Intraocular lens with high strength haptics

Abstract: An intraocular lens (10) is integrally machined from a blank of polymethylmethacrylate (PMMA) or copolymers thereof to provide an optic having a plurality of strong and resilient haptics. The lens is made from a PMMA material that is specially processed to provide greatly increased tensile strength and flexibility. The material surface is carefully polished to minimize irregularities. It is wrapped in a protective cover and heated to a range of between 35°C and 150°C. It is then clamped in a machine which multi-axially stretches the heated sheet, which is held in its stretched condition as it is cooled to room temperature. Multi-axis stretching under these conditions provides highly oriented polymer chains in the material which greatly increases tensile strength, flexibility and resiliency, thereby imparting these same qualities to the lens haptics (12,14).

Description: Background of the Invention The present invention relates to intraocular lenses and more particularly concerns intraocular lenses in which the lens optic and/or the supporting haptics have significantly increased strength, flexibility, and resiliency. Among the procedures for treating cataracts in the human eye is replacement of the clouded damaged natural lens with a plastic intraocular lens. Some exemplary configurations of intraocular lenses and their haptics are shown in the U. S. Patents to Feaster, 4,842,600, Sheets, 4,813,953, Walman, 4,504,981, Burk, 4,504,982, Cozean, Jr., 4,441,217, Kelman, 4,451,938, Lopez, 4,494,254, Arnott, 4,476,591, Anis, 4,804,361, and Jaffe, 4,822,358. All of these patents, except the patent to Burk, show intraocular lenses with different types of long slender haptics which may be formed integrally with the optic or lens portion of the device. The intraocular lens is generally implanted, often during the same procedure in which the clouded lens is removed, in either the anterior or posterior chamber of the eye. The intraocular lens, which includes a portion termed the "optic", which is the lens portion itself, and supporting legs or loops termed "haptics", is introduced into in the eye through a small incision and then appropriately positioned within the eye itself. For example, to place a lens in the posterior chamber of the eye, where the lens has an inferior and superior haptic, the inferior haptic is first passed through the pupil and into the posterior chamber. The superior haptic is then grasped with a suitable instrument and compressed or bent to a position close to the optic and pushed into the posterior chamber with the optic while held in this compressed position. Thereafter, the superior haptic is released and the lens is then finally positioned, to be held in place by engagement of the haptics with the eye tissue. Thus it will be seen that flexibility and resiliency of the haptics is desirable to facilitate at least the described type of implantation of the intraocular lens. In addition, such flexibility is highly desirable for proper support of the optic after implantation. A sudden motion of the head or a blow to the head may tend to move the intraocular lens relative to the supporting tissue of the eye, thus causing the haptics to place increased strain on supporting tissue. The greater the stiffness of the haptic, the greater the possibility that repetition of such motions or blows will cause the haptic to damage the tissue. Accordingly, to accommodate the sometimes severe bending which may be experienced by the haptics during implantation of the lens, and to minimize adverse interaction of the haptics and eye tissue during various motions of the head after implantation, it is highly desirable that the intraocular lens have haptics that are strong, flexible and resilient, yet are sufficiently stable to provide secure placement of the lens within the eye. This has not been possible heretofore because of the limited types of materials available and approved for such use. Polymethylmethacrylate (PMMA), or co-polymers thereof, are widely used for making intraocular lenses, largely because of two important considerations. These are, first, the fact that the material is inert and will not react with the tissue of the eye, and, second, the fact that the materials have been approved by the Federal Food and Drug Administration for such use. However, the PMMA generally employed in the manufacture of intraocular lenses, because it is relatively inflexible, may not react as well as desired to the relatively large amount of bending required during various lens insertion techniques nor to the stresses imposed thereafter by sudden head motion or impact. While PMMA subjected to biaxial stretch has been utilized for several purposes, there has been no suggestion to utilize these materials in the production Of high strength intraocular lenses with haptics of increased flexibility. U. S. Patent 4,278,723 discloses the formation of shaped bodies from sheet material biaxially stretched more than 40%, preferably 60- 80% to produce a more corrosion-resistant and weather-resistant product. U. S. Patent 4,228,267 discloses a process for stretching PMMA film to produce scratch-resistant optical articles. The common intraocular lens known in the art has an optic disc on the order of about 6mm in diameter, with the haptics extending outwardly therefrom to provide a total outside diameter in the range of 9 to 14mm. The haptics are typically long, slender elements having cross sectional diameters on the order of 0.1mm to 0.18mm when round or similar dimensions per side when polygonal or elliptical in shape and generally radially extending from the optic for several millimeters. Lacking great strength and flexibility, these hair-like supporting structures may not withstand bending or repetitive flexing. Some repetitive bending may be encountered in the implantation procedure itself if the operating surgeon should release his instrument's grip on the superior haptic after it has been bent toward the optic, thereby requiring a second or possibly still further major bending of this element. Because the PMMA material previously used for intraocular lenses has a relatively high rigidity, it is highly desirable to improve its strength, flexibility and resiliency in order to decrease the possibility of fracture and to improve the positioning of the lens in the eye. Accordingly, it is an object of the present invention to provide an intraocular lens which avoids or minimizes the above mentioned problems. Summary of the Invention In carrying out the principles of the present invention in accordance with preferred embodiments thereof, an intraocular lens having an optic body and at least one haptic integral with and extending from the lens body is formed of a material selected from the class of polymethylmethacrylate and co-polymers thereof, which material has been processed by stretching along at least two mutually angulated axes, to provide highly oriented polymer chains and increased tensile strength and flexibility, whereby the lens haptic exhibits unexpectedly increased strength, flexibility, and resiliency. Brief Description of the Drawings Figure 1 is a front view of a typical intraocular lens including optic and haptic made of a material embodying principles of the present invention; Figure 2 is a side view of the lens of Figure 1; and Figure 3 shows a blank from which a lens is to be machined. Detailed Description of the Invention Many different intraocular lens configurations and sizes may be employed in the present invention. For purposes of exposition, a typical and exemplary intraocular lens configuration is illustrated in Figure 1, which shows an optic or lens disc 10 suitably shaped for proper focusing and having integrally formed supporting elements or haptics 12 and 14 extending therefrom. Exemplary lens 10 may be bi-convex, as shown, or plano-convex, concavo-convex or any other optical configuration as desired. The optic may also have refractive and diffractive optic portions, several refractive curves or an aspheric surface to give bifocal or multifocal capabilities and by be formed of a material having different optical or physical properties than the material forming haptics 12 and 14. Some of the many other configurations of lenses and their haptics, which may be used in practice of the present invention, are shown in the U. S. patents identified above, and those skilled in the art will appreciate that virtually any haptic shape, configuration, or number may be utilized in accordance with the teachings of the present invention. The haptics 12 and 14 of exemplary lens 10 are oppositely directed, curved, partially looped supports emanating from optic 10 at substantially diametrically opposed portions of the optic. While the haptic may have various configurations, as shown in the U. S. patents identified above, it is common for the haptic to form a curve roughly parallel to but spaced from the edge of the haptic. Small apertures 18,20 are provided in the optic adjacent the connection with haptics for reception of implantation tools. However, apertures 18,20 are not essential to the practice of the present invention and may be deleted or positioned in different locations. In lenses of this type the central portion or optic is a solid body of approximately 6mm in diameter, with the very small cross section haptics extending outwardly to create an overall size, between the free ends of the haptics, of between about 9 and 14mm. Typically each haptic has a generally square cross section that is about 0.15mm on each side. Other configurations and dimensions may also be employed. It will be readily appreciated that any forces exerted on the haptics create high stress at the junction between the haptics and the optic from which the haptics are generally cantilevered. Thus high strength and flexibility are major considerations. According to principles of the present invention, as carried out in an exemplary embodiment thereof, an intraocular lens of any one of a variety of known configurations is made of a material which, by virtue of its use in an intraocular lens, enables the lens to provide highly desirable qualities of increased strength and flexibility. Instead of being made of a material, such as conventional PMMA which has generally randomly oriented polymer chains, as occurs in the standard condition of the material, lenses of the present invention are made from PMMA or copolymers thereof, as will be more particularly described below, that has been processed to provide highly oriented polymer chains, greatly increasing the strength and flexibility of the material. In general, to obtain these properties, PMMA material in sheet form is stretched along a plurality of different axes so as to provide highly oriented polymer chains and improved strength and flexibility. While not required, the axes along which the material is drawn preferably intersect in the center of the sheet of material. Material processed in this matter has not heretofore been employed in the manufacture of intraocular lenses. Use of such material to make an intraocular lens provides surprising and unexpected improvements in the physical properties of the lens, in the procedures for implantation and in the actual service of such lenses. Materials which can be employed to provide the unexpectedly high tensile strength, flexibility and resiliency of the intraocular lenses of the present invention include, in addition to polymethylmethacrylate, co-polymers of polymethylmethacrylate, such as butyl acrylate, ethyl acrylate, and lauryl acrylate. In the processing of material, such as a polymethylmethacrylate, in order to provide a blank from which an intraocular lens is to be machined in accordance with the teaching of the present invention, a sheet of substantially pure, transparent and colorless polymethylmethacrylate is first treated to minimize surface irregularities which may give rise to points of stress concentration during subsequent processing, for example, by buffing, polishing or other known finishing techniques. The material is then covered with a protective cover, such as, for example, by placing the sheet between lint free towels and wrapping it to prevent foreign particle build-up and to protect the polished and buffed surface. The wrapped sheet is then placed in an oven to uniformly heat the material to a temperature of between 35°C and 150°C, said temperature preferably near but below the glass transition temperature of the material. Preferably the sheet material initially has a thickness in the range of from 1/8 of an inch (0.32 cm) to 1 inch (2.54 cm), preferably about 3/8 of an inch (0.95 cm). While the material is uniformly maintained at a fixed temperature within the indicated range, it is clamped in a machine that stretches the heated sheet simultaneously along several different axes in a plane. It is important that the stretching be done at least bi-axially, that is, along two mutually angulated and preferably mutually orthogonal axes. It is also contemplated that stretching may occur along a larger number of axes, such as, for example, six or more axes which are uniformly angularly spaced from one another. The material is stretched by synchronously pulling along each of the several axes with a force of at least 250 pounds (1110 N) (for a sheet of 3/8 of an inch (0.95 cm) thickness, for example). Stretching may be accomplished by clamping opposite edges in a machine that simultaneously exerts oppositely directed tensile forces on opposite edges of the material in the directions of each of the several axes along which the material is to be stretched. Each tensile force is exerted in an amount sufficient to stretch the material and to increase its dimension along the axis of stretch by an amount in the range of 20% to 65%. Stretching by 40% is presently preferred. Preferably the amount of stretch that is achieved is the same along all of the axes of stretch. The stretched sheet, still held in its stretched condition by the clamps of the stretching machine, is then cooled at a controlled rate to room temperature (about 21°C to 22°C). This process results in a very high degree of uniformity of orientation of the polymer chains within the plastic sheet material, which imparts to the material greatly increased strength and flexibility. One or more blanks or buttons are then cut from the stretched and cooled material, at an area thereof that is displaced at least two inches inwardly from the edges to avoid areas of distortion and ripples or clamping stresses at the edges. Preferably, for stretching a material having a thickness in the order of about 3/8 of an inch, a rectangular material sheet, having dimensions before stretching of between 12 and 20 inches on each side is employed. After stretching and cooling the central area of between 8 and 14 inches is used to cut a plurality of blanks for manufacture of a number of intraocular lenses. The stretched blank, which is thicker and larger in all dimensions than the lens to be made therefrom, or a sheet from which a large number of complete lenses and haptics may be machined, is produced using a numerically controlled milling machine and/or lathe, and one or more integral intraocular lenses, including center optic 10 and its integral haptics 12,14, are machined therefrom, as is known in the art. Without removing or disturbing the machined blanks, all chips, cutting fluids and other foreign matter are preferably removed by means of ultrasonic vibration, and the machined parts are air dried using clean, dry compressed air. The fully machined lenses are then polished until all sharp edges and corners have been removed and properly rounded. After further cleaning, the lenses can be individually covered by wrapping for protection during storage and handling until ready for sterilization and shipping prior to use. The use of the material processed as described herein, provides the haptics of these lenses with markedly improved tensile strength, on the order of 350 to 600 grams for haptics of the described dimensions. This may be compared with haptics of a conventional polymethylmethacrylate of the same dimensions which exhibit a tensile strength of only approximately 100 to 200 grams, usually in the range of 120 grams. Further, it has been observed that haptics of the stretched PMMA material are considerably more flexible and resilient than haptics made of conventional non-stretched PMMA material. It is found that intraocular lenses made of the stretched materials as described herein have haptics that can be bent as much as 180° to either side through approximately 20 cycles before failure. In a lens made of conventional unstretched PMMA material haptics of the same configuration and dimension will frequently fracture upon being subjected to a single 90° bend. Also all lenses of the same design machined from a sheet of stretched material or different sheets stretched under the same condition have physical properties substantially the same. It is also contemplated that manufacture of the described intraocular lens may start with a sheet of a thickness greater than one inch (2.54 cm). For example, a five inch (12.7 cm) thick sheet of polymethylmethacrylate is heated, stretched and cooled, substantially as described above, and cylindrical plugs which may be up to five inches (12.7 cm) thick (depending upon the thickness of the starting material and the amount of stretch imparted) are cut from interior portions of the sheet or other portions displaced,inwardly from the edges. These plugs are then cut into slices, each slice forming a blank from which an improved intraocular lens as described herein will be machined. There have now been described intraocular lenses and their manufacture which provide haptics of unexpectedly great strength, flexibility and resiliency. Such haptics help to minimize motion of the center portion or optic of the lens on sudden motion of the head or a blow to the head, while the greater flexibility of the optics minimizes the possibility of breakage upon a shock or bending during implantation. The increased flexibility not only improves the life and operation of the lens, but also greatly facilitates surgical implantation procedures, which may itself be further facilitated by use of colored haptics provided by the present invention. Having thus described preferred embodiments of the present invention, it should be noted by those skilled in the art that the disclosures herein are exemplary only and that other alternatives, adaptations, and modifications may be made within the scope of the present invention. Thus, by way of example and not of limitation, it is contemplated that alternative lens configurations may be used other than the specific optic and haptic configurations as shown. Accordingly, the present invention is not limited to the specific embodiment illustrated herein.

Claims: An intraocular lens including a transparent optic body (10) defining a plane and at least one haptic member (12, 14) fixed to and extending outwardly from said optic body, said at least one haptic member being integral with said optic body and defining an integral portion of said intraocular lens; characterised in that said integral haptic portion is formed of a single piece of multi-axially stretched material selected from the group consisting of polymethylmethacrylate and copolymers thereof; said multi-axially stretched material has an elongation in the direction of stretching in the range of from twenty percent to sixty-five percent along at least two mutually angulated and generally coplanar axes of elongation as determined by deformation of said material from its un-stretched condition, and said material is oriented relative to said plane of said optic body with said axes of elongation of said material being generally parallel with said plane of said optic body to provide oriented polymer chains in said haptic portion of said intraocular lens which has increased tensile strength and flexibility in comparison with un-stretched material. The intraocular lens of claim 1 wherein said material is stretched to provide an elongation of forty percent. The intraocular lens of claim 1 wherein said lens includes a pair of integral haptic portions both of which are formed of a single piece of said material. A method of making an intraocular lens comprising the steps of: stretching a sheet of material selected from the class of materials including polymethylmethacrylate and co-polymers thereof along at least two different and mutually angulated axes disposed in the plane of the sheet to provide highly oriented polymer chains within said material and increased strength and flexibility for said material; selecting a blank from a section of said stretched sheet material which is spaced from edges thereof; and forming from said blank of material an intraocular lens including a transparent optic body defining a plane which is disposed generally in the plane defined by said mutually angulated axes of stretching and which includes at least one integral haptic. The method of claim 4 including the step of polishing said sheet prior to said stretching to minimize irregularities of the surfaces thereof. The method of claim 4 wherein said step of stretching said sheet comprises stretching said sheet along at least four different and mutually angulated axes. The method of claim 4 wherein said step of stretching said sheet includes the steps of: wrapping said sheet in a protective cover; heating said wrapped sheet; stretching said heated and wrapped sheet; and uniformly cooling said stretched sheet. The method of claim 4 further including the steps of: selecting a thickness for said sheet before stretching thereof of from about 4 mm (0.125 inch) to about 25 mm (1 inch); and stretching said sheet by an amount in the range of from about twenty percent to about sixty-five percent. The method of claim 8 including the steps of: selecting a thickness for said sheet before stretching of about 10 mm (3/8 inch); and stretching said sheet by an amount providing an elongation along each stretching axis of about forty percent. The method of claim 8 further including the step of exerting at least 1110 N (250 pounds) of tensile force upon said sheet to effect said stretching thereof. The method of claim 4 further including the step of: selecting a thickness for said sheet of at least 25 mm (1 inch); cutting a plug from said sheet after stretching thereof, which plug has a length equal to the thickness of said stretched sheet; and slicing said plug transversely into sections each one of which provides a blank for a respective intraocular lens.

Inventors: HEALY DONALD D, KOENIG DON, KRUG JOHN A, SCHWARTZ ALAN, YANG STAN, HEALY, DONALD D., KOENIG, DON, KRUG, JOHN A., SCHWARTZ, ALAN, YANG, STAN
Applicants: PHARMACIA AB
Application year: 1990
Main classification: A61F2
IPC codes: A61F2, A61L27