Intraocular lens
The Invention concerns an intraocular lens with negative spherical aberration and a method of determining the refractive power of intraocular lenses. In the environment of immersion medium the intraocular lens refracts an incoming wave with an elliptically oblongly curved wave front into an outgoing wave with a substantially spherical wave front.
The invention concerns an intraocular lens (IOL) and a method of determining the imaging properties of intraocular lenses.
Lenses of that kind are known. The topology of conventional intraocular lenses generally involves spherical curved surfaces whose imaging properties however are not ideally suited to producing an image on the retina of the human eye. Known methods of determining the imaging properties of intraocular lenses therefore presuppose basically spherically curved surfaces.
The object of the invention is to provide an intraocular lens whose imaging properties produce an image of improved quality on the retina. A further object of the invention is to provide a method of determining the imaging properties of the intraocular lens, which method provides reliable results independently of the topological nature of the lens.
According to the invention that object is attained by an intraocular lens with negative spherical aberration. Conventional, spherically curved intraocular lenses of positive refractive power have a positive spherical aberration, that is to say they refract an incoming wave with a plane wave front into an outgoing wave with an elliptically oblongly curved wave front. The focus of such a lens is accordingly not punctiform. In comparison the intraocular lens according to the invention is preferably of such a configuration that, in the environment of immersion medium, in particular the in vivo environment (refractive index 1.336) in the eye it refracts an incoming wave with an elliptically oblongly curved wave front into an outgoing wave with a wave front which is substantially spherical. In that way the imaging properties of the cornea of the eye, which is in front of the IOL, are better taken into consideration and the effect is that more accurate focusing on the retina is possible.
Such imaging properties are preferably achieved by the refractive index and the curvature of the lens surfaces being so selected that the lens at the centre has a refractive power D of greater than or equal to +3. dioptres (dpt) in the immersion medium and that in the environment of air an incoming wave with a substantially plane wave front is refracted into an outgoing wave with a hyperbolic wave front.
The shape of the curvature of the wave fronts and also the curvature of the lens surfaces can be described by the following function:
y2=px−(1+asph)x2 (1)
wherein x coincides with the direction of light propagation or the lens thickness, y specifies the direction perpendicular thereto, radially outwardly with respect to the lens centre, p is any parameter and asph is so-called asphericity, that is to say a measurement in respect of the deviation of the curvature of the lens surface from a spherical shape. The shape of the lens surface or wave front is shown in section for different asphericities in
Preferably the hyperbolic wave front of a wave produced from an incoming plane wave by the lens according to the invention has an asphericity (asphOUT) of less than or equal to −1. Also the intraocular lens preferably has at least one convexly curved surface whose curvature is of an asphericity (asphL) of less than or equal to −1.
The invention is described in greater detail hereinafter by means of embodiments by way of example with reference to the Figures in which:
The imaging conditions taken into account in relation to the IOL according to the invention, in the human eye, are investigated hereinafter. As is known the cornea has a refractive index of about 1.37 topographically it essentially represents an aspheroidal dish. It has a negligibly slight influence on refraction of an incoming wave. Refraction of the incident light depends rather on the one hand on the curvature which is predetermined by the topography of the cornea but on the other hand on the refractive index of the immersion medium behind the cornea (aqueous humour). As is known, that has a refractive index of 1.336. The topography of the cornea is characterised by the asphericity (asphc), for which the literature specifies values of asphc=−0.26±0.18 (Kiely et al, in G Smith et al, Vision Research 41, 2001, 235-43) and asphc=−0.18±0.15 (Guillon et al, loc. cit.). In accordance with those literature values it can be assumed that the cornea of the human eye is generally elliptically curved. For the following considerations, a value range of asphc=−0.56 to 0 is therefore assumed for the asphericity of the cornea, in order to ensure that practically all human cornea asphericities occurring in nature are embraced. In that respect it is to be observed that the upper limit value (asphc=0) corresponds to a cornea with spherical curvature.
In addition the topography of the cornea is characterised by its surface refractive power at the centre point, that is to say on the optical axis. A range of 40 to 50 dioptres (dpt) is assumed for that purpose, whereby the range of the surface refractive power of the cornea, which actually occurs in nature and which in accordance with knowledge at the present time is at 43 dpt as an average value, is masked both towards higher and also lower values.
Preferably the IOL according to the invention is so designed that, in the environment of immersion medium, an incoming wave with an elliptically oblongly curved wave front is refracted into an outgoing wave with a substantially spherical wave front, wherein the refractive power of the IOL is to be so selected in dependence on the eye of the patient that the centre of the outgoing waves is on the retina of the eye.
The IOL according to the invention can assume various configurations: in accordance with a first embodiment, at its centre, in the environment of the immersion medium, it has a refractive power DI of at least +3 dpt and the refractive power decreases towards the edge of the lens. In addition by way of example a refractive index of 1.46, a lens diameter of 6 mm and an axis-parallel edge thickness of 0.25 mm is assumed to apply.
In addition
It is to be seen from
It can also be seen from
A conventional IOL with spherically curved surfaces in contrast has a positive spherical aberration, that is to say it refracts an incoming wave with a plane wave front into an outgoing wave with an elliptically oblongly curved wave front. That basically applies in regard to the positive refractive power of the lens, that is to say both in air and also in the immersion medium, insofar as the refractive index of the lens material is greater than that of the environment medium, with a refractive index of the lens material of 1.46 in particular therefore also in the immersion medium.
By virtue of a measurement of the waveform of the outgoing wave, with a known refractive index therefore, an IOL according to the first embodiment of the present invention can be distinguished from an intraocular lens according to the state of the art when it is illuminated with a plane wave. And more specifically suitable measurement can be effected in vitro in a standardised measuring structure and does not need to be implemented in the human eye. An example of such a measuring structure is shown in
In both of
The IOL in accordance with a second embodiment of the present invention has a central refractive power in the immersion medium DI of a maximum of −2 dpt. Such an IOL according to the invention also refracts an incoming wave with an elliptically oblongly curved wave front into an outgoing spherical wave, with suitable curvature for the lens surface, that is to say with a refractive power which decreases towards the lens edge (negative spherical aberration). Such a lens according to the invention converts an incoming plane wave into an outgoing wave with an elliptically oblongly curved wave front.
As already mentioned a conventional spherical lens of positive refractive power converts an incoming plane wave into a wave with an elliptically oblongly curved wave front, that is to say the refracted edge beams experience greater deflection than the central beams. In other words spherical lenses with a positive refractive power have a positive spherical aberration. Accordingly, aberration is negative in the case of a spherical lens with negative refractive power. Such a lens converts an incoming plane wave into an outgoing wave with an also elliptically oblongly curved wave front.
That can be seen from following Table 1 comparing the asphericities of outgoing waves after refraction of an incoming plane wave by intraocular lenses which are in accordance with the invention and which are conventional, each with the same central (nominal) refractive power in immersion DI both for the environment medium air and also for the immersion medium.
In accordance with the data assembled in Table 1 the wave fronts measured in the immersion medium, in investigation of the IOL according to the invention, have a positive asphericity which is 1600 to 20 times greater in comparison with a conventional spherical IOL, in each case in dependence on the refractive power of the lenses. In the case of refraction in air the wave fronts produced by the IOL according to the invention, in comparison with the wave fronts produced by a conventional spherical lens, have a positive asphericity which is increased by 500 to 8.5 times, once again dependent in each case on the refractive power of the lenses. In particular the asphericity of a spherical IOL with negative refractive power in air, independently of the magnitude thereof, does not reach any values which are greater than +10. The two outgoing waves can therefore be easily distinguished by measurement of their asphericity. With the refractive power of the lenses being known therefore once again it is possible by means of the apparatus shown in
An intraocular lens according to a third embodiment of the present invention has a central refractive power of between +2 dpt and −1 dpt in the immersion medium. In this case also the refractive power of the IOL according to the invention is lower at the edge than at the centre.
Claims
1. An intraocular lens
- having a configuration such that, in each an immersion medium environment, an incoming wave with an elliptically oblongly curved wave front is refracted into an outgoing wave with a substantially spherical wave front.
2. An intraocular lens according to claim 1,
- wherein the lens has a positive refractive power in the environment of immersion medium and a negative spherical aberration.
3. An intraocular lens according to claim 2,
- wherein the lens has a refractive power at the center of the lens which in the environment of immersion medium is greater than or equal to +3 dpt, and wherein the lens is so configured that, in an air environment, an incoming wave with a substantially plane wave front is refracted into an outgoing wave with a hyperbolic wave front.
4. An intraocular lens according to claim 3.
- wherein the hyperbolic wave front has an asphericity of less than or equal to −5.
5. An intraocular lens according to claim 3,
- wherein the lens has at least one convexly curved surface whose curvature has an asphericity of less than or equal to −1.
6. An intraocular lens according to claim 1.
- wherein the lens has a refractive power at the center of the lens which in the immersion medium environment is at most +2 dpt and at least −1 dpt, and wherein the lens is so configured that an incoming wave with a substantially plane wave front is refracted into an outgoing wave whose apex surface has a meridian with an inflexion point.
7. An intraocular lens according to claim 1,
- wherein the lens has a refractive power at the center of the lens which in the immersion medium environment is less than or equal to −2 dpt, and wherein the lens is so configured that an incoming wave with a substantially plane wave front is refracted into an outgoing wave with an elliptically oblongly curved wave front whose aspericity measured in air is greater than +10.
8. A method of determining imaging properties of an intraocular lens, comprising:
- producing a parallel light beam,
- orienting the light beam on to the intraocular lens,
- breaking the light beam refracted by the intraocular lens down into a plurality of focused beams via a lens arrangement, and
- detecting the local distribution of the focus beams focused by the lens arrangement.
9. An intraocular lens according to claim 5, wherein the hyperbolic wave front has an asphericity of less than or equal to −5.
Type: Application
Filed: Jun 4, 2004
Publication Date: Jul 27, 2006
Inventors: Werner Fiala (Wien), Christine Kreiner (Munich)
Application Number: 10/559,376
International Classification: A61F 2/16 (20060101);