WO2001089378A1

WO2001089378A1 - Needle electrode for electromyography

Info

Publication number: WO2001089378A1
Application number: PCT/DK2001/000357
Authority: WO
Inventors: Jan Paustian
Original assignee: Medicotest A/S
Priority date: 2000-05-24
Filing date: 2001-05-23
Publication date: 2001-11-29
Also published as: AU2001260088A1

Abstract

The invention concerns a needle electrode for electromyography (EMG), comprising an electrically conductive core separated from an external cannula by an electrically insulating material, said needle electrode, at a first end, being adapted for introduction into a tissue and comprising an exposed surface of the core and, at a second end, being adapted for establishing electrical contact with an electrical appliance suitable for providing an electromyogram, wherein, at the first end, the surface of the core is provided with a layer of a metal shaft. The needle electrode possesses a low impedance.

Description

TITLE

Needle electrode for electromyography.

INTRODUCTION The present invention relates to a needle electrode suitable for electromyography (EMG) . Furthermore, the invention relates to a process for the preparation of a needle electrode.

BACKGROUND FOR THE INVENTION

The needle electrodes of the kind disclosed in the present description with claims is generally referred to as concentric needle electrodes (CNE) . CNEs was first described in 1929 by Adrian and Bronk in J^". Physiol . (Lond) 1929, 67:119-151 and have found widely acceptance in diagnostic clinical electromyogram (EMG) . The CNE is used to gather information about the bio-electrical condition of a muscle. When using the CNE, the needle end of the CNE is introduced through the skin and into the muscle fibre of interest. If the CNE is properly inserted into a motor unit of a muscle, the action potential signal can be measured by the tip of the needle. The electrode is connected to an electrical appliance suitable for converting electrical signals measured by the CNE into an electromyogram.

The parameters of the motor unit action potential examined in diagnostic EMG are known to vary with the physical properties of the CNE. Also, properties of the CNEs contribute to the technical quality of EMG recordings. Therefore, to obtain a high quality recording, it is essential to use a CNE having a design and choice of construction optimising the measurement of the individual parameters. The electrical characteristics of a CNE influencing the final electromyogram are inter alia impedance and noise generation.

The design and use of materials at the tip of the CNE is of major importance for the impedance. For instance, it is known that an increased area of the core surface decreases the impedance. However, increasing the diameter of the needle to allow for an increase of the core surface area is only suitable to a certain extent due to patience . compliance. Furthermore, the desirability to record from only a small number of muscle fibres establish an upper boundary on the suitable core surface area.

Another way to improve the impedance is to treat a CNE electrolytically before used, as first described by Buchthal et al : Action potentials in normal human muscle and their dependance on physical variable, Acta

Physiol Scand, 1954, 32:200-218. As an example, the needle electrode can be connected as cathode and treated in 10 seconds at a frequency of 20 to 100 Hz. However, such electrolytical treatment is in many cases transient and the effect of the treatment is quickly reduced during handling or wetting of the electrode tip. To be effective, the electrolytical treatment should be performed within a few hours prior to the use of the CNE and the CNE should be subjected to a minimum of handling during the time interval between treatment and application. If the electrolytical treatment is prepared immediately prior to use an improvement of the reduction of the impedance of 1.5 to 4 times can be reached.

The electrolytical treatment was extensively used for the multi-use CNEs. However, to-day most clinics uses presterilised, disposable, ready-for-use CNEs. The preferential use of disposable (or single-use) CNEs has halted the procedure of electrolytical treatment, properly because the procedure is time consuming and involves an extra procedural step before the recording of an electromyogram. Therefore, the disposable electrodes essentially show the same relatively high impedance as the CNEs for multi-use. The high impedance and the variation thereof among individual electrodes from the same manufacture generates noise on the measurements and uncertainties on the results. The generation of noise is at least partly due to a high impedance because the CNE functions as an antenna.

The major sources of noise influencing the final electromyogram are ambient, i.e. external to the patient . The electro-magnetic radiations from the various electrical appliances and lights in a modern clinic is recorded by the CNE. Usually, the noise is in the lower frequence range, i.e. about 50 or 60 Hz.

Thus, to be an effective CNE, a low impedance should preferentially be present at the lower frequency range.

It has been attempted to reduce the significance of the noise generation by shielding the connecting leads. However, the shielding usually does not reduce the noise generation effect more than 30%.

In the remote technical area of measuring electrical signals in the cardiac cavity, it has been suggested in US 4.922.912 to use a monophasic action potential (MAP) catheter comprising a pair of Ag-AgCl electrodes. The MAP catheter is adapted to be guided into the body through a flexible liner and is hardly usable as a needle electrode because one of the Ag-AgCl electrodes is projecting at a peripheral surface. This design will hamper insertion of the electrode or at least be unpleasant for the patient. Furthermore, MAP catheters are not suitable for skeletal muscles . They have their major use in the measuring of electrical signals in smooth muscles, such as the heart. Also, the geometrical design of the electrode may change the shape of the recorded electromyogram, such that a direct comparison with a recording from a CNE electrode would not be possible. In one aspect of the invention it is the object to provide a needle electrode having reduced impedance and thus, reduced sensitivity toward electromagnetic radiation. Furthermore, in another aspect of the invention, it is the object to obtain a process for production a needle electrode having a durable relatively low impedance.

DISCLOSURE OF THE INVENTION

The present invention concerns a needle electrode for electromyography, comprising an electrically- conductive core separated from an external cannula by an electrically insulating material, said needle electrode, at a first end, being adapted for introduction into a tissue and comprising an exposed surface of the core and, at a second end, being adapted for establishing electrical contact with an electrical appliance suitable for providing an electromyogram, wherein, at the first end, the surface of the core is provided with a layer of a metal salt. The electrically conductive core can be of any- suitable conducting material . Examples of suitable materials are platinum, platinum alloy, stainless steel, nickel/chromium alloy, iridium and silver. Silver is generally preferred due to the eminent conductive abilities thereof and the relatively low price .

The electrically conductive core is separated from the external cannula by an electrically insulating material. Suitably, an electrically insulating material, such as polyethylene, polyvinylchloride or a varnish, is applied to the core throughout the entire longitudinal direction. The core covered by the electrically insulating material is fixed inside the external cannula by a suitable adhesive. A suitable adhesive is preferably an electrically insulating material. A preferred adhesive is of the epoxy type.

As the external cannula suitably is used as the reference electrode an electrically conductive material having the desired electrical and physical characteristics is generally used. Preferably, the electrically conductive material for the cannula is prepared of metal or metal alloy. An example of a preferred material for the cannula is stainless steel. If an external reference electrode is used, however, any non-conducting material having the desired physical characteristics may, in principle, be used.

The exposed surface of the core is suitable provided by cutting the assembly provided of the core, electrically insulating material and the external cannula at a desired angle. If the angle relative to the longitudinal direction of the assembly is 90° the area of the exposed core surface is the lowest possible ensuring a local measurement. However, it is in general desired to cut the cannula in an angle of 10° to 45° relative to the longitudinal direction of the assembly to increase the core surface area and to provide the needle with a sharp point making it easier to introduce the needle into the skin. When cut at an angle of less than 90° the geometric form of the core surface is an ellipse. The elliptic core is coaxially surrounded by the electrically isolating material and the external cannula, hence the reference electrode. The surface area of the core is typically 0.02 to 2 mm², preferably 0.05 to 1 mm². According to the invention the surface of the core is provided with a layer of metal salt. The metal salt is preferably a metal halide, such as metal chloride, metal bromide, or metal iodide. The metal of the metal salt is preferably of a type also present in the core. Examples of the metal in the metal salt are silver, chromium, iron, nickel, copper, and platinum. Specific examples of suitable metal salts are AgCl , AgBr, CuCl₂, CuBr₂, ZnCl₂, and FeCl₃. When the core contains silver, preferably the metal salt is AgCl .

In designing the needle electrode of the invention a proper balance of geometry and material should be considered, since the area as well as the material contribute to the impedance. Suitably, the cannula is constructed of a metal or a metal alloy and has an area larger that the surface of the core, due to the fact that the impedance per square metre is higher for the metal or metal alloy compared to the metal salt . Optimally, the area of the cannula and the core is selected so as to compensate for any irregularity in impedance .

The metal salt can be provided on the core in any suitable way. As examples, the metal salt can be provided on the core by compressing an amorphous metal salt, sputtering or electrolytical application. Preferably, the metal salt is provided on the core by electrolytical application.

Thus, the invention does also pertain to a process for the preparation of a needle electrode, comprising the steps of i) providing a needle electrode comprising an electrically conductive core separated from an external cannula by an electrically insulating material, said needle electrode, at a first end, being adapted for introduction into a tissue and comprising an exposed surface of the core and, at a second end, being adapted for establishing electrical contact with an electrical appliance suitable for providing an electromyogram, ii) immersing a part of the needle electrode comprising the first end into an electrolyte, iii) connecting the core of the needle electrode to a power source as anode and an auxiliary electrode as cathode, and iv) energising the power source to produce a layer of a metal salt at the exposed core surface.

The electrolyte may be any media with the ability to conduct the current therein. Preferably, the electrolyte is aqueous. Furthermore, the electrolyte preferably contains the anion of the metal salt to be formed on the exposed core surface . The anion may be any one which forms a metal salt on the core surface suitable for measurement of a motor unit action potential. Suitable anions are halide ions, such as chloride, bromide and iodide ions. A preferred anion in the electrolyte solution is the chloride ion (Cl^") . The anion can be provided in the electrolyte solution in any suitable way. By way of example the anion may be provided by dissolution of a salt containing the anion or by providing the corresponding acid. Preferably the anion is provided in the electrolyte solution by adding the corresponding acid to a suitable amount of water, because the acidic nature of the electrolyte accelerates the deposition of metal salt during the electrolysis. Suitable acids are HC1, HBr, and HI, preferably HCl .

The electrically conductive core can be of any suitable conducting material . Examples of suitable materials are platinum, platinum alloy, stainless steel, nickel/chromium alloy, iridium, and silver. Silver is generally preferred due to the eminent conductive abilities thereof and the relatively low price. In a preferred embodiment of the present invention, the metal of the core is converted during the electrolysis process to the cation of the metal salt formed on the surface of the core .

The auxiliary electrode may be any electrically conductive material. -Furthermore, the auxiliary electrode may be insoluble or soluble during energising of the power source. Suitable examples of insoluble auxiliary electrodes are electrodes containing carbon, silver, palladium and/or platinum. Suitable examples of soluble auxiliary electrodes are electrodes containing aluminium, zinc, magnesium and/or lead. Soluble electrodes may be suitable if a metallic anion is desired. However, preferably the auxiliary electrode is insoluble and the metal of the core is used as the metal cation of the metal salt to be formed on the surface of the core. The auxiliary electrode is present in the same electrolyte solution as the needle electrode, however, without direct physical contact. The electrical contact between the needle electrode and the auxiliary electrode is conveyed by the electrolyte solution. The power source for the electrolytical process is a DC power source . The core of the needle electrode is connected to the positive pole as anode and the auxiliary electrode is connected to the negative pole as the cathode. Following the connection of the electrodes the power source is energised. A suitable current and voltage is selected in accordance with the selected materials and design of the needle electrode, the auxiliary electrode as well as the composition of the electrolyte. The skilled person can easily select a suitable current and voltage through simple trial and error experimentation.

With the purpose of illustrating the present invention only it is believed, when silver is selected as the core material, aqueous hydrochloric acid is selected as the electrolyte and silver is selected as the material of the auxiliary electrode, that the anode reaction (I) and the cathode reaction (II) is as follows :

Ag + Cl^" -> AgCl + e^" (I)

H⁺ + e^~ -> % ₂ (II)

Thus, in accordance with the anode reaction (I) , AgCl is deposited at the surface of the core of the needle electrode and, in accordance with the cathode reaction (II) , hydrogen gas is liberated at the cathode . In the preferred embodiment of the present invention, in which the metal of the core is transformed to the cation of the metal salt during the electrolysis process, a suitable electrical contact between the metal salt and the core metal is believed to occur.

During the electrolysis process, that is following the energising of the power source, the treatment time is decisive for the amount of metal salt formed on the surface of the core if the current and the voltage has been fixed. Thus, the thickness of the layer of metal salt provided on the surface of the core can be estimated as the charge supplied per surface area of the core . According to a preferred embodiment of the present invention the charge supplied per surface area of the core is at least 200, preferably 400 mC/cm², while the maximum charge supplied per surface area of the core preferably does not extent 4000, preferably 1500, and most preferred 1000 mC/cm².

The initial needle electrode, used for application of a layer of metal salt at the surface of the core, may be provided in any suitable way securing that the core is electrically insulated from the external cannula. A conveniently way of producing the initial electrode is to cover a wire of the material desired for the core with an electrically insulating material of, e.g. a plastic material such as polyvinylchloride

(PVC) or polyethylene (PE) or a varnish. Generally a varnish is used for covering the wire. The covered wire is provided in the interior of the cannula and the intervening space between the covered core and the interior of the cannula is at least partly filled with an adhesive having a sufficient low viscosity to be able to flow from the outside into the intervening space. Subsequently the assembly is allowed to cure and the end thereof intended to be introduced into the subject is cut in a desired angle and ground to expose the core surface. The second end of the assembly is provided with a connecting plug able to engage with a corresponding connecting wire. The connecting wire connects the assembly with the appliance able to transform the measured signals into an electromyogram. Suitably, the external cannula is adapted for connection as a reference electrode, while the core is adapted for connection as a measurement electrode. However, if so desired, it is possible to use an external electrode as reference electrode. The external electrode may be introduced in the tissue next to the needle electrode or elsewhere in the body of the subject, or the external electrode may be immersed in a reference solution. Furthermore, a non-invasive electrode may be used as reference electrode. The non- invasive reference electrode may be placed on the skin or a mucous membrane of the subject. As an example, when the non-invasive reference electrode is placed on the skin, electrodes of the type used for electrocardiography may be used.

The needle electrode according to the present invention shows excellent low impedances at high as well as low frequencies. The improvement of the needle electrodes according to the invention is most pronounced for the low frequencies, however, improvements throughout the whole frequency range is observed. Moreover, the variation among electrodes prepared in accordance with the same procedure is remarkable low indicating that less variability among the individual needle electrodes occur when a layer of metal salt is provided on the exposed core.

The invention is illustrated in the examples below. The examples are intended for elucidation of the invention only and must not be construed as limiting the same .

EXAMPLES

Example 1

Preparation of needle electrode having chlorinated core surface.

A needle comprising a silver core and an external cannula of stainless steel was prepared by inserting a silver wire covered by an electrical isolating varnish into the lumen of the cannula and subsequently fixing the silver wire to the interior of the cannula walls with an epoxy adhesive. The tip of the cannula was cut and ground in a angle of about 15 degrees relative to the longitudinal direction of the needle. The area of the exposed surface of the core on the tip was 0.00068 cm². In this and the subsequent examples said electrode is referred to as an untreated needle electrode. The tip of the needle was immersed in a solution of 21 ml HC1 (37%) in 500 ml demineralised water. The pH of the solution was 0.42 and the temperature was 22°C. Also, an auxiliary electrode of silver was provided in the solution. A DC power supply unit was provided and the core of the cannula was connected as anode and the auxiliary silver electrode was connected as cathode. A current of 3.3 μA at a voltage of 32.8 V in different time intervals as depicted in table I was supplied to each needle. The charge supplied to the exposed area of the core of the needle was used as an indicator for the amount of deposited silver chloride at the tip of the cannula and calculated as:

(Current [mA] ) x (time [s] ) _ _.rh____ _.„ _„„ _[τr._{r / nπ}.2-, (Area of exposed core [cm²] ) ^{" char}9^e P^{er area} LmC/cm ]

The impedance of the needles treated as described above was measured for two frequencies, viz. 10 Hz and 10kHz. A signal generator generating a sinus curve was used. In the circuit a resistor of 1 MOhm was included.

TABLE I

It may be inferred from table I that a lower impedance is obtained for all treated needles, compared to the untreated needles, irrespective of the amount of deposit and the frequency used. However, the lowest impedances at a frequency of 10 Hz are obtained in the interval from 200 to 1500 mC/cm² deposit, with the lowest impedance level at 400 mC/cm² deposit. Also, it may be inferred from table I that the impedance of the treated as well as the untreated needle electrodes are more sensitive to low than high frequencies. Example 2

To study the effect of the frequencies on the impedance, treated and untreated needle electrodes was subjected to various frequencies. First, an untreated needle was arranged in an electrical circuit involving a signal generator and a resistor of 1 MOhm. The signal generator was set to perform a sinus curve . The impedance was measured for the frequencies 10, 50, 100, 500, 1000 and 10,000 Hz. Next, the needle was released from the above circuit and treated in accordance with experiment 4 of Example 1, i.e. with a charge density of 500 mC/cm². The impedance of the treated needle electrode was then measured again for the frequencies indicated. The above procedure was repeated for a total of 4 needle electrodes. The results are indicated table II below:

Table II:

It may be inferred form table II that a treatment with 500 mC/cm² lowers the impedance for all frequencies investigated. The most significant improvements are obtained at the low frequencies. Example 3

Measurement of noise generated by the needle electrode itself before and after chlorination.

An EMG apparatus (Medtronic Keypoint) was used to measure the noise generated by 10 untreated needle electrodes as well as the same 10 needle electrodes treated in accordance with experiment 4 of Example 1. The noise was measured as the amount of mV from the lowest to the highest peak (mV p-p) of the base line. The results are shown in table III below:

Table III

It can be derived from table III that the noise is approximately 1/10 for the treated needle electrodes compared to the untreated. Moreover, the standard derivation is less for the treated needle electrodes compared to the untreated ones, indicating that less variability among the individual needle electrodes occur for the treated ones .

Claims

P A T E N T C L A I M S

1. A needle electrode for electromyography, comprising an electrically conductive core separated from an external cannula by an electrically insulating material, said needle electrode, at a first end, being adapted for introduction into a tissue and comprising an exposed surface of the core and, at a second end, being adapted for establishing electrical contact with an electrical appliance suitable for providing an electromyogram, wherein, at the first end, the surface of the core is provided with a layer of a metal salt .

2. The electrode according to claim 1, wherein the core contains silver.

3. The electrode according to claim 1 or 2 , wherein the layer of metal salt provided on the surface of the core corresponds to a charge per area of 200 to 1500 mC/cm².

4. The electrode according to any of the preceding claims, wherein the metal salt provided on the surface of the core is silver chloride.

5. The electrode according to any of the preceding claims, wherein the outer cannula is of stainless steel .

6. Process for the preparation of a needle electrode comprising the steps of i) providing a needle electrode comprising an electrically conductive core separated from an external cannula by an electrically insulating material, said needle electrode, at a first end, being adapted for introduction into a tissue and comprising an exposed surface of the core and, at a second end, being adapted for establishing electrical contact with an electrical appliance suitable for providing an electromyogram, ii) immersing a part of the needle electrode comprising the first end into an electrolyte, iii) connecting the core to a power source as anode and an auxiliary electrode as cathode. iv) energizing the power source to create a metal salt at the exposed core surface.

7. The process according to claim 6, wherein the electrically conductive core comprises a metal which is converted during step iv) to the cation of the metal salt formed at the exposed core surface .

8. The process according to claim 6 or 7, wherein the core contains silver.

9. The process according to any of the claims 6, 7 or 8, wherein the electrolyte contains chloride ions .

10. The process according to any of the preceding claims, wherein the electrolyte contains hydrochloric acid.

11. The process according to any of the claims 6 to 8, wherein the power source in step iv) is energized such that a charge per core surface area of 200 to 1500 mC/cm² is obtained.