Abstract:
A separator constructed of a polymeric material is used with a surgical device that separates the epithelium of a cornea from the underlying Bowman&#39;s layer of an eye of a patient. The surgical device includes a positioning ring for temporary attachment to the eye and is structured to present and expose the cornea to be separated. The separator support is structured and disposed to carry the separator. A drive is operably connected to the separator support for causing movement of the separator across the positioning ring and for causing oscillating movement of said separator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/432,305, filed Dec. 10, 2002, which application is hereby incorporated herein by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to a device for optical surgery, and in particular to a disposable separator for separating the epithelium layer of a cornea from the underlying Bowman&#39;s layer.  
         BACKGROUND OF THE INVENTION  
         [0003]    Microkeratome blades are widely used in LASIK (Laser-Assisted In Situ Keratomilousis) procedures. LASIK permanently changes the shape of the cornea, the clear covering of the front of the eye, using an excimer laser. The microkeratome is used to cut a corneal flap containing the epithelium, Bowman&#39;s layer, and a portion of the stroma by slicing through the stroma, dividing it into at least two distinct portions. A hinge of uncut corneal tissue is typically left at one end of this flap. The flap is folded back revealing the penetrated stroma, the middle section of the cornea. Pulses from a computer-controlled laser vaporize a portion of the stroma and the flap is replaced. It is important that the blade used during the LASIK procedure is sharp, otherwise the quality of the procedure and the healing time are poor. Additionally, the blade has to be exceedingly sharp in order to produce consistent and reproducible flaps.  
           [0004]    Known microkeratome blades are typically formed of either stainless or low-carbon steel. A variety of other materials, including diamond, sapphire, tungsten, ceramic, and silicon carbide, have been proposed for use in microkeratome blades. Among the known materials, diamond is believed to have the best cutting capacity due to its great hardness, because the cutting edge can be sharpened to a very small radius of curvature, for example in the nanometer range. Disadvantages are, however, the high material cost and the difficulties in applying the diamond as a cutting edge on a blade.  
           [0005]    A blade made of stainless steel, on the other hand, can be manufactured in a comparatively simple way, and offers considerable cost advantages. However, while stainless steel blades are cheaper to manufacture than diamond blades, they are not so inexpensive as to render them “disposable” in all instances. Stainless steel blades are, therefore, sometimes autoclaved after a use and reused on another patient. While autoclaving is generally considered an effective method of sterilization, it is not foolproof, and only one-time use of blades can ensure that each blade is entirely free of infection or physical defects.  
           [0006]    Because the “sharpness” of the blade, until now, has been considered to be the most important characteristic of the blade for achieving a precise and consistent corneal resection, materials cheaper than stainless steel, such as plastics, have not been considered for use in microkeratome blades, as these materials are typically too soft to achieve the required edge sharpness. Instead, the art has focused on various methods of manufacturing ever-more sharper steel blades, resulting in more complex and expensive manufacturing processes, and rendering re-use of blades more tempting economically for practitioners. For instance, it has been proposed to melt the cutting edge of a metallic blade body by laser beam treatment and to rapidly cool it off in a water bath. In this way, the cutting edge is amorphized and can then be sharpened to a radius of curvature less than several ten nanometers. It has also been proposed to produce sharper blades by providing a blade having a carrier portion and a thin-walled cover portion made of amorphous metal, which is joined to the carrier portion and forms a cutting edge. To prevent multiple uses of a blade, it has been proposed to magnetically encode the blade upon its first use, and to provide a microkeratome machine that will not accept a blade if a subsequent use is attempted. However, such proposed equipment is complex and expensive.  
           [0007]    Thus, there is a need in the art for a device that effects separation of the corneal epithelium, and that can be manufactured in a simple fashion from inexpensive polymeric raw materials. Additionally, it is advantageous for the blade to be configured for one time use by virtue of its material composition. It is to the provision of a device meeting these and other needs that the present invention is primarily directed.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a disposable separator or blade for separating the epithelium of a cornea from the underlying Bowman&#39;s layer, the device comprising a separator fabricated from a polymeric material. The separator comprises a front portion that includes a separating edge, a rear trailing portion having a rear edge, and a pair of side edges that extend from the front and rear portions. The separating edge is sharp enough to separate the epithelium layer from Bowman&#39;s layer, but not sharp enough to cut into Bowman&#39;s layer when in contact therewith. The blade may include a blade holder that is preferably, but not necessarily, a polymeric material.  
           [0009]    In another aspect, the invention provides a separator to be used with a handpiece drive tool or other surgical device that separates the epithelium of a cornea from the underlying Bowman&#39;s layer of an eye of a patient, the surgical device preferably including a positioning ring for temporary attachment to the eye and structured to present and expose the cornea to be separated, a separator head assembly structured and disposed to releasably engage and carry said separator, and a drive operably connected to the separator head assembly for causing movement of the separator across the positioning ring and for causing oscillating movement of said separator, said separator comprising a separating edge, said separator having a polymeric separating edge. The separator preferably includes one or more coupling features for engagement with cooperating coupling features of the drive tool. Example drive tools and methods suitable for use in connection with the separator of the present invention are disclosed in U.S. Provisional Patent Application Ser. No. 60/435,009, filed Dec. 19, 2002, and U.S. Provisional Patent Application Ser. No. 60/500,874, filed Sep. 5, 2003, which applications are hereby incorporated herein by reference.  
           [0010]    In a preferred aspect of the invention, the polymeric material of the separator is transparent. A transparent separator will not obstruct the visual field when observing the progress of the separator through the cornea. More preferably, the polymeric material comprises a slight tint so that it is visibly different in perceived color than the epithelium.  
           [0011]    In another preferred aspect, the separator is constructed of a polymeric material that will undergo dimensional changes if exposed to temperatures exceeding about 100° C. This can be accomplished, for example, with a polymeric material that has a Vicat softening point below about 100° C. This prevents the blade from being used after either autoclaving or steam sterilization, thus ensuring that a new, pristine and sterile blade is used on each patient. In this manner, the quality and safety of the separator can be guaranteed.  
           [0012]    In yet another aspect of the present invention a method is provided for separating at least a portion of an epithelium from a cornea of an eye, so that an intact Bowman&#39;s layer of the cornea is exposed. The method comprises the steps (a) fixing a positioning ring to an eye so that the cornea at least partially extends therethrough; (b) moving a separator having a polymeric separating edge along a travel path that intersects at least a portion of the cornea so as to separate the epithelium from the cornea, leaving Bowman&#39;s layer intact; and (c) retracting the separator out of contact with the cornea. The edge of the separator is preferably sufficiently blunt so as not to sever Bowman&#39;s layer, but rather to separate and peel back the corneal epithelium from Bowman&#39;s layer.  
           [0013]    In example embodiments, the present invention provides a separator that is able to separate the epithelium of a cornea from the underlying Bowman&#39;s membrane in such a way that the epithelium can be easily and precisely aligned back into its original position following the reshaping of the cornea. Example embodiments of the separator of the present invention can be manufactured cheaply and easily, such that the separator is economically disposable, thus reducing the incentive to reuse the device and create a chance of infection due to inadequate sterilization. Example embodiments of the separator of the present invention are incapable of being sterilized by autoclaving or steam sterilization after use. Optionally, however, the separator is capable of being sterilized by other means, such as, for example, exposure to electromagnetic radiation, or to chemical agents. Example embodiments of the separator of the present invention do not obstruct the visual field of the surgeon as the separator progresses through the cornea.  
           [0014]    These and other aspects, features and advantages of the invention will be understood with reference to the drawing figures and detailed description herein, and will be realized by means of the various elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following brief description of the drawings and detailed description of the invention are exemplary and explanatory of preferred embodiments of the invention, and are not restrictive of the invention, as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0015]    [0015]FIG. 1 is a perspective view of a separator according to one embodiment of the present invention.  
         [0016]    [0016]FIG. 2 is a cross-sectional view of the first three layers of tissue of the cornea of an eye.  
         [0017]    [0017]FIG. 3 is a partial side view of a flat leading edge portion of a separator according to an embodiment of the present invention.  
         [0018]    [0018]FIG. 4 is a partial side view of a rounded leading edge of a separator according to another embodiment of the present invention.  
         [0019]    [0019]FIG. 5 is a partial side view of an angled leading edge of a separator according to yet another embodiment of the present invention.  
         [0020]    [0020]FIGS. 6A-6C are cross-sectional views of separators according to different embodiments of the present invention.  
         [0021]    [0021]FIG. 7 is a side view of a separator assembly according to an embodiment of the present invention.  
         [0022]    [0022]FIG. 8 is a side view of a hand piece useful in practicing the present invention.  
         [0023]    [0023]FIG. 9 is a side view of the separator assembly in a first position slidably engaged with a hand piece secured to the eye by vacuum.  
         [0024]    [0024]FIG. 10 is a side view of the separator assembly of FIG. 9 in a second position.  
         [0025]    [0025]FIG. 11 is a side view of the separator assembly of FIG. 9 in a third position.  
         [0026]    [0026]FIG. 12 is a top view of portions of the hand piece and separator assembly of FIG. 9 after the epithelium has been separated from the eye.  
         [0027]    [0027]FIG. 13 is a cross-sectional side view of a portion of the separator assembly showing the spatial relationship between the separating edge and the applanator.  
         [0028]    [0028]FIGS. 14A-14C show various possible configurations of the separated epithelium as the separating edge engages the cornea and causes separation of the epithelium from the Bowman&#39;s layer. 
     
    
     DETAILED DESCRIPTION  
       [0029]    The present invention may be understood more readily by reference to the following detailed description of the invention taken in connection with the accompanying drawing figures, which form a part of this disclosure. It is to be understood that this invention is not limited to the specific devices, methods, conditions or parameters described and/or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of the claimed invention. Also, as used in the specification including the appended claims, the singular forms “a,” “an,” and “the” include the plural, and reference to a particular numerical value includes at least that particular value, unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” or “approximately” one particular value and/or to “about” or “approximately” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment.  
         [0030]    The disclosed epithelium separator is especially suited for use in excimer laser reshaping of the cornea. It is safer than standard microkeratomes used in eye surgery, and is inexpensive enough to be a disposable, single use device, which eliminates the need for sterilization between procedures, and thus reduces the possibility of infection.  
         [0031]    The disclosed separator is ideally suited to the unique requirements for separating the epithelium layer from the underlying Bowman&#39;s layer. While microkeratomes developed to sever the stroma for laser in situ keratomileusis were required to be extremely hard and sharp to maintain a radius of curvature as low as 1 micron at the edge, the present separator is not intended for applications requiring severing of Bowman&#39;s layer or the stroma, and therefore has no such stringent sharpness requirements and can be constructed of cheaper, softer materials. In fact, in example embodiments, the edge of the separator is sufficiently “blunt” so as not to be capable of severing Bowman&#39;s layer under normal operating conditions, but instead, only has sharpness sufficient to cleave the boundary between the epithelium and Bowman&#39;s layer. For example, the “blunt” leading edge of the separator of the present invention preferably has a radius of curvature of at least about 5 microns, and no more than about 100 microns. In further preferred embodiments, the radius of curvature of the separator edge is between about 10 microns and about 30 microns, and most preferably is between about 15 microns to about 25 microns.  
         [0032]    Referring now to FIG. 1, the separator  100  comprises a separator body  102  having a leading separating edge  104 , a rear edge  106 , and first and second side edges  108 ,  110  that extend from the separating edge  104  to the rear edge  106 , thereby defining the body  102 . In a preferred embodiment, the rear edge  106  is generally parallel to the separating edge  104 . The separating edge  104  is the first portion of the separator  100  to come into contact with the cornea and effects the separation of the epithelium therefrom. At least the separating edge  104  of the separator  100  is formed of a plastic or polymeric material. In example embodiments, the entire body  102  is formed of a plastic or polymeric material.  
         [0033]    While the dimensions and configuration of the separator are largely determined by the instrument in which they are to be used, the separator  100  is preferably less than 1000 microns in thickness. However, because the separating edge  104  must not be sharp enough to cut into Bowman&#39;s layer under normal operating conditions, it should not be so thin that excision of Bowman&#39;s layer would occur. The thickness of the separating edge  104  is preferably greater than about 200 microns, sufficient to prevent cutting into Bowman&#39;s layer under normal operating conditions.  
         [0034]    While the separator  100  can be flat, having a rear edge  106  substantially the same width as the separating edge  104 , more preferably, the rear edge  106  is thicker in dimension than the separating edge  104 , the body  102  being tapered toward the separating edge. In some cases, the rear edge  106  may be an order of magnitude thicker than the separating edge  104 , and even up to two orders of magnitude thicker. Such dimensions may make it easier for the surgeon to handle the separator prior to insertion into the surgical device and also aid in its stability once installed.  
         [0035]    The cornea  200  of the human eye includes five layers, the outer three of which are illustrated in FIG. 2. The outer-most layer is known as the epithelium layer  202  and is typically 50 to 90 microns thick. The epithelial layer  202  is stratified, possessing 5 to 6 layers of epithelial cells, which are held together by desmosomes (not shown). Bowman&#39;s layer  204  separates the epithelium from the stroma layer  206 . Bowman&#39;s membrane  204  is typically about 12 microns thick, while the stroma  206  is from 400 to 450 microns thick and makes up most of the thickness of the cornea. While the example embodiments of the present invention disclosed herein are considered optimal for use upon a human eye, it is to be understood that such a separator is easily modified for use on similar animal eyes, including eyes of most mammals and many vertebrates, such as horses, dogs, cats, elephants, sheep, and swine.  
         [0036]    [0036]FIG. 3 shows a side view of a flat separating edge  302  portion of a separator  100  according to one embodiment of the invention. The polymeric separating edge  302  of the separator  100  should not be too thick, such that it will reduce the consistency with which the epithelial layer  202  is penetrated. The separating edge  302  preferably is about 5 to 25 micrometers thick, and more preferably about 15 micrometers thick. FIG. 4 shows a side view of a rounded separating edge  402  according to another embodiment of the separator  100 . As shown in FIG. 5, the separating edge can also come to an angled point  502 , provided, however, that it is not sufficiently sharp to sever Bowman&#39;s layer when used as intended.  
         [0037]    As shown in FIG. 6A, the separator  600  need not be the flat rectangular shape shown in FIG. 1. In alternate embodiments, the separator  600  comprises a separator body  602  having a polymeric separating edge  604 , a rear edge  606 , and a pair of side edges (not shown) that extend from the polymeric separating edge  604  to the rear edge  606  defining the body. A notch, projection, or other surface feature  605  is preferably provided on the underside of the separator  600  or elsewhere for coupling with a cooperating support member of a surgical device for stability.  
         [0038]    While in some embodiments the separator  600  comprises a solid body of polymeric material and optionally includes a reinforcing material therein, in alternate embodiments the separator  600  is fabricated as a polymeric coated metallic or ceramic body. For example, a metallic core  618  can be employed as a base structure upon which a polymeric or polymeric-composite material  616  is disposed. While FIG. 6C, shows a polymeric coating  616  over only the separating edge, the coating  616  may alternatively cover the entire metallic core  618 . In this manner, the metal core will provide rigidity to the separator  600  whereas the polymeric material  616  will provide the blunt separating edge  614  for interaction with the cornea without the risk of severing Bowman&#39;s layer.  
         [0039]    [0039]FIG. 6B shows another alternate embodiment of the present invention in which the separator  600  comprises a polymeric front portion  610  that includes a separating edge  612 , and a metallic rear portion  608  comprising a rear edge  609 . The front portion  610  is joined to the rear portion  608  in any one of a variety of known ways, such as by adhesive, thermal or solvent welding, interengaging surface features, one or more fasteners, or the like, for example. As in the embodiment shown in FIG. 6C, the metal portion  608  will provide rigidity to the separator  600  whereas the polymeric portion  610  will provide the blunt separating edge  612  for contact with the cornea.  
         [0040]    Referring to FIGS. 7-9 and  12 , one embodiment of a surgical device according to the present invention comprises a hand piece  800  with an integral vacuum ring  802  and a separator assembly  700 . (Note that, for simplicity, the separator cover  706  is not shown in FIGS. 9-11 and that the figures are not necessarily drawn to scale.) Separator assembly  700  comprises a drive shaft  710  that engages a motor (not shown) through a bushing  806  in the hand piece  800  to move the separator assembly  700  transversely and to oscillate the separator  600 . Vacuum is applied to the vacuum ring  802  through vacuum port  804  to secure the eye thereto.  
         [0041]    Preferably, one or more motors (not shown) provide two types of motion to the separator assembly  700  and the separator  600 . The first type of motion is side-to-side oscillation along an axis parallel to the separating edge  604  of the separator  600  to assist in the separation process. The second type of motion is longitudinal motion generally perpendicular to the separating edge  604  of the separator  600  to advance the separation along the cornea. The rotational motion of the motor is transferred from the drive shaft  710  to the plunger assembly  712 , through which it is translated to oscillations in the separator  600 . Under action from the plunger assembly  712 , the separator  600  is oscillated by the motor. The separator  600  can oscillate either transversely, vertically, or longitudinally with frequency ranging from about 10 Hz to about 10 KHz. Electromagnetic or piezoelectric forces on the separator  600  can alternatively provide the oscillation, or external rotating or vibrating wires can provide the oscillation. The separator  600  is preferably oscillated along the separator support  703  in a direction perpendicular to the plane of the figure.  
         [0042]    An applanator  702  is connected to the separator assembly  700  in a position forward of the separator  600 . Separator  600  is held firmly within the separator assembly  700  by separator cover  706 , which is preferably hingedly connected to the hand piece  700  moveable in the direction of the arrow in FIG. 7. The cover  706  is secured in place through a locking screw  708 , which can be tightened by hand through the locking screw head  704 .  
         [0043]    Separator assembly  700  is slidably associated with hand piece  800  through grooves  1208   a ,  1208   b  (see FIG. 12). FIG. 9 shows a cross-sectional side view of an eye  902  of a patient and an epithelial separator device comprising the hand piece  800  associated with the separator assembly  700 . When the eye  902  is placed within the vacuum ring  802  and a vacuum is applied to vacuum port  804 , the surface of the eye  902  is tightened and pulled through the ring  802  to expose the cornea  200  at a position forward of the applanator  702 . As shown in FIG. 9, the separator assembly  700  begins in a first position located away from the eye  902 .  
         [0044]    Referring now to FIG. 10, as the applanator  702  moves forward under action of the drive shaft  700  through tracks  1208   a ,  1208   b , the cornea  200  is forced against the undersurface of the applanator  702 . This results in a flattening of the cornea  200  before it comes into contact with the separator  600 . As the separator assembly  600  moves along the cornea  200  of the eye  902 , the separator  600  engages the cornea  200  and removes the epithelium layer  202  located at the surface of the cornea  200  of the eye  902 . However, the separator  600  is not sharp enough to excise Bowman&#39;s layer  204  during operation of the epithelial separator device, and the separator passes over the intact Bowman&#39;s layer as it separates the epithelium.  
         [0045]    Referring now to FIG. 13, the separating edge  604  is preferably positioned or angled such that it is located at a height h below the bottom surface of the applanator  702 . This spacing or distance between the separating edge and the bottom surface of the applanator does not determine the depth of the cut, as with previously known methods and devices for severing the cornea for LASIK procedures. Therefore, the exact value of this distance is not as critical to performance of the separator as it can be with previously known devices and procedures, where tens of microns can be the difference between a successful flap and a medical emergency. While prior art LASIK microkeratomes typically cut at a distance of 130-150 microns, the present separator can be set at a depth (h) from between about 40 microns to about 300 microns, more preferably from about 40 to about 100 microns. Surprisingly, consistent epithelium removal has been demonstrated at depths of about 240 microns.  
         [0046]    The separator  600  is preferably fabricated from a synthetic polymeric material. The preferred polymeric material is a thermoplastic or thermoset polymer or ionomer. There are presently available a wide variety of durable, resilient polymers which may be employed to fabricate the separator. Included among such materials are, but are not limited to, acetals, (meth)acrylates, acrylics, alkyds, polycarbonates, polyolefins, polyesters and co-polyesters, polymethylpentene, polypropylene, polysulfones, cellulosics, styrene acrylic co-polymers, fluoropolymers, nylons, polystyrene, polyetheretherketones (PEEK), polyarylates, polyetherimides, styrene acrylonitrile, silicones, epoxys, polyvinyl chloride, urethanes, acrylonitrile-butadiene-styrene (ABS), methylmethacrylate-acrylonitrile-butadiene-styrene (MABS), allyl diglycolcarbonate, as well as combinations or blends of these polymers. The preferred polymeric materials are polycarbonates, PEEK, polystyrenes, MABS, acetal homopolymers, and poly(methyl methacrylate) (PMMA). It has in fact been found, in accord with the principles of the present invention, that many of these materials can retain a sufficiently sharp edge and have sufficient durability and resiliency to function as a separator.  
         [0047]    Preferably, the separator has a flexural modulus of at least about 1.5 GPa according to ASTM D790-02, more preferably at least about 2.0 GPa, and most preferably at least about 3.0 GPa. Furthermore, the separator preferably has a tensile strength at yield of at least about 25 MPa according to ASTM D638-02, more preferably at least about 40 MPa, and most preferably at least about 50 MPa. Additionally, the separator preferably has either a Rockwell M hardness greater than or equal to 70 or a Rockwell R hardness greater than or equal to 90, according to ASTM D785-98e1. Most preferably, the material has a Rockwell M hardness of greater than 90. Such relatively stiff materials (compared to other plastics) are preferred in order to avoid deformation of the separator during normal operation. However, it is indeed surprising that such materials having strength and hardness less than stainless steel are nonetheless suitable for use in a separator in the present invention. Commercially available materials meeting the above preferred criteria include various grades and formulations of PEEK, PMMA, acetal homopolymer, polystyrene, MABS, and polycarbonate.  
         [0048]    In addition to the stiffness of the material, the toughness of the material can be important in the use of the separator. Accordingly, the separator preferably has a toughness of at least about 1 J/cm 2 , more preferably at least about 2 J/cm 2 , most preferably at least about 3 J/cm 2 , according to ISO 179-1 (15 Dec. 2000) Charpy Impact Test. When this test method is referenced to herein it is meant to refer only to the portion of the test performed at 23° C. using unnotched specimens. Such relatively tough materials (compared to other plastics) are preferred in order to avoid cracking or shattering of the separator during normal operation. However, it is indeed surprising that such materials having toughness less than stainless steel are nonetheless suitable for use in a separator in the present invention. Commercially available materials meeting the above preferred criteria include various grades and formulations of PEEK, PMMA, acetal homopolymer, polystyrene, MABS, and polycarbonate. However, while unmodified polystyrene has moderate strength, it is rigid and brittle. Impact strength is increased significantly by blending the polymer with rubbers such as polybutadiene. The preferred MABS is available commercially from BASF as Terlux® 2802 and the preferred polystyrene is commercially available from Nova Chemicals as Crystal PS 3500. Table 1 below presents data provided by the manufacturer of various polymers.  
                                                         TABLE 1                                   Tensile   Flexural   Charpy   Vicat           Strength at   Modulus   Impact   Softening           Yield (MPa)   (GPa)   (J/cm 2 )   (° C.)                                    Terlux ® 2802   48   *   15   91       Crystal PS 3500   36   3.5   *   92       Victrex PEEK   97   4.1   *   *       450G       BASF Lucryl ® KR   60   *   5   106       2008/1 PMMA                          
 
         [0049]    In example embodiments, the polymeric material is reinforced by incorporation of various inorganic filler materials. For example, carbon and glass fibers and powders have been incorporated into various polymeric materials to greatly increase flexural strength. Such materials typically have high degrees of strength and are capable of taking and maintaining a sufficient separating edge, as well as providing sufficient toughness to allow for their use in fabricating the separating device.  
         [0050]    In other example embodiments of the invention, the polymeric material of the separator is transparent. A transparent separator will not obstruct the visual field when observing the progress of the separator through the cornea. The polymeric material preferably exhibits a light transmission greater than 50 percent, more preferably greater than 75 percent, and a haze factor less than about 25 percent, more preferably less than about 5 percent, in accordance with ASTM D1003-00. More preferably, the polymeric material comprises a slight tint so that there it is visibly different in perceived color than the epithelium. This is easily accomplished, for example, by addition of a tinting agent to the polymer before manufacture. The slight tint will provide a contrast between the blade and the epithelium enabling the surgeon to differentiate therebetween, but yet, still providing optical clarity for observation of the cornea during use. The tint, by increasing the visibility of the separator during use, will also make it easier for the surgeon to handle the blade prior to insertion into the surgical device.  
         [0051]    The tinting agent can include one or more pigments. Preferably, the pigment is a white pigment, a black pigment, a blue pigment, a brown pigment, a cyan pigment, a green pigment, a violet pigment, a magenta pigment, a red pigment, or a yellow pigment, or shades or combinations thereof. Suitable classes of colored pigments include, for example, anthraquinones, phthalocyanine blues, phthalocyanine greens, diazos, monoazos, pyranthrones, perylenes, heterocyclic yellows, quinacridones, diketopyrolo-pyroles, and (thio) indigoids. Representative examples of phthalocyanine blues include copper phthalocyanine blue and derivatives thereof (Pigment Blue 15). Representative examples of quinacridones include Pigment Orange 48, Pigment Orange 49, Pigment Red 122, Pigment Red 192, Pigment Red  202 , Pigment Red  206 , Pigment Red 207, Pigment Red 209, Pigment Violet 19 and Pigment Violet 42. Representative examples of anthraquinones include Pigment Red 43, Pigment Red 194 (Perinone Red), Pigment Red 216 (Brominated Pyanthrone Red) and Pigment Red 226 (Pyranthrone Red). Representative examples of perylenes include Pigment Red 123 (Vermillion), Pigment Red 149 (Scarlet), Pigment Red 179 (Maroon), Pigment Red 190 (Red), Pigment Violet, Pigment Red 189 (Yellow Shade Red) and Pigment Red 224. Representative examples of thioindigoids include Pigment Red 86, Pigment Red 87, Pigment Red 88, Pigment Red 181, Pigment Red 198, Pigment Violet 36, and Pigment Violet 38. Representative examples of heterocyclic yellows include Pigment Yellow 1, Pigment Yellow 3, Pigment Yellow 12, Pigment Yellow 13, Pigment Yellow 14, Pigment Yellow 17, Pigment Yellow 65, Pigment Yellow 73, Pigment Yellow 74, Pigment Yellow 110, Pigment Yellow 117, Pigment Yellow 128, Pigment Yellow 138, and Pigment Yellow 151. A representative example of diketopyrolo-pyroles include Pigment Red 254. Such pigments are commercially available in either powder or press cake form from a number of sources including, BASF Corporation, Engelhard Corporation and Sun Chemical Corporation. Examples of other suitable colored pigments are described in the Colour Index, 3rd edition (The Society of Dyers and Colourists, 1982).  
         [0052]    In further example embodiments, the separator is constructed of a polymeric material that will undergo dimensional change if exposed to temperatures exceeding about 121° C., and more preferably if exposed to temperatures exceeding about 100° C. Such a separator is incapable of being re-used if it is autoclaved after use, because the dimensional change will render the separator incompatible for coupling with the surgical device, thereby ensuring that separators are not reused. In example embodiments, the polymeric material has a Vicat softening point, measured by ASTM D1525-00, of less than about 121° C., and more preferably of less than about 100° C. The Vicat softening point is the temperature at which a flattened needle of 1 mm 2  cross-section, and under a specified constant load, penetrates a specimen of the plastic to a depth of 1 mm. It is useful as a rough comparative guide to a resin&#39;s resistance to elevated temperatures.  
         [0053]    Referring to FIG. 14A, the separator  600  is used with a surgical device that separates the epithelium  1206  of a cornea from the underlying Bowman&#39;s layer  204  of an eye of a patient. As the separator  600  is positioned in contact with the eye, the separator edge  604  will cleave the fibrils connecting the epithelium  1206  to Bowman&#39;s layer  204 , but will not slice into Bowman&#39;s layer  204 . The separator  600  pushes the epithelial cells  1206  and preferably, does not exert a force that could disrupt the intercellular bonds, such as the desmosomes. As the separator edge  604  progresses along the eye, the epithelium  1206  is preferably left free to assume an unhindered position and configuration. Often, the epithelium  1206  will progress along the top surface of the applanator  702 . Referring to FIG. 14B, depending, in part, on the angle of incidence of the separator  100  and the depth of encounter (h), the epithelium  1206  may be pushed out in front of the separator  100 , forming multiple folds  1400   a ,  1400   b  as it progresses. Alternatively, the epithelium may progress up the front surface  1402  of the separator  600  as shown in FIG. 14C.  
         [0054]    By not constraining the epithelium  1206  during separation, the epithelium  1206  encounters minimal stress and strain and will suffer less cell death. This is particularly important when the separator  600  is oscillated. If the epithelium  1206  is constrained or otherwise prevented from moving freely (such as being held against a surface post-separation), the oscillatory energy of the separator  600  will be absorbed, at least partially, by the epithelium  1206 , causing cell disruption or death. However, a freely moving epithelium  1206  will not absorb as much energy from the oscillatory movement of the separator  600  and will maintain structural integrity.  
         [0055]    Referring back to FIG. 12, when the separator assembly  700  is retracted from the cornea after separation has occurred, the separated epithelium layer  1206  is preferably left partially attached to the cornea of the eye by a hinge  1202 . The hinge  1202  is preferably about 1 cm in length, but can differ significantly from this, provided enough of Bowman&#39;s layer  1204  is exposed to perform laser ablation. The separated epithelium  1206  typically will be laid out flat upon the exposed Bowman&#39;s layer  1204  after the separator assembly  700  is retracted. In this case, the epithelium is carefully moved to the side with forceps to the position shown prior to laser ablation.  
         [0056]    While the invention has been described by reference to various embodiments, it will be understood that many changes and modifications can be made without departing from the scope of the invention. It is therefore intended that the foregoing detailed description be understood as an illustration of example embodiments of the invention, and not as a limiting definition of the invention. It is only the following claims, including all equivalents, which are intended to define the scope of this invention.