Abstract:
Systems and methods for securing a screen-type active electrode to the distal tip of an electrosurgical device used for selectively applying electrical energy to a target location within or on a patient&#39;s body. A securing electrode is disposed through the screen electrode and mechanically joined to an insulative support body while also creating an electrical connection and mechanical engagement with the screen electrode. The electrosurgical device and related methods are provided for resecting, cutting, partially ablating, aspirating or otherwise removing tissue from a target site, and ablating the tissue in situ. The present methods and systems are particularly useful for removing tissue within joints, e.g., synovial tissue, meniscus, articular cartilage and the like.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to the field of electrosurgery, and more particularly to apparatus and methods for applying high frequency voltage to ablate tissue. More particularly, the present invention relates to apparatus and methods for securing a substantially flat screen-type active electrode to the distal tip of the shaft of an electrosurgical instrument. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventional electrosurgical methods are widely used since they generally achieve hemostasis and reduce patient bleeding associated with tissue cutting operations while improving the surgeon&#39;s visibility of the treatment area. Many of the electrosurgical devices used in electrosurgery make use of a screen-type active electrode which is typically cut, or etched, from a sheet of conductive material. These electrosurgical devices and procedures, however, suffer from a number of disadvantages. For example, screen-type active electrodes typically require some method of securement to an insulative body and furthermore to the distal tip of the device itself. Failure to adequately secure the screen electrode to the insulative body may result in improper device function and possible patient harm during the electrosurgical procedure. 
         [0003]    Prior attempts to secure the screen active electrode to the insulative body have involved mechanical, thermal, and chemical means or various combinations thereof. Numerous mechanical forms of securement have been utilized, while adhesives have been used as a chemical form of joining, and welding the screen may provide one thermal method of joining. These mechanical joining methods may also include the use of plastic, or non-recoverable, deformations of the materials being used for securement. However, even in combination with other joining methods, the above-listed methods for fixation provide only marginally effective solutions that typically are challenged over extended periods of use. 
         [0004]    Accordingly, devices and methods which allow for the securement of flat screen active electrodes to the insulative body of an electrosurgical instrument while maintaining electrical connections through the insulative body are desired. In particular, mechanical methods for providing reasonable and durable securement of an electrically connected screen active electrode to the insulative body at the distal tip of an electrosurgical device while providing enhanced electrosurgical operating parameters are desired. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides systems, apparatus and methods for mechanically securing a screen type active electrode to the insulative body at the distal tip of an electrosurgical device. In particular, methods and apparatus are provided for reliably securing the screen electrode over extended periods of use. Further, the methods and systems of the present invention are particularly useful for providing expanded and enhanced electrosurgical operating parameters. 
         [0006]    In one aspect of the invention, the method of securement comprises inserting a securing electrode through a channel or slot in both the screen electrode and insulative body. In a configuration where the screen electrode is supported by the insulative body, the securing electrode functions to mechanically couple the screen electrode to the insulative body, and are also electrically coupled the screen electrode and a high frequency power supply via electrical connectors. Thus, the securing electrode provides a mechanical method of joining the screen electrode to the insulative body while also allowing for an electrical connection to transmit RF energy from the screen electrode to the securing electrode. 
         [0007]    Another configuration of the electrosurgical device according to the present disclosure comprises an active screen electrode having at least two bilateral channels therethrough. At least two bilateral securing electrodes are provided and are respectively inserted through the channels of the screen electrode. Additionally, the device comprises an insulative support member having at least two bilateral channels correspondingly positioned with regard to the screen electrode channels. The bilateral securing electrodes are inserted through the support member and screen electrode channels and may be oriented symmetrically to thereby allow for creation of a zone for RF ablation between the two securing electrodes. 
         [0008]    In certain configurations, the securing electrodes may be characterized by a saw tooth pattern on a superior surface. Additionally, the securing electrodes may be formed in the shape of a staple or bridge, thereby allowing for the creation of another zone of RF ablation in a space between the staple securing electrode and the screen electrode. The added edges formed on the securing electrode in these configurations may result in increased current density and thus promote the formation of improved zones of RF ablation. 
         [0009]    In yet another configuration, the active electrode comprises a conductive screen having a plurality of holes and is positioned over the insulative body at the distal tip of an electrosurgical device in relation to the distal opening of an aspiration lumen. In the representative embodiment, the screen electrode is supported by the insulating support member such that the one of the plurality of holes on the screen is aligned with the aspiration lumen opening, thereby allowing for the aspiration of unwanted tissue and electrosurgery byproducts from the target site. Additionally, the screen and the distal opening of the aspiration lumen may be positioned on a lateral side of the instrument (i.e., facing 90 degrees from the instrument axis). 
         [0010]    In open procedures, the system may further include a fluid delivery element for delivering electrically conducting fluid to the active electrode(s) and the target site. The fluid delivery element may be located on the instrument, e.g., a fluid lumen or tube, or it may be part of a separate instrument. Alternatively, an electrically conducting gel or spray, such as a saline electrolyte or other conductive gel, may be applied to the tissue. In addition, in arthroscopic procedures, the target site will typically already be immersed in a conductive irrigant, i.e., saline. In these embodiments, the apparatus may not have a fluid delivery element. In both embodiments, the electrically conducting fluid will preferably provide a current flow path between the active electrode terminal(s) and the return electrode(s). In an exemplary embodiment, a return electrode is located on the instrument and spaced a sufficient distance from the active electrode terminal(s) to substantially avoid or minimize current shorting therebetween and to isolate the return electrode from tissue at the target site. 
         [0011]    In another aspect of the invention, a method comprises positioning one or more active electrode(s) (which may include an active screen electrode and securing electrode) at the target site within a patient&#39;s body and applying a suction force to a tissue structure to draw the tissue structure to the active electrode(s). High frequency voltage is then applied between the active electrode(s) and one or more return electrode(s) to ablate the tissue structure. Typically, the tissue structure comprises a flexible or elastic connective tissue, such as synovial tissue. This type of tissue is typically difficult to remove with conventional mechanical and electrosurgery techniques because the tissue moves away from the instrument and/or becomes clogged in the rotating cutting tip of the mechanical shaver or microdebrider. The present invention, by contrast, draws the elastic tissue towards the active electrodes, and then ablates this tissue with the mechanisms described above. 
         [0012]    In another aspect of the invention, an electrosurgical instrument is disclosed for applying energy to a target site within or on a patient&#39;s body. The instrument has a shaft with a proximal end and a distal end and an active electrode assembly disposed at the distal end of the instrument shaft. The active electrode assembly includes a substantially flat active screen electrode and at least one securing electrode, which is in electrical contact with the screen electrode. The instrument also has at least one electrically conductive connection element, mechanically and electrically coupled to the screen electrode proximal end, and the connection element is part of the electrical conduit between the active electrode assembly and an energy source. The instrument also includes an electrically insulating support member upon which the screen electrode is mounted and the support member has at least one securing electrode slot to receive a stem of the at least one securing electrode. The support member also has at least one conductive connection element opening to receive a portion of the conductive connection element. In some aspects of the invention, the instrument may also have at least one return electrode positioned on the shaft or as part of the shaft itself, and spaced away from the active electrode assembly. The insulative support member may also be formed as a one piece component, formed from a material operable to be resistant to electrosurgical plasma degradation such as an inorganic material (i.e., ceramic). 
         [0013]    In another aspect of the invention, an electrosurgical instrument is disclosed for applying energy to a target site within or on a patient&#39;s body, the instrument having a shaft defining a proximal end and a distal end and an insulating support member disposed at shaft distal end. The insulating support member includes a screen electrode support surface, at least one securing electrode opening on the screen electrode support surface, and at least one conductive connection element channel adjacent to the screen electrode support surface. An active electrode assembly is also disposed adjacent to the insulating support member, with a substantially flat active screen electrode disposed on the screen electrode support surface and the active assembly also includes at least one securing electrode, located adjacent the support member securing electrode opening. The active electrode assembly also includes at least one electrically conductive connection element electrically connected with the screen electrode proximal end, and a portion of the conductive connection element may be inserted into the conductive connection element channel. 
         [0014]    In yet another aspect of the invention, a method for securing a substantially flat active screen electrode to a distal end of an electrosurgical instrument is disclosed, the method including the steps of electrically connecting an electrically conductive connection element to the proximal end of the screen electrode, followed by inserting the connection element into a support member connection element channel. The screen electrode is then disposed upon the support member and at least one stem portion of a securing electrode is extended through the screen electrode and into a slot in the support member. This stem portion is then fixedly engaged within the slot in the electrically insulating support member, so that the securing electrode mechanically secures the screen electrode to the support member. The securing electrode is also electrically connected to the screen electrode. The connection element is then electrically coupled to an electrical connector that is electrically connected with a high frequency power supply. In some aspects of this disclosure, the step of inserting the connection element may involve bending the connection element so as to insert the connection element into the channel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a perspective view of an electrosurgical system incorporating a power supply and an electrosurgical probe; 
           [0016]      FIG. 2  is a perspective view of another electrosurgical system incorporating a power supply, an electrosurgical probe and a supply of electrically conductive fluid for delivering the fluid to the target site; 
           [0017]      FIG. 3  is a side view of an electrosurgical probe for ablating and removing tissue; 
           [0018]      FIG. 4  is a cross-sectional view of the electrosurgical probe of  FIG. 3 ; 
           [0019]      FIG. 5  is a detailed view illustrating ablation of tissue; 
           [0020]      FIG. 6  is an exploded view of the distal end portion of the probe of  FIG. 3  according to at least some embodiments; 
           [0021]      FIG. 7  is a perspective view of the distal end portion of a probe according to at least certain embodiments described in the present disclosure; and 
           [0022]      FIG. 8  is a perspective view of the securing electrodes and screen electrode. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The present invention provides systems and methods for selectively applying electrical energy to a target location within or on a patient&#39;s body. The present invention is particularly useful in procedures where the tissue site is flooded or submerged with an electrically conducting fluid, such as arthroscopic surgery of the knee, shoulder, ankle, hip, elbow, hand or foot. In other procedures, the present invention may be useful for collagen shrinkage, ablation and/or hemostasis in procedures for treating target tissue alone or in combination with the volumetric removal of tissue. More specifically, the embodiments described herein provide for electrosurgical devices characterized by a substantially flat screen active electrode disposed at the distal tip of the device. Additionally, the present embodiments include apparatus and methods for the mechanical securement of the screen electrode to the insulative body located at the distal tip of the device. Such methods of mechanical securement of the screen electrode may extend the operating period of the electrosurgical device by providing a more secure method of attachment. 
         [0024]    Before the present invention is described in detail, it is to be understood that this invention is not limited to particular variations set forth herein as various changes or modifications may be made to the invention described and equivalents may be substituted without departing from the spirit and scope of the invention. As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process act(s) or step(s) to the objective(s), spirit or scope of the present invention. All such modifications are intended to be within the scope of the claims made herein. 
         [0025]    Methods recited herein may be carried out in any order of the recited events which is logically possible, as well as the recited order of events. Furthermore, where a range of values is provided, it is understood that every intervening value, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. Also, it is contemplated that any optional feature of the inventive variations described may be set forth and claimed independently, or in combination with any one or more of the features described herein. 
         [0026]    All existing subject matter mentioned herein (e.g., publications, patents, patent applications and hardware) is incorporated by reference herein in its entirety except in so far as the subject matter may conflict with that of the present invention (in which case what is present herein shall prevail). The referenced items are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such material by virtue of prior invention. 
         [0027]    Reference to a singular item, includes the possibility that there are plural of the same items present. More specifically, as used herein and in the appended claims, the singular forms “a,” “an,” “said” and “the” include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. Last, it is to be appreciated that unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 
         [0028]    The electrosurgical device of the present embodiments may have a variety of configurations as described above. However, at least one variation of the embodiments described herein employs a treatment device using Coblation® technology. 
         [0029]    As stated above, the assignee of the present invention developed Coblation® technology. Coblation® technology involves the application of a high frequency voltage difference between one or more active electrode(s) and one or more return electrode(s) to develop high electric field intensities in the vicinity of the target tissue. The high electric field intensities may be generated by applying a high frequency voltage that is sufficient to vaporize an electrically conductive fluid over at least a portion of the active electrode(s) in the region between the tip of the active electrode(s) and the target tissue. The electrically conductive fluid may be a liquid or gas, such as isotonic saline, blood, extracellular or intracellular fluid, delivered to, or already present at, the target site, or a viscous fluid, such as a gel, applied to the target site. 
         [0030]    When the conductive fluid is heated enough such that atoms vaporize off the surface faster than they recondense, a gas is formed. When the gas is sufficiently heated such that the atoms collide with each other causing a release of electrons in the process, an ionized gas or plasma is formed (the so-called “fourth state of matter”). Generally speaking, plasmas may be formed by heating a gas and ionizing the gas by driving an electric current through it, or by shining radio waves into the gas. These methods of plasma formation give energy to free electrons in the plasma directly, and then electron-atom collisions liberate more electrons, and the process cascades until the desired degree of ionization is achieved. A more complete description of plasma can be found in Plasma Physics, by R. J. Goldston and P. H. Rutherford of the Plasma Physics Laboratory of Princeton University (1995), the complete disclosure of which is incorporated herein by reference. 
         [0031]    As the density of the plasma or vapor layer becomes sufficiently low (i.e., less than approximately 1020 atoms/cm3 for aqueous solutions), the electron mean free path increases to enable subsequently injected electrons to cause impact ionization within the vapor layer. Once the ionic particles in the plasma layer have sufficient energy, they accelerate towards the target tissue. Energy evolved by the energetic electrons (e.g., 3.5 eV to 5 eV) can subsequently bombard a molecule and break its bonds, dissociating a molecule into free radicals, which then combine into final gaseous or liquid species. Often, the electrons carry the electrical current or absorb the radio waves and, therefore, are hotter than the ions. Thus, the electrons, which are carried away from the tissue towards the return electrode, carry most of the plasma&#39;s heat with them, allowing the ions to break apart the tissue molecules in a substantially non-thermal manner. 
         [0032]    By means of this molecular dissociation (rather than thermal evaporation or carbonization), the target tissue structure is volumetrically removed through molecular disintegration of larger organic molecules into smaller molecules and/or atoms, such as hydrogen, oxygen, oxides of carbon, hydrocarbons and nitrogen compounds. This molecular disintegration completely removes the tissue structure, as opposed to dehydrating the tissue material by the removal of liquid within the cells of the tissue and extracellular fluids, as is typically the case with electrosurgical desiccation and vaporization. A more detailed description of these phenomena can be found in commonly assigned U.S. Pat. No. 5,697,882, the complete disclosure of which is incorporated herein by reference. 
         [0033]    In some applications of the Coblation® technology, high frequency (RF) electrical energy is applied in an electrically conducting media environment to shrink or remove (i.e., resect, cut, or ablate) a tissue structure and to seal transected vessels within the region of the target tissue. Coblation® technology is also useful for sealing larger arterial vessels, e.g., on the order of about 1 mm in diameter. In such applications, a high frequency power supply is provided having an ablation mode, wherein a first voltage is applied to an active electrode sufficient to effect molecular dissociation or disintegration of the tissue, and a coagulation mode, wherein a second, lower voltage is applied to an active electrode (either the same or a different electrode) sufficient to heat, shrink, and/or achieve hemostasis of severed vessels within the tissue. 
         [0034]    The amount of energy produced by the Coblation® device may be varied by adjusting a variety of factors, such as: the number of active electrodes; electrode size and spacing; electrode surface area; asperities and sharp edges on the electrode surfaces; electrode materials; applied voltage and power; current limiting means, such as inductors; electrical conductivity of the fluid in contact with the electrodes; density of the fluid; and other factors. Accordingly, these factors can be manipulated to control the energy level of the excited electrons. Since different tissue structures have different molecular bonds, the Coblation® device may be configured to produce energy sufficient to break the molecular bonds of certain tissue but insufficient to break the molecular bonds of other tissue. For example, fatty tissue (e.g., adipose) has double bonds that require an energy level substantially higher than 4 eV to 5 eV (typically on the order of about 8 eV) to break. Accordingly, the Coblation® technology generally does not ablate or remove such fatty tissue; however, it may be used to effectively ablate cells to release the inner fat content in a liquid form. Of course, factors may be changed such that these double bonds can also be broken in a similar fashion as the single bonds (e.g., increasing voltage or changing the electrode configuration to increase the current density at the electrode tips). A more complete description of these phenomena can be found in commonly assigned U.S. Pat. Nos. 6,355,032; 6,149,120 and 6,296,136, the complete disclosures of which are incorporated herein by reference. 
         [0035]    The active electrode(s) of a Coblation® device may be supported within or by an inorganic insulating support member positioned near the distal end of the instrument shaft. The return electrode may be located on the instrument shaft, on another instrument or on the external surface of the patient (i.e., a dispersive pad). The proximal end of the instrument(s) will include the appropriate electrical connections for coupling the return electrode(s) and the active electrode(s) to a high frequency power supply, such as an electrosurgical generator. 
         [0036]    Further discussion of applications and devices using Coblation® technology may be found in commonly assigned U.S. Pat. Nos. 6,296,638; 7,241,293 and 7,429,260, each of which are incorporated herein by reference. 
         [0037]    In one example of a Coblation® device for use with the presently-described embodiments, the return electrode of the device is typically spaced proximally from the active electrode(s) a suitable distance to avoid electrical shorting between the active and return electrodes in the presence of electrically conductive fluid. In many cases, the distal edge of the exposed surface of the return electrode is spaced about 0.5 mm to 25 mm from the proximal edge of the exposed surface of the active electrode(s), preferably about 1.0 mm to 5.0 mm. Of course, this distance may vary with different voltage ranges, conductive fluids, and depending on the proximity of tissue structures to active and return electrodes. The return electrode will typically have an exposed length in the range of about 1 mm to 20 mm. 
         [0038]    A Coblation® treatment device for use according to the present descriptions may use a single active electrode or an array of active electrodes spaced around the distal surface of a catheter or probe. In the latter embodiment, the electrode array usually includes a plurality of independently current-limited and/or power-controlled active electrodes to apply electrical energy selectively to the target tissue while limiting the unwanted application of electrical energy to the surrounding tissue and environment resulting from power dissipation into surrounding electrically conductive fluids, such as blood, normal saline, and the like. The active electrodes may be independently current-limited by isolating the terminals from each other and connecting each terminal to a separate power source that is isolated from the other active electrodes. Alternatively, the active electrodes may be connected to each other at either the proximal or distal ends of the catheter to form a single wire that couples to a power source. 
         [0039]    In certain configurations, each individual active electrode in the electrode array may be electrically insulated from all other active electrodes in the array within the instrument and is connected to a power source which is isolated from each of the other active electrodes in the array or to circuitry which limits or interrupts current flow to the active electrode when low resistivity material (e.g., blood, electrically conductive saline irrigant or electrically conductive gel) causes a lower impedance path between the return electrode and the individual active electrode. The isolated power sources for each individual active electrode may be separate power supply circuits having internal impedance characteristics which limit power to the associated active electrode when a low impedance return path is encountered. By way of example, the isolated power source may be a user selectable constant current source. In this embodiment, lower impedance paths will automatically result in lower resistive heating levels since the heating is proportional to the square of the operating current times the impedance. Alternatively, a single power source may be connected to each of the active electrodes through independently actuatable switches, or by independent current limiting elements, such as inductors, capacitors, resistors and/or combinations thereof. The current limiting elements may be provided in the instrument, connectors, cable, controller, or along the conductive path from the controller to the distal tip of the instrument. Alternatively, the resistance and/or capacitance may occur on the surface of the active electrode(s) due to oxide layers which form selected active electrodes (e.g., titanium or a resistive coating on the surface of metal, such as platinum). 
         [0040]    The Coblation® device is not limited to electrically isolated active electrodes, or even to a plurality of active electrodes. For example, in certain embodiments the array of active electrodes may be connected to a single lead that extends through the catheter shaft to a power source of high frequency current. 
         [0041]    The voltage difference applied between the return electrode(s) and the active electrode(s) will be at high or radio frequency, typically between about 5 kHz and 20 MHz, usually being between about 30 kHz and 2.5 MHz, preferably being between about 50 kHz and 500 kHz, often less than 350 kHz, and often between about 100 kHz and 200 kHz. In some applications, applicant has found that a frequency of about 100 kHz is useful because the tissue impedance is much greater at this frequency. In other applications, such as procedures in or around the heart or head and neck, higher frequencies may be desirable (e.g., 400-600 kHz) to minimize low frequency current flow into the heart or the nerves of the head and neck. 
         [0042]    The RMS (root mean square) voltage applied will usually be in the range from about 5 volts to 1000 volts, preferably being in the range from about 10 volts to 500 volts, often between about 150 volts to 400 volts depending on the active electrode size, the operating frequency and the operation mode of the particular procedure or desired effect on the tissue (i.e., contraction, coagulation, cutting or ablation). 
         [0043]    Typically, the peak-to-peak voltage for ablation or cutting with a square wave form will be in the range of 10 volts to 2000 volts and preferably in the range of 100 volts to 1800 volts and more preferably in the range of about 300 volts to 1500 volts, often in the range of about 300 volts to 800 volts peak to peak (again, depending on the electrode size, number of electrons, the operating frequency and the operation mode). Lower peak-to-peak voltages will be used for tissue coagulation, thermal heating of tissue, or collagen contraction and will typically be in the range from 50 to 1500, preferably 100 to 1000 and more preferably 120 to 400 volts peak-to-peak (again, these values are computed using a square wave form). Higher peak-to-peak voltages, e.g., greater than about 800 volts peak-to-peak, may be desirable for ablation of harder material, such as bone, depending on other factors, such as the electrode geometries and the composition of the conductive fluid. 
         [0044]    As discussed above, the voltage is usually delivered in a series of voltage pulses or alternating current of time varying voltage amplitude with a sufficiently high frequency (e.g., on the order of 5 kHz to 20 MHz) such that the voltage is effectively applied continuously (as compared with, e.g., lasers claiming small depths of necrosis, which are generally pulsed about 10 Hz to 20 Hz). In addition, the duty cycle (i.e., cumulative time in any one-second interval that energy is applied) is on the order of about 50% for the present invention, as compared with pulsed lasers which typically have a duty cycle of about 0.0001%. 
         [0045]    The preferred power source of the present invention delivers a high frequency current selectable to generate average power levels ranging from several milliwatts to tens of watts per electrode, depending on the volume of target tissue being treated, and/or the maximum allowed temperature selected for the instrument tip. The power source allows the user to select the voltage level according to the specific requirements of a particular neurosurgery procedure, cardiac surgery, arthroscopic surgery, dermatological procedure, ophthalmic procedures, open surgery or other endoscopic surgery procedure. For cardiac procedures and potentially for neurosurgery, the power source may have an additional filter, for filtering leakage voltages at frequencies below 100 kHz, particularly voltages around 60 kHz. Alternatively, a power source having a higher operating frequency, e.g., 300 kHz to 600 kHz may be used in certain procedures in which stray low frequency currents may be problematic. A description of one suitable power source can be found in commonly assigned U.S. Pat. Nos. 6,142,992 and 6,235,020, the complete disclosure of both patents are incorporated herein by reference for all purposes. 
         [0046]    The power source may be current limited or otherwise controlled so that undesired heating of the target tissue or surrounding (non-target) tissue does not occur. In a presently preferred embodiment of the present invention, current limiting inductors are placed in series with each independent active electrode, where the inductance of the inductor is in the range of 10 uH to 50,000 uH, depending on the electrical properties of the target tissue, the desired tissue heating rate and the operating frequency. Alternatively, capacitor-inductor (LC) circuit structures may be employed, as described previously in U.S. Pat. No. 5,697,909, the complete disclosure of which is incorporated herein by reference. Additionally, current-limiting resistors may be selected. Preferably, these resistors will have a large positive temperature coefficient of resistance so that, as the current level begins to rise for any individual active electrode in contact with a low resistance medium (e.g., saline irrigant or blood), the resistance of the current limiting resistor increases significantly, thereby minimizing the power delivery from said active electrode into the low resistance medium (e.g., saline irrigant or blood). 
         [0047]    Referring now to  FIG. 1 , an exemplary electrosurgical system for resection, ablation, coagulation and/or contraction of tissue will now be described in detail. As shown, certain embodiments of the electrosurgical system generally include an electrosurgical probe  20  connected to a power supply  10  for providing high frequency voltage to one or more electrode terminals on probe  20 . Probe  20  includes a connector housing  44  at its proximal end, which can be removably connected to a probe receptacle  32  of a probe cable  22 . The proximal portion of cable  22  has a connector  34  to couple probe  20  to power supply  10  at receptacle  36 . Power supply  10  has an operator controllable voltage level adjustment  38  to change the applied voltage level, which is observable at a voltage level display  40 . Power supply  10  also includes one or more foot pedals  24  and a cable  26  which is removably coupled to a receptacle  30  with a cable connector  28 . The foot pedal  24  may also include a second pedal (not shown) for remotely adjusting the energy level applied to electrode terminals  42 , and a third pedal (also not shown) for switching between an ablation mode and a coagulation mode. 
         [0048]    Referring now to  FIG. 2 , an exemplary electrosurgical system  211  for treatment of tissue in ‘dry fields’ will now be described in detail. Of course, system  211  may also be used in ‘wet field’, i.e., the target site is immersed in electrically conductive fluid. However, this system is particularly useful in ‘dry fields’ where the fluid is preferably delivered through electrosurgical probe to the target site. As shown, electrosurgical system  211  generally comprises an electrosurgical handpiece or probe  210  connected to a power supply  228  for providing high frequency voltage to a target site and a fluid source  221  for supplying electrically conducting fluid  250  to probe  210 . The system  211  may also include a vacuum source (not shown) for coupling to a suction lumen disposed in probe  210  (not shown) via a connection tube (not shown) on probe  210  for aspirating the target site, as discussed below in more detail. 
         [0049]    As shown, probe  210  generally includes a proximal handle  219  and an elongate shaft  218  having an array  212  of electrode terminals  258  at its distal end. A connecting cable  234  has a connector  226  for electrically coupling the electrode terminals  258  to power supply  228 . The electrode terminals  258  are electrically isolated from each other and each of the terminals  258  is connected to an active or passive control network within power supply  228  by means of a plurality of individually insulated conductors (not shown). A fluid supply tube  215  is connected to a fluid tube  214  of probe  210  for supplying electrically conducting fluid  250  to the target site. 
         [0050]    Similar to the above embodiment shown in  FIG. 1 , power supply  228  has an operator controllable voltage level adjustment  230  to change the applied voltage level, which is observable at a voltage level display  232 . Power supply  228  also includes first, second and third foot pedals  237 ,  238 ,  239  and a cable  236  which is removably coupled to power supply  228 . The foot pedals  237 ,  238 ,  239  allow the surgeon to remotely adjust the energy level applied to electrode terminals  258 . In an exemplary embodiment, first foot pedal  237  is used to place the power supply into the “ablation” mode and second foot pedal  238  places power supply  228  into the “coagulation” mode. The third foot pedal  239  allows the user to adjust the voltage level within the “ablation” mode. In the ablation mode, a sufficient voltage is applied to the electrode terminals to establish the requisite conditions for molecular dissociation of the tissue (i.e., vaporizing a portion of the electrically conductive fluid, ionizing charged particles within the vapor layer and accelerating these charged particles against the tissue). As discussed above, the requisite voltage level for ablation will vary depending on the number, size, shape and spacing of the electrodes, the distance in which the electrodes extend from the support member, etc. Once the surgeon places the power supply in the “ablation” mode, voltage level adjustment  230  or third foot pedal  239  may be used to adjust the voltage level to adjust the degree or aggressiveness of the ablation. 
         [0051]    It will be recognized that the voltage and modality of the power supply may be controlled by other input devices. However, applicant has found that foot pedals are convenient methods of controlling the power supply while manipulating the probe during a surgical procedure. 
         [0052]    In the coagulation mode, the power supply  228  applies a low enough voltage to the electrode terminals (or the coagulation electrode) to avoid vaporization of the electrically conductive fluid and subsequent molecular dissociation of the tissue. The surgeon may automatically toggle the power supply between the ablation and coagulation modes by alternatively stepping on foot pedals  237 ,  238 , respectively. This allows the surgeon to quickly move between coagulation and ablation in situ, without having to remove his/her concentration from the surgical field or without having to request an assistant to switch the power supply. By way of example, as the surgeon is sculpting soft tissue in the ablation mode, the probe typically will simultaneously seal and/or coagulation small severed vessels within the tissue. However, larger vessels, or vessels with high fluid pressures (e.g., arterial vessels) may not be sealed in the ablation mode. Accordingly, the surgeon can simply step on foot pedal  238 , automatically lowering the voltage level below the threshold level for ablation, and apply sufficient pressure onto the severed vessel for a sufficient period of time to seal and/or coagulate the vessel. After this is completed, the surgeon may quickly move back into the ablation mode by stepping on foot pedal  237 . 
         [0053]    Now referring to  FIGS. 3 and 4 , an exemplary electrosurgical probe  300  incorporating an active screen electrode  302  is illustrated. Probe  300  may include an elongate shaft  304  which may be flexible or rigid, a handle  306  coupled to the proximal end of shaft  304  and an insulative electrode support member  308  coupled to the distal end of shaft  304 . Probe  300  further includes active screen electrode  302  and at least one securing electrode  303 . Return electrode  310  is spaced proximally from screen electrode  302  and provides a method for completing the current path back to a power source (similar to those shown in  FIGS. 1 and 2 ). Return electrode  310  may comprise an annular exposed region of shaft  304  slightly proximal of insulative support member  308 , typically about 0.5 to 10 mm and more preferably about 1 to 10 mm. Securing electrode  303  and return electrode  310  are each coupled to respective electrical connectors  328  disposed in handle  306  (as illustrated in  FIG. 4 ) that extend to the proximal end of probe  300 , where connectors  328  are suitably electrically connected to a power supply (e.g., power supply  10  in  FIG. 1  or power supply  228  in  FIG. 2 ). As shown in  FIG. 4 , handle  306  defines an inner cavity  326  that houses the electrical connectors  328 , and provides a suitable interface for connection to an electrical connecting cable (e.g., cable  22  in  FIG. 1  or cable  234  in  FIG. 2 ). 
         [0054]    Still referencing  FIGS. 3 and 4 , in certain embodiments screen electrode  302 , securing electrode  303  and insulative support member  308  are configured such that screen electrode  302  and securing electrode  303  are positioned on a lateral side of the shaft  304  (e.g., 90 degrees from the shaft axis) to allow the physician to access tissue that is offset from the axis of the portal or arthroscopic opening into the joint cavity in which the shaft  304  passes during the procedure. 
         [0055]    Shaft  304  preferably comprises an electrically conducting material, usually metal, which is selected from the group consisting of tungsten, stainless steel alloys, platinum or its alloys, titanium or its alloys, molybdenum or its alloys, and nickel or its alloys. Shaft  304  may include an electrically insulating jacket  309 , which is typically formed as one or more electrically insulating sheaths or coatings, such as polytetrafluoroethylene, polyimide, and the like. The provision of the electrically insulating jacket over the shaft prevents direct electrical contact between these metal elements and any adjacent body structure or the surgeon. Such direct electrical contact between a body structure and an exposed electrode could result in unwanted heating and necrosis of the structure at the point of contact causing necrosis. 
         [0056]    The probe  300  further includes a suction connection tube  314  for coupling to a source of vacuum, and an inner suction lumen  312  ( FIG. 4 ) for aspirating excess fluids, tissue fragments, and/or products of ablation (e.g., bubbles) from the target site. Preferably, connection tube  314  and suction lumen  312  are fluidly connected, thereby providing the ability to create a suction pressure in lumen  312  that allows the surgeon to draw loose tissue, e.g., synovial tissue, towards the screen electrode  302 . Typically, the vacuum source is a standard hospital pump that provides suction pressure to connection tube  314  and lumen  312 . As shown in  FIGS. 3 and 4 , internal suction lumen  312 , which preferably comprises peek tubing, extends from connection tube  314  in handle  306 , through shaft  304  to an axial opening  316  in support member  308 , through support member  308  to a lateral opening  318  in support member  308 . Lateral opening  318  may be positioned adjacent to screen electrode  302 , which further includes a suction port (not shown) disposed on the surface of screen electrode  302  and fluidly connected to lateral opening  318  for allowing aspiration therethrough. 
         [0057]      FIG. 5  representatively illustrates in more detail the removal of a target tissue by use of an embodiment of electrosurgical probe  50  according to the present disclosure. As shown, the high frequency voltage is sufficient to convert the electrically conductive fluid (not shown) between the target tissue  502  and active electrode terminal(s)  504  into an ionized vapor layer  512  or plasma. As a result of the applied voltage difference between electrode terminal(s)  504  and the target tissue  502  (i.e., the voltage gradient across the plasma layer  512 ), charged particles  515  in the plasma are accelerated. At sufficiently high voltage differences, these charged particles  515  gain sufficient energy to cause dissociation of the molecular bonds within tissue structures in contact with the plasma field. This molecular dissociation is accompanied by the volumetric removal (i.e., ablative sublimation) of tissue and the production of low molecular weight gases  514 , such as oxygen, nitrogen, carbon dioxide, hydrogen and methane. The short range of the accelerated charged particles  515  within the tissue confines the molecular dissociation process to the surface layer to minimize damage and necrosis to the underlying tissue  520 . 
         [0058]    During the process, the gases  514  will be aspirated through a suction opening and suction lumen to a vacuum source (not shown). In addition, excess electrically conductive fluid, and other fluids (e.g., blood) will be aspirated from the target site  500  to facilitate the surgeon&#39;s view. During ablation of the tissue, the residual heat generated by the current flux lines  510  (typically less than 150° C.) between electrode terminals  504  and return electrode  511  will usually be sufficient to coagulate any severed blood vessels at the site. If not, the surgeon may switch the power supply (not shown) into the coagulation mode by lowering the voltage to a level below the threshold for fluid vaporization, as discussed above. This simultaneous hemostasis results in less bleeding and facilitates the surgeon&#39;s ability to perform the procedure. Once the blockage has been removed, aeration and drainage are reestablished to allow the sinuses to heal and return to their normal function. 
         [0059]    Referring now to  FIG. 6 , an exploded view of an alternative embodiment of a probe distal end portion  1204  is described, according to at least certain embodiments described in the present disclosure. Probe distal end portion  1204  may include an active electrode assembly  1220 , an insulating electrode support member  1230  and a shaft distal end  1250 . Shaft distal end  1250  preferably comprises a predominantly metal or electrically conductive material and may provide a portion of an electrical conduit between the shaft distal end  1250  and an energy source (not shown here). In some embodiments, an exposed portion of shaft distal end  1250  may comprise a return electrode  1251 , while the remaining portion of the shaft distal end  1250  may have an electrically insulative coating or sheath  1253  to prevent unwanted tissue effect to the patient. Shaft distal end  1250  may have an axial opening  1252 , to provide an internal conduit for electrically conductive means such as cables, wires or ribbons. Shaft axial opening  1252  may also provide housing or a protective conduit for fluid lumens, these lumens operable to supply or aspirate fluids, such as electrically conductive fluids, ablative byproducts and patient tissue and fluids to or from the probe distal end portion  1204 . Shaft axial opening  1252  may also be formed so as to provide a portion of the structural coupling between the shaft distal end  1250  and insulating electrode support member  1230 . As illustrated in  FIG. 12 , a proximal portion or neck  1248  of support member  1230  may fit within shaft axial opening  1252 . Support member  1230  may be press fit into shaft axial opening  1252 . Additional or alternative mechanical coupling means may also be used such as adhesives, bolts, clamps or deformable portions such as snaps. 
         [0060]    As further illustrated in  FIG. 6  shaft axial opening  1252  may have a non uniform or tapered opening, to form a distal extension  1254  of the shaft distal end  1250  on the probe&#39;s inferior side  1206 . Shaft extension  1254  may improve the mechanical connection between shaft distal end  1250  and support member  1230 . Additionally, shaft extension  1254  may provide additional structural support to the probe distal end  1204  during use, such as to add some rigidity to the probe distal end, and provide increased resistance to bending as a user applies force on the active electrode assembly  1220  during use. The inferior shaft extension  1254  may also form a portion of a return electrode  1251 , potentially improving the probe&#39;s electrosurgical functionality. Generally a return electrode that maintains close proximity to an active electrode assembly allows for reduced power input requirements compared with return electrodes with comparatively larger spacing from an active electrode and consequently, a lower power input may generally decrease the potential for unwanted or widespread tissue effect. Additionally, a return electrode&#39;s performance and overall probe performance may be improved when a substantial portion of a return electrode is consistently in fluid contact with a treatment site, so that is it consistently providing a return path to a power supply. Therefore the more distal a portion of the return electrode  1251  is, the more likely the return electrode  1251  will be within a tissue treatment site and the better it may function. Shaft extension  1254  may allow at least a portion or the return electrode  1251  to be disposed closer to or adjacent to the active electrode assembly  1220 . A portion of the extension  1254  may preferably axially overlap the electrode assembly  1220 . 
         [0061]    An electrosurgical probe may also perform better when the spacing between a substantial portion of an active electrode and a substantial portion of a return electrode is consistent, as the tissue effect may be more consistent between the two electrodes. As illustrated in  FIG. 12 , return electrode  1251  may comprises an annular exposed region of shaft distal end  1250  spaced from active screen electrode  1222 , typically about 0.5 mm to 10 mm and more preferably about 1 mm to 3 mm. The curve or taper on shaft extension  1254  as shown may approximately follow an adjacent curved edge on the adjacent electrode assembly  1220  so that there is a more consistent spacing between the electrodes. In this embodiment the proximal side of active screen electrode  1222  curve may approximate the taper on shaft extension  1254 . 
         [0062]    Insulating electrode support member  1230  preferably comprises an inorganic material, such as glass, ceramic, silicon nitride, alumina, ceramic-filled epoxy, or the like that are substantially resistant to plasma degradation. Support member  1230  may preferably comprise a one piece member with multiple slots, channels or conduits within the member  1230 , the one piece member being preferable as it may provide a higher strength component over a multi-piece support member  1230 . Support member  1230  may have at least one lateral fluid lumen opening  1232  in communication with an axial fluid lumen opening  1234  to form an internal fluid channel within support member  1230 . The at least one lateral fluid opening  1232  may be sized and positioned adjacent to active electrode assembly  1220  to aspirate patient tissue, fluids and gases from the treatment site. Providing suction in the area around treatment may pull patient tissue towards the active electrode assembly  1220  and improve tissue effect, as well as remove much or the debris from the area to improve treatment site visibility. In alternative embodiments, the fluid opening  1232  may supply electrically conductive fluids to the active electrode assembly  1220 . Axial fluid opening  1234  may couple with a fluid lumen that extends proximally to deliver patient tissue and aspirated materials to the proximal portion of probe. Axial opening  1234  may be sized to fit standard sized nylon tubing (not shown here). 
         [0063]    Support member  1230  may also comprise at least one securing electrode lateral opening or slot  1236  for receiving and potentially coupling with a portion of the at least one securing electrode  1224 . A portion of securing electrode  1224  may be secured within said slot  1236  using any number of mechanical fixing means such as adhesives, threaded attachments, snap fits, press fits, interlocking crossbars and polymer press sleeves. As illustrated in  FIG. 12  securing electrode  1224  comprises a stem portion  1225  which may have at least one stem aperture  1227 . Stem portion  1225  may fit within slot  1236  and be mechanically coupled using adhesive, and the at least one aperture  1227  may improve fixation between the adhesive and slot. Support member  1230  may also comprise a lateral connection element opening  1238  in communication with axial connection element opening  1239 , forming a connection element channel or groove in-between that houses an electrically conductive connection element  1226 . Support member  1230  may have an axial proximal portion or neck  1248  that is sized to couple with or interface with shaft distal end  1250  using mechanical means as described earlier. Insulating support member distal tip  1237  may be blunt or rounded to provide atraumatic access to treatment site. Support member  1230  may also comprise a flat support surface  1233  for nesting or receiving the active screen electrode  1222 . Support surface may have a circumferential ridge or locating pin (not shown) to help retain screen electrode  1222  in place during assembly or during use. 
         [0064]    Active electrode assembly  1220  may comprise an active screen electrode  1222 , at least one securing electrode  1224  and an electrically conductive connection element  1226  mechanically and electrically connected to the active screen electrode  1222 . Wires or electrically conductive means such as ribbons or cables may be electrically coupled to the proximal end of connection element  1226  using electrically coupling means such as soldering, crimping, laser welding or any other means known to those skilled in the art. These wires may extend proximally and internally through the probe shaft to electrical connectors (i.e., electrical connectors  328  in  FIG. 4 ), thereby electrically coupling the screen electrode  1222  to a high frequency power supply. The at least one securing electrode  1224  may be electrically coupled to the active screen electrode  1222  through direct contact with the active screen electrode  1222 , or supplemented with other electrically coupling means such as laser welding or soldering. In alternative embodiments, additional internal wiring or electrically conductive means may connect electrical connectors disposed within the probe handle to the at least one securing electrode  1224 . 
         [0065]    Electrically conductive connection element  1226  preferably comprises an electrically conductive material, such as tungsten, niobium or stainless steel. It may be preferable for the connection element material to be a ductile material so that curves may be more readily formed in the connection element shape and so that the connection element  1226  may easily be inserted into support member lateral opening  1238  and through a channel to axial opening  1239 . Connection element  1226  may elastically or non-elastically deform or bend so as to assemble with the support member  1230 ; however, the connection element  1226  is preferably a sufficiently rigid element so as to provide part of the active screen electrode  1222  securement and limit any screen electrode  1222  motion should other securement points fail. Additionally there is preferably some rigidity to the mechanical and electrical connection between the screen electrode  1222  and connection element  1226 , so as to provide some additional movement limitation between the support member  1230  and screen electrode  1222 . Insulating support member  1230  may house a portion of the connection element  1226  to secure the electrode assembly  1220  in place and also to electrically insulate the connection element  1226  from the patient tissue and cause any unwanted tissue effect. Additional electrical insulative means may also be used to electrically insulate the connection element  1226 , such as shrink tubing or coatings. At least a portion of the electrically conductive connection element  1226  may be formed to preferentially degrade or wear with energy application. A portion of the connection element  1226  may have a reduced width or reduced cross section to create a preferential weak point within the connection element  1226 . Alternatively all of the connection element  1226  may be of a substantially consistent width or cross section and the plasma generation may slowly degrade or etch away at a portion of the connection element  1226 . This may create a preferable failure mode for the probe, as once a substantial portion the connection element  1226  has been reduced in dimension, energy may no longer be delivered to the active electrode assembly  1220 , and the probe will no longer function. However if this failure occurs, the electrode assembly  1220  may still be secured due to other securing points such as the at least one securing electrode  1224 . 
         [0066]    Active screen electrode  1222  may comprise a conductive material, such as niobium, tungsten, titanium, molybdenum, stainless steel, aluminum, gold, copper or the like. Screen electrode  1222  may have a diameter in the range of about 0.5 mm to 8 mm, preferably about 1 mm to 4 mm, and a thickness of about 0.05 mm to about 2.5 mm, preferably about 0.1 mm to 1 mm. Screen electrode  1222  may have a variety of different shapes, such as the shape shown in  FIG. 5 . Screen electrode  1222  may have at least one securing electrode aperture or port  1221 , and may comprise at least one fluid port  1223  having sizes and configurations that may vary depending on the particular application. Fluid port  1223  may typically be large enough to allow ablated tissue fragments to pass through any downstream suction conduits including lateral fluid opening  1232 , axial fluid opening  1234  and fluid lumens extending from these. Fluid port  1223  may preferably be about 2 mm to 30 mm in diameter, preferably about 5 mm to 20 mm in diameter. In some applications, it may be desirable to only aspirate fluid and the gaseous products of ablation (e.g., bubbles) so that the there may be multiple apertures, which may be much smaller, e.g., on the order of less than 10 mm, often less than 5 mm. In certain configurations, fluid port  1223  may be formed in the shape of a zigzag or lightning bolt. Active electrode screen  1222  may be electrically and mechanically coupled to the distal end of connection element  1236  using electrically connecting means such as soldering, crimping, brazing or laser welding. Active electrode screen  1222  may be secured using multiple means as described earlier, and via multiple contact points which is preferable and may improve the general probe robustness and resistance to electrode assembly  1220  dislodgement during use. For example, active screen electrode  1222  may be partially secured via the mechanical connection with the connection element  1226 . Additionally, at least one securing electrode  1224  may be used to mechanically secure active screen electrode  1222  in place, as illustrated in  FIG. 12 . Securing electrode stem  1227  may be mechanically coupled to insulator slot  1236  using an epoxy or ceramic filled adhesive, such as Loctite  3984 . Stem apertures  1227  may improve fixation with an adhesive. Stem aperture may also provide some space within lateral opening  1236  for any expansion or thermal shock reactions the stem or adhesive may have to the energy being applied. In alternative embodiments, securing electrode stems may have teeth or roughened surfaces, or snaps or cross bars (not shown) to improve mechanical fixation. The at least one securing electrode  1224  may further be coupled to the screen electrode  1222  using mechanical means such as laser welding, brazing, soldering or using adhesives, preferably electrically conductive adhesives. 
         [0067]      FIG. 7  illustrates an assembled representation of distal end portion  1204  of embodiment as described in  FIG. 3 . Distal end portion  1204  of a representative probe is shown with at least two bilateral securing electrodes  1224  thereon, said securing electrode  1224  providing both a portion of the tissue treatment and securement of screen electrode  1222 . In this configuration, securing electrodes  1224  may be oriented symmetrically about the central axis of shaft distal end  1250 , and may thereby allow for creation of a zone for RF ablation or plasma chamber between the symmetrically oriented bilateral securing electrodes  1224  as well as between securing electrodes  1224  and screen electrode  1222  (as shown in  FIG. 8 ). Incorporation of symmetrical securing electrodes  1224  may allow for the creation of a three dimensional zone represented by plasma zone  1000  shown in  FIG. 8  for carrying out RF ablation. The placement of the bilateral securing electrodes  1224  not only provides for mechanical fixation but also provides a plasma forming feature. The securing electrodes surface may have a saw tooth, serrated, or rasp-like feature for enhanced plasma formation and increased tactile feedback during soft tissue ablation.  FIG. 7  also shows the assembly of connection element  1226  within connection element lateral opening  1239  for additional screen electrode securement. 
         [0068]    Other modifications and variations can be made to disclose embodiments without departing from the subject invention as defined in the following claims. For example, it should be noted that the invention is not limited to an electrode array comprising a plurality of electrode terminals. The invention could utilize a plurality of return electrodes, e.g., in a bipolar array or the like. In addition, depending on other conditions, such as the peak-to-peak voltage, electrode diameter, etc., a single electrode terminal may be sufficient to contract collagen tissue, ablate tissue, or the like. 
         [0069]    In addition, the active and return electrodes may both be located on a distal tissue treatment surface adjacent to each other. The active and return electrodes may be located in active/return electrode pairs, or one or more return electrodes may be located on the distal tip together with a plurality of electrically isolated electrode terminals. The proximal return electrode may or may not be employed in these embodiments. For example, if it is desired to maintain the current flux lines around the distal tip of the probe, the proximal return electrode will not be desired. 
         [0070]    While preferred embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teaching herein. The embodiments described herein are exemplary only and are not limiting. Because many varying and different embodiments may be made within the scope of the present teachings, including equivalent structures or materials hereafter thought of, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.