Ambu Neuroline needles for
neurophysiological examination:
Review of clinical data
The present clinical review is based on published
literature within the field of needle electrodes used for
neurophysiological examinations and data generated
during clinical investigations performed by Ambu A/S
(See Annex I). The present document covers the Ambu
Neuroline Concentric Needle, Ambu Neuroline
Monopolar Needle, Ambu Neuroline Subdermal needle
and Ambu Neuroline Twisted Par needles.
the tip of the needle are also critical factors since they
have been thought to be related to the discomfort
cause to the patient; however, some data does suggest
that patient discomfort is more related to the physicians
technique to manipulate the needle and the patients
psychological state during the examination (iFi).
cannula
1. Introduction
The measurement of the body’s electrical signals
involves a sensor, an amplification system, and a display.
Within the last decades, the development of electronics
and specially amplifiers made it possible to record
signals of very small amplitude (micro Volts). However,
the sensor element, which is a needle in the case of
several neurophysiological examinations, is a critical
component of the recording process. In fact, the design
of the two most widely used types of EMG needle
electrodes, the concentric and monopolar needles,
remains almost unchanged since they were introduced
in the forties [1].
Ambu A/S manufactures the following needles used
for neurophysiological examinations: concentric,
monopolar, subdermal and twisted pair needles.
2. Needles for neurophysiolgical
examinations
The concentric needle
Adrian and Bronk first introduced the concentric needle
electrode in 1929. The concentric needle consists of a
cannula (reference electrode) and a core (active
electrode), which are generally made of different
materials (See figure 1). The outer diameter of the
cannula ranges between 0.45 to 0.7 mm diameter
(AMBU has sizes 0.3 to 0.65 mm (22 and 30 G
respectively)), and the core approximately 0.1 mm of
diameter. The Ambu neuroline concentric needle is
made of stainless steel (cannula) and silver (core).
The core is embedded in an insulating material, so that
the two metal parts (electrodes) are electrically
insulated. The tip of concentric needles is typically a flat
elliptical shape and is generally ground to an angle of
approximately 15 degrees. The shape and sharpness of
core
Figure 1: Schematic of the concentric needle showing
the cannula (reference electrode) and core (active
electrode) components.
Concentric needles are used during Electromyographic
(EMG) examinations. The EMG measurement is
performed by inserting the needle, which contains both
the active (core) and the reference (cannula) electrodes
as close as possible to the potential of interest. Finally,
a ground electrode is placed relatively away from the
needle. (See examinations for more detail).
The monopolar needle
The use of a monopolar needle to measure both normal
and denervated muscle activity was first introduced by
Jasper and Ballem in 1949, and the design has not
changed much since that first prototype. Nowadays,
most monopolar needles are made of stainless steel
with a gauge ranging from 26 to 31 G (AMBU has sizes
25 to 28 G). The needle is covered with an insulating
material layer. This insulation covers the whole needle
except for the tip, which is the conductive (measuring)
area of the needle. The tip of monopolar needles is
traditionally sharpened to a conical shape, and the
length of the electrical conductive tip varies among
manufacturers. The area of the conductive tip is one of
the most critical factors in the recording, as it is directly
related to the pickup field and the amplitude of the
recorded activity.
The AMBU monopolar needle is insulated and its surface
is specially treated with the aim of decreasing the friction
between the needle and the biological tissue, facilitating
the penetration and decreasing the discomfort for the
patient. Moreover, the shape of the tip is not conical as
conventional monopolar needles, but it has a back-bevel
cut aiming to improve the sharpness and thus the needle
penetration (see figure 2).
Sensor (tip with a back-bevel cut)
Insulated area
Figure 2: Schematic of the monopolar needle showing
the tip, which is the sensor element and the insulated
area of the needle.
Monopolar needles are used for EMG examinations. The
EMG measurement is performed by inserting the
monopolar needle (active electrode) as close as possible
to the potential of interest. The reference electrode,
generally a surface electrode, is placed close to the
active electrode but not on the active tissue, and finally,
a ground electrode is placed relatively away from these
two electrodes. (See examinations for more detail).
Both, monopolar and concentric needles are used for
EMG examinations. However, the signals recorded by
these two types of needles are significantly different
and therefore the norm data used by physicians to
evaluate the patients’ data depends on the needle type.
Subdermal and Twisted Pair Subdermal
Subdermal
needles
are
usually
used
for
Electroencephalography (EEG), Evoked Potentials (EP)
and during Intra-Operative Monitoring (IOM). Subdermal
Twisted pair needles are typically used for EP and IOM.
Ambu needle are made of stainless steel with a gauge
of 27, and have a tip with lancet cut. (See figure 3).
Figure 3: Subdermal (left) and subdermal twisted
pair needle.
The active recording surface is the entire needle and
the area depends on the inserted part. The configuration
depends on the neurophysiological examination to
be performed.
3. Examinations
The present section briefly explains the methodology
of the most typical neurophysiological examinations
and the use of the corresponding needle during these
examinations.
Electromyography (EMG) is a technique for evaluating
and recording the activation of muscles. EMG is
performed using an instrument called electromyography,
that produces a record called electromyogram. The
electromyograph detects the electrical potential
generated by muscle cells. The EMG signal is analyzed in
order to detect medical abnormalities in the muscle or
nerve.
The EMG can be measured by means of surface
electrodes or intramuscularly by using needle
electrodes. Ambu Neuroline concentric and monopolar
needles can be used with this purpose. To perform
intramuscular EMG, a needle electrode is inserted
through the skin into the muscle tissue. The muscle
activity at insertion, resting, and activation provide
valuable information about the state of the muscle and
its innervating nerve. The shape, size and frequency of
the motor unit potentials are evaluated for the
diagnosis. During the examination, the needle electrode
may be retracted and re-inserted repeatedly to find the
position where the muscle fibers activity is clearly
detected. Because skeletal muscles differ in the inner
structure, the electrode has to be placed at various
locations to obtain an accurate study.
Nerve conduction velocity (NCV) is a common
measurement made during EMG examinations. Surface
and needles electrodes can be used in this test. Nerve
conduction studies are used mainly for evaluation of
paresthesias (numbness, tingling, burning) and/or
muscle weakness. Some of the common disorders,
which can be diagnosed by nerve conduction studies,
are: Peripheral neuropathy, Carpal tunnel syndrome,
Ulnar
neuropathy,
Guillain-Barré
syndrome,
Facioscapulohumeral muscular dystrophy and Spinal
disc herniation.
Electroencephalography (EEG) is the recording of
electrical activity along the scalp produced by the firing
of neurons. In clinical contexts, EEG refers to the
recording of the brain’s spontaneous electrical activity
over a short period of time, usually 20–40 minutes, as
recorded from multiple electrodes placed on the scalp.
In neurology, the main diagnostic application of EEG is
in the case of epilepsy, as epileptic activity can create
clear abnormalities on a standard EEG study. Other
clinical use of EEG is in the diagnosis of coma,
encephalopathies, and brain death. EEG used to be a
first-line method for the diagnosis of tumors, stroke
and other focal brain disorders, but this use has
decreased with the advent of anatomical imaging
techniques such as MRI and CT.
EEG examinations are usually performed by using cup
electrodes. However, there are cases in which subdermal
needle electrodes are recommended to ensure
attachment and avoid movement artifacts.
Evoked Potential is an electrical potential recorded
from the nervous system following a certain stimulus
(visual, auditory, somatosensory) and is detected by
EEG or EMG. For example, visual evoked potentials are
elicited by a flashing light or changing pattern on a
monitor; auditory evoked potentials by a click or tone
stimulus presented through earphones, somatosensory
evoked potential are elicited by tactile or electrical
stimulation of a sensory or mixed nerve in the periphery,
and motor evoked potentials are recorded from muscles
following direct stimulation of exposed motor cortex,
or transcranial stimulation of motor cortex, either
magnetic or electrical. Combination of needle and
surface electrodes can be used in this examination.
These EPs have been widely used in clinical diagnostic
medicine since the 1970s, and also in intraoperative
neurophysiology monitoring (IOM).
Intraoperative monitoring (IOM) refers to diverse
procedures performed during surgery to monitor the
nervous system. For example, somatosensory evoked
potentials is used to monitore the spinal cord and the
brain during surgeries that may place these structures at
risk. Electrical stimulation of the scalp is used to produce
an electrical current within the brain that activates the
motor pathways of the pyramidal tracts thus allowing the
monitoring of the motor pathways. Surface or needle
electrodes may be used during IOM procedures.
4. Analysis and evaluation of
clinical data
This section presents a brief discussion of the following
topics evaluated as having significance for the clinical
performance and safety of the needles:
• Monopolar and concentric needle measurements
• Effect of the needles tip geometry on the
recorded signal
• Patient discomfort
• Effect of the needle material on the recorded signal
The review is based on scientific published literature,
clinical data generated during Clinical Investigations
performed by Ambu A/S and post marketing surveillance.
5.1 Published literature
Monopolar and concentric needles measurements
Monopolar and concentric needles can be used to
perform EMG examinations. However, the signal
measured with these two types of needles is significantly
different. The next section presents a discussion of the
characteristics, pros and cons of monopolar and
concentric needles:
Monopolar needles:
-
E xposure area: the recording area of monopolar
needles is the whole tip as compared to concentric
needles where the recording area is only a flat elliptical
face of the core wire. Thus, the measuring area of
monopolar needles (average area of 0.10 to 0.50
mm2) is approximately three times larger than the
total area of exposure of the concentric needles
(approximately 0.02 to 0.1 mm2), and the impedance
is also lower by a factor of several times. Thus
monopolar needles recruit activity from a larger
amount of muscle fibres (larger pickup field) than do
concentric needles [2]. As a result, the amplitude of
potentials recorded with monopolar needles is larger
than those recorded with concentric needles.
- Recording characteristics 1: The monopolar needles with
conical tip record symmetrical activity from fibres on all
directions, however, concentric needle record mainly
signals in the direction of the elliptical surface (bevel) and
must be rotated to pickup activity from fibers in other
directions. Thus, monopolar needles record the electrical
activity associated to a larger amount of muscle fibres
and therefore the amplitude of the recorded potential is
higher than the concentric needle.
-R
ecording characteristics 2: In the concentric needle,
the active electrode (core) and reference electrode
(cannula) are located close to each other, so both may
be recording the information of interest at the same
time. Theoretically, if the cannula has a potential
equally to the mean potential of the tissue in contact
with the cannula surface, the potentials will average
out and the cannula will end up being electrically
indifferent. However, if the cannula does not reach
the zero potential, there will be some loss of the
active signal due to cancellation between the core
and cannula potentials [2].
Concentric needles:
- Recording characteristics: In the concentric needles
the active (core) and reference (cannula) electrodes
are located very close to each other. This produces an
excellent common mode rejection and a more stable
signal. However, in monopolar needles, it may take
some time to find a good location for the reference
electrode in order to get a stable signal [3]. Moreover,
the measurement of concentric needles is more focal
and specific.
-
Reproducibility: Studies have shown that the
reproducibility of recordings performed with concentric
needles is higher than those performed with monopolar
needles [3]. As a consequence, there is a larger amount
of reference data (normal values) available of different
muscles and age class for concentric measurements
than for monopolar measurements.
Although monopolar and concentric needles present
different characteristics, the choice for clinical use
might depend on the training of the physicians
performing the studies, and their personal preference.
Effect of the needles tip geometry on the
recorded signal
Most monopolar needles are made of stainless steel
with a tip traditionally sharpened to a conical shape (or
bullet shape). The length of the electrical conductive tip
varies among manufacturers. The area of the conductive
tip is one of the most critical factors in the recording, as
it is directly related to the pickup field and the amplitude
of the recorded activity.
The EMG measurement is performed by inserting the
monopolar needle (active electrode) as close as possible
to the potential of interest. The reference electrode,
generally a surface electrode, is placed close to the
active electrode but not on the active tissue, and finally,
a ground electrode is placed relatively away from these
two electrodes. The recording area of monopolar
needles is the whole tip, and the bullet-shaped tip
results in the symmetrical recording of potential
traveling from all directions. Thus monopolar needles
recruit activity from a large amount of muscle fibers
(large pickup field) [2].
The area of exposure of the tip is the critical factor
affecting the potential recording [1,5] as this exposure
area is related to the amount of fibers that will be in
contact or in the proximity of the recording tip. The
exposure area of the needle tip is inversely related to
the needle impedance (Z=k/Area). Therefore, in principle,
tips with the same exposure area should record the
same potentials, independently of the tip shape.
Only one publication investigating the effect of the tip
geometry on the potentials was found. Wiechers et al.
[6] made electrical models of inter electrode impedance
for electrode systems with different geometries.
However, the electrode systems and geometries
compared were extremely different from each other
and the examples cannot be extrapolated to bullet and
elliptical needle tips in this case. The study used some
empirical data to suggest that two electrodes of same
exposure area but different tip shapes would influence
the recorded potentials. The data compares two bulletshaped monopolar needles from different fabricants.
One of the needle tips was blunt while the other was
longer and slender. The author mentions that the
impedances of these needles were different by a factor
of 4. Such a difference in impedance is likely due to
differences in the material of the needles (different
stainless steel used by the 2 fabricants) than the
geometry of them, and therefore, the data should be
interpreted cautiously.
Clinical data from the Ambu monopolar needles, shows
that the signal shapes are identical to bullet-shaped
needles and that the sharpness of the needle tip is
significantly superior.
Patient Discomfort
Regarding patient discomfort, there is not clear
evidence showing differences between needles types
of the same diameter [8]. Discomfort depends primarily
on the sharpness and diameter of the needle, and as
the diameters of concentric and monopolar needles are
very close, there is not clear evidence on this matter.
Some studies however, indicate that the pain perception
is different for concentric and monopolar needles
depending on the insertion technique and the size of
the needle movement [9]. Results indicate that when
small repositioning movements are used, the concentric
needle cause less discomfort than do monopolar
needles, but the opposite has been reported when
large repositioning movements are performed [9].
Some studies suggest [3] that when using a standard
insertion technique characterized by large needle
movements, the monopolar needles are less painful
than concentric needle electrodes. However, when
using small needle movements (1 mm or less), pain
associated with concentric needles was significantly
reduced with no difference between the needle types.
Other studies suggest that there is no difference in
pain perception between monopolar and concentric
needles [4,6]. These studies found that the examiner
experience, previous EMG examination, reported pain
tolerance, pain on nerve conduction studies and other
factors did not predict the amount of pain [4]. Regarding
gender, some studies found no difference in pain
perception between men and women [4] while others
found that females reported higher discomfort than
males [6]. Due to the variability and inconsistency of
the data available it is generally concluded that pain is
not an important selection criterion for type of needle
electrode. The patients’ perception is subjective and
more dependent on other factors than needle
characteristic.
In 2004 Ambu performed a clinical investigation of the
current concentric needle on 270 patients distributed
on 5 centres [i]. One aim of the study was to measure
pain as perceived by the patients. Pain was measured
on a Visual Analog Scale (scale length: 100 mm; scores:
0=no pain – 100=worst imaginable pain). The results
were (mean, (median)): Insertion of needle 21(14),
repositioning of needle 29(23), contraction of muscle
30(22), and withdrawal of needle 11(5). Compared to an
earlier study on concentric needles and comparable
studies found in the literature, this was considered a
low pain score. Females experienced more pain than
males during insertion of the needle.
In 2007 Ambu performed a Clinical Investigation to
evaluate the modified Neuroline concentric needle as
compared to 3 competitor needles [ii]. The
parameters
evaluated
were
the
discomfort
experienced by the subjects during a simulated-EMG
examination and the neurophysiologists’ opinions
regarding the needles performance.
Ten healthy male volunteers were recruited for the test,
and three neurophysiologists performed the simulatedEMG examinations to classify the needles. The
neurophysiologists and the volunteers were blind to
what needles was used. The investigators evaluated
the same 10 subjects in 3 different sessions.
Considering data from the investigators opinion
regarding the needles’ performance, the results indicate
that the neurophysiologists could feel a difference in
penetration and friction for the needles that had a
higher sharpness and reduced friction. Among the high
performing needles was the new Ambu Neuroline
concentric needle. However, data from the subjects’
evaluation were unclear. While the volunteers expressed
feeling a difference between the needles in each
session, their classification was not consistent across
the sessions and did not provide significant differences.
The tendency, suggests that there was no relationship
between the classification done by the investigators
and the one done by the subjects. Interestingly, these
results suggest that patient discomfort is likely more
related to other factors than specific technical
characteristics of the needles. Particularly, when the
needles compared have similar diameters and sharpness.
Effect of the needle material on the recorded signal
The Ambu Neuroline concentric needle is made of
stainless steel and silver. Most concentric needles on
the market have a core of platinum. The main advantage
of using silver (Ag) as compared with Platinum (Pt) is the
way Ag reacts with the biological tissue during the
recording of electrical signals (metal/saline, electrode/
electrolyte interface).
The electrode/ electrolyte interface can be modelled as
an electric circuit with a resistance (R) and capacitance
components (C). The electrical signal, generated in the
muscle runs though this circuit and therefore, the
properties of this electric circuit, i.e. the value of the
resistance and capacitance are very important. These
values depend partly on the metal used for the sensors.
Platinum is an inert material. This means, that Pt does
not react or reacts very slowly with electrolytes. As a
result of this, when a Pt-needle is inserted in the muscle
tissue, there is almost no reaction between the
electrode and biological tissue and therefore, there is a
relatively high polarization impedance in the interface.
Ag, however, is a more reactive metal and therefore the
polarization potential for low frequency signals is lower
as compared to Pt, while the values are similar for high
frequency signals [11].
The EMG signal has low and high frequency components
(mainly 0,1 – 500 Hz). This means that an Ag sensor
(with low impedance) affects much less the low
frequency components of the EMG, while for higher
frequencies both metals behave similarly. In conclusion,
studies demonstrate that the capability of transmitting
electrical current (conductance) is similar for both
materials [10]. However, Ag presents advantages with
respect to Pt for the lower frequency components of
the EMG signal.
5. Conclusion
The design of Ambu Neuroline needles has been optimized
during the last years to achieve a high performance. The
clinical data suggest that Ambu Neuroline needles are
among the best needles on the market.
The modifications introduced to the Ambu needles,
such as the sensor material of the concentric needle,
the shape of the monopolar needle, surface treatments
to reduce friction, and increase in sharpness of needle
tips, has resulted in a significant superior performance
as compared to other needles available on the market.
6. Reference list
Scientific literature:
1- Joynt RL. The Selection of Electromyographic Needle Electrodes. 1994,
Arch Phys Med Rehabil 75: 251-258.
2- Joynt RL. The concentric versus the monopolar needle electrode. The
case for the Monopolar Needles. 1998, Muscle and Nerve: Issues and
Opinions 1804-1806.
3- Trojaborg W. The concentric versus the monopolar needle electrode.
The case for the Concentric Needles. 1998, Muscle and Nerve: Issues
and Opinions 1806-1808.
4- Strommen JA and Daube JR. Determinants of pain in needle
Electromyography. 2001. Clinical Neurophysiology 112:1414-1418.
5- Balbierz JM, Petajan JH, Alder SC, Vlach SA. Differences in pain
perception in women using concentric and monopolar needles.
Arch Phys Med Rehabil. 2006 Oct;87(10):1403-6
6- Wiechers DO, Blood JR, Stow RW. EMG needle electrodes: electrical
impedance. Arch Phys Med Rehabil. 1979 Aug;60(8):364-9
7- Walker WC, Keyser-Marcus LA, Johns JS, Seel RT. Relation of
lectromyography-induced pain to type of recording electrodes. 1:
Muscle Nerve. 2001 Mar;24(3):417-20.
8- Trojaborg W. The concentric versus the monopolar needle electrode.
The case for the Concentric Needles. 1998, Muscle and Nerve: Issues
and Opinions 1806-1808.
9- Strommen JA and Daube JR. Determinants of pain in needle
Electromyography. 2001. Clinical Neurophysiology 112:1414-1418.
10- Ragheb and Geddes, Electrical Properties of metallic electrodes,
Medical and Biological Engineering & Computing (28): 182-186, 1990.
11- Ragheb and Geddes, The polarization impedance of common
electrode metals operated at low current density, Ann Biomedical
Engineering (19):151-163, 1991.
Clinical Investigations performed by Ambu A/S
(i). A MBU Clinical Investigation Report: A new improved design of the
Neuroline Concentric EMG needle-A Multicentre, Post-marketing
Trial. (2004) N-4-III-01.03.
(ii). A
mbu Clinical Investigation Report: “Sammenlignende undersøgelse
af 5 koncentriske EMG nåle” Post Marketing Trial (2006) N-PS-5-2006.