GB2641270A - Improvements in or relating to contact lenses - Google Patents

Abstract

A soft contact lens formed of a material having a Young’s modulus (“E”) value in the range 0.8 – 3.0 MPa, adapted for use in orthokeratology (ortho-K) such that, in use, when the wearer closes their eye, the contact lens exerts a pressure on the eye having a distribution profile in which the pressure on the cornea is relatively high in a central zone of the cornea and is relatively low in an intermediate zone of the cornea. The lens has a central portion 10 which contacts the central zone of the cornea; an intermediate portion 12 which overlies the intermediate zone of the cornea, wherein at least part of the intermediate portion having a radius of curvature which is smaller than the radius of curvature of the intermediate zone of the cornea; and a peripheral portion 16, which contacts at least part of a peripheral zone of the cornea.

Description

Title: Improvements in or Relating to Contact Lenses

Field of the Invention

The present invention relates to a contact lens, a method of making said contact lens, and to a method of using the contact lens, in particular using the contact lens to treat a myopic condition in an individual.

Background of the Invention

It is known to use contact lenses for the purpose of orthokeratology, also known as 0 "ortho K". This is the use of specially designed and fitted contact lenses to temporarily reshape the cornea to improve vision. Ortho K contact lenses are generally worn at night, and apply pressure to the cornea (which has the effect, as explained below, of reshaping the front surface of the eye) while the subject is asleep, and are removed during the day. The re-shaping of the cornea, and consequent vision is improvement, is reversible but can be maintained if the subject persists with the use of the ortho K lenses. Orthokeratology can be considered as the optical analogue of dental braces. Orthokeratology can be used to treat hyperopia, but is normally used to reduce myopic defects in a subject. In addition, it is believed that the use of ortho K lenses can slow "myopic progression", a condition in which myopia worsens over time, and which is increasing in incidence. Current trends indicate that the global prevalence of myopia will be around 49% by 2050, and the prevalence of high myopia (i.e. with optical errors greater than 6D) is expected to reach 10% by the same year. High myopia is very serious because it is associated with increased risk of other conditions, such as glaucoma, retinal detachment, and cataract.

It is believed that the application of pressure to certain areas of the corneal anterior surface affects the process by which corneal epithelial cells replicate. Specifically, the pressure affects the process by which the corneal epithelial cells migrate from the limbus (where they are created) to the corneal centre. If this pressure is not uniform, it can lead to changes in the thickness of the corneal epithelium: areas of increased pressure tending to reduce the thickness of the epithelium, and conversely with relative thickening of the epithelium in areas of the cornea under relatively lower pressure. Conventional ortho K contact lenses apply non-uniform pressure to the cornea, and thus induce re-shaping of the cornea by inducing differential epithelial thickening. If the pressure acting in the corneal central region is higher than the pressure acting on the peripheral portions of the cornea, this will induce a thinning and flattening of the central corneal surface. The flattened central corneal surface shifts the focal point of light passing therethrough, pushing the focal point backward, further into the eye and away from the cornea. In subjects with myopia, the focal point of light passing through the eye sits in front of the retina, causing blurred vision. Thus, shifting the focal point backwards, towards the retina, by orthokeratology, can help correct or reduce the refractive error associated with myopia.

Ortho K lenses are particularly useful in treating children with high levels of myopia, since they not only reduce the refractive error but can also slow the rate of myopia progression, which is very important when myopia begins at a young age.

Conventional ortho K lenses are all rigid gas permeable (abbreviated as "RGP") lenses. A typical RGP lens is about 4000 times stiffer, or more rigid, than the human cornea, and has a larger central radius of curvature than the cornea (i.e. the central part of the lens is substantially flatter than the central part of the cornea). Accordingly, when an RGP contact lens is worn and contacts the cornea, the relatively rigid lens forces the cornea to adapt to the shape of the contact lens, and the central portion of the cornea is flattened. A conventional RGP ortho K contact lens also has a paracentral region (immediately adjacent to the central region of the lens), overlying the paracentral region of the cornea. The radius of curvature of the paracentral region of the contact lens is smaller (i.e. more steeply curved) than that of the paracentral region of the cornea. Thus, the paracentral region of the cornea undergoes steepening as a result of the lens contacting the cornea. It is believed that the consequent change in peripheral vision is involved in the reduction in myopic progression associated with the use of RGP ortho K lenses.

A conventional RGP ortho K contact lens has a central region (typically about 3-5 millimetres in diameter) which is markedly flatter than the cornea. This central region has a curvature referred to as the "base curve". Moving radially outwards, the conventional RGP ortho K lens then has a steeply-curved region, known as the "reverse zone", and then an "alignment zone", having a curvature similar to that of the cornea.

Figure la and Figure lb depict, in exaggerated form for the purposes of illustration, a sectional view of a conventional RGP ortho K contact lens in situ on the cornea of a subject before (Figure la) and after (Figure lb) the application of eyelid pressure (by the closing of the subject's eyelid) on the contact lens/cornea system. Due to the inflexibility of the RGP lens, under application of eyelid pressure the RGP lens presses onto the cornea, forcing it to conform (in a central area) to the shape of the io back surface of the lens, which latter surface remains essentially unaltered. The contact pressure between the lens and the corneal surface can be modelled by computer modelling systems (e.g. using finite element analysis), with a good degree of accuracy, enabling the prediction of the consequent re-shaping of the corneal front surface.

Whilst RGP lenses can be very effective in the context of orthokeratology, they can be uncomfortable for a wearer, due to their rigidity (i.e. the very property which makes them good at re-shaping the cornea). This is especially troublesome if the subject is unable to sleep because of discomfort caused by the ortho K contact lenses.

The present invention aims to provide a contact lens which can be used effectively in orthokeratology applications but which is more comfortable for a wearer than an RGP contact lens.

Summary of the Invention

In a first aspect the invention provides a soft contact lens, especially a soft contact lens formed of a material having a Young's modulus ("E") value in the range 0.8 3.0 MPa, adapted for use in orthokeratology such that, in use, when the contact lens is worn and the wearer closes their eye, the contact lens exerts a pressure on the eye having a distribution profile in which the pressure on the cornea is relatively high in a central zone of the cornea and is relatively low in an intermediate zone of the cornea outside of, but substantially adjacent to, the central zone, the contact lens comprising, sequentially, from the centre of the lens outwards: a) a central portion which contacts the central zone of the cornea; b) an intermediate portion which overlies the intermediate zone of the cornea, at least part of the intermediate portion having a radius of curvature which is smaller than the radius of curvature of the intermediate zone of the cornea; and c) a peripheral portion, which contacts at least part of a peripheral zone of the cornea, which peripheral zone is outside of the intermediate zone.

Typically (but not necessarily) a contact lens in accordance with the present invention has a substantially circular central portion. Typically (but not necessarily) in a contact to lens in accordance with the present invention the intermediate portion forms a substantially circular concentric annulus around a substantially circular central portion. Typically (but not necessarily), the peripheral portion forms a concentric substantially circular annulus around the intermediate portion.

The central portion of the contact lens may desirably have a wholly or substantially uniform radius of curvature, which radius of curvature will be similar to, or slightly greater than, that of the central zone of the cornea of the subject (i.e. the curvature of the central portion of the lens will be similar to or slightly flatter than the central zone of the cornea). In general, the intermediate portion of the contact lens will be more steeply curved than the intermediate zone of subject's cornea, and also more steeply curved than the central portion of the contact lens. The front surface of the intermediate portion of the contact lens will tend to rise above the general profile of the front surface of the contact lens as a whole, and may, for example, form a small (typically circular) raised "ridge" around the central portion, which ridge will interact with the wearer's eyelids when the wearer closes their eye.

The preferred radius of curvature for the central portion of the contact lens will depend at least in part on the curvature of the central zone of the cornea of the individual for whom the lens is intended. Typical suitable values for the radius of curvature of the central portion of the contact lens will be in the range 7.0 mm to 10.0 mm The intermediate portion may have a plurality of radii of curvature. Typically the geometry of the intermediate portion (b) is such that, when the contact lens is positioned on the cornea of the subject and the subject closes their eye, at least a region of the intermediate portion does not contact the cornea and, in a highly preferred embodiment, at least said region of the intermediate portion does not contact the cornea even after normal eyelid pressure is applied to the contact lens. In a preferred embodiment, the closing of the subject's eyelid creates a force on at least the intermediate portion of the contact lens, which force may tend to decrease the maximum separation between the rear surface of the intermediate portion of the to contact lens and the cornea, the closing of the eyelid acting on the contact lens to create an area of relatively high pressure on a central portion of the cornea underlying the central portion (a) of the contact lens, and an area of relatively low pressure on an intermediate portion of the cornea underlying the intermediate portion (b) of the contact lens.

As with the central portion of the contact lens, the preferred radius of curvature of the intermediate portion will depend at least in part on the curvature of the intermediate zone of the cornea of the individual for whom the lens is intended. However, at least part of the intermediate portion of the contact lens will typically have a radius of curvature in the range 4.0 mm to 8.0 mm.

In some embodiments of the invention the intermediate portion (b) may form a generally annular region around the circumference of the central portion (a) but this is not essential. In a preferred embodiment the intermediate portion comprises a region having a rear surface which does not contact the cornea (when the subject closes their eye) at any angular displacement around the optical axis of the contact lens. Alternatively stated, in a preferred embodiment there is a region of the intermediate portion, which region extends continuously around the circumference of the central portion (a), in which the rear surface of the contact lens does not contact the cornea, even after normal eyelid pressure is applied to the contact lens. However, this constitutes a preferred feature of the invention and is not essential. Further, there is no requirement that the amount of separation between the rear surface of the intermediate portion and the cornea be constant for all values of angular displacement around the optical axis of the contact lens. Thus, the intermediate portion (b) may vary, in terms of its radius of curvature and/or in terms of the separation of its rear surface from the cornea, at different angular displacements around the optical axis of the lens.

Indeed, in some embodiments in which the contact lens is intended for use by the subject exhibiting an astigmatic visual defect, in addition to a myopic visual defect, it may be preferred that the separation between the rear surface of the intermediate portion (b) and the subject's cornea varies with angular displacement around the o optical axis of the lens, such that reshaping of the corneal epithelium may not be uniform in the intermediate zone. The thickness of the corneal epithelium may be altered by different amounts at different angular displacements around the optical axis of the subject's eye, thereby ameliorating or moderating an astigmatic visual defect.

Application of eyelid pressure to the contact lens, and especially to the intermediate portion (b) thereof, pushes the central portion of the lens (a) against the corneal surface and also tends to extend the contact lens radially outwards, thus in essence tending to spread the lens across the centre of the cornea. The inventors have found that a soft contact lens with the aforementioned geometry, in accordance with the invention, can (when the subject's eyelids are shut) closely reproduce the profile of pressure distribution on the cornea which is created by a conventional RGP ortho K lens, and thus produce substantially the same corneal epithelium re-shaping. Accordingly, whilst a conventional RGP ortho K lens and a soft lens in accordance with the invention have the same effect, they act in different ways. The RGP forces the corneal surface to comply with the geometry of the RGP lens due to the rigidity of the latter, whilst a soft lens in accordance with the invention interacts with the cornea (with the shape of parts of both the cornea and the contact lens being altered by the interaction) to create the same pressure distribution profile as that experienced by the cornea when an RGP ortho K lens is in position.

In a typical preferred embodiment of the invention a soft contact lens in accordance with the first aspect of the invention has a substantially circular central portion which is about 3 -6 mm in diameter, preferably about 3.5 -5.5 mm in diameter, and most preferably about 4.0 -5.5 mm in diameter.

A soft contact lens in accordance with the invention, when worn on the eye of a subject (and the subject closes their eye) the lens will exert a contact pressure on the cornea of the subject, as described previously. In the central zone of the cornea, lying beneath the central portion of the contact lens, the average contact pressure will generally be in the range 1000 -3000Pa. The contact pressure on the central zone will typically be lowest at the very centre of the central zone, and the contact pressure in the central zone tends to be at its peak just within the circumference of the central zone. As a guide, a typical contact pressure at the very centre of the central zone might be about 1000 -1500Pa, and a typical contact pressure just within the circumference of the central zone might be about 2500 -3000Pa.

The typical embodiment of a contact lens in accordance with the first aspect of the invention will comprise a substantially annular intermediate portion having an inner radius in the range 3.5 -5.5 mm, and an outer radius in the range 7.5 -8.5 mm. When the subject closes their eye, the average contact pressure on the intermediate zone of the cornea, underlying the intermediate portion of the contact lens, will be less than the average contact pressure acting on the central zone of the cornea. More especially, in preferred embodiments, the average contact pressure on the intermediate zone of the cornea will typically be less than 100Pa, preferably less than 50Pa, more preferably less than 10Pa, and in the most preferred embodiments will be zero (wherein the intermediate portion of the contact lens does not come into contact with the intermediate zone of the cornea) or may even be a negative value (wherein the contact lens exerts a slight suction on the intermediate zone of the cornea).

The peripheral portion of a preferred embodiment of the contact lens of the invention will desirably form a substantially concentric annulus around the intermediate portion.

The inner radius of the peripheral portion will typically be in the range 7.5 -8.5 mm and the outer radius will typically be in the range 12 -15 mm. A major function of the peripheral portion as to facilitate correct positioning of the contact lens on the subject's eye. Accordingly it is preferred that, when the subject closes their eye the peripheral portion will exert a contact pressure on the peripheral zone of the cornea underlying the peripheral portion of the contact lens. The average contact pressure on the peripheral zone of the cornea will desirably be significantly higher than the contact pressure on the intermediate zone of the cornea. The peak contact pressure in the peripheral zone may be higher than the peak contact pressure in the central zone and may, for example, be 4000Pa or even higher, but this peak pressure is exerted over a small portion only of the peripheral zone, such that the average contact pressure on the peripheral zone of the cornea may be generally similar to the average contact pressure on the central zone of the cornea. The person skilled in the art will appreciate however that the magnitude of the contact pressure in the peripheral zone of the cornea is not overly critical, since the peripheral zone of the cornea is not significantly involved in vision, and reshaping of the corneal epithelium in the peripheral zone is unlikely to have much if any impact on myopic or other vision defects.

Preferably the peripheral portion of the contact lens of the invention will have a substantially uniform radius of curvature, which simplifies design and manufacture of the lens, but this is not essential. Preferred values for the radius of curvature of the peripheral portion of the contact lens will depend at least in part on the curvature of the peripheral zone of the cornea of the individual for whom the contact lens is intended, but typical suitable values for the radius of curvature of the peripheral portion will be in the range 8.0 mm to 11.0 mm.

Despite reproducing the corneal surface pressure effects of an RGP contact lens on the cornea, the contact lens of the invention, being much softer, is far more comfortable for the subject than an RGP lens.

For the purposes of the present specification, a soft contact lens is considered to be a lens having a Young's modulus ("E" value) of 3.0 MPa or less. It is a highly preferred feature of the invention that the contact lens is formed of a material having a Young's modulus ("E") value in the range 0.8 -3.0 MPa, more preferably 1.0 -3.0 MPa, even more preferably 1.2 -3.01\iPa, and most preferably 1.5 -3.0MPa. This range is very much lower than that of the material used to make a conventional ortho K RGP contact lens, which typically has a Young's modulus of about 3 GPa (i.e. about 103 higher). The inventors found that soft lenses formed from materials with a Young's modulus below 0.8 MPa were unable to reproduce the same pressure distribution profile as RGP lenses, whilst lenses formed from materials having a Young's modulus above 3.0MPa could be uncomfortable to wear.

Equally, the contact lens of the invention is highly preferably more rigid than a conventional soft contact lens, which is typically formed from a material having a Young's modulus in the range 0.2 -0.7 IVII3a.

Further, although not essential, a contact lens in accordance with the invention will preferably have a thickness (especially, a thickness in the central portion) of about 250 pm. This is significantly thicker than a conventional soft contact lens, which will generally have an average thickness of about 70 pm.

In preferred embodiments, a contact lens in accordance with the present invention has an average thickness in the range 200 -300 pm (more especially 220 -280 gm), and is formed from a material having a Young's modulus in the range 1.5 -3.0 MPa.

The person skilled in the art will appreciate that the aforementioned range of values stated as preferred for the thickness of the lens, and the stiffness of the material used to form the lens, may be interrelated. Thus, for example, a relatively stiffer material may permit the lens thickness to be reduced; and use of a relatively thicker lens may permit the stiffness of the material to be reduced. With the benefit of the present disclosure, using simple trial-and-error, it will be possible for the person skilled in the art to model the behaviour of soft contact lenses having different thickness and stiffness, and select those combinations which have the desired properties.

The front surface of the contact lens (i.e. that surface further from the cornea) may generally follow the profile of the back surface although this is not critical. Indeed, for some parts of the front surface, the radius of curvature may depart significantly from the radius of curvature of the back surface.

A function of the peripheral zone is to facilitate correct positioning of the contact lens on the subject's eye.

The inventors have found that, in some embodiments, it may be desirable if the surface of the contact lens is provided with a coating. The coating may preferably be provided on both the front and back surfaces of the lens. The coating is preferably formed of a layer comprising a hydrophilic polymer. A preferred hydrophilic polymer comprises a polyethylene glycol (PEG) polymer or co-polymer, preferably polyethylene glycol with an average molecular weight in the range 200-9,500. The 0 coating is very thin and forms a very minor constituent of the contact lens. The thickness of any coating on the lens is in addition to the preferred average lens thickness of 200 -300 um noted above (i.e. the average total thickness of the lens will be 200 -300 um plus any coating thickness). The coating may be applied to the lens using conventional application techniques known to those skilled in the art, and such PEG-coated hydrogel materials are readily available commercially. Other hydrophilic polymer materials are known and could be used in place of PEG, or in admixture therewith (e.g. as co-polymers), such as polyvinylpyrrolidone (PVP), 2- methacryloyloxyethyl phosphorylcholine (MIPC), and 2-hydroxyethylmethacrylate (EMMA). Less preferably, the surface of the lens may be plasma-treated to cause chemical modification, to render the lens material less hydrophobic at its surface.

In one embodiment the contact lens of the invention may comprise or be wholly or predominantly formed from a hydrogel material, especially a silicone hydrogel. In an embodiment the contact lens of the invention may comprise or be wholly or predominantly formed from a non-hydrated or hydrated polymer of (poly)dimethylsiloxane (PDMS), and/or its related compounds and derivatives such as 3-(methacryloyloxy)propyltris(trimethylsiloxy)silane. Polymers formed from PDMS and its derivatives have a high oxygen permeability, which is a desirable characteristic. Oxygen permeability is a parameter of a contact lens that expresses the ability of the lens to let oxygen reach the eye by diffusion. The oxygen permeability of the lens is determined by the thickness of the lens and the material of the lens. Because of this dependence on thickness, transmissibility level (abbreviated as Dk/t), the Dk per thickness of the lens, is commonly used. Silicone hydrogels have a very high oxygen permeability, typically in the range 100-120. Such materials tend to have rather hydrophobic surfaces and, for this reason, it is generally preferred to provide the contact lens material with a coating of hydrophilic polymer (such as PEG, noted above) to improve the wetting characteristics of the lens. Other coatings to improve wetting are known (e.g. as disclosed in US2017-0165932). Other materials which may also be used (e.g. to form a co-polymer, typically together with a dimethylsiloxane monomer, in the lens manufacture) include the following: dimethyl acryl ami de (DMA), N-vinyl pyrrolidinone (NVP), and 2-hydroxyethylmethacrylate (HEMA) and the like. Incorporation of such hydrophilic materials into the body of the lens may confer sufficient wettability without requiring the application of a hydrophilic coating to the surface of the lens.

In a second aspect, the invention provides a method of making a soft contact lens Is adapted for use in orthokeratology, the contact lens being in accordance with the first aspect of the invention, the method comprising the steps of (i) designing or selecting a rear surface geometry for the contact lens, based on data describing the topography of the surface of the cornea for an individual subject's eye, or based on data describing the averaged topography of the surface of the cornea for the eye of a plurality of subjects; (ii) optionally, modelling the optical characteristics and/or modelling the fit characteristics of the designed or selected rear surface geometry of the lens on eye, and altering the designed or selected near surface geometry if required for better conformity with the desired optical characteristics and/or fit characteristics; and manufacturing a contact lens in accordance with the first aspect of the invention having the designed or selected rear surface geometry from step (i) or as altered in step (ii).

The data describing the topography of the surface of the cornea may relate to the eye of an individual subject, which allows the manufacture of a "bespoke" contact lens, specifically adapted and configured for an individual subject. Alternatively, the data may relate to an average topography of the surface of the cornea, using information relating to a plurality of subjects. The average topography may be obtained by standard Orbscan or Scheimpflug techniques employed by corneal topography devices such as the Medmont E300, the Oculus Keratograph 5M, and the Oculus Pentacam®.

The data describing the topography may, in some embodiments, be obtained by conventional methods comprising, for example, one or more of keratometry, corneal mapping, and the like. The data may relate to (i) the topography of the whole surface of the cornea, the limbus and the anterior sclera; (ii) the topography of the central io cornea; or (iii) the central corneal radius (i.e. radius of best fit sphere) and shape factor. (i) and (ii) consider the actual corneal topography, whilst (iii) assumes rotational symmetry of the cornea. Use of (i) may be preferred since a soft contact lens in accordance with the invention will typically have a diameter larger than that of a conventional RGP ortho K lens, and hence may extend beyond the cornea to the front surface of part of the sclera.

These data may relate to a specific individual subject, or may relate to a data from a plurality of subjects, the latter being useful in manufacturing a "generic" soft contact lens in accordance with the invention, which may have acceptable optical and/or fit characteristics, whilst being sub-optimal. Data for a specific individual subject may be useful in manufacturing a "bespoke" contact lens which may have optical and/or fit characteristics which are optimal, or closer to optimal than a generic contact lens.

Once the desired geometry of the rear surface of the contact lens has been determined, an appropriate geometry for the front surface can be ascribed. It is important for the comfort of the wearer that the front surface of the lens has a profile which is smoothly curved, as is well known to those skilled in the art.

After the appropriate front and back surfaces of the contact lens have been determined, one or more contact lenses may be manufactured having the desired surfaces. Lenses may be produced with small dimensional variations, allowing the selection of a lens which best fits the subject's eye. The actual manufacturing may be accomplished using conventional manufacturing techniques, such as moulding, casting and the like, which are well-known to those skilled in the art.

In a third aspect, the invention provides a method of treating a myopic condition in an individual in need of such treatment, the method comprising the steps of obtaining information on the shape of the surface of the cornea of the subject's eye requiring treatment and preferably also obtaining information on the nature or extent of the myopic condition; and making a soft contact lens in accordance with the first aspect of the invention to fit the eye of the subject and to cause a desired pattern of o epithelial remodelling and corneal reshaping, thereby to reduce the myopic condition in the subject. Information on the shape of the surface of subject's cornea (and of the nature and extent of the myopic condition) can be obtained by conventional methods and means well known to those skilled in the art.

is Typically, the contact lens of the invention will be worn by the subject at night whilst the subject is sleeping, typically for at least 6 hours per night, more typically for at least 7 hours, up to about 8 or even 9 hours (which length of sleep is not unusual for young children). Preferably, the subject should wear the contact lens every night until the desired pattern of corneal epithelial remodelling and corneal reshaping has been obtained. This can be ascertained by simple performance of standard visual acuity assessments on the subject, to monitor treatment of the myopic condition. It will be apparent to those skilled in the art that the myopic condition is one which is caused in the individual as a result of an inappropriately-shaped corneal, such that appropriate reshaping of the cornea can bring about improvement in the myopic condition.

The invention will now be further described by way of illustrative embodiment and with reference to the accompanying drawings, in which: Figures la and lb are schematic representations of a median sectional view of a conventional, prior art RGP ortho K contact lens in situ on the eye of a subject before (I a), and after (lb), the application of eyelid pressure on the lens; Figures 2a and 2b are schematic representations of a median sectional view of soft ortho K contact lens, in accordance with the invention, in situ on the eye of a subject before (2a), and after (2b), the application of eyelid pressure onto the lens; Figure 3 is a composite figure including a schematic representation of part of a soft ortho K contact lens in accordance with the invention in situ on the eye of a subject (with eye closed) showing the locations in which the contact lens comes into contact with the subject's cornea, the schematic representation being aligned with a graph of contact pressure against distance from the centre of the cornea (in arbitrary units); Figures 4a and 4b show lens-eye finite element computer models of a soft ortho K lens in accordance with the invention, before (Figure 4a) and after (Figure 4b) eyelid pressure application; Figure 5 is a graph showing contact pressure (in kilo Pascals) plotted against distance from corneal apex (in mm) for both a typical RGP ortho K lens and for a soft ortho K lens in accordance with the present invention; Figure 6 is a graph of actual refractive correction of conventional RGP contact lenses on eye (in Dioptres), against predicted refractive correction; Figure 7 shows plots of pressure (in kPa) against R (mm) -the distance away from the centre of the cornea and lens, illustrating the distribution of contact pressure on the cornea (estimated, by using finite element analysis) for various soft ortho K contact lens geometries in accordance with the invention, and how these translate into actual measured changes in corneal epithelium for a plurality of different subjects; Figure 8 is a graph of contact pressure distribution (kPa) against displacement from the centre of the eye/contact lens (mm) for two soft contact lenses in accordance with the present invention being formed form materials having differing Young's modulus values (0 8MPa, circle symbols; and 1.5MPa, cross symbols); and Figure 9 is a schematic representation of a median section through one embodiment of a contact lens in accordance with the invention, showing the front and back surfaces of the contact lens and the surface of the cornea, the latter depicted in the figure as being part of a spherical-curved surface.

Examples

Example 1

Referring to the prior art RGP contact lens illustrated in Figures la and lb, before to (Fig la) or after (lb) the subject closes their eye, the lens 2 is situated on the subject's cornea 4. Whilst the Figure is exaggerated for improved clarity, it can be seen that the central portion of the RGP lens is much flatter (i.e. has a larger radius of curvature) than the corresponding central zone of the subject's cornea. Accordingly, in Figure la it is seen that a central portion 6 of the RGP contact lens is in contact with the cornea over a relatively small central part, and a substantial part of the lens does not contact the cornea. However, when the subject closes their eye, the eyelids press against the front surface of the contact lens (pressure indicated in Fig. lb by the downward pointing arrows). The contact lens is very much more rigid than the subject's cornea. Accordingly, the force exerted by the eyelids is transmitted to the cornea through the lens, and the cornea tends to become flattened against the rear surface of the contact lens. In doing so, the cornea comes into contact with the contact lens over a greater area in a larger central portion 8, but the periphery of the cornea does not contact the contact lens and is not subjected to any significant eyelid pressure. The result is that there is a pressure distribution profile across the cornea, with little or no pressure in an outer part of the cornea, but high pressure across a major central zone of the cornea. The effect of this pressure distribution profile is a thinning of the corneal epithelium in the central zone exposed to high pressure, and a relative thickening of the corneal epithelium, outside the central zone, where there is little or no pressure.

Figures 2a and 2b illustrate a soft orthokeratology lens, in accordance with the present invention, in situ on a subject's eye before (Figure 2a) or after (Figure 2b) the subject closes their eye. As can be seen in Figure 2a, the lens 2 tends generally to drape across the surface of the subject' s cornea 4. similar to a conventional soft contact lens. In particular, a central portion 10 (denoted by a double line) of the lens 2, comes into contact with the cornea over a central part which is similar or greater in area than that for the RGP lens seen in Figure la. Substantially immediately adjacent to the central portion 10, is an intermediate portion 12, shown with a dotted line in Figure 2a. The intermediate portion 12 has at least one region 14 in which the rear surface of the contact lens has a concave section and so is raised or separated from the cornea 4. This results in a corresponding protruding ridge in the front surface of the intermediate portion 12 of the contact lens 2. In the illustrated embodiment, the raised or separated region 14 forms a continuous, symmetrical annulus around the central portion 10 of the lens -this constitutes a preferred embodiment of the invention, but is not essential.

Beyond the intermediate portion 12 is a peripheral portion 16, also shown with a is dotted line. The curvature of the peripheral portion 16 more closely conforms to the curvature of the cornea.

When the subject closes their eye, as shown in Figure 2b, pressure is exerted by the subject's eyelids on the lens (again, denoted by downward arrows, as in Figure lb).

This pressure has several effects. The central portion 10 of the lens is brought into contact with the cornea over a similar or slightly increased area compared to that in figure 2a and the cornea is slightly flattened. More noticeably, the ridge or "bulge" 14 in the intermediate portion 12 is greatly reduced in height as the lens is pushed against the cornea. Desirably however, as illustrated in the embodiment shown in Figure 2b, the bulge is not removed entirely and the intermediate portion 12 of the lens 2 remains raised or separated from the cornea, at least over a part of the intermediate portion 12. The peripheral portion 16 of the lens 2 does come into contact with the cornea and exerts some pressure thereon.

The net effect is that, when the subject closes their eye, the pressure distribution profile across the cornea when using a soft orthokeratology lens in accordance with the invention is very similar to that produced on the cornea by a conventional RGP contact lens, as demonstrated by computer modelling.

Referring to Figure 3, the Figure shows a schematic representation of a part of a soft ortho K lens in accordance with the invention, in situ on the eye of a subject, when the subject's eye is closed, and in the upper part of the figure there is shown a graph of contact pressure (arbitrary units) against distance from the centre of the cornea (the graph being aligned with the representation of the contact lens). The arrows in the schematic representation indicate contact between the soft lens and the subject' s cornea, with the length of the arrows being proportional to the contact pressure on the cornea at the indicated location. It can be clearly seen that there is contact pressure on to the cornea where the cornea contacts the central portion of the soft ortho K lens.

There is also significant contact pressure on the cornea at the extreme outer periphery of the soft ortho K lens. However, where the cornea underlies the intermediate portion 12 of the soft ortho K lens, there is no contact pressure. The pressure distribution profile illustrated in Figure 3 for the soft lens of the invention is closely similar to that observed for conventional RGP ortho K contact lenses (see Figure 5).

Example 2 -Finite Element Analysis (FEA) Referring to Figures 4a and 4b, there is shown a representation of FE analysis of a soft ortho K lens in accordance with the invention in situ, before (Figure 4a) and after (Figure 4b) the subject's eye is closed. Just over half of the lens profile is shown. Of particular note in this image are; the draping of the lens around the cornea; the radial stretching of the lens towards the limbus; the contact between the lens and the cornea (4) in the central (10) and peripheral (16) portions; and the gap between the lens and the cornea at the intermediate portion (12). Further, it can be seen how the front surface profile of the contact lens becomes smoother when the subject closes their eye, which smooth profile is desirable for optimum wearer comfort.

In addition to this, a figure illustrating how the meridian-averaged contact pressure (when the subject's eye is closed) varies with distance away from the corneal apex is shown for this model, (Figure 5). It is clear from Figure 5 that, for the illustrated embodiment of a soft ortho K contact lens in accordance with the invention, the lens contacts the central and peripheral zones of the cornea and applies pressure thereto, whilst there is no contact with, or pressure on, the subject' s cornea in the intermediate zone. For comparison, an example RGP contact lens contact pressure curve is also presented. By comparing the two curves it is evident that essentially the same pressure distribution profile as observed for a conventional RGP ortho K contact lens can be generated using a soft orthokeratology lens. More specifically, the soft ortho K lens in accordance with the invention is able to generate pressure on the cornea in the central zone, which is both generally uniform and of a magnitude similar to that exerted by the conventional RGP ortho K lens, and both lenses exert zero contact pressure in the intermediate zone.

In order to effectively design orthokeratology lenses using finite element analysis (FEA performed using Simulia AbaqusTM, available from Dassault Systemes), a link between the mechanical response of the cornea and the associated change in refractive power was sought. To achieve this, 42 RGP lens designs, with known intended refractive correction, were simulated on eye and the associated contact pressure Is distributions were recorded. A machine learning approach using a trained feed forward neural network with two hidden layers (see Bebis, George, and Michael Georgiopoulos. "Feed-forward neural networks." Ieee Potentials 13.4 (1994): 27-31) was then employed to identify the link between the contact pressure distribution on the cornea and the magnitude and distribution of corneal epithelial remodelling. The resulting change in corneal profile could then be used to compute the corneal refractive power. This approach provided a mapping tool linking the contact pressure distribution to the associated refractive power change. Predictions made by the tool, using the contact pressure distributions produced by the 42 RGP designs. These were plotted against the actual refractive correction for which these lenses would be prescribed in line with current prescription methods. The results are shown in Figure 6. With this tool, providing a link between contact pressure distribution, epithelial remodelling, and refractive changes in the cornea, it was possible to simulate soft ortho K designs and estimate the induced refractive power change, and design appropriate soft ortho K lenses in accordance with the invention.

The model was also developed in line with epithelial changes identified in a clinical study, where the corneal topography and tomography data of 17 clinical cases were measured before and after ortho K wear and were then modelled directly using finite element analysis. A comparison between the clinically measured epithelial thickness changes and the contact pressure curves outputted by FEA, for some of these clinical cases, is shown in Figure 7. In the Figure, for each clinical case, there is provided a graph of contact pressure (in kPa) against radial distance (in mm) from the centre of the cornea/contact lens, and the resulting change in epithelial thickness (in pm, determined by analysis of ocular coherence tomography images taken of the cornea before and after wearing the lens).

In general, it can be seen that the maximum increase in epithelial thickness occurred in the intermediate zone, where the contact pressure was lowest, or towards the junction between the central zone and the intermediate zone; whilst without exception, the smallest increase in corneal epithelial thickness (or, in some cases, the biggest decrease in the thickness) was observed in the central zone, where the contact pressure was relatively steady Example 3 -Preferred Contact Lens Rigidity The inventors modelled the contact pressure distribution (in kPa) on the cornea of a subject (with their eye closed) at increasing radial displacement (in mm) from the centre of the cornea/lens for a lens made of a silicone hydrogen material with a Young's modulus value of 0.8MPa (circle symbols), and for a contact lens with a Young's modulus value of 1.5MPa (cross symbols). The results are shown in Figure 8. It can be seen that for the less rigid contact lens (0.8MPa), a zone of zero contact [denoted by the shorter double headed horizontal arrow) between the cornea and the contact lens is very narrow (restricted to about 2.6mm to 2.8mm radial displacement from the centre). Accordingly, from the results described above, the zone of corneal epithelial thickening would be expected to be correspondingly very narrow, and if a material were used with a Young's modulus less than 0.8MPa, the zero contact zone would be expected to disappear entirely. In contrast, the lens having a Young's modulus of 1.5MPa provides a zero contact zone [denoted by the longer double headed horizontal arrow] from about 2.5mm to 4mm radial distance from the centre of the cornea/lens, which should result in a substantial zone of corneal epithelial thickening.

Example 4 -an embodiment in accordance with the invention One embodiment of a contact lens in accordance with the invention is now described with reference to Figure 9, which is a schematic representation of a median section through a contact lens 2, showing the front (2') and back (2") surfaces of the contact lens, and the surface of the cornea 4, the latter depicted in the figure as being part of a spherical-curved surface. In reality, the surface of the cornea will typically deviate from perfect spherical curves. The contact lens has infinite rotational symmetry about its central optical axis (which is a preferred feature, as it simplifies manufacture of the lens, but this is not essential). The lens is nominally divided into 5 sections, numbered io 31-35 in Figure 9. Section 31 corresponds to the central portion 10 of the contact lens. Sections 32 -34, in combination, correspond to the intermediate portion 12; and section 35 corresponds to the peripheral portion 16.

The central portion 31 has a single, uniform radius of curvature. In the illustrated embodiment, the radius of curvature of the central portion is 8 mm, but suitable values for the radius of curvature of the central portion may be greater or less than this and will depend at least in part on the curvature of the central zone of the cornea of the individual for whom the contact lens is intended. Typical values for the radius of curvature of the central portion of the contact lens may be in the range 7.5 mm to 9.5 MITI.

The intermediate portion 12 of the contact lens comprises three sections, numbered 32-34, each section having a different radius of curvature. Of significance, at least one section (34) of the intermediate portion has a radius of curvature which is notably less than the radius of curvature of the underlying intermediate zone of the cornea. A consequence of this is that numbered sections 32 & 33 of the intermediate portion, which are closer to the central axis of the contact lens than is section 34, effectively "bulge" upwards, away from the cornea and, when the subject closes their eyelids, please sections of the intermediate portion do not contact the underlying zone of the cornea.

A suitable value for the average radius of curvature of the intermediate portion (i.e. averaged over sections 32, 33 and 34) will, as with the central portion, depend in part on the curvature of the intermediate zone of the cornea of the individual for whom the lens is intended. However, typical values for the average radius of curvature for the intermediate portion will be in the range 5.0 mm to 7.0 mm.

The peripheral portion 16 (equivalent to numbered section 35 in Figure 9) has a substantially uniform radius of curvature. Again, the radius of curvature of the peripheral portion of the contact lens will depend in part on the curvature of the peripheral zone of the cornea of the individual for whom the lens is intended. A typical value for the radius of curvature of the peripheral portion of the contact lens o will be in the range 8.5 mm to 9.5 mm.

Claims (16)

1. Claims I A soft contact lens, especially a soft contact lens formed of a material having a Young's modulus CE") value in the range 0.8 -3.0 MPa, adapted for use in orthokeratology such that, in use, when the contact lens is worn and the wearer closes their eye, the contact lens exerts a pressure on the eye having a distribution profile in which the pressure on the cornea is relatively high in a central zone of the cornea and is relatively low in an intermediate zone of the cornea outside of, but substantially adjacent to, the central zone, the contact lens comprising, sequentially, from the centre io of the lens outwards: a) a central portion which contacts the central zone of the cornea; b) an intermediate portion which overlies the intermediate zone of the cornea, at least part of the intermediate portion having a radius of curvature which is smaller than the radius of curvature of the intermediate zone of the cornea; and is c) a peripheral portion, which contacts at least part of a peripheral zone of the cornea, which peripheral zone is outside of the intermediate zone.
2. 2 A contact lens according to claim 1, being wholly or predominantly formed from a material having a Young's modulus in the range 0.8 -3.0 MN, preferably in the range 1.0-3.0 MPa, more preferably in the range 1.2 -3.0MPa, and most preferably in the range 1.5-3.0 MPa.
3. 3 A contact lens according to claim 1 or 2, having an average thickness in the central portion in the range 200 -300jtm, preferably in the range 220 -280jim
4. 4 A contact lens according to any one of the preceding claims, comprising or being predominantly formed of one or more of the following: non-hydrated PDMS; a hydrogel material, preferably a silicone hydrogel material;
5. 5 A contact lens according to any one of the preceding claims, comprising a hydrophilic polymeric coating on the front and/or rear surface.
6. 6. A contact lens according to claim 5, having a coating comprising one or more of the following: polyethylene glycol, polyvinylpyrrolidone, 2-methacryloyloxyethyl phosphorylcholine; and 2-hydroxyethylmethacrylate (HEMA).
7. 7. A contact lens according to claim 5 or 6, wherein the coating comprises polyethylene glycol with an average molecular weight in the range 200-9,500.
8. 8. A contact lens according to any one of the preceding claims, having a circular, or substantially circular, central zone.
9. 9. A contact lens according to claim 8, having an annular intermediate zone, concentric with the central zone.
10. 10. A contact lens according to claim 9, having an annular peripheral zone, concentric Is with the central zone.
11. 11. A contact lens according to any one of the preceding claims, wherein the average radius of curvature of the central portion is in the range 7.0 mm to 10.0 mm, preferably in the range 7.5 mm to 9.5 mm.
12. 12. A contact lens according to any one of the preceding claims, wherein the average radius of curvature of the intermediate portion is in the range 5.0 mm to 7.0 mm, preferably in the range 4.0 mm to 8.0 mm.
13. 13. A contact lens according to any one of the preceding claims, wherein the average radius of curvature of the peripheral portion is in the range 8.0 mm to 11.0mm, preferably in the range 8.5 mm to 9.5 mm.
14. 14. A method of making a soft contact lens adapted for use in orthokeratology, the contact lens being in accordance with any one of the preceding claims, the method comprising the steps of: i) designing or selecting a rear surface geometry for the contact lens, based on data describing the topography of the surface of the cornea for an individual subject's eye, or based on data describing the averaged topography of the surface of the cornea for the eye of a plurality of subjects; ii) optionally, modelling the optical characteristics and/or modelling the fit characteristics of the designed or selected rear surface geometry of the lens on eye, and altering the designed or selected near surface geometry if required for better conformity with the desired optical characteristics and/or fit characteristics; and iii) manufacturing a contact lens in accordance with the first aspect of the invention having the designed or selected rear surface geometry from step (i) or as altered in step (ii).
15. 15. A method of treating a myopic condition in an individual in need of such treatment, the method comprising the steps of: obtaining information on the shape of the surface of the cornea of the subject's eye requiring treatment and preferably also obtaining information on the nature or extent Is of the myopic condition; and making a soft contact kens in accordance with any one of claims 1-13 to fit the eye of the subject and to cause a desired pattern of corneal epithelial reshaping.
16. 16. A method according to claim 15, wherein the contact lens in accordance with claims 1-H is worn at night, preferably every night, until the myopic condition has been successfully treated.