ORIGINAL RESEARCH

Can Triggered Electromyography Be Used to Evaluate Pedicle Screw Placement in Hydroxyapatite-Coated Screws: An Electrical Examination Timothy T. Davis,* Stephanie Tadlock,† Johannes Bernbeck,‡ Daniel A. Fung,* and Diana M. Molinares*

Objectives: To assess if hydroxyapatite (HA)-coated titanium pedicle screws exhibit the same electroconductive characteristics as non–HA-coated screws. Methods: Resistance measurements were obtained from a random sampling of 10 HA-coated pedicle screws and 10 non–HA-coated screws, and surgical conditions simulated. Surface resistivity measurements were taken for each screw to determine voltage drop over its entire length. Results: The non–HA-coated screws tested showed low resistive properties and proved to be an ideal conductor of electrical current. The resistive properties associated with the HA-coated pedicle screws were found to be similar to those of commonly used insulators removing the effectiveness of triggered electromyographic responses. Conclusions: Based on test results, these data suggest that the resistance value of the HA-coated screw is large enough to prevent modern IntraOperative Monitoring (IOM) equipment from delivering the necessary current through the shank of the screw to create a diagnostic electromyographic response. Any response that would be produced would be because of shunting of electric current from the non-coated head of the screw into adjacent tissue and not through the shank of the screw. These study results suggest that HA-coated screws cannot be stimulated to assist in determining the accuracy of pedicle screw placement. Key Words: Lumbar spine surgery, Hydroxyapatite, Pedicle screw, Electromyography, EMG. (J Clin Neurophysiol 2014;31: 138–142)

osterior instrumented spinal fusion was first introduced in the 1940s but gained popularity in Europe during the 1970s (Esses et al., 1993). The introduction of pedicle screws as a means of posterior fixation was introduced in 1959 (Boucher, 1959). Pedicle screw fixation has since been applied to a variety of spinal pathologies. The placement of pedicle screws is most commonly performed under fluoroscopic guidance. Various techniques for placing pedicle screws have been described (Kantelhardt et al., 2009; Ludwig et al., 2000a; Ludwig et al., 2000b; Rajasekaran et al., 2007; Wiesner et al., 1999) (Fig. 1). Assistive devices to aid in proper placement of pedicle screws continue to be developed. One of the most common complications in posterior spinal fixation is unrecognized screw misplacement (5.2%) that can lead to postoperative pain or paresthesias in mild cases and

P

From the *Orthopedic Pain Specialists, Santa Monica, California, U.S.A.; †Alpha Diagnostics, Santa Monica, California, U.S.A.; and ‡Orthopedic Surgery Department, Kaiser Medical Group Los Angeles, California, U.S.A. Address correspondence and reprint requests to Daniel A. Fung, MD, Orthopedic Pain Specialists, 2811 Wilshire Boulevard, Suite 850, Santa Monica, CA, 90403; e-mail: [email protected]. Copyright  2014 by the American Clinical Neurophysiology Society

ISSN: 0736-0258/14/3102-0138

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paralysis in the most severe situations (Esses et al., 1993). Avoiding these types of complications is of utmost importance. Triggered electromyography (tEMG) is one assistive technique that is used to verify appropriate screw placement. It was first introduced in 1992 and subsequent studies set forth standards and guidelines for practical application (Calancie et al., 1992; Calancie et al., 1994; Raynor et al., 2007; Shi et al., 2003; Whang and Vaccaro, 2006). Along with intraoperative imaging, the use of tEMG testing has become a common means of confirming proper placement of pedicle screw (Whang and Vaccaro, 2006). Triggered electromyography can be used before or after screw placement to assist with the identification of pedicle breach. A monopolar probe is used to deliver a current to the head of the pedicle screw which acts as an extension of the stimulating probe into deeper tissues. A stimulation threshold is determined as being the minimum current necessary applied to the pedicle screws, which evokes an electromyographic response. Previous studies show that medial or inferior cortical breach is associated with stimulation thresholds of ,9 mA (Rampersaud et al., 2005; Raynor et al., 2007). Thresholds of 10 mA or greater usually indicate a screw is placed well within the confines of the cortices of the pedicle (Raynor et al., 2007). In any electrical circuit, current requirements will change depending on the electroconductive properties of the parts of that circuit. A pedicle screw with a higher electroconductive resistance would produce current thresholds higher than expected. This elevated stimulation threshold could be interpreted as indicating proper screw placement when in fact the pedicle screw could be misplaced. This would result in a false-negative conclusion and may be associated with intraoperative complications. The resistive value of a pedicle screw may vary depending on the material from which it is constructed. Previous studies have shown that screws constructed from stainless steel have resistive properties nearly identical to titanium pedicle screws (Anderson et al., 2012; Raynor et al., 2007; Von Knoch et al., 2004). Industry has continued to make advancements in pedicle screw construction, design, and materials, as our understanding of spinal pathology has expanded. In the strive to develop more useful and reliable implants, hydroxyapatite (HA) was applied as a coating for the pedicle screw shank as an attempt to promote bone in-growth and increase “pullout” strength (Hasegawa et al., 2005; Sandén et al., 2001; Spivak et al., 1994; Yildirim et al., 2006). Such improvement in materials, however, can at times hinder other properties. Hydroxyapatite is a naturally occurring mineral form of calcium apatite. Some orthopedic implants are coated with HA because of its porous molecular structure, which improves the interface with bone (Baramki et al., 2000). Manufacturers of HA-coated screws have warned of inconsistent stimulation thresholds during tEMG testing. There is a lack of published data to

Journal of Clinical Neurophysiology  Volume 31, Number 2, April 2014

Journal of Clinical Neurophysiology  Volume 31, Number 2, April 2014

FIG. 1.

Triggered EMG and HA-Coated Pedicle Screws

X-ray of implanted pedicle screws.

explain this inconsistency, although some authors have suggested the potential disparity secondary to the coating of an HA screw (Isley et al., 2012). In clinical practice, the authors noted two clear instances in which there was a medial pedicle breach using an HA-coated pedicle screw, and the tEMG stimulation thresholds were noted to be .30 mA. In both cases, the misplacement was not detected during the surgery. Both patients awoke with unilateral leg pain and postprocedure computed tomography verified medial breach. Revision surgery was performed. Before the removal of the misplaced pedicle screws, thresholds were tested again and results were similar. A review of all parts of the circuit was performed in each case. The one unknown in both instances was the electroconductive properties of an HA-coated pedicle screw versus a non-coated screw. The purpose of this study is to assess if HA-coated titanium pedicle screws exhibit the same electroconductive characteristics as non–HA-coated screws.

MATERIALS AND METHODS Resistance and surface resistivity measurements were obtained from a random sampling of 10 HA-coated pedicle screws and 10 non–HA-coated screws. All screws were of the same diameter (6.5 mm) and length (45 mm) (Fig. 2). An independent laboratory of California Institute of Electronics and Materials Science (CIEMS, www.ciems.com) was commissioned to perform the testing on the pedicle screws. The measurements were performed only on the cylindrical part of the screws. The average diameter was calculated from the dimensions measurements of the thread outer and inner diameters, taking in consideration the thread shape. The conical part of the samples was used as one of the current contacts only. The experimental error evaluated by the partial derivatives and least squares methods does not exceed 5% and 4% for samples. The equipment used by CIEMS meets the applicable National Institute of Standards and Technology (NIST), American Society for Testing and Materials (ASTM), American Society of Mechanical Engineers (ASME), Occupational Safety and Health Administration (OSHA), and State requirements and was calibrated with the standards traceable to the NIST. The calibration was performed per ANSI/ISO/ ASQ Q9004-2000, ANSI/ASQC M1-1996, ISO 10012:2003, MILSTD-45662, MIL-I-45208, NAVAIR-17-35-MTL-1, CSP-1/03-93, and the instrument manufacturers’ specifications. The equipment passed Copyright  2014 by the American Clinical Neurophysiology Society

FIG. 2. Polyaxial head pedicle screws 6.5 · 45 mm. On the left is the hydroxyapatite-coated screws, and the right is the non–HA-coated screws.

a periodic accuracy test. The linear measures and weight measuring instruments were calibrated. The equipment used was as follows: 1. Precision Kelvin Bridge: Adjustable Standard Low-Resistance Bridge Model 4300 L&N with Bridge Ratio Box Model 4320 L&N. 2. General Purpose Kelvin Bridge Model 4306 L&N. 3. Precision Milliohm Meter Model 380460 ExTech. 4. Electrometers: Model 610C KTL and Model 616 KTL. 5. High-Resistance Meters Model 4329A HP, Model 4339B Agilent, and Model 5105 ARI. 6. Picoammeter Model 3503 RU with Metrologic Laser Model ML869S/C MII. 7. 50A-6V Stabilized Power Supply Model SC-506FAVD HBC. 8. Starrett Dial Indicator Model 25-109 (1.27 mm/div). 9. Digital Hygrothermometer Model 63-844 MI. 10. Barometer Model 602650 SB.

RESULTS Resistance and resistivity measurements for samples of HA-coated and non–HA-coated pedicle screws are summarized in Tables 1 and 2. The non–HA-coated screws tested showed low resistive properties. The medical grade titanium-based alloy screws were constructed from proved to be an ideal conductor of electrical current, with an average value of 3.97 to 3 U. These values are similar to those of commonly used conductors like gold, copper, and aluminum, requiring ,40.0 mV to deliver 10 mA of current to the most distal part of the pedicle screw. 139

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T. T. Davis et al.

TABLE 1.

Electrical Resistance and Resistivity Test: Hydroxyapatite-Coated Pedicle Screws Average Axial Electrical Resistance (U)

No. 1 2 3 4 5 6 7 8 9 10

Sample Pedicle screw No. 1 Pedicle screw No. 2 Pedicle screw No. 3 Pedicle screw No. 4 Pedicle screw No. 5 Pedicle screw No. 6 Pedicle screw No. 7 Pedicle screw No. 8 Pedicle screw No. 9 Pedicle screw No. 10 Average

Per Turn (Per Pitch) 2.65 2.76 2.52 2.70 2.59 2.49 2.79 2.60 2.62 2.73 2.65

· · · · · · · · · · ·

1024 1024 1024 1024 1024 1024 1024 1024 1024 1024 1024

The resistive properties associated with the HA-coated pedicle screws are similar to those of commonly used insulators like plastic, glass, or porcelain with values exceeding 1.0 · 1011 U. To scale, to effectively deliver 10 mA of current to the most distal part of the coated pedicle screw, more than 100 million volts must be applied to the screws hexagonal head. The International Annealed Copper Standard percentage is a value placed on materials based on its ability to conduct electricity compared with that of annealed copper, which is 100%. Materials with the rating of 50% or higher are considered to be very good electrical conductors. Materials that fall below 1% are considered insulators. As seen in Table 3, the non-coated pedicle screws are rated 72%. Figure 3 shows the relationship between non–HA-coated pedicle screws and other well-known conductors used today in applied engineering. Hydroxyapatite-coated pedicle screws have a 1.66 · 10212% IACS rating, which qualifies it as an electrical insulator. Other materials that contain values similar to this are glass (1.72 · 10213%) and rubber (1.70 · 10215%), which would also be considered as insulators (Fig. 4).

DISCUSSION Posterior spinal fusion using pedicle screws as a means of fixation is a relatively common procedure for a variety of spinal TABLE 2.

Effective Electrical Resistivity Total

3.98 4.13 3.78 4.05 3.89 3.74 4.19 3.90 3.94 4.09 3.97

· · · · · · · · · · ·

1023 1023 1023 1023 1023 1023 1023 1023 1023 1023 1023

Volume (U$m) 2.39 2.48 2.27 2.43 2.33 2.24 2.51 2.34 2.36 2.46 2.38

· · · · · · · · · · ·

1 2 3 4 5 6 7 8 9 10

140

1026 1026 1026 1026 1026 1026 1026 1026 1026 1026

Surface (U/sq) 1.71 1.77 1.62 1.74 1.67 1.60 1.79 1.67 1.69 1.76 1.70

· · · · · · · · · · ·

1023 1023 1023 1023 1023 1023 1023 1023 1023 1023 1023

pathologies. Triggered electromyography can be used in conjunction with intraoperative imaging techniques to assist in the placement of pedicle screws. Triggered electromyography is only valuable during this process if the parts of the electrical circuit have consistent electroconductive properties. Donohue et al. found screws made of titanium alloys had higher resistance and impedance at tested frequencies compared with similar stainless steel screws (Donohue et al., 2012). In the study by Donohue et al., the titanium screws were not standardized to one type, and there was a lot of variability in their results; in addition, resistance and impedance levels were tested with an off-the-shelf ohmmeter (BK LCR/ESR Model 885; BK Precision, Yorba Linda, CA) which may have led to further inaccuracies. In the study we are presenting, each of the HA-coated and non–HA-coated screws were of the same manufacturer and model to eliminate material and size variability. Our resistance and impedance testing was contracted out to an independent laboratory with professional grade instruments to ensure the highest degree of accuracy and standards were maintained. The results of our study reveal that there is an exponential difference in the electroconductive nature of an HA-coated screw versus a non-coated screw. Hydroxyapatite-coated screws were found to be greater than one billion times more resistive than non–HA-coated screws. These results would indicate that HA-coated screws cannot be used as a means of conducting electrical current because of the insulative nature that HA possesses as a material.

Electrical Resistance and Resistivity Test: Non–Hydroxyapatite-Coated Pedicle Screws Average Axial Electrical Resistance (U)

No.

1026

Effective Electrical Resistivity

Sample

Per Turn (Per Pitch)

Total

Volume (U$m)

Pedicle screw No. 1 Pedicle screw No. 2 Pedicle screw No. 3 Pedicle screw No. 4 Pedicle screw No. 5 Pedicle screw No. 6 Pedicle screw No. 7 Pedicle screw No. 8 Pedicle screw No. 9 Pedicle screw No. 10 Average

· · · · · · · · · · ·

· · · · · · · · · · ·

· · · · · · · · · · ·

1.17 1.11 1.23 1.18 1.12 1.19 1.12 1.22 1.14 1.16 1.16

1010 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010

1.76 1.67 1.84 1.77 1.68 1.79 1.68 1.83 1.71 1.74 1.75

1011 1011 1011 1011 1011 1011 1011 1011 1011 1011 1011

1.06 1.00 1.11 1.06 1.01 1.07 1.01 1.10 1.03 1.04 1.04

108 108 108 108 108 108 108 108 108 108 108

Surface (U/sq) 7.57 7.10 7.85 7.55 7.18 7.63 7.17 7.78 7.28 7.38 7.38

· · · · · · · · · · ·

1010 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010

Copyright  2014 by the American Clinical Neurophysiology Society

Journal of Clinical Neurophysiology  Volume 31, Number 2, April 2014

Triggered EMG and HA-Coated Pedicle Screws

TABLE 3. Electrical Resistance and Resistivity Test: Comparison of Hydroxyapatite-Coated and Non–Hydroxyapatite-Coated Screw Averages Average Axial Electrical Resistance (U) Sample Non–HA-coated screw HA-coated screw

Per Turn (Per Pitch)

Total

1.16 · 1010 2.65 · 1024

1.75 · 1011 3.97 · 1023

Effective Electrical Resistivity Volume (U$m) 1.04 · 108 2.38 · 1026

Surface (U/sq)

IACS%

7.38 · 1010 1.70 · 1023

7.2 · 101 1.66 · 10212

HA, hydroxyapatite; IACS%, International Annealed Copper Standard percentage.

of an HA-coated screw. Once an HA-coated screw is placed, any further use of a stimulating probe applied to the screw is of no value in determining proximity to neural structures.

KEY POINTS 1. Hydroxyapatite-coated pedicle screws have an exponentially higher electrical resistance than non–HA-coated screws. 2. Hydroxyapatite coating precludes the use of modern IntraOperative Monitoring (IOM) equipment as useful tool during screw placement. 3. Responses generated by stimulating HA-coated screws are because of electrical current shunting to adjacent tissues. 4. It is not recommended to stimulate HA-coated screws as an assistive tool during pedicle screw placement. FIG. 3.

International Annealed Copper Standard percentage.

The increased resistance of the HA-coated screw is large enough to prevent any Intra-Operative Monitoring (IOM) equipment from delivering the necessary current to create tEMG responses safely. Any response that is produced would be because of shunting of electrical current from the non-coated head of the screw across adjacent tissues, not through the shank of the screw. Hydroxyapatitecoated screws should not be stimulated to assist in determining pedicle screw placement secondary to the high probability of false negatives. The current tEMG standards and guidelines cannot be applied to the placement of HA-coated screws. The authors would further recommend using a stimulating pedicle probe to evaluate for a pedicle wall breach before placement

FIG. 4.

Insulation values.

Copyright  2014 by the American Clinical Neurophysiology Society

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Can triggered electromyography be used to evaluate pedicle screw placement in hydroxyapatite-coated screws: an electrical examination.

To assess if hydroxyapatite (HA)-coated titanium pedicle screws exhibit the same electroconductive characteristics as non-HA-coated screws...
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