scan, failed to evoke “pedicle screw” responses @ 30mA. This technique requires delivery of the current through a ball tip probe held obliquely to press against the medial wall of the pedicle track at the entry point, mid point, and floor. In our experience, a ball tip of <2.3 mm is necessary to fully assess thoracic pedicle tracks as described in their technique. Also, the technique is unfortunately not applicable in minimally invasive placement of thoracic screws in which guide wires are placed in the pedicle track for cannulated screw position.

    Of note in this study, regarding your comments on multimodality monitoring: while pulse-train pedicle track stimulation findings were highly sensitive in the detection of thoracic medial breaches, SSEP and TceMEP failed to detect any of the medially malpositioned screws.

  • Reply Chris Baehr December 13, 2014 at 18:46

    great information. Thank you

  • Reply Kunal December 16, 2014 at 18:57

    Great article indeed. At our institution all pedicle screws are navigated and thereafter tested by a neurophysiologist. We strongly believe when a navigated screw(visualizing no lateral breach during placement) is tested using pedicle screw stimulation (>20 mA, no medial breach) on a healthy individual, the need to perform a 2nd O-arm spin is redundant. This significantly shortens the length of the case and prevents patients from being exposed to additional radiation.

  • Reply Mushtaq Shaikh December 28, 2014 at 06:23

    Dear Richard

    Thank you for this excellent article. There are many neurosurgeons and spinal surgeons who are quite blasé about neuromonitoring, probably because their lack of understanding of its positive use.

    The other point to reiterate is that there are many staff carrying out neuromonitoring who are not qualified or without any basic understanding carrying out neuromonitoring and this has resulted in devastating outcome for the patient.

    This is probably the best article I have come across in recent times and it is a must read for any monitoring staff or student.

    Thank you once again for this excellent article.

    Best wishes

    Mushtaq Shaikh

    • Reply Richard Vogel December 28, 2014 at 19:34

      Hi Mushtaq,

      Thank you so much…I appreciate the feedback! I hope you keep reading. Please tell your colleagues about the blog.

      Cheers,

      Rich

  • Reply Jeremy M. Bartel May 29, 2015 at 19:15

    Very well written article that touched on many important concepts to consider during cases. I use a Tof 4 before testing screws/nerves too and agree that stimming until threshold is determined is critical. Especially with chronically compressed roots with elevated thresholds. Its not enough to get up to 20mA and give the okay. Also liked the mention of pulse width with regard to direct nerve stim vs screw stim.

    Has anyone seen a difference in threshold with cannulated vs non-cannulated screws? The last several cannualted ones I’ve tested seem to all have elevagted thresholds.

    • Reply Richard Vogel June 8, 2015 at 10:09

      Hi Jeremy,

      Thanks for the feedback! There’s some data out there on thresholds for different types of metal, different coatings (e.g., hydroxyapetite), and for the MIS systems (where I recall there is ~60% accuracy rate using a breach criteria of 20 mA). Let me know if you come across anything for cannulated screws. Best, Rich

  • Reply Dan June 5, 2015 at 15:42

    hey Rich,
    I enjoyed this article I found it well written unlike my response 🙂 Anyway the first question that i have and feel I must pose is What documentation do you have about myth #4? I have never heard this before and I try very hard to not incorporate things especially if they sound like a personal preference or superstition pushed as fact.
    Thank you in advance for your response.

    • Reply Richard Vogel June 8, 2015 at 10:06

      Hi Dan,

      Great question. In fact, the literature is rather sparse on the subject. If you look at the ASNM position statement, which was written by Leppanen, it says you should use a large grounding pad between the scapulae as an anode. I’m quite sure no one uses this method. I’m listing one other reference below. As for thoracic screw testing, I worked with a group that routinely tested thoracic screws in pediatric scoliosis correction surgery. They’ve been honing this technique for a decade with excellent, but unpublished, results. One of the key factors that they’ve uncovered in the course of refining their technique is that the very best place for the anode is directly across from the screw being tested. Also, in general, it makes sense to have the return contralateral to the screw being tested because you want the current to flow in that direction. So…short story: some literature, lots of unpublished research, perhaps a little superstition clothed in logic. Cheers,

      Ricci WM, Padberg AM, Borrelli J. The significance of anode location for stimulus-evoked electromyography during iliosacral screw placement. J Orthop
      Trauma. 2003 Feb;17(2):95-9.

  • Reply hector August 12, 2015 at 16:35

    Hi Richard very good article , your tips are useful for my practise, I want to know your opinion about testing the hole first?? there are surgeons that do it first and other dont

    • Reply Richard Vogel August 12, 2015 at 19:08

      Hi. I certainly don’t think it can hurt to test the hole first, but I think it’s best if the screw is also tested after placement.

  • Reply Peter December 11, 2015 at 14:08

    Can you comment or write a post on the testing of cervical pedicle screws. I believe many people would love your insight on this. Your site provides an excellent current recourse to the field. Thank you so much for all your work.

    • Reply Richard Vogel January 10, 2016 at 18:47

      Hi Peter, thanks for your comment. I plan to write something in the coming months about testing cervical and thoracic pedicle screws. Stay tuned!

  • Reply Kirk Mason April 25, 2017 at 13:09

    Hi Dr. Vogel,

    Have you written anythinng about Cervical screw testing? Would love to read it.

    Thanks.

    Sincerely,
    Kirk Mason

    • Reply Richard Vogel April 25, 2017 at 13:27

      I haven’t written anything yet, but it may be a topic in the future.

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