the method of the present invention is directed to reversible removal of an epithelial flap or sheet from the cornea in such a manner as to create a smooth surface of exposed corneal tissue optimized for both subsequent far - uv laser ablation and rapid , firm attachment of the removed epithelial layer , while at the same time , by maintenance of the basal epithelial cell layer and at least one intact layer of the basement membrane , eliminating or significantly reducing the type of cellular damage that triggers a cascade of biochemical events involved in wound healing that are recognized as contributing directly to some of the most significant complications of laser refractive surgery . a growing body of data from clinical and experimental studies indicates that a critical factor in improving outcomes after laser vision correction is avoidance of basal epithelial cellular and / or tear film interaction with the stroma in order to prevent the triggering of normal cellular “ repair ” responses in the stroma , which responses are strongly associated with opacification ( loss of corneal transparency ) and post - operative “ haze ”. integrity of the basement membrane can act as a “ fibrotic switch ” and maintain stromal homeostasis . thus , a goal associated with the practice of the present invention is avoidance or absolute minimization of disruption of basal epithelial cell membranes through a removal epithelial layer based on attack at binding sites posterior to the basal epithelial cell layer . the majority of refractive procedures currently performed on the cornea have injury to the epithelium in common . epithelial injury initiates a sequence of events that occur as part of a protective system for preserving vision . for example , keratocyte apoptosis , the first detectable event after any type of epithelial injury , is associated with either mechanical trauma , corneal surgical procedures , or herpetic ( hsv ) keratitis , where cellular suicide may provide an early firewall to viral penetration into the eye and central nervous system . animal studies have demonstrated that superficial keratocytes undergo programmed cell death mediated by cytokines released from the injured epithelium , such as interleukin ( il )- i alpha , fas / fas - ligand , bone morphogenic protein ( bmp ) 2 , bmp4 , and tumor necrosis factor ( tnf ) alpha . redundancy is probably intended to augment the natural defense system by making it difficult for viral pathogens to overcome a single apoptosis activation system . these cytokines are also present in the tear film , thus making it important to prevent exposure of the treated corneal surface or epithelial cells to the tear film so as to avoid adverse responses triggered by cytokines . keratocyte apoptosis is followed by a complex cascade of events that takes place in the corneal epithelium and stroma . these events are regulated by cytokine - mediated interactions between epithelial cells , stromal cells , inflammatory cells , nerves , and lacrimal glands . although some apoptosis cannot be prevented by even the practice of the current invention , the goal of the instant invention is to eliminate , or at least significantly reduce , this cascade of events following apoptosis , and allow for a rapid return to a normal physiologic state of the cornea , with normal regenerative activities rather than repair activities . following keratocyte death , the remaining keratocytes surrounding the zone of depletion begin to undergo proliferation within twelve to 24 hours of epithelial injury . at this point , inflammatory cells are also attracted by chemotactic factors such as the monocyte chemotactic and activating factor ( mcaf ). mcaf production is upregulated in keratocytes by il - i alpha . il - i is released from the epithelium after injury , but is also present in the tear film . it appears to be a master modulator of many of the events involved in this cascade . in experiments performed on eyes from patients scheduled to undergo enucleation because of intraocular melanoma , it was confirmed that keratocyte apoptosis and proliferation occur in the human cornea after epithelial scrape ( prk ). these events occur in parallel with the closure of the epithelial defect , which is enhanced by growth factors produced by both the lacrimal glands and keratocytes , such as epidermal growth factor ( egf ), hepatocyte growth factor ( hgf ) and keratinocyte growth factor ( kgf ). myofibroblasts are keratocyte - derived cells that are present in the repopulated stromata that are characterized by the expression of alpha smooth muscle actin ( sma ). these cells , along with other activated keratocytes , produce disorganized collagen , glycosaminoglycans and growth factors that stimulate healing of the overlying epithelium . myofibroblasts also have altered transparency in vivo , related to corneal crystallin expression . they are thought to be responsible for , or at least implicated in , the creation of post - operative stromal haze . differentiation of myofibroblasts is induced by transforming growth factor ( tgf ) beta , and reversal to fibroblast phenotype has been observed in vitro in the presence of fibroblast growth factor ( fgf ). tgf - beta , found in the basal layer of the epithelium during its closure , seems to control stromal myofibroblast transformation during corneal repair . in addition , basement membrane formation seems to have an indirect effect on the myofibroblast transformation by regulating the extent of tgf - beta release into the corneal stroma . there is a return to a normal physiologic state in the corneal stroma several months after injury . this process is associated with eradication of myofibroblasts via programmed cell death or phenotype reversal to quiescent keratocytes . remodeling of disordered collagen that was produced by myofibroblasts or activated keratocytes during the wound healing process is also mediated by keratocytes . the corneal epithelium may undergo hyperplasia following corneal injury , as a result of the growth factors produced by activated keratocytes and myofibroblasts . stromal remodeling and epithelial hyperplasia are thought to be the most important mechanisms for regression of the refractive effect of prk or lasik surgery . immunohistochemical analysis of tissue from the underside of an epithelial flap has shown it to contain the structural elements collagen vii and heparin sulfate , as well as components involved in attachment of the overlying cell to the underlying stroma by hemidesmosomes . these include laminin , fibronectin and entactin - nidogen . hemidesmosomes are specialized transmembrane cell - matrix junctions between the cytoskeleton of epithelial cells and the extracelleular matrix of basement membranes . the principal component of the hemidesmosomes involved in cell - matrix adhesion is the integrin heterodimer α6β4 , a transmembrane protein that can attach to laminin in the basement membrane . referring now to fig2 , hemidesmosomes ( hd ) 10 comprise very small stud - or rivet - like structures on the inner basal surface of keratinocytes . specifically , the hd comprises two rivet - like placques ( the inner and the outer placques ). together with anchoring fibrils 12 and anchoring filaments 14 , these are collectively referred to as the hd - stable adhesion complex , or hd - anchoring filament complex . the outer plaque contains alpha6 - beta4 ( α6β4 ) integrin 16 that functions as the principal structural component involved in adhesion by attaching to laminin 18 in the basement membrane . the anchoring network is composed by anchoring filaments 14 ( laminin 5 ), anchoring fibrils 12 ( collagen vii ) and anchoring plaque ( collagen iv ) 20 . together , the hd - anchoring filament complex forms a continuous structural link between the basal keratinocyte filaments and the adjacent basement membrane . anchoring filaments traverse the lamina lucida and appear to insert into the lamina densa . beneath the lamina densa , anchoring fibrils can extend within the stroma and physically interact or encircle collagen fibers within the lamellae of the stroma to achieve their anchoring effect between the corneal layers . thus , the intended cleavage plane of the epithelial flap occurs at the level of anchoring fibrils , either between the epithelium basement membrane and anterior stroma ( bowman &# 39 ; s layer ), or between the lamina lucida and the lamina densa of the basement membrane . the hd - mediated interactions between these layers of the cornea are not based on strong covalent links between epithelial cell membrane components and extracellular matrix components . in contrast , the molecular interactions involved are essentially based on much weaker attractions such as hydrogen bonds , van der waals interactions , and hydrophobic interactions . these interactions are reinforced by mechanical entanglement of fibrillar macromolecules with cell - membrane receptors and other membrane components . it is important to appreciate that the chemical nature and the energetic magnitude of these interlamellar forces are responsible for both the utility of chemical delamination and the criteria for selection of appropriate delamination agents . removal of the corneal epithelium has been found to cause damage to stromal keratocytes , which changes start within 15 to 30 minutes of mechanical disepithelialisation in rabbit and monkey corneas . other studies have shown that an early decrease in the density of keratocytes is followed by an increased number of these cells in the underlying stroma and production of collagen and extracellular matrix . there is evidence to suggest that keratocyte changes are influenced by the regenerating epithelium ( cytokines ). covering of the denuded surface of the cornea directly after a surface - type procedure with the corneal epithelial flap can decrease changes in the stromal keratocytes and minimize the likelihood of the production of extracellular matrix and collagen and the undesirable opacification of the cornea arising from such processes . although procedures such as lasek and epi - lasek involve replacement of an epithelial flap over the photoablated stromal surface , the gain from such a step is severely limited due to the lack of viability of the resulting epithelial cell layer ( with disruption of the basal epithelial cells ) due to the very process of creating the layer . in the almost inevitable demise of such reattached epithelial layers , these techniques devolve to variations of prk , with the previously discussed limitations of same . demonstrating potentially advantageous diminished wound healing response obtained from lasek , reduced keratocyte loss was observed after lasek when compared to prk in a preliminary rabbit study . this may occur due to a barrier effect from the basement membrane against pro - apoptotic cytokines that are also present in the tear film . for example , the use of a collagen shield diminished keratocyte loss after corneal epithelial scrape in rabbits . also , lower levels of tgf - beta were detected in the tear film during the first week after lasek when compared with prk in the contralateral eye . in addition , a recent study pointed out the importance of the tgf - beta liberated from the healing epithelium for myofibroblast transformation , as well as the effect of the basal membrane as a barrier for this interaction . thus , it is logical to hypothesize that if an epithelial flap is properly created , less myofibroblast transformation would occur , resulting in less haze . as recognized in the practice of the present invention , it is important to create an epithelial flap by methods that minimally damage the basal epithelial cells , and that such minimal damage becomes a mere incidental effect of the procedure and not an inherent , and unavoidable , result of the techniques used . successfully preventing induction of optical aberrations and at least limiting , if not altogether preventing , avoidable repair / wound healing responses after refractive surgery , through use of procedures according to the present invention utilizing epithelial flaps , is a vital goal that cannot now be achieved through procedures of the prior art , even through use of ethanol disepithelialisation as in lasek procedures . moreover , ethanol - assisted epithelial separation has been confirmed to be toxic to epithelial cells in both a dose - and time - dependent manner ( see invest opthalmol vis sci 43 : 2593 - 2602 ( 2002 ); and j cataract refract surg 28 : 1841 - 46 ( 2002 )). an increase of only a few seconds beyond the minimal exposure necessary for separation leads to cell death , since ethanol is a solvent of the lipid components of the cellular membrane and causes shrinkage of the cell walls . ethanol enters the epithelial cells and produces disorganization of the cellular chemistry . numerous clinical studies have documented that the theoretical goals of repositioning an epithelial flap to facilitate epithelial healing , decrease chemotaxis , reduce inflammation , diminish pain , decrease haze formation , and expedite visual recovery are defeated or at least counteracted due to the effects of ethanol toxicity . the health and viability of epithelial cells , particularly basal epithelial cells , must be maintained in order to obtain optimal clinical outcomes , including reduction or elimination of the most common side effects of uv laser refractive correction procedures . before development of the methods of the present invention , it has not been possible to achieve this . it has proven to be impossible to reproducibly separate an epithelial flap mechanically while , at the same time , maintaining a viable epithelial layer because of the varying thicknesses and curvatures of the cornea at different axes . delamination at relatively constant thickness would inevitably result in separation of tissue sheets at different levels in different areas of the cornea . only cryofracture can reproducibly separate the epithelium from the basement membrane , but that laboratory - only process cannot be performed in vivo . thus , it is an element of the present invention that reproducible creation of a viable , non - damaged epithelial flap can optimally be accomplished pharmacologically , rather than mechanically . thus , the method of the present invention is directed to reversible removal of a corneal epithelial flap or sheet in such a manner as to expose both a smooth surface for photoablation and a smooth opposing surface posterior to the epithelial flap , with both surfaces optimized for rapid , strong , stable reattachment of a viable epithelial flap . under this method , the epithelial cells remain viable with intact cell membranes and intact intracellular junctions . in addition , the basal epithelial cell layer and at least one layer of the basement are optimally preserved intact , diminishing typical wound healing responses , whether such responses are triggered by mechanical damage to epithelial cells or by the cytotoxic effect of chemical agents used to remove the epithelium , or by the tear film . only in this manner is it possible to maintain epithelial viability which , in turn , can prevent or significantly minimize the most common negative side effects of laser refractive surgery . furthermore , effective removal of the epithelium should have the added benefit of significantly expanding the pool of patients susceptible to vision correction through laser surgery , including patients with myopia , hyperopia , astigmatism , and presbyopia . the method of the present invention comprises chemical / pharmacologic separation of the epithelium either from between the basement membrane and bowman &# 39 ; s layer , or from between the lamina lucida and the lamina densa of the basement membrane , leaving : 1 .) a very smooth surface to be laser - ablated ; and 2 .) an epithelial flap with at least one layer of basement membrane and viable , undamaged basal epithelial cells enabling rapid hemidesmosome reformation with firm attachment of the epithelial flap to the underlying surface . the practices of the prior art have almost exclusively focused on the use of ethanol as a delamination agent . although this renders it possible to achieve many of the goals of creation of a replaceable epithelial layer , such as a smooth surface for refractive correction , and minimization of damage to the stroma and other elements of corneal anatomy , these achievements do not come without a price . in an empirical fashion , other chemical agents for disepithelialisation have been investigated , but have not demonstrated the same utility in creation of an optimal epithelial flap . thus , it is an element of the practice of the method of the present invention to utilize chemical agents or compositions selected for a similar activity for attacking the relatively weak , non - covalent interactions through which hemidesmosomes act to bind the layers within the basement membrane , as well as to bind the basement membrane to the underlying bowman &# 39 ; s layer . chemical / pharmacologic agents may be chosen based on possessing chemical activity similar to ethanol where the agents can interrupt the relatively weak binding forces associated with hydrogen bonding and / or hydrophobic / van der waal &# 39 ; s forces . of course , these agents must be selected so as to avoid the deleterious effects ascribed to alcoholic agents such as entering the epithelial cell with resulting damage to major cellular components . relevant among these mechanisms of cytotoxicity is the breakdown of the lipid bi - layer within cells by ethanol , which mechanism is presumed to be correlated to the relatively short carbon chain length of the alcohol . thus , longer chain - length alcohols , as well as polyhydroxy alcohols , are preferable candidates for the chemical agent of the present invention . at the same time , these agents and / or compositions must be selected on the basis of their relative lack of cytotoxic activity . by way of example , and without limitation to the scope of the present invention , suitable chemical / pharmacological agents , as would be recognized by one of ordinary skill in the relevant art , could be selected from long - chain , high molecular weight organic solvents displaying milder hydrolytic activities on peptide bonds , along with efficient destabilization of strong molecular hydrophobic interactions . alternatively , effective epithelial delamination agents could be selected on the basis of an enzymatic approach related to the specific cleavage sites of the fibrillar macromolecules responsible for adhesion of the basement membrane of the epithelium to the bowman &# 39 ; s layer of the stroma , or in combinations of both approaches , in a single or sequentially administered agent or composition . suitable delamination agents of the present invention would be selected from long - chain , high molecular weight organic solvents . such species would display mild hydrolytic activities on peptide bonds , thus acting far less cytotoxic than the current agent of choice for lasek procedures , ethanol . at the same time , such species would display an ability for efficient destabilization of molecular hydrophobic interactions and / or hydrogen bonds sufficient to counteract the anchoring function of hemidesmosomal complexes . by way of illustration , and without limitation to the scope of the invention disclosed herein , polyhydroxy alcohols and / or polymers of same , meet the necessary chemical criteria for interruption of the binding forces involving hd anchoring between layers of the cornea . potential cytotoxic effects of such species can be modulated to optimal levels through control / selection of carbon chain length and number of hydroxyl groups on the molecular chain for small molecule species , and molecular weight for polymeric species . preferably , for polyhydroxy alcohols , optimal carbon chain length would be 4 - 6 carbon atoms , with 2 - 3 hydroxyl groups on the carbon chain . for polymeric species , preferred molecular weight ranges would be on the order of 6 , 000 to 90 , 000 da . a number of compounds useful in the present invention , which have already received fda approval for ophthalmic use in other applications , include benzyl alcohol , cetyl alcohol , lanolin alcohols , phenethyl alcohol , and polyvinyl alcohol . see cder inactive ingredient search for approval drug products , a copy of which is incorporated herein by reference in its entirety . polyhydroxy alcohols within the scope of the present invention are known in the art . they include ethylene glycol , propylene glycol , glycerol 1 , 2 - propanediol , sorbital , mannitol , inosital , pentaerythritol and the like , the last five being examples of polyhydroxy alcohols having between 4 and 6 carbon atoms . likewise , polymers of these polyhydroxy alcohols , which also come with the scope of the present invention , are known in the art . they include polyethylene glycol ( peg , such as peg 300 and peg 400 ), polypropylene glycol , polyglycol , polysorbital , polymamitol and the like . other alcohols are also suitable for the present invention . these other alcohols include aliphatic alcohols of between three to five carbon atoms in length , including isomers of these alcohols , for example primary , secondary and tertiary alcohols . thus , alcohols such as n - propyl alcohol , isopropyl alcohol , sec - butyl alcohol , tert - butyl alcohol , n - pentyl alcohol , isopentyl alcohol and other isomers of pentyl alcohol , including cyclopentyl alcohol . also envisioned are alcohols with six carbon atoms , such as hexanol and cyclohexanol . see n . kornfield - paullain , et . al . effect de différents solvants organiques sur la dégredation alcaline de l &# 39 ; élastine , 50 bull . soc . chem . biol . 759 - 771 ( 1968 ), which is incorporated herein by reference in its entirety . the concentration of the above delaminating agents should be sufficient to cause loosening of the hemidesmosonal links within the cornea . these links function between the anterior epithelial layer of the cornea and the stomal layer of the cornea posterior to the epithelial layer . the concentration should not be so high , however , as to be cytotoxic . as envisioned , the concentration of the delaminating agent should be from about 0 . 1 % to about 60 %. more preferably , the concentration should be from about 5 % to about 30 %. in one embodiment , the ph of the delaminating agent is made to be about the same as the ph of the eye , which is about 7 . 4 . in another embodiment , the delaminating agent is isotomic . practice of the method of the present invention , as will be recognized by one of skill on the appropriate art , can be accomplished through procedures adapted from those currently utilized for ethyl alcohol disepithelialisation ( lasek ). accordingly , procedures such as those described below , if utilized with chemical delamination agents selected according to the disclosures and teachings herein , will provide optimal patient outcomes ( as defined above ) for refractive vision correction with far uv laser radiation . patient is seated comfortably in an appropriate treatment chair . for those patients with a heightened sense of anxiety concerning the impending procedure , pre - administration of approved anti - anxiety medications may be indicated . typically , a mechanical aid such as a speculum is utilized to allow the treating physician unhindered access to the patient &# 39 ; s eye . with or without such an aid , one or more doses of a suitable topical anesthetic are applied the eye to be treated . preferably , in addition to the topical anesthetic , the eye is also treated with an ophthalmic antibiotic . the regimen of antibiotic therapy may be limited to in situ administration concurrently with pre - operative medications , or it may involve a course of administration begun some days prior to surgery . in addition , non - steroidal anti - inflammatory agents may be used or may be applied topically . once the patient and the eye to be treated are prepared , one or more of the chemical agents or pharmacological compositions of the present invention may be applied to the eye . the method or system of application of the delaminating agent do not necessarily comprise a component of the present invention but may rely on techniques and apparatus currently used in similar procedures for laser vision correction . concentrations of any compositions of the invention , diluent , time of application , method of cessation of application , duration and rigor of rinsing of the treated eye , are all factors , as would be recognized by one of skill in the appropriate art , that would depend on the specific chemical identity of the delaminating agent used . by way of comparison , lasek procedures , as discussed above , typically utilize solutions of ethanol in a concentration range of 15 - 20 % by volume . at this concentration , exposure times necessary to optimize delamination of the epithelium are in the range of 20 - 30 seconds . any longer exposure significantly increases the risk , or likelihood , that the epithelium will experience irreversible cytotoxic damage that could have significant negative impact not only on the continued viability of the epithelium upon reattachment , but also the ultimate outcome of the vision correction procedure . however , one of the advantages of the practice of the method and compositions of the present invention is that the delamination agents are not cytotoxic so that the criticality of time of exposure of the cornea to the agent is no longer an issue . exposure times are then dictated solely , on one end of the time spectrum , by the exposure duration necessary for the agent to act on the hemidesmosomal links to insure delamination and , on the other end , by consideration of the convenience of the patient and / or treating physician . the present invention also contemplates that the delaminating agents of the present invention will possess a range of efficacies in their delamination function so that exposure times , as well as other parameters of use , will have to be adjusted in accord with choice of specific agent . however , as addressed above , the essential lack of toxicity of these agents removes time of exposure as a critical determinant of the procedural protocol . at termination of exposure to the delaminating agent , the treated eye is rinsed with an appropriate lavage and / or excess delamination agent may be removed by gentle blotting with a merocel sponge . the lavage could also comprise an nsaid analgesic , such as diclofenac sodium or ketoroloac tromethamine . an alternative , additional step to the procedure would be application of a suitable antibiotic , either before of after treatment with the delaminating agent . at this point in the procedure , the now loosened epithelial flap or sheet is carefully removed and stored or , for procedures involving flap creation , flipped over , for that duration of time necessary for laser treatment of the exposed stroma . the specifics of the devices used and the procedure to be followed are analogous to those used in prior art procedures involving removal of corneal layers , such as epi - lasik , and do not necessarily comprise an essential component of the practice of the method of the present invention . the exposed stromal surface of the eye to be treated is now subject to far - uv radiation ( preferably from an excimer laser at a wavelength of 193 nm ). at this point , either the stored epithelial flap or sheet is returned to the surface of the treated eye and appropriately repositioned with the aid of devices conveniently available , ranging from metallic spatulas or canulas to methylcellulose sponges , or the epithelial flap is repositioned . optionally , a non - steroidal anti - inflammatory ( nsaid ) composition may be added to the treated eye . another option , in place of , or in combination with , nsaid treatment involves administration of an ophthalmologically effective steroid . finally , a bandage contact lens is applied and the patient is instructed on follow - up care of the treated eye ( s ). in a preferred embodiment , the chemical delamination agent is packaged in a pre - portioned , single - dose as part of a kit that may , optionally , contain other compositions or devices with utility in the practice of the present invention . one particularly preferred alternative embodiment comprises a bandage contact lens adapted to function as an aid to removal , temporary storage and re - application of the delaminated epithelial sheet . this embodiment provides particular utility in that the essential bandage contact lens , when used in conjunction with the removal , handling , storage and re - application of the epithelial sheet can significantly reduce the extent of handling or manipulation of the tissue . as would be particularly appreciated by one of skill in the art , any reduction in handling or manipulation of the delaminated epithelial layer will significantly reduce the chances the tissue will suffer damage that would limit , or even prevent , it from providing the advantages of reduction in discomfort and shorter time for visual recovery that can only be realized through reattachment of a viable epithelial layer . in one embodiment , the bandage contact lens of the invention could be affixed to a mechanical device or surgical tool adapted to draw a slight vacuum through which affixation of the lens to the tool is accomplished or aided . if the bandage contact lens is further adapted in a manner to enhance its porosity , then the vacuum drawn through tool can be applied across the lens so that , when the lens is applied to the loosened epithelial layer on the patient &# 39 ; s eye , the suction is sufficient to draw the layer off of the eye and reversibly affix it to the inner surface of the bandage contact lens . in this manner , the mechanical handling of the tissue layer is further reduced providing the benefit of reduced possibility of damage to the delaminated epithelial layer . these embodiments are provided to aid in illustration of the practice of the present invention only and in no manner are intended , or will serve to , limit in any way the scope of the present invention , which scope is defined in the claims that follow .
disclosed herein are methods and compositions for use in surgical procedures for refractive ablation of the cornea to achieve vision correction with a minimum of undesirable side effects and for a broad range of optical conditions such myopia , hyperopia , presbyopia and astigmatism . specifically disclosed are compositions , and methods involving their use , wherein the compositions act as agents for the reversible removal of corneal epithelial layers to provide access for uv radiation in manipulation of the refractive properties of the cornea . the methods and compositions of the present invention are capable of achieving desirable results in corrective surgery not possible with current methods for exposing the corneal stroma to far - uv laser radiation .