Deformable-elastic intraocular lens

Abstract

A deformable-elastic intraocular lens comprising a deformable-elastic lens body of crosslinked acrylic material formed of copolymers of methacrylate and acrylate esters which are relatively hard and relatively soft at body temperature, crosslinked with a diacrylate ester to produce an acrylic copolymer having a substantially tack-free surface, a crosslink density of between .[∅5×10^{-2 and 1.5×10-2 #x2205;23×10-1 and 1.66×10-1 moles per liter, and glass transition temperature in the range of -30°C to 25°C, a tensile modulus between 1000 and 3000 psi and a elongation a break of 100% or greater.}

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to improvements in intraocular lenses (IOLs) designed for surgical implantation into the eye, for example, as a replacement for a cataractous or injured natural lens. More specifically, the invention relates to improvements in deformable IOLs which can be folded or rolled to a relatively low profile size to fit into the eye through a relatively small incision, and then within the eye naturally return to an internal nondeformed shape with predetermined optical properties.

IOLs are well known in the art for implantation into the eye as a replacement for a natural crystalline lens which has been surgically removed typically due to opacification, commonly referred to as a cataract condition. Such IOLs have been formed from a small disk of transparent glass or plastic material having appropriately shaped lens surfaces to achieve a desired set of optical properties. The IOL is implanted directly into the eye, typically after removal of the natural crystalline lens, via an incision formed in ocular tissue such as the scalera outside the normal line of sight. Many IOLs are designed for implantation into the so-called posterior chamber of the eye behind the iris and pupil, whereas other IOLs are adapted for placement into the anterior chamber in front of the iris and pupil. In most IOL designs, support structures are attached to or formed integrally with a central lens body or optic and project outwardly therefrom to contact eye tissue at the periphery of the posterior or anterior chamber, thereby retaining the lens body or optic in generally centered relation with the line of sight passing through the pupil.

In the past, most IOLs have been formed from polymethylmethacrylate (PMMA) which is relatively light in weight, possesses excellent optical properties, and is generally considered to be relatively inert when implanted into the eye, thereby avoiding adverse tissue reactions. However, PMMA comprises a plastic matrix which, when formed into the shape of a lens, possesses high rigidity and cannot be deformed by folding, rolling, compression, etc. Accordingly, the use of PMMA lenses requires a relatively large incision in the ocular tissue sufficient to accommodate the entire diametric size of the lens body; which is typically six millimeters or larger, together with the accompanying lens supports structures. Although the resilient lens support structures such a polypropylene loops or haptics are commonly used and advantageously may be folded over the lens body during insertion, such resilient haptics are anchored into the periphery of the hard plastic lens body and thus tend to spring back to their initial unfolded shape with a rapid snap like action during IOL implantation, resulting in undesired trauma to sensitive eye tissues.

While IOLs with rigid PMMA lens bodies have gained widespread acceptance and use, it has been recognized that deformable IOLs have the potential of providing medical benefits well beyond those associated with current IOLs including rigid lens bodies. More particularly, an IOL including a deformable transparent lens body which may be folded or rolled into a reduced profile size may fit through a relatively small incision in ocular tissue and after insertion and release within the eye return to its original size and shape by virtue of its natural resilience. The use of a smaller incision would beneficially result in a safer overall surgical procedure requiring fewer stitches and reduced likelihood of postoperative complications such as infections. In addition, a smaller incision would reduce the incidence of postoperative astigmatism and substantially reduce rehabilitation time. Second, it is anticipated that IOLs with deformable lens bodies may reduce the potential for complications secondary to contact or rubbing against delicate uveal tissues. Also, deformable IOLs may decrease the potential for pigmentary dispersion or pigmentary glaucoma. Finally, it is anticipated that the formable IOLs will provide an added margin of safety for patients with blood dyscarsias, coagulopalthies and hematologic matogrant disease as well as those patients being given anti-coagulant therapy.

Accordingly, deformable IOLs formed of silicones and hydrogels have been proposed for implantation. For example, in 1983, Fyodorov reported on chemical testing of a silicone IOL (Fyodorov, S. W. et al "Initial Clinical Testing of a Silicone Intraocular Lens" Interzonal Scientific/Practical Conference of Ophthalmologists of Western and Eastern Siberia and the Far East, Conference Proceedings 4: 22-24, 1983, Vladivostock). Also in 1983, Mazzacco and Davidson presented initial data on the implantation of silicone IOLs with 6 mm optical zones through 3 mm incisions (Mazzacco, T. R. and Davidson, V. A. "6 mm Optic for a 3 mm Wound" presented at the A.I.O.I.S. United States Intraocular Lens Symposium, New Orleans, La., March 1983). Wichterle and his associates developed a hydrogel of hydrophilic polyacrylates for orbital and intracameral implants in 1960 while Epstein implanted flexible IOLs comprised of poly(hydro hydroxyethyl methacrylate) in 1976 and 1977. The condition of some patients implanted with such lens was followed until 1984 ("Insertion Techniques and Clinical Experience with HEMA Lenses" Soft Implant Lenses in Cataract Surgery T. R. Mazzacco, G. M. Rajacich, E. Epstein, published by Slack Inc., 1986, pp. 11).

Unfortunately, silicones and hydrogels have several well documented deficiencies which hinder their use as IOL materials. In particular, silicones cause complement activation leading to the production of C-4 proteins, a symptom of bio-incompatibility. Also, while silicones may be folded, when released they tend to snap back or regain their unfolded shape too rapidly, posing a threat to the integrity of the endothelial cell layer of the eye. In addition, the long term stability of UV-absorbing silicone formulations is uncertain. As for hydrogels, it has been found that hydrogel materials when hydrated vary in composition including water content from lot to lot. Such variability induces a corresponding variability in the refractive power of IOL lens bodies formed of hydrogel material. Therefore, hydrogel IOLs need to be hydrated in order to determine their refractive power in implanted state. Unfortunately, hydrated lenses cannot be safely stored in the wet state without losing sterilization. If they are dehydrated subsequently, the process of hydrothermal cycling reduces the tensile strength of the IOL material and may cause cracks or crazes to develop in the lens body.

Other deformable IOLs have been described in U.S. Pat. Nos. 4,573,998 and 4,608,049. More specifically, the '998 patent is directed to methods for implantation of deformable IOLs. The patent describes an IOL having an optical zone portion composed of materials such as polyurethane elastomers, silicone elastomers, hydrogel polymer collagen compounds, organic or synthetic gel compounds and combinations thereof. In practice, such materials possess the disadvantages previously attributed to silicone and hydrogen materials.

The '049 patent describes two basic types of deformable IOLs. The firs type includes a lens body of one or more rigid portions hinged or otherwise connected to overlap each other when it is desired to reduce the profile of thelens body as during implantation of the lens. Such lens configurations are difficult to construct and to manipulate during implantation and further suffer from the limitations associated with rigid IOLs. The second type of IOL described by the '049 patent includes a deformable lens body characterized as being capable of return to an undeformed configuration after insertion into the eye. The lens body may be of silicone rubber or an acrylate polymer with ethylene glycol dimethacrylate as a crosslinking agent producing a material of a rubber consistency. The deformable lens body is secured to an L-shaped fixation member around which it may be curled during insertion into the eye. The silicone rubber IOL of the '049 patent suffers from the limitations previously attributed to silicone IOLs. The acrylate polymer lens body described in the '049 patent is a hydrogel of a relatively hard consistency (subject to the foreign problems attributed to hydrogels) while other acrylate polymers known to be pliable are prone to mechanical failure upon compression or folding and are subject to degradation in the eye.

In viewing of the foregoing, it is apparent that there is a need for an intraocular lens and lens material having an improved balance of superior optical characteristics, flexibility, elasticity, elastic memory, and tensile strength. The present invention satisfies such needs.

SUMMARY OF THE INVENTION

Generally speaking, the present invention comprises an IOL having a deformable-elastic transparent lens body of crosslinked acrylic material having a tensile strength sufficient to resist deformation after implantation into the eye as by forces exerted by growing tissue around the IOL; a flexibility as measured by elongation at break sufficient to allow the lens body to be readily folded, rolled or otherwise deformed to a low profile condition for implantation through a small incision into the eye; an elastic memory which enables the folded lens body to naturally and at a controlled rate return to its original shape and optical resolution without damaging or otherwise traumatizing eye tissue; and a low-tack surface which will not stick to surgical instruments used to hold and guide the lens body during insertion and positioning within the eye. In particular, the crosslinked acrylic material comprises copolymers of methacrylate and acrylate esters which are relatively hard and relatively soft at body temperature, crosslinked with a diacrylate ester to produce an acrylic material having a substantially tack-free surface, a crosslink density between .[∅5×10-2 and 1.5×10-2 #x2205;23×10-1 and 1.66×10-1 moles per liter. Such a crosslinking density provides the resulting polymer with the desired elastic memory and elasticity. In particular, upon being folded, the resulting lens bodies 16 will return to its initial state naturally in about 20 to 180 seconds and preferably about 30 seconds.

To produce an IOL 10 with the lens body 16 having the foregoing characteristics, and as further depicted in FIGS. 7, 8A and B and 9, the syrup, crosslinking agent and initiator (in the indicated percent by weight concentrations) are mixed, deareated and the resulting mixture poured into a mold such as mold number 1 or 2 illustrated in FIG. 8A and B or the mold illustrated in FIG. 9. With respect to molds of FIG. 8, the resulting mixture is poured onto an aluminum plate 1 bounded by rubber gaskets 2. A glass plate 3 is placed on top of the rubber gaskets and the combination clamped together by clamps 4. The mold is placed in an oven, heated to about 60°C and cured for about 16 hours. The mold is then post cured at about 90°C for 24 hours.

After curing, the mold is disassembled and the sheets formed therein made ready for cutting into cylindrical lens blanks in the case of mold number 1 for deflashing into lens bodies in the case of mold number 2. Alternatively, the mold bottom shown in FIG. 9 may be used. As illustrated, the mold has slots machined into its aluminum base to accommodate the haptics at an appropriate angle. The molded part from the mold of FIG. 9 comprises the optic and the haptic elements encased in a thin sheet of flash which may be machined off to produce the finished IOL.

Such cutting and machining to produce the desired IOL may involve conventional milling and lathe techniques with the exception that the part is held at a temperature well below room temperature and preferably between -80° and -10°C Specifically, it is desired that the material be held below its Beta-relaxation temperature during cutting. Preferably, during cutting, the low temperature environment is formed by exposing the part to a liquid nitrogen spray which maintains the part within the desired temperature range and provides the desired moisture for the cutting operation. As previously noted, at or below its Beta-relaxation temperature, the copolymer material possesses a particularly hard characteristic suitable for high speed and efficient cutting.

An example of a procedure used to fabricate a multi-piece IOL as shown in FIG. 1 including separate haptics is as follows. First, flat sheets of the crosslinked acrylic are molded at a thickness of between 2 mm and 8 mm as described above and mounted on holders. The material is then cut into disks which are lathe cut at the low temperatures previously described to form the curved planar surfaces and edge cut. The resulting lens bodies are soaked in Freon and chlorofluoro hydrocarbon solvents for 20 minutes and then dried for 30 minutes in a vacuum oven at 60°C The curved surfaces of the lens bodies are then polished at a low temperature. Next, the lens bodies are mounted for drilling of the positioning holes 32 as well as the edge holes for receiving the haptics 18. The positioning holes are typically 0.3 mm while the edge holes for receiving the haptics are typically 0.1 mm in diameter. To mount the haptics into the edge holes, the haptics are located in a stainless steel needle and one end of the haptic melted to form a thickened blunt tip. The needle is then inserted into the edge hole to force the blunt end of the haptic into the hole at room temperature. The needle is carefully withdrawn allowing the walls of the edge hole to collapse back to their normal position clamping the haptic in place. This operation is then repeated for the other haptic.

Alternatively, for lens bodies molding using mold number 2 illustrated in FIG. 8B, the sheet is cored in the area of the lens bodies to cut the lens bodies from the sheet. The resulting lens bodies are then mounted in suitable holders and the foregoing procedure repeated.

Finally, for parts molded from the mold illustrated in FIG. 9, the flash may be removed on a mill to form the desired one piece IOL.

From the foregoing, it should be appreciated that the IOLs of the present invention may be provided in various geometries adapted for folding or rolling, etc. to a reduced profile configuration thereby permitting implantation into the eye through an incision of reduced size. Within the eye, the deformed lens returns to its original nondeformed state. However, according to the invention, the lens is formed from a material having a combination of excellent elastic memory and slow speed of retraction characteristics. The lens thus returns slowly to the nondeformed state without injuring eye tissue while achieving the final nondeformed state without creases, wrinkles, or other structural deviations which would otherwise result in optical distortions.

A variety of further modifications and improvements to the invention describe herein are believed to be apparent to those skilled in the art. Accordingly, no limitation is intended by way of the description herein, except as set forth in the appended claims.

Claims

1. A deformable-elastic intraocular lens (IOL), comprising:

a deformable-elastic lens body of crosslinked acrylic material comprising copolymers of methacrylate and a acrylate esters which are relatively hard and relative relatively soft at body temperature, crosslinked with a diacrylate ester wherein the crosslinked acrylic material has a substantially tack-free surface, a crosslink density of between .[∅5×10^{-2 and 1.5×10-2 #x2205;23×10-1 and 1.66×10-1 moles per liter, a glass transition temperature between -30° and 25° C., a tensile modulus between 1000 and 3000 psi and an elongation at break of at least 100%; and}

flexible haptics attached to the lens body to position the lens body in the eye.

2. The IOL of claim 1 wherein the lens body is formed by chemically crosslinking the diacrylate ester with a partially polymerized mixture of the copolymers, curing the crosslinked acrylic and holding the cured crosslinked acrylic at a temperature below its Beta-relaxation temperature while machining the lens body.

3. The IOL of claim 2 wherein each haptic is attached by forcing an enlarged end thereof into a smaller hole in an edge of the lens body.

4. A deformable-elastic intraocular lens (IOL), comprising:

a deformable-elastic lens body of crosslinked acrylic material formed by mixing copolymers of methacrylate and acrylate esters which are relatively hard and relatively soft at body temperature, with a diacrylate ester to produce an acrylic material having crosslinked density of between .[∅5×10^{-2 and 1.5×10-2 #x2205;23×10-1 and 1.66×10-1 moles per liter and a glass transition temperature of between -30° and 25` C.; and}

flexible haptics attached to the lens body to position the lens body in the eye.

5. The IOL of claim 4 wherein the copolymers are mixed and partially polymerized before mixing with the diacrylate ester.

6. A deformable-elastic intraocular lens body of a crosslinked acrylic material comprising copolymers of methacrylate and acrylate esters which are relatively hard and relatively soft at body temperature, crosslinked with a diacrylate ester wherein the acrylic material has a substantially tack-free surface, a crosslink density of between .[∅5×10^{-2 and 1.5×10-2 #x2205;23×10-1 and 1.66×10-1 moles per liter, a glass transition temperature between -30° and 25°C, a tensile modulus between 1000 and 3000 psi and an elongation at break of at least 100%.}

7. A deformable-elastic intraocular lens body of a crosslinked acrylic material formed by reacting copolymers of methacrylate and acrylate esters which are relatively hard and relatively soft at body temperature to produce a reaction product having a glass transition temperature between -30° and 25°C, partially polymerizing the reaction product and mixing it with a diacrylate ester to produce a crosslinked acrylic having a crosslink density of between .[∅5×10^{-2 and 1.5×10-2 #x2205;23×10-1 and 1.66×10-1 moles per liter, curing the acrylic and machining the lens body therefrom.}

8. The lens body of claim 7 wherein the relatively hard methacrylate ester is a fluoroacrylate.

9. The lens body of claim 7 wherein reaction product comprises ethyl methacrylate, trifluoro ethyl methacrylate and an acrylate ester present in percent by weight concentrations of 25 to 45, 5 to 25 and 30 to 60%, respectively.

10. The lens body of claim 9 wherein the acrylate ester is selected from n-butyl acrylate, ethyl acrylate and 2-ethyl hexyl acrylate.

11. The lens body of claim 10 wherein the diacrylate ester is present in a percent by weight concentration of 0.5 to 3.0%.

12. The lens of body of claim 11 wherein the diacrylate ester is selected from ethylene glycol dimethacrylate, propylene glycol dimethacrylate, and ethylene glycol diacrylate.

13. A method of forming a deformable-elastic intraocular lens body comprising the steps of:

(a) mixing copolymers of methacrylate and acrylate ester which are relatively hard and relatively soft at body temperature;

(b) partially polymerizing the product of Step (a);

(c) chemically crosslinking the product of Step (b) with a diacrylate ester;

(d) curing the product to Step (c); and

(e) forming a lens body having a predetermined optical characteristic from the product of Step (d).

14. The method of claim 13 wherein Step (e) comprises holding the product of Step (d) at a temperature below its Beta-relaxation temperature while machining the lens body.

15. The method of claim 13 wherein the methacrylate and acrylate esters are mixed together in approximately a 45 to 55% by weight ratio.

16. The method of claim 15 wherein the diacrylate ester of Step (C) is present in a percent composition by weight of 0.5 to 3.0%.

17. The method of claim 16 further including the mixing of a UV-absorber and a free radical initiator in Step (a).

18. The method of claim 13 wherein the relatively hard methacrylate ester is a fluoroacrylate.

19. The method of claim 18 wherein Step (a) further including mixing the fluoroacrylate in a concentration range by weight of between 5 and 25% with ethyl methacrylate in a concentration range by weight of between 25 and 45% and an acrylate ester selected from n-butyl acrylate, ethylacrylate or 2-ethyl hexyl acrylate in a concentration range by weight of between 30 and 60%.

20. The method of claim 19 wherein the fluoroacrylate is trifluoro ethyl methacrylate.

21. The method of claim 20 wherein Step (a) further includes the mixing of a UV-absorber in a concentration range by weight of between 0 and 10% and a free radical initiator in a concentration range by weight of 0.05 and 0.2%.