CA2242352A1

CA2242352A1 - An electrosurgical instrument

Info

Publication number: CA2242352A1
Application number: CA002242352A
Authority: CA
Inventors: Nigel Mark Goble; Colin Charles Owen Goble
Original assignee: Individual
Current assignee: Gyrus Medical Ltd
Priority date: 1996-01-09
Filing date: 1997-01-09
Publication date: 1997-07-17
Also published as: US6234178B1; DE69734612D1; DE69734612T2; ES2250820T3; CN1209736A; DE69728794T2; WO1997024993A1; EP0959784A1; AU1390297A; BR9706946A; US6013076A; AU720807B2; EP0959784B1; JP2000515776A; DE69728794D1

Abstract

An electrosurgical instrument is disclosed for the treatment of tissue in the presence of an electrically-conductive fluid. The instrument comprises an instrument shaft (32), and a tissue treatment electrode (31) at one end of the shaft, the tissue treatment electrode being constructed to define a plurality of pockets for trapping electrically-conductive fluid. Alternatively, the tissue treatment electrode (81) is made from an electrically-conductive material (81a) and is coated with a resistive inert material (81b) which is effective to increase the local power density within the tissue treatment electrode.

Description

AN Fl FcTRosuRGIcAL INSTRUMENT

This invention relates to an electrosurgical instrument for the treatment of tissue in the ~l~sence of an electrically conductive fluid medium. to electrosurgical a{J~ s including 5 such an instrument. and to an electrode unit for use in such an instrument.

Fnt~oscopic electrosurgery is useful for treating tissue in cavities of the body, and is norm~lly perforrned in the p.es~"ce of a distension mediurn. When the ~ ~nsion m~ m is a liquid, this is commonly .~fell~d to as underwater electrosurgery, this terrn denoting 10 electrosurgery in which living tissue is treated using an electrosurgical instrument with a treatment electrode or electrodes inLIl,e~ed in liquid at the operation site. A gaseous medium is cornrnonly employed when endoscopic surgery is performed in a ~ t~n~ible body cavity of larger potential volume in which a liquid medium would be unsuitable as is often the case in laparoscopic or gastroenterological surgery.
Undc.vv~ surgery is co~ llollly ~c,rulllled using endoscopic techniques, in which the endoscope itself may provide a conduit (commonly referred to as a working channel) for the passage of an electrode. ~It~n~tively, the endoscope may be specifically adapted (as a resectoscope) to include means for mounting an electrode, or the electrode may be 20 introduced into a body cavity via a separate access means at an angle with respect to the endoscope - a technique comrnonly referred to as triangulation. These variations in technique can be subdivided by surgical speciality, where one or other of the techniques has particular ad-v~ ages given the access route to the specific body cavity. Endoscopes with integral working Ch~rln~lc, or those characterised as resectoscopes, are generally 25 employed when the body cavity may be ~cc~ossed through a natural body opening - such as the cervical canal to access the en~orn~ial cavity of the uterus, or the urethra to access the prostate gland and the bladder. Endoscopes specifically designed for use in the endometrial cavity are referred to as hysterocopes, and those designed for use in the urinary tract include cystoscopes, urethroscopes and resectoscopes. The procedures of 30 transurethal resection or vaporisation of the prostrate gland are known as TURP and EVAP les~el;lively. When there is no natural body opening through which an endoscope may be passed, the techni~ue of triangulation is cornrnonly employed. Triangulation is comrnonly used during underwater endoscopic surgery on joint cavities such as the knee and the 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 bipo}ar instrurnent. With monopolar electrosurgery, an active electrode is used in the operating region, and a conductive return plate is secured to the patient's skin. With this arr~ngem~rlt, current passes from the active electrode through the patient's tissues to the 10 t xtern~l return plate. Since the patient re~ se~ a $ignific~nt portion of the circuit, input power levels have to be high (typically 150 to 250 watts~, to compensate for the resistive current limiting of the patient's tissues and, in the case of underwater electrosurgery, power losses due to the fluid medium which is rendered partially conductive by the presence of blood or other body fluids. Using high power with a monopolar arrangement is also hazardous, due to the tissue heating that occurs at the return plate, which can cause severe skin burns. There is also the risk of c~r~citive coupling between the i~lsl~ e.ll and patient tissues at the entry point 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 tne safety standpoint. due to the relative proximity of the two electrodes so that radio frequency currents are limited to the region ~ ell the electrodes. However, the depth of effect is directly related to the cl;~t~nce between the two electrodes; and, in applications requiring very small electrodes, the inter-electrode spacing becomes very small, thereby limitin~ tissue effect and the output power. Spacing the electrodes further apart would often obscure vision of the application site, and would require a modification in surgical technique to ensure correct contact of both electrodes with the tissue.

There are a number of variations to the basic design of the bipolar probe. For exarnple, U.S. Patent No.4706667 describes one of the fi~ c ~ of the design, namely that the ratio of the contact areas of the return electrode and of the active electrode is greater than CA 022423~2 1998-07-06 7:1 and smaller than 20:1 for cutting purposes. This range relates only to cutting electrode configurations. When a bipolar instrument is used for desiccation or coagulation, the ratio of the contact areas of the two electrodes may be reduced to approximately 1:1 to avoid differential electrical stresses occurring at the contact between the tissue and the electrodes.

The electrical junction between the retum electrode and tissue can be supported by wetting of the tissue by a conductive solution such as normal saline. This ensures that the surgical effect is limited to the needle or active electrode. with the electric circuit between 10 the two electrodes being completed by the tissue. One of the obvious lirnitations with the design is that the needle must be completely buried in the tissue to enable the return electrode to complete the circuit. Another problem is one of the orientation; even a relatively small change in application angle from the ideal perpendicular contact with respect to the tissue surface will change the contact area ratio, so that a surgical effect can 15 occur in the tissue in contact with the return electrode.

Cavity distension provides space for gaining access to the operation site, to improve vic--~li.c~tion, and to allow for manipulation of instrurnents. In low volume body cavities, particularly where it is desirable to distend the cavity under higher ~le~ c;, liquid rather 20 than gas is more cornmonly used due to better optical characteristics, and because it washes blood away from the operative site.

Conventional under~vater electrosurgery has been perforrned using a non-conductive liquid (such as 1.5% glycine) as an irrigant, or as a dicte.lcion medium to elimin~t~o 2~ electrical con~ ction losses. Glycine is used in isotonic col.cellLIdlions to prevent osmotic changes in the blood when intra-vascular absorption occurs. In the course of an operation, veins may be severed, with resnlt~nt infusion of the liquid into the circulation, which could cause, among other things, a dilution of serum sodium which can lead to a condition known as water intoxication.

CA 022423~2 1998-07-06 The applicants have found that it is possible to use a conductive li4uid medium, such as normal saline, in underwater endoscopic electrosurgery in place of non-conductive, electrolyte-free solutions. NolTnal saline is the pl~r~ d distension medium in underwater endoscopic surgery when electrosurgery is not contemplated, or a non-electrical tissue S effect such as laser treatment is being used. Although norrnal saline (0.9%w~v;
1 50mmol/1) has an e}ectrical conductivity somewhat greater than that of most body tissue, it has the advantage that displacement by absorption or extravasation from the operative site produces little physiological effect, and the so-called water intoxication effects of non-conductive, electrolyte-free solutions are avoided.
The applicants have developed a bipolar instrument suitable for underwater electrosurgery using a conductive liquid or gaseous medium. This electrosurgical instrument for the tre~tm~ nt of tissue in the presence of a fluid medium, comprises an instrument body having a handpiece and an instrument shaft and an electrode assembly, at one end of the 15 shaft. The electrode assembly comprises a tissue trei.7tmçrlt electrode which is exposed at the extreme distal end of the instrument, and a return electrode which is electrically insulated from the tissue treatment electrode and has a fluid contact surface spaced proximally from the exposed part of the tissue treatment electrode. In use of the instrument. the tissue treatment electrode is applied to the tissue to be treated whilst the ~0 retum electrode~ being spaced proximally from the exposed part of the tissue treatment electrode, is normally spaced from the tissue and serves to complete an electrosurgical current loop from the tissue Irci7~ .t electrode through the tissue and the fluid medium.
This electrosurgical instrument is described in the specification of the applicants' co-pending ~nt~rn~tional Patent Application No. PCT/GB96/0 1473, the colllcn~ of which are 25 incol~olaLed in this application by reference.

The electrode structure of this instrurnent, in combination with an electrically conductive fluid medium largely avoids the problems experienced with monopolar or bipolar electrosurgery. In particular. input power levels are much lower than those generally 30 necess~y with a monopolar arrangement (typically 100 watts). Moreover, because of the CA 022423~2 1998-07-06 relatively large spacing between its electrodes. an improved depth of effect is obtained compared with a conventional bipolar arrangement.

Figure 1 illustrates the use of this type of instrument for tissue removal by vaporisation.
5 The electrode assembly 12 of this instrument comprises a tissue treatment (active) electrode 14 which is exposed at the distal end of the instrument, and a return electrode which is spaced from the exposed part of the tissue tre~tm~nt electrode by an insulation sleeve 16. This electrode assembly is powered to create a sufficiently high energy density at the tissue t~ electrode 14 to vaporise tissue 22, and to create a vapour pocket 24 10 surrounding the active tip. The formation of the vapour pocket 24 creates about a 1 0-fold increase in contact impe~nre, with a consequent increase in output voltage. Arcs 26 are created in the vapour pocket 24 to complete the circuit to the return electrode 1~. Tissue 22 which contacts the vapour pocket 24 will le~les~ a path of least electrical resi~t~nre to co,ll~lcte the circuit. The closer the tissue 22 comes to the electrode 14 the more energy 15 is co,l~e~ te~ to the tissue, to the extent that the cells explode as they are struck by the arcs 26, because the return path through the conductive fluid (saline in this case) is blocked by the high impedance barrier of the vapour poclcet 24. The saline solution also acts to dissolve the solid products of vaporisation.

20 The power threshold required to reach vaporisation is an hll~Jol ~ parameter of this type of instrurnent. and it is the aim of the invention to provide a bipolar electrosurgical instrument having improved vaporisation power threshold p~o~el~ies.

In its broadest aspect, the invention provides an electrosurgical instrurnent having an 25 electrode which is so constructed as to have a better vaporisation power threshold than Icnown electrodes.

Thus, according to a first aspect, the present invention provides an electrosurgical ent for the tre~tmerlt of tissue in the presence of an electrically-conductive fluid,~0 the ina~ c~ll comprising an instrument shaft, and a tissue II~A~ electrode at one end of the shaft. the tissue tr~tment electrode being constructed to define a plurality of pockets for trapping electrically-conductive fluid and vapour.

In use. the tissue trç~tn~ent electrode traps electricallv-conductive fluid, the trapped fluid 5 thereby absorbing more electrical power for conversion to vapour than would otherwise be the case. This leads to a reduction in the power threshold for vaporisation at the tissue tre~tment electrode.

The electrically conductive fluid trapped within the irregularities (pockets) of the tissue 10 treatment electrode progressively absorbs more power as it becGil,es hotter and is not refreshed by fluid from the surrounding envh~ lclll. As the fluid approaches boiling point. vapour pockets begin to form on the surface of the electrode. The vapour pockets effectively insulate regions of the electrode from the con~rtive fluid and, as a result, power beco~l~es concentrated at regions of the electrode not enveloped in vapour. Fluid 15 adjacent to these exposed regions then rapidly reaches a point of val,o,ia~lion such that the whole tissue tlcAI..,rnt electrode beco...f s coated in vapour. The vapour is t~ a~ed by the irregular form of the active electrode such that, if an area of the electrode becomes exposed to the fluid medium during use, then the vapour pocket is rapidly reestablished with minim~l power dissipation to the surrounding fluid. This leads to a reduction in the ~0 power threshold re~uired both to initiate and sustain the vapour pocket during use.

In a preferred embodiment. the tissue l~ ...rnt electrode is constituted by a plurality of interlaced strands of electrically-conductive material. In this case, the pockets are defined by the interlacing of the strands. Each strand may be formed as a helix, the helices preferably having a common central axis, and being of equal rl;~meter and equal pitch.
They may be so interlaced that the pockets formed bet~,veen them take the form of helical apertures providing fluid communication between an axially extending space within the helices and the space outside the helices. In another variant, the helices may be tightly wound together so that each helix lies against other helices and the above-mentioned 30 pockets are simply helical lecesses bet~veen neighbouring helices~ little or no comrnunication being available between an interior space and the outside of the electrode It is possible to achieve a similar function to the tightlv wound interlaced strand variant ~,vith a single piece of conductive material with helical ridges about its outer surface, either created by moulding, m~ ininE~, or by twisting the piece of material about its longitudinal axis, with the twisting c~--sing helical ridges about the outer surface of the material.

Alternatively, the tissue llc;~ .1 electrode is constituted by a generally helical coil made of electrically-conductive material. Here, the pockets are formed between ~cent turns of the helical coil. Again, the turns of the coil may be spaced apart to allow communication between the interior of the coil and the outside, or they may be tightly 10 abutting with the pockets comprising a single helical recess on the outer surface of the electrode.

The tissue treatment electrode may also be constituted by a plurality of fil~rn~-nt.~ made of an electrically-conductive m~t~ri~l . In this case, the spaces bet~,veen the fil~m~ontc define 15 the pockets.

In any of these cases, the ins~ n~ may further comprise an inc~ tine shroud which exterl~c along, and partially surrounds, the tissue treatment electrode. The shroud traps electrically-conductive fluid and vapour against the tissue tre~sm~ns electrode, thereby 20 enhancing its power absorption capabilities.

In another preferred embodiment, the tissue ~ .e.-l electrode is con~ ed by a spherical mP~nber made of electrically-con~ ctive material. the spherical member being mounted on the shaft of the insl,u~,.ent by means of an electrically-conductive support 25 member, the instrument further colllpl;~ing an in~ul~ting shroud which partially surrounds the spherical member.

Advantageously1 the tissue treatment electrode is made of tllng~ten~ a noble metal such as pl~tim-m, or of a pl~tinl.m alloy such as platinum/iridium, pl~tin-lmitl-ng~ten or 30 pl~tinllm/cobalt.

CA 022423~2 1998-07-06 Preferably, the instrument filrther comprises a retum electrode which is electrically inc~ t~d from the tissue tre~tment electrode by means of an insulation member, the tissue treatment electrode being exposed at the extreme distal end of the instrument, and the return electrode having a fluid contact surface spaced proximally from the exposed end 5 of the tissue tre~tm~nt electrode by the insulation member. Conveniently, the fluid contact surface of the return electrode is a smooth polished surface.

According to a second aspect, the present invention provides an ele~ us lrgical insl~ c.lL
for the L~ ..,el~t of tissue in the presence of an electrically-conductive fluid, the 10 instrument comprising an instrument shaft, and a tissue treatment electrode at one end of the shaft, the tissue treatment electrode being made from an electrically- conductive material and being coated with a resistive inert material which is effective to increase the local power density within the tissue treatm~nt electrode.

15 Preferably, the resistive inert material is constituted by a conductive ceramic material.

According to a third aspect, the present mention provides an electrosurgical instrument for the treatment of tissue in the ~,les~nce of an electrically-conductive fluid, the instrument comprising an instrument shaft, and an electrode assembly at one end of the ~0 shaft, the electrode assembly comprising a tissue treatment electrode and a return electrode which is electrically inS~ tt~ from the tissue treatrnent electrode by means of an insulation member, the tissue tre~tm~t electrode being exposed at the extreme distal end of the instrument, and the return electrode having a smooth, polished. fluid contact surface spaced proximally from the exposed end of the tissue treatment electrode by the 25 insulation member.

In this case the instrument may further comprise means for feeding electrically conductive fluid over the fluid contact surface of the return electrode.

30 The electrosurgical instrument of the invention is useful for dissection, resection, vaporisation, desiccation and coagulation of tissue and combinations of these functions WO 97/24993 PCT~GB97/00065 with particular application in hysteroscopic surgical procedures Hysteroscopic operative procedures may include removal of submucosal fibroids, polyps and m~lign~nt neoplasms; resection of congenital uterine anoma}ys such as a septum or subsepturn;
division of synechi~e ~adhesiolys is): ablation of ~iceacefl or h~/lJell~ù,ohic endometrial 5 tissue; and haemostasis The instrument of the invention is also useful for dissection, resection, vaporisation, desiccation and coagulation of tissue and combinations of these functions with particular application in arthroscopic surgery as it pertains to endoscopic and pcl.;uL~leous 10 procedures l~clrullllcd on joints of the body including, but not limited to, such techniques as they apply to the spine and other non-synovial joints Arthroscopic op~.~live procedures may include partial or complete meniscectomy of the knee joint including m~nicc~l cy~l~clo .ly; lateral retinacular release of the knee joint; removal of anterior and posterior cruciate lig~m~ntc or ~ thereof; labral tear resection, acromioplasty, 15 b~euLw-ly and subac.u,llial ~fCO-- ~ s~ion of the shouider joint; anterior rele~e of the te...l)e.c,...alldibular joint; synovectomy, cartilage debril1ement chondroplasty, division of intra-articular adhesions, rlaLlu.e and tendon debri~l~ment as applied to any of the synovial joints of the body; inrillcin~ the~nal shrinkage of joint capsules as a l,e~t . l Ut for .ecul~e~l dislocation, subluxation or .~pe~ e stress injury to any articulated joint of ~0 the body; ~icrectomy either in the trearment of disc prolapse or as part of a spinal fusion via a posterior or anterior approach to the cervical, thoracic and lurnbar spine or any other fibrous joint for similar purposes; excision of llice~ecl tissue; and haemostasis.

The instrument of the invention is also useful for dissection, resection, vaporisation, 25 desiccation and coagulation of tissue and combinations of these functions with particular application in urological endoscopic (urethroscopy, cystoscopy, ureteroscopy andnephroscopy) and p~-cul~leous surgery Urological procedures may include: electro-val~o..salion of the plu~LlaLe gland (EVAP) and other variants of the procedure cornmonly ~e~ d to as transurethral resection of the ~lusL~e (I URP) including, but not limited to, 30 i~ iLial ablation of the prostate gland by a percutaneous or perurethral route whether performed for benign or m~lign~nt disease: transurethral or ~ ;uL~eOUS resection of CA 022423~2 1998-07-06 urinary tract tumours as they may arise as primary or secondary neoplasms, and further as they may arise anywhere in the urological tract from the calyces of the kidney to the external urethral meatus: division of strictures as they may arise at the pelviureteric junction (PUJ), ureter, ureteral orifice, bladder neck or urethra; correction of ureterocoele shrinkage of bladder diverticular~ cystoplasty procedures as they pertain to corrections of voiding dysfunction; thermally intlnre~l shrinkage of the pelvic floor as a corrective treatment for bladder neck descent; excision of ~ice~etl tissue; and haemostasis.

Surgical procedures using the instrument of the invention include introducing the 10 electrode assembly to the surgical site whether through an artificial conduit (a c-AnnlllA), or through a natural conduit which may be in an anatomical body cavity or space or one created sur~ically. The cavity or space may be distended during the procedure using a fluid, or may be naturally held open by anatomical structures. The surgical site may be bathed in a continuous flow of conductive fluid such as saline solution to fill and distend 15 the cavity. The procedures may include simultaneous viewing of the site via an endoscope or using an indirect visualisation means.

The invention also provides an electrode unit for an electrosurgical instrument for the tre~tmrnt of tissue in the presence of an electrically-conductive fluid medium, the 20 electrode unit comprisin_ a shaft having at one end means for cormection to an in~ ent handpiece~ and. mounted on the other end of the shaft, a tissue l~AI~ t electrode. the tissue treatrnent electrode being constructed to define pockets for trapping electrically-conductive fluid and vapour.

25 The invention further provides an electrode unit for an electrosurgical instrument for the treAtment of tissue in the presence of an electrically-conductive fluid medium, the electrode unit comprising a shaft having at one end means for connection to an instrurnent handpiece. and. mounted on the other end of the shaft, a tissue ~ electrode, the tissue ll~AI-..rnt electrode being made from an electrically-conductive material and being 30 coated with a resistive inert mAtrriAI which is effective to increase the local power density within the tissue treatment electrode.

The invention still further provides electrosurgical a~pa~ s comprising a radio frequency gcllc.aLol and an electrosurgical instrument for the treatment of tissue in the pressure of an electrically-condu~ fe fluid medium. the instrument comprising an instrument shaft, and an electrode assembly at one end of the shaft, the electrode assembly comprising a 5 tissue ~o~ t electrode and a retum electrode which is electrically insulated from the tissue lle<~ cnt electrode by means of an insulation member, the tissue ~le~ t electrode being exposed at the distal end portion of the instrument, the retum electrode having a fluid contact surface spaced proximally from the exposed end of the tissue tre~tment electrode by the insulation member, and the radio frequency generator having 10 a bipolar output connected to the electrodes, wherein the exposed end of the tissue n.~ electrode is constructed to define a plurality of pockets for trapping electrically-conductive fluid and vapour.

r~he invention also provides electrosurgical aplJal~Lus comprising a radio frequency 15 ge~ alol and an clccllu~ ,ical instrument for the tre~tm~nt of tissue in the plese.~ce of an ele.,l-ically-corl~uctive fluid medium, the instrument comprising an instrument shaft, and an electrode assembly at one end of the shaft, the electrode assembly comprising a tissue treatment electrode and a retum electrode which is electrically ins~ ted from the tissue tr~tm~nt electrode by means of an insulation member, the tissue tre~tment'O electrode being exposed at the distal end portion of the instrument, the return electrode having a fluid contact surface spaced proximally from the exposed end of the tissue lle~ ,r." electrode by the insulation member, and the radio frequency generator having a bipolar output connected to the electrodes, wherein the exposed end of the tissue tre~tmPnt electrode is made from an electrically-conductive material and is coated with a resistive inert material which is effective to increase the local power density within the tissue tleA~...ent electrode.

Advantageously, the radio frequency generator includes control means for varying the output power delivered to the electrodes. Preferably, the control means is such as to 30 provide output power in first and second output ranges, the first output range being for powering the electrosurgical instrurnent for tissue desiccation, and the second output range CA 022423~2 1998-07-06 being for powering the electrosurgical instrument for tissue removal by vaporisation.
Conveniently, the first output range is from about 150 volts to 200 volts. and the second output range is from about 250 volts to 600 volts, the voltages being peak voltages.

5 The invention will now be described in greater detaih by way of exarnple, with lere~ ce to the drawings, in which:-Figure I is a diagrarnrnatic side elevation of an electrode unit, showing the use of such aunit for tissue removal by vaporisation;
Figure 2 is a diagrarn showing an electrosurgical a~ ~dL~ls constructed in accor~ cc with the invention;

Figure 3 is a longitudinal sectional view of the distal end of a first form of electrode unit 15 constructed in accordance with the invention;

Figure 4 is a diagr~mm~tic side elevation of the electrode assembly of a second form of electrode unit constructed in accordance with the invention;

20 Figure S is a diagramrnatic side elevation of a modified electrode assembly similar to that of Figure 4;

Figure 6 is a diagr~mm~tic side elevation of the electrode assembly of a third forrn of electrode unit constructed in accordance with the invention;
Figure 7 is a diagr~rnm~tic side elevation of the electrode assembly of a fourth form of electrode unit constructed in accordance with the invention;

Figure 8 is a diagr~mm~tic side elevation of the electrode assembly of a fifth forrn of 30 electrode unit constructed in accordance with the invention;

WO 97t24993 PCT/GB97/00065 Figure 9 is a diagramsnatic side elevation of the electrode assembly of a sixth form of electrode unit constructed in accordance with the invention;

~ igure 10 is a diag~ .latic side elevation of the electrode assembly of a seventh form of S electrode unit constructed in accordance ~vith the invention; and Figures 11 and 12 are sch~m~t-c side elevations of the distal end portion of an electrode assembly similar to that of Figure 7, showing di~.~ stages in the forrnation of a vapour pocket around conductive eleckode filaments.
Each of the electrode units described below is intended to be used with an electrically conductive fluid medium such as normal saline, and each instrument has a dual-electrode structure. with the conductive mediurn acting as a conductor between the tissue being treated and one of the electrodes, hereinafter called the retum electrode. The other 15 electrode is applied directly to the tissue, and is hereinafter called the tissue ~.~a~ e.
active) electrode.

Referring to the drawings, Figure 2 shows electrosurgical al,y~al~s including a g~ncldtor I having an output socket 2 providing a radio frequency (RF) output for an instrument in 20 the form of a handpiece 3 via a connection cord 4. Activation of the generator 1 may be pe~ollllcd from the handpiece 3 via a control connection in the cord 4, or by means of a footswitch unit 5, as shown, collneeled separately to the rear of the generator I by a footswitch cGr~le~;~ion cord 6. In the illustrated embo~lim~nt the footswitch unit S has two footswitches 5a and 5b for selecting a desiccation mode and a vaporisation mode of the 25 gen~.alo~ 1 rc~ecli.~ely. The generator front panel has push buttons 7a and 7b for respectively setting desiccation and vaporisation power levels. which are indicated in a display 8. Push buttons 9a are provided as an alternative means for selection between the iec~tion and vaporisation modes. The h~n~lpiece 3 mounts a ~let~h~hle electrode unit E, such as the electrode units El to E7 to be described below.

- Figure 3 shows the distal end of the first form of electrode unit El for ~let~ ble fzt~tenin~
to the electrosurgical instrument handpiece 3. The electrode unit El is formed with an electrode assembly at the distal end thereof, the electrode assembly comprising a central tissue treatrnent (active) electrode 31 and a tubular return electrode 32. The active 5 electrode 31 is made of a twisted metal such as tllrtg~tt~n a noble metal such as pl~tinllm or a pl~tinllm alloy such as pl~tin~lmtiridium, pl~tinllm~cobalt or plzttinllrnttungsten~ and the return electrode 32 is a stainless steel tube. The return electrode 32 is completely enveloped by an polyimide in.~ ing sheath 33. The return electrode 32 extends the entire length of the electrosurgical i~ ,n~, and co~ s the shaft of the insL~ cllt. Thus, 10 the return electrode 32 is m~int~inerl at a relatively low tC;~ e due to the th~rrnz conduction therealong.

The electrodes 31 and 32 are provided with cuITent from the radio frequency (RF)generator 1, the return electrode 32 being directly connecte~l to the g.,lle.alor and the 15 active electrode 31 being con~r~ d via a copper conductor 34. The generator may be as described in the specification of our co-pending European Patent Application No.96304558.8. The active electrode 31 is held centrally within the return electrode 32 by means of a cerarnic in~ul~tn~ Jacel 35. The in~ul~tor/spacer 35 has a generally cylindrical portion 35a surrounding the ~unction between the active electrode 31 and the conductor ~0 34 and the adjacent re~ions of these two members, and four radially-extending, equi~pace.l wings 35b which contact the internal circumferential wall of the return electrode 32 to hold the insulator/spacer, and hence the active electrode 31, centrally within the retum electrode.

~5 A tube 36, made of an inc~ ting material such as PTFE, is a friction fit around the proximal end of the cylindrical portion 35a of the insulator/spacer 35, and extends y along the entire length of the i~ ell~. The tube 36 defines, together with the return electrode 32, a coaxial saline supply channel 37~ the interior of the tube 36 defining a saline return channel 38. In use, saline is fed to the channel 37 under gravity 30 (no ~ pi,1g being reyuired), and saline is removed via the eh~t.~lfl 38 and apertures (not shown3 in the cylindrical portion 35a of the insulatorlspacer 35 by means of suction.

Preferably, the suction is carried out by a low noise pump (not shown) such as a moving vane pump or a diaphragm pump, rather than by using a high speed impeller. As the tubing leading to the pump will intermittently contain small quantities of saline, a large vacuum (at least 500mBar) is required. However, the 4llallLily of gas and liquid to be S removed is co~ Jdld~ ely small, and this permits the use of a moving vane or diaphragm pump, although a high volume peristaltic pump could also be used.

To circumvent the requirement for pump sterilisation, the pump Op~,laLts via a disposable fluid trap (not shown) inco-~ulaling a 1011m PTFE filter. This filter prevents both 10 exh~ tecl fluids and gas particulates from being drawn in by the pump and col~ z.
its workings and the surrounding environrnent.

The in~ .lt described above is int~n~ed for use in open air or gas filled envin~in body fluids, or by insertion into tissue by the creation of a conductive fluid en~,i,ùnlllcllL
15 around the tip of the instrument, and it is so arrdnged that it is possible to create a local saline field at a distal end of the hlsll.~ ent. This instrument can, the,. fole, be used for laparoscopic applications. In use, saline is fed to the active electrode 3 I via the channel 37, the saline providing a conductive medium to act as a conductive path between the tissue being treated and the return electrode 32. By varying the output of the gt;n~ldtor 1, 20 the instrument can be used for tissue removal via vaporisation~ for cutting or for desiccation. In each case, as saline contacts the active electrode 31, it heats up until it reaches an equilibrium te~ .d~llre dependent upûn the power output of the generator 1 and the flow rate of the saline. ln equilibrium, as fresh saline is fed via the cll~nn~ol 37 to the active electrode 31, the exterior tcn.,ucldl~lre of the shaft is m~int~in~ri at the sarne 25 te.llu~.dlL~re as of that of the surrounding saline. As the inc~ ting sheath 33 completely covers the external surface of the return electrode 32, accidental contact between the return electrode and tissue is avoided.

One of the advanta~es of using a low saline flow rate, is that the sahne telll,u~ld~ulc can 30 reach boiling point. However, as there is a continuous flow of saline, there is a te~llp~dlLlre gr~rlient rise in the saline from the return electrode 32 to the active electrode CA 022423~2 1998-07-06 31. This temperature gradient is important, as the hotter saline adjacent to the active electrode 31 reduces the power threshold requirement to reach vaporisation. Although the flow rate re~uirement can be calc~ ted on the basis of the input power. the flexibility of the generator I in m~int~ininy optimurn power density means that the flow rate is non-5 critical. For example, if the generator I is set for 100 W~ then the maximurn flow rate istheoretically calculated as follows:
Flow rate = power/specific heat capacity 100/4.2 x 75 cc~s 0.32 cc/s = 19cc/min This assumes an initial saline temperature of 25~C. and a heat capacity of 4200 J/kg/CC.

Although during vaporisation saline is brought into the vapour state, the vapour is only 15 stabie around the active electrode 31. Thus, the energy absorbed by virtue of the latent heat of vaporisation can be ignored~ as this energy is recovered by freshly-arriving saline.

Another hl.~o~ t factor is that. due to the very short circuit path of the saline~ the current may be regarded as flowing along a nurnber of different paths, which. therefore, do not ~0 have the same power densitv. Consequently, vaporisation can occur at flow rates higher than the calculated ma~cimum~ due to the unequa} power densities within the saline environment. However, the amount of vaporisation occurring along the length of the active electrode 31 will depend upon the flow rate.

25 As the saline is heated up by the active electrode 31, it is potentially ~m~ging to tissue as it can cause thermal necrosis. It is important, therefore. that all the heated saline is recovered and e~h~l-sted from the patient before coming into contact with the tissue adjacent to the application site. It is for this reason that there is suction from the active electrode 3 I to an exhaust reservoir (not shown). However, by ensuring that the suction 30 occurs in excess, no saline can then escape from region of the active electrode 31 other than via the saline return channel 38. Any saline which escapes transversely beyond the exterior shaft falls away from the current path, and so is not heated. The priority is~
therefore. to ensure that the hottest saline is removed. As the thermal gradient is at a ma~m~ dj~cent to the active electrode 31 this is the most ap~ iate ~yh~llct point for the saline. It is for this reason that the saline is exh~l-cte~ through the cylindrical portion 5 35a of the insulatortspacer 35.

Another hll~o~ t consiciP~tinn in deciding the point of saline evacuation is the potential for blockage of the exhaust path. This could occur when cutting or vaporising tissue in such a way as to free small tissue particles which could easily block the exhaust. The 0 ~xh~lct point is, therefore, selectecl to be at the highest energy density point on the active electrode 31. This measure ensures that any tissue appro~ching the exhaust point is instantly vaporised into solution. thereby avoiding the potential for blockage.

Another significant advantage of ensuring a high degree of suction during tissue removal 15 by vaporisation, is that any smoke which has not been absorbed by the saline is also eV~cn~t~ This is illlpOl~lt, because smoke is capable of transmitting viable biological particles, and this could lead to infection.

As mentioned above, the power threshold for vaporisation is not well defined. If the 20 insL~lle.ll were operating in a static conductive medium~ then the vaporisation threshold would be well defined by an impedance switching point where the electrode impedance sn~ nly rises as a result of vapour pockets forrning around the active electrode 31. The threshold is normally dependent upon the flicsir~tion mer.h~ni~m of the saline. In a static e"~iro~ e.ll, the fiiCcir~tion mecl~ is predomin~ntly by convection currents within 2~ the saline. Under these cirC-~rnct~nres the power threshold for vaporisation is define~3 by ~he input power into the electrode active region being in excess of the dissipation from the saline. However, in the embodiment, described above, the saline around the active electrode 31 is continually refreshed. If it were not, then the only dissipation mech~nicm would be by latent heat of vaporisation, and the saline would quickly evaporate. By 30 providing a flow, the threshold power level is increased. However, the threshold power level is dependent on the saline refresh rate at the very periphery of the active electrode CA 022423~2 1998-07-06 31. The refresh rate at this boundary layer can be modified by altering the surface finish of the active electrode 31. For example, if the active electrode 31 had a smooth surface, then saline would be rapidly refreshed, as a rapid flow rate would be established.
However. as the active electrode 31 has an irregular finish, the refresh rate of pockets S within the irregular surface is r~imini~hecl Thus~ the irregular surface traps saline (or at least delays the refresh) and vapour. and so absorbs more power before being replaced.
In other words, the power threshold is decreased by the irregular active electrode surface.
This is a highly desirable ~)lUp~ y, as the electrode power requirement drops ~ lly without adversely effecting tissue perforrnance. The threshold power is further reduced 10 because the active electrode 31 is constructed so as to provide a capillary action. Thus, even in the vaporised state. the active electrode 31 is interrnittently wetted. By en~u,illg that this wetting wets the entire active electrode 31 by capillary action, there is a con~
source of vapour which minimices the intermittent wetting, and so further reduces the power clem~n~
The return electrode 32 has a smooth polished surface which has no impe~imerlt to convection currents. Conse~uently, the return electrode 32 does have a coll~l~ltly ch~ngin~ saline boundary layer which is replaced at a high rate, and the return electrode has a high power threshold. Moreover, the return electrode 32 forrns one edge surface of ~0 the saline feed channel 37, so that there is a turbulent flow of saline along the retum electrode. This results in the boundary layer replacement being very rapid, and the electrode 32 itself being cooled by the flow. The reslllt~nt h~ ase in the power threshold of the return electrode 32 means that vaporisation can never occur at the return electrode.
Indeed, the power threshold of the return electrode 32 is increased in this way so that it 25 is considerably in excess of the maximurn available power. This ensures that, even if the return electrode 32 is partially obscured~ or the flow of saline impeded, the power threshold at the return electrode will never be rP~c~P~ As the power threshold for vaporisation at the return electrode 32 cannot be re~clle~ there is no risk of tissue being vaporised by the return electrode. Collateral tissue darnage is, therefore, avoided.
30 Moreover. as the saline exhaust channel 38 is inside the return electrode 32, the hottest saline is removed efficiently, therebv precluding tissue darnage by plumes of heated saline leaving the active electrode 31.

By varying the output of the generator 1~ the electrode unit El can also be used for 5 desiccation (coagulation). In this case, the generator I is controlled so that small vapour bubbles form on the surface of the active electrode 3 l, but insufficient vapour is produced to provide a vapour bubble (pocket) surrounding the active tip of the electrode, the vapour bubble being e~.cPnti~l for tissue removal by vaporisation.

10 The generator I is controlled in such a manner that it has ~cs~eclive output ranges for tissue desiccation and for tissue removal by vaporisation. The former range is from 150 volts to 200 volts, and the latter range is from 250 volts to 600 volts, the voltages being peak voltages. In the vaporisation mode, the generator I is controlled in such a ~ el as to prevent the active electrode 31 ov~,l.e~l;n~ This requires a reduction in the output 15 voltage of the ~ elalor I once a vapour pocket has been established. The g. ,l~ ul I and its control means are described in greater detail in the specification of our co-pending European Patent Application No. 963045~8.g.

The coagulation from this electrode is vastly superior to any conventional bipolar 20 electrode. The reasons are t~,vo-fold. Firstly, the coagulation mech~nicm is not merely by electrical current in the tissue, but is also due to the heated saline. Secondly, under normal ch.;~ .r~s~ the weakest link in providing electrical power to the tissue is the electrode interface, as this is the point of highest power density, and so imposes a power limit. If too high a power level is alL~ )tt:d, the tissue at the int~rf~rP~ quickly desiccates, far faster 25 than the larger cross-section of tissue forming the rem~inin~ circuit. If a lower power is selected, the interface can dissipate the te~ c~ rise by mP~nicmc other than vaporisation. Conse~uently, the int~,~ce leln~,s intact longer, and so a greater depth of effect can be achieved. In this embodiment, the electrical interface is much stronger by virtue of the saline, and it is not possible completely to desiccate the target tissue. Thus, 30 power can be delivered at a higher rate and for a longer period, resulting in a depth of effect which is purely time and power related.

CA 022423~2 1998-07-06 ~0 Vaporisation threshold control is an important aspect of such a multi-functional active electrode, the active electrode area being maximised for desiccation, whilst still being capable of vaporisation or cutting functions by retaining the vapour pocket and heated saline in the interstices of the active electrode.

As mentioned above, a fundamental feature of the design of a bipolar electrosurgical instrument is the ratio of the contact areas of the return electrode and of the active electrode. This ratio should be high for vaporisation and low for desiccation. A b~l~nre must, therefore. be struck for multi-functional electrodes. The electrode unit El achieves 10 this balance by minimicing the ratio to ensure efficient desiccation, and by providing vaporisation threshold control to ensure efficient vaporisation.

Figure 4 shows the electrode assembly of the second forrn of electrode unit E2. This unit E2 has a shaft (not shown) for detachably f~stening the unit to the electrosurgical 15 instr~ment handpiece 3 . The electrode assembly is positioned at the distal end of the shaft, means (not shown) being provided at the other end of the shaft for conn~ctinE the electrode assembly to the handpiece 3 both me-~h~nically and electrically.

The electrode assembly includes a centraL tissue contact (active) electrode 41 which is ~0 exposed at the e~ctreme distal end of the in~ ent. The active electrode 41 is made of twisted strands of a metal such a tnngcten or a noble metal such as platinum, or a pl~tinnm alloy such as pl~tinn~n cobalt, pl~tinllm/iridium or pl~tinllmltnngcten The active electrode 41 is electrically connected to the RF generator by a central conductor ~not shown). An incnl~ting sleeve 42 surrounds the active electrode 41 and the inner conductor, ~5 the distal end of the insulating sleeve being exposed p,.~xil"ally of the exposed part of the electrode 41. The sleeve 42 is made of a ceramic material, silicone rubber or glass. A
return electrode 43 surrounds the sleeve 41, the return electrode being in the form of a st~inlçss steel tube. The return electrode 43 is constituted by the distal end portion of the shaft of the in~llul~lellt, and is electrically cor~n~cted to the RF generator. An outer 30 inc~ ting polyamide coating (not shown) surrounds that portion of the shaft adjacent to the return electrode 43.

CA 022423~2 1998-07-06 The electrode ur~it E2 of Figure 4 is int~n~te~i for tissue removal by a vaporisation within a ~lictçn.~ion medium in the form of an electrically conductive liquid such as saline. In this case, the power threshold required to reach vaporisation is dependent on the power diccip~tion capability ofthe active electrode 41 and the flow characteristics around it. As S the electrode assembly is h.llllc.~ed in saline, power ~ ip~tion is by electrical conversion to heat. The heated saline rises as a plume from the active electrode 41 by the action of convection. Under these circ~Tm~t~nces~ the power threshold of vaporisation is dependent on the maximum rate of convection from the active electrode.

10 The highest power density exists at the surface boundary of the active electrode 41.
Power density falls off at a rate pl~yol donal to 1 /d' where d is the ~ist~nre away from the active electrode 41. Therefore, it is the ssline at the surface of the electrode 41 which defines the power threshold. The rate of saline repl~ce.n~nt by convection and condllctiQn losses at this point defines the power threshold. As soon as this boundary layer vaporises, 15 then the electrode 41 becomes stable in vaporisation with a lower power level.

The irregular surface of the active electrode 41 traps saline, and so absorbs more power before being replaced. A highly polished active electrode would have a constantly ch~nging saline boundarv layer, due to the convection currents "washing" its surface. In 20 this case. the boundary layer would be replaced at a high rate, so there would be a high power threshold. The irregular surface of the active electrode 41, however, results in the trapping of saline tand vapour) so that the saline boundary layer changes at a low rate.
Thus, the irregular surface of the active electrode 41 defines a number of peaks and troughs. The saline at the boundary layer of the peaks will be replaced readily by the 25 convection currents. However, the convection of saline in the troughs will be impeded.
Thus, the saline in the troughs will not be replaced as quiclcly, and so will absorb more power before being replaced. In other words, the power threshold is decreased by the irregular surface of the active electrode 41. As with the embodiment of Figure 2, this is desirable as the electrode power requirement drops subst~nti~lly without adversely 30 affecting tissue perforrn~n~e. The threshold power is further reduced because the active electrode 41 is constructed so as to provide a capillary action. Thus, even in a vaporised state, the active electrode 41 is intermittent}y wetted. By ensuring that this wetting wets the entire active electrode 41 by capillary action. there is a continual source of vapour which minimicec the intermittent wetting, and so further reduces the power ~1em~nrl In the electrode ~t E2 of Figure 4. the strands are shown loosely twisted so that ~ rent strands touch each other either at spaced positions or not at all. Such a structure leaves a series of openings in the electrode that connect to a central axial cavity within the electrode structure Iying along the longitudinal axis of the electrode. To prevent the electrode from fraying at its tip, the distal ends of the strands may be connected together, 10 such as by welding or another fusing method.

Referring to Figure ~, in a variation on the embodiment of Figure 4. an altemative electrode unit E3 has a plurality of conductive strands which are twisted or otherwise interlaced tightly about each other, so that adjacent strands press tightly against each 15 other, causing any cavities Iying along the electrode longitudinal axis within the twisted structure to be small or non-existent. In this embo~im~ns subst~nti~lly all the pockets for trapping conductive fluid are located at tne outer surface of the electrode, in and along the joins between adjacent strands. The ~,~ef~ d material for the strands is an alloy of pl~tinllm and iridium. The tightly wound configuration provides a more rigid structure 20 than that of electrode unit E shown in Figure 4. Again, the strands are welded together at the extreme distal end of the electrode.

As yet a further alternative electrode structure, not shown in the drawings, the central-tissue contact (active) electrode 41 may be formed from a single length of conductive 25 material with helical ridges forrned in its outer surface, either created by moulding, m~rhining, or by twisting a piece of the material (preferably of non-circular cross section) about its longin~lin~l axis to cause spiralling ridges about the outer surface. As before, the ridges create pockets therebetween. Formation of spiralling ridges from a non-circular cross-section length of material may be l,~.ro~.lled by twisting the material so that the 30 ridges are formed in the same way as ridges are formed when an elastic band is twisted about itS own axis.

I he above described altematives to the twisted and interlaced structure of Figure 4 may also be used in the embodiment of Figure 3.

Figures 6 to 8 show modified versions E4 to E6 of the electrode units E2 and E3 of 5 Figures 4 and 5, so iike reference nurnerals will be used for like parts, and only the moflific~tionc will be described in detail. ~hus, the electrode unit E4 of Figure 6 includes an active electrode 51 in the form of a helical coil, the active electrode being made of , a noble metal such as pl~timml~ or of a pl~tinllm alloy such as pl~tinnm/iridium, pl~tinum/cobalt or pl~tint-m/tl-ngctçn In use, saline is trapped between ~ nt turns of 10 the coil, so here again the saline boundary layer changes at a low rate, thereby ensuring that the active electrode 51 has a low power threshold. The active electrode S } has the additional advantage that saline is trapped within the coil itself, thereby leading to a further reduction in the repl~rem~nt rate of saline at the boundary layer, and a consequent further reduction in the power threshold.
Figure 7 shows an electrode unit E5 having an active electrode 61 in the form of a brush col~sliluled by a plurality of fil~mentc made of tlm~cten~ a noble metal such as platinum~
or a pl~tinum alloy such as pl~tinllm/iridium~ pl~tinumlcobalt or pl~tinllm/tlln~sten In use, saline is trapped within the strands of the fiT~m~nt~ once again leading to a reduction ~0 in the repl~rennpnt of saline at the boundary layer, and a reduction in the power threshold.
The fil~mentc of the brush electrode 61 also provide a capillary action, further reducing the power threshold.

The electrode unit E6 of the embodiment of Figure 8 is similar to that of Figure 6, having 25 an active electrode 51 is in the forrn of a coil made of tlm~tçn, a noble metal such as platinum, or a pl~tinum alloy such as pl~tinnmliridium, platinum/cobalt or platinum/~ In tnis embodiment however, the ins~ tin~ sleeve 42 is formed with an arcuate extension 42a which co~ iLules a shroud. The irmer surface of the shroud 42a closely overlies the turns of the coil electrode 51 over about half its circumference. The 30 shroud 42a does, therefore, impede convection current flow? thereby illeleds?illg the ability of the electrode assembly to trap saline. and so leads to a further decrease in the power CA 022423~2 1998-07-06 threshold. This electrode assembly benefits from a secondary mech~ni~m Thus, when in the vaporising state, tissue destruction yields gaseous products. The shroud 42a captures these gaseous products, and so excludes conduction by virtue ofthe incul~ting plO~JC~lieS
of these gaseous products.
s Figure g shows a further form of electrode unit E7 having an active electrode 71 in the form of a roller ball. The roller ball electrode 71 is made of stainless steel, and is rotatably supported on an arrn 72 made of an electrically-conductive material such as copper. A
generally h~micrh~rical shroud 73 is fixed to the arm 72 so as to closely surround about 10 half ofthe area of the ball electrode 71. The shroud 73 is made of an insulating material such as a ceramic material. silicone rubber or glass. A return electrode 74 made of stainless steel is mounted on that side of the shroud 73 remote from the ball electrode 71.
Here again, the shroud 73 traps saline between its inner surface and the outer surface of the roller ball electrode 71. so the power threshold of the active electrode is re~luce~l The 1 S shroud 73 also traps the products of vaporisation to reduce the effective size of the large active electrode 71. Moreover, by excluding a direct return path through the saline, the return: active area ratio is effectively i~ ased. This feature reduces the amount of power required to support vaporisation, and enables the use of a much larger active electrode 71 than would otherwise be possible. Another advantage of the shroud 73 is that it preserves 20 the environrnent in the immediate region of the active electrode 71 from disturbances which otherwise would be created by the flow of saline.

Figure 10 shows another forrn of electrode unit E8 having an active electrode 81 which is con~titllte~l by a needle electrode 81 a made of t m~sten~ a noble metal such as pl~tinltm, 25 or a pl~tinnm alloy such as pl~finllrn/iridiurn, pl~tinllm/cobalt or pl~timlm/tllng~ten coated with a conductive ceramic material 81b. The coating 81b increases the power rli.~sip~tjon at the saline boundary layer, by increasing the local power density within the active electrode 81. This results in an increase in the interfacing impedance between the electrode 81 and the saline. This increase in power ~ ip~tion leads to a reduction in the 30 power threshold of the electrode 81. This method of reducing the power threshold of an active electrode 81 is particularlv useful for situations where active electrode is WO 97~24993 PCT/GB97/00065 necessztrily very small due to the limitzttions imposed by certain operational requirements.
Obviously, the electrode 81 a could be coated with any other highIy resistive inert material, such as a highly resistive metal plating which is capable of with~t~tnrling the elevated te",~e.~ res associated with the vaporisation of tissue. Alternatively, the local power 5 density of the electrode 81a could be increased by spraying it with a porous incul,tting material such as a ceramic material, the spraying being such as to produce spots of insulation on a conductive s~lrfz~ce.

The return electrode of each of the embo-lim~ntc of Figures 4 to 10 has a smooth polished 10 surface which has no impe~timPnt to convection currents. As with the embodiment of Figure 2, therefore, each of these return electrodes has a high power threshold for vaporisation, so that there is no risk of tissue being vaporised by the return electrode, and no risk of collateral tissue damage. 7'he electrode assembly of each of these embot1;r..~
could be positioned zttlj~c~nt to the saline supply port of an endoscope so that saline will 15 flow over the return electrode to provide a turbulent flow of saline along that electrode.
This would result in the boundary layer replace.nel1t at the return electrode being very rapid. and further increase the power threshold of the return electrode.

As mentioned above, mulLirl~t.clional electrode units require vaporisation threshold ~0 control, and a minimum for the ratio of the contact areas of the return electrode and the active electrode. The minimum ratio depends on four h..~olLallt criteria. narnely:
1. The intrinsic il~.l,e;l~re of the target tissue;
2. The volume of the body cavity;
3. The configuration of the active electrode.
25 4. The maximum output power from RF generator.

The configuration of the active electrode obviously influences the ratio, with cylindrical forrns lep~se.~ the lowest ratio for a given length, but the other factors relate to the ability of the electrode to retain the vapour bubble. The fil~n~entc of the brush-type 30 e}ectrodes retain vapour bubbles, which helps m~int~in the vaporisation condition. As a result, the ratio for this type of electrode can be lowest of the multifunctional electrodes;

and, when combined with application to tissue with high impedance, the ratio is similar to that for desiccate functions, that is in the region of 1:1 to 2:1. With solid electrode forms~ however. the transition and m~intPrl~nre of the vaporisation condition at similar ratios ~ ~les very high power levels ~greater than 150W at l.5rnm diameter) for a given S electrode size. As a result~ the ratio must be elevated for these forms to the region of 2:1 to 3 :1. Ch~ ing the exterior surface with a variety of grooves or cuts, or by using coiled wire to produce a similar form, assists vaporisation perfoll,l~lce by stim~ ting the vapour pocket retention of the brush-type electrodes, thereby allowing a reduction in the ratio.

An arthroscopic electrode may be characterised as short ( 100-1 40rnrn), rigid, and having a worlcing diameter up to 4mm. If can be introduced through a stab incision into a joint cavity (with or without a cannula) using the triangulation technique. It is operated with a motion which commonly moves the electrode between the 9 o'clock and 3 o'clock positions on the arthroscopic image. As a result, the tissue to be treated is commonly 15 approached at a shallow working angle with respect to the axis of the electrode. The active electrode, lhc~ e, needs to include a range of end-effect to side-effect ~lo~,.Lies.
In certain circumstances~ an end-effect is desirable, particularly as an end-effect is very difficult to obtaining using a shaver device wherein the centre of rotation represents the desired point of application. The tissue to be treated (such as meni~c~l cartilage) is 20 commonly dense and of a high electrical impedance w~th a free edge of the cartilage le~ s~ the common site of injury where tre~tment is required. ~he electrode units E1, E2, E3, E4, E5 and E8 are end-effect electrode units suitable for arthroscopic use.

Either extensions or side-effect configurations of the in~ tor material assist with 25 engagement~ and prevent unwanted effects occurring in ~ rent s~ ;Lu~s - usually the articular surfaces of the femur and tibia. In addition, the extension or side-effect electrode forrns (of Figures 8 and 9) also assist in r~ g tne vapour pocket, and prevent cooling ofthe saline in the imme~i~tP vicinity of the active electrode by the flow of saline irrigant commonly from the endoscope.

The risk of heating distension fluid within the joint cavity occurs primarily during power application to reach the vaporisation threshold. Once the threshold has been reached, power requirements typically fall by 30-50%. Reducing the ratio increases the power re~uirement to reach the threshold so that, despite the high impedance of the target tissue, S it is undesirable to reduce the ratio to the lowest value capable of sluhJOl lhlg vaporisation.
The feature of ~,~oli~alion threshold control retains vapour pockets and heated saline in the interstices of the electrode, and configures the jn~ tor to reduce the effect of irrigant flow, thereby assisting in re~-lrin~ the power required to establish vaporisation and hence the risk of unwanted he~ting By way of exarnple, the coiled wire-forrn electrode of Figure 6 entraps vapour products, as does the electrode of Figure 8 (a side-effect forrn with the added feature of the in~ tor shrouding the non-contact region of the active electrode). The addition of the insulator shrouding feature can halve the power re~uired to reach the vaporisation threshold.
Typically, in arthroscopic use, the primary fimction comprises rapid debulking of dense, avascular tissue. The volume of tissue removed can be increased for a given size of electrode by a colllbinalion of the vaporisation threshold control feature and by inc~asillg the output voltage from the RF generator I . Figure I 1 shows a scll~m~tic of the brush-~0 type electrode of Figure 8, wherein the vapour threshold is excee~t ~ and a vapour pocket,in~ir~e(~ by the l~rel~nce P, is established around each of the filaments. When applied to tissue, particularly fi~n, dense tissue such as that comprising meniscal cartilage, the result will be vaporisation of a series of grooves in the tissue co~ ,oilding each of the f;l~m~ntc Increasing the RF output voltage will increase the size of the vapour pockets 25 around each of the fil~mtonts which, because of the retention will reach the stage, shown in Figure 12, where they merge to ~orm a contiguous vapour pocket, indicated by the reference P', so that tissue which may otherwise have passed bet~,veen the fil~m~ntc is also vaporised.

Our co-pending European Patent Application No. 96304558.8 discloses discrimination between desiccation and vaporisation output functions. It also discloses that a blended function can be created by constantly alternating between these output states.
Vaporisation threshold control is particularly advantageous in these circ~-m~t~nl~es, as the hot saline created by the desiccate output phase is retained in proximity to the active electrode such that the v~l,ulis~lion threshold is rapidly exceeded during the vaporisation 5 cycle. This is useful as a method to achieve simultaneous desiccation when detaching muscle from bony ~ rhm~nt~, such as is ~rulllled in an acromioplasty of the shoulder joint, or when debulking ~ice~cecl tissue with a vascular component such as synovium.

The embodiment of Figure 9 is particularly useful with a resectoscope to ~.,.ÇOllll 10 electrosurgical vaporisation of the plu~Late (EVAP). This particular configuration comprises a roller bar (cylindrical) active electrode 71, typically 2.4 to 3rnm in ~ rnPter by 3 to 4 mm in width. It is evident that the return electrode ?4 could be mounted in an axially-separated arrangement on the shaft 72. Under these circ.~ ..ces, however, the size of the active electrode 71, and the exposure of the complete surface area to the 15 con~ tive environment as well as the cooling effect of irrigant flow over the electrode, would re~uire a very high power to reach the vaporisation threshold.

It will be appreciated that the electrode 71 can be grooved or ridged so as to further reduce the vaporisation threshold. Similarly, the side-effect active electrode of Figure 8 (which 20 could be axially or transversely mounted with respect to the axis of the resectoscope), could be substituted for the electrode assembly of Figure 9. In this case, the active electrode would not provide a mechanical rolling function.

This instrument can also be used to perform electrosurgical vaporisation of soft tissue 2~ tumours, such as a prostatic adenoma, without use of a dispersive return plate in a conductive fluid environment. It can also be applied to fibroids using a resectoscope in the uterine cavity.

The electrosurgical instruments described above also have irrigated electrode applications.
30 Thus, each utilises a method of creating a localised saline working envin)~ enl as a means of completing the electrical circuit of axially sepa~aled active and return electrodes WO 97t24993 PCT/GB97/00065 to perforrn tissue vaporisation, cutting and desiccation in a gas or air filled body cavity whether of natural origin or created surgically, or at a tissue surface of the body whether of natural origin or created surgically.

5 More specifically, each such instrurnent utilises a method of removing tissue by vaporisation wherein the products of vaporisation are aspirated from the site of application by suction through, or adjacent to, the active electrode assembly. Diseased tissue can be also removed by vaporisation from natural body cavities such as sin--ses, nasal cavities and the o~ ha~c. Similarly, ~ e~ l tissue can be removed by vaporisation from the 10 abdominal cavity under gaseous ~icten~ion.

Such an instrument can also be used to create the surgical access to an interstitial site where the tissue to be treated is Iying deep to the tissue surface.

Claims

1. An electrosurgical instrument for the treatment of tissue in the presence of an electrically-conductive fluid, the instrument comprising an instrument shaft. and a tissue treatment electrode at one end of the shaft, the tissue treatment electrode being constructed to define a plurality of pockets for trapping electrically-conductive fluid and vapour.

2. An electrosurgical instrument as claimed in claim 1, wherein the tissue treatment electrode is constituted by a plurality of interlaced strands of electrically-conductive material.

3. An electrosurgical instrument as claimed in claim 1 or claim 2, wherein the tissue treatment electrode comprises a plurality of strands of electrically conductive material, with the strands being wound about each other.

4. An electrosurgical instrument as claimed in claim 1, wherein the tissue treatment electrode comprises a shaft of electrically conductive material having spiralling ridges.

5. An electrosurgical instrument as claimed in claim 1. wherein the tissue treatment electrode is constituted by a generally helical coil made of electrically-conductive material.

6. An electrosurgical instrument as claimed in claim 1, wherein the tissue treatment electrode is constituted by a plurality of filaments made of an electrically-conductive material.

7. An electrosurgical instrument as claimed in any one of claims 1 to 6, furthercomprising an insulating shroud which extends along, and partially surrounds, the tissue treatment electrode.

8. An electrosurgical instrument as claimed in claim 1, wherein the tissue treatment electrode is constituted by a spherical member made of electrically-conductive material, the spherical member being mounted on the shaft of the instrument by means of anelectrically-conductive support member, the instrument further comprising an insulating shroud which partially surrounds the spherical member.

9. An electrosurgical instrument as claimed in any one of claims 1 to 8, wherein the tissue treatment electrode is made of a noble metal such as platinum.

10. An electrosurgical instrument as claimed in any one of claims 1 to 8, wherein the tissue treatment electrode is made of a platinum alloy such as platinum/iridium,platinum/tungsten or platinum/cobalt.

11. An electrosurgical instrument as claimed in any one of claims 1 to 8, wherein the tissue treatment electrode is made of tungsten.

12. An electrosurgical instrument as claimed in any one of claims 1 to 11, further comprising a return electrode which is electrically insulated from the tissue treatment electrode by means of an insulation member, the tissue treatment electrode being exposed at the extreme distal end of the instrument, and the return electrode having a fluid contact surface spaced proximally from the exposed end of the tissue treatment electrode by the insulation member.

13. An electrosurgical instrument as claimed in claim 12, wherein the fluid contact surface of the return electrode is formed with a smooth polished surface.

14. An electrosurgical instrument as claimed in claim 13, further comprising means for feeding electrically-conductive fluid over the fluid contact surface of the return electrode.

15. An electrode unit for an electrosurgical instrument for the treatment of tissue in the presence of an electrically-conductive fluid medium, the electrode unit comprising a shaft having at one end means for connection to an instrument handpiece, and, mounted on the other end of the shaft, a tissue treatment electrode, the tissue treatment electrode being constructed to define pockets for trapping electrically- conductive fluid and vapour.

16. Electrosurgical apparatus comprising a radio frequency generator and an electrosurgical instrument for the treatment of tissue in the pressure of an electrically-conductive fluid medium, the instrument comprising an instrument shaft, and an electrode assembly at one end of the shaft, the electrode assembly comprising a tissue treatment electrode and a return electrode which is electrically insulated from the tissue treatment electrode by means of an insulation member, the tissue treatment electrode being exposed at the distal end portion of the instrument, the return electrode having a fluid contact surface spaced proximally from the exposed end of the tissue treatment electrode by the insulation member, and the radio frequency generator having a bipolar output connected to the electrodes, wherein the exposed end of the tissue treatment electrode is constructed to define a plurality of pockets for trapping electrically-conductive fluid and vapour.

17. Apparatus as claimed in claim 16, wherein the radio frequency generator includes control means for varying the output power delivered to the electrodes.

18. Apparatus as claimed in claim 17, wherein the control means is such as to provide output power in first and second output ranges, the first output range being for powering the electrosurgical instrument for tissue desiccation, and the second output range being for powering the electrosurgical instrument for tissue removal by vaporisation.

19. Apparatus as claimed in claim 18, wherein the first output range is from about 150 volts to 200 volts, and the second output range is from about 250 volts to 600 volts, the voltages being peak voltages.