Contact Lens Having Improved Dimensional Stability

The present invention relates to a contact lens having improved dimensional stability and, more particularly, relates to a spheric contact lens, a toric contact lens, and a multifocal contact lens constructed from a copolymer of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate.

The prior art describes many polymers, based on acrylates or methacrylates, for use in contact lenses. For instance, U.S. Pat. No. 4,056,496 to Mancini et al. discloses a hydrogel formed by bulk polymerization of a dihydroxyalkyl acrylate or methacrylate, such as GMA; an alkyl acrylate or methacrylate; and a minor amount of an epoxidized alkyl acrylate or methacrylate. Additionally, U.S. Pat. No. 3,985,697 to Urbach teaches terpolymer hydrogels formed from hydroxyalkyl acrylate or hydroxyalkyl methacrylate, a non-water-soluble acrylate or methacrylate diester as a cross-linking agent, and an alkenoic carboxylic acid, such as acrylic or methacrylic acid. U.S. Pat. No. 3,947,401 to Stamberger discloses a bulk-polymerized water-insoluble, but water-swellable, copolymer formed from a polymerizable monoester of acrylic or methacrylic acid, such as HEMA, and glycidyl methacrylate. Macret et al. in Polymer, 23(5), 748-753 (1982) discloses hydrogels prepared by radical polymerization of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate, but this document neither discloses nor suggests a contact lens having improved dimensional stability or superior water balance.

Conventional non-ionic hydrogels constructed from methyl methacrylate (MMA) copolymers derive their strength from the methacrylate polymer backbone, but depend upon the pendant hydrophilic groups of the comonomers for water content. An exemplary hydrophilic comonomer is N-vinylpyrrolidone (NVP). The structure and amount of these hydrophilic components are limited by their compatibility with the hydrophobic MMA.

HEMA-based hydrogels have a hydrophilic core that permits a water content of 38%. Higher water contents are achieved by inclusion of either methacrylic acid (MAA) comonomer in ionic hydrogels, or hydrophilic comonomers in non-ionic systems. NVP has been a key monomer in attaining water contents up to 70%, but use of this comonomer results in temperature sensitivity during manufacturing. Moreover, progressive yellowing with age and changes in optical parameters as a result of temperature-dependent dimensional changes have also been observed with lenses constructed from these compositions.

The ability of a hydrogel lens to maintain its water-saturated state is essential for maximum lens stability. All hydrogel lenses dehydrate, for water evaporates continuously from the surface of a hydrogel lens. Dehydration of a contact lens results in a change in the dimensions of the lens, hence dehydration has a direct effect upon dimensional stability. Conventional contact lenses undergo a significant degree of dehydration during use and, accordingly, have a significant degree of dimensional instability, particularly at higher water contents.

Further, rehydration is important to the dimensional stability of a contact lens. If a lens material can be constructed which absorbs water more rapidly, then the lens will more closely return to a water-saturated state during each blink, when the lens is bathed in tear fluid. Therefore, as a lens begins to dehydrate, a characteristic of rapid rehydration is extremely advantageous for maintaining saturation and maximum stability. Unfortunately, conventional contact lens development either has ignored the effect of rehydration rate upon lenses or has constructed lenses of materials with a less than optimal rate of rehydration.

As such, there remains a need for a contact lens possessing superior dimensional stability and having a low rate of dehydration coupled with a high rate of rehydration.

Accordingly, an object of the present invention is to provide a novel contact lens having improved dimensional stability.

Another object of the present invention is to provide a spheric contact lens, a toric contact lens, and a multifocal contact lens, each having superior dimensional stability.

Another object of the present invention is to provide a contact lens with a low rate of dehydration coupled with a high rate of rehydration, relative to lenses currently available.

Still another object of the present invention is to provide a contact lens with a superior water balance, or ratio of dehydration to rehydration.

These objects, among others, have been accomplished by means of a contact lens constructed from a copolymer of 2,3-dihydroxypropyl methacrylate (glyceryl methacrylate hereinafter referred to as "GMA") with 2-hydroxyethyl methacrylate ("2-HEMA").

In contrast to recent developments in rigid gas permeable contact lenses, no significant improvement has lately been achieved in hydrophilic contact lenses. All hydrophilic lenses introduced in recent years have been based on either existing materials employing new production technology or slight modifications of known compositions. This lack of progress in the soft lens field has resulted in a large variety of lens designs, but a narrow choice of lens materials and a narrow range of lens stabilities as measured by water balance ratio.

Limited choice in lens material is problematic when attempting to fit contact lenses on patients subject to a wide variety of physiological and environmental conditions. For example, an array of factors affect contact lens comfort and stability, such as, tear quantity, ambient humidity, prolonged open eye periods, and air flow around the eye. Especially difficult cases are posed by patients with dry eyes.

The dehydration of hydrophilic lenses is a major problem, affecting lens movement, lens power, oxygen permeability and comfort. Various factors including patient physiology, environment, lens design, and lens material significantly influence the rate of dehydration, as described in Andrasko, Hydration Levels and Oxygen Transmissivities of Ophthalmic Polymer In Situ, Thesis, Ohio State University, 1980, and McCarey et al. pH, Osmolarity and Temperature Effects on the Water Content of Hydrogel Contact Lenses, Contact and Intraocular Lens Medical Journal 8, 158-167, 1982. Thicker lenses also appear to dehydrate less than thinner lenses, as described in Businger et al., Die Beeirflussung der Dehydratation yon hydrophilen Kontaktlinsen durch verschiedene Linsenparameter, Deutsche Optiker Zeitung 40, 99-102 (1985).

While a variety of hydrophilic lens materials are available, they differ only slightly in their rates of dehydration, as described in Helton et al., Hydrogel Contact Lens Dehydration on Rates Determined by Thermogravimetric Analysis CLAO 17, 59-61 (1991). These factors are particularly pronounced during the cold season or in dry environments, see Andrasko et al., The Effect of Humidity on the Dehydration of Soft Contact Lenses on the Eye, Int. Cl. Clinic 7, 30 (1982) and Eng et al., The Wearing of Hydrophilic Contact Lenses Aboard a Commercial Jet Aircraft: 1 Humidity Effects on Fit, Aviat. Space Environ. Med. 53, 235 (1982).

Conventional materials containing glyceryl methacrylate have been reported to have improved internal water retention over poly-HEMA, see Pescosolido, et al., Nuclear Magnetic Resonance Study of Dehydration in Glyceryl-methyl-methacrylate Contact Lens, Contactologia 15D, 64 (1993). However, this polymeric material contained MMA as a hydrophobic component, which precludes increasing the water content beyond 38%.

The ability of a hydrogel lens to maintain its saturated state is essential for lens stability. All hydrogel lenses dehydrate. Water evaporates from the surface of a hydrogel lens continuously. The amount of water loss that a lens will experience depends upon the dehydration/rehydration behavior of the particular lens material, the quantity of tears deposited on the lens with each blink, the ambient humidity, temperature and air flow around the eye.

Superior dehydration/rehydration behavior of soft lens materials provides the material with increased dimensional stability. If a soft lens material can be made to dehydrate (allow evaporation) more slowly, then the lens will remain closer to its saturated state. Equally important, but not recognized in the prior art, is the importance of rehydration. If a lens material can be made to re-absorb water more rapidly, then the lens can return to a state closer to saturation during each blink, when the lens is bathed in tear fluid. Thus, an "ideal" soft contact lens is one constructed from a composition that is both slow to dehydrate and quick to rehydrate.

The present inventors undertook a program of research to develop a material composition for soft contact lenses having enhanced dimensional stability and a superior water balance ratio (a low rate of dehydration coupled with a high rate of rehydration). They discovered that copolymers of 2,3-dihydroxypropyl methacrylate with 2-hydroxyethyl methacrylate possess a low rate of dehydration coupled with a high rate of rehydration and, accordingly, would furnish contact lenses having enhanced dimensional stability.

Moreover, with the introduction of this material composition, the water content can be increased from 40% up to 70% by weight, depending on the ratio of the comonomers.

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a graph illustrating the lens rehydration behavior of various commercially available hydrophilic lenses in vitro tests conducted at 35.degree. C.;

FIG. 2 is a graph depicting the lens rehydration behavior of various commercial lenses of 0.1 mm. wet thickness dehydrated by 10% of their original saturation weight;

FIG. 3 is a graph illustrating a comparison of the water balance ratio with the nominal water content of various polymers, including those of the present invention;

FIG. 4 is a chart depicting a comparison of the water balance ratio of various polymers, including those of the present invention;

FIG. 5 is a chart illustrating the staining behavior of lenses of the present invention with conventional lenses;

FIG. 6 is a chart depicting the subjective rating of lenses of the present invention with conventional lenses; and

FIG. 7 is a chart illustrating the lens preference of subjects, when choosing between lenses of the present invention and conventional lenses.

Other features of the invention will become apparent in the course of the following descriptions of exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof.

The copolymer is made by any known method of polymerizing GMA with a 2-HEMA comonomer. Suitable polymerization methods are the free radical bulk polymerization methods taught by U.S. Pat. No. 4,056,496 to Mancini et al., U.S. Pat. No. 3,985,697 to Urbach, and U.S. Pat. No. 3,947,401 to Stamberger, all of which are incorporated here by reference. The proportions of the reactants and the reaction conditions can be varied so as to optimize conditions. Suitable catalysts are also taught by the cited U.S. patents incorporated by reference. Known cross-linking agents, as taught in U.S. Pat. No. 4,038,264 to Rostoker et al., which is incorporated herein by reference in its entirety, are used in proportions ascertainable by one skilled in the art.

It is important to the present invention that the GMA be pure and free of impurities such as glycidyl methacrylate and dimethacrylate. Thus, while the GMA is synthesized by the known method as taught by Mancini, it is further purified by vacuum distillation.

As referred to hereinafter, the term "purified by distillation" refers to this process in which GMA is purified by vacuum distillation, by which process impurities such as glycidyl methacrylate and dimethacrylate are removed.

2-HEMA is commercially available and is preferably purified by vacuum distillation as taught by Urbach. The invention contemplates copolymers of GMA and 2-HEMA.

In a preferred embodiment of the present invention, GMA constitutes between 20 mole % and 90 mole % of the copolymer and in a still more preferred embodiment of the present invention, GMA constitutes between 50 mole % and 90 mole % of the copolymer.

The polymers of the present invention are copolymers of GMA which have been purified by vacuum distillation. The copolymers contain, as comonomers, 2-HEMA and GMA.

A hydrogel is formed from the polymers in a known manner, whether by polymerizing the monomers in bulk, in an aqueous solvent, or in an organic solvent and hydrating the polymer after polymerization. The composition has a water content ranging from about 40% to about 70%. Table I shows the water content of some of the polymers according to the present invention having various proportions of the monomers shown (any water content from 40% to 70% is readily formulated).

Glyceryl methacrylate (GMA) can be prepared according to the methods presented in U.S. Pat. No. 4,056,496 to Mancini et al., which is incorporated herein by reference. The resultant GMA can be purified using standard methods, such as, thin film evaporation, distillation, and chromatography. For the present invention, the preferred method of purification of GMA was distillation so as to achieve the purity criteria listed in Table II.

857.3 grams of GMA were mixed with 142.7 grams of 2-HEMA and 0.6 gram of 2,2-azobis(2,4-dimethylvaleronitrile) were added. The total diester concentration was adjusted to 0.6% by weight with ethylene glycol dimethacrylate. The mixture was degassed by applying vacuum with vigorous stirring. The mixture was dispensed into cylindrical molds, polymerized at 35.degree. C. for 5 hours, and post-cured at 110.degree. C. for 5 hours. The polymer was removed from the molds and cut into cylinders approximately 0.5 inch (approximately 1.27 cm.) in diameter and approximately 0.2 inch (approximately 0.508 cm.) in height suitable for contact lenses. The blanks were then cured at 110.degree. C. for 5 hours. After curing, the blanks were ground and lapped to right cylinders of 0.5 inch in diameter and 0.2 inch in height.

551.7 grams of GMA were mixed with 448.3 grams of 2-HEMA and 0.6 gram of 2,2-azobis(2,4-dimethylvaleronitrile) was added. The total diester concentration was adjusted to 0.6% by weight with ethylene glycol dimethacrylate. The mixture was degassed by applying vacuum with vigorous stirring. The mixture was dispensed into cylindrical molds, polymerized at 35.degree. C. for 5 hours, and post-cured at 110.degree. C. for 5 hours. The polymer is removed from the molds and cut into cylinders approximately 0.5 inch (approximately 1.27 cm.) in diameter and approximately 0.2 inch (approximately 0.508 cm.) in height suitable for contact lenses. The blanks were then cured at 110.degree. C. for 5 hours. After curing, the blanks were ground and lapped to right cylinders of 0.5 inch in diameter and 0.2 inch in height.

246.5 grams of GMA were mixed with 753.5 grams of 2-HEMA and 0.6 gram of 2,2-azobis(2,4-dimethylvaleronitrile) was added. The total diester concentration was adjusted to 0.6% by weight with ethylene glycol dimethacrylate. The mixture was degassed by applying vacuum with vigorous stirring. The mixture was dispensed into cylindrical molds, polymerized at 35.degree. C. for 5 hours, and post-cured at 110.degree. C. for 5 hours. The polymer was removed from the molds and cut into cylinders approximately 0.5 inch (approximately 1.27 cm.) in diameter and approximately 0.2 inch (approximately 0.508 cm.) in height suitable for contact lenses. The blanks were then cured at 110.degree. C. for 5 hours. After curing, the blanks were ground and lapped to right cylinders of 0.5 inch in diameter and 25 0.2 inch in height.

A study was conducted to determine the dehydration and rehydration behavior of lenses of similar thicknesses constructed from a variety of materials. Table III lists various polymer compositions and their nominal water content. In this table, the prefix "p" designates a polymer constructed from the monomer or comonomers indicated.

Dehydration experiments were carried out in a thermogravimetric analysis instrument which measures small changes in weight. All samples were run under identical conditions. p-HEMA was run as a standard with every sample set. The results are graphically presented in FIG. 1, which compares a GMA/2-HEMA-II contact lens of the present invention with conventional materials of the same thickness.

The rate of dehydration is indicated for the first 10% of weight loss from saturation, because this initial loss is of the greatest physiological importance. Isothermal conditions at 35.degree. C. were chosen to approximate physiological milieu. The slower dehydration rate in the GMA/2-HEMA copolymer is apparent, even compared with polymeric compositions of higher water content.

The rate of rehydration of a contact lens after partial dehydration is of particular importance, because the time that a lens has to rehydrate over its entire surface is that of a blink or possibly a few blinks in succession. The lens has only a few seconds, at most, in which to bathe in tear fluid. A lens composition material with rapid rehydration behavior would be advantageous for maintaining a state of saturation, or near saturation, through the wearing cycle of the lens, and would, therefore, maintain its saturated dimensions.

FIG. 2 graphically illustrates the rehydration behavior of lenses of the same thickness (0.1 mm wet thickness) of various compositions. The data were obtained using gravimetric analysis to measure weight gain after immersion in borate buffer saline solution. Again, the graph shows rehydration of lenses that had been dried to 90% of their saturated weight. Note that GMA/2-HEMA lenses have the fastest rehydration rate and the shortest time-to-saturation of the lenses examined.

Copolymers of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate show the fastest rehydration behavior and the shortest time-to-saturation of any soft lens material. This rapid rehydration behavior, when combined with their slow dehydration rates, allows these materials to maintain a hydrated state much closer to saturation during the entire wearing cycle relative to conventional contact lens compositions. This "hydro-equilibrium" is termed "water balance".

"Water balance" is an inherent property dependent on the material's ability to bind water and can be examined by the rates of dehydration and rehydration. The ratio of these two parameters serves as a guide to lens stability and comfort. FIG. 3 compares the in vitro performance of various commercial materials. The data shown are based on the ratio of the time it takes a lens (0.1 mm hydrated thickness) to lose 10% of its weight as water from its saturated state to the time it takes to re-saturate. The graph shows the water balance ratio plotted against nominal water content for each material.

With this integration of glyceryl methacrylate and 2-HEMA, new copolymers with a water content in the range from 40 to 70% can be synthesized. Lenses from the new material showed good strength, fine handling and excellent water retention properties.

This copolymer of GMA/2-HEMA material remains, even in vivo, closer to the water-saturation point than any other material currently available. This result is provided in FIG. 4.

A clinical study was established to determine if the laboratory findings result in enhanced performance with patients and if patients with marginal dry eyes would benefit from lenses constructed from GMA/2-HEMA copolymer.

Thirty patients, who were previously diagnosed as having marginal dry eyes were fitted with lenses made out of GMA/HEMA. The criteria for marginal dry eyes was set either by a non-invasive breakup time between 10 to 20 seconds as measured with a tear scope or by fluorescein staining greater than grade 1 during normal wear time with conventional soft contact lenses, which staining could not be reduced by varying the lens design.

During the study, only one eye was fitted with a lens of GMA/HEMA, while the other eye was fitted with a conventional lens as a control. All lenses constructed of GMA/2-HEMA had a continuous offset bi-curve design with a 11.25 mm back optical zone and an overall diameter of 14.0 mm. The lenses had a center thickness of 0.12 mm. The lenses used as control were made out of VP/MMA, VA/MMA, or 2-HEMA. Except for the lenses made out of 2-HEMA and VP/MMA, all other lenses had a similar center thickness as the test lenses.

Patients who were previous contact lens wearers used the same care system as they were using before. All new fitted patients were using Oxysept Comfort (Allergan), a peroxide system that leaves the lens at the end of the disinfection cycle in unpreserved saline.

Lens performance was evaluated by visual acuity, staining of the cornea, hyperaemia of the conjunctiva, ophthalmometer reading, refraction, wearing time, and subjective rating of lens comfort and stability of visual acuity. Staining was evaluated by slitlamp observation using a yellow filter to enhance the appearance, and was graded by intensity and extension. The clinical data is provided in FIG. 5.

The subjective rating was rated by whole numbers ranging from 1 to 5, with 5 representing a very comfortable lens with no irritation and 1 representing intolerable irritation. When patients were asked to select their preferred lens, they could choose between the test lens, the control lens, or indicate no preference.

For the purpose of evaluation the findings at the three month visit were used. Of the 30 patients fitted with lenses made out of GMA/2-HEMA material, 20 patients had a non-invasive breakup time of 20 or less seconds and 10 were previously wearing lenses that caused more than grade 1 staining and could not be reduced by changing to a different lens design or/and lens material. After wearing the test lens in one eye for at least three months the staining was reduced to at least 1, in all cases where previously a different lens was worn on the same eye. When the staining of two eyes were compared, the average eye with the lens constructed of GMA/2-HEMA measured 0.2 relative to a measurement of 0.88 with the eye that was wearing a different lens. No corneal or conjunctival problems were observed by slitlamp observation. The visual acuity was, in all cases, the same or slightly better when compared with that observed with previously worn lenses on the same eye. Many patients commented about the more stable acuity obtained with the new lens, particularly during intensive near tasks (VDT tasks).

The subjective rating of the GMA/2-HEMA lens by the patients, on a scale from 1 to 5, showed 53.3% rated the lens at 5 and 36.7 rated the lens at 4, as compared with the rating of the conventional lens in the other eye, which was rated at 5 by 15.4% of the patients and at 4 by 57.7%. Wearing time, for all patients and all lens types, was 12 hours or more. When the overall satisfaction of the lens made out of GMA/2-HEMA was compared to the lens worn on the other eye, 69.2% preferred the GMA/2-HEMA lens and only 11.6% chose the lens on the other eye as the better lens, 19.2% remained undecided. This clinical data is provided in FIG. 6.

At the conclusion of the clinical trial, 26 of the 30 patients chose to be fitted with GMA/2-HEMA lenses on both eyes because of reduced staining and/or more stable acuity. This clinical data is provided in FIG. 7.

Moreover, there were no changes in appearance from the initial findings 10 days after fitting of the lens up to the three month visit.

An interesting finding was the more stable acuity during intensive near tasks. When patients were further questioned if they felt dryness during their work on VDT, they often reported that they felt the dryness but within a few complete blinks that feeling was gone in the eye having the GMA/2-HEMA lens, while dryness remained in the eye with the control lens. It was also observed during the evaluation of the new lens material that only a short time was required for the GMA/2-HEMA lens to settle. After a few minutes of lens wear, movement and rotation of this lens reached a stable state that did not change even after several hours.

Compared to the lens parameters that have been employed in conventional lens design, it was striking that lenses could be fitted steeper than previously and still showed better movement after several hours of lens wear. This is because of the unusual dimensional stability of the lenses. This results in less chair time for the patient and greater predictability about lens movement and rotation after only a few minutes on the eye during the initial fitting.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.