US 20050080485 A1
Prosthetic implants designed to be implanted in the cornea for modifying the cornea curvature and altering the corneal refractive power for correcting myopia, and myopia with astigmatism, such implants formed of a micro-porous hydrogel material.
1. A corneal implant, comprising:
a body formed of an optically clear, biocompatible material having an index of refraction ranging from 1.36 to 1.39, the body having anterior and posterior surfaces, the biocompatible material having micropores sized to act as a barrier against tissue in growth, the micropores adapted to permit nutrient and fluid transfer to prevent tissue necrosis.
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27. A corneal implant; comprising:
a body formed of an optically clear, biocompatible material having an index of refraction ranging from 1.36 to 1.39,
the body having anterior and posterior surfaces, the body having a center that is no greater than 50 micrometers thick,
the biocompatible material having micropores sized to act as a barrier against tissue in growth, the biocompatible material having a water content greater than 40% up to approximately 90%, and
the micropores adapted to permit nutrient and fluid transfer to prevent tissue necrosis.
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This application is a continuation of application Ser. No. 10/047,726, filed Jan. 15, 2002, which is a continuation of U.S. patent application Ser. No. 09/385,103, filed Aug. 27, 1999, now U.S. Pat. No. 6,361,560, which is a continuation-in-part of U.S. patent application Ser. No. 09/219,594, filed Dec. 23, 1998, now U.S. Pat. No. 6,102,946.
The field of this invention relates to prosthetic implants designed to be implanted in the cornea for modifying the cornea curvature and altering the corneal refractive power for correcting myopia, hyperopia, astigmatism, and presbyopia, and, in addition, to such implants formed of a micro-porous hydrogel material.
It is well known that anomalies in the shape of the eye can be the cause of visual disorders. Normal vision occurs when light that passes through and is refracted by the cornea, the lens, and other portions of the eye, and converges at or near the retina. Myopia or near-sightedness occurs when the light converges at a point before it reaches the retina and, conversely, hyperopia or far-sightedness occurs when the light converges a point beyond the retina. Other abnormal conditions include astigmatism where the outer surface of the cornea is irregular in shape and effects the ability of light to be refracted by the cornea. In addition, in patients who are older, a condition called presbyopia occurs in which there is a diminished power of accommodation of the natural lens resulting from the loss of elasticity of the lens, typically becoming significant after the age of 45.
Corrections for these conditions through the use of implants within the body of the cornea have been suggested. Various designs for such implants include solid and split-ring shaped, circular flexible body members and other types of ring-shaped devices that are adjustable. These implants are inserted within the body of the cornea for changing the shape of the cornea, thereby altering the its refractive power.
These types of prostheses typically are implanted by first making a tunnel and/or pocket within the cornea which leaves the Bowman's membrane intact and hence does not relieve the inherent natural tension of the membrane.
In the case of hyperopia, the corneal curvature must be steepened, and in the correction of myopia, it must be flattened. The correction of astigmatism can be done by flattening or steepening various portions of the cornea to correct the irregular shape of the outer surface. Bi-focal implants can be used to correct for presbyopia.
It has been recognized that desirable materials for these types of prostheses include various types of hydrogels. Hydrogels are considered desirable because they are hydrophilic in nature and have the ability to transmitting fluid through the material. It has been accepted that this transmission of fluid also operates to transmit nutrients from the distal surface of the implant to the proximal surface for providing proper nourishment to the tissue in the outer portion of the cornea.
However, while hydrogel lenses do operate to provide fluid transfer through the materials, it has been found that nutrient transfer is problematic because of the nature of fluid transfer from cell-to-cell within the material. Nutrients do not pass through the hydrogel material with the same level of efficacy as water. Without the proper transfer of nutrients, tissue in the outer portion of the cornea will die causing further deterioration in a patient's eyesight.
Thus, there is believed to be a demonstrated need for a material for corneal implants that will allow for the efficacious transmission of nutrients from the inner surface of a corneal implant to the outer surface, so that tissue in the outer portion of the cornea is properly nourished. There is also a need for a more effective corneal implant for solving the problems discussed above.
The present invention is directed to a corneal implant formed of a biocompatible, permeable, micro-porous hydrogel with a refractive index substantially similar to the refractive index of the cornea. The device, when placed under a lamellar dissection made in the cornea (such as a corneal flap), to relieve tension of Bowman's membrane, alters the outer surface of the cornea to correct the refractive error of the eye. By relieving the pressure and subsequent implantation of the device, the pressure points which typically are generated in present corneal surgeries are eliminated, and hence reduced risk to patients of extrusion of implants.
The implant is preferably generally circular in shape and is of a size greater than the size of the pupil in normal or bright light, and can specifically be used to correct hyperopia, myopia, astigmatism, and/or presbyopia. Due to the complete non-elastic nature of the corneal tissue, it is necessary to place the implant in the cornea with Bowman's membrane compromised, such as through a corneal lamellar dissection, to prevent extrusion of the implant from the cornea over the lifetime of the implant. Extrusion is undesirable because it tends to cause clinical complications and product failure.
Preferably, for the correction of hyperopia, the implant is formed into a meniscus-shaped disc with its anterior surface radius smaller (steeper) than the posterior surface radius, and with negligible edge thickness. This design results in a device-that has a thickness or dimension between the anterior and posterior surfaces along the central axis greater than at its periphery. When such an implant is placed under the corneal flap, the optical zone of the cornea is steepened and a positive optical power addition is achieved.
For the correction of myopia, the implant is shaped into a meniscus lens with an anterior surface curvature that is flatter than the posterior surface. When the implant is placed concentrically on the stromal bed the curvature of the anterior surface of the cornea in the optic zone is flattened to the extent appropriate to achieve the desired refractive correction.
For astigmatic eyes, implants are fabricated with a cylindrical addition along one of the axes. This device can be oval or elliptical in shape, with a longer axis either in the direction of cylindrical power addition or perpendicular to it. The implant preferably has a pair of markers such as, for example, protrusions, indentations or other types of visual indicators, in the direction of the cylindrical axis to easily mark and identify this direction. This indexing assists the surgeon in the proper placement of the implant under the flap with the correct orientation during surgery to correct astigmatism in any axis.
For simple or compound presbyopia, the implant is made by modifying the radius of curvature in the central 11.5-3 mm, thereby forming a multi-focal outer corneal surface where the central portion of the cornea achieves an added plus power for close-up work. The base of an implant designed for compound presbyopia can have a design to alter the cornea to achieve any desired correction for the myopic, hyperopic, or astigmatic eye.
The material from which any one or more of these implants are made is preferably a clear, permeably, microporous hydrogel with a water content greater than 40% up to approximately 90%. The refractive index should be substantially identical to the refractive index of corneal tissue. The permeability of the material is effected through a network of irregular passageways such as to permit adequate nutrient and fluid transfer to prevent tissue necrosis, but which are small enough to act as a barrier against the tissue ingrowth from one side of the implant to another. This helps the transmembrane tissue viability while continuing to make the implant removable and exchangeable.
The refractive index of the implant material should be in the range of 1.36-1.39, which is substantially similar to that of the cornea (1.376). This substantially similar refractive index prevents optical aberrations due to edge effects at the cornea-implant interface.
The microporous hydrogel material can be formed from at least one (and preferably more) hydrophilic monomer, which is polymerized and cross-linked with at least one multi- or di-olefinic cross-linking agent.
The implants described above can be placed in the cornea by making a substantially circular lamellar flap using any commercially available microkeratome. When the flap is formed, a hinge is preferably left to facilitate proper alignment of the dissected corneal tissue after the implant is placed on the exposed cornea.
The implants described above which can be used for correcting hyperopia or hyperopia with astigmatism are preferably made into a disc shape that is nominally about 4.5 mm in diameter and bi-meniscus in shape. The center of the lens is preferably no greater than 50 micrometers thick. The edge thickness should be less than two keratocytes (i.e., about 15 micrometers).
An improvement over the lenses described above for correcting myopia with astigmatism includes forming a lens in the shape of a ring with one or more portions in the center being solid and defining voids in the center section for shaping the astigmatic component by providing solid portions under the flatter meridian of the astigmatic myopic eye. An example of such a shape includes a ring with a rib extending across the center that is either squared off or rounded where it contacts the ring. Another example is a ring with one or more quadrants filled in, with the other ones forming voids. Other shapes can used to provide a solid portion under the flatter meridan.
A better understanding of the invention can be obtained from the detailed description of exemplary embodiments set forth below, when considered in conjunction with the appended drawings, in which:
Referring first to
The outer-most covering is a fibrous protective portion that includes a posterior layer which is white and opaque, called the sclera 16, which is sometimes referred to as the white of the eye where it is visible from the front. The anterior ⅙th of this outer layer is the transparent cornea 12.
A middle covering is mainly vascular and nutritive in function and is made up of the choroid 18, the ciliary 20 and the iris 22. The choroid generally functions to maintain the retina. The ciliary muscle 21 is involved in suspending the lens 24 and accommodating the lens. The iris 22 is the most anterior portion of the middle covering of the eye and is arranged in a frontal plane. The iris is a thin circular disc corresponding to the diaphragm of a camera, and is perforated near its center by a circular aperture called the pupil 26. The size of the pupil varies to regulate the amount of light that reaches the retina 14. It contracts also to accommodate, which serves to sharpen the focus by diminishing spherical aberrations. The iris 22 divides the space between the cornea 12 and the lens. 24 into an anterior chamber 28 and posterior chamber 30.
The inner-most covering is the retina 14, consisting of nerve elements which form the true receptive portion for visual impressions that are transmitted to the brain. The vitreous 32 is a transparent gelatinous mass which fills the posterior ⅘ths the globe 10. The vitreous supports the ciliary body 20 and the retina 14.
If the outer surface of the cornea 12 is caused to steepen, as shown by dotted lines 40, such as through the implantation of a corneal implant of an appropriate shape as discussed below, the rays of light 36 are refracted from the steeper surface at a greater angle as shown by dotted lines 42, causing the light to focus at a shorter distance, such as directly on the retina 14.
A hyperopic eye of the type shown in
The lens 50 shown in
The anterior surface has a greater radius than the posterior surface. The lens 50 preferably has a nominal diameter of about 4.5 mm. The center of the lens is preferably no greater than 50 micrometers thick to enhance the diffusion characteristics of the material from which the lens is formed, which allows for more effective transmission of nutrients through the lens material and promotes better health of the anterior corneal tissue. The outer edge of the lens 50 has a thickness that is less than the dimensions of two keratocytes (i.e., about 15 micrometers) juxtaposed side-by-side, which are the fixed flattened connective tissue cells between the lamellae of the cornea. An edge thickness as specified prevents stacking and recruitment of keratocytes in the lens material so that keratocyte stacking and recruitment does not take place. This in turn eliminates unorganized collagen that forms undesirable scar tissue and infiltrates the lens, which tends to compromise the efficacy of the lens.
On the other hand, in order to cure myopia, an implant 56 having the shape shown in
Alternatively, instead of using a solid implant as shown in
Implants of the type shown in
As is also known in the art, the flap is cut deeply enough to dissect the Bowman's membrane portion of the cornea, such as in keratome surgery or for subsequent removal of the tissue by laser or surgical removal. A corneal flap of 100 to 200 microns, typically 160 to 180 microns, will be made to eliminate the Bowman's membrane tension. This reduces the possibility of extrusion of the implants due to pressure generated within the cornea caused by the addition of the implant. Implants of the type shown in
Implants can also be formed with a cylindrical addition in one axis of the lens in order to correct for astigmatism, as shown in the implants in
Alternatively, as shown in
Because implants of the type identified by reference numeral 72, 74 are relatively small and transparent, it is difficult for the surgeon to maintain proper orientation along the x and y axes. In order to assist the surgeon, tabs 76 a, 76 b or indentations 78 a, 78 b are used to identify one or the other of the axis of the implant to maintain proper alignment during implantation. This is shown in
The central power add portions 82, 84, and 88 are preferably within the range of 1.5-3 mm in diameter, most preferably 2mm, and which provide a multi-focal outer corneal surface where the central portion of the cornea achieves an added plus power for close-up work. In addition to the based device having no correction, or corrections for hyperopia or myopia, the base device can have a simple spherical correction for astigmatism as shown in
As shown in
Implantation of the device shown in
Another example of a design for correcting myopia with astigmatism is a lens 170 as shown in
The implants described above are preferably formed of a microporous hydrogel material in order to provide for the efficacious transmission of nutrients from the inner to the outer surface of the implants. The hydrogels also preferably have micropores in the form of irregular passageways, which are small enough to screen against tissue ingrowth, but large enough to allow for nutrients to be transmitted. These microporous hydrogels are different from non-microporous hydrogels because they allow fluid containing nutrients to be transmitted between the cells that make up the material, not from cell-to-cell such as in normal hydrogel materials. Hydrogels of this type can be formed from at least one, and preferably more, hydrophillic monomer which is polymerized and cross-linked with at least one multi-or di-olefinic cross-linking agent.
An important aspect of the materials of the present invention is that the microporous hydrogel have micropores in the hydrogel. Such micropores should in general have a diameter ranging from 50 Angstroms to 10 microns, more particularly ranging from 50 Angstroms to 1 micron. A microporous hydrogel in accordance with the present invention can be made from any of the following methods.
Hydrogels can be synthesized as a zero gel by ultraviolet or thermal curing of hydrophillic monomers and low levels of cross-linking agents such as diacrylates and other UV or thermal initiators. These lightly cross-linked hydrogels are then machined into appropriate physical dimensions and hydrated in water at elevated temperatures. Upon complete hydration, hydrogel prosthesis are flash-frozen to temperatures below negative 40° c., and then gradually warmed to a temperature of negative 20° c. to negative 10° c. and maintained at the same temperature for some time, typically 12 to 48 hours, in order to grow ice crystals to larger dimensions to generate the porous structure via expanding ice crystals. The frozen and annealed hydrogel is then quickly thawed to yield the microporous hydrogel device. Alternatively, the hydrated hydrogel device can be lyophilized and rehydrated to yield a microporous hydrogel.
Still further, the microporous hydrogel can also be made by starting with-a known formulation of monomers which can yield a desired cross-linked hydrogel, dissolving in said monomer mixture a low molecular weight polymer as a filler which is soluble in said mixture and then polymerizing the mixture. Resulted polymer is converted into the required device shape and then extracted with an appropriate solvent to extract out the filled polymer and the result in a matrix hydrated to yield a microporous device.
Still further and alternatively, microporous hydrogels can also be made by any of the above methods with the modification of adding an adequate amount of solvent or water to give a pre-swollen finished hydrogel, which can then be purified by extraction. Such formulation can be directly cast molded in a desired configuration and do not require subsequent machining processes for converting.