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The selection of electromyographic needle electrodes
251
The Selection of Electromyographic
Needle Electrodes
Robert L. Joynt, MD ABSTRACT. Joynt RI. The selection of electromyographlc electrodes. Arch Phys Med Rehabil 1994:75:251-S. a Monopolar and concentric reusable and disposable EMG needle electrodes from several manufacturers were studied at various stages of usage. Needle tips were examined and measured microscopically, and needles were tested for spontaneous noise and resistive and capacitative characteristics at four different test signal frequencies. Resistive and capacitative characteristics were found to be frequency dependent. Needle tip areas differed by as much as 10 times among the various electrodes. The amount of noise correlated most highly with resistive characteristics. Monopolar and concentric needles differed in respect to tip area, noise, and all resistive and capacitative measures. Coaxial disposable and reusable electrodes did not differ significantly and minor differences were noted in electrical characteristics between reusable and disposable monopolar needles, even from the same manufacturer. There was considerable variability among manufacturers for both needle types. In choosing an electrode, consistent tip area is probably the most important consideration to ensure repeatable quantifiable results, but minimizing needle impedance is useful to reduce noise. 0 1994 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation Evaluation of the body’s electrical characteristics involves recording low voltage events from tissue, using a recording probe (in electromyography, a needle electrode) and an amplification and display system. Currently, the electronics are very sophisticated, with adequate amplification of very small signals. The needle electrode, however, is a critical component in the electromyographic (EMG) recording process and is a component that has remained essentially unchanged for many years in spite of the advances in electronics. Textbooks on electrodiagnosis describe different types of needle electrodes and indicate that standard clinical EMG is commonly performed with monopolar or coaxial (concentric) electrodes. Little further advice is provided, however. on how to choose an electrode for clinical use. Decisions need to be made regarding whether to use monopolar or concentric needles, whether to use disposable or reusable needles, which manufacturer’s needles to use, how to sterilize the reusable needles, whether some needle treatment should be used prior to the electromyographic examination, and when to discard a needle. To make these decisions. it is important to know what differences exist between the various needles and conditions, how these differences affect the information recorded, and whether certain methods of handling affect the ability of the electrodes to record events accurately. The accuracy of recording the electrical event in the tissue with a needle electrode is, to a large extent, related to the electrical characteristics of the needle, particularly to resistance and capacitance. These components produce an impedance to the electrical signal and also affect the amount of noise in the system.” An impedance imbalance between Department trott.
of PhyGcal
Medicine
and Rehabilitation,
Wayne
State University.
De-
Ml.
Submitted
for publication
March
3. IY93
Accepted
in revised
form
September
Y.
lYY3. Thk
research
supported
No commercial wpporting
this
organization Reprint 0
in pat
party having article
wth requests
has or will
which
to R. L. Joynt.
of Physical
Medicine
Foundation. interat
a beneht
in the results
of the research
upon the authora
or upon
any
are associated.
MD.
Congress
0003-Y993/94/7503-0106$3.00/O
finanaal
confer
the authors
1994 by the American
Academy
by the Umted
a direct
I148
Olden
Road, Ann
of Rehabilitatmn
and Rehahihtation
Medicine
Arbor.
MI
48103.
and the American
recording and reference electrodes will affect common mode rejection. Also, resistance and capacitance, coupled together, create a filter effect that influences the frequency spectrum of the signal recorded. Few studies have described the characteristics of clinically-used EMG needles:“,“-’ most of those studies have involved only small numbers of new needles. The purpose of the first phase of this study was to establish a reliable method for needle evaluation, to measure the characteristics of EMG needles from several manufacturers to determine how they differed, and to determine if the differences were of clinical significance. Further phases of the study will be directed at the problems of needle handling and durability. METHOD New needles were selected from those regularly purchased in the EMG laboratory, and were also solicited from various manufacturers. Because some companies did not donate needles of all types, the sample does not include the complete lines of all manufacturers. In addition, some needles that were in routine use in the laboratory, and some needles that had been previously used but were not in current use, were tested. Those not in use had been discarded because of changes in preferences of the physicians performing examinations, or because there may have been a problem noted during use. All needles were tagged and numbered. Many needles were tested several times while in use as a step in the ongoing study of the effects of use on the needle proper-ties, and values for all tests were included to compile the overall statistics. To compile data comparing different needles and manufacturers (tables l-5), needles that had been treated prior to that test (current passage, tip abrasion, or saline soaking) were excluded. The “H” in the tables reflects the number of tests done to compile the information in a given table. Reusable needles were used until the electromyographer decided they were unsuitable because of some difficulty in insertion, a feeling of irregularity on insertion, obvious insulation peeling, or mechanical damage such as bending or loss of continuity in a wire. All needles were examined in an identical way for each
Arch Phys Med Rehabil Vol75,
March 1994
252
EMG NEEDLES,
Joynt
Table 1: Needle Type Test Frequency n
Tip Area (mm’)
10
10 2
Resistance Coaxial Monopolor
Monopolor
104
10
10 2
10 3
Capacitance
104
(C) (nF)
10
,074
777
353
96.4 *
22.0
22.6
16.5
11.6
5.3
1,043 *
135
.;50
t1
2;1
59.2
11.0
48.7
31.0
19:4
IO:8
638
Resist/area
(RCM) (M ohm/cm’)
27
4.19 *
1,570
765
135
1.94
;46
1;1
212 3*4.5
Capac/area
48.1
36.2
24.6
i.1
22.3
1:.5
(CCM) @f/cm’) 17.1 9:22
10 2
Cap Reactance
27
Noise (,uV/3s) Coaxial
10 3
(R) (kilo ohms)
189
563
5131
;98
30.4 *
8i.5
Impedance
8.54
10 3
10 4
(XC) (kilo ohms)
12.1
4.81 * 1.93
(IMP) (kilo ohm)
147 * 76.5
28.5
4.35 *
A.5
1.I7
*p < .05.
The needles were first observed under the microscope. They were then placed in a saline bath and spontaneous noise was measured. Resistance and capacitance were then measured in the same saline bath, using the same signal frequencies in the same order at each session. Under the microscope, the length and width of the exposed recording surface of the needle tip were determined using the microscope’s micrometer gauge. Measurements were approximated to the nearest 50pm. The Teflon at the base of the monopolar tip was often quite irregular, so a site in the middle of the irregular area was chosen to measure length. Tip areas (TA) were calculated for concentric needles using the formula for the area of an ellipse (7rHW/4), and for monopolar needles using the area of a cone (7rHW/2), where H is the long axis of the concentric recording surface or the length of the side of the tip of the monopolar electrode, and W is the width of the short axis of the concentric tip or the width of the base of the monopolar electrode. Concentric needles often had prominent grinding marks on the surface that would increase the surface area, but no attempt was made to quantify this effect. The EMG needle was then suspended in the center of a grounded stainless steel bowl filled with a 0.9% saline solution and placed inside a shielded grounded cage. A special needle holder was constructed to ensure that the needle tip was suspended at exactly the same spot in the saline solution in each instance. The spontaneous noise in the needles was measured using an EMG integratof with a frequency band test.
Table 2: Monopolar
from 25Hz to 1,OOOHz. The amount of electrical activity between the needle and the bowl in microvolts per 3 seconds was measured. After the needle was placed in the saline, noise measurements were taken as soon as the values had stabilized. Resistance (R) and capacitance (C) were measured by a balanced bridge system with adjustable resistance and capacitance in parallel (fig). Four frequencies (f) of sine wave (10, 100, 1,000, 10,OOOHz) were applied to the bowl by a signal generator.b The signal was monitored by an oscilloscope’ with input impedance of approximately 50 megaohms, and maintained at 5,OOOmV peak to peak (V = 1.77mV RMS). To make bridge balancing easier and more accurate, equal resistors were placed in the first section of both sides of the bridge and set so as to cause approximately i the voltage drop across the needle. Needle R and C were measured by adjusting the resistance and capacitance in the bridge so that the potential difference measured with an AC digital multimeter between the two sides of the bridge was minimal (usually less than O.OS/_LV).After balancing, the adjustable resistance was measured by a digital multimeter, and capacitance determined by adding the values of the capacitors used. Because the bridge had been balanced, these values were assumed to be equal to those of the needle being tested. Several additional electrical parameters were then calculated. Capacitative reactance (XC) was calculated using the
Needles-Dependent
Variables
by Disposability
Test Frequency n
Tip Area (mm*)
10
28
,235
629
306
70.4
22.2
33.7
30.3
107
,254
631
262
56.3
8.2
52.6
31.2
10 2
Resistance Disposable Reusable
Noise (,uV/3s) Disposable Reusable
Resist/area
10 3
10 4
10
10 2
10 3
Capacitance
(R) (kilo ohms)
(RCM) (M ohm/cm’)
Capac/area
(0
10 4
10
13.5 * 10.1
625
74.5
9.1
1.4
641
87.1
12.9
2Tl
(nF)
24.2 * 18.2
10 2
Cap Reactance
Impedance
(CCM) (PWcm’)
10 3
10 4
(XC) (kilo ohms)
(IMP) (kilo ohm)
28
1.03
273
139
32.4
8.8
13.1
12.0
9.7
5.6
401
68.9
8.9
1.3
107
2.15
363
154
34.9
5.5
24.4
15.1
9.1
5.2
397
78.5
12.1
lT9
*p < .05.
Arch Phys Med Rehabil Vol75,
March 1994
EMG NEEDLES,
Joynt
Table 3: Coaxial Needles Dependent
253
Variables
by Manufacturer -
Test Frequency
Tip Area (mm’)
10
10 2
LO 3
10 4
10
10 2
Resistance (R) (kilo ohms)
Teca
.098 :I
814
357
D1sa
,049
740
342
Chalgren Medelec
2
X
108
104
(C’) (nF)
Cap Reactance -1.8
978
159
23.5
Ii 8
31.0
17.3
10.9
6.X
2.3
I.042
232
47.5
8 2*
682
85.9
Il.7
2.2
I .om
15.0
lT.0
52.2
4.2
28.7
21.6
15.5
,064
830
427
95.5
6.2
t 9.8
IS.1
Il.8
X.6
Disa
7
7.91
848 * 2.909
Chalgren
6
I .68
1,250
554
Medelec
7
1,890
I.020
,700
37s 1.467
ohm/cm’)
81.8 * 413
10 4 (XC) (kilo ohms)
X.6
326
3.06
10 3
12.8
754
(RCM) (M
10 2
17.7
,062
Resist/area
10
13.3
:i 7.4
Noise t,uV/3s) Teca
Capacitance
76.6
-
10 3
14.6
Capac/area
18.0
(CCM) (pF/cm’)
I?
Impedance
(IMP) (kilo ohm)
17.9
X.6
4.8
612
II?.
22.3
3.6
54.3
3(;.8 I
lY.5
9.4
567
168
47.8
7.2
7.6
45.3
34.2
24.6
12.0
475
11.4
I.8
16.4
32.3
25.9
20.8
15.4
618
14.8
1.9 -
:i 107
9i.l 257
82.X 117
* ,’ < .05.
formula $rfC. The phase angle (theta) was calculated using theta = - tan--’ (2irfCR). The total current flow was calculated (for resistance and capacitance in parallel, I = square root of (V/R)’ + (V/XC)“). Total impedance (IMP) was then determined using Ohm’s law. Current density at the needle tip (current/tip area), and the resistance and capacitance per square centimeter of needle tip area (RCM, CCM) were also calculated. R and C values were also calculated for an equivalent series circuit to permit comparison of these values to those reported in the literature using the series method. These numbers are presented in table 2 but are not used elsewhere in the analysis. Statistics were generated using the SPSS/PCi program on a desktop computer. Tests included f tests, correlations using Pearson’s r, and one-way analysis of variance. Several data transformations were performed to determine optimal correlations. To analyze the relationship between the independent variables of needle type, manufacturer, and disposability. ANOVAs were performed between each independent Table 4: Monopolar
Disposable
variable and the dependent variables (R, C, XC, RCM, CCM, IMP). Needles that had received any special treatment such as saline soaking, abrasion, or current passage were excluded from these analyses. A p value of less than 0.05 was considered to be significant. In the tables, values showing significant differences are marked with an asterisk. RESULTS Seventy-three different needles were examined with 1 to 19 tests per needle, producing a total of 241 tests for analysis. The longest follow-up on one needle was 269 days. Of the 73 needles, 33 were new, 16 were in use, and 24 had been used previously but were not currently in use. There were 51 monopolar and 22 coaxial needles; 33 were disposable and 40 were reusable. Needle length was from 20 to 75mm. Forty-six were manufactured by TECAd, 12 by Chalgren’, 4 by DISA’, 3 by Nicolet”, 3 by Rocheste?‘, 3 by Dantec’, 2 by Medelec’, and 1 was not identified by manufacturer. Microscopic examination of concentric needles showed
Needles-Dependent
Variables
by Manufacturer
-
Test Frequency n
Tip Area (mm?
10
10 2
Resistance Teca
20
568
254
Nicolet
3
,184
947
688*
Chalgren
5
292
678
285
Teca
Resist/area
10 4
10
(R) (kilo ohms)
234
Noise (p,V/3s)
10 3
68.4 151 29.8
10 2
10 3
Capacitance
(0
10 4
(nFJ
30.1
28.2
23.4
16.9
10.3
1.7
6.3
4.6”
61.9*
52.4*
38.3*
IfI.?+
(RCM) (M ohm/cm’)
Capac/area
10 2
Cap Reactance
28.3
1.9
10
14.2
(CCM) (pF/cm’)
sso* 1.567” 358°C
10 4
(XC) (kilo ohms)
61 .o ‘14’ A-I.3
Impedance
10 3
7.1
1.1
26.2*
3.t1*
5.7
I .(I
(IMP) (kilo ohm)
20
1.01
239
104
27.4
9.9
13.0
12.2
10.1
6.2
367
54.9
7.2
1.1
Nicolet
3
.09
527*
383*
84.2
9.1
4.3
807”
204*
25.8*
3..5*
5
234
114
12.2
3.5 * 13.3
2.6
Chalgren
5.7 * 21.2
5.2
312
“p
1.2
.71
18.6 *
‘13.7
5.5
.X8 -
< .05.
Arch Phys Med Rehabil Vol75,
March 1994
EMG NEEDLES,
254 Table 5: Monopolar
Reusable
Joynt
Needles-Dependent
Variables
by Manufacturer
Test Frequency n
Tip Area (mm’)
10
10 2
Resistance Teca
98
,241
6 3
,530 ,131
Roth Chalgren
665 * 189 404
Noise (pV13s) Teca Roth Chalgren
98 6 3
Resist/area
2.30 .70 .633
384 40.7 312
10 3
10 4
10
10 2
10 3
Capacitance
(R) (kilo ohms)
10 4
(C) (nF)
10
10 2
Cap Reactance
10 3
10 4
(XC) (kilo ohms)
280
60.1
8.6
46.8
29.5
18.0
10.3
682 *
91.6
13.2
2.0
3i.9 105
10.1 23.5
3.6 2.9
149.2 41.7
57.0 35.5
17.8 23.3
6.0 10.4
112 340
33.2 48.6
10.3 8.48
2.9 1.9
(RCM) (M ohm/cm’) 166 8.4 80.9
31.5 2.1 18.5
5.9 .75 2.1
Capac/area 23.6 31.4 36.9
14.9 12.0 27.5
(CCM) @F/cm’) 9.1 3.8 18.3*
Impedance 5.4 1.3* 8.2
420 89.6* 248
(IMP) (kilo ohm)
82.8 24.3* 43.6
12.6 7.0 1.4
1.9 2.2 1.5
*p < .05.
some variation from manufacturer to manufacturer with slight differences in the bevel angle and other minor difference in tip construction detail. Scratches were evident across the needle tip as a result of the grinding process that had been used to create the bevel. Monopolar needles showed much more variability between needles; some brands had very thin insulation and it was occasionally difficult to determine the exact area that might be electrically responsive; in others, the insulation at the base of the tip was irregular, and with use, the Teflon was noted to occasionally have peeled back from the tip. The average total current flow in the test circuit through the needles was 7.7 nanoamps at 1OHz and rose to 1.34 microamps at 10,OOOHz (table 6). This resulted in an average current density at the needle tip of 4.1 microamps per cm2
100K +A/ T
1
1 OOK
10K
at 1OHz and 748 microamps per cm2 at the 10,OOOHz (table 6). The area of the exposed tip differed considerably between concentric and monopolar needles, and among the different brands. These values are shown in the various comparison tables (tables l-5). Correlations comparing the tip area, TA, (and transformations of this data) and IMP showed the best correlation with log TA (r = -.45 to -.49 at the various frequencies). The values for all tests for all dependent variables are shown in table 7. It is noted that as the signal frequency (f) increased, all the variables except the phase angle showed a progressive decrease. The correlation between impedance and frequency using the raw frequency, f, log f, v?, and l/& showed the highest correlation between the log f and log IMP with Pearson r value being -.941. The amount of noise in the needles as measured by the integrator was consistently correlated with IMP at all frequencies (r = ,322 to .556) suggesting that the amount of noise in the needles is related to their resistive parameters. Concentric and monopolar electrodes showed significant differences in almost all parameters measured (table 1). The average tip area of monopolar electrodes was more than three times that of concentric electrodes. The concentric electrodes also had more noise, higher resistive components (R, RCM, XC, IMP), and lower total C, although CCM was actually higher.
-v/b
Table 6: Test Current and Current Density Test Current (nA)
500K BOWL
1 OOK
k
Rehabil Vol75, March 1994
1OHz 1OOHz 1,OOOHz 10,OOOHr
n
Mean
SD
237 241 238 231
7.73 42.18 242.38 1,334.84
14.37 65.41 210.53 735.25
10K
Balanced Bridge Test Circuit. EMG needle in bowl of physiological saline. Abbreviations: SG, signal generator; OSC, oscilloscope (Cadwell 5200 EMG machine); VOM, digital multimeter.
Arch Phys Med
At At At At
Min
I .87
Max
3.22 13.86 134.67
160.69 127.97 1,915.55 4,602.57
.I4 1.05 8.66 52.77
72.70 301.85 730.07 3,728.96
Current Density kAhxn*) At At At At
1OHz 1OOHz 1,OOOHz 10,000Hz
233 231 234 234
4.07 21.60 130.27 748.48
6.02 26.65 99.28 456.75
EMG NEEDLES, Table 7: Dependent
Resistance (R-Knhm) At IOHz At I OOHz At 1.OOOHr At lO.OOOHz Capacitance tC-nF) At IOHz At IOOHL At I.OOOHz At lO.OOOHr Phase angle (radians) At IOHz At IOOHz At I.OOOHz At 1O.OOOHz Capacitative reactance (
Variables
n
Mean
SD
Min
Max
237 241 238 237
637.0 266.8 59.9 I I.2
254.5 165.8 71.9 37.4
19.6 4.5 I.2 .44
975 791 551 486
237 241 238 237
54.6 34.7 19.7 10.2
Ill.0 50.1 14.1 5.0
2.5 1.3 1.2 I.2
1,195 551 116 23.2
237 341 238 237
.86 1.22 1.26 I.13
.25 20 .I9 21
.I5 .44 Sl .44
1.38 I .46 1.55 1.57
237 241 238 337
660.6 95.2 14.2 2.3
638.8 117.9 16.8 2.0
13.3 2.9 1.37 .69
6,366 1,224 132.6 13.3
233 237 234 233
483.5 215.0 s2. I 10.0
640.7 341.2 107.5 34. I
5.1 I.2 .33 .05
5.88 I 3,368 755 324
233 237 234 234
27.3 17.8 10.7 5.8
45.7 20.8 7.3 3.4
1.1 .93 .77 .41
578.8 231.8 53.7 28.5
231 241 238 237
403.9 81.9 13.4 2.1
209.9 72.9 15.9 1.9
11.0 2.4 .92 .38
948.3 549.7 127.7 13.1
XC-Kohm)
At 1OHr At 1OOHz At I,OOOHr At I O.OOOHz Resistance/Area (RCM-
Mohmkm’) At IOHT At IOOHL At 1.OOOHz At 10,000Hz Capacitance/Area (CCMmicrofaradskm’) At IOHL At 1OOHz At I.OOOHL At IO,OOOHz Impedance (IMP-Kohm) At 1OHz At IOOHz At 1.OOOHI At 10.000Hr Equivalent series resistance (Kohms 1 At IOHz At IOOHz At l.OOOHr At I O.OOOHz Equivalent series capacitance (nF) At IOHr At IOOHz At i.OOOHz At IO,OOOHz
237 241 238 237
274.2 31.8 3.8 .70
195.7 52.0 5.4 .48
5.9 1.3 .I5 .Oa
931 481 44.7 4.1
237 241 238 237
96.0 42.0 24.8 14.4
155.9 68.8 28.3 10.3
33.5 4.9 1.3 I.2
I .746 777 256 87.8
The disposable and reusable concentric needles did not differ significantly in any parameter, so the data are not presented in a table. Monopolar disposable and reusable needles showed occasional differences (table 2). It was noted that values for C, RCM, and CCM tended to have less variation through the frequency range for disposable than for reusable needles. XC and IMP, however, tended to be higher in reusable needles at all frequencies, but statistically significantly so only at the highest frequency. Because statistical differences between disposable and reusable concentric needles were not evident, these two groups were combined for evaluation of the differences that might exist among different manufacturers (table 3). In particular, it was noted that DISA needles had the smallest tip area,
Joynt
255
highest RCM, XC, and IMP, and the lowest C, although the lowest CCM values were seen in the TECA needles that had the largest tip area. It was interesting to note that although there were significant differences in the RCM values, the gross R values did not show significant differences. Monopolar needles also showed significant differences among manufacturers. and differences between disposable and reusable types. Of the disposable monopolar electrodes (table 4), the Nicolet electrodes had the smallest tip. the lowest C and CCM. and the highest XC and IMP. The Chalgren electrodes had the largest tip with the highest C and the lowest XC and IMP. Many of these differences were significant, as shown in the table. For reusable monopolar needles (table 5). there were also significant differences between manufacturers. The Rochester needle had a much larger tip exposure than the other electrodes. with lower resistive components (R, RCM) and higher C at the low frequencies. The Chalgren electrodes. however, tended to have the highest CCM. The Rochester needles also tended to have the lowest XC and IMP at the lower frequencies. The differences between disposable and reusable monopolar needles for a single manufacturer were tested by examining the TECA electrodes, of which there were sufticient samples available for satisfactory statistical comparisons. These comparisons showed some significant differences, with the disposable needles showing somewhat lower RChl and higher C values along with generally lower XC and IMP, although the tip areas were the same. This suggest% that reusable and disposable monopolar needles, even from the same manufacturer, may not have identical characteristics. DISCUSSION Measurement of bioelectrical events with needle electrodes involves a fairly complex interaction with metal in an electrolyte solution. ie. the electrical event is recorded indirectly through a tissue and electrolyte medium. The metal-electrolyte contact creates ionic movements with metal tending to go into solution and ions in solution tending to combine with the metal. If ions are flowing, the interface acts as a voltage source. Also the layering of positive and negative ions on the surface of the electrode, in effect, produces a capacitor. The magnitudes of the resistance, capacitance, and potential at the needle tip depend on the specific metal, the area of the metal exposed, the specific electrolyte, temperature. current density, and the frequency of the signal being recorded.’ Most studies of needle properties use nor-. ma1 saline as the electrolyte for testing to mimic as closely as possible the physiological state of living tissue, and although actual values in muscle differ somewhat because the electrolyte composition is different. studies have shown close approximation between in vitro studies with saline and in viva studies.? The resistance and capacitance of the recording electrode affect the signal recorded, both in the amplitude and in cre ation of noise and distortion. High impedance will decrease the signal amplitude, and the capacitance, besides having impedance (capacitative reactance. XC), will also cause a phase shift, and in combination with resistance, acts as a Arch Phys Med Rehabil Vol75,
March 1994
EMG NEEDLES,
filter, the nature of which is dependent upon specific resistance and capacitance values. The evaluation of resistance and capacitance for a given electrode is difficult, and measurement techniques necessarily involve these factors on two electrodes and an intervening medium. Several methods may be used to estimate resistance and capacitance.‘.’ Some investigators suggest a model with resistance and capacitance in series3 but the real situation is obviously different, in that direct current can pass through the needle, whereas if the system were truly a resistancecapacitance series situation, direct current would be blocked. The method chosen in this study allows direct evaluation of resistance and capacitance independently and a model of resistance and capacitance in parallel was used. Although the real situation is likely much more complex and involves several resistances and capacitances in series and parallel, it was believed that the single resistance and capacitance in parallel would produce adequate information about these parameters, and reflect conditions reasonably close to reality. The values of the parameters measured, however, are likely somewhat different than those actually present at the needle tip. An attempt was made to minimize the effects of the electrolyte and the second electrode. A large stainless steel bowl was used as the second electrode. It was filled with physiologic saline to the same level for each test, and the needle was suspended in the same location in the bowl, to ensure that this part of the system was consistent. It was believed that the large metal bowl surface had a very low resistance and high capacitance (and therefore low capacitative reactance). Impedance effects of the bowl therefore should be small. The effects of the saline solution could also be assumed to be relatively small, and these factors should remain constant throughout the various tests if the needle electrode was placed in exactly the same location for each test. This testing method has the advantage of simplicity and reproducibility, and allows measurement of a wide range of resistance and capacitive values separately, while maintaining a constant input voltage, and a current within the range where resistance and capacitance are not greatly dependent upon the current density. It has the disadvantage of being somewhat time consuming. Previous investigators’,’ have indicated that it is important to perform testing with low current density at the needle tip, because resistance and capacitance depend on the current density as well as the signal frequency. Studies2 have shown a fairly flat response of resistance and capacitance to current density changes if the current density is kept below OSmA per square centimeter. The effects of higher current density, however, are mainly seen at low frequencies. At frequencies above l,OOOHz, there appears to be minimal effect on resistance and capacitance with current density up to 10 milliamps per square centimeter. In this study, the signal was regulated by control of the input voltage. A peak-to-peak sine wave of 5mV was chosen to mimic the amplitude of a large motor unit action potential. This input voltage resulted in an average current density at 10,OOOHz of .748 milliamps per square centimeter (table 6) with maximum current density of 3.7mAfcm2. At the lower frequencies, however, where the effect of increased current density would be the greatest,
Arch Phys Med Rehabil Vol75,
March 1994
Joynt
the average current density was 4.1,~Alcrn~ with the maximum value of .073mA/cm*. It was believed, therefore, that, for the most part, these currents were within ranges expected to have little effect on the resistance and capacitance values. Other investigators have reported using different techniques and different test signals, so observed values may differ accordingly. Weichers et al3 attempted measurements using a constant current of .025mA per square centimeter but values were unable to be recorded at high frequencies because of the inability to obtain this current density. Nandedkar et al5 used voltages of 50mV and IV, so current densities would be much higher. Dorfman et al6 used 1OmV peak-to-peak sine waves in their study, so it would be expected that current densities would be approximately twice those used in our study. Ackmann et al’ used a constant current source to ensure maximum current densities of 0.7mA/cm’ for concentric electrodes and 0.4lmA/cm’ for monopolar needles. It has been previously reported3,6 that the area of exposure of the tip of the recording electrode has a major effect on the needle electrical parameters. It has been suggested2.3 that the needle impedance should be inversely proportional to the tip area. Weichers et al” reported that electrodes with larger tip areas had lower impedance, although they did not report specific correlations. In addition, they indicated that the shape of the tip also had a significant effect, and that two electrodes with different shapes but the same area showed different impedances. Dorfman et al’ also indicated that in concentric needles, smaller core surfaces showed higher impedance and noise, although mathematical correlations were not performed. They also indicated, however, that this finding was not invariable and suggested that variations were likely attributable to differences in core material in different electrodes. Our studies also showed a relationship between tip area and impedance (r = -.47) with log TA showing the best relationship. The average values for tip area in this study for all tests were .236mm’ for monopolar and .074mm2 for concentric electrodes. These values are somewhat larger than those reported by Weichers et aI although the methods of estimating tip exposure were slightly different. Other investigators have reported needle tip exposure areas similar to ours.5-7 To allow for the possibility that differences in electrical parameters were mainly a result of different tip exposures of the various needles tested, resistance and capacitance were also calculated per unit area of exposed surface. Although this evaluation is somewhat imprecise because of the measurement technique used and the difficulty in exact measurement of the tip exposure, it does attempt normalization in the face of considerable variation in tip exposure among the different electrodes. If various parameters are dependent only upon tip exposure, it would be expected that calculating values per unit area should neutralize any effect of area alone. However, in our study, when values per unit area were used to determine differences between needle conditions, the results of the ANOVA statistics often showed more significant differences than with R and C alone, suggesting that the effects of R and C are dependent upon changes at the level of the interface between the metal and the surrounding electrolyte, rather than being related merely to the total area of metal exposed.
EMG NEEDLES,
Previous investigators’-7 have indicated that both resistance and capacitance decrease as signal frequency increases. This was also consistently observed in our study. An attempt was made in this study to evaluate noise or spontaneous generation of electrical potentials at the needle tip by recording voltage between the needle tip and the bowl while no signal was being applied. It is known that a metal in an electrolyte solution will develop a voltage at the interface. and this voltage is greatest if the two electrodes are of dissimilar metals.’ In this study, the bowl was stainless steel, as were many of the needles tested, so differences in metal should be minimal. Also the solution was constant, the temperature controlled. the amplifier system constant, and the external electrical interference excluded by shielding, so it was assumed that changes in recorded noise originated from the needles. Spontaneous noise develops when current flow occurs, ie, something disturbs the equilibrium between the two electrodes. The amount of noise measured appeared statistically related most to the resistive property of the electrodes (in particular RCM, XC, and IMP), so concentric needles with their higher resistive properties were noisier. It is known that electrode or thermal noise is affected by electrode impedance.‘.? The amount of noise, however, in most instances, did not appear to be a major problem that would interfere with satisfactory clinical recording. Our studies showed marked differences in all properties between concentric and monopolar needles, with the concentric needles having generally higher resistive characteristics and lower capacitative characteristics (table 1). In Weichers studies,’ values for total impedance for monopolar and concentric needles were somewhat higher than ours for needles that had not been presoaked in saline. Ackmann’ reported impedance in concentric needles greater than monopolar types at frequencies of 10 and 1OOHz but no other differences. In Dorfman’s study,’ the impedance values for a variety of concentric needles tended to be in the same range as ours, although somewhat more variable, with some needles showing extremely high values (3 megaohms at 10Hz). Nandedkar’ reported R values for concentric needles in the range of 1 to 3 kilo ohms and capacitance values of 20 to 70 nanofarads. These R values are different than those reported by others and may reflect the high test currents used. The concentric needles on average have a tip exposure approximately f that of the monopolar electrodes, so it might be expected that these differences are related to the area of tip exposure. However, if the two types of needles are compared per square centimeter of exposed surface, the differences are even more significant, suggesting that the local factors at the metal-electrolyte interface are responsible rather than the gross effect of the amount of tip exposed. The fact that monopolar needles have less total impedance and less noise than concentric needles suggests that, in respect to these factors, there may be some advantages to recording with monopolar electrodes. When disposable and reusable concentric needles were compared, there appeared to be no significant difference between the two, suggesting that clinical findings with either type of concentric needle will be reasonably comparable. Ackmann’ also reported no significant differences between
Joynt
the concentric types. Some differences were observed in some parameters between needles from different manufacturers (table 3), but the total number of concentric needles examined was rather small and, on the whole, concentric needIes from the various manufacturers showed relatively few differences. Some of the differences may be related to the differences in the needle tip area. as a result of different wire gauge and bevel angle used. Dorfman” did find differences among manufacturers of concentric needles and suggested that these differences may be related to differences in core material. design. and construction. Nandedkar5 found that disposable concentric needles had a generally higher capacitance and lower resistance than reusable needles at all test frequencies, although the resistance-capacitance product was similar for both groups. In our study. disposable and reusable monopolar electrodes had greater variability in construction and electrical parameters than did concentric ones. The tip exposure depends on the gauge of the wire used and the extent to which the insulation is removed at the tip, and it was noted that tip exposures tended to be somewhat inconsistent. The Teflon or other insulation material may be somewhat irregular at the base of the tip and may peel back with use. Although monopolar disposable and reusable electrodes did not show systematic differences between the two groups (table 3). several isolated parameters did show differences, and the range of values found for some parameters through the various frequencies of testing was less in the disposable electrodes than in the reusable ones. These differences are probably not of clinical significance. Ackmann’ found no significant differ-ences between the monopolar types. There would appear to be several major differences among the monopolar needles of different manufacturers (tables 4,5). Tip exposures varied by as much as a factor of 4 between manufacturers and statistically significant differences were observed for almost all parameters. Electromyogra phers, therefore, must be aware that when doing quantitative studies, findings may not be comparable between different electrodes, particularly if they are from different manufacturers. Even monopolar disposable and reusable needles from the same manufacturer (TECA) showed significant differences in electrical parameters, although the tip area of both types was identical. During clinical EMG evaluation, the electromyographer is interested in the size and shape of the electric signals being recorded, and this is particularly important when quantitative studies of the motor unit action potential are being performed. The characteristics of the recorded potentials are dependent on the impedance of the needle, but variation of the impedances among needles can be accommodated by the use of high input impedance amplifiers, which set input impedance at 50 to 250 times needle impedance, and thereby minimize the effect of variability in impedances in the needles used. In no instances was the magnitude of the impedance such that the size of the tissue electrical event would be materially altered. The clinical effect of different impedances in different needles in altering recorded potential size and shape, therefore, should be minimal. The relationship ot noise to impedance however suggests that care in minimizing impedance is likely a clinically useful undertaking.
Arch Phys Med Rehabil Vol75,
March 1994
EMG NEEDLES, Joynt
258
In this study, needle tip area varied by a factor of 10 from .049mm* in Disa concentric needles to .530mm2 in Rochester monopolar needles. It can be seen, therefore, that if one recording area is 10 times larger than another, based on the geometry of the needle and adjacent muscle fibers, there is a significant chance that the recorded uotentials would be diff&ent. For repeatable quantitative &dies, it would be important that the tip exposure be the same at each usage to enable establishment of normal values for a particular electrode and to be able to have a basis for consistent comparison in patients with possible pathology. Results in quantitative studies would appear to be much more dependent on the tip exposure of the electrode than on the electrical characteristics, so to obtain consistent reproducible quantitative results, the most critical consideration in electrode choice may be the constancy of the tip area of the electrode. CONCLUSIONS Evaluation of EMG needle electrodes by the method described suggests that, in choosing EMG needles, the following facts should be considered: (1) large tip exposure results in lower impedance and less noise; (2) monopolar needles have larger tip exposures than concentric needles; (3) for quantitative studies, to ensure consistent tip exposure, the same size electrodes from the same manufacturer should be used for all examinations. Concentric needles are somewhat more likely to be consistent from needle to needle. The desired area of exposure needs to be decided based on the geometry of the needle in the tissue, ie, how many muscle fibers should be in contact with the needle tip at one time; (4) disposable and reusable needles coaxial needles show
Arch Phys Med Rehabil Vol75,
March 1994
few significant differences in electrical characteristics. Differences found between disposable and reusable monopolar needles are probably not of clinical significance. Acknowledgment: The author acknowledges Nichols. Bertram Ezenwa, and Hisham Bismar testing apparatus and analysis of the results.
the assistance of Angelo in the development of the
References Basmajian JV, DeLuca CJ. Muscles alive. ed 5. Baltimore: Williams and Wilkins, 1985. Geddes LA, Baker LE. Principles of applied biomedical instrumentation. New York: Wiley, 1989:315-39. Wiechers DO, Blood JR, Stow RW. EMG needle electrodes: electrical impedance. Arch Phys Med Rehabil 1979$X):364-9. Buchthal F, Guld C, Rosenfalck P. Action potential parameters in normal human muscle and their dependence of physical variables. Acta Physiol Stand 1954;32:20@ 18. 5. Nandedkar SD, Tedman B. Sanders DB. Recording and physical characteristics of disposable concentric needle EMG electrodes. Muscle Nerve 1990;13:909-14. 6. Dorfman LJ. McGill KC, Cummins KL. Electrical properties of commercial concentric EMG electrodes. Muscle Nerve 1985;8:1-8. Ackmann JJ, Lomas JN, Hoffmann RG, Wertsch 53. Multifrequency characteristics of disposable and nondisposable EMG needle electrodes. Muscle Nerve 1993;16:616-23.
Suppliers
i: c.
d. e.
h.
RMS-J&J EMG M57, J&J Enterprises, Poulsbo, WA. Dynascan Corporation, Chicago, IL. Cadwell 5200 electromyograph, Cadwell Laboratories, Incorporated, Kennewick, WA. TECA Corporation, 3 Campus Drive, Pleasantville, NJ 10570 Chalgren. The Electrode Store. PO Box 188. Enamclan. WA. DISK Electronics. 779 Susquehana Avenue,‘Franklin Lakes, NJ. Nicolet Instrument Company, 5225 Verona Road, PO Box 4445 1, Madison, WI. Rochester Fleck0 Medical Incorporated, 13708 Attley Place, Tampa, FL. Dantec Medical Incorporated, 590 Division Street, Campbell, CT. Medelec. UK Medelec Incorporated, Maur Way. Old Woking, Surrey, England.
The Selection of Electromyographic
Needle Electrodes
Robert L. Joynt, MD ABSTRACT. Joynt RI. The selection of electromyographlc electrodes. Arch Phys Med Rehabil 1994:75:251-S. a Monopolar and concentric reusable and disposable EMG needle electrodes from several manufacturers were studied at various stages of usage. Needle tips were examined and measured microscopically, and needles were tested for spontaneous noise and resistive and capacitative characteristics at four different test signal frequencies. Resistive and capacitative characteristics were found to be frequency dependent. Needle tip areas differed by as much as 10 times among the various electrodes. The amount of noise correlated most highly with resistive characteristics. Monopolar and concentric needles differed in respect to tip area, noise, and all resistive and capacitative measures. Coaxial disposable and reusable electrodes did not differ significantly and minor differences were noted in electrical characteristics between reusable and disposable monopolar needles, even from the same manufacturer. There was considerable variability among manufacturers for both needle types. In choosing an electrode, consistent tip area is probably the most important consideration to ensure repeatable quantifiable results, but minimizing needle impedance is useful to reduce noise. 0 1994 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation Evaluation of the body’s electrical characteristics involves recording low voltage events from tissue, using a recording probe (in electromyography, a needle electrode) and an amplification and display system. Currently, the electronics are very sophisticated, with adequate amplification of very small signals. The needle electrode, however, is a critical component in the electromyographic (EMG) recording process and is a component that has remained essentially unchanged for many years in spite of the advances in electronics. Textbooks on electrodiagnosis describe different types of needle electrodes and indicate that standard clinical EMG is commonly performed with monopolar or coaxial (concentric) electrodes. Little further advice is provided, however. on how to choose an electrode for clinical use. Decisions need to be made regarding whether to use monopolar or concentric needles, whether to use disposable or reusable needles, which manufacturer’s needles to use, how to sterilize the reusable needles, whether some needle treatment should be used prior to the electromyographic examination, and when to discard a needle. To make these decisions. it is important to know what differences exist between the various needles and conditions, how these differences affect the information recorded, and whether certain methods of handling affect the ability of the electrodes to record events accurately. The accuracy of recording the electrical event in the tissue with a needle electrode is, to a large extent, related to the electrical characteristics of the needle, particularly to resistance and capacitance. These components produce an impedance to the electrical signal and also affect the amount of noise in the system.” An impedance imbalance between Department trott.
of PhyGcal
Medicine
and Rehabilitation,
Wayne
State University.
De-
Ml.
Submitted
for publication
March
3. IY93
Accepted
in revised
form
September
Y.
lYY3. Thk
research
supported
No commercial wpporting
this
organization Reprint 0
in pat
party having article
wth requests
has or will
which
to R. L. Joynt.
of Physical
Medicine
Foundation. interat
a beneht
in the results
of the research
upon the authora
or upon
any
are associated.
MD.
Congress
0003-Y993/94/7503-0106$3.00/O
finanaal
confer
the authors
1994 by the American
Academy
by the Umted
a direct
I148
Olden
Road, Ann
of Rehabilitatmn
and Rehahihtation
Medicine
Arbor.
MI
48103.
and the American
recording and reference electrodes will affect common mode rejection. Also, resistance and capacitance, coupled together, create a filter effect that influences the frequency spectrum of the signal recorded. Few studies have described the characteristics of clinically-used EMG needles:“,“-’ most of those studies have involved only small numbers of new needles. The purpose of the first phase of this study was to establish a reliable method for needle evaluation, to measure the characteristics of EMG needles from several manufacturers to determine how they differed, and to determine if the differences were of clinical significance. Further phases of the study will be directed at the problems of needle handling and durability. METHOD New needles were selected from those regularly purchased in the EMG laboratory, and were also solicited from various manufacturers. Because some companies did not donate needles of all types, the sample does not include the complete lines of all manufacturers. In addition, some needles that were in routine use in the laboratory, and some needles that had been previously used but were not in current use, were tested. Those not in use had been discarded because of changes in preferences of the physicians performing examinations, or because there may have been a problem noted during use. All needles were tagged and numbered. Many needles were tested several times while in use as a step in the ongoing study of the effects of use on the needle proper-ties, and values for all tests were included to compile the overall statistics. To compile data comparing different needles and manufacturers (tables l-5), needles that had been treated prior to that test (current passage, tip abrasion, or saline soaking) were excluded. The “H” in the tables reflects the number of tests done to compile the information in a given table. Reusable needles were used until the electromyographer decided they were unsuitable because of some difficulty in insertion, a feeling of irregularity on insertion, obvious insulation peeling, or mechanical damage such as bending or loss of continuity in a wire. All needles were examined in an identical way for each
Arch Phys Med Rehabil Vol75,
March 1994
252
EMG NEEDLES,
Joynt
Table 1: Needle Type Test Frequency n
Tip Area (mm’)
10
10 2
Resistance Coaxial Monopolor
Monopolor
104
10
10 2
10 3
Capacitance
104
(C) (nF)
10
,074
777
353
96.4 *
22.0
22.6
16.5
11.6
5.3
1,043 *
135
.;50
t1
2;1
59.2
11.0
48.7
31.0
19:4
IO:8
638
Resist/area
(RCM) (M ohm/cm’)
27
4.19 *
1,570
765
135
1.94
;46
1;1
212 3*4.5
Capac/area
48.1
36.2
24.6
i.1
22.3
1:.5
(CCM) @f/cm’) 17.1 9:22
10 2
Cap Reactance
27
Noise (,uV/3s) Coaxial
10 3
(R) (kilo ohms)
189
563
5131
;98
30.4 *
8i.5
Impedance
8.54
10 3
10 4
(XC) (kilo ohms)
12.1
4.81 * 1.93
(IMP) (kilo ohm)
147 * 76.5
28.5
4.35 *
A.5
1.I7
*p < .05.
The needles were first observed under the microscope. They were then placed in a saline bath and spontaneous noise was measured. Resistance and capacitance were then measured in the same saline bath, using the same signal frequencies in the same order at each session. Under the microscope, the length and width of the exposed recording surface of the needle tip were determined using the microscope’s micrometer gauge. Measurements were approximated to the nearest 50pm. The Teflon at the base of the monopolar tip was often quite irregular, so a site in the middle of the irregular area was chosen to measure length. Tip areas (TA) were calculated for concentric needles using the formula for the area of an ellipse (7rHW/4), and for monopolar needles using the area of a cone (7rHW/2), where H is the long axis of the concentric recording surface or the length of the side of the tip of the monopolar electrode, and W is the width of the short axis of the concentric tip or the width of the base of the monopolar electrode. Concentric needles often had prominent grinding marks on the surface that would increase the surface area, but no attempt was made to quantify this effect. The EMG needle was then suspended in the center of a grounded stainless steel bowl filled with a 0.9% saline solution and placed inside a shielded grounded cage. A special needle holder was constructed to ensure that the needle tip was suspended at exactly the same spot in the saline solution in each instance. The spontaneous noise in the needles was measured using an EMG integratof with a frequency band test.
Table 2: Monopolar
from 25Hz to 1,OOOHz. The amount of electrical activity between the needle and the bowl in microvolts per 3 seconds was measured. After the needle was placed in the saline, noise measurements were taken as soon as the values had stabilized. Resistance (R) and capacitance (C) were measured by a balanced bridge system with adjustable resistance and capacitance in parallel (fig). Four frequencies (f) of sine wave (10, 100, 1,000, 10,OOOHz) were applied to the bowl by a signal generator.b The signal was monitored by an oscilloscope’ with input impedance of approximately 50 megaohms, and maintained at 5,OOOmV peak to peak (V = 1.77mV RMS). To make bridge balancing easier and more accurate, equal resistors were placed in the first section of both sides of the bridge and set so as to cause approximately i the voltage drop across the needle. Needle R and C were measured by adjusting the resistance and capacitance in the bridge so that the potential difference measured with an AC digital multimeter between the two sides of the bridge was minimal (usually less than O.OS/_LV).After balancing, the adjustable resistance was measured by a digital multimeter, and capacitance determined by adding the values of the capacitors used. Because the bridge had been balanced, these values were assumed to be equal to those of the needle being tested. Several additional electrical parameters were then calculated. Capacitative reactance (XC) was calculated using the
Needles-Dependent
Variables
by Disposability
Test Frequency n
Tip Area (mm*)
10
28
,235
629
306
70.4
22.2
33.7
30.3
107
,254
631
262
56.3
8.2
52.6
31.2
10 2
Resistance Disposable Reusable
Noise (,uV/3s) Disposable Reusable
Resist/area
10 3
10 4
10
10 2
10 3
Capacitance
(R) (kilo ohms)
(RCM) (M ohm/cm’)
Capac/area
(0
10 4
10
13.5 * 10.1
625
74.5
9.1
1.4
641
87.1
12.9
2Tl
(nF)
24.2 * 18.2
10 2
Cap Reactance
Impedance
(CCM) (PWcm’)
10 3
10 4
(XC) (kilo ohms)
(IMP) (kilo ohm)
28
1.03
273
139
32.4
8.8
13.1
12.0
9.7
5.6
401
68.9
8.9
1.3
107
2.15
363
154
34.9
5.5
24.4
15.1
9.1
5.2
397
78.5
12.1
lT9
*p < .05.
Arch Phys Med Rehabil Vol75,
March 1994
EMG NEEDLES,
Joynt
Table 3: Coaxial Needles Dependent
253
Variables
by Manufacturer -
Test Frequency
Tip Area (mm’)
10
10 2
LO 3
10 4
10
10 2
Resistance (R) (kilo ohms)
Teca
.098 :I
814
357
D1sa
,049
740
342
Chalgren Medelec
2
X
108
104
(C’) (nF)
Cap Reactance -1.8
978
159
23.5
Ii 8
31.0
17.3
10.9
6.X
2.3
I.042
232
47.5
8 2*
682
85.9
Il.7
2.2
I .om
15.0
lT.0
52.2
4.2
28.7
21.6
15.5
,064
830
427
95.5
6.2
t 9.8
IS.1
Il.8
X.6
Disa
7
7.91
848 * 2.909
Chalgren
6
I .68
1,250
554
Medelec
7
1,890
I.020
,700
37s 1.467
ohm/cm’)
81.8 * 413
10 4 (XC) (kilo ohms)
X.6
326
3.06
10 3
12.8
754
(RCM) (M
10 2
17.7
,062
Resist/area
10
13.3
:i 7.4
Noise t,uV/3s) Teca
Capacitance
76.6
-
10 3
14.6
Capac/area
18.0
(CCM) (pF/cm’)
I?
Impedance
(IMP) (kilo ohm)
17.9
X.6
4.8
612
II?.
22.3
3.6
54.3
3(;.8 I
lY.5
9.4
567
168
47.8
7.2
7.6
45.3
34.2
24.6
12.0
475
11.4
I.8
16.4
32.3
25.9
20.8
15.4
618
14.8
1.9 -
:i 107
9i.l 257
82.X 117
* ,’ < .05.
formula $rfC. The phase angle (theta) was calculated using theta = - tan--’ (2irfCR). The total current flow was calculated (for resistance and capacitance in parallel, I = square root of (V/R)’ + (V/XC)“). Total impedance (IMP) was then determined using Ohm’s law. Current density at the needle tip (current/tip area), and the resistance and capacitance per square centimeter of needle tip area (RCM, CCM) were also calculated. R and C values were also calculated for an equivalent series circuit to permit comparison of these values to those reported in the literature using the series method. These numbers are presented in table 2 but are not used elsewhere in the analysis. Statistics were generated using the SPSS/PCi program on a desktop computer. Tests included f tests, correlations using Pearson’s r, and one-way analysis of variance. Several data transformations were performed to determine optimal correlations. To analyze the relationship between the independent variables of needle type, manufacturer, and disposability. ANOVAs were performed between each independent Table 4: Monopolar
Disposable
variable and the dependent variables (R, C, XC, RCM, CCM, IMP). Needles that had received any special treatment such as saline soaking, abrasion, or current passage were excluded from these analyses. A p value of less than 0.05 was considered to be significant. In the tables, values showing significant differences are marked with an asterisk. RESULTS Seventy-three different needles were examined with 1 to 19 tests per needle, producing a total of 241 tests for analysis. The longest follow-up on one needle was 269 days. Of the 73 needles, 33 were new, 16 were in use, and 24 had been used previously but were not currently in use. There were 51 monopolar and 22 coaxial needles; 33 were disposable and 40 were reusable. Needle length was from 20 to 75mm. Forty-six were manufactured by TECAd, 12 by Chalgren’, 4 by DISA’, 3 by Nicolet”, 3 by Rocheste?‘, 3 by Dantec’, 2 by Medelec’, and 1 was not identified by manufacturer. Microscopic examination of concentric needles showed
Needles-Dependent
Variables
by Manufacturer
-
Test Frequency n
Tip Area (mm?
10
10 2
Resistance Teca
20
568
254
Nicolet
3
,184
947
688*
Chalgren
5
292
678
285
Teca
Resist/area
10 4
10
(R) (kilo ohms)
234
Noise (p,V/3s)
10 3
68.4 151 29.8
10 2
10 3
Capacitance
(0
10 4
(nFJ
30.1
28.2
23.4
16.9
10.3
1.7
6.3
4.6”
61.9*
52.4*
38.3*
IfI.?+
(RCM) (M ohm/cm’)
Capac/area
10 2
Cap Reactance
28.3
1.9
10
14.2
(CCM) (pF/cm’)
sso* 1.567” 358°C
10 4
(XC) (kilo ohms)
61 .o ‘14’ A-I.3
Impedance
10 3
7.1
1.1
26.2*
3.t1*
5.7
I .(I
(IMP) (kilo ohm)
20
1.01
239
104
27.4
9.9
13.0
12.2
10.1
6.2
367
54.9
7.2
1.1
Nicolet
3
.09
527*
383*
84.2
9.1
4.3
807”
204*
25.8*
3..5*
5
234
114
12.2
3.5 * 13.3
2.6
Chalgren
5.7 * 21.2
5.2
312
“p
1.2
.71
18.6 *
‘13.7
5.5
.X8 -
< .05.
Arch Phys Med Rehabil Vol75,
March 1994
EMG NEEDLES,
254 Table 5: Monopolar
Reusable
Joynt
Needles-Dependent
Variables
by Manufacturer
Test Frequency n
Tip Area (mm’)
10
10 2
Resistance Teca
98
,241
6 3
,530 ,131
Roth Chalgren
665 * 189 404
Noise (pV13s) Teca Roth Chalgren
98 6 3
Resist/area
2.30 .70 .633
384 40.7 312
10 3
10 4
10
10 2
10 3
Capacitance
(R) (kilo ohms)
10 4
(C) (nF)
10
10 2
Cap Reactance
10 3
10 4
(XC) (kilo ohms)
280
60.1
8.6
46.8
29.5
18.0
10.3
682 *
91.6
13.2
2.0
3i.9 105
10.1 23.5
3.6 2.9
149.2 41.7
57.0 35.5
17.8 23.3
6.0 10.4
112 340
33.2 48.6
10.3 8.48
2.9 1.9
(RCM) (M ohm/cm’) 166 8.4 80.9
31.5 2.1 18.5
5.9 .75 2.1
Capac/area 23.6 31.4 36.9
14.9 12.0 27.5
(CCM) @F/cm’) 9.1 3.8 18.3*
Impedance 5.4 1.3* 8.2
420 89.6* 248
(IMP) (kilo ohm)
82.8 24.3* 43.6
12.6 7.0 1.4
1.9 2.2 1.5
*p < .05.
some variation from manufacturer to manufacturer with slight differences in the bevel angle and other minor difference in tip construction detail. Scratches were evident across the needle tip as a result of the grinding process that had been used to create the bevel. Monopolar needles showed much more variability between needles; some brands had very thin insulation and it was occasionally difficult to determine the exact area that might be electrically responsive; in others, the insulation at the base of the tip was irregular, and with use, the Teflon was noted to occasionally have peeled back from the tip. The average total current flow in the test circuit through the needles was 7.7 nanoamps at 1OHz and rose to 1.34 microamps at 10,OOOHz (table 6). This resulted in an average current density at the needle tip of 4.1 microamps per cm2
100K +A/ T
1
1 OOK
10K
at 1OHz and 748 microamps per cm2 at the 10,OOOHz (table 6). The area of the exposed tip differed considerably between concentric and monopolar needles, and among the different brands. These values are shown in the various comparison tables (tables l-5). Correlations comparing the tip area, TA, (and transformations of this data) and IMP showed the best correlation with log TA (r = -.45 to -.49 at the various frequencies). The values for all tests for all dependent variables are shown in table 7. It is noted that as the signal frequency (f) increased, all the variables except the phase angle showed a progressive decrease. The correlation between impedance and frequency using the raw frequency, f, log f, v?, and l/& showed the highest correlation between the log f and log IMP with Pearson r value being -.941. The amount of noise in the needles as measured by the integrator was consistently correlated with IMP at all frequencies (r = ,322 to .556) suggesting that the amount of noise in the needles is related to their resistive parameters. Concentric and monopolar electrodes showed significant differences in almost all parameters measured (table 1). The average tip area of monopolar electrodes was more than three times that of concentric electrodes. The concentric electrodes also had more noise, higher resistive components (R, RCM, XC, IMP), and lower total C, although CCM was actually higher.
-v/b
Table 6: Test Current and Current Density Test Current (nA)
500K BOWL
1 OOK
k
Rehabil Vol75, March 1994
1OHz 1OOHz 1,OOOHz 10,OOOHr
n
Mean
SD
237 241 238 231
7.73 42.18 242.38 1,334.84
14.37 65.41 210.53 735.25
10K
Balanced Bridge Test Circuit. EMG needle in bowl of physiological saline. Abbreviations: SG, signal generator; OSC, oscilloscope (Cadwell 5200 EMG machine); VOM, digital multimeter.
Arch Phys Med
At At At At
Min
I .87
Max
3.22 13.86 134.67
160.69 127.97 1,915.55 4,602.57
.I4 1.05 8.66 52.77
72.70 301.85 730.07 3,728.96
Current Density kAhxn*) At At At At
1OHz 1OOHz 1,OOOHz 10,000Hz
233 231 234 234
4.07 21.60 130.27 748.48
6.02 26.65 99.28 456.75
EMG NEEDLES, Table 7: Dependent
Resistance (R-Knhm) At IOHz At I OOHz At 1.OOOHr At lO.OOOHz Capacitance tC-nF) At IOHz At IOOHL At I.OOOHz At lO.OOOHr Phase angle (radians) At IOHz At IOOHz At I.OOOHz At 1O.OOOHz Capacitative reactance (
Variables
n
Mean
SD
Min
Max
237 241 238 237
637.0 266.8 59.9 I I.2
254.5 165.8 71.9 37.4
19.6 4.5 I.2 .44
975 791 551 486
237 241 238 237
54.6 34.7 19.7 10.2
Ill.0 50.1 14.1 5.0
2.5 1.3 1.2 I.2
1,195 551 116 23.2
237 341 238 237
.86 1.22 1.26 I.13
.25 20 .I9 21
.I5 .44 Sl .44
1.38 I .46 1.55 1.57
237 241 238 337
660.6 95.2 14.2 2.3
638.8 117.9 16.8 2.0
13.3 2.9 1.37 .69
6,366 1,224 132.6 13.3
233 237 234 233
483.5 215.0 s2. I 10.0
640.7 341.2 107.5 34. I
5.1 I.2 .33 .05
5.88 I 3,368 755 324
233 237 234 234
27.3 17.8 10.7 5.8
45.7 20.8 7.3 3.4
1.1 .93 .77 .41
578.8 231.8 53.7 28.5
231 241 238 237
403.9 81.9 13.4 2.1
209.9 72.9 15.9 1.9
11.0 2.4 .92 .38
948.3 549.7 127.7 13.1
XC-Kohm)
At 1OHr At 1OOHz At I,OOOHr At I O.OOOHz Resistance/Area (RCM-
Mohmkm’) At IOHT At IOOHL At 1.OOOHz At 10,000Hz Capacitance/Area (CCMmicrofaradskm’) At IOHL At 1OOHz At I.OOOHL At IO,OOOHz Impedance (IMP-Kohm) At 1OHz At IOOHz At 1.OOOHI At 10.000Hr Equivalent series resistance (Kohms 1 At IOHz At IOOHz At l.OOOHr At I O.OOOHz Equivalent series capacitance (nF) At IOHr At IOOHz At i.OOOHz At IO,OOOHz
237 241 238 237
274.2 31.8 3.8 .70
195.7 52.0 5.4 .48
5.9 1.3 .I5 .Oa
931 481 44.7 4.1
237 241 238 237
96.0 42.0 24.8 14.4
155.9 68.8 28.3 10.3
33.5 4.9 1.3 I.2
I .746 777 256 87.8
The disposable and reusable concentric needles did not differ significantly in any parameter, so the data are not presented in a table. Monopolar disposable and reusable needles showed occasional differences (table 2). It was noted that values for C, RCM, and CCM tended to have less variation through the frequency range for disposable than for reusable needles. XC and IMP, however, tended to be higher in reusable needles at all frequencies, but statistically significantly so only at the highest frequency. Because statistical differences between disposable and reusable concentric needles were not evident, these two groups were combined for evaluation of the differences that might exist among different manufacturers (table 3). In particular, it was noted that DISA needles had the smallest tip area,
Joynt
255
highest RCM, XC, and IMP, and the lowest C, although the lowest CCM values were seen in the TECA needles that had the largest tip area. It was interesting to note that although there were significant differences in the RCM values, the gross R values did not show significant differences. Monopolar needles also showed significant differences among manufacturers. and differences between disposable and reusable types. Of the disposable monopolar electrodes (table 4), the Nicolet electrodes had the smallest tip. the lowest C and CCM. and the highest XC and IMP. The Chalgren electrodes had the largest tip with the highest C and the lowest XC and IMP. Many of these differences were significant, as shown in the table. For reusable monopolar needles (table 5). there were also significant differences between manufacturers. The Rochester needle had a much larger tip exposure than the other electrodes. with lower resistive components (R, RCM) and higher C at the low frequencies. The Chalgren electrodes. however, tended to have the highest CCM. The Rochester needles also tended to have the lowest XC and IMP at the lower frequencies. The differences between disposable and reusable monopolar needles for a single manufacturer were tested by examining the TECA electrodes, of which there were sufticient samples available for satisfactory statistical comparisons. These comparisons showed some significant differences, with the disposable needles showing somewhat lower RChl and higher C values along with generally lower XC and IMP, although the tip areas were the same. This suggest% that reusable and disposable monopolar needles, even from the same manufacturer, may not have identical characteristics. DISCUSSION Measurement of bioelectrical events with needle electrodes involves a fairly complex interaction with metal in an electrolyte solution. ie. the electrical event is recorded indirectly through a tissue and electrolyte medium. The metal-electrolyte contact creates ionic movements with metal tending to go into solution and ions in solution tending to combine with the metal. If ions are flowing, the interface acts as a voltage source. Also the layering of positive and negative ions on the surface of the electrode, in effect, produces a capacitor. The magnitudes of the resistance, capacitance, and potential at the needle tip depend on the specific metal, the area of the metal exposed, the specific electrolyte, temperature. current density, and the frequency of the signal being recorded.’ Most studies of needle properties use nor-. ma1 saline as the electrolyte for testing to mimic as closely as possible the physiological state of living tissue, and although actual values in muscle differ somewhat because the electrolyte composition is different. studies have shown close approximation between in vitro studies with saline and in viva studies.? The resistance and capacitance of the recording electrode affect the signal recorded, both in the amplitude and in cre ation of noise and distortion. High impedance will decrease the signal amplitude, and the capacitance, besides having impedance (capacitative reactance. XC), will also cause a phase shift, and in combination with resistance, acts as a Arch Phys Med Rehabil Vol75,
March 1994
EMG NEEDLES,
filter, the nature of which is dependent upon specific resistance and capacitance values. The evaluation of resistance and capacitance for a given electrode is difficult, and measurement techniques necessarily involve these factors on two electrodes and an intervening medium. Several methods may be used to estimate resistance and capacitance.‘.’ Some investigators suggest a model with resistance and capacitance in series3 but the real situation is obviously different, in that direct current can pass through the needle, whereas if the system were truly a resistancecapacitance series situation, direct current would be blocked. The method chosen in this study allows direct evaluation of resistance and capacitance independently and a model of resistance and capacitance in parallel was used. Although the real situation is likely much more complex and involves several resistances and capacitances in series and parallel, it was believed that the single resistance and capacitance in parallel would produce adequate information about these parameters, and reflect conditions reasonably close to reality. The values of the parameters measured, however, are likely somewhat different than those actually present at the needle tip. An attempt was made to minimize the effects of the electrolyte and the second electrode. A large stainless steel bowl was used as the second electrode. It was filled with physiologic saline to the same level for each test, and the needle was suspended in the same location in the bowl, to ensure that this part of the system was consistent. It was believed that the large metal bowl surface had a very low resistance and high capacitance (and therefore low capacitative reactance). Impedance effects of the bowl therefore should be small. The effects of the saline solution could also be assumed to be relatively small, and these factors should remain constant throughout the various tests if the needle electrode was placed in exactly the same location for each test. This testing method has the advantage of simplicity and reproducibility, and allows measurement of a wide range of resistance and capacitive values separately, while maintaining a constant input voltage, and a current within the range where resistance and capacitance are not greatly dependent upon the current density. It has the disadvantage of being somewhat time consuming. Previous investigators’,’ have indicated that it is important to perform testing with low current density at the needle tip, because resistance and capacitance depend on the current density as well as the signal frequency. Studies2 have shown a fairly flat response of resistance and capacitance to current density changes if the current density is kept below OSmA per square centimeter. The effects of higher current density, however, are mainly seen at low frequencies. At frequencies above l,OOOHz, there appears to be minimal effect on resistance and capacitance with current density up to 10 milliamps per square centimeter. In this study, the signal was regulated by control of the input voltage. A peak-to-peak sine wave of 5mV was chosen to mimic the amplitude of a large motor unit action potential. This input voltage resulted in an average current density at 10,OOOHz of .748 milliamps per square centimeter (table 6) with maximum current density of 3.7mAfcm2. At the lower frequencies, however, where the effect of increased current density would be the greatest,
Arch Phys Med Rehabil Vol75,
March 1994
Joynt
the average current density was 4.1,~Alcrn~ with the maximum value of .073mA/cm*. It was believed, therefore, that, for the most part, these currents were within ranges expected to have little effect on the resistance and capacitance values. Other investigators have reported using different techniques and different test signals, so observed values may differ accordingly. Weichers et al3 attempted measurements using a constant current of .025mA per square centimeter but values were unable to be recorded at high frequencies because of the inability to obtain this current density. Nandedkar et al5 used voltages of 50mV and IV, so current densities would be much higher. Dorfman et al6 used 1OmV peak-to-peak sine waves in their study, so it would be expected that current densities would be approximately twice those used in our study. Ackmann et al’ used a constant current source to ensure maximum current densities of 0.7mA/cm’ for concentric electrodes and 0.4lmA/cm’ for monopolar needles. It has been previously reported3,6 that the area of exposure of the tip of the recording electrode has a major effect on the needle electrical parameters. It has been suggested2.3 that the needle impedance should be inversely proportional to the tip area. Weichers et al” reported that electrodes with larger tip areas had lower impedance, although they did not report specific correlations. In addition, they indicated that the shape of the tip also had a significant effect, and that two electrodes with different shapes but the same area showed different impedances. Dorfman et al’ also indicated that in concentric needles, smaller core surfaces showed higher impedance and noise, although mathematical correlations were not performed. They also indicated, however, that this finding was not invariable and suggested that variations were likely attributable to differences in core material in different electrodes. Our studies also showed a relationship between tip area and impedance (r = -.47) with log TA showing the best relationship. The average values for tip area in this study for all tests were .236mm’ for monopolar and .074mm2 for concentric electrodes. These values are somewhat larger than those reported by Weichers et aI although the methods of estimating tip exposure were slightly different. Other investigators have reported needle tip exposure areas similar to ours.5-7 To allow for the possibility that differences in electrical parameters were mainly a result of different tip exposures of the various needles tested, resistance and capacitance were also calculated per unit area of exposed surface. Although this evaluation is somewhat imprecise because of the measurement technique used and the difficulty in exact measurement of the tip exposure, it does attempt normalization in the face of considerable variation in tip exposure among the different electrodes. If various parameters are dependent only upon tip exposure, it would be expected that calculating values per unit area should neutralize any effect of area alone. However, in our study, when values per unit area were used to determine differences between needle conditions, the results of the ANOVA statistics often showed more significant differences than with R and C alone, suggesting that the effects of R and C are dependent upon changes at the level of the interface between the metal and the surrounding electrolyte, rather than being related merely to the total area of metal exposed.
EMG NEEDLES,
Previous investigators’-7 have indicated that both resistance and capacitance decrease as signal frequency increases. This was also consistently observed in our study. An attempt was made in this study to evaluate noise or spontaneous generation of electrical potentials at the needle tip by recording voltage between the needle tip and the bowl while no signal was being applied. It is known that a metal in an electrolyte solution will develop a voltage at the interface. and this voltage is greatest if the two electrodes are of dissimilar metals.’ In this study, the bowl was stainless steel, as were many of the needles tested, so differences in metal should be minimal. Also the solution was constant, the temperature controlled. the amplifier system constant, and the external electrical interference excluded by shielding, so it was assumed that changes in recorded noise originated from the needles. Spontaneous noise develops when current flow occurs, ie, something disturbs the equilibrium between the two electrodes. The amount of noise measured appeared statistically related most to the resistive property of the electrodes (in particular RCM, XC, and IMP), so concentric needles with their higher resistive properties were noisier. It is known that electrode or thermal noise is affected by electrode impedance.‘.? The amount of noise, however, in most instances, did not appear to be a major problem that would interfere with satisfactory clinical recording. Our studies showed marked differences in all properties between concentric and monopolar needles, with the concentric needles having generally higher resistive characteristics and lower capacitative characteristics (table 1). In Weichers studies,’ values for total impedance for monopolar and concentric needles were somewhat higher than ours for needles that had not been presoaked in saline. Ackmann’ reported impedance in concentric needles greater than monopolar types at frequencies of 10 and 1OOHz but no other differences. In Dorfman’s study,’ the impedance values for a variety of concentric needles tended to be in the same range as ours, although somewhat more variable, with some needles showing extremely high values (3 megaohms at 10Hz). Nandedkar’ reported R values for concentric needles in the range of 1 to 3 kilo ohms and capacitance values of 20 to 70 nanofarads. These R values are different than those reported by others and may reflect the high test currents used. The concentric needles on average have a tip exposure approximately f that of the monopolar electrodes, so it might be expected that these differences are related to the area of tip exposure. However, if the two types of needles are compared per square centimeter of exposed surface, the differences are even more significant, suggesting that the local factors at the metal-electrolyte interface are responsible rather than the gross effect of the amount of tip exposed. The fact that monopolar needles have less total impedance and less noise than concentric needles suggests that, in respect to these factors, there may be some advantages to recording with monopolar electrodes. When disposable and reusable concentric needles were compared, there appeared to be no significant difference between the two, suggesting that clinical findings with either type of concentric needle will be reasonably comparable. Ackmann’ also reported no significant differences between
Joynt
the concentric types. Some differences were observed in some parameters between needles from different manufacturers (table 3), but the total number of concentric needles examined was rather small and, on the whole, concentric needIes from the various manufacturers showed relatively few differences. Some of the differences may be related to the differences in the needle tip area. as a result of different wire gauge and bevel angle used. Dorfman” did find differences among manufacturers of concentric needles and suggested that these differences may be related to differences in core material. design. and construction. Nandedkar5 found that disposable concentric needles had a generally higher capacitance and lower resistance than reusable needles at all test frequencies, although the resistance-capacitance product was similar for both groups. In our study. disposable and reusable monopolar electrodes had greater variability in construction and electrical parameters than did concentric ones. The tip exposure depends on the gauge of the wire used and the extent to which the insulation is removed at the tip, and it was noted that tip exposures tended to be somewhat inconsistent. The Teflon or other insulation material may be somewhat irregular at the base of the tip and may peel back with use. Although monopolar disposable and reusable electrodes did not show systematic differences between the two groups (table 3). several isolated parameters did show differences, and the range of values found for some parameters through the various frequencies of testing was less in the disposable electrodes than in the reusable ones. These differences are probably not of clinical significance. Ackmann’ found no significant differ-ences between the monopolar types. There would appear to be several major differences among the monopolar needles of different manufacturers (tables 4,5). Tip exposures varied by as much as a factor of 4 between manufacturers and statistically significant differences were observed for almost all parameters. Electromyogra phers, therefore, must be aware that when doing quantitative studies, findings may not be comparable between different electrodes, particularly if they are from different manufacturers. Even monopolar disposable and reusable needles from the same manufacturer (TECA) showed significant differences in electrical parameters, although the tip area of both types was identical. During clinical EMG evaluation, the electromyographer is interested in the size and shape of the electric signals being recorded, and this is particularly important when quantitative studies of the motor unit action potential are being performed. The characteristics of the recorded potentials are dependent on the impedance of the needle, but variation of the impedances among needles can be accommodated by the use of high input impedance amplifiers, which set input impedance at 50 to 250 times needle impedance, and thereby minimize the effect of variability in impedances in the needles used. In no instances was the magnitude of the impedance such that the size of the tissue electrical event would be materially altered. The clinical effect of different impedances in different needles in altering recorded potential size and shape, therefore, should be minimal. The relationship ot noise to impedance however suggests that care in minimizing impedance is likely a clinically useful undertaking.
Arch Phys Med Rehabil Vol75,
March 1994
EMG NEEDLES, Joynt
258
In this study, needle tip area varied by a factor of 10 from .049mm* in Disa concentric needles to .530mm2 in Rochester monopolar needles. It can be seen, therefore, that if one recording area is 10 times larger than another, based on the geometry of the needle and adjacent muscle fibers, there is a significant chance that the recorded uotentials would be diff&ent. For repeatable quantitative &dies, it would be important that the tip exposure be the same at each usage to enable establishment of normal values for a particular electrode and to be able to have a basis for consistent comparison in patients with possible pathology. Results in quantitative studies would appear to be much more dependent on the tip exposure of the electrode than on the electrical characteristics, so to obtain consistent reproducible quantitative results, the most critical consideration in electrode choice may be the constancy of the tip area of the electrode. CONCLUSIONS Evaluation of EMG needle electrodes by the method described suggests that, in choosing EMG needles, the following facts should be considered: (1) large tip exposure results in lower impedance and less noise; (2) monopolar needles have larger tip exposures than concentric needles; (3) for quantitative studies, to ensure consistent tip exposure, the same size electrodes from the same manufacturer should be used for all examinations. Concentric needles are somewhat more likely to be consistent from needle to needle. The desired area of exposure needs to be decided based on the geometry of the needle in the tissue, ie, how many muscle fibers should be in contact with the needle tip at one time; (4) disposable and reusable needles coaxial needles show
Arch Phys Med Rehabil Vol75,
March 1994
few significant differences in electrical characteristics. Differences found between disposable and reusable monopolar needles are probably not of clinical significance. Acknowledgment: The author acknowledges Nichols. Bertram Ezenwa, and Hisham Bismar testing apparatus and analysis of the results.
the assistance of Angelo in the development of the
References Basmajian JV, DeLuca CJ. Muscles alive. ed 5. Baltimore: Williams and Wilkins, 1985. Geddes LA, Baker LE. Principles of applied biomedical instrumentation. New York: Wiley, 1989:315-39. Wiechers DO, Blood JR, Stow RW. EMG needle electrodes: electrical impedance. Arch Phys Med Rehabil 1979$X):364-9. Buchthal F, Guld C, Rosenfalck P. Action potential parameters in normal human muscle and their dependence of physical variables. Acta Physiol Stand 1954;32:20@ 18. 5. Nandedkar SD, Tedman B. Sanders DB. Recording and physical characteristics of disposable concentric needle EMG electrodes. Muscle Nerve 1990;13:909-14. 6. Dorfman LJ. McGill KC, Cummins KL. Electrical properties of commercial concentric EMG electrodes. Muscle Nerve 1985;8:1-8. Ackmann JJ, Lomas JN, Hoffmann RG, Wertsch 53. Multifrequency characteristics of disposable and nondisposable EMG needle electrodes. Muscle Nerve 1993;16:616-23.
Suppliers
i: c.
d. e.
h.
RMS-J&J EMG M57, J&J Enterprises, Poulsbo, WA. Dynascan Corporation, Chicago, IL. Cadwell 5200 electromyograph, Cadwell Laboratories, Incorporated, Kennewick, WA. TECA Corporation, 3 Campus Drive, Pleasantville, NJ 10570 Chalgren. The Electrode Store. PO Box 188. Enamclan. WA. DISK Electronics. 779 Susquehana Avenue,‘Franklin Lakes, NJ. Nicolet Instrument Company, 5225 Verona Road, PO Box 4445 1, Madison, WI. Rochester Fleck0 Medical Incorporated, 13708 Attley Place, Tampa, FL. Dantec Medical Incorporated, 590 Division Street, Campbell, CT. Medelec. UK Medelec Incorporated, Maur Way. Old Woking, Surrey, England.