Field of the Invention

The invention relates to contact lenses, and more specifically to contact lenses adapted to correct for the eye condition, keratoconus of the cornea.

Background to the Invention

Contact lenses have been used to treat a wide variety of eye conditions since at least the 1970s. The most common lenses are designed to incorporate positive or negative meniscus lens shapes, to correct refractive error conditions such as long or short sightedness (myopia and hypermytropia). As light passes through the corrective outer surface of the lens, it is refracted to a greater or lesser extent than the underlying cornea, ensuring correct optical power for clear vision.

More recent advances have sought to correct for the condition known as astigmatism. This is where a deviation from spherical curvature, caused by a defect in the cornea or lens, results in distorted and blurred images at all distances. In these cases, the cornea or lens is curved so that the refraction of the eye differs between the vertical and horizontal planes. Spherical lenses, as described above, bend light evenly in every direction, making them suitable for the correction of hypermetropia or myopia. The inherently inconsistent deformation associated with astigmatism, however, requires different refractive powers on the vertical and horizontal orientations. As a result, corrective lenses for astigmatisms, known as 'Toric lenses', must have a greater radius of curvature in one axis than the other. Those skilled in the art will realise that the loss of rotational symmetry caused by this inconsistent curvature, creates the need to maintain the correct orientation of the lens relative to the eye, whilst in use. Several solutions to this problem have been developed, such as having thicker ballasting zones or truncated portions of the lens, which create a weight imbalance in the lens that ensures correct orientation.

Keratoconus, is a disorder of the eye in which the normally round dome-shaped cornea progressively thins causing a cone-like bulge to develop. The loss of correct corneal refraction associated with keratoconus, can cause blurring, distortion of vision and increased sensitivity to light. Traditional treatment for this disorder, utilises rigid, gas permeable (RGP) lenses, with an anterior face which corrects the visual acuity and a posterior surface which is specifically designed to permit the diseased portion of the cornea to conform itself to the lens surface. The hard lenses are designed to rest on a layer of tears, limiting the points of contact between the lens and the users eye. Many hard lenses have regions of the posterior surface so shaped to put pressure on the underlying cornea to conform to a normal shape. The design of these zones and the deviance from the current level of corneal curvature creates varying amounts of pressure on the cornea. The limitation of such designs is that the pressure created on the cornea can become painful for the user. In addition, although designed to limit contact between the contact lens and the surface of the eye, continual use of hard lenses often causes discomfort or even corneal scarring.

(EP0235328) describe a soft contact lens, with an anterior surface of radius of curvature required for clear vision, and posterior surface with distinct zones suitable for

accommodating the conical shape of the cornea. Unlike, the RGP lenses, the distinct zones are designed to overlay the conical corneal region without making contact. In this way, they can correct the vision of the user without creating the discomfort associated with RGP lenses. However, the lenses described do not account for the asymmetrical nature of the condition. The apex of the ectatic cornea is often not centred, rather it is displaced to an off-centre location, and often with an inferior protrusion. As a result, the central domed zone of such lenses would not adequately match the specific topology of the corneal protrusion and would create regions of discomfort. The current invention provides a contact lens for use in correcting the vision of users with the condition keratoconus which seeks to overcome the limitations of the prior art as set forth.

Summary of the Invention

In a first broad independent aspect, the invention provides a contact lens for a patient with keratoconus of the cornea, the lens having a substantially continuous arcuate anterior surface and a posterior surface comprising at least three distinct zones: a substantially circular outer zone with a consistent arcuate surface, a central domed zone with smaller radius of curvature than said outer zone, said central domed zone of dimensions suitable for accommodating the patient's corneal protrusion, and an arcuate intermediate region connecting said outer zone with said central zone.

This is particularly advantageous because the designed zones allow the lens to overlay the outer surface of the user's eye without making contact with the surface of the eye. The domed region of the lens is designed to have a diameter slightly greater than the diameter of the diseased portion of the cornea with the same radius of curvature. The outer zone will rest on a thin layer of tears raising the lens from the surface of the eye and causing the central domed region to enclose the diseased conical portion of the cornea without making contact with the eye. In practice, by removing the points of contact between the lens and the surface of the user's eye, the current lens can limit the levels of discomfort that the user experiences. In addition, the intermediate zone provides a gradual transition in the angle of curvature between the outer zone and the central domed zone which further increase comfort. Preferably, at least one of the lens and/or the central domed zone is rotationally asymmetrical, due to one or both of: the central domed zone is offset from the centre of the lens in the horizontal and/or vertical axis, the circumference of the central domed zone is not spherical, the apex of the central domed zone is offset from the centre of the circumference of the dome, defining a shorter semi-meridian and a longer semi-meridian.

This is particularly advantageous because by understanding that the diseased protrusion is often asymmetrical, and recording its topology accordingly, it is possible to provide a lens with posterior surface of greater similarity to the outer surface of the eye. The result of this increased synergy between the contours of the eye and the zones of the lens, is an increase in comfort for the user. Preferably, with an unbalanced element to the lens ensuring that the lens maintains its angular orientation relative to the eye whilst in use. This is particularly advantageous because it enables the asymmetrically designed lens to maintain correct orientation on the eye of the user. Preferably, said unbalanced element is one or more truncated portions of the outer zone, intended in use to occupy, the upper and lower peripheries of the lens creating a weight imbalance.

Preferably, the contact lens is one or more of: a rigid gas permeable (RGP) lens, a hard or soft contact lens, or some combination thereof, made of a plastics, polycarbonate, hydrophilic material, and/or hybrid material, pressed, formed, moulded milled or otherwise cut on a lathe.

Preferably, the anterior surface of the lens incorporates one or more zones for bifocal or multifocal correction.

Preferably, the lens is a corneo-scleral lens of diameter within the range of from about 13 mm to about 15 mm. This is advantageous because the increased surface area of the outer zone provides a greater area of support for the central domed zone and improves comfort for the user. Brief Description of the Figures

Embodiments of the invention will now be described in detail, with reference to the figures, of which:

Figures 1a and 1 b show a diagram of a first eye with keratoconus of the cornea, in front and cross sectional side views.

Figures 2a and 2b show a diagram of a second eye with keratoconus of the cornea, in front and cross sectional side views.

Figure 3 is a first embodiment of a contact lens in vertical cross section.

Figure 4 is a second embodiment of a contact lens in front and side views.

Detailed Description of the Figures

The figures illustrate a contact lens for correcting the vision of a user suffering with the condition keratoconus of the cornea. Keratoconus, is a disorder of the eye in which the normally round dome-shaped cornea progressively thins causing a cone-like bulge to develop. By mapping the topology of the eye using known techniques such as

photokeratoscopy or videokeratography, the disclosed contact lens will provide a posterior surface with distinct regions so shaped to accommodate the contours of the eye without making contact, and an anterior surface of required curvature to produce clear vision at all distances.

To provide clarity, the components of the system which are common to different Figures have retained identical numerical references throughout all figure descriptions. Figure 1a shows an eye with keratoconus of the cornea in cross-sectional side view.

Thinning of the cornea has resulted in the formation of the domed or conical protrusion 2. The regions 3a and 3b represent healthy cornea. In the current example, the corneal protrusion is a central nipple protrusion, with healthy regions of cornea 3a and 3b of equal radius, on both the vertical and horizontal axis. The apex 1 of the protrusion is also central within the circular margin 7 of the protrusion, creating identical semi-meridians on all axes.

Figure 1b shows the same eye with keratoconus of the cornea in front view. The corneal protrusion 2 is centred in the cornea and has a circular margin 7. In most cases of keratoconus, however, the corneal protrusion is asymmetrical. The entire protrusion could be off centred in regard to the eye, being closer to the sclera on one axis than the other. The circumference of the protrusion could be circular, elliptical, ovate or irregular in shape. The apex of the corneal protrusion could also be off centred within the circumference of the protrusion, creating one shorter and one longer semi-meridian.

Figure 2a shows a second eye with keratoconus of the cornea in cross-sectional side view. This example is known as an inferior protrusion 4 and is characterised by an apex 6 off centred in the circumference 5 of the protrusion 4 creating a longer superior semi- meridian and a shorter inferior semi-meridian, as oriented in Figure 2a.

Figure 2b shows a second eye with keratoconus of the cornea in front view. The inferior protrusion 4 is also characterised by the protrusions inferior location in the cornea, creating a shorter radius of healthy cornea at the inferior and a greater radius of healthy cornea at the superior of the protrusion 4, as oriented in Figure 2b. The circumference 5 of the protrusion 4 is ovate in shape.

Figure 3 shows an embodiment of a contact lens 10 in vertical cross-section. The contact lens 10 has an anterior surface 12 and a posterior surface 11. The anterior surface 12 is a substantially continuous arcuate surface, of curvature required to provide the correct power of refraction for clear vision at all distances. The posterior surface 11 incorporates several distinct regions designed to follow the contours of the user's eye to provide the greatest fit. The central domed region 15 is of diameter slightly larger than that of the conical diseased portion of the user's cornea, with identical radius of curvature. For example, if the diameter of the corneal protrusion was 5 mm, then the central domed region 15 would be 6 mm in diameter. In use, the central domed region 15 would be centred on the corneal protrusion creating a 0.5 mm gap between the corneal protrusion and the central domed region 15 on either side. In use, this space would be filled with a thin layer of tears which would support the lens. In practice, the specific topology of the eye and corneal protrusion would be measured to ensure optimal fit of the lens. These measurements include at least; the location of the corneal protrusion, the shape and parameters of the margin of the protrusion, the height and location of the apex of the protrusion and the length and radius of curvature of each of the semi-meridians.

The outer zone 13 is a substantially circular region with a consistent arcuate face. The radius of curvature is much greater than the central domed region 15 and is the same as the region of the eye on which it lies. The outer zone 13 bears the weight of the lens and is supported by a thin layer of tears which prevents it from contacting the surface of the eye.

An intermediate region 14 connects the outer zone 13 with the central domed region 15. The intermediate region 14 has an arcuate face of smaller radius of curvature than the outer zone 13 but greater radius of curvature than the central domed region 15. This intermediate region 14 provides a smooth transition between the steep curvature of the central domed region 15 and the shallow curvature of the outer zone 13. In the displayed embodiment, the lens is a corneo-scleral lens in which the outer zone 13 overlays the sclera and the intermediate region 14 overlays regions of healthy cornea, to provide greater comfort. In a preferred embodiment these lenses are of diameter 13 to 15 mm. The radius of curvature of the intermediate region 14 is the same as the healthy region of cornea on which it overlays.

Figure 4 shows a second embodiment of a contact lens in front and side views. The regions 21 and 22 incorporate a thickness differential to create dynamic stabilisation and maintain correct orientation of the toric posterior surface. The regions 21 and 22 are thinner than the intermediate region 14 at their peripheries but become thicker towards the medial locations. The action of the user's eyelids on these chamfered regions 21 and 22, serves to stabilize the lens in the correct orientation. Those skilled in the art will realise that this is only one method of maintaining toric orientation, alternative embodiments could incorporate one or more of; prism ballasts, peri-ballasts or truncated inferior and superior portions. In a preferred embodiment the anterior surface 12 can incorporate one or more refractive surfaces to correct for visual conditions distinct from keratoconus.

The contact lenses of the current embodiment are made from a soft polymer material such as silicone hydrogels. Alternatively, the contact lenses could be any combination of; a rigid gas permeable (RGP) lens, a hard or soft contact lens, or some combination thereof, made of a plastics, polycarbonate, hydrophilic material, and/or hybrid material. The contact lenses of the current embodiment could be; pressed, formed, moulded milled or otherwise cut on a lathe.