Contact lens for correcting of astigmatism

Abstract: A contact lens for correcting astigmatism. The front surface has a central optical zone and an outer carrier having two points of maximum thickness located at an oblique angle of about 110° to 130° as measured from the vertical axis of the lens. The anterior surface of the lens is symmetrical about its vertical axis. The carrier tapers from the points of maximum thickness to thin portions at the top and bottom of the lens. The rear surface of the lens has an outer spherical base curve zone, an inner toric zone, and an intermediate transition zone having thickness that gradually increase from the base curve zone to the toric zone. By making the radius of the base curve zone substantially equal to that of the toric zone's major meridian, a thinner, more comfortable lens is obtained.

Description: BACKGROUND OF THE INVENTION Astigmatism is a defect in the eye that is corrected by a lens with a non-spherical prescription. The prescription, which is usually expressed as cylinder on the patient's prescription order, causes at least a portion of the surface of the lens to have the shape of a toric segment. Hence, such lenses are called toric lenses. Soft contact lenses which have been designed for use to correct astigmatism are well known in the art. An ideal lens for correcting astigmatism provides, besides good rotational orientation and good rotational stability comfort to its wearer. In general the thinner the lens and the smoother the lens surface, the more comfort it will provide. Lenses in accordance with this invention are preferably soft contact lenses, which may be formed of many materials including hydroxyethylmethacrylate, metallo-organic substances, silicone rubbers and various other materials such as described in U.S.-A-3,503,942 and 3,639,524. The preferred soft contact lenses are hydrophilic; that is, they absorb water and, in fact, the water becomes an integral part of their structure. Hydrophilic contact lenses made in accordance with U.S.-A-4,405,773 are especially preferred for practicing this invention. U.S.-A-4,084,890, discloses a contact lens which is alleged to have improved adherence to the cornea and improved visual acuity as a result of its configuration. The lens tapers from a thin outer edge to an intermediate point of maximum thickness and then tapers to the center of the lens. The peripheral surface portion is wedge shaped in cross section. However this lens is believed to be uncomfortable because of its thickness. U.S.-A-4,324,461, discloses a contact lens for correcting astigmatism. However the thicker ballast portion of the lens is believed to cause discomfort compared to the lens of the present invention. While the posterior surface of a contact lens is generally spherical in configuration, where the lens is to used to correct astigmatism it will have a toric configuration. That is, the curved portion of the posterior of the lens has a major and minor axis; the radius of curvature of the posterior surface of the lens being longer in the major-axis direction than in the minor-axis direction. The result is that rather than being of a spherical configuration, the posterior of the lens has a toric configuration with the major axis running orthogonal to the minor axis. The major diameter of the toric curve is generally smaller in diameter than the overall lens, and is cut into a starting base curve which has a spherical configuration. In the toric lenses of the prior art the junction between the base curve of the posterior surface of the lens and the toric curve portion is in an area of sharp inflection resulting in a sometimes uncomfortable fit to the cornea. SUMMARY OF THE INVENTION The aspect of the invention may be summarized as a contact lens for correcting astigmatism of general concavo-convex configuration having a posterior surface comprising: (a) an outer annular spherical base curve zone; (b) an inner toric zone having a major meridian and a minor meridian orthogonal to each other; and (c) a transition zone between the toric zone and the base curve zone having thickness that gradually increases from the base curve zone to the toric zone; wherein the radius of the base curve zone is substantially equal to the radius of the major meridian; the intersection of the minor meridian and the border between the transition zone and the inner toric zone (14) is located no more than 0.12mm below an imaginary extension of the spherical base curve zone passing above that intersection; and the intersection of the major meridian and the border between the transition zone and the inner toric zone (14) is located no more than 0.07 mm below an imaginary extension of the spherical base curve zone passing above that intersection. The invention is especially suitable for hydrophilic contact lens which are normally shaped in the dry state and then hydrated. Upon hydration, such lenses expand. All deminsions given in the specification and claims are for a full sized lens; i.e., for a hydrophilic lens, a fully hydrated lens; unless specifically stated to be for a lens in the dry state. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional view of the lens showing its anterior surface taken along the major axis of the lens's toric curve. Figure 2 is a sectional view of the lens showing its anterior surface taken along the minor axis of the lens's toric curve. Since Figures 1 and 2 are intended to show only the posterior surface of the lens, the details of the anterior surface do not appear in those figures. Figure 3 is a view of the posterior surface of the lens. Figure 4 is s sectional view of the edge of the lens of this invention showing details of an optional edge configuration. DETAILED DESCRIPTION OF THE INVENTION As used in the specification and claims the term "vertical" is used with reference to the lens as it would be, if ideally seated on the cornea of eye without any shift in orientation resulting by movement caused by the eye lid. The vertical axis is represented by the center line A-A. Referring to Figure 1, in cutting the posterior surface of the lens of this invention a base curve is cut into the posterior surface of the lens. Unlike the prior art, however, this base curve is constructed of two distinct zones, a spherical base curve zone 11 and a pad zone 10. The pad zone is made up of pad curve 13 and a transition zone 9. The radius of the base curve zone 11 on an outer annulus of the lens is approximately equal to the radius that the toric section of the lens is to have along its major meridian. This is much different from the prior art wherein the radius of the base curve, which initially extends over the entire anterior surface of the prior art lens is longer than the radius which the toric section is to have in its major meridian. Pad curve 13 is cut into the lens at a radius that is smaller than the radius of base curve zone 11 by 0.08 to 0.15 mm, preferably 0.1 mm. Curves 13 and 11 have the same center of curvature. Transition zone 9 is 0.5 to 1.5 mm wide, and in cross section is essentially a linear ramp connecting base curve zone 11 with pad curve 10. Curves 13 and 9 are both spherical. Thus the thickness of the transition zone generally increases from base curve zone 11 to pad curve 13. A toric curve, 14, whose dimensions are determined by the prescription, is then cut into the previously described posterior surface. As described above this toric curve has a major and minor axis. The radius of curvature of the major curve of the major meridian is generally the same as that of the base curve zone 11. This major median of the toric curve may lie on vertical line A-A (Figure 3). However, alternatively, it may be an an oblique angle to that line depending on the users prescription or the rotation of the lens resulting from movement caused by the eye lid, or both. In cutting the major meridian of the toric curve, 14, the depth of cut is controlled so that it intersects the transition zone 9 near its outer edge, 12, as best seen in figure 3 along axis A-A. Referring to Figure 3, the outer edge of the transition zone, 12, is the outer edge of the pad zone 10. Generally, the major meridian 15 of the toric curve will be slightly smaller than the outer dimension of the pad zone. Preferably, the major meridian of the toric curve is 0.03 to 0.5 mm smaller than the pad zone diameter. The minor meridian, 15', of the toric curve which is at right angles to the major meridian and may lie on line E-E, is less than the pad zone diameter by 0.2 to 2.5 mm. Figure 2 is a sectional view showing the posterior lens configuration taken along the minor axis of the toric curve, i.e. along section E-E of figure 3. Here the outer edge 17 of the minor axis can be seen, as well as the edge 16 of the pad zone, and pad curve 13. This configuration results in a smoother blend between the transition zone 9 and toric curve 14 as contrasted with the prior art which has a steeper inflection point between the toric curve and the base curve. For lenses in accordance with this invention, the intersection of the minor meridian 17 and transition zone 9 is located no more than 0.12 mm below an imaginary extension of spherical base curve zone 11 passing above that intersection. Preferably this intersection will be between 0.04 and 0.12mm below the imaginary extension of the spherical base curve zone. Furthermore, the intersection of the major axis 15 and transition zone 9 will be located no more than 0.07 mm below an imaginary extension of spherical base curve zone 9. For prescription, wherein the outer edge of the toric curve at the major meridian and the outer edge of transition zone 9 coincide, the intersection will be located on spherical base curve zone 9. The configuration of the posterior of the lens of this invention results in a lens which along its vertical meridian is thinner at the junction of the carrier with the optical zone than prior art lenses and thus is thinner in the carrier as a whole. This results in a lens which on average is lighter in weight. Prior art lenses are about 0.18 mm in thickness in the dry state at their thickest point along the vertical meridian. The lenses of this invention will range in maximum thickness from 0.13 to 0.15 in the dry state at the corresponding point. The intersection of the outer edge of the toric curve with the transition zone is more obtuse than that between the toric curve and the base curve of the prior art. The more obtuse junction contributes to greater lens comfort, and lessens the possibility that the lens will make a pressure indentation on the corneal surface. This contributes both to comfort because of reduced weight, and to improved oxygen transmissibility of the lens, thereby improving corneal health. The edge thickness 19 of the lens as shown in Figure 4, is 0.03 to 0.11 mm. This edge, optionally, is displaced from the base curve zone, 10, by an edge lift, 18, of 0.05 to 0.12 mm. The effect of the lift is to give a more comfortable feel to the wearer of the lens. The edge thickness, 19, of the lens of this invention as measured at the edge of the lens, 8, is generally thinner than that of the prior art as a result of the concentric trim, previously described. While the manufacture of the lens of this invention has been described in terms of cutting of the various curves, it will be appreciated by those skilled in the art having access to this disclosure that the configuration of the anterior of the lens and the base curve as well as the pad of the posterior of the lens can be formed by molding the lens in an appropriately shaped cavity mold.

Claims: A contact lens for correcting astigmatism of general concavo-convex configuration having a posterior surface comprising: (a) an outer annular spherical base curve zone (11); (b) an inner toric zone (14) having a major meridian (15) and a minor meridian (15') orthogonal to each other; and (c) transition zone (12) between the toric zone (14) and the base curve zone (11) having thickness that gradually increases from the base curve zone to the toric zone; wherein the radius of the base curve zone (11) is substantially equal to the radius of the major meridian (15); the intersection (17) of the minor meridian (15') and the border between the transition zone (12) and the inner toric zone (14) is located no more than 0.12 mm below an imaginary extension of the spherical base curve zone (11) passing above that intersection; and the intersection of the major meridian (15) and the border between the transition zone (12) and the inner toric zone (14) is located no more than 0.07 mm below an imaginary extension of the spherical base curve zone (11) passing above that intersection. A contact lens in accordance with claim 1 wherein the intersection of the minor meridian (15') and the transition zone (12) is located from 0.04 to 0.12 mm below an imaginary extension of the spherical base curve zone (11).

Inventors: TOWNSLEY MALCOLM, TOWNSLEY,MALCOLM
Applicants: JESSEN WESLEY CORP, WESLEY-JESSEN CORPORATION
Application year: 1991
Main classification: G02C7
IPC codes: G02C7, G02B3