WO2017006113A1

WO2017006113A1 - Intraocular lens

Info

Publication number: WO2017006113A1
Application number: PCT/GB2016/052032
Authority: WO
Inventors: Daniel Purchase
Original assignee: Rayner Intraocular Lenses Limited
Priority date: 2015-07-06
Filing date: 2016-07-06
Publication date: 2017-01-12
Also published as: GB201511794D0; GB2540144A

Abstract

A multifocal intraocular lens 'IOL' comprises an optic having an optical centre and an optical axis through the optical centre, the IOL optic comprising a first portion having a first refractive index and a second portion having a second refractive index. The first and second portions are contiguous with one another and are non-circularly symmetric about the optical axis. The first refractive index is different than the second refractive index so as to provide different refractive powers for the first and second portions, and thereby multifocal capability. Also provided is a method of manufacture of the multifocal IOL.

Description

INTRAOCULAR LENS

Field of the Invention

The present invention relates to an intraocular lens (IOL), and in particular to a multifocal intraocular lens (MIOL) with varying refractive index for the optic.

Background of the Invention

Intraocular lenses (lOLs) are now widely used in the treatment of ophthalmic conditions, both to replace the natural lens of the subject or as a secondary lens to supplement the natural lens or primary replacement IOL. Such lOLs have steadily become more sophisticated with improved designs for the IOL haptics and for the annular rims of the IOL optic to ensure accurate and stable location within the lens cavity of the eye. Moreover, the design of the IOL optic itself has evolved, with the introduction of aspheric and toric designs to supplement the more conventional spherical design. Another development has been the realisation of multifocal functionality, which may also employ or complement aspheric or toric IOL designs.

Typically, multifocal IOL designs have employed a zonal configuration for the optic to provide differing refractive and/or diffractive power in order to achieve multifocal operation, such as described in WO 01/04667A1. Annular zones of different curvature provide different refractive power (different focal length) for each of the zones. This enables the lens to have at least two foci which can provide both distance and near vision for a patient. Whilst relatively effective, such designs can be complicated to manufacture, often requiring high tolerance machining, and can introduce unwanted diffractive effects.

More recently, another approach to multifocal functionality has been the so- called "sector" lens, whereby the refractive power of adjacent sectors of the lens optic differs due to a differing surface shape. Typically, this is achieved through a recessed sector, such as described in US8696746B and EP2219065B. Designs of this type can be more complex to manufacture than zonal types, but generally introduce less unwanted diffractive effects. However, such designs still place certain demands on machining tolerances and there can be unwanted artefacts due to the transition region between adjacent sectors.

A further design of MIOL employs gradient refractive index optics, whereby an inner central lens is embedded within an outer lens having a different refractive index, a so-called "lens within a lens". RU2234417C2 describes such a lens, which provides an effective radial gradation in refractive index of the composite lens due to the internal variation in structure of the lens. This approach mitigates problems associated with designs that require a relief structure on the outer lens surface, but introduces other complexities associated with forming a lens within a lens.

Accordingly, there is a need for an alternative design of multifocal intraocular lens (MIOL), which addresses some of the problems associate with existing MIOL designs.

Summary of the Invention

According to a first aspect of the present invention there is provided a multifocal intraocular lens, IOL, comprising an optic having an optical centre and an optical axis through said centre, the IOL optic comprising a first portion having a first refractive index and a second portion having a second refractive index, said first and second portions contiguous with one another and non-circularly symmetric about said optical axis, wherein the first refractive index is different than the second refractive index, whereby to provide different refractive powers for the first and second portions.

Preferably, the first portion and the second portion are bounded on one side of the IOL optic by a first region and a second region respectively of a first surface of the IOL optic, said first and second regions contiguous with one another and non-circularly symmetric about said optical axis.

In this way a sector type multifocal IOL is realised, which has the benefits of providing different refractive focussing powers without the complications associated with either surface relief structures or embedded structures. Since the outer surface of the lens can be continuous in shape, unwanted diffractive effects are also avoided. The concept can be extended to a plurality of sectors by having more than two portions of the IOL optic of different refractive index. For example, the lens may include three or four different portions, thereby providing for further different optical focussing powers within the same IOL. Therefore, in some embodiments, the multifocal IOL further comprises a third portion having a third refractive index, said third portion contiguous with at least one of said first and second portions and non-circularly symmetric about said optical axis, wherein the third refractive index is different than the first refractive index and the second refractive index, whereby to provide a different refractive power for the third portion. Preferably, the third portion is bounded on the one side of the IOL optic by a third region of the first surface of the IOL optic that is contiguous with the at least one of said first and second portions and is non- circularly symmetric about said optical axis. In many embodiments the first surface of the optic is curved and unmodulated. In some embodiments the first surface of the optic is spherical or aspherical. The surface curvature provides for refractive focussing and the refractive index of the underlying lens material determines the precise focal power, which can thus vary between the first and second portions of the optic.

The relative size and positioning of the differing refractive index portions can vary according to the application. In some embodiments the first and second regions are configured as sectors of the first surface of the optic bounded by two radii from the optical axis and a peripheral edge of the optic. The relative size of the two sectors can be adjusted according to particular requirements.

In some preferred embodiments the first portion comprises between 30% and 70% of the optic and the second portion comprises between 70% and 30% of the optic. This provides an optimum split for many applications. Where the lens is divided into just two portions, these add up to 100% of the optic.

In some embodiments the first refractive index is lower than the second refractive index, whereby the second portion provides an add-on refractive power relative to the refractive power of the first portion.

In such embodiments it is preferred that the refractive power of the first portion is between 10 and 30 dioptres and the add-on power provided by the second portion is between 2 and 5 dioptres. It is also preferred that the first portion comprises between 55% and 65% of the optic and the second portion comprises between 45% and 35% of the optic.

In many embodiments the first portion of the IOL optic forms a lens for distance vision and the second portion of the IOL optic forms a lens for near vision.

In some preferred embodiments there is a transition region at a junction of the first and second portion, the transition region characterised by a gradation in actual or effective refractive index between the first refractive index and the second refractive index. This can provide a smoother transition in refractive index between the two portions and thereby alleviate optical artefacts associated with abrupt changes in refractive index.

In some such embodiments the first and second portions interlock, whereby to provide an effective gradation in refractive index via the shape and refractive index of the two interlocking portions. Alternatively, the transition region may comprise a varying material composition, whereby to provide an actual gradation in refractive index. The first and second portions of the optic may be formed from different materials, whereby to provide the different refractive indices. Alternatively, the first portion and the second portion are formed from the same host material but are differently doped, whereby to provide the different refractive indices. The IOL optic may be formed by bonding together first and second portions formed of materials having different refractive index, or at least of different composition that can subsequently be altered to change the refractive index. Alternatively, the IOL optic may be formed from a unitary piece of material, which is then altered though a chemical process to form first and second portions of different refractive index. Such process may include photopolymerization, whereby components incorporated into a host matrix are photopolymerized upon exposure to optical radiation of a particular wavelength, for example visible or UV radiation. Selective spatial exposure can be used to define the first and second portions, for example through masking.

Similar processes may be employed to define a transition region with graded refractive index between the first and section portions.

According to a second aspect of the present invention there is provided a method of fabricating a multifocal IOL comprising the steps of:

forming a monofocal IOL optic of a material having a refractive index and being capable of refractive index change upon exposure to light characterised by a particular wavelength;

exposing a section of the monofocal IOL optic to light characterised by the particular wavelength, whereby to change the refractive index of said section and thereby form the multifocal IOL. The monofocal lens may be fabricated via lathe turning or moulding to provide a desired surface form factor and monofocal optical power for the IOL optic.

As will be appreciated by those skilled in the art, the present invention provides a new type of multifocal IOL based on a sector design which may take many forms. Any number of different focal lengths may be accommodated by providing a commensurate number of sections of different optical power by virtue of a different refractive index. The shape and proportions of the sections can be adapted according to the particular design and application. Moreover, the multifocal lOL can be fabricated with a common surface shape for the different sections, thereby simplifying the manufacturing process. Instead the multiple optical powers are achieved through a change in refractive index, which can be realised through a number of different processes. Further variations and embellishments to the multifocal lOL and its method of fabrication will become apparent to the skilled person in light of this disclosure.

Brief Description of the Drawings

Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows an example of an lOL comprising an optic and two haptics;

Figures 2A and 2B show, respectively, a side view and a plan view of a conventional monofocal lOL optic;

Figures 3A and 3B show schematics of a multifocal lOL optic according to the invention with two equal sectors;

Figures 4A and 4B show schematics of a multifocal lOL optic according to the invention with two unequal sectors in the ratio 60:40;

Figures 5A and 5B show schematics of a multifocal lOL optic according to the invention with three sectors in the ratio 60:20:20;

Figure 6 shows the variation in optical power with material refractive index for an equi-bi-convex lens with a base optical power of approximately 20 D;

Figure 7 shows the variation in optical power with material refractive index for an equi-bi-convex lens with a base optical power of approximately 10 D;

Figure 8 shows the variation in optical power with material refractive index for an equi-bi-convex lens with a base optical power of approximately 30 D;

Figure 9 shows the refractive index profile of a two-sector lOL optic according to the invention with a step change in refractive index;

Figure 10 shows the refractive index profile of a two-sector lOL optic according to the invention with a gradient refractive index transition region; and, Figure 11 illustrates a method of manufacturing a two-sector IOL optic according to the invention with a step change in refractive index.

Detailed Description

Figure 1 illustrates an example of an IOL 10 having an optic 1 1 , comprising convex faces (only one face 12 is shown) and two haptics 13a and 13b. Each haptic comprises an aperture, respectively 14a and 14b. Opposed points of each aperture, at 15a and 16a, and at 15b and 16b, are shown. These haptics are designed to be compressible for ease of insertion of the IOL in a subject's eye. Furthermore, in this particular example, the lens comprises an annular rim

17 on the face 18 of the optic 1 1 , although this need not be present. The face

18 of the optic 1 1 can have various surface profiles according to the particular application. As will now be described, the present invention provides a multifocal intraocular lens (MIOL) based on a new type of sector design for the MIOL optic, which utilizes regions of different refractive index to achieve different refractive power, rather than relying on varying surface shape. Although not without its own technical and manufacturing challenges, the invention provides for generally uniform curvature across its surface, thus negating many issues relating to known MIOLs, of both sector and zonal type designs.

Also, since the implementation of differing refractive indices is not subject to the same physical manufacturing constraints associated with surface relief lenses (e.g. tool tip radii), this means that the size and form of the transition zones between sectors can be better controlled to reduce light wastage. This is particularly true where the different refractive indices are achieved using refractive index modification technologies.

The invention thus provides a multifocal ophthalmic lens with at least two focal lengths (i.e. having at least two optical powers), which can provide both distance and near vision for a patient. The lens generally has a smooth surface, which may be of constant curvature. The change in optical power is obtained via different refractive indices at different locations across the lens.

Figure 2A and 2B illustrate, respectively, a side view and a plan view of a conventional monofocal IOL optic 20 having an equi-bi-convex lens form for the two optic faces, 21 and 22 around the optical axis OA. The optical power of such a lens can be deduced from the lens maker equation:

P is the power of the lens;

/ is the focal length of the lens;

n is the refractive index of the lens material;

is the radius of curvature of the lens surface closest to the light source;

R₂ is the radius of curvature of the lens surface farthest from the light source (= ?_x for an equi-bi-convex lens); and

d is the thickness of the lens (the distance along the optical axis of the lens between the two surface vertices).

It is noted that this formula can be adjusted for lenses where the surrounding medium is not air by replacing the factor (n-1 ) with (n - n_meciium), where n_meciium is the refractive index of the surrounding medium. Thus, it can be seen that the lens power is a function of the lens material refractive index, particularly for a fixed form factor. When length dimensions are specified in metres, the formula gives the focussing power of the lens in dioptres (D).

Sector Multifocal IOL

Figures 3A and 3B show a simple embodiment of the invention, in which the lens 30 has a constant surface curvature, such as shown in Figure 2A, but is effectively split into two halves, 31 and 32, through its centre by virtue of a different refractive index for the two halves. Figure 3A shows the basic spatial form of the lens in terms of refractive index boundary 33, and Figure 3B illustrates schematically the different refractive index of the two sections, 31 and 32. In this embodiment, the refractive index of the bottom half 32 of the lens is higher than the top half 31 of the lens. Thus, according to the lens maker formula, the focal length of the bottom half will be shorter than the top half due to its high optical focussing power, thereby providing a bi-focal functionality for the IOL. Figures 4A and 4B show another embodiment of a MIOL according to the invention, in which the IOL optic 40 is split into two sectors, 41 and 42, of different refractive index to provide an intended energy split of 60:40 in terms of the ratio of far to near focus sectors. Figure 4A shows the basic spatial form of the lens in terms of refractive index boundary 43, and Figure 4B illustrates the different refractive index of the two sections, 41 and 42. As shown, when looking at the lens along the optical axis, the surface area (and the body) of the lens is split into two sectors, with a 60% upper sector 41 to provide for far focus and a 40% lower sector 42 to provide for near focus. The difference in refractive index effectively provides a desired 'add power' between the two sectors, for example 3.0 D relative to the base power.

Figures 5A and 5B show another embodiment of a MIOL according to the invention, in which the IOL optic 50 is split into three sectors, 51 , 52 and 53, each of different refractive index, so as to provide a trifocal MIOL. The arrangement is similar to the bifocal MIOL of Figures 4A and 4B, except that the lower sector is effectively split into two equal sectors 52 and 53 to provide an overall energy split of 60:20:20 between sectors 51 , 52 and 53. The relative focal power of each sector depends on its relative refractive index. Figure 5A shows the basic spatial form of the lens in terms of refractive index boundaries 54, 55 and 56, and Figure 5B illustrates the different refractive index of the three sections, 51 , 52 and 53. As shown, when looking at the lens along the optical axis, the surface area (and the body) of the lens is split into three sectors, with a 60% upper sector 51 to provide for far focus, for example, and two 20% lower sectors 52 and 53 to provide for different near focus, for example. The difference in refractive index effectively provides a desired 'add power' between the three sectors relative to the base power.

Of course, the relative size and positioning of the three sectors in an embodiment of the type shown in Figures 5A and 5B could be adapted according to the particular application. Moreover, the concept can be extended to MIOLs having four or more sectors of varying size and position, some having the same refractive index as other sectors or all having a different refractive index. In this way, a quadrifocal or higher order MIOL can be achieved.

Generally speaking, an lOL power range of 10 D to 30 D covers the majority of powers required for average human eyes to reach emmetropia, and it is possible with current lOL materials to achieve the necessary refractive index required to provide such focussing power in a multifocal lens whilst maintaining the lens surface radius.

Referring to the lens maker equation above, if the radii of curvature of the far and near sectors of the lOL are kept the same, then the optical power of the respective sectors can be controlled using the refractive index. A relatively common optical power for an lOL is 20.0 D. For such a lens having an equi-bi- convex lens form with a 16.9 mm radius of curvature and a 1 mm thickness, and located in situ in aqueous humour {n_meciium =1.336), the lens power varies with refractive index of the lens material in the manner shown graphically in Figure 6. Thus, as shown, the optical power of the lens varies from about 19.4 D to about 22.9 D as the refractive index of the lens material varies from 1.5 to 1.53.

Therefore, if the far focus sector of the lOL optic were made with refractive index 1.505 and the near focus sector made with material refractive index 1.531 , then the lens would have two optical powers of approximately 20 D and 23 D. This is a multifocal lens with a base power of 20 D and an add power of + 3 D, the latter being achieved through a refractive index change of about 0.026 between the two sectors. Figure 7 shows graphically how lens power varies with refractive index for an equi-bi-convex IOL with a 33.0 mm radius of curvature and a 1 mm thickness, and located in situ in aqueous humour {n_meciium =1.336). As shown, the optical power of the lens varies from about 9.9 D to about 13.25 D as the refractive index of the lens material varies from 1.5 to 1.555.

Therefore, if the far focus sector of the IOL optic were made with a refractive index of a little over 1.5 and the near focus sector made with material refractive index a little over 1.55, then the lens would have two optical powers of approximately 10 D and 13 D. This is a multifocal lens with a base power of 10 D and an add power of + 3 D, the latter being achieved through a refractive index change of about 0.05 between the two sectors.

Figure 8 shows graphically how lens power varies with refractive index for an equi-bi-convex IOL optic with a 1 1.0 mm radius of curvature and a 1 mm thickness, and located in situ in aqueous humour {n_meciium =1.336). As shown, the optical power of the lens varies from about 29.7 D to about 33.3 D as the refractive index of the lens material varies from 1.5 to 1.52. Therefore, if the far focus sector of the IOL optic were made with a refractive index of about 1.502 and the near focus sector made with material refractive index a little over 1.518, then the lens would have two optical powers of approximately 30 D and 33 D. This is a multifocal lens with a base power of 30 D and an add power of + 3 D, the latter being achieved through a refractive index change of about 0.018 between the two sectors.

Graded Transition Region

In the embodiments described above, the implication has been that the change in refractive index between the between the far and near sections of the lens is an abrupt step change, and this is certainly the case in some simpler embodiments. Figure 9 shows such a step change in the refractive index profile of a 6 mm diameter IOL along a diameter at 90° to the boundary in refractive index change. For example, this may be the refractive index profile along the vertical diameter from top to bottom through the lenses shown in Figures 3A and 3B or Figures 4A and 4B.

The IOL optic whose refractive index profile is shown in Figure 9 has a base power of about 20 D in the far section and an add power of +3 D in the near section, similar to the embodiment described with reference to Figure 4. As illustrated, the refractive index is constant at about 1.5 in the near half of the lens and steps up at the midpoint to about 1.53, remaining at this level for the near half of the lens.

While simpler in form and easier to manufacture, the abrupt change in refractive index can lead to some undesirable optical artefacts and optical losses. Therefore, in some embodiments it is desirable to have a transition region between the two sections of the IOL, in which the refractive index is spatially graded so as to vary gradually from the refractive index of one segment to the refractive index of the other segment.

Figure 10 shows the refractive index profile of an IOL optic similar to that shown in Figure 9, but in which there is a gradient refractive index change over a transition region in the centre of the lens. Specifically, as shown, the refractive index increases linearly from a value of 1.5 at a position of 2.5 mm along the lens diameter to a value of 1.53 at a position of 3.5mm along the lens diameter. Thus, the transition region occupies 1 mm at the centre of the lens and gives rise to a gradually increasing optical power from the far section to the near section, resulting in a final add power of +3 D in the near section. By employing a gradient profile and allowing the refractive index to increase over a controlled 'transition zone', it is possible to direct some light to an intermediate focus position, i.e. to a position between the far and near focus positions of the lens. In use in the eye, this would provide the ability for the IOL to have objects at an intermediate distance in focus on the retina. This may be particularly useful for users working with computer monitors and the like, which are typically at an intermediate distance from the user.

IOL Materials and Fabrication

The refractive indices specified in connection with the embodiments described above are possible with a number of currently available IOL materials. For example, considering two common IOL materials available from Alcon and Abbot Medical Optics (AMO), it is noted that the Alcon material has a refractive index of n=1.55 and the AMO material a refractive index of n=1.47. The refractive index difference of 0.08 between these materials is more than adequate to give the optical power difference required for the multifocal lOLs described previously.

The Alcon material is a copolymer of phenylethyl acrylate and phenylethyl methacrylate, whereas the AMO material is a copolymer of ethyl acrylate, ethyl methacrylate and 2,2,2-trifluoroethyl methacrylate. By mixing these monomers in different proportions it is possible to obtain polymers with different refractive index. Other alternative materials might be based on a polymer containing oligomers of urethane methacrylate(s), oligomers of carbonate methacrylate(s) and methacrylic acid in different proportions. Such materials can be produced with a refractive range of approximately 1.48 to 1.52.

Using materials of the type described above, the different sections of the lens could be moulded at the same time in the same mould, or alternatively joined together after fabrication. In the case of a two sector IOL, these would correspond to the far focus and near focus sections.

It is also possible to obtain localized refractive index change in some polymers via exposure to certain wavelengths of light. An example of such a process is the diffusion of higher index monomers or initiator to the light exposed area locally, which increases the refractive index. These changes can be made permanent by uniform exposure to consume the photosensitive species without driving diffusion. For some materials the magnitude of the change in refractive index may be partly determined by the strength and/or duration of the light exposure. This allows for careful control of the refractive index change within a potential range. Indeed, a spatially graded exposure could be utilised to achieve a gradient refractive index change, such as described above in connection with embodiments of the multifocal IOL having a gradient refractive transition region.

It is necessary to utilise a suitable polymer for fabricating an implantable IOL. One example of a material with suitable behaviour is Omnidex photopolymer from Dupont.

Using such techniques, a multifocal IOL according to the invention can be fabricated in an efficient and cost effective manner. In one embodiment a monofocal lens is first fabricated via lathe turning or moulding to provide the desired surface form factor. The resulting monofocal lens is then exposed in a spatially selective manner to modify the local refractive index, thereby creating the desired multifocal sector IOL. The spatially selective exposure can be achieved by scanning the light source over the IOL or by spatial masking parts of the IOL during exposure to the light.

In a simple embodiment of the type shown in Figures 2A and 2B, the IOL can be fabricated by forming a monofocal lens with the desired base power for the far focus section of the lens and then spatially exposing half of the lens with light illumination to modify and increase the refractive index of the exposed half by the amount required to achieve the desired add power. Figure 1 1 illustrates this technique, whereby the lower half 1 12 of the monofocal IOL optic 1 10 is exposed to optical radiation 1 13 to increase its refractive index, whilst the upper half 1 1 1 of the monofocal IOL optic 1 10 is shielded from the optical radiation 1 13 by mask 1 14, such that its refractive index remains unchanged.

In this way, the near focus section of the lens is formed. The same approach can be adopted to fabricate the IOL shown in Figures 3A and 3B by modifying the spatial light exposure to match the shape of the near focus section. In addition to modifying the local refractive index it may also be possible to "fine tune" the spatial form of the lOL via a light exposure process that causes local and permanent swelling of the lOL material, thereby adjusting the surface shape and curvature of the lens. Such processes may include photopolymerization, whereby components incorporated into a host matrix are photopolymerized upon exposure to UV radiation. By appropriate choice of materials, both refractive index and spatial form of the lOL may be modified.

In summary, the present invention provides a multifocal lOL which can take a range of different forms broadly based on sections of different refractive index. Moreover, the multifocal lOL may be fabricated by a number of techniques according to the materials used. As will be appreciated by those skilled in the art, various modifications of the invention are possible based on the foregoing teaching.

Claims

1. A multifocal intraocular lens, IOL, comprising an optic having an optical centre and an optical axis through said centre, the optic comprising a first portion having a first refractive index and a second portion having a second refractive index, said first and second portions contiguous with one another and non- circularly symmetric about said optical axis, wherein the first refractive index is different than the second refractive index, whereby to provide different refractive powers for the first and second portions.

2. A multifocal IOL according to claim 1 , wherein the first portion and the second portion are bounded on one side of the optic by a first region and a second region respectively of a first surface of the optic, said first and second regions contiguous with one another and non-circularly symmetric about said optical axis.

3. A multifocal IOL according to claim 2, wherein the first surface of the optic is curved and unmodulated.

4. A multifocal IOL according to claim 2 or claim 3, wherein the first surface of the optic is spherical or aspherical.

5. A multifocal IOL according to any one of claims 2 to 4, wherein the first and second regions are configured as sectors of the first surface of the optic bounded by two radii from the optical axis and a peripheral edge of the optic.

6. A multifocal IOL according to any preceding claim, wherein the first portion comprises between 30% and 70% of the optic and the second portion comprises between 70% and 30% of the optic.

7. A multifocal IOL according to any preceding claim, wherein the first refractive index is lower than the second refractive index, whereby the second portion provides an add-on refractive power relative to the refractive power of the first portion.

8. A multifocal IOL according to claim 7, wherein the refractive power of the first portion is between 10 and 30 dioptres and the add-on power provided by the second portion is between 2 and 5 dioptres.

9. A multifocal IOL according to claim 7 or claim 8, wherein the first portion comprises between 55% and 65% of the optic and the second portion comprises between 45% and 35% of the optic.

10. A multifocal IOL according to any one of claims 1 to 4, further comprising a third portion having a third refractive index, said third portion contiguous with at least one of said first and second portions and non-circularly symmetric about said optical axis, wherein the third refractive index is different than the first refractive index and the second refractive index, whereby to provide a different refractive power for the third portion.

1 1. A multifocal IOL according to any preceding claim, wherein the first portion forms a lens for distance vision and the second portion forms a lens for near vision.

12. A multifocal IOL according to any preceding claim, wherein there is a transition region at a junction of the first and second portion, the transition region characterised by a gradation in actual or effective refractive index between the first refractive index and the second refractive index.

13. A multifocal IOL according to claim 12, wherein the first and second portions interlock, whereby to provide an effective gradation in refractive index.

14. A multifocal IOL according to claim 12, wherein the transition region comprises a varying material composition, whereby to provide an actual gradation in refractive index.

15. A multifocal IOL according to any of claims 1 to 14, wherein the first portion and the second portion are formed from different materials.

16. A multifocal IOL according to any of claims 1 to 14, wherein the first portion and the second portion are formed from the same host material but are differently doped.

17. A method of fabricating a multifocal IOL comprising the steps of:

forming a monofocal IOL optic of a material having a refractive index and capable of refractive index change upon exposure to light characterised by a particular wavelength;

exposing a section of the monofocal IOL optic to light characterised by the particular wavelength, whereby to change the refractive index of said section and thereby form the multifocal IOL.

18. A method according to claim 17, wherein the step of forming the monofocal IOL optic includes the step of providing a desired surface form factor for the optic by lathe turning or moulding.