GB2132785A

GB2132785A - Thin aphakic contact lens

Info

Publication number: GB2132785A
Application number: GB08236601A
Authority: GB
Inventors: Dr William Albert Sammons; Dr Michel Lucien Guillon
Original assignee: National Patent Development Corp
Current assignee: National Patent Development Corp
Priority date: 1982-12-23
Filing date: 1982-12-23
Publication date: 1984-07-11

Abstract

A thin aphakic contact lens has a front surface which is in cross section an aspheric curve having a radius of curvature which increases substantially continuously away from the lens axis. <IMAGE>

Description

SPECIFICATION Thin aphakic contact lens The present invention relates to a contact lens for use in the correction of aphakia. In order to produce the high plus power required in a lens to be used for an aphakic patient it is necessary for the lens to have a relatively high curvature on its front surface in the area of its central axis. It has been necessary in the past for aphakic lenses to have a central area of the front surface having a spherical contour with a small radius of curvature to produce the necessary power and a flatter surrounding zone with which the central spherical curve makes a sharp intersection. This is illustrated in Figure 1 of the accompanying drawings.

In order to produce a lens which has sufficient optical zone such that the abrupt power change at such a junction does not occur within the pupil area, it has previously been necessary to provide a lens with substantial centre thickness, in the range of 0.40 to 0.65mm.

The present invention now provides a contact lens for use in the correction of aphakia and having a front surface which is in cross section an aspheric curve having a radius of curvature which increases substantially continuously away from the lens axis. The lens is preferably a semi-scleral lens and will therefore preferably have a diameter of from 13 to 16mm. The invention is not however restricted to semi-scleral lenses and includes corneal lenses.

In order to have a suitable power for use in the correction of aphakia the lens will preferably have a power in the central region of from + 10 to 20 dioptres, for instance + 14 to 16 dioptres.

The continuously flattening curve used for the front surface of the lens is preferably a hyperbolic curve, at least in the central region of the lens. By virtue of adopting a continuously flattening curve of this type, for instance a hyperbola, oblate ellipse or parabola, it is possible to produce a lens having an adequateiy high plus power in its central regions which is considerably thinner than has previously been achieved.

The power of the lens will change continuously away from the central axis but this will not be noticeable by the wearer or troublesome if it is kept within reasonable limits. Preferably, the central area of the lens bounded by a circle whose diameter is a chord of the lens of length about 5mm should exhibit a power which is relatively constant eg. constant to within 1.5 to 2 dioptres.

The invention includes a lens according to the previous description in which the power in the central 1-4mm is 1-5 dioptres higher than the patient's required correction. This arrangement, of higher power in the centre, and diminishing power towards the periphery, causes the lens to act as a varifocal lens and enhances the patient's reading visual acuity with the lens, in which the central zone is allocated to the near correction.

The front surface of the lens need not be a totally continuous curve but may contain small discontinuities.

However, the curve should be continuous in the central region where discontinuities would be likely to affect vision.

The back curve of the lens may be of any suitable kind for contact lenses generally. It may be a spherical back curve with one or more peripheral curves of an aspheric back curve comprising one aspherical curve or two or more spherical or aspherical curves in combination. The back curve of a contact lens is of course chosen to fit the shape of the front of the eye of the wearer. A lens according to the present invention may be produced by any of the methods conventionally used for the production of contact lenses. In particular, the desired front curve shape can be cut on a Gfeller copying lathe in the manner described in detail hereafter.

The invention will be illustrated by the following description of a preferred embodiment with reference to the accompanying drawings in which Figure 1 is a cross-section through a conventional aphakic lens and Figure 2 is a cross section through a lens according to the present invention.

As shown in Figure 1, a conventional form of lens for the correction of an aphakic patient is a semi-scleral lens having a back curve which may be spherical. The lens has a central region which has a spherical front curve of a radius sufficiently small to produce the required power of from + 10 to 20 dioptres. The front to back thickness of the lens is limited by what can be tolerated by the wearer and this constrains the central front curve to tend to cut the back curve close to the centre of the lens. It is therefore necessary for the outer regions of the front curve of the lens to be formed by a separate large diameter spherical curve which joins the central small diameter central curve at a discontinuity. If the lens were made thinner, the discontinuity would be closer to the centre of the lens and more likely to interfere with vision.

Figure 2 shows a lens according to the present invention having the same power in the central region as the lens shown in Figure 1. It will immediately be observed that the lens shown in Figure 2 is considerably thinner. The back curve of the lens shown in Figure 2 is the same as that shown in Figure 1, ie. a spherical curve of suitable radius.

The front curve of the lens shown in Figure 2 however, is aspheric. The front curve is continuous and flattens, ie. its radius increases, continuously moving away from the central axis. The central part of the front curve approximates to a hyperbola having at the centre of the lens the same radius of curvature as the spherical front surface of the lens in Figure 1. The lens therefore has the same power as the lens of Figure 1.

However, because of the continuously flattening curve adopted for the front surface, the central thickness of the lens can be considerably less than that of the lens shown in Figure 1.

It should be noted that because of the continuously flattening nature of the front curve it can be arranged that the lens is thinnest in an intermediate zone between the centre and the edges of the lens and this intermediate minimum lens thickness may be maintained at a constant value independent of the power of the lens. This has beneficiai consequences in the behaviour of the lens on the eye giving good positional stabiiity. The lens may be made of any of the materials conventionally used for the production of soft contact lenses, such as poly 2-hydroxy-ethyl methacrylate. However the invention is not restricted to soft or hydrophillic lenses, but may include lenses made from rigid materials such as PMMA or gas permeable materials.

Because the lens is thinner, its oxygen transmission is higher, which is beneficial to the patient.

Any continuously flattening curve may be adopted for the front curve of the lens.

Such curves may be cut using a Gfeller copying lathe. Such a lathe can be set to cut a curve determined by the shape of a cam provided. The lathe also has a control known as a "lever arm" which determines to what extent the shape actually cut deviates progressively from the shape of the cam as the cutter moves away from the central axis of the lens being cut. A suitable lens according to the present invention may be cut by using a hyperbolic cam and a lever arm setting of between +9.0 and +13.6.Examples of the characteristics of lenses cut using a Gfeller lathe with a hyperbolic cam and various lever arm settings are shown the following table: BCOR Power CT Lever Arm 8.1 +10.00 0.20 + 9.00 8.1 +15.00 0.20 +10.85 8.1 +20.00 0.26 +12.70 8.4 +10.00 0.20 + 9.25 8.4 +15.00 0.20 +11.30 8.4 +20.00 0.26 +13.45 9.0 +10.00 0.20 +10.60 9.0 +15.00 0.20 +12.10 9.0 +20.00 0.26 z 13.55 BCOR = Back central optic radius CT = Centre thickness The cam used in producing the above described lenses was characterised by having a region corresponding to the optical zone of the lens which was in section hyperbolic of eccentricity 1.2 and a region corresponding to the carrier of the lens which was in section substantially a curve chosen so as to produce a desired thickness of the lens in each case described above was about 0.1 mm which was considered optimum for comfort and handleability.

The back curve of the lens can be made by any of the conventional methods.

Claims

1. A contact lens for use in the correction of aphakia and having a front surface which is in cross section an aspheric curve having a radius of curvature which increases substantially continuously away from the lens axis.

2. A lens as claimed in claim 1 havingadiameteroffrom 13to 16mm.

3. A lens as claimed in claim 1 or claim 2 having a power in its central region of from + 10 to 20 dioptres.

4. A lens as claimed in claim 3 having a power in its central region offrom + 14to 16 dioptres.

5. A lens as claimed in any preceding claims having in the central zone bounded by a circle whose diameter is a chord of the lens of length 5mm a power which is constant to within 1.5 to 2 dioptres.

6. A lens as claimed in any preceding claim wherein the said aspheric curve, at least in the central region of the lens, is a hyperbola.

7. A lens as claimed in any one of claims 1 to 6 composed of a rigid material.

8. A lens as claimed in any one of claims 1 to 7 which is a corneal lens.

9. A lens as claimed in any one of claims 1 to 7 which is a semi-scleral lens.

10. A lens as claimed in any preceding claim which is composed of a soft contact lens material.

11. A lens as claimed in claim 10 composed of poly 2-hydroxyethyl methacrylate.

12. A contact lens substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.