The invention concerns an electrode arrangement for a surgical instrument for electrothermal coagulation of tissue, which includes a front cylinder at the distal end of the instrument having a distal tip, a carrier proximally adjoining the front cylinder, and two electrodes which are connectable to an ac voltage source.
Using the application of high-frequency alternating currents (for example in the frequency range of between 300 KHz and 2 MHz) to generate high temperatures for tissue coagulation and for tissue division has long been known in surgery. In a practical context, so-called monopolar electrode arrangements or bipolar electrode arrangements are used for introducing the HF-current into the tissue.
In the case of the monopolar arrangements, an electrode—also referred to as the neutral electrode—is applied in the form of an electrode of large area to the skin of the patient in the proximity of the treatment location and fixed there and earthed or connected to ground. A second electrode which is manipulated by the operator—also referred to as the active electrode—is connected to the ac voltage source. In terms of its shape, the electrode is so adapted to the respective use, in particular the size of the tissue area to be treated, that both the operational time and also the thermal loading of the region of the body or organ involved are reasonable and they coagulate only the desired area of tissue.
In the case of arrangements for bipolar HF-thermotherapy, both electrodes are connected to an HF-generator and arranged with dimensions which are fixed with respect to each other, for example on an insulating carrier, and are placed by the operator in the immediate proximity of the treatment location and generally also actively guided.
WO 97/17009 discloses a bipolar electrode arrangement with a fluid duct by way of which flushing fluid can be introduced into the operational area. Two or three electrodes are arranged in the form of a cone portion on a conical distal tip of the instrument which can be introduced into the tissue, wherein the electromagnetic HF-field is formed between the electrodes and is intended to coagulate the surrounding tissue.
WO 96/34569 and the documents referred to in the associated international search report disclose systems and processes for the coagulation of body tissue while maintaining a pre-calculated maximum tissue temperature, in which fluid cooling or thermoelectric cooling is provided during the actual tissue coagulation procedure. Those known arrangements are intended for the introduction into body cavities by way of natural accesses.
U.S. Pat. No. 4,832,048 as well as WO 95/10320 and WO 99/11186 or EP 96 945 879.3 and WO 98/19613, WO 96/18349 and WO 81/03272 also disclose surgical instruments which treat tissue by means of HF-thermotherapy by means of a bipolar electrode arrangement.
The known surgical instruments for bipolar HF-thermotherapy are often expensive to manufacture and, for the various areas of use involved, they often suffer from disadvantages which frequently result in locally inaccurate tissue treatment which in particular in part does not reach the tissue to be treated or thermally overloads sound tissue.
Therefore the object of the present invention is to develop an electrode arrangement for a surgical instrument of the kind set forth in the opening part of this specification, such that it is simple to manufacture and use and permits precise localizable treatment of the tissue, while at the same time sparing surrounding sound tissue.
That object is attained by an electrode arrangement for a surgical instrument for thermoelectric coagulation of tissue, including an electrically conductive front cylinder at the distal end of the instrument, with a distal tip and with a cylindrical first electrode, a tubular outer conductor proximally adjoining the front cylinder and with a cylindrical second electrode, an insulator element between the front cylinder and the outer conductor, wherein the electrodes are connectable to an ac voltage source, characterised by a bar-shaped inner conductor in the outer conductor and an insulating tube between the inner conductor and the outer conductor.
The advantages of the invention are in particular that the electrode arrangement is of a particularly simple structure, wherein more specifically the front cylinder forms an electrode and that the proximally adjoining outer conductor which is insulated by way of an insulator element forms the second electrode so that the ac voltage source is connectable to the front cylinder and from the outside directly to the outer conductor by way of the bar-shaped inner conductor which is separated by an insulating tube outer conductor.
The insulating element has between the front cylinder and the outer conductor a radial partition which at the outside goes into a cylindrical casing wall which surrounds the front cylinder and/or the outer conductor in closely fitting relationship therewith over a predetermined axial lengthwise portion. The inner conductor is connectable to the front cylinder through that partition of the insulating element. Preferably, embodied between the front cylinder and the inner conductor is a releasable screw connection in which the inner conductor carries at its distal end a male screwthread which can be screwed to a corresponding axial female screwthread in the front cylinder. The advantages of this embodiment are in particular that the cup-like insulator element can be applied in the form of an insulating layer directly to the metal front cylinder and/or the metal outer conductor, in such a way that the separating surface between the front cylinder and the outer conductor and a lengthwise portion, axially adjoining same, of the outside surface has a coating with an insulating material. A coating of that kind can be produced for example in accordance with the invention by anodization of the surfaces in question in an electrolyte bath if the front cylinder and/or the outer conductor comprise an anodizable metal, for example titanium or aluminum.
Depending on the respective purpose of use the electrode arrangement for an instrument can be of a flexible nature so that in that case the inner conductor, the outer conductor, the insulating tube and possibly also the insulator element comprise an elastic material. In the case of a flexible surgical instrument of that kind, the bipolar electrode arrangement can under some circumstances be more easily moved to the specific treatment location. Alternatively however the inner conductor and the outer conductor can be of a straight and rigid configuration, in which case then the front cylinder and the outer conductor are arranged in mutually coaxially aligned arrangement and can then be moved to the treatment location by a rectilinear translatory movement. In regard to certain treatment locations it may also be particularly advantageous to angle the instrument in the lengthwise direction.
In all embodiments the outer conductor and the front cylinder are of substantially the same outside diameter in order to provide for unimpeded sliding movement of the electrode arrangement in the tissue.
Preferably the front cylinder, over its axial lengthwise portion which is not covered by the insulator element, forms the first electrode and the outer conductor, over the entire axial lengthwise portion, insofar as same is not covered by the insulator element, forms the second cylindrical electrode. The axial length of the electrodes is preferably greater than the axial length of the insulator element and it is also greater than the outside diameter of the front cylinder and the outer conductor. Preferably the length of the outer conductor is a multiple of the length of the front cylinder. If, in this embodiment, the tissue adjoining the outside surface of the instrument is coagulated and as a result becomes of high resistance, then in this embodiment of the invention the electromagnetic field can be displaced outwardly into adjoining tissue regions because there is a suitably long second electrode so that, if the tissue has become of high resistance adjoining the outside surface, the electromagnetic field can move radially outwardly and in so doing still ends at the second electrode. With this embodiment therefore it is possible to implement a coagulation effect which moves in a defined fashion into the tissue and which comes to an end when the field extends from the first electrode to the proximal end of the second electrode.
Conversely it has been found that the commencement of coagulation is optimal when the two electrodes are at a relatively small axial spacing from each other, which is approximately of the order of magnitude of the outside diameter or only slightly larger.
In a preferred embodiment of the invention the inner conductor and the front cylinder are provided with a central hollow duct which issues from the distal tip of the front cylinder and which contains an optical waveguide which can be acted upon with visible laser light. In that way light can be passed to the tip of the applicator. If then for example that applicator is inserted into thin-walled regions of the body, for example in the nasal concha for therapy of concha hyperplasia, then the light issuing at the tip makes it possible to locate the position of the tip in the nasal concha by the doctor implementing the treatment using the naked eye. The doctor can therefore see at any time where the tip of the electrode arrangement is disposed, in the nasal concha. The above-described surgical instrument is suitable for dynamic use, for example it is inserted into the enlarged nasal concha and then, with HF-power activated, withdrawn from the nasal concha, thereby producing a tubular coagulation zone which has formed around the path of the electrode arrangement.
In accordance with a further preferred embodiment of the invention, the front cylinder which is electrically connected to the inner conductor and the outer conductor are separated from each other by an insulating annular body. Preferably the insulating annular body is made from translucent or partially translucent material and disposed in the annular body is a light source which discharges its light to the exterior through the annular body, preferably in the form of stray or scattered light. In accordance with a preferred embodiment of this instrument the inner conductor has a hollow duct which terminates in the insulating annular body and which accommodates an optical waveguide. In the region of the annular body radially into the fiber core of the optical waveguide the inner conductor is provided with incisions so that light issues radially at the incisions from the optical waveguide and, through the annular body, makes that zone of the bipolar electrode arrangement which is between the two electrodes and in which therefore coagulation of the tissue respectively occurs, visible to the doctor carrying out the treatment. In the case of thin-walled bodies therefore the doctor performing the treatment always sees directly with his own eyes that location at which coagulation is taking place. It is thereby possible to treat the tissue in a particularly accurate and local fashion. Depending on the requirements involved the tip of the front cylinder may be conical or wedge-shaped.
The object of the invention is further attained by an electrode arrangement for a surgical instrument for electrothermal coagulation of tissue, including a front cylinder at the distal end of the instrument with a distal tip, an elongate, electrically insulating carrier proximally adjoining the front cylinder, at least two spaced electrodes on the carrier, which are connectable to an ac voltage source, characterised in that the electrodes are of a strip-shaped configuration and extend along the carrier.
The advantages of the invention lie in particular in the simplicity of structure and the electrodes which extend in a strip-shaped configuration along the carrier. Particularly when the two electrodes extend parallel to the longitudinal axis of the carrier, this electrode arrangement is suitable for therapy in respect of concha hyperplasia. The applicator, that is to say the electrode arrangement, is used statically in that case, in other words, the electrode arrangement is pierced into the enlarged nasal concha and remains fixed in a position with active HF-power. In that procedure, due to the specific electrode configuration involved, the desired tubular coagulation zone is produced without the need to move the applicator in the tissue. A further advantage is the particularly simple structure which makes it possible to implement connection of the HF-generator at the proximal end of the carrier directly—from the exterior—to the electrodes.
If the electrodes extend parallel to the longitudinal axis of the carrier and for example are disposed in mutually diametrally opposite relationship on the preferably circular carrier cross-section, then the result is two coagulation zones extending in the longitudinal direction, between the two electrodes. If in contrast the electrodes are disposed on the carrier along spaced helical lines, then a correspondingly helical zone of the tissue is treated and coagulated. In the event of an additional axial movement of the electrode arrangement, a circular coagulated enclosing passage is then formed around the carrier.
In accordance with a particularly preferred embodiment of the invention the carrier is in the form of a metal tube carrying an externally disposed insulating layer on which the strip-shaped electrodes are disposed. If the metal tube used comprises a metal which can be anodically oxidized in an electrolysis bath, for example therefore titanium or aluminum, then production of the insulating carrier is particularly simple, more specifically if the outside surface of the carrier is electrolytically anodized to form a layer of titanium oxide or aluminum oxide. In this embodiment also it is possible to draw through a hollow duct which passes axially through the carrier and issues at the tip of the front cylinder, an optical waveguide which makes the position of the tip of the front cylinder visible to the operator—when dealing with correspondingly thin tissue—or in the event of implementing a treatment just under the skin, whereby the operator can guide the arrangement in properly targeted fashion. The optical waveguide can be supplied for example with visible laser light. The distal tip of the front cylinder is advantageously either of a conical or a wedge-shaped configuration and the electrodes are applied in the form of thin conducting metal layers to the carrier.
In accordance with a preferred embodiment of the invention the carrier can be made from a flexible material on which the strip-shaped electrodes are carried. The carrier used can be for example an optical waveguide, on the insulating outer sheath of which the electrodes are disposed elastically and in a strip-shaped configuration. The electrode arrangement can then more easily be introduced through body openings to the treatment location.
In this embodiment also the axial length of the electrodes is preferably greater than the outside diameter of the front cylinder and carrier which are both preferably of the same outside diameter in order to permit the electrode arrangement to be easily introduced by a sliding movement into the tissue.
The object of the invention is further attained by an electrode arrangement for a surgical instrument for electrothermal coagulation in the tissue, including a front cylinder of metal or insulating material at the distal end of the instrument, the front cylinder is frontally pointed or rounded-off, an elongate carrier proximally adjoining the front cylinder, two electrodes which extend in the longitudinal direction of the carrier and which are connectable to an ac voltage source, characterised in that the carrier includes externally disposed, self-supporting metal bar profile members which extend in the longitudinal direction and which are connected together by means of one or more insulating spacer elements and form the electrodes.
The advantages of the last-mentioned embodiment of the invention are in particular that the electrodes extend in the longitudinal direction of the carrier and are self-supporting metal bar profile members forming the carrier, whereby manufacturing steps can be omitted in manufacture of the electrode arrangement. In a particularly preferred configuration the one or more insulating spacer elements between the electrodes is or are optical waveguides which are guided in the longitudinal direction between the bar profile members and give off light to the distal tip of the instrument so that the operator can at any time locate the tip of the instrument by visual means when the instrument is used in thin-walled parts of the body. In a particularly preferred feature the externally disposed visible optical waveguides can also be provided with radial countersinks or incisions ground therein, which provide that light also issues radially at those locations. That accordingly shows the operator over what axial distance the instrument—with HF-energy activated—is also coagulating tissue. The cross-section of the bar profile members corresponds, when considered together, preferably to a circular surface area, but alternatively the cross-section of the bar profile members can also be in the form of a portion of the periphery of a tube, in which case then the bar profile members are for example preferably fixed in mutually opposite relationship on the outside sheath of an optical waveguide and in that way form a rigid carrier with externally disposed strip-shaped electrodes extending in the longitudinal direction.
The object of the invention is further attained by an electrode arrangement for a surgical instrument for thermoelectrical coagulation in tissue, including a front cylinder of metal or insulating material at the distal end of the instrument, wherein the front cylinder is frontally pointed or rounded-off, an elongate carrier proximally adjoining the front cylinder, and two electrodes which extend in the longitudinal direction of the carrier and which are connectable to an ac voltage source, characterised in that the electrodes (2, 4) are cylindrical tube portions (82, 84) of metal which are arranged at a predetermined spacing from each other in axial alignment with the carrier in the longitudinal direction.
In accordance with this preferred embodiment of the invention the first electrode can be in the form of a self-supporting tube portion which is carried between the front cylinder and an insulating tubular first carrier and the second electrode can also be in the form of a self-supporting tube portion arranged between the first carrier and a second tubular carrier, wherein the end portions of the electrodes are supported on the front is cylinder, the first and the second carrier over a predetermined lengthwise portion. Alternatively it is also possible for the second electrode to extend as far as the proximal end portion. Particularly preferably in this embodiment the lengthwise portions of the electrodes which bear on the first and/or the second carrier are covered with an insulating layer. In addition, provided within the hollow duct is a flushing tube which extends from the proximal end of the instrument to the front cylinder, that is to say also through the tube portions which form the electrodes, and extends as far as the front cylinder and discharges fluid at the distal end into the hollow duct in which the fluid—in contact with the electrodes—flows back to the proximal end of the instrument.
Cooling of the electrode surfaces by means of a flushing fluid means that the so-called “hot-spot” of coagulation is displaced by approximately between two and three millimeters from the surface of the instrument into the tissue. The cooling action ensures that the tissue-electrode contact surface is always kept below a predetermined temperature and therefore does not dry out to such a severe degree, so that the introduction of energy into the adjoining tissue is also guaranteed, over a relatively long period of time. It is quite particularly advantageous in that respect that the lengthwise portions of the electrodes which rest on the insulating carriers—or in a specific embodiment also on the front cylinder which is in the form of an insulating body—and are therefore not directly cooled by the cooling fluid, are covered with an insulating layer. In that way those lengthwise portions which are cooled to a lesser degree and which consequently heat up to a greater degree than the cooled electrode portions are covered over by the insulator and therefore come into contact with the adjoining tissue only by way of the comparatively cooler insulating layer. Thus, the consequence of covering over the end portions of the electrodes, which are not cooled by the flushing fluid, by means of insulating layers, is that the adjoining tissue does not become too hot even in those longitudinal portions, and thus it does not dry out.
In a preferred embodiment of the invention a self-supporting metal tube is provided between the front cylinder and the carrier. A distal portion of the metal tube serves as the first electrode, an adjoining proximal tube portion is surrounded by a cylindrical insulating layer and carries on that insulating layer a metal layer which serves as the second electrode. Alternatively, the proximal tube portion may also be used as the second electrode and then disposed on the distal tube portion is a cylindrical insulating layer which is coated with a metal layer, wherein that distal metal layer then serves as the first electrode.
This embodiment of the invention has the advantage of affording ease of manufacture and assembly of the bipolar electrode arrangement. This applies in particular when the metal used is an anodizable material, for example titanium or aluminum and the cylindrical insulating layer applied to the tube portion is produced by anodic oxidation (anodization) of the metal surface, in which case the metal layer deposited thereon can be produced for example by vapor deposition or electrolytic coating. In this embodiment the carrier has a hollow duct which is extended into a hollow duct through the metal tube. The connecting lines for the electrodes extend from the electrodes through the hollow duct to the proximal end of the instrument. In accordance with the invention this preferred embodiment thereof also provides that a flushing tube is taken through the hollow duct, which extends as far as the front cylinder and discharges cooling fluid at the distal end into the hollow duct.
Advantageously, provided as a prolongation of the hollow duct of the carrier in the front cylinder is an opening in which there is disposed a temperature sensor whose connecting line is passed through the hollow duct to the proximal end of the instrument. In that way it is possible—for example for therapy in respect of benign prostate hyperplasia to fit a temperature sensor or thermistor into the tip of the front cylinder, which can be used to measure the tissue temperature. Preferably the carrier is formed from a flexible insulating hoze or tube and the electrodes are cylindrical self-supporting tube portions of metal which are fixed at predetermined spacings on the carrier. In this embodiment of the invention the outside diameter of the front cylinder also corresponds to the outside diameter of the electrodes and the axial length of the electrodes is greater than the diameter, while the axial spacing of the two electrodes from each other is approximately equal to or smaller than the outside diameter thereof. It has been found that, with that dimensioning, the electrical field which causes coagulation of the tissue can be sufficiently strongly produced and also—after the tissue adjoining the outside surface is coagulated—can be propagated sufficiently far into the tissue so that an advantageous large coagulation zone is produced.
The temperature sensor in the opening in the front cylinder is preferably embedded in synthetic resin or in an adhesive bed which provides for good conduction of the temperature of the metal front cylinder to the temperature sensor. The outside diameter of the electrodes and the outside diameter of the front cylinder are identical and the intermediate space between the electrodes is filled with insulating material so that this lengthwise portion is also of the outside diameter which occurs elsewhere. That provides for a uniform cross-section over the distal tip to the proximal end of the second electrode, in contrast to which the flexible carrier, adjoining same, can be of a reduced outside diameter. The consequence of the constant dimension of the outside diameters in the region of the front cylinder and the electrodes is that the instrument can be introduced into the tissue easily and in particular without impediment.
The object of the invention is further attained by an electrode arrangement for a surgical instrument for electrothermal coagulation of tissue, including a front cylinder of metal at the distal end of the instrument, an elongate cylindrical carrier proximally adjoining the cylinder and comprising insulating material, two spaced cylindrical electrodes, characterised in that the metal front cylinder is rounded-off at the distal end and adjoining the round end portion has a cylinder portion of predetermined length, that the front cylinder forms the first electrode, and that a metal layer as the second electrode is disposed on the carrier at a predetermined axial spacing from the front cylinder.
In this preferred embodiment of the invention, extending through the carrier is a hollow duct which is extended into the front cylinder and which accommodates a flushing hoze or tube which at its distal end discharges flushing fluid which flows back along the inside wall of the front cylinder and finally between the carrier and the flushing tube to the proximal end.
The front cylinder can be fixed with its proximal end in an annular opening in the carrier and the overlap region between the carrier and the front cylinder has on its outside an oxide layer on the metal front cylinder, which ensures that the metal of the front cylinder which is uncooled at that location does not come into contact in an overheated condition with the adjoining tissue.
The carrier can be of both a flexible and a rigid nature and, as the front cylinder is formed from metal material, this instrument—by virtue of the active tip—makes it possible to treat edge tumors which are disposed immediately in front of the rounded-off front cylinder. A prerequisite for advantageous functioning of this electrode arrangement is that the dissipation of heat by the cooling circulation is as far as possible the same at both electrodes. That is achieved if the mean current density at the first, that is to say distal electrode is greater than or equal to the current density at the second, proximal electrode. That condition is met when the surface area A1 of the first electrode is smaller than or equal to the surface area A2 of the second electrode. Insofar as the surface area component of the round end portion—with its radius R—is also taken into consideration, that gives the following relationship in respect of the lengths L1, L2 of the first and second electrodes: L1+R<L2. If very great lengths L2 of the second electrode are to be implemented and if nonetheless the degree of flexibility of the applicator is to be maintained, then the second electrode can be constructed by coating the flexible carrier with a metal layer.
The object of the invention is further attained by an electrode arrangement for a surgical instrument for electrothermal coagulation of tissue including: an elongate cylindrical carrier comprising a metal tube or a metal bar, two spaced cylindrical electrodes on the carrier, characterised in that the first electrode is a distal portion of the carrier, that on a portion axially adjoining same an insulating layer is applied to the carrier, and that a cylindrical metal layer as the second electrode is arranged on the insulating layer at a predetermined axial spacing from the first electrode.
In this embodiment of the invention the front cylinder and the carrier are embodied in the form of an integral metal tube or metal bar whose distal end is pointed. A distal portion of the metal tube or metal bar forms the first electrode. Adjoining same, an insulating layer is applied to the carrier and then a cylindrical metal layer is deposited on the insulating layer in the proximal region of the insulating layer and forms the second cylindrical electrode. The insulating layer can be embodied by a plastic hoze or tube to which a metal coating is applied, as the second electrode. The metal carrier with the distal tip represents a bipolar electrode arrangement in the form of a cannula or needle and is suitable in particular for therapy in respect of enlarged terminal vessels such as for example finely mottled varicose veins. The electrode arrangement is pierced with its tip in the longitudinal direction into the enlarged vessel. Upon activation of the HF-power the blood and the vessel wall coagulate primarily around the first electrode. When that happens the vessel contracts so as to afford a closure effect with the result than then no further blood can flow into the vessel whereby the vessel is no longer perceptible through the skin and the desired cosmetic effect is achieved.
It is particularly advantageous to employ insulating layers which are used in bipolar electrode arrangements, comprising ceramic material. The advantage of this material is that it has a high level of mechanical strength and can be easily produced by means of electrolytic anodization (eloxation) for example on titanium in the form of titanium oxide or in the case of aluminum in the form of aluminum oxide. The thickness of the layer depends on the electrical voltage used in the electrolysis operation. Instead of titanium, various titanium alloys on which the ceramic layer is produced by anodic oxidation are also suitable as the starting material involved. In order to implement complete or partial coating in that way with titanium or suitable titanium alloys or aluminum, firstly the corresponding metal body is subjected to a preliminary chemical cleaning operation in order to obtain grease-free and oxide-free surfaces. Then, the places which are not to be coated are masked. Masking can be effected by means of special lacquers or layers but also by shrink tubes. For anodically applying a ceramic layer, the starting material, that is to say titanium, titanium alloys or aluminum is to be electrically contacted and subjected to voltage as the anode.
In order for example—based on titanium as the starting material —to apply a titanium oxide ceramic layer, the following steps are to be taken: in order to convert the titanium at its surface into its ion phase, a suitably molar acid in aqueous solution is to be used. The molar solutions in question are between 0.1 and 1 molar H2SO4 (sulfuric acid) and H3PO4 (phosphoric acid) respectively. By the application of a suitable dc voltage, oxygen is deposited at the electrode, the titanium electrode which is to be coated in this case, and bonds to the ionized titanium surface and is converted to titanium oxide. Depending on the respective layer thickness involved, the dc voltages and currents to be used are between 10V and 500V at maximum currents of 1 A. As a result the oxidation procedure passes through a plurality of oxidation stages (titanium oxides) depending on the respective length of the procedure. The layer thicknesses which are to be achieved with these processes are of the order of magnitude of between 20 and 30 μm. By means of the layer thicknesses which are to be represented by way of interference colors, due to the differing light refraction at the interface with the metal (the oxide layer is transparent), the latter can be represented proportionally over a color spectrum. With that method it is possible efficiently to provide specific paramagnetic electrodes of titanium—or titanium alloys such as TiAl6V4—with a dielectric ceramic layer, being variable in terms of layer thickness and/or color.
Besides the good dielectric properties of the ceramic layers produced in that way, the tribological properties are also excellently well suited to possibly increasing the levels of abrasion strength and surface quality. These colored ceramic layers are also suitable for the stable marking of needles, cannulae or probes. A proportional interference color is to be selected by way of the choice of the layer thickness. It is possible in that way to set colors of grey, gold, violet and on to blue.
In accordance with a further preferred embodiment of the invention at least one of the connecting lines which serve to connect the electrodes has at its end a portion of spring metal, preferably spring wire, which is of such a configuration that it is clamped in the hollow duct—within the electrodes—radially outwardly against the inside surface of the electrodes and thereby sufficiently reliably and securely produces electrical contact. The spring metal portion of the connecting lines of that configuration is preferably wound to form a spiral or coil spring which is acted upon by a predetermined tensile stress in the spiral wire, the winding of which therefore under that tensile stress is of a reduced diameter so that it can be easily introduced from the exterior into the cavity of the electrode. The tensile stress acting on the spring wire is then removed, the spiral spring then attains its full outside diameter and in so doing bears in self-clamping relationship from the interior against the inside surfaces of the electrodes. In order easily to be able to implement introduction of the corresponding spring portions at the end of the connecting lines, it is possible to use a corresponding special tool which makes it possible to insert the spiral spring of reduced diameter, then remove the tensile biasing of the spiral spring wire, and thus cause the spiral spring to bear against the inside surface of the electrode.
Advantageous developments of the invention are characterised by the features of the appendant claims.