# Patent

## Title
Internally located return electrode electrosurgical apparatus, system and method

## Abstract

A bipolar, plasma-generating electrosurgical apparatus and system wherein the return electrode is enclosed within an electrosurgical shaft, and the active electrode is located on the outside surface of the shaft such that in treating the tissue, the tissue is exposed to plasma generated on the active electrode, but is minimally exposed to electric fields generated between the active and return electrodes. Due to the configuration of the electrodes, electric fields generated between the electrodes are directed away from the target tissue and inwardly towards the return electrode within the shaft, thereby electrical stimulation of neuromuscular structures in the tissue by the electric fields is minimized.

## Background
FIELD OF INVENTION
This invention pertains to an electrosurgical apparatus, system and method of treating body tissue, in particular a bipolar, plasma-generating electrosurgical apparatus and system wherein the return electrode is enclosed within an electrosurgical shaft, and the active electrode is located on the outside surface of the shaft such that in treating the tissue, the tissue is exposed to plasma generated on the active electrode, but minimally exposed to localized electric fields generated between the active and return electrodes. In various embodiments, due to the configuration of the electrodes, the electric fields are directed away from the target tissue, as they are oriented inwardly towards the return electrode within the shaft, thereby avoiding electrical stimulation of neuromuscular structures in the tissue by these electric fields.
BACKGROUND
Electrosurgical instruments and systems comprising an active and return electrode and powered by a radio-frequency voltage supply as is illustrated for example in FIG. 1, are widely used in procedures for treating target tissues in the body. Treatment of the target tissues involves placing the electrodes (10) in close proximity to a target tissue (12) and applying power to the electrodes to cause CoblationTM, heating, ablation, coagulation, cutting, removal, puncturing, probing, and otherwise stimulating the tissue. In some systems an electrically conductive fluid is supplied between the electrodes to generate plasma to treat the tissue; in other systems, the body's fluids are used as the conductive fluid. An example of such system for treating tissues with plasma is described in commonly assigned U.S. patent application Ser. No. 10/661,118, hereby incorporated herein by reference herein for all purposes.
In an electrosurgical system as illustrated in FIG. 1, the electrodes are located on the distal end portion of the shaft (14). In one configuration of the distal end portion of the shaft as is illustrated in detail FIG. 2, the return electrode (16) is positioned on the outside perimeter of the shaft and, in various embodiments, surrounds the active electrode (18) which is within the shaft. To ensure that plasma (20) generated on the active electrode is closest to the tissue, the distal tip of the active electrode projects beyond the return electrode. Also, in the embodiment illustrated in FIG. 2, the active electrode is separated from the return electrode by an insulator (24), and electrically conductive fluid (26) is supplied between the electrodes by a fluid lumen (27) circumferentially positioned on the shaft around the return electrode. This conductive fluid as is illustrated in FIG. 2 forms a conductive fluid pathway (38) between the electrodes.
Also in an electrosurgical system as is illustrated in FIG. 2 and as will be appreciated by one ordinarily skilled in the art, when power is applied across the electrodes, an electric field (22) sometimes in the order of 30,000 V/cm is generated which, for some procedures, is not desired as these fields can interact with the tissue and cause electrical stimulation of neuromuscular structures (28) within the tissue.
Accordingly, there is a need for apparatus and systems for use in electrosurgical procedures wherein unwanted electrical stimulation of the tissue is avoided, and which can be used in confined spaces within the body.
SUMMARY OF THE INVENTION
The present electrosurgical apparatus in one embodiment comprises an electrosurgical shaft having a proximal end portion and a distal end portion. The shaft includes an active electrode disposed on the surface of the distal end portion, and a return electrode disposed within the distal end portion of the shaft. Positioned between the active and return electrode is an insulating member that prevents direct electrical contact between the active and return electrodes. The shaft includes an interconnecting passageway within the distal end portion of the shaft between the active and return electrode. The electrodes are connected to a radio-frequency voltage supply by connectors such that on applying a radio-frequency voltage difference across the active and return electrodes, plasma is generated on the active electrode, and electrical fields generated between the electrodes are directed from the active electrode to the return electrode in the shaft, to avoid electrical stimulation of the tissue. The plasma can be used to treat the tissue by coblating, heating, ablation, coagulation, cutting, removal, puncturing, probing, and otherwise stimulating the tissue.
The present electrosurgical system in one embodiment is a system for performing an electrosurgical procedure on a body tissue using plasma such that electrical stimulation of the tissue is minimized, the system comprising: an electrosurgical instrument comprising a shaft; an electrically conductive fluid supply having a discharge port on a distal end portion of the shaft; and a radio-frequency voltage supply connected to the electrosurgical instrument. In one embodiment, the shaft has: an active electrode on the distal end portion; a return electrode recessed within the shaft; an electrical insulator separating the active and return electrode. Within the shaft is a chamber in communication with the active and return electrodes such that on applying the radio-frequency voltage supply to the active and return electrodes in the presence of an electrically conductive fluid, plasma is generated on the active electrode on the surface of the shaft, and electric fields generated between the active and return electrodes are directed within the shaft, and thus away from the tissue.
The present electrosurgical method in one embodiment is a method of treating tissue that avoids nerve stimulation, comprising the steps of: positioning a distal end portion of an electrosurgical instrument in close proximity to the tissue, the distal end portion comprising an active electrode and a return electrode; applying a radio frequency voltage across the active and return electrodes in the presence of an electrically conducting fluid sufficient to generate plasma on the active electrode; and contacting the tissue with the plasma such that the tissue is exposed to plasma but minimally exposed to electric fields generated between the active electrode and the return electrode.
In various embodiments the present apparatus and system can be used in procedures for treating highly neutralized tissue, and other tissues located in confined targets within the body. An example of such targets is tissue in the intervertebral discs.
Details of embodiments of the present apparatus, system and methods are illustrated and described the following Figures and specifications.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of a bipolar electrosurgical system.
FIG. 2 is an illustration of a cross-section of the distal end portion of an electrosurgical instrument showing plasma and electrical fields between the active and return electrodes.
FIG. 3 is an illustration of lateral view of a partial spinal column.
FIG. 4 is an illustration of a perspective view of an intervertebral disc.
FIG. 5 is an illustration of an embodiment of the present apparatus within a herniated intervertebral disc for treating the disc in accordance with one orientation of the apparatus.
FIG. 6 is an illustration of a perspective cut-away view of the distal end portion of the present electrosurgical shaft.
FIG. 7 is an illustration of a longitudinal cross-sectional view of the distal end portion of the present electrosurgical shaft.
FIG. 8 is an illustration of the cross-section of the distal end portion of the present apparatus showing conductive fluid and electrical fields between the active and return electrodes, without plasma.
FIG. 9 is an illustration of the cross-section of the distal end portion of the present apparatus showing conductive fluid flow, electrical field lines between the active and return electrodes, with plasma on the active electrode.
FIG. 10 is an illustration of a perspective view of an embodiment of the present active electrode.
FIG. 11 is as illustration the present electrosurgical system wherein the return electrode is enclosed with the distal end portion of an electrosurgical shaft.
FIG. 12 is an algorithm of the present method of treating tissue with the present apparatus and system.



DETAILED DESCRIPTION
With reference to FIGS. 1 and 2, a plasma-generating bipolar electrosurgical system typically comprises an electrosurgical shaft (14) having proximal (30) and distal (32) end portions; one or more active electrode(s) (18) located on the distal end of the shaft; a return electrode (16) located on the shaft of the return electrode and separated from the active electrode by an insulator (24); electrical connectors (34) coupling the active and return electrodes (18, 16) to a source of radio-frequency voltage supply (36); and a supply of electrically conductive fluid (26, 26a) adapted to be discharged between the active and return electrodes. On application of the radio-frequency voltage across the electrodes in the presence of the conductive fluid, plasma is generated which can be used to treat tissue as described for example in U.S. patent application Ser. No. 10/661,118, supra.
A bipolar electrosurgical apparatus, as is illustrated for example in FIGS. 1 and 2, is an electrosurgical apparatus wherein both the active and return electrodes (18, 16) are positioned on the shaft (14). In this regard, a bipolar apparatus is distinguishable from a monopolar apparatus in that on a monopolar apparatus only the active electrode is positioned on the shaft; in a monopolar apparatus the return electrode is located off the shaft but is in electrical contact through the patient to the target site and the electrically conductive fluid.
Examples of an electrically conductive fluid include isotonic saline, a conductive gel, Ringer's solution and the biocompatible electrolytes as described for example in U.S. patent application Ser. No. 10/661,118, supra.
In a bipolar electrosurgical apparatus as is illustrated for example in one embodiment in FIG. 2, the electrodes are separated from each other by an insulator (24) to prevent short-circuiting of the electrodes on the distal end portion of the shaft. However, to establish a closed electrical circuit across the electrodes on the shaft and generate plasma, an electrically conductive fluid pathway (38) is provided between the electrodes. This electrically conductive fluid pathway can be provided in several ways including placing the conductive fluid on the shaft such that the fluid is in contact with both electrodes; or placing the conductive fluid on the target tissue such that the fluid is in contact with both electrodes and the target tissue at the same time; or inserting the shaft into the tissue such that the electrical circuit between the electrodes is established through the tissue by conductive body fluids in the tissue.
In both bipolar and monopolar plasma-generating apparatus, however, regardless of how the conductive pathway is established between the electrodes, for the instrument to generate plasma it is necessary to maintain a closed electrical circuit on the distal end of the shaft comprising the electrodes, the electrically conductive fluid and the power supply, as described for example in U.S. patent application Ser. No. 10/661,118, supra.
On a bipolar plasma-generating systems and apparatus as illustrated in FIGS. 1 and 2 and as is described in commonly assigned U.S. patent application Ser. No. 10/661,118, supra, plasma is generated on the electrodes by applying a radio frequency voltage across the electrodes in the presence of the electrically conductive fluid (26, 38). With these systems and apparatus, plasma (20), comprised of energized charged species such as ions and electrons, is used to treat the target tissue by Coblation® as described in U.S. patent application Ser. No. 10/661,118, supra.
On a plasma-generating bipolar apparatus, in order to generate and use plasma to treat the tissue, the electrodes are designed such that only the active electrode generates the plasma, and that in use this electrode is located as close as possible to the target tissue. Conversely, the return electrode is designed such that it does not generate plasma, and that in use it is away from the target tissue to avoid contacting the tissue, but it is in electrical contact with the active electrode through the electrically conductive fluid. One way by which the plasma is generated on the active electrodes but not on the return electrode is to maintain the surface area of the active electrode smaller relative to the surface area of the return electrode.
In this regard it should be noted that during use, ablated tissues and other materials may accumulate on the return electrode thereby causing a reduction of its exposed surface area relative to the exposed surface area of the active electrode, thereby undesirably causing the return electrode to also generate plasma.
In a plasma-generating bipolar apparatus and system as is illustrated for example in FIGS. 1 and 2 and described in U.S. patent application Ser. No. 10/661,118, supra, a convenient way by which a relatively large return electrode is maintained is to use the shaft proximal of the active electrode as the return electrode. Typically this involves using an outer metallic portion of the shaft that is insulated from the active electrode. Thus, as is illustrated in FIG. 2 in a bipolar systems, in one embodiment, the active electrode is the distal tip of the shaft, whereas the return electrode is the shaft's outer surface insulated from the active electrode.
Also as is illustrated for example in FIGS. 1 and 2 and described in U.S. patent application Ser. No. 10/661,118, supra, in using a plasma-generating bipolar apparatus for some procedures it is necessary to supply the electrodes with an electrically conductive fluid to form the conductive pathway (26, 38) between the electrodes (18, 16), and in some embodiments also to flush the target site and the electrodes with fluid.
In procedures requiring a conductive fluid, this fluid can be provided by a fluid supply lumen located on the shaft. In this arrangement the fluid supply lumen is attached to a conductive fluid supply at the proximal end, such that the fluid is available for discharge at the distal end through an opening in the lumen near the electrodes and the target site.
Further, in a plasma-generating bipolar apparatus as is illustrated for example in FIGS. 1 and 2, for some procedures it is necessary to remove excess fluids and ablated tissue away from the target site. Where such fluid and tissue removal is necessary, an aspiration lumen is provided. In various embodiments the fluid aspiration lumen is located on the shaft but it can also be placed off the shaft in other embodiments. In an arrangement wherein the aspiration lumen is on the shaft, the lumen may comprise of a fluid inlet port disposed at the distal end of shaft, and a fluid discharge port at the proximal end where it is connected to a vacuum system for suctioning fluids, gases and ablated tissue from the target site through the aspiration lumen.
One procedure wherein a bipolar, plasma-generating apparatus is used for treating tissue is in treating an intervertebral disc as is described for example in U.S. patent application Ser. No. 10/656,597, incorporated herein by reference herein for all purposes. In one procedure as illustrated in FIGS. 3-5, the distal end of the shaft (40) is inserted in the disc (42) and thereafter radio-frequency voltage is applied across the electrodes to generate plasma to treat the disc. In FIG. 5, the wand (50) is shown inserted anteriorily into the disc, however as will be appreciated to one ordinarily skilled in the art, in other procedures not shown in FIG. 5, the wand is also insertable posteriorily into the disc.
As can be appreciated in the art in using a bipolar apparatus in treating a 10 intervertebral disc as is illustrated in FIGS. 3-5, besides generating plasma as described above, the apparatus also generates electrical fields (22) across the electrodes as is illustrated in FIG. 2, and these fields can be as high as 30,000 V/cm. A problem with these electrical fields is that in sensitive tissues such as in the disc and around the spine, the electric fields can cause undesired stimulation of regional nerve or nerve fibers (28) as is illustrated in FIG. 2.
Also as can be appreciated by one ordinarily skilled in the art, in treating tissue in confined spaces such as in the intervertebrate disc, it can be difficult to avoid contacting the tissue with the return electrode thus causing a short circuit across the electrodes or reducing the surface area of the electrode relative to the area of the active electrode.
Another problem with using a bipolar apparatus in confined spaces such as in the invertebrate disc is that since the shaft may include a fluid supply lumen and an aspiration lumen, the shaft can get too bulky for easy access and use.
Accordingly, the present apparatus, system and apparatus in various embodiments are adapted to electrosurgically treat tissue, while minimizing exposure of the tissue to electrical stimulation. The apparatus, as will be appreciated from the present description, is also reduced in size in part because of the placement of the return electrode with the shaft; thus, with a smaller profile on the distal end portion the present apparatus and system provides improved access flexibility for applying electrosurgical procedures in restricted areas of the body, as for example, within an intervertebral disc.
With reference to FIGS. 5-11, in one embodiment the electrosurgical apparatus (50) comprises a shaft (52) having a proximal end portion (54) and a distal end portion (56). On the distal end portion of the shaft is disposed an active electrode (58) on the surface of the shaft. Also disposed on the distal end portion but within the shaft is a return electrode (60) that is insulated from the active electrode by an insulating member (62) positioned on the distal end portion of the shaft. In this position, the insulating member prevents direct electrical contact between the active and return electrodes. Also included in the shaft in the present embodiment are electrical conductors (64, 66) that are adapted for applying a radio-frequency voltage difference across the active and return electrodes.
In one embodiment the electrosurgical apparatus comprises a lumen (70) within the shaft through which an electrically conductive fluid such as saline, Ringer's solution or another acceptable other biocompatible ionic solutions can be supplied to the distal end portion of the shaft in the vicinity of the electrodes and the target tissue. As is illustrated in FIGS. 1 and 11, the electrically conductive fluid can be supplied from a reservoir (26A) attached to the apparatus at the proximal end; in other embodiments not shown the reservoir is located on another apparatus.
In the embodiment illustrated in FIGS. 6-9, for example, the lumen is connected to an interconnecting passage (68) formed within the distal end portion of the shaft in between the electrodes. Within this interconnecting passage as is illustrated in FIG. 9, when a high frequency voltage is applied across the electrodes in the presence of an electrically conductive fluid, for example within the interconnecting passageway (68), plasma (74) which can be used to treat tissue is generated on the active electrode (58). Also, as noted above, when the power is applied to the electrodes, an electric field (76) is generated between the active electrode (58) and the return electrode (60) located within the shaft (52). Thus, since these electric fields are directed inwards, their effect on neighboring tissue is at least minimized, or eliminated.
In an embodiment of the active electrode illustrated in FIGS. 6-10, and in particular in FIG. 10, a plurality of apertures or holes (78) are provided on the electrode for passing an electrically conductive fluid between the outside of the shaft to the return electrode. In one embodiment the apertures are in the form of a mesh made of interwoven wires. Thus with this embodiment, both electrodes can be kept in electrical contact with an electrically conductive fluid within the interconnecting passageway (68).
Also in the embodiment of the apparatus illustrated in FIGS. 6-10, the return electrode is connected to a conductive cap (77) having an exposed surface on the outer surface of the shaft (52), such that the cap is spaced sufficiently far from the active electrode to minimize generation of an electric circuit between the active electrode and the cap. An advantage of using this cap is that if the cap is conductive, since it is connected to the return electrode, its conductive area contributes to the area of the return electrode and thus helps to ensure that the charge density on the surface of the return electrode is lower than the charge density on the surface of active electrode.
With reference to FIGS. 1 and 11, the electrosurgical apparatus in one embodiment comprises an aspiration lumen having an inlet in the proximity of the electrodes for removing fluids form the distal end portion of the shaft. The fluids may include fluids that flush the site as well as fluids that result from treatment of the tissue.
Also provided in the present application is a system for performing an electrosurgical procedure on a body tissue using plasma, as is illustrated for example in FIG. 11. The system (50) in one embodiment comprises an electrosurgical instrument comprising a shaft (52); an electrically conductive fluid supply having a discharge port on a distal end portion (56) of the shaft; and a radio-frequency voltage supply (36) connected to the electrosurgical instrument. In one embodiment, and as described above with reference to FIGS. 6-9, the shaft has: an active electrode (58) on the distal end portion; a return electrode (60) recessed within the shaft; an electrical insulator (62) separating the active and return electrode; and an interconnecting passageway (68) in communication with the active and return electrodes within the shaft, wherein on applying the radio-frequency voltage supply (36) to the active and return electrodes in the presence and electrically conductive fluid (72), plasma (74) is generated on the active electrode on the surface of the shaft, and electric fields (76) generated between the active and return electrodes are directed within the shaft.
Further provided is a method of treating body tissue including nerve-sensitive tissue in the body, as set forth in FIG. 12, comprising the steps of: positioning a distal end portion of an electrosurgical instrument in close proximity to the tissue (122), the distal end portion comprising an active electrode and a return electrode; applying a radio frequency voltage across the active and return electrodes in the presence of an electrically conduct fluid sufficient to generate plasma on the active electrode; contacting the tissue with the plasma (124) and thereby avoiding exposing the tissue to electric fields generated between the active electrode and the return electrodes.
By the present description and Figures it is to be understood that the terms used herein are descriptive rather than limiting, and that changes, modifications, and substitutions may be made without departing from the scope of the invention. Also it will be appreciated that although the present apparatus, system is described in the context electrosurgery on an intervertebral disc, the apparatus and its use is not restricted to treating discs but is applicable in general for electrosurgical procedures wherein is desired to minimize exposure of the tissue to electrical stimulation, and where access to the tissue is limited. Therefore the invention is not limited to the embodiments described herein, but is defined by the scope of the appended claims.

## Claims

1. An electrosurgical apparatus comprising:
a shaft having a proximal end portion and a distal end portion;
an active electrode disposed on a surface of the distal end portion of the shaft;
a return electrode disposed within the distal end portion of the shaft;
wherein the shaft comprises a conductive cap disposed at a distal end of the shaft and in electrical communication with the return electrode and wherein the cap is spaced sufficiently from the active electrode to minimize generation of an electric arc between the active electrode and the cap;
wherein the shaft comprises an insulating member positioned on the distal end portion of the shaft and preventing direct electrical contact between the active and return electrodes; and
electrical conductors adapted for applying a radio-frequency voltage difference across the active and return electrodes,
wherein the active electrode and return electrode define an interconnecting passageway within the distal end portion of the shaft such that the interconnecting passageway is between the active electrode and the return electrode, the interconnecting passageway allowing for electrical communication therein between the active electrode and the return electrode in the presence of an electrically conductive fluid, and so that upon application of the radio-frequency voltage difference across the active and return electrodes electric fields generated between the active and return electrodes are mostly contained within the shaft.

2. The electrosurgical apparatus of claim 1, wherein the return electrode comprises a lumen within the shaft.

3. The electrosurgical apparatus of claim 2, wherein the lumen and the interconnecting passageway comprise a continuous fluid conduit within the shaft.

4. The electrosurgical apparatus of claim 3, wherein the lumen is fluidly connected with a fluid supply reservoir to supply the electrically conductive fluid to the interconnecting passageway at the distal end portion of the shaft.

5. The electrosurgical apparatus of claim 4, wherein the lumen is adapted for receiving electrically conducting fluid at the proximal end portion of the shaft.

6. The electrosurgical apparatus of claim 4, wherein the electrically conductive fluid is selected from the group consisting of isotonic saline, Ringer's solution and other biocompatible ionic solutions.

7. The electrosurgical apparatus of claim 1, wherein plasma is generated on the active electrode upon application of the radio-frequency voltage across the active and return electrodes in the presence of an electrically conductive fluid.

8. The electrosurgical apparatus of claim 1, wherein the electric field is directed inward within the shaft from the active to the return electrode in the shaft.

9. The electrosurgical apparatus of claim 1, wherein the active electrode comprises one or more apertures therethrough for passing an electrically conductive fluid between the outside of the shaft to the return electrode.

10. An electrosurgical apparatus comprising:
a shaft having a proximal end portion and a distal end portion;
an active electrode disposed on a surface of the distal end portion of the shaft;
a return electrode disposed within the distal end portion of the shaft;
wherein the shaft comprises an insulating member positioned on the distal end portion of the shaft and preventing direct electrical contact between the active and return electrodes; and
electrical conductors adapted for applying a radio-frequency voltage difference across the active and return electrodes,
wherein the active electrode and return electrode define an interconnecting passageway within the distal end portion of the shaft;
wherein the shaft comprises a conductive cap disposed at a distal end of the shaft and exposed on an outer surface of the shaft, wherein the return electrode is electrically connected to the conductive cap, wherein the cap is spaced sufficiently from the active electrode to minimize generation of an electric arc between the active electrode and the cap.

11. The electrosurgical apparatus of claim 1, wherein the distal end of the shaft is sized for insertion into an intervertebral disc of a patient.

12. The electrosurgical apparatus of claim 1, wherein the shaft comprises an aspiration lumen having an inlet in the proximity of the electrodes for removing fluids from the distal end portion of the shaft.

13. An electrosurgical instrument for treating tissue within a body, comprising:
a shaft having a proximal end portion and a distal end portion;
an active electrode comprising a tissue-contacting surface on the distal end portion of the shaft;
a return electrode recessed within the distal end portion of the shaft;
wherein the tissue-contacting surface and the return electrode define an interconnecting passageway within the distal end portion of the shaft such that the interconnecting passageway is between the tissue-contacting surface and the return electrode, the interconnecting passageway allowing for electrical communication therein between the tissue-contacting surface and the return electrode in the presence of an electrically conductive fluid; and
wherein the shaft comprises an insulator separating the active and return electrodes,
wherein the tissue-contacting surface is adapted for generating plasma upon application of a radio frequency voltage across the active and return electrode in the presence of an electrically conducting fluid, and wherein upon application of the radio-frequency voltage across the active and return electrodes electric fields generated between the active and return electrodes are mostly contained within the shaft; and
wherein the shaft comprises a return electrode cap disposed at a distal end of the shaft and exposed on an outer surface of the shaft, wherein the return electrode is electrically connected to the return electrode cap at a location sufficiently spaced from the tissue-contacting surface to prevent an electrical circuit between the active electrode and the return electrode cap.

14. The instrument of claim 13, wherein the shaft comprises a fluid discharge lumen having a discharge port into the chamber.

15. The instrument of claim 13, wherein the shaft comprise a suction lumen having a suction port from the chamber.

16. The instrument of claim 13, wherein the active electrode defines an opening into the chamber.

17. The instrument of claim 16, wherein the active electrode comprises one or more holes opening into the chamber for fluid flows between the active and return electrodes.

18. The instrument of claim 15, wherein the return electrode comprises the suction lumen.

19. The instrument of claim 13, wherein the active and return electrodes are coupled to the radio-frequency power supply by one or more connectors.

20. A system for performing an electrosurgical procedure on a body tissue using plasma, the system comprising:
an electrosurgical instrument comprising a shaft;
an electrically conductive fluid supply having a discharge port on a distal end portion of the shaft; and
a radio-frequency voltage supply connected to the electrosurgical instrument, wherein the instrument further comprises:
an active electrode on the distal end portion;
a return electrode recessed within the shaft;
wherein the shaft comprises an electrical insulator separating the active and return electrode;
wherein the shaft comprises a conductive cap exposed on a distal end of the shaft, the cap electrically connected to the return electrode and sufficiently spaced from the active electrode to minimize generation of an electric arc between the active electrode and the cap; and
a chamber in communication with the active and return electrodes within the shaft, wherein the chamber is defined between the active electrode and the return electrode, and wherein on applying the radio-frequency voltage supply to the active and return electrodes in the presence of an electrically conductive fluid, plasma is generated on the active electrode, and electric fields generated between the active and return electrodes are mostly contained within the shaft.

21. The system of claim 20, wherein the body tissue comprises an intervertebral disc.

22. The system of claim 20, wherein the conductive fluid supply comprises a fluid delivery element that forms an electrically conductive fluid path in contact with the active electrode and the return electrode.

23. The system of claim 20, wherein the distal end portion of the shaft is sized for insertion into an intervertebral disc.

24. The system of claim 20, wherein the active electrode comprises a portion of an external surface of the shaft.

25. The system of claim 22, wherein the fluid delivery element comprises a fluid supply lumen extending through the shaft, the fluid supply lumen comprising an inlet port into the conductive fluid supply, and a discharge port in the proximity of the active electrode.

26. The system of claim 22, wherein the fluid delivery element comprises a fluid supply instrument separate from the shaft.

27. The system of claim 20, wherein a fluid aspiration lumen for aspirating fluid from a region around the active electrode is provided for removing fluid in the vicinity of the active electrode.

28. The system of claim 27, wherein the fluid aspiration element comprises a suction lumen extending through the shaft, the suction lumen having an inlet in the vicinity of the active electrode.

29. The apparatus of claim 20, wherein the active electrode comprises one or more openings positioned over the chamber for passing electrically conductive fluid into the chamber.

30. The apparatus of claim 20, wherein an electrically insulating support member is located within the shaft for isolating the active electrode from the return electrode.

31. An electrosurgical instrument for treating tissue within a body, comprising:
an elongate shaft, the shaft comprising a proximal end portion and a distal end portion;
an active electrode disposed on the distal end portion of the shaft;
a return electrode disposed within the distal end portion of the shaft;
wherein the shaft comprises an insulating member disposed on the distal end portion of the shaft and separating the active electrode and the return electrode;
wherein the shaft comprises a return electrode cap exposed on a distal end of the shaft at a location sufficiently spaced from the active electrode to prevent an electrical circuit between the active electrode and the return electrode cap, wherein the cap is electrically connected to the return electrode; and
an interconnecting passageway disposed within the distal end portion of the shaft, wherein the interconnecting passageway is formed between the active electrode and the return electrode and allows for electrical communication therein between the active electrode and the return electrode in the presence of an electrically conductive fluid, and so that upon application of a radio-frequency voltage difference across the active electrode and the return electrode electric fields generated between the active electrode and the return electrode are mostly contained within the distal end portion of the shaft.