Abstract:
A partially hydrated lens is provided for implantation into a cornea. The partially hydrated lens has a water content less than the water content of the cornea, resulting in an osmotic pressure differential between the lens and the cornea which improves adhesion between the lens and the corneal tissue. It also causes the lens to expand and flatten when implanted, and to naturally center itself while flattening. In the preferred embodiment of the invention, the lens is maintained in its partially hydrated state by storage in a hypertonic hydration medium.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to methods of improving the adherence and/or centering of intra-corneal implants to the corneal bed. More particularly, the invention relates to methods for pre-treating intra-corneal implants so that the implants, after being placed in contact with the cornea, effectively self-center and adhere to the corneal bed, for example, without sutures.  
           [0002]    Various treatments are known for correcting corneal refractive errors. The use of lasers, for instance, to reshape the cornea by removing corneal tissue, has become increasingly popular in recent years. However, the removal of tissue can result in loss of the structural integrity of the cornea, and can also cause bulging. Furthermore, once corneal tissue has been removed, it can not easily be restored. Thus, laser vision correction is substantially irreversible.  
           [0003]    The need for a reversible treatment which does not adversely affect the structural integrity of the cornea has led to the use of intra-corneal implants, which do not require the removal of tissue. Instead, a single small incision is made in the cornea to make a flap or hinge, which is then folded back to expose the middle layer of corneal tissue known as the stromal bed. A corrective lens, typically formed of hydrogel material, is placed on the stromal layer Then the flap is returned to its initial position and smoothed over the lens.  
           [0004]    Various techniques have been used for affixing the lens to the cornea once it has been embedded. In some cases, sutures are used. Other techniques eliminate sutures, but rely on a very close match between the curvature of the lens and the curvature of the recipient&#39;s eye. None of these techniques have been entirely satisfactory, resulting in some slippage between the lens and the cornea, and improper positioning in the eye.  
           [0005]    Therefore, it would be advantageous to develop methods and apparatus for enhancing the adherence and/or centering of intra-corneal implants on the corneal bed.  
         SUMMARY OF THE INVENTION  
         [0006]    New apparatus and methods for treating intra-corneal implants to enhance centering and adherence have been discovered. The present invention addresses one or more of the concerns of the prior art systems, such as those noted above. Moreover, the present methods are straightforward, relatively easy to produce, use and practice, and provide substantial benefits to both the surgeon implanting the intra-corneal implants, and the patient receiving the implants.  
           [0007]    In one broad aspect of the invention, a partially hydrated lens is provided for implantation into the stroma of a cornea. The partially hydrated lens comprises an amount of fluid selected to create an osmotic pressure gradient with respect to fluids in the cornea. This osmotic pressure gradient enhances adhesion of the lens to the cornea. In addition, the lens is configured to expand and flatten as it absorbs water from the cornea, naturally centering itself as it flattens.  
           [0008]    In another broad aspect of the invention, the partially hydrated lens is packaged in a hypertonic hydrating solution.  
           [0009]    In still another broad aspect of the invention, a method of treating refractive errors in a cornea comprises implanting a partially hydrated lens formed of a hydrophilic polymeric material lens into the cornea, wherein the osmotic pressure differential between the partially hydrated implant and the cornea causes the implant to adhere to the cornea. In one embodiment, the step of implanting the partially hydrated lens is preceded by a step of wetting a pre-hydrated lens with a hypertonic aqueous hydration medium. In another embodiment, the step of implanting the partially hydrated lens is preceded by a step of drying a more hydrated lens.  
           [0010]    Each and every feature described herein, and each and every combination of two or more of such features, is included within the scope of the present invention provided that the features included in such a combination are not mutually inconsistent.  
           [0011]    These and other broad aspects and advantages are set forth in the following detailed description and claims, particularly when considered in conjunction with the accompanying drawings in which like parts bear like reference numerals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a front view of a lens according to the present invention in its equilibrium state;  
         [0013]    [0013]FIG. 2 is a transverse sectional view taken through line  22  of FIG. 1.  
         [0014]    [0014]FIG. 3 is a transverse sectional view, similar to FIG. 2, showing the lens according to the present invention in a partially hydrated state.  
         [0015]    [0015]FIG. 4 is an enlarged axial, cross-sectional view showing the partially hydrated lens of FIG. 3 immediately after implantation into the stroma of a cornea.  
         [0016]    [0016]FIG. 5 is an enlarged axial, cross-sectional view, similar to FIG. 4, showing the implanted lens in its equilibrium state.  
         [0017]    [0017]FIG. 6 is a perspective view showing a packaging arrangement including the lens of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Referring now to the drawings, FIGS. 1 and 2 show an exemplary lens or intra-corneal implant  10  structured to be implanted in a cornea, preferably in the stroma of the cornea. For the purposes of this disclosure, the terms “lens” and “implant” are used substantially interchangeably, with “lens” generally being used to refer to the lens  10  in its pre-implanted state, and “implant” being used to refer to the lens  10  after implantation. Also for the purposes of this disclosure, the term “cornea” is used somewhat interchangeably with “stroma”, and with the implicit understanding that, while the stroma is the presently preferred location for an intra-corneal implant, other layers of the cornea may also be suitable.  
         [0019]    The lens  10  is illustrated as circular in plan, with a diameter DF and radius of curvature RF. The lens  10  includes a convex anterior surface  12  approaching the optical axis  13 , and a concave posterior surface  14 . However, the principles of the invention may also be applied to lenses of other shapes and curvature.  
         [0020]    The lens  10  may be made of any material, preferably a hydrophilic material, that swells when wetted with an aqueous liquid medium, has a suitable index of refraction, and is known to be compatible with corneal tissue. In a preferred embodiment, the lens  10  comprises a composition including a hydrophilic polymeric material, preferably a hydrogel-forming polymeric material, and water.  
         [0021]    Specific examples of useful hydrophilic polymeric materials include polymers derived in whole or in part from monomers which possess an unsaturated vinyl or allyl group and produce polymers which exhibit hydrophilicity. Such monomers include, but are not limited to, acryl type monomers, methacryl type monomers, unsaturated amide type monomers, diene type monomers, and triene type monomers which meet the requirements mentioned above. Typical examples of such monomers include (meth)acrylamides, N-methyl(meth)acrylamides, N,N-dimethyl(meth)acrylamides, N,N-methylethyl (meth)acrylamides, N,Ndiethyl(meth)acrylamides, (meth)acrylicacids, 2-hydroxyethyl(meth)acrylates, N,N-dimethylaminoethyl(meth) acrylates, N,N-diethyl-amino-ethyl(meth)acrylates, N-vinylpyrrolidone, p-styrene sulfonic acid, vinyl sulfonic acid, 2-methyacryloyloxethyl acid, 2-acrylamide-2-methylpropane sulfonic acid, and the like, and mixtures thereof.  
         [0022]    In a prior art procedure, a pre-hydrated lens is wetted to a final hydrated state prior to implantation in the stroma of the cornea. This prior art wetting procedure typically consists of dipping or soaking and storing the lens in an isotonic hydration medium, which is understood in the art to mean a solution having an osmotic pressure approximately equal to fluids in the cornea. In the final hydrated state, also defined here as the equilibrium state, there is substantially no osmotic pressure differential between the fluids in the lens  10  and the fluids in the stroma into which the lens is to be implanted.  
         [0023]    In the method according to the present invention, the lens  10  is only partially hydrated before implantation in the cornea. The degree to which the pre-hydrated lens should be hydrated prior to implantation can be determined in a variety of ways. One way is to consider the curvature of the lens. In its final hydrated state, the lens should have a curvature substantially conforming to the uniformly curved central area of a patient&#39;s stroma. Thus, the lens in the partially hydrated state should be more steeply curved than the stromal bed, yet not so steeply curved as to cause excessive discomfort in the patient when it is first implanted.  
         [0024]    Another way of determining the extent of hydration is to consider imbibition pressure, which is defined as the tendency of the lens to absorb water. The partially hydrated lens should have an imbibition pressure higher than the osmotic pressure of stromal fluids, while the lens in its final, or equilibrium, state should have an imbibition pressure approximately equal to the osmotic pressure of stromal fluids. Thus, the lens  10  substantially ceases to absorb fluid from a cornea having normal fluid levels when the lens  10  reaches its equilibrium state, thereby reducing the possibility of dry eye in a normal patient.  
         [0025]    The hydration level can also be expressed in terms of the colloidal osmotic pressure of fluids in the lens material. When the lens  10  is in its partially hydrated state, the colloidal osmotic pressure of fluids in the lens should be less than the osmotic pressure of fluids in the stroma, so that an osmotic pressure gradient is established, which enhances adhesion of the implant to the stroma. In the equilibrium state, the colloidal osmotic pressure of fluids in the lens should be approximately equal to the osmotic pressure of the stromal fluids.  
         [0026]    In one embodiment of the invention, the lens  10  is maintained in the partially hydrated state by soaking and storage in a hypertonic hydration medium  18 , which is understood in the art to mean a solution having an osmotic pressure greater than fluids in the cornea. For the sake of convenience, the partially hydrated lens  10  is preferably supplied to the ophthalmic surgeon pre-packaged with the hypertonic hydration medium  18 . This relieves the surgeon and/or technicians of the need to hydrate the lens before surgery, and to determine whether the proper degree of hydration has been achieved. The packaging arrangement may comprise any suitable sealed vessel, such as a vial  16 , containing the partially hydrated lens suspended in the hydration medium  18 , as shown in FIG. 6.  
         [0027]    The hydration medium may be selected from any suitable material effective to maintain the lens  10  in the desired partially hydrated state. Advantageously, the hydration medium is an aqueous-based liquid. Preferably, the hydration medium comprises an ophthalmically acceptable acceptable aqueous carrier, such as sterile purified water, to which a tonicity adjusting agent has been added. Suitable such agents include alkali metal halides, phosphates, hydrogen phosphate, and borates. Preferred are sodium chloride, potassium chloride, sodium phosphate monobasic and sodium phosphate dibasic and combinations thereof. The amount of the tonicity adjusting agent needed to achieve the desired hydration of the lens  10  can vary greatly depending on such properties as the permeability and the ratio of the water diffusion coefficient to the ion diffusion coefficient of the lens material. It can also vary based on the presence of other components in the carrier, including stabilizers, buffering agents, disinfectants, pH adjusters, and the like, all of which would be determined by the specific type of lens and the needs of the individual patient. Accordingly, there is no upper or lower critical limitation upon the amount of the tonicity adjusting agent. The required quantity to be employed in the present invention can be determined clinically by those skilled in the art.  
         [0028]    In the presently preferred embodiment, the hydration medium is an aqueous solution having an osmotic pressure which is higher than the osmotic pressure of fluids in the cornea. Although the hydration medium is preferably hypertonic, the tonicity of the hydration medium should not be so high as to cause dry eye or other detrimental effects when placed in a cornea having normal fluid levels. Again, the maximum desirable tonicity will vary depending on numerous factors, but can be readily ascertained by a skilled practitioner of the ophthalmic arts.  
         [0029]    [0029]FIG. 3 shows the lens  10  in its partially hydrated state, just prior to implantation in the cornea. The partially hydrated lens  10  has a generally spherical surface, but has a smaller diameter d P  and a smaller radius of curvature R P  than the lens  10  in its final state. For purposes of illustration, the steepness of the curvature of the lens  10  has been exaggerated. In actuality, the difference between the radii of curvature R F  and R P , as well as the difference between the lens diameters d F  and d P , would be almost imperceptible.  
         [0030]    [0030]FIG. 4 shows the partially hydrated lens  10 , immediately after having been implanted into the stromal bed  20  of a cornea  22  using prior art surgical techniques. Specifically, a small incision was made in the cornea  22  to make a flap or hinge (not shown), which was then folded back to expose the middle layer of corneal tissue known as the stromal bed  20 . The lens  10  was then placed on the stroma  20  and the flap returned to its initial position and smoothed over the lens  10 .  
         [0031]    Initially, the partially hydrated lens  10  is more steeply curved than the cornea. In addition, the lens  10  may be slightly off-center with respect to the cornea  22 . However, the difference in hydration between the lens  10  and the cornea creates an osmotic pressure gradient, causing the lens  10  to absorb water from the cornea  22  until a state of equilibrium is reached. As the lens  10  gradually absorbs water, it expands and flattens until it adheres to the stromal bed, centering itself as it flattens. After it adheres and centers, the lens  10  becomes fully hydrated and assumes a configuration substantially conforming to the curvature of the cornea  22 , as shown in FIG. 5.  
         [0032]    While this invention has been described with respect to various specific examples and embodiments, it is to be understood that the invention is not limited thereto and that it can be variously practiced within the scope of the following claims. For example, the partially hydrated state of the lens  10  could be achieved by fully hydrating the lens with a conventional isotonic solution, and then either air-drying the lens  10  for a prescribed time period, or treating the lens  10  with a dehydrating agent. In some instances, such methods may be preferred, since a fully hydrated lens is more easily inspected for imperfections than a partially hydrated one.  
         [0033]    In addition, while the intra-corneal implants and methods disclosed herein are believed to be particularly effective for treating hyperopia, the teachings of the foregoing disclosure could well be adapted to lenses for correcting myopia, astigmatism and other refractive errors, without departing from the spirit of the invention.