MXPA98000249A - An electroquirurg instrument - Google Patents

Abstract

The present invention relates to an electrosurgical instrument for the treatment of tissue in the presence of a fluid electrically conductive medium, the instrument comprises an instrument shaft (30) defining a longitudinal axis (42), and an assembly of electrodes (32) in an axial end of the shaft, wherein the electrode assembly comprises: a single active electrode (34) fixedly positioned in the electrode assembly and having an exposed tissue treatment portion (34A), a return electrode (38) ) having a contact surface with the exposed one, the active electrodes and the return electrodes are separated in the direction of the longitudinal axis (42), and an insulating member (36) placed between and electrically isolating the active electrode (34) and the return electrode (38), the return electrode ends short of a distal end of the insulating member (36) so that the insulating member separates the exposed tissue treatment portion ( 34A) of the active electrode and the exposed fluid contact surface of the return electrode (38) in the direction of the longitudinal axis, whereby, when the exposed tissue treatment portion is brought adjacent to a tissue surface immersed in the fluid medium, the exposed fluid contact surface is separated from the tissue surface and the fluid medium completes a conduction path between the active electrode and the return electrode, and wherein the dimensions and configurations of the treatment portion of exposed tissue (34A), the exposed fluid contact surface and the insulating member (36) are such, that when the electrode assembly (32) is immersed in a conductive fluid medium, the ratio of (i) the length of the shortest driving path (P1) through the fluid medium between the exposed fluid contact surface and part of the exposed tissue treatment portion (34A) that is further away from the fluid contact surface exposed to, a (ii) the length of the shorter conduction path (P2) through the fluid medium between the exposed fluid contact surface and the exposed tissue treatment portion ( 34A), lies substantially within the range of from 1.25: 1 to 2

Description

AN ELECTROOUROPING INSTRUMENT The invention relates to an electrosurgical instrument for the treatment of tissue in the presence of a fluid electrically conductive medium, and to an electrosurgical system apparatus including that instrument. Endoscopic electrosurgery is useful for treating tissue in the body cavities, and is normally performed in the presence of a distention medium. When the distension medium is a liquid, the technique is commonly referred to as under-electrosurgery, denoting this term electrosurgery in which living tissue is treated using an electrosurgical instrument with an electrode or treatment electrodes submerged in liquid at the operating site . A gaseous medium is commonly used when performing endoscopic surgery in a distensible body cavity of larger potential volume, in which a liquid medium would be inadequate, as is often the case with laparoscopy or gastroenterological surgery. Underwater surgery is commonly performed using endoscopic techniques, in which the same endoscope can provide a conduit (commonly referred to as a working channel) for the passage of an electrode. Alternatively, the endoscope can be specifically adapted (as in a resectoscope) to include elements for mounting an electrode, or the electrode can be introduced into a body cavity via a separate access element at an angle to the endoscope - a technique which is commonly referred to as triangulation. These variations in the technique can be subdivided by surgical specialty, where one or the other of the techniques has particular advantages given the access route for the specific body cavity. Endoscopes with integral working channels, or those characterized as resectoscopes, are generally used where the body cavity can be accessed through a natural opening-such as the cervical canal to access the endo -tric cavity of the uterus, or the urethra to access the prostate gland and the bladder. Endoscopes designed specifically for use in the endometrial cavity are referred to as hysteroscopes, and those designed for use in the urinary tract include cytoscopes, urethroscopes and resectoscopes. Procedures for transurethral resection or vaporization of the prostate gland are known as TURP and EVAP, respectively. When there is no natural bodily opening through which an endoscope can be passed, the triangulation technique is commonly employed. Triangulation is commonly used during endoscopic underwater surgery in joint cavities such as the knee and shoulder. The endoscope used in these procedures is commonly referred to as an arthroscope. Electrosurgery is usually carried out using either a monopolar instrument or a bipolar instrument. With monopolar electrosurgery, an active electrode is used in the region of operation, and a return plate is secured to the patient's skin. With this arrangement, the current passes from the active electrode through the patient's tissues to the external return plate. Since the patient represents a significant portion of the circuit, the energy input levels have to be high (typically 150 to 250 watts), to compensate for the limiting resistive current of the patient's tissues, and in the case of electrosurgery under the water, the energy losses due to the fluid medium that becomes partially conductive due to the presence of blood or other bodily fluids. Using high energy with a monopolar array is also risky, because tissue heating occurs on the return plate, which can cause severe skin burns. There is also the risk of capacitive coupling between the instrument and the patient's tissues at the point of entry into the body cavity. With bipolar electrosurgery, a pair of electrodes (an active electrode and a return electrode) are used together at the tissue application site. This arrangement has advantages from the point of view of safety, due to the relative proximity of the two electrodes, so that the radiofrequency currents are limited to the region between the electrodes, and, during applications requiring very small electrodes, the separation between electrodes becomes very small, thus limiting the effect on the tissue and the output energy. Further separating the electrodes would often obscure the vision of the application site, and would require a modification in the surgical technique to ensure the correct contact of both electrodes with the tissue. There are many variations to the basic design of the bipolar probe. For example, U.S. Patent Specification Number: 4706667 describes one of the design fundamentals, i.e., the ratio of the contact areas of the return electrode and the active electrode is greater than 7: 1 and less than 20: 1 for cutting purposes. This range relates only to the cutting electrode configurations. When a bipolar instrument is used for desiccation or coagulation, the radius of the contact areas of the two electrodes can be reduced to approximately 1: 1 to avoid the differential electrical tensions that occur at the contact between the tissue and the electrodes. The electrical connection between the return electrode and the tissue can be supported by wetting the tissue by a conductive solution such as normal saline solution. This ensures that the surgical effect is limited to the needle or active electrode, with the electrical circuit between the two electrodes being completed with the tissue. One of the obvious limitations is that the active electrode must be completely buried in the tissue to allow the return electrode to complete the circuit. Another problem is of orientation, a change even if it is relatively small in the application angle from the ideal perpendicular contact with respect to the surface of the tissue, will change the proportion of the contact area of the electrode, so that a surgical effect can occur in the tissue that is in contact with the return electrode. The distension of the cavity provides space to gain access to the site of operation, to improve visualization, and to allow manipulation of the instruments. In low volume cavities, particularly where it is desirable to distend the cavity under higher pressure, liquid is commonly used in place of gas due to the best optical characteristics, and because it washes the blood out of the operating site. Conventional underwater electrosurgery has been performed using a non-conductive liquid (such as 1.5 percent glycine) as irrigant, or as a distention medium to eliminate electrical conduction losses. Glycine is used in isotonic concentrations to prevent osmotic changes in the blood when intravascular absorption occurs. During the course of an operation the veins can be cut, with the resulting infusion of the liquid into the circulation, which could cause, among other things, the dilution of serum sodium which can lead to a state of water intoxication. Applicants have found that it is possible to use a conductive liquid medium, such as normal saline, in endoscopic electrosurgery under water instead of solutions without non-conductive electrolytes. Normal saline solution is the preferred distention medium in endoscopic surgery when the water is not considered electrosurgery, or when an effect on non-electrical tissue is used, such as laser treatment. Although normal saline solution (0.9 percent weight / volume); 150 mmol / l) has a conductivity somewhat greater than that of most body tissues, has the advantage that the displacement by absorption or extravasation from the operative site produces a small physiological effect, and the so-called effects of water intoxication of electrolyte-free, non-conductive solutions. Applicants have developed a bipolar instrument suitable for underwater electrosurgery using a conductive liquid medium. A first aspect of the invention is as defined in claim 1 which accompanies this description. Other aspects of the invention are as defined in claim 7, which relates to an electrosurgical system including an instrument and a generator, claims 12, 19 and 23, each addressed to an electrosurgical instrument, and claims 31 and 37 are directed to methods of tissue desiccation and vaporization. Some of the preferred features of the different aspects of the invention are set forth in the dependent claims. The electrode structure of this instrument, in combination with a fluid electrically conductive medium, greatly avoids the problems experienced with monopolar or bipolar electrosurgery. In particular, the input energy levels are much lower than those generally needed, with a monopolar array (typically 100 watts). Moreover, due to the relatively large separation between their electrodes, an increased depth of effect is obtained compared to conventional bipolar arrays. The invention will now be described in greater detail, by way of example, with reference to the drawings, in which: Figure 1 is a diagram showing an electrosurgical system constructed in accordance with the invention. Figure 2 is a side view of a portion of an electrosurgical instrument that is part of the system of Figure 1.

Figure 3 is a cross section of part of an alternative electrosurgical instrument according to the invention, the instrument being cut along a longitudinal axis. Figure 4 is a graph illustrating the hysteresis of the electric charge impedance and the dissipated radiofrequency energy that occurs between the use of an instrument according to the invention in the desiccation and vaporization modes. Figure 5 is a block diagram of the generator of the electrosurgical system shown in Figure 1. Figure 6 is a diagrammatic side view of the instrument of Figure 3 showing the use of the instrument for tissue removal by vaporization. Figure 7 is a diagrammatic side view of an instrument similar to that shown in Figure 6, showing the use of the instrument for tissue drying or coagulation, and Figures 8, 9 and 10 are side views of other electrosurgical instruments of according to the invention, showing different configurations of the electrode and the insulator. Referring to the drawings, Figure 1 shows the electrosurgical apparatus including a generator 10 having an output hub IOS that provides a radio frequency (RF) output for a bipolar instrument, in the form of a manual part 12 and a unit of removable electrodes 28 via a connecting cord 14. Activation of the generator 10 can be performed from the hand piece 12 via a control connection on the cord 14, or by means of a pedal switch unit 16 as shown , separately connected to the rear of the generator 10 by means of a connecting cord of the foot switch 18. In the illustrated embodiment the foot switch unit 16 has two foot switches 16A and 16B for selecting a drying mode or a mode of vaporization of the generator 10, respectively. The front panel of the generator has squeeze buttons 20 and 22 to respectively set the drying and vaporization energy levels, which are indicated in a visual display 24. The squeeze buttons 26 are provided as alternate elements for the selection between the drying and drying modes. vaporization. The instrument does not need to include a manual part, but can simply include a connector to be mounted to another device such as a resectoscope. In Figure 1 the instrument has an electrode unit 28 that is shown mounted to the hand piece 12. The electrode unit 28 may have a number of different shapes, some of which are described below.

In a basic configuration, shown in Figure 2, an electrode unit for releasably securing a manual piece of instrument comprises a shaft 30 which may be a conductive tube covered with an insulating cover 30S, with an assembly of electrodes 32 in a distal end of the shaft 30. At the other end of the shaft (not shown) there are provided elements for connecting the unit to a handpiece both mechanically and electrically. The electrode assembly 32 comprises a central active electrode 34 that is exposed at the terminal distal end of the unit to form an electrode treatment portion. Preferably, the active electrode is a metal wire that extends as a central conductor through the entire shaft 30 to a contact at the proximal end (not shown in the drawing) Surrounding the electrode 34 and the inner conductor is an insulating sleeve 36 whose distal end is proximally exposed to the exposed treatment portion of the electrode 34. Typically, this sleeve is made of a ceramic material to withstand the damage of an arching. Surrounding the sleeve 36 is the return electrode 38 in the form of a metal tube that is electrically (and optionally also mechanically) integral with the metal tubular body of the shaft 30. This return electrode terminates at a short point on the end of the sleeve 36 so that it is returned from the exposed treatment portion of the active electrode 34 and is separated both radially and axially from the latter. It will be appreciated that, mainly due to the very large diameter of the return electrode compared to that of the active electrode, the return electrode provides a contact surface exposed to the fluid having a surface area much larger than that of the treatment portion of the active electrode exposed . The insulating cover 30S terminates at a location spaced proximally from the distal end of the return electrode 38 in order to provide the surface area for the contact surface with the fluid of the return electrode. At the distal end of the electrode unit, the diameter of the return conductor is typically in the region of from 1 millimeter to 5 millimeters. The longitudinal extension of the contact surface with the fluid of the exposed part of the return electrode 38 is typically between 1 millimeter and 5 millimeters with the longitudinal separation from the return electrode 38 being the treatment portion of the active electrode exposed between 1 millimeter and 5 mm. Other aspects of the configuration and dimensions of the electrode assemblies are set forth in more detail below. In effect, the electrode structure shown in Figure 2 is bipolar, with only one of the electrodes (34) actually extending to the distal end of the unit.

This means that, during normal use, when the electrode assembly is immersed in a conductive fluid medium, the return electrode 38 remains separate from the tissue being treated and there is a current path between the two electrodes via the tissue and the electrode. means of conductive fluid that is in contact with the return electrode. The axial separation of the electrodes allows a very fine electrode structure in terms of diameter since the isolation path is considerably longer than a bipolar electrode having only separation between the surfaces of the exposed electrodes. This allows higher energies to be used than with conventional electrode structures without causing undesired toning, or in the case of electrosurgical cutting or vaporizing treatment, without causing damage to the electrode unit due to excessive arcing at high temperatures. The particular stepped arrangement shown provides the surgeon with a view of the tip of the electrode in contact with the tissue, and allows a large range of angles applied with respect to the surface of the tissue, which is particularly important in the confined spaces typical of surgery. endoscopic With reference to Figure 3, an alternative electrode unit for releasably securing the manual part of the electrosurgical instrument 12 shown in Figure 1 comprises an axis 30, which is constituted by a semiflexible tube made of stainless steel or finox electroplated in copper or gold, with an assembly of electrodes 32 at a distal end thereof. At the other end (not shown) of the shaft 30, there are provided elements for connecting the electrode unit to the handpiece both mechanically and electrically. The electrode assembly 32 includes an active or tissue contact electrode 34 which is made of platinum, platinum / iridium or platinum / tungsten, and is constituted by a generally hemispherical exposed tip 34A and an integral central conductor 34B. The conductor 34B is electrically connected to a copper center conductor 34C ensuring a thin stainless steel re-cut over the adjacent terminal portions of the conductors 34B and 34C, thereby providing an electrical connection between the handpiece part of the instrument and the exposed tip 34A . An insulating ceramic sleeve 36 surrounds the conductor 34B, the spring 34D and the adjacent end portion of the copper conductor 34C. The sleeve 36 has an exposed portion 36A surrounding the distal end portion of the conductor 34B. A return electrode 38, which forms a distal end portion of the shaft 30 providing a contact surface with the cylindrical fluid, closely surrounds the sleeve 36 and extends over the copper conductor 34C separated from the latter by an insulating sleeve 40. A shrinkable or polyimide heat insulating outer coating 30S surrounds the shaft 30 and the proximal portion of the return electrode 38. When used in combination with the electrosurgical generator as shown in Figure 1, the electrode unit of Figure 3 is It can be used in a conductive fluid medium for tissue removal by vaporization, for sculpting and turning the meniscus during arthroscopic surgery, or for desiccation, depending on the way in which the generator is controlled. Figure 4 illustrates how the generator can be controlled to have the advantage of the hysteresis that exists between the desiccation and vaporization modes of the electrode unit. Thus, assuming that the electrode assembly 32 of the unit is immersed in a conductive medium such as saline, there is an initial load impedance "r" at point "0", whose magnitude is defined by the geometry of the electrode assembly and the electrical conductivity of the fluid medium. The value of "r" changes when the electrode 34 comes into contact with the tissue, at a higher value of "r" the greater the propensity of the electrode assembly 32 to enter the vaporization mode. When radiofrequency energy is applied to the electrode assembly 32 the fluid medium heats it. Assuming that the fluid medium is normal saline solution (0.9 percent weight / volume), the temperature coefficient of the conductivity of the fluid medium is positive, so that the corresponding impedance coefficient is negative. Thus, as energy is applied, the impedance initially falls and continues to fall with the increasing dissipation of energy to point "B", at which point the saline in intimate contact with the electrode assembly 32 reaches its boiling point. Small bubbles of steam are formed on the surface of the active tip 34A and the impedance then begins to rise. After point "B", as the energy dissipation increases more, the positive energy coefficient of the impedance is dominant, so that the increasing energy now effects an increasing impedance. As a vapor pocket forms from the vapor bubbles, there is an increase in energy density at the residual electrode / salt interface. There is, however, an exposed area of the tip of the active electrode 34A not covered by vapor bubbles, and this reinforces the interface, producing more vapor bubbles and thus also a higher energy density. This is a leakage condition, with a point of equilibrium that occurs only once the electrode is completely wrapped in steam. For a given set of variables, there is an energy threshold before this new equilibrium can be reached (point "C"). The region of the graph between points "B" and "C", therefore, represents the upper limit of the desiccation mode. Once in the equilibrium state of vaporization, the impedance rapidly increases to approximately 1000 ohms, with the absolute value depending on the system variables. The vapor pocket is then held by discharges through the vapor pocket between the tip of the active electrode 34A and the vapor / saline interface. Most energy dissipation occurs inside this bag, with consequent heating of the tip 34A. The amount of energy dissipation, and the size of the bag, depends on the output voltage. If this is too low, the bag will not be supported, and if it is too high the electrode assembly 32 will be destroyed. Thus, in order to avoid the destruction of the electrode assembly 32, the energy output of the generator must be reduced once the impedance has reached point "D". It should be noted that, if the energy is not reduced at this point, the energy / impedance curve will continue to rise and destruction of the electrodes would occur. The dotted line E indicates the level of energy above which the destruction of electrodes is inevitable. As the energy is reduced, the impedance drops until, at point "A", the vapor pocket collapses and the electrode assembly 32 reverts to the drying mode. At this point, the dissipation of energy within the vapor pocket is insufficient to sustain it, so that the direct contact between the tip of the active electrode 34A and the saline is re-established, and the impedance drops dramatically. The energy density at tip 34A also falls, so that the temperature of the saline solution falls below the boiling point. The electrode assembly 32 is then in a stable desiccation mode. The energy control of the generator to achieve the functions of desiccation, tissue cutting and vaporization is carried out by detecting the radiofrequency voltage peak that appears through the generator output connections and rapidly reducing the output energy supplied each time a previously selected peak voltage threshold is reached. In a desiccation mode at least, this energy reduction is significantly greater than that required merely to obtain the peak output voltage below the threshold. Preferably the energy reduction is at least 50 percent to have the advantage of the characteristic hysteresis described above with reference to Figure 4. Referring to Figure 5, the generator comprises a radiofrequency energy oscillator 60 having a pair of output connections 60C for coupling via output terminals 62 to the load impedance 64 represented by the electrode assembly during use. The energy is supplied to the oscillator 60 by a switched mode power supply 66. In the preferred embodiment, the radio frequency oscillator 60 operates at approximately 400 kHz, with any frequency from 300 kHz upwards within the range of HF that is possible. The switched-mode power supply typically operates at a frequency in the range of from 25 to 50 kHz. Coupled through the output connections 60C is the voltage threshold detector 68 having a first output 68A coupled to the switched mode power supply 16 and a second output 68B coupled to an "on" time control circuit 70. A microprocessor controller 72 (shown in Figure 1), coupled to the operator controls and visual display is connected to a control input 66A of the power supply 66 to adjust the output power of the generator by varying the voltage supply and for an established threshold input 68C of the threshold detector 68 for detecting the peak radio frequency output voltage limits. During operation, the microprocessor controller 72 causes the energy to be applied to the switched-mode power supply 66 when electrosurgical energy is demanded by the surgeon in operation, an activation switch arrangement that can be provided in a manual part or in a foot switch (see Figure 1) . A constant output voltage threshold is established independently of the supply voltage via the input 68C according to the control parameters on the front control of the generator (see Figure 1). Typically, for desiccation or coagulation the threshold is set as a desiccation threshold value between 150 volts and 200 volts. When a cut or vaporization output is required, the output is set to a value in the range of 250 or 300 volts to 600 volts. These voltage values are peak values. The fact of being peak values means that for desiccation at least it is preferable to have a low ridge factor output radiofrequency waveform to give the maximum energy before the value is set at the given values. Typically a crest factor of 1.5 or less is achieved. When the generator is activated first, the state of the control input 601 of the radio frequency oscillator 60 (which is connected to the "on" time control circuit 70) is "on", so that the power switching device which forms the oscillation element of oscillator 60 is switched on for a maximum driving period during each operation cycle. The power supplied to the load 64 partially depends on the supply voltage applied to the radio frequency oscillator 60 from the mode switched mode power supply 66 and partly from the load impedance 64. If the supply voltage is sufficiently high, the temperature of the liquid medium surrounding the electrode in the electrosurgical instrument (or within the gaseous medium, the temperature of the liquids contained within the tissue) can grow to a degree in which the liquid medium vaporizes, leading to a rapid increase in the load impedance and a rapidly increasing increase in output voltage across terminals 62. This is an undesirable state of affairs if a desiccation outlet is required. For this reason, the drying out threshold is set to cause trigger signals to be sent to the "on" time control circuit 70 and to the switched mode power supply 66 when the threshold is reached. The "on" time control circuit 70 has the effect of virtually instantaneously reducing the "on" time of the switching device of the radio frequency oscillator. Simultaneously, the power supply of the switched mode is disabled so that the voltage supplied to the oscillator 60 begins to fall. The output voltage of the generator is important for the operation mode. In fact, the output modes are purely defined by the output voltage, specifically the peak output voltage. The absolute measurement of the output voltage is only necessary for control of multiple terms. However, a simple single-term control (i.e., using a control variable) can be used in this generator in order to confine the output voltage to predetermined limit voltages. Thus, the voltage threshold detector 68 shown in Figure 5 compares the radio frequency peak output voltage with a preset direct current threshold level, and has a sufficiently fast response time to produce a reset pulse for the circuit "on" 70 time control with a half cycle of radiofrequency. The maximum absorbed energy coincides with the state of the electrode that exists immediately before the formation of vapor bubbles, since this coincides with the maximum energy distribution and the largest wetted electrode area. It is therefore desirable that the electrode Stay in its wet state for maximum drying energy. The use of voltage limit detection effects a reduction in energy that allows vapor bubbles to collapse which in turn increases the ability of the active electrode to absorb energy. It is for this reason, that the generator includes a control cycle that has a large surplus, because the peak voltage feedback stimulus that reaches the predefined threshold causes a large instantaneous energy reduction causing a reduction in the peak output voltage to a level significantly below of the voltage level established by the threshold detector 68. This surplus of control ensures a return to the required wetting state. Other details of the generator and its operation are described in European Patent Application No. 0754437A. In light of the foregoing, it will be apparent that the electrode unit of Figure 3 can be used for drying by operating the unit in the region of the graph between point "0" and a point in the region between points "B" and "C". In this case, the electrode assembly 32 is inserted into a selected operating site with the active tip 34A adjacent to the tissue to be treated, and with the tissue and the active tip and the return electrode submerged in the saline solution. . The generator is then activated (and controlled cyclically as described above) to supply sufficient power to the electrode assembly 32 to maintain the saline solution adjacent to the active tip 34A at, or just below, its boiling point without creating a vapor bag surrounding the active tip. The electrode assembly is manipulated to cause heating and drying of the tissue in a required region adjacent to the active tip 34A. The electrode unit can be used for vaporization in the region of the graph between the "D" point and the dotted line F that constitutes the level below which the vaporization is no longer stable. The upper part of this curve is used for the removal of tissue by vaporization. In this mode, a slight application of the instrument to the tissue to be treated allows the sculpting and turning to be carried out. The electrode stack 32 preferably has unitary electrodes with a return: the ratio of the surface area of the active electrode in the range of from 5: 1 to 40: 1 (i.e., the ratio of the surface areas of the exposed portions of the two electrodes are in this range). Figure 6 illustrates the use of the electrode unit of Figure 3 for tissue removal by vaporization, the electrode unit being immersed in the conductive fluid 78. Thus, the electrode unit creates a sufficiently high energy density in the active tip 34A to vaporize tissue 80, and to create a vapor pocket 82 surrounding the active tip. The formation of the vapor pocket 82 creates approximately a 10-fold increase in contact impedance, with a consequent increase in the output voltage. The arcs 34 are created in the steam bag 82 to complete the circuit to the return electrode 38. The fabric 80 which is in contact with the steam bag 82 will represent a path of electrical resistance to complete the circuit. The closer the tissue 80 is to the active tip 34A, the more energy is concentrated in the tissue to the extent that the cells explode as they are touched by the arcs 84, because the return path through the connective fluid (solution saline in this case) is blocked by the high impedance barrier of the steam bag 82. The saline solution also acts to dissolve or disperse the solid products of the vaporization. During use, the electrode assembly 32 is inserted into a selected operating site with the tip of the active electrode 34A adjacent to the tissue to be vaporized, and with the tissue, the active tip and the return electrode submerged in the saline solution 78. The radio frequency generator is activated to supply sufficient energy (as described above with reference to Figure 4) to the electrode assembly 32 to vaporize the saline solution and to maintain a vapor pocket surrounding the contact electrode with the tissue. When the electrode unit is used to sculpt or rotate menisci during arthroscopic surgery, the electrode assembly is applied with slight pressure at the selected operating site, and is manipulated so that the partly spherical surface of the active tip 34A moves through the surface to be treated by smoothing the tissue, and in particular the meniscus with an action of sculpting or turning. Figure 7 illustrates the use of an electrode unit similar to that of Figure 3 used for tissue desiccation. In the desiccation mode, the output energy is euminietra to the electrodes in a first output range, so that the current flows from the active electrode 34 to the return electrode 38. As described above, the output energy causes the saline solution adjacent to the active electrode 34 is heated, preferably to a point at or near the boiling point of the saline solution. This creates small vapor bubbles on the surface of the active electrode 14 that increase the impedance around the active electrode 34. The body tissue 80 typically has an impedance smaller than the impedance of the combination of vapor bubbles and the saline solution adjacent to the active electrode. 34. When an active electrode 34 surrounded by small vapor bubbles and saline solution comes into contact with the tissue 80, the tissue 80 becomes part of the preferred electric current path. Accordingly, the preferred current path leaves the active electrode 34 at the point of contact with the tissue through the tissue 80, and then back to the return electrode 38 via the saline solution, as shown in Figure 7. The invention has a particular application for drying tissue. For tissue drying, a preferred approach form is to contact only a portion of the active electrode with the tissue, the remainder of the active electrode remaining away from the tissue and surrounded by saline so that the current can pass from the active electrode to the return, via the saline solution, without passing through the tissue. For example, in the embodiment shown in Figure 7 only the dietal portion of the active electrode is brought into contact with the tissue, with the proximal portion remaining separate from the tissue. The invention can achieve desiccation with nothing or minimal carbonization of the tissue, when the active electrode 34 is brought into contact with the tissue 80, the current passes through the tissue, causing the tissue in and around the contact point to dry out. The area and volume of the dried tissue generally expands radially outward from the point of contact. In the embodiment shown in Figure 7, the exposed treatment portion of the active electrode 34 is longer than wide. This allows the tip of the electrode to contact the tissue surface while keeping most of the exposed treatment portion out of contact with the tissue even when the instrument is angled with respect to the surface of the tissue. Because the majority of the exposed portion of the electrode is out of contact with the tissue, the path of the current will be more easily changed, upon desiccation of a sufficient volume of tissue, from the path through the tissue to a trajectory that goes directly from the active electrode to the saline solution. In the electrode unit shown in Figure 3 the exposed portion of the active electrode 34 is relatively short compared to the length of the isolation member 36 between the active electrode 34 and the return electrode 38. With that electrode configuration, the operation bistable of the instrument inherent in hysteresis characteristics described above with reference to Figure 4 applies, because the instrument can be used in the desiccation mode or in a low energy vaporization mode. In some circumstances, particularly if the exposed treatment portion of the active electrode is long, the bistable operation can make it difficult to achieve it. The measures for overcoming this difficulty will now be described with reference to Figure 8, which shows an electrode unit comprising an axis 30 constituted by a semiflexible tube made of stainless steel or electroplated finox in copper or gold, with an electrode assembly 32 at a distal end thereof. The electrode assembly 32 includes an active central electrode 34 having an elongate exposed treatment portion 34A (referred to as a "needle" electrode), and an integral center conductor 34B. A cylindrical ceramic insulating sleeve 36 surrounds the conductor 34B, and a return electrode 38, which is constituted by the distal end portion of the shaft 30, engages a proximal end of the sleeve 36. An outer insulating polyimide coating 40 surrounds the proximal portion of the shaft adjacent to the return electrode 38, thereby providing the return electrode with an annular fluid contacting surface extending from the edge of the coating 40 to the insulating sleeve 36. The insulating sleeve 36 has a distal end face 36A of a diameter such that the pitch radius (ie, the distance between the edge of the circumference of the end face 36A and the outside diameter of the active electrode 34) is at least 1/20 of the length of the electrode treatment portion active 34a. The insulating sleeve 36 thus has a shoulder (or step) which is coaxial with the active electrode 34. During use, the step prevents local arcing that would otherwise occur at the proximal end of the treatment portion of the active electrode 34A , making the die end of the treatment portion 34A ineffective. To consider the operation of the electrode in greater detail, when the electrode unit is operated in a tissue cutting or vaporizing mode, a vapor bubble forms around the treatment portion of the active electrode 34A. This bubble is held by arcing within it. The higher the voltage applied, the larger the size of the bubble. The energy dissipated by each arc is limited by the impedance by the fluid remaining in the conduction path and by the impedance of the generator source. However, an arc behaves like a negative impedance because if the energy in the arc is sufficiently high, a very low impedance ionized path is formed. This can lead to an unstable impedance condition of ionized path always decreasing unless the impedance of the fluid between the bubble and the return electrode is sufficient to act as a limit on the dissipated energy. It is also possible for the vapor pocket around the treatment portion of the active electrode to invade the return electrode. In these circumstances, the arc energy is limited only by the impedance of the generator source, but those energy limitations are few and can not be adjusted according to the size of the electrode. For these reasons, the dimensions and configuration of the insulation sleeve 36 must be such to define a minimum path length of 1 millimeter between the treatment portion of the active electrode 34A and the contact surface with the fluid of the return electrode. 38. This minimum length of trajectory is, in the case of the modality shown in Figure 8, the length a of the sleeve 36 plus the step radius c, as shown in Figure 8. A further consideration is the possibility of forming a vapor pocket only on part of the exposed treatment portion 34A of the active electrode 34. When the applied voltage and the energy is high enough, a vapor pocket will form around the exposed treatment portion of the active electrode. Preferably, the bag is formed uniformly over the entire length of the treatment portion. In this situation, the load impedance presented to the generator can change by as much as a factor of 20. However, when there are significant differences in the length of the conduction path between the fluid contact surface of the return electrode and different parts of the treatment portion of the exposed active electrode 34A, a voltage gradient is established over the length of each electrode. Preferably, the contact surface of the fluid is sufficiently large and has an aspect ratio such that its length is at least as large as its diameter so as to minimize a voltage gradient on its surface. However, with some insulation sleeve and active electrode configurations, the voltage gradient may be large enough to allow the formation of the vapor pocket only on that part of the exposed treatment portion closest to the contact surface with the fluid, leaving the distal end of the exposed treatment portion still in contact with the conductive fluid. Thus, the voltage gradient is established within the conductive fluid wherein the edge of the vapor pocket intersects the surface of the treatment portion of the active electrode 34A. The electrical behavior of that treatment portion of the partially wrapped active electrode is very different from that of a fully covered treatment portion. The transition of the impedance from the wet state to the steam-wrapped state is much less marked than that described above with reference to Figure 4. In terms of the output of the controlling generator detecting the peak voltage, the behavior of the electrode assembly no longer It is more bistable. However, the energy demand is considerably higher as a result of the vaporization voltage presented through the wetted region of low impedance of the treatment portion of the active electrode. The clinical effect is not only the required vaporization, but also an undesirable thermal damage effect that results from the increased energy dissipation. The partial wrapping of the treatment portion of the active electrode can be largely avoided by ensuring that the ratio of the length of the conductive path between the furthest point of the treatment portion of the active electrode and the contact surface of the fluid is less than or equal to 2: 1, that is, b / (a + c) = 2. In some circumstances, it can be found that the length of the path between the active and return electrodes is too large to allow vaporization of the conductive fluid due to to the long series of consequent impedances represented by the fluid. A too large voltage drop may result in a previously established voltage threshold being reached before vaporization is achieved. Then, preferably, the proportion of the length of the largest conduction path with respect to the annular peripheral length of the contact surface with the fluid of the return electrode is not greater than 1.43: 1. In the case of a contact surface of the cylindrical fluid which is coaxial with the active electrode, the proportion of the length of the largest conduction path with respect to the diameter of the surface in contact with the fluid is less than or equal to 4.5. :1. A) Yes, with reference to Figure 8, b / d = 4.5. The main use of the electrode unit shown in Figure 8 is to cut tissue, with at least part of the treatment portion 3A buried in the tissue to be treated and with the generator operated in the vaporization portion of the features of impedance / energy shown in Figure 4. Alternative active electrode configurations include forming the exposed treatment portion 34A as a hook, as shown in Figure 9.

In this case, the insulation sleeve is conical, tapering from the contact surface with the fluid of the return electrode 38 to the face of the distal end 36A. Another alternative, shown in Figure 10 has a treatment portion of the active electrode 34a in the form of a curling hook. In the embodiments of Figures 8, 9 and 10, it will be seen that the dimensions a, b, c, d are such that they fall within the proportion limits described above. Furthermore, in each case, the electrode eamble can be visualized as having a treatment axis 42, the axis along which the instrument can be introduced towards the tissue, the return electrode 38 being fixed back in the direction of the treatment axis from the exposed portion of treatment of the active electrode 34A. In order to compare the different conduction path lengths between the return electrode and the different portions of the active electrode portion, the paths in a common plane, the plane containing the treatment axis 42, should be considered. In case of the views of Figures 8, 9 and 10, the lengths of the trajectories shown are, of course, on the piano of the paper bearing the views.

Claims (44)

1. CLAIMS 1. An electrosurgical instrument for the treatment of tissue in the presence of a conductive fluid medium, comprising an instrument shaft and an assembly of electrodes at a distal end of the shaft, wherein the electrode assembly comprises: a single active electrode having a portion of exposed tissue treatment, a return electrode having a contact surface with the fluid, and an insulating member positioned between, and electrically isolating the active electrode and the return electrode and serving to separate the exposed treatment portion of the electrode active and the contact surface of the exposed fluid of the return electrode, the fluid contact surface of the return electrode being returned in the direction of an assembly treatment axis from the exposed treatment portion of the active electrode. and wherein the dimensions and configuration of the exposed treatment portion, the contact surface of the exposed fluid and the isolation member are such that when the electrode assembly is immersed in a conductive fluid medium the ratio of (i) the length of the shorter conduction path (P1) through the fluid medium between the contact surface of the exposed fluid and that portion of the exposed treatment portion that is further away from the contact surface of the exposed fluid and that portion of the portion of the exposed treatment that is furthest from the contact surface of the exposed fluid, to (ii) the length of the shortest conduction path (P2) through the fluid medium between the contact surface of the expueeta fluid and the treatment portion exposed, is less than or equal to 2 to 1.
2. 2. An instrument in accordance with the claim 1, wherein the exposed treatment portion of the active electrode projects in a first direction from the isolation member, the contact surface of the return electrode is fixed back from the treatment portion of the active electrode, and the insulating member surrounds to the active electrode and, between the exposed portion of the active electrode and the fluid contact surface of the return electrode, projects outwardly in a second direction perpendicular to the first direction to define an isolation barrier to divert the flow of current electrical through the fluid medium, thereby increasing the length of the shortest driving path (P2) between the contact surface of the exposed fluid and the exposed treatment portion.
3. 3. An instrument according to claim 1, wherein the first direction defines a treatment axis and the two shorter conduction paths (P1 (p2) fall in a plane containing the treatment axis. according to claim 1, wherein the length of the shortest conduction path (P2) through the fluid medium between the contact surface of the fluid and the exposed treatment portion is at least 1 millimeter. according to any of the preceding claims, wherein the contact surface of the exposed fluid is generally cylindrical and has a length and a diameter, the length of the fluid contact surface being at least as large as its diameter and wherein the of (i) the shortest driving path (P?) through the fluid medium between the fluid contact surface and that portion of the exposed treatment portion that is s the furthest from the contact surface, a (ii) the diameter of the fluid contact surface, is at most 4.5 to 1. 6. An instrument according to any of the preceding claims, wherein the proportion of ( i) the length of the shortest driving path (P?) through the fluid medium between the contact surface of the fluid and that part of the exposed treatment portion that is farthest from the contact surface of the exposed fluid, a (ii) the length of the shortest driving path (P2) through the fluid medium between the contact surface of the expueeta fluid and the exposed treatment portion, is greater than or equal to 1.25. An electrosurgical device comprising an instrument according to any of the preceding claims, and further comprising an electrosurgical generator for supplying radiofrequency energy to the instrument, the generator including an output state that it has when a pair of output connections is reduced. connectable respectively to the active electrode and to the return electrode of the instrument, a detector circuit for deriving a detection signal representative of the radio frequency peak output voltage developed between the output connections, and a power adjustment circuit for automatically causing a reduction in the output power supplied when the detector signal is indicative of a predetermined peak radio frequency output voltage that has been reached. A system according to claim 7, wherein the power adjustment circuit is operated to cause at least a 50 percent reduction in the output power delivered when the detector signal is indicative that the threshold has been reached , said reduction being carried out with a period of 100 μs or less. 9. A system according to claim 8, wherein the energy adjustment circuit is operated to effect reduction in a period of 20 μe or less. 10. A system according to any of claims 7 to 9, wherein the departure date includes at least one radiofrequency energy device and wherein the control circuits are arranged so that at least 50% reduction is effected. percent of the output energy by reducing the period of device conduction during the individual radiofrequency oscillation cycles independently of the supply voltage to the device. A seventh according to claim 10, wherein the sensing circuit and the energy adjusting circuitry can be repeatedly operated to effect a rapid reduction in the driving period cycle by cycle in the energy device from one level peak at a level from beginning to end followed by a less rapid progressive increase in the driving period until the driving period again reaches its peak level, with the sequence of rapid reduction and repeated progressive increase while simultaneously reducing the supply voltage to the output state until the peak driving period level can be reached without the output voltage exceeding the predetermined ~ .brai. An electrosurgical instrument for the treatment of tissue in the presence of a conductive fluid medium, comprising an instrument body, an elongate instrument shaft and, at a dietal end of the shaft, an array of electrodes, wherein the electrode assembly comprises a only active electrode having an exposed tissue treatment portion, and a return electrode having an exposed fluid contact surface that recedes from the treatment portion of the active electrode and is separated from the treatment portion by an insulating member when The treatment portion is brought adjacent to a tissue surface immersed in a fluid medium, the contact surface of the fluid is normally separated from the surface of the tissue and the fluid medium completes a conduction path between the active electrode and the return electrode. 13. An instrument according to claim 12, wherein the return electrode comprises a conductive sleeve located around the isolation member behind the treatment portion of the active electrode. 14. An instrument according to claim 12 or claim 13, wherein the treatment portion of the active electrode is located at the terminal distal end of the assembly and the fluid contact surface of the return electrode is proximally separated from the portion of the electrode. treatment of the active electrode, and wherein the exposed portion of the active electrode has a length and an amplitude, the length being greater than at least half the amplitude. 15. An instrument according to the claim 14, wherein the longitudinal separation of the exposed portion of the active electrode and the fluid contact surface of the return electrode is at least 1 millimeter. 16. An instrument according to the claim 15, wherein the proportion of (i) the longitudinal distance between the distal end of the exposed portion of the active electrode and the most distal portion of the return electrode, a (ii) the shortest longitudinal distance between the exposed portion of the active electrode and the most distal part of the return electrode is less than or equal to 2 to 1. 17. An instrument according to claim 15 or claim 16, wherein the return electrode has a fluid contact surface surrounding the member. of isolation and wherein the ratio of (i) the longitudinal distance between the distal end of the exposed portion of the active electrode and the distal edge of the fluid contact surface of the return electrode with respect to (ii) the circumference of the contact surface in the region of its distal edge is less than or equal to 1.43: 1. 18. An instrument according to claim 12, wherein the instrument shaft comprises a metal tube as its main structural element, and the return electrode is an integrally distal end portion of the tube. 19. An electrosurgical instrument for the treatment of tissue in the presence of an electrically conductive fluid, comprising an instrument body, an elongated instrument shaft and, at a distal end of the shaft, an assembly of electrodes, wherein the assembly of The electrodes comprise an exposed active electrode treatment interface and a fluid interface of the return electrode exposed behind the treatment interface and separated therefrom by an isolation member, the treatment interface projecting outwardly from the sealing member in the where the surface area of the fluid interface is larger than that of the treatment interface, and wherein the treatment interface extends outward from the isolation member by a distance that is greater than, or equal to, the half of its width in a direction perpendicular to the outward direction. 20. An instrument according to claim 19, wherein the axis defines a longitudinal axis, the treatment interface is a conductive axial projection whose axial length is greater than half its lateral width, the isolation member is a sleeve of ceramic located proximally of the projection, and the fluid interface is a conductive outer sleeve surrounding the isolation member and spaced from the treatment interface by an axial separation of at least 1 millimeter. 21. An instrument according to claim 19 or claim 20, wherein the treatment interface extends outwardly from the isolation member by a distance greater than its width in a direction perpendicular to the outward direction. 22. An instrument according to any of claims 19 to 21, wherein the treatment interface of the active electrode comprises a tip member of the active electrode whose length is the outward direction is at least half its width, and in wherein the isolation member has an end face adjacent to the tip member, the face of which does not extend laterally beyond the tip member by more than half the length of the tip member. 23. An electrosurgical instrument for treating tissue in the presence of an electrically conductive fluid medium, the instrument comprising: an instrument shaft, and an electrode assembly at a distal end of the shaft, the electrode assembly having a distal end and which includes an active electrode and a return electrode, with an exposed portion of the active electrode at the distal end of the electrode assembly and a fluid contact surface of the return electrode positioned proximally of the exposed portion of the active electrode, further including a insulation member positioned between, and electrically insulating the active electrode and the return electrode, wherein the exposed portion of the active electrode has a length and an amplitude, and the length of the exposed portion of the active electrode is greater than the amplitude of the exposed portion of the active electrode. 24. An instrument according to claim 23, wherein the exposed portion of the active electrode extends longitudinally from the distal end of the shaft. 25. An instrument according to the claim 23 or 24, wherein the isolation member comprises a generally cylindrical sleeve and the return electrode is positioned on the outside of the sleeve longitudinally separated from the exposed portion of the active electrode by a distance of at least 1 millimeter. 26. An instrument according to claim 25, wherein the -r. The isolation bead has an annular distal end face defining a shoulder, and the exposed portion of the active electrode is centrally positioned with respect to, and projecting from the end face of the isolation member, the depth of the shoulder being in one direction laterally out from the active electrode between 0.051 and 0.51, where 1 is the length of the exposed portion of the active electrode. 27. An instrument according to the claim 26, wherein the dimensions and configuration of the exposed portion of the active electrode, the fluid contact surface of the return electrode and the isolation member are such that when the electrode assembly is immersed in a conductive fluid medium the proportion of (i) the length of the shortest conduction path through the fluid medium between the fluid contact surface of the return electrode and part of the exposed portion of the active electrode that is farthest from the fluid contact surface, a (ii) the length of the shortest driving path through the fluid medium between the fluid contact surface of the return electrode and the portion of the active electrode is less than or equal to 2 to 1. 28. An instrument of according to claim 27, wherein the length of the shortest path of conduction through the fluid medium between the contact surface of the fluid of the r and the exposed portion of the active electrode is at least 1 millimeter. 29. An instrument according to claim 27 or claim 28, wherein the contact surface of the fluid of the return electrode is annular and has a length and a diameter, the length of the fluid contact surface being at least as large. as its diameter, and where the proportion of (i) the shortest conduction path through the fluid medium between the contact surface of the return electrode fluid and that portion of the exposed portion of the active electrode that is furthest from the the contact surface of the fluid, a (ii) the diameter of the contact surface of the fluid is at least 4.5 to 1. 30. An intrument in accordance with the claim 24, wherein the isolation member comprises a generally tapered member tapering towards the distal end of the instrument. 31. A method of tissue desiccation using a bipolar electrode assembly, including the assembly of an active electrode and a return electrode, the active electrode having an exposed treatment portion, and the return electrode having an exposed fluid contact surface and which is retracted from the exposed treatment portion, the method comprises the steps of: (a) introducing the electrode assembly into a selected operating site; (b) surrounding the electrode assembly with a conductive fluid so that the conductive fluid defines an electrical path between the active and return electrodes; (c) applying sufficient radiofrequency output energy to the electrode assembly to increase the temperature of the conductive fluid adjacent to the treatment portion of the active electrode without creating a vapor pocket surrounding the treatment portion; (d) contacting the treatment portion with the tissue while maintaining the contact surface of the fluid of the return electrode from contact with the tissue. 32. A method according to claim 31, wherein step (d) includes keeping a portion of the exposed treatment portion of the active electrode out of contact with the tissue. 33. A method according to claim 32, wherein step (d) includes an additional step of (e) moving the active electrode through a tissue surface. 34. A method according to claim 33, wherein step (e) includes moving the electrode across the tissue surface in a side-to-side movement. 35. A method according to any of claims 31 to 34, wherein step (c) includes maintaining the temperature of the conductive fluid adjacent to the treatment portion of the active electrode substantially at the boiling point of the conductive fluid. 36. A method, according to any of claims 31 to 35, wherein the conductive fluid comprises a saline solution. 37. A method according to any of claims 31 to 35, wherein the conductive fluid comprises a solution of sodium lactate. 38. A method for vaporizing tissue using a bipolar electrode assembly, including the eneamble an active electrode and a return electrode, the active electrode having an exposed treatment portion, and the return electrode having an exposed fluid contact surface and which is returned from the exposed treatment portion, the method comprises the steps of. (a) introduce the electrode assembly in a selected operation site; (b) surrounding the electrode eamble with a conductive fluid so that the conductive fluid defines an electrical path between the active and return electrodes; (c) applying sufficient radiofrequency output energy to the electrode assembly to increase the temperature of the conductive fluid adjacent to the treatment portion of the active electrode without creating a vapor pocket surrounding the treatment portion; (d) contacting the treatment portion with the tissue while maintaining the contact surface of the fluid of the return electrode from contact with the tissue. 39. A method according to claim 38 wherein step (d) includes the additional step of: (e) moving the treatment portion of the active electrode onto a tissue surface. 40. A method according to claim 39, wherein step (e) includes moving the electrode over the tissue surface in a side-to-side movement. 41. A method according to any of claims 38 to 41, wherein step (e) includes moving the active electrode on the surface of the tissue to turn the tissue. 42. A method according to any of claims 38 to 41, wherein the conductive fluid comprises a saline solution. 43. A method according to any of claims 38 to 43, wherein the conductive fluid comprises a compound of a sodium lactate solution. 44. The method of claim 38, wherein the treating portion of the active electrode is a distal end portion and the contact surface of the exposed fluid of the return electrode is spaced proximally from the treatment portion, and where the pae ( a) includes placing a portion of the adjacent treatment portion and from time to time, in contact with the tissue.