GB2301196A

GB2301196A - Contact lens having spherical central zone and aspheric annular zone

Info

Publication number: GB2301196A
Application number: GB9510528A
Authority: GB
Inventors: Denis Anthony Hough
Original assignee: Bausch and Lomb Inc
Current assignee: Bausch and Lomb Inc
Priority date: 1995-05-24
Filing date: 1995-05-24
Publication date: 1996-11-27
Anticipated expiration: 2015-05-24
Also published as: GB9510528D0; GB2301196B

Abstract

A rigid contact lens 1 has a back surface comprising a generally spherical central zone 2, surrounded by a generally aspheric concentric annular zone 3, having a radius of curvature which decreases towards the outer circumferential edge of the annular zone, the circumferential edge of the central zone forming a junction 4 with the inner circumferential edge of the annular zone and having a common tangent at the junction between the zones. The lens is particularly suitable for use in correcting defects of sight in an eye which has been subjected to corneal alteration, especially photo-refractive keratectomy. There may be a further common tangent at junction 8 between zone 3 and peripheral zone 7. Edge lift 9 is shown.

Description

CONTACT LENS This invention relates to a rigid contact lens which is particularly suitable for use in correcting defects of sight in an eye which has been subjected to corneal alteration, especially photo-refractive keratectomy.

Many people suffer from ametropia, that is, an abnormal condition of the refraction of the eye, with myopia (short-sightedness) being especially common. The most common way of correcting such defects of sight-is to provide a corrective lens in the form of spectacles or contact lenses. However, many people dislike spectacles and contact lenses or, in the case of contact lenses, cannot tolerate them for physiological reasons. For such people, refractive surgery to correct the defect on a permanent basis may be a desirable option.

One surgical procedure that has been developed in recent years is known as photo-refractive keratectomy (PRK) and involves the use of an Excimer laser to reshape the corneal surface. Specifically, the laser is used to remove a portion of the surface of the cornea thereby changing the refraction of the eye. However, the success rate of this procedure is variable, being generally higher in lower refractive errors and lower in higher refractive errors. Moreover, the success rate falls dramatically for those having the procedure repeated for a second time.

Clearly, those people not obtaining total correction following the PRK procedure will still require some form of correction and this is frequently sought in the form of contact lenses. However, the cornea following the PRK procedure is an atypical shape since the procedure has the characteristic of producing an ablated central zone which is significantly flatter in its curvature than the original cornea, superimposed on the original peripheral cornea. It is therefore impossible to obtain a satisfactory lens-cornea relationship in such circumstances using a conventional rigid contact lens which generally comprises a series of spherical or aspheric curves which flatten increasingly towards the periphery of the lens.For instance, if a conventional rigid lens is fitted to approximate to the ablated central cornea, there would be an excessive edge stand off at the periphery with the result that the lens would fall off the cornea. Such a lens could not be worn. However, if a conventional rigid lens is fitted to the normal peripheral cornea, the central zone is too steep resulting in an excessively large apical tear layer between the lens and the cornea. Since the lens is covering a zone of altered cornea, this compromise is clearly undesirable.

There is therefore a need for a new design of rigid contact lens which will fit a cornea that has been subjected to the PRK procedure or similar corneal surgery in such a way that the lens-cornea relationship is similar in its character to that of a conventional rigid lens on a normal eye and which will also maintain the necessary on-eye dynamics which are required for the lens to function correctly over a long period of time and to maintain corneal health.

According to the invention there is therefore provided a rigid contact lens comprising a back surface, which has a generally spherical central zone, surrounded by a generally aspheric concentric annular zone, having a radius of curvature which decreases towards the outer circumferential edge of the annular zone, the circumferential edge of the central zone forming a junction with the inner circumferential edge of the annular zone and having a common tangent at the junction between the zones.

When the central zone is described as being generally spherical, this means that the central zone has a shape corresponding to the surface of a sphere having its centre of curvature on the same side of the lens as the back surface.

When the concentric annular zone is described as being a generally aspheric, this means that the annular zone has a shape defined by the surface of an asphere, that is, a shape which departs to a greater or lesser degree from a sphere, for instance, a shape having a parabolic or elliptical section, the asphere having its centre of curvature of its vertex radius on the same side of the lens as the back surface.

As stated above, the generally aspheric back surface also has a radius of curvature which decreases towards the outer circumferential edge of the annular zone. In other words, the aspheric back surface has a shape factor selected to produce a surface which has a steepening gradient towards the outer circumferential edge of the annular zone. This steepening aspheric geometry can also be specified in terms of negative axial edge lift as will be described below.

The requirement that the back surfaces of the central zone and annular zone have a common tangent at the junction of the zones means that the back surface at the junction is smooth and continuous, rather than discontinuous.

Preferably the annular zone is surrounded by a concentric peripheral zone in the form of an annulus which flattens, in a manner according to the desired lens/cornea relationship, outwardly towards the outer circumferential edge of the peripheral zone.

More preferably, the annular zone is surrounded by a concentric peripheral zone in the form of an annulus which is generally spherical, that is, having a shape corresponding to the shape of a sphere, having a centre of curvature on the opposite side of the lens to the centres of curvature of the central zone and annular zone. Such a shape is sometimes referred to as a reverse spherical bevel and can also be specified in terms of positive relative edge lift as will be described below.

It is also preferred that the outer circumferential edge of the annular zone forms a junction with the inner circumferential edge of the peripheral zone with the annular zone and the peripheral zone having a common tangent at the junction between the zones. The presence of a common tangent at the junction of the zones again ensures that the back surface at the junction is smooth and continuous.

The presence of a peripheral zone as defined above is particularly advantageous in that, despite the unusual topography of a cornea subjected to the PRK procedure, it promotes tear pooling at the edge of the lens which assists with the removal of normal corneal debris and the interchange of tears beneath the lens while the lens is being worn.

Preferably the rigid contact lens is gas permeable.

The geometry of the front surface of the lens and the lens thickness are determined by the nature of the visual correction required and other lens design criteria in a conventional manner.

Each patient's corneal topography will depend on the pre-PRK treatment corneal topography and the amount of refractive error which is intended to be corrected by the PRK procedure. Accordingly, when making a rigid contact lens according to the invention, it is desirable to prepare topographical maps of the patient's cornea before and after the PRK procedure. In the absence of such data, the pre-PRK Keratometry may be calculated from the pre- and post-operative refractive error and the post-PRK Keratometry. In such cases, the corneal eccentricity is assigned a typical value for the pre-PRK corneal model. Once the shape of the cornea and the lens dimensions have been established, a computer-controlled lathe may be programmed to cut the desired generally spherical and aspheric surfaces.

According to another aspect of the invention there is provided a method for correcting defects of sight which comprises applying a rigid contact lens as defined above to the cornea of an eye, particularly a cornea which has been surgically altered and, especially, a cornea which has been subjected to photo-refractive keratectomy.

According to a further aspect of the invention there is provided the use of a rigid contact lens as defined above for correcting defects of sight in an eye, particularly an eye which has been subjected to corneal alteration and, especially, an eye which has been subjected to photo-refractive keratectomy.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is plan view of a rigid contact lens according to the invention; and Figure 2 is a vertical section on the line A-A in Figure 1 In Figures 1 and 2, the back surface of rigid contact lens 1 has a spherical central zone 2, and an aspheric annular zone 3. Central zone 2 and annular zone 3 have a common tangent at junction 4 of these zones.

The negative axial edge lift 5, that is, the decrease in axial lift at the total diameter 6 of the lens, of annnuar zone 3, is calculated from an extrapolation of spherical central zone 2 and aspheric annular zone 3. In Figures 1 and 2, lens 1 also has a peripheral zone 7 in the form of a reverse spherical bevel curve. Annular zone 3 and peripheral zone 7 have a common tangent at junction 8 of these zones. The positive relative axial edge lift 9, that is, the increase in axial lift due to the reverse spherical bevel of peripheral zone 7 is calculated from an extrapolation of spherical central zone 2 and reverse spherical zone 7 at the total diameter 6 of the lens.

Claims

1. A rigid contact lens comprising a back surface, which has a generally spherical central zone, surrounded by a generally aspheric concentric annular zone, having a radius of curvature which decreases towards the outer circumferential edge of the annular zone, the circumferential edge of the central zone forming a junction with the inner circumferential edge of the annular zone and having a common tangent at the junction between the zones.

2. A rigid contact lens according to claim 1 in which the annular zone is surrounded by a concentric peripheral zone in the form of an annulus which has a back surface which flattens outwardly towards the outer circumferential edge of the peripheral zone.

3. A rigid contact lens according to claim 1 or claim 2 in which the annular zone is surrounded by a concentric peripheral zone in the form of an annulus which has a generally spherical back surface having a centre of curvature on the opposite side of the lens to the centres of curvature of the central zone and annular zone.

4. A rigid contact lens according to claim 3 in which the outer circumferential edge of the annular zone forms a junction with the inner circumferential edge of the peripheral zone and the annular zone and the peripheral zone have a common tangent at the junction between the zones.

5. A rigid contact lens according to any one of the preceding claims which is gas permeable.

6. A rigid contact lens substantially as hereinbefore described and with reference to Figures 1 and 2 of the accompanying drawings.

7. A method for correcting defects of sight which comprises applying a rigid contact lens as defined in any one of the preceding claims to the cornea of an eye.

8. A method according to claim 7 in which the cornea has been surgically altered.

9. A method according to claim 7 or claim 8 in which the cornea has been subjected to photo-refractive keratectomy.

10. Use of a rigid contact lens as defined in any one of claims 1 to 6 for correcting defects of sight in an eye which has been subjected to corneal alteration.

11. Use of a rigid contact lens as defined in any one of claims 1 to 6 for correcting defects of sight in-an eye which has been subjected to photo-refractive keratectomy.