Patent Publication Number: US-2002013579-A1

Title: Rotating electrosurgical blade for corneal reshaping

Description:
FIELD OF INVENTION  
       [0001] The present invention relates to the field of correcting refractive errors of the eye, and more particularly to corneal electrosurgery.  
       DESCRIPTION OF THE RELATED ART  
       [0002] Anomalies in the overall shape of the eye can cause visual disorders. Hyperopia (“farsightedness”) occurs when the front-to-back distance in the eyeball is too short. In such a case, parallel rays originating greater than 20 feet from the eye focus behind the retina. In contrast, when the front-to-back distance of the eyeball is too long, myopia (“nearsightedness”) occurs and the focus of parallel rays entering the eye occurs in front of the retina. Astigmatism is a condition which occurs when the parallel rays of light do not focus to a single point within the eye, but rather have a variable focus due to the fact that the cornea refracts light in a different meridian at different distances. Some degree of astigmatism is normal, but where it is pronounced, the astigmatism must be corrected. Hyperopia, myopia, and astigmatism are usually corrected by glasses or contact lenses.  
       [0003] Another method for correcting those disorders is by reshaping the corneal surface through an operative procedure. Such methods include radial keratotomy (see, e.g., U.S. Pat. Nos. 4,815,463 and 4,688,570) and laser corneal ablation (see, e.g., U.S. Pat. No. 4,941,093). Other surgical techniques involve scraping or cutting the exterior corneal surface. Lieberman (U.S. Pat. No. 4,807,623) employs a pair of angled cutting blades that are rotated around the corneal center to excise an annular wedge from the cornea to correct refractive errors. Kilmer, et al. (U.S. Pat. No. 5,318,044) provides curved rotating blades that scrape the corneal surface to correct refractive errors. That patent is incorporated by reference herein.  
       [0004] Some other corneal reshaping techniques do not involve surgery, but rather apply a radio frequency electrical signal to remove corneal tissue noninvasively. One technique, conductive keratoplasty, described in U.S. Pat. No. 5,533,999, issued to Hood, et al., applies an RF current directly to symmetrical spots on the cornea. This technique heats the corneal tissue to shrink and steepen the tissue in order to correct hyperopia and astigmatism. Similarly, both Doss, et al. (U.S. Pat. No. 4,326,529) and Doss (U.S. Pat. No. 4,381,007) employ an electrode that is placed near but not physically touching the anterior corneal surface. An electrically conductive coolant is placed over the corneal surface and circulated around the electrode as RF energy is applied through the electrode. The RF apparently heats various stroma within the cornea and thereby reshapes it as a biological response to the heat generated by the RF.  
       [0005] Two related patents, Dobrogowski, et al. (U.S. Pat. No. 5,025,811) and Latina, et al. (U.S. Pat. No. 5,174,304) illustrate noninvasive methods for focal transcleral destruction of living human eye tissue. In general, these devices and their underlying procedures involve the use of electric currents for ablating eye tissue, particularly the ciliary process. No mention of corneal reshaping is made. These references also relate to the application of a DC signal to the eye employing an ionic solution within the electrosurgical probe. The use of RF is not disclosed. Further, the ablation process is performed by repeatedly applying the probe to 10-30 spots around the circumference of the eye.  
       SUMMARY OF THE INVENTION  
       [0006] The present invention provides a rotatable electrosurgical apparatus for reprofiling a cornea. The apparatus includes one or more electrosurgical electrodes that extend radially outward from a center point. The electrodes are shaped to reform at least a portion of an anterior surface of the cornea. The electrodes are disposed on an electrode support, which is rotatable. The electrodes project from the bottom of a rotary handle, which rotates the electrodes about a central visual axis of the cornea. The rotary handle has a hollow bore and a viewing port. The apparatus includes a support base having a base ring for positioning on the eye. The base ring can hold a solution against the eye to even out irregularities in the cornea.  
       [0007] The apparatus may include pads to exert pressure on the cornea to cause the cornea to bulge in desired areas. These bulged areas are more easily modified by the electrodes when energized. The pressure pads take different forms depending upon whether they are used for the correction of myopia, hyperopia or astigmatism.  
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0008] The present invention provides a rotary electrosurgical blade assembly for corneal reshaping and a related method. In the following description, numerous details are set forth in order to enable a thorough understanding of the present invention. However, it will be understood by those of ordinary skill in the art that these specific details are not required in order to practice the invention. Further, well-known elements, devices, process steps and the like are not set forth in detail in order to avoid obscuring the present invention.  
     [0009] Prior to explaining the details of the inventive procedures and devices, a short explanation of the physiology of the eye is provided.  
     [0010]FIG. 1 shows a horizontal cross-section of the eye with the globe  11  of the eye resembling a sphere with an anterior bulged spherical portion representing the cornea  12 .  
     [0011] The globe  11  of the eye consists of three concentric coverings enclosing the various transparent media through which the light must pass before reaching the light-sensitive retina  18 . The outermost covering is a fibrous protective portion, the posterior five-sixths of which is white and opaque and called the sclera  13 , and sometimes referred to as the white of the eye where visible to the front. The anterior one-sixth of this outer layer is the transparent cornea  12 .  
     [0012] A middle covering is mainly vascular and nutritive in function and is made up of the choroid, ciliary body  16 , and iris  17 . The choroid generally functions to maintain the retina  18 . The ciliary body  16  is involved in suspending the lens  21  and accommodation of the lens. The iris  17  is the most anterior portion of the middle covering of the eye and is arranged in a frontal plane. It is a thin circular disc similar in function to the diaphragm of a camera, and is perforate near its center by a circular aperture called the pupil  19 . The size of the pupil varies to regulate the amount of light which reaches the retina  18 . The iris divides the space between the cornea  12  and the lens  21  into an anterior chamber  22  and the posterior chamber  23 . The innermost portion of covering is the retina  18 , consisting of nerve elements which form the true receptive portion for visual impressions.  
     [0013] The retina  18  is a part of the brain arising as an outgrowth from the fore-brain, with the optic nerve  24  serving as a fiber tract connecting the retina part of the brain with the fore-brain. A layer of rods and cones, lying just beneath a pigmented epithelium on the anterior wall of the retina serve as visual cells or photoreceptors which transform physical energy (light) into nerve impulses.  
     [0014] The vitreous body  26  is a transparent gelatinous mass which fills the posterior four-fifths of the globe  11 . At its sides it supports the ciliary body  016  and the retina  18 . A frontal saucer-shaped depression houses the lens.  
     [0015] The lens  21  of the eye is a transparent bi-convex body of crystalline appearance placed between the iris  17  and vitreous body  26 . Its axial diameter varies markedly with accommodation. A ciliary zonule  27 , consisting of transparent fibers passing between the ciliary body  16  and lens  21  serves to hold the lens  21  in position and enables the ciliary muscle to act on it.  
     [0016] Referring again to the cornea  12 , this outermost fibrous transparent coating resembles a watch glass. Its curvature is somewhat greater than the rest of the globe and is ideally spherical in nature. However, often it is more curved in one meridian than another, giving rise to astigmatism. Most of the refraction of the eye takes place through the cornea.  
     [0017]FIG. 2 is a more detailed drawing of the anterior portion of the globe showing the various layers of the cornea  12  making up the epithelium  31 . An anterior limiting lamella  33 , referred to as Bowman&#39;s membrane or layer, is positioned between the epithelium  31  and the stroma  32  of the cornea. The term “corneal mass” refers to the various stroma  32  between Bowman&#39;s layer  33  and Descemet&#39;s membrane  34 . The corneal stroma  32  are made up of lamellae having bands of fibrils parallel to each other and crossing the whole of the cornea. While most of the fibrous bands are parallel to the surface, some are oblique, especially anteriorly. A posterior limiting lamella  34  is referred to as Descemet&#39;s membrane. It is a strong membrane sharply defined from the stroma  32  and resistant to pathological processes of the cornea.  
     [0018] The endothelium  36  is the most posterior layer of the cornea and consists of a single layer of cells and function to maintain transparency of the cornea  12 . These epithelial cells are rich in glycogen, enzymes and acetylcholine and their activity regulates the transport of water and electrolytes through the lamellae of the cornea  12 . The limbus  37  is the transition zone between the conjunctiva  38  and sclera on the one hand and the cornea  12  on the other.  
     [0019] In general, there are two distinct electrosurgical delivery probe types: the monopolar probe and the bipolar probe. An in-between electrosurgical configuration applicable to this invention also exists and is known as sesquipolar. In each instance, some section of the human body is used to complete a circuit between one pole and the other. In the monopolar probe device, there is a single active contact which is inserted or otherwise contacted with the human body and it is the site at which some body activity, e.g., desiccation, ablation, necrosis, fulguration, or the like, takes place. To complete the circuit in a monopolar device, there must be another contact which is inactive and placed against the body in a location remote from the active contact. By “inactive” is meant that only an insignificant temperature rise occurs at that contact point. One such method of ensuring that the inactive electrode is in fact “inactive” is to make it quite large in area. This causes the current to spread over a large area for completion of the circuit.  
     [0020] A bipolar electrode typically has two equal-area active electrodes contained in the same electrode probe-handle structure. This symmetric bipolar electrode design produces a significant temperature rise at both electrodes.  
     [0021] In a monopolar or sesquipolar configuration, only one electrode has an area of tissue contact producing a significant temperature rise. Unlike the monopolar configuration, however, the sesquipolar return electrode is not so remote, thereby limiting current flow through the body to the nearby return electrode. The return electrode area in the sesquipolar configuration electrode is usually at least three times the area of the active electrode and produces little or no tissue effect. For ocular surgery, the sesquipolar return electrode may be located on a non-remote region of the body, such as on the sciera or on a shaved area at the back of the patient&#39;s head.  
     [0022] There are a variety of effects that may occur depending upon the electrosurgical mode desired. For instance, there are both high temperature and low temperature desiccation effects when the active electrosurgical probe contact(s) are used to promote tissue desiccation. The resistance of the tissue in contact with the active probe electrode obviously varies with the tissue temperature and water content. A low temperature desiccation effect involves heating such that the temperature-time product causes tissue necrosis with little immediate denaturation or discoloration of the tissue. There is a transient decrease in local tissue impedance with little drying of tissue. In high temperature desiccation, there are significant increases in local tissue impedance and also in local tissue desiccation.  
     [0023] In the ablation mode, the electrosurgical energy density delivered largely causes the tissue near the probe contact to vaporize. The temperature at the electrode/tissue interface is increased significantly past the point of steam formation. The effect of electrical resistance varies during a specific radio frequency cycle and although there is sparking, carbonization is not usually significant and the effects of the device are relatively rapid.  
     [0024] Electrosurgical ablation and cutting produce an effect where a thin layer of tissue is vaporized (cutting) or where a larger section of tissue is vaporized (ablation). The line between “cutting” and “ablation” is not always clear.  
     [0025] Blended mode is essentially a combination of the cutting and coagulation (desiccation) modes. In blended mode, cutting or ablation with hemostasis is achieved.  
     [0026] The present invention employs electrosurgical ablation to reprofile the anterior surface of the cornea  301 . For techniques that employ electrosurgery to modify the cornea from below the surface, please refer to U.S. patent application Ser. Nos. 08/194,207, 08/513,589, and 08/698,985, all of which are incorporated by reference herein.  
     [0027]FIG. 3A illustrates a rotatable electrosurgical apparatus of the present invention, where the basic parts of the assembly are shown in an exploded view. In one embodiment, the components of the assembly include a generally cylindrical support base  300  having an annular base ring  302  and a cylindrical bore  304  extending through the support base. The base ring  302  may be implemented as a circumcorneal vacuum ring. A vacuum hose  306  connects the vacuum ring  302  to a vacuum pump (not shown). The vacuum ring  302  is configured so that it meets with and seals to the front of the eye, rendering the support base  300  relatively immobile when the support base  300  is applied to the front of the eye and a suitable vacuum is applied to the vacuum hose.  
     [0028] A rotary handle  305  having an electrosurgical blade assembly  307  is adapted for insertion into the support base  300 . The rotary handle  305  has a hollow bore  309  to allow viewing of the corneal surface during operation of the apparatus. The side of the bore is also open to provide a viewing port  313 . The inner diameter of the handle bore  309  is at least large enough so that the surgeon can see the blade assembly  307  by looking down into the bore  309 . The bore  309  is desirably a length such that the ratio of the bore&#39;s length to its diameter is between 0.25:1 and 15:1; specifically between 0.4:1 and 1:1, at least about 1:1 and less than about 3:1; or at least 3:1 up to about 15:1. Preferably, the ratio is about 2.5:1. This sizing allows easy manipulation by the surgeon.  
     [0029]FIG. 3B illustrates a bottom view of the base ring  302  to show its internal structure when implemented as a vacuum ring. The vacuum ring  302  comprises an inner wall  308  having an inner diameter that allows the outer diameter of the rotary handle tube  311  to fit into the base ring  302 .  
     [0030] The outer vacuum ring wall  310  forms the outside of the base ring  302 . Interior to the vacuum ring  302  may be one or more ridges  312  which extend down to the corneal surface when the support base  300  is attached to the eye. These ridges  312  may be made of conductive material, whereas the surrounding support base structure, such as the inner wall and outer wall, are made of insulative material. The ridges may be coupled to an electrosurgical generator. Using this configuration, the ridges may act as return electrodes when operating in sesquipolar mode. These return electrodes may be positioned to rest on the sclera  314  or translimbal region of the eye.  
     [0031]FIG. 3C shows an alternative arrangement of return electrodes comprising radial vanes  316  that extend downward through the vacuum ring  302  to make contact with the sclera  314  or translimbal region.  
     [0032] Similar vacuum ring configurations for other purposes are described in U.S. Pat. No. 5,403,335, issued to Loomas et al., and assigned to the assignee of the present invention. That patent is incorporated by reference herein.  
     [0033] Alternatively, the support base  300  can rest on the sclera  314  without use of a vacuum ring. In its place, a base ring  302  of resilient material can be used as a substitute for the hollow annular vacuum ring. As another alternative, the bottom of the base ring  302  can be serrated to hold the ring in place.  
     [0034] The support base  300  may include two standoffs  318 , shown here as one behind the other on opposite sides of the support base  300 . The standoffs  318  are topped by a support ring  320 . The support ring  320  may have an inner diameter greater than or equal to that of the base ring  302 . As shown in FIG. 3D the support ring  320  may be threaded and screwed into a calibrated micrometer-like adjustment ring  322 , similar to that used in the Kilmer &#39;044 patent. A collar  324  of the handle  305  rests on top of the adjustment ring  322 . By rotating the adjustment ring  322 , the adjustment ring  322  controls the axial depth of the blades  307 .  
     [0035] Because only two thin standoffs  318  are employed to support the adjustment ring  322 , the surgeon is provided with a relatively large viewing port area to allow observation of the operational steps taking place at the corneal surface. The base  300  may have substantially more open area than closed area to maximize visibility. As an alternative, the support base may not include the standoffs  318  and support ring  320 . The base ring  302  alone may serve as a guide for the handle to increase viewing area. Further, the entire support base may be omitted when performing the surgical procedure. In that case, the surgeon essentially performs the operation “free hand.” 
     [0036] The electrode blade assembly  307  is coupled through one lead  326  to an electrosurgical generator  328  so as to act as an active electrode. In a sesquipolar configuration, the other lead  330  of the generator  328  may be coupled to return electrodes  312  or  316  disposed on the bottom of the support base  300 , as shown in FIGS. 3B and 3C. The return electrodes  312  or  316  rest on the scleral portion  314  of the eye. Alternatively, in a monopolar configuration, the return electrode may be placed elsewhere on the patient&#39;s body.  
     [0037] When using the complete support base as a guide, the surgeon positions the support base  300  on the eye so that it is centered over the central visual axis of the cornea  301 . A vacuum is applied to hold the base in place if the vacuum ring embodiment is employed. The surgeon inserts the rotary handle  305  into the support base  300  so that the collar  324  rests on the adjustment ring  322 . The surgeon rotates the adjustment ring  322  so that the electrode blade assembly  307  contacts the cornea  301 . Because the invention employs electrical energy, the blades  307  need only lightly touch the corneal surface.  
     [0038] Alternatively, the blades  307  may be positioned near the corneal surface without touching the surface when a conducting medium such as saline is present. For this purpose, the blades  307  may be placed within a range of approximately 50-500 microns from the eye. It is the electrical contact, not the mechanical contact, between the blades and the cornea that achieve modification of the corneal surface. Initial electrical contact may be indicated by a continuity tester, as is well known in the art. The proper distance to achieve local conduction between the blades and the cornea can instead be determined by the surgeon by energizing and slowly lowering the energized blade assembly  307  towards the cornea while viewing the effects on the corneal surface  301 . To aid in blade placement, the distance from the cornea may be measured with a traveling scale, such as an electronic dial caliper manufactured by Mitsutoyo, Inc. The scale can be zeroed when the blades touch the cornea.  
     [0039] The surgeon energizes the rotary blade assembly  307  with an RF current from the generator  328  to achieve volume modification of the cornea  301 . Preferably, the procedure should be performed while the eye is bathed in a solution, such as saline, in order to even out irregularities in the tissue caused by uneven hydration of corneal tissue. The solution is held in the bore  332  of the base ring  302 , and does not leak because of the tight fit between the base ring  302  and the eye.  
     [0040] The current employed by the present invention to achieve volume modification is typically a radio frequency current approximately on the order of 500 KHz or more. Additionally, the RF energy is often delivered in a pulsed or a continuous, non-pulsed operation depending on the exact effects desired. For further information concerning the electrical characteristics of electrosurgical waveforms, and electrosurgery in general, please refer to J. A. Pearce,  Electrosurgery , John Wiley &amp; Sons, 1986; U.S. Pat. No. 4,438,766 issued to Bowers; the SSE2K Electrosurgical Generator Service and Instruction Manuals (1982, 1980), the SSE2L Electrosurgical Generator Instruction Manual (1991), and the Force 2 Electrosurgical Generator Instruction Manual (1993), Valleylab. All of these references are incorporated by reference herein.  
     [0041] The rotary blades  307  may be energized by a common electrosurgical generator such as the Force 2, manufactured by Valleylab, Inc. The generator  328  includes settings for providing the appropriate electrosurgical waveforms for cutting, coagulation or blended modes. The wave shape for each mode is specified in the Valleylab generator manual. Cutting or ablation is performed with a 510 KHz continuous sinusoid. Coagulation (desiccation) employs a 510 KHz damped sinusoidal burst with a repetition frequency of 31 KHz. In blended modes, the generator outputs a 510 KHz sinusoidal burst at various duty cycles recurring at 31KHz. Those skilled in the art will recognize that the present invention is not limited to the generators, particular wave shapes or electrical characteristics disclosed herein.  
     [0042] The blades  307  initially may be energized at a low power setting (e.g., 0-5 watts) for approximately 1-5 seconds or longer. During energization of the blades, the surgeon rotates the blade assembly  307  and observes the volume reduction process to ensure that tissue is being safely removed or shrunk from the proper corneal regions. Typically, this observation may be performed through an ophthalmic microscope commonly used in opthalmological surgical procedures. The observation is conducted through the viewing ports or by removing the entire apparatus after each iteration of the procedure.  
     [0043] After completion of the corneal volume reduction step, the support base  300  and rotary handle assembly  305  are removed and the curvature of the corneal surface is then measured. One common method for measuring corneal curvature employs the Placido ring technique embodied in the Corneal Topography System manufactured by Eyesys of Houston, Tex. Curvature may also be measured using the technique described in allowed U.S. patent application Ser. No. 08/200,241, assigned to the assignee of the present invention, and incorporated by reference herein. The procedure may be repeated if insufficient correction has occurred. When repeating the procedure, the surgeon may increase the output power to reduce a greater volume of tissue until the desired effect is achieved. The surgeon may also lower the blades  307  by adjusting the adjustment ring  322 .  
     [0044] FIGS.  4 - 10  illustrate side and bottom views of various configurations of the rotary blade assembly. FIG. 4 illustrates an embodiment of a single blade assembly for correction of myopia. A single active blade electrode  400  is disposed on an insulating electrode blade support  401  and extends radially outward from a center point  402 . The broken lines of the bottom views of FIGS.  4 - 10  illustrate the full circles that can be swept by the blades and blade supports of those figures. In FIG. 4, the electrode is shaped to flatten the central portion of the anterior surface of the cornea  301 . By rotating the electrode  400  in ablation mode, a surgeon may modify the volume of the central corneal region in order to correct myopia.  
     [0045] Selecting the proper blade shape for the desired correction is relatively easy using well-known relationships between the radius of corneal curvature and refractivity. The patient is given an eye exam to determine the degree of correction necessary. The refractive power correction is then correlated to a desired radius of corneal curvature, as is known in the art. A blade, such as that of FIG. 4, is chosen with this radius to reform the cornea to the correct radius. Blade selection may be refmed by conforming the blade shape to the shape determined by known topographical techniques as necessary for proper correction.  
     [0046]FIG. 5 illustrates a single blade embodiment for the correction of hyperopia. An active electrode  500  is disposed on an insulating blade support  501  and extends radially outward from a center point  502 . The active electrode  500  is disposed near the periphery of the rotary blade assembly  307 . When the blade  500  is rotated by the surgeon in ablation mode, the blade removes an annulus of corneal tissue in order to steepen the central corneal region so as to correct hyperopia.  
     [0047] Generally, the blade electrode of FIG. 4 is rotated 360 degrees to correct myopia. Similarly, the blade electrode  500  of FIG. 5 is rotated 360 degrees to correct hyperopia. Those skilled in the art will recognize that the blades can be rotated over smaller angular sectors in order to vary the correction of refractive error. For example, the blades of any of the embodiments described herein may be rotated through various angular sectors to correct astigmatism.  
     [0048]FIG. 6 illustrates side and bottom views of a dual blade embodiment of the blade assembly  307  for correcting myopia. The assembly  307  includes two active electrodes  600  and  602  disposed on an insulating blade support  603  along a curved diameter line  604  passing through a center point  606 . Each of the blade electrodes  600  and  602  is curved to reform the shape of the central corneal region to correct myopia. The blade electrodes  600  and  602  may be separated by an insulator  608 . The blades  600  and  602  may be electrically coupled together by a wire (not shown) in the rotary handle. The wire itself is connected to the active lead of the generator. Alternatively, one integrated conducting blade electrode (not shown) that is symmetric about the center point may replace the two separate electrodes  600  and  602 .  
     [0049] The blade assembly  307  may also fit into an annular peripheral pressure pad  610 , which is shown in cross-section in the side view of FIG. 6. The insulative pad is placed inside the bore  332  of the base ring  302 , and allowed to move freely in the axial direction. The pad  610  may include a vertical groove on its outer side to accept a pin (not shown) in the base  302  so that the pad is fixed in the direction of rotation, but still allowed to move in the axial direction. Alternatively, the pad  610  may be mounted to the interior of the tube  311 . The pad may rotate with the tube  311  or loosely placed in the tube  311  so that it is held in place on the eye while the tube  311  rotates. When the peripheral pad  610  is applied to the peripheral area on or near the cornea, the central corneal region bulges to provide a more well-defined region for ablation. Those skilled in the art will recognize that the peripheral pad may be employed with any of the blade assemblies described herein for modifying tissue near the center of the cornea.  
     [0050] The blade support  603  is mounted to the interior of the tube  311  of the rotary handle  305 , for example, by thin brackets  605 , so that the blade support  603  (and the blades  600  and  602 ) rotates as the handle  305  is rotated. (Generally, all blade assemblies described herein are mounted to the interior of the tube  311 .)  
     [0051] The brackets  605  act as a stop to prevent upward movement of the pad  610 . Thus, by using pads of different heights, the relationship between the bottom of the pad  610  and the edge of the blades  600  and  602  may be adjusted. This, in turn, adjusts the size of the corneal bulge when the assembly is placed on the eye, thereby giving a different resulting corneal curvature for the same blade. That is, the higher the bulge, the deeper the resulting tissue modification.  
     [0052] This dual blade configuration allows the surgeon to ablate a 360 degree region by rotating the assembly  307  through only 180 degrees because each blade ablates half of the total 360 degree region. Similarly, the blade assembly can be reproduced and orthogonally combined so that the electrodes are separated by 90 degrees. Further combinations can be made for smaller angular separations. Those skilled in the art will recognize that any of the blade assemblies  307  disclosed herein may be combined in this manner.  
     [0053] To effectively achieve multiplexing, each blade can also be independently energized to provide a higher current density per blade for the same amount of power. For example, the surgeon can rotate the dual blade assembly in one direction with only one blade energized, and then rotate the assembly back in the other direction with only the other blade energized.  
     [0054]FIG. 7 illustrates a dual blade assembly  307  for correcting hyperopia. Blades  700  and  702  are disposed on an insulating blade support  703  along a diameter line  704  passing through a center point  706 . The blade electrodes  700  and  702  may be electrically coupled together in the same manner as in FIG. 6. The blade assembly may also include an insulative central pressure pad  708 . The pad extends slightly below, about 0.1 mm, the portion of the blade support  703  adjacent the pad  708 . The blade support  703  is mounted on the rotary handle  305  so that the blade support (and the blades) rotate as the handle is rotated. The pad  708  is rotatably coupled to the blade support so that when the blade assembly  307  is applied to the eye, the pad  708  is held stationary against the cornea  301  by friction as the blade support  703  swivels around the pad  708  when the handle  305  is rotated. Alternatively the pad  708  may be fixed to the handle  305 . When the pad  708  is applied to the central area of the cornea, the peripheral corneal surface bulges to provide a more well-defined region for ablation. The size of the bulge is governed by the relative distance between the bottom of the pad  708  and the edge of the blades  700  and  702 . Those skilled in the art will recognize that a central pressure pad may be employed in any of the blade assemblies described herein for modifying tissue outside the center area of the cornea.  
     [0055]FIG. 8 illustrates another embodiment of the dual blade myopic correction assembly  307 . In this embodiment, the active electrode assembly is divided into four active electrodes  800 ,  802 ,  804  and  806 . The electrodes are separated by insulative portions  808 ,  810  and  812 , respectively, of a blade support  814 . The electrodes  802  and  804  may be electrically coupled to each other to form a first set of coupled electrodes, and electrodes  800  and  806  may be electrically coupled together to form a second set of coupled electrodes. The four electrodes of this embodiment are configured to have effectively the same blade area for contact with the cornea as the two electrodes of the embodiment of FIG. 6.  
     [0056] By employing this configuration, the sets of electrodes can be energized independently of each other using a simple switching circuit between the generator and the blades. For example, the surgeon can ablate the central corneal region with the first set of coupled electrodes through a given angular sector using a given axial pressure and power setting. Then, the surgeon can ablate a concentric region with the second set of coupled electrodes through the same or another angular sector using the same or a different axial pressure and the same or different power. In this manner, the surgical procedure is effectively multiplexed.  
     [0057]FIG. 9 illustrates another embodiment of the dual blade assembly for hyperopic correction. This embodiment features four blades  900 ,  902 ,  904  and  906  mounted on an insulating blade support  912 . The blades  902  and  904  may be electrically coupled to form a first coupled set of electrodes, and electrodes  900  and  906  may be electrically coupled to each other to form a second set of coupled electrodes. Electrodes  900  and  902  are separated by an insulative portion  908  of the blade support  912 . Electrodes  904  and  906  are separated by an insulative portion  910 . These blades may be operated by the surgeon in a manner similar to that described with respect to FIG. 8, and may include a central pressure pad (not shown) such as that illustrated in FIG. 8.  
     [0058]FIG. 10 illustrates a combination electrode blade assembly  307 . This embodiment includes eight blade electrodes  1000 ,  1002 ,  1004 ,  1006 ,  1008 ,  1010 ,  1012 , and  1014 , separated by insulative portions  1016 ,  1018 ,  1020 ,  1022  and  1024 , respectively, disposed on a blade support  1026 . These electrodes may be electrically coupled in any manner and energized in any sequence to correct myopia, hyperopia, astigmatism or any other error correction desired by the surgeon.  
     [0059] As mentioned above, the present invention may be employed to treat astigmatism. Referring to FIG. 11, astigmatism occurs, generally, when the curvature of the anterior surface is not uniform along the circumference of the cornea, resulting in a steep axis  1100  and a flat axis  1101  along perpendicular meridians. The steeper axis is known as the axis of astigmatism  1100 . A butterfly or figure-8-shaped region  1103  about the astigmatic axis  1100  is steeper than the surrounding region  1102  of the cornea. To correct astigmatism, the region  1103  must be flattened to cause the cornea to become reasonably symmetrical and more spherical in shape.  
     [0060] Blade assemblies such as those shown in FIGS. 6 and 8 may be employed to flatten the steepened region  1103  along the astigmatic axis  1100 . Using those blade assemblies, a surgeon would not rotate the assemblies through a full 360 degree angle, but rather would only rotate them through angular sectors A and B to ablate the steepened tissue.  
     [0061] FIGS.  12 - 14  illustrate pressure pads that may be employed in the correction of astigmatism. The pads create bulges in the corneal regions adjacent the point of contact between the pads and the cornea. By making those regions more prominent, the pads make it easier for the surgeon to ensure that the correct areas of the cornea are modified.  
     [0062]FIG. 12 illustrates a bottom view of a central astigmatic pressure pad  1200  similar to the central pressure pad of FIG. 7 along with a blade  1202  and a blade support  1204 . The pad  1200  is rotatably coupled to the blade support  1204 . Unlike the pad of FIG. 7, this pad  1200  does not apply a uniform disc of pressure to the central corneal region. Instead, the pad has a butterfly shape to complement the steep butterfly region  1103  of the astigmatic cornea. The pad  1200  is applied to the flatter regions near the corneal center in order to cause the steep areas near the center to bulge. A first axis  1206  of the pad  1200  is applied to the flat corneal axis  1101 . Wings  1208  of the pad limit rotation of the blade to the angular sectors A and B about the steep astigmatic axis  1100 . The dashed lines indicate the limit angular sector swept by the blades  1202  and blade support  1204 .  
     [0063]FIGS. 13A and 13B illustrate a side cross-sectional view and a bottom view, respectively, of an annular peripheral astigmatic pressure pad  1300  similar to the peripheral pad of FIG. 6, along with a blade  1302  and a blade support  1304 . The pad  1300  is mounted to the base ring  302 . However, unlike the pad of FIG. 6, this pad does not circumscribe a complete 360 degree annulus. Instead, the pad  1300  is shaped so that no pressure is applied to the angular sectors A and B, thereby causing those regions to bulge when pressure is applied. The pad comprises first and second annular segments or wings  1306  and  1308 , respectively. FIG. 13C is a side view (not sectional) of FIG. 13A rotated 90 degrees to show the side of wing  1308 . A first axis  1310  is disposed along the flat corneal axis  1101 . The annular segments limit rotation of the blade  1302  to the angular sectors A and B about the steep astigmatic axis  1100 . The pad  1300  also allows the blade to contact the center of the cornea.  
     [0064]FIGS. 14A and 14B illustrate a variation of FIGS. 13A and 13B, wherein pressure is applied not only to a peripheral annular region, but also to the central corneal region in which the corneal surface is relatively flat. The pad  1400  is mounted to the base ring  302 . The pad  1400  comprises first and second wings  1402  and  1404 , respectively. FIG. 14C is a side view (not sectional) of FIG. 14A rotated 90 degrees to show wing  1404 . A first axis  1406  is disposed along the flat axis  1101 . The wings apply pressure to both the central and peripheral corneal regions to limit rotation. As a result, the corneal surface bulges in the angular sectors A and B, almost as if a combination of the central astigmatic and peripheral astigmatic pressure pads were applied.  
     [0065] Of course, any of the pad configurations disclosed herein may be varied to cause different corneal regions to bulge.  
     [0066] As apparent from the discussion above, the present invention exhibits advantages over prior art mechanical techniques. Because the electrical blade assembly requires only light or no mechanical contact, the invention does not traumatize the corneal surface and provides a more controlled tissue removal procedure than mechanical methods. When a mechanical blade scrapes a cornea, tissue in the path of the advancing blade can bulge, leading to a possible gash in the bulge or other non-uniformity in the surface modification. Further, debris resulting from mechanical scraping in the path of the advancing blade can jam the blade, also leading to non-uniformities. In contrast, electrical ablation by the blade assembly of the present invention vaporizes tissue cleanly in the path of the blades.  
     [0067] While the invention has been described with reference to numerous specific details, one of ordinary skill in the art will recognize that the invention can be embodied in other specific forms without departing from the spirit of the invention. Further, all patents, applications and other references cited herein are incorporated by reference herein. One of ordinary skill in the art will understand that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.