U.S. patent application number 12/289789 was filed with the patent office on 2009-05-14 for aspheric intraocular lens and method for making the same.
Invention is credited to Mario Gerlach, Cedric Lesage.
Application Number | 20090125105 12/289789 |
Document ID | / |
Family ID | 38319564 |
Filed Date | 2009-05-14 |
United States Patent
Application |
20090125105 |
Kind Code |
A1 |
Lesage; Cedric ; et
al. |
May 14, 2009 |
Aspheric intraocular lens and method for making the same
Abstract
The invention relates to a novel artificial intraocular lens
(IOL) and a method for improving such a lens in the field of
opthalmology, with surface shape modifications that differ from
perfect spherical geometries. The intraocular lens takes into
account the natural optical configuration of the human vision
apparatus, for example, visual axis tilt and pupil decentration. In
addition, the method accounts for potential positioning errors
caused by implantation and surgery effects.
Inventors: |
Lesage; Cedric;
(Sainte-Soulle, FR) ; Gerlach; Mario; (Altenberga,
DE) |
Correspondence
Address: |
WALTER OTTESEN
PO BOX 4026
GAITHERSBURG
MD
20885-4026
US
|
Family ID: |
38319564 |
Appl. No.: |
12/289789 |
Filed: |
November 4, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2007/003674 |
Apr 26, 2007 |
|
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12289789 |
|
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Current U.S.
Class: |
623/6.23 ;
703/1 |
Current CPC
Class: |
A61F 2240/002 20130101;
G09B 23/28 20130101; G09B 23/30 20130101; A61F 2/164 20150401; A61F
2/1613 20130101 |
Class at
Publication: |
623/6.23 ;
703/1 |
International
Class: |
A61F 2/16 20060101
A61F002/16; G06F 17/50 20060101 G06F017/50 |
Foreign Application Data
Date |
Code |
Application Number |
May 5, 2006 |
DE |
10 2006 021 521.4 |
Claims
1. A method for designing an intraocular lens capable of adjusting
the aberrations of the eye in order to provide optimal vision
correction to patients, the method comprising the steps of:
providing a mathematical model eye that describes the optical setup
and performance of the natural human eye including at least one
aspherical corneal surface; a gradient index and/or aspherical
crystalline lens model; a visual axis that is tilted with respect
to the axis of symmetry of the eye; and, a decentered iris that
represents a decentered entrance pupil; determining the
performances of the mathematical model eye with respect to image
quality and spherical aberrations as a function of pupil diameter;
using a mathematical model describing the statistics of potential
lens misalignments and positioning errors induced by surgery or
wound healing processes; calculating the optical performance and
resulting aberrations employing said mathematical eye model
convoluted with the statistical model for lens displacements; and,
optically modeling an aspherical lens shape that replaces the
natural human crystalline lens in the eye that provides optical
power restoration while providing optical characteristics of the
human lens in order to cause the eye with the intraocular lens
having aspherical shape to have the same amount of spherical
aberrations and the same level of image quality than the
mathematical model eye as a function of the pupil diameter.
2. The method of claim 1, wherein the radial distribution of
refractive optical power is divided in at least three functional
zones that account for photopic, mesopic and scotopic vision.
3. The method of claim 1, wherein modeling and optimization of the
lens shape includes selecting the radii of base curvature of the
anterior and posterior surfaces as well as the central thickness,
the edge thickness and the refractive index.
4. The method of claim 1, wherein the amount of spherical
aberration of a mathematical model eye having the intraocular lens
is maintained at the same level as the mathematical model eye
having a human crystalline lens for pupil diameters ranging from
greater 0 to 4 mm.
5. The method of claim 1, wherein the modified lens shape is
defined in terms of a linear combination of polynomials.
6. The method of claim 1, wherein the modified lens shape is
defined by the equation: z = cr 2 1 + 1 - ( 1 + Q ) c 2 r 2 + k 2 r
2 + k 4 r 4 + k 6 r 6 + k 8 r 8 ##EQU00003## wherein:
c=r.sub.curv.sup.-1 (curvature=1/base radius of curvature)
r=independent variable, radius about optical axis Q=conic constant
k.sub.n=polynomial coefficient of order n.
7. The method of claim 6, wherein k.sub.2 is 0.
8. An intraocular lens capable of adjusting the aberrations of the
eye in order to provide optimal vision correction to patients, the
intraocular lens being produced by a method which comprises the
steps of: providing a mathematical model eye that describes the
optical setup and performance of the natural human eye including at
least one aspherical corneal surface; a gradient index and/or
aspherical crystalline lens model; a visual axis that is tilted
with respect to the axis of symmetry of the eye; and, a decentered
iris that represents a decentered entrance pupil; determining the
performances of the mathematical model eye with respect to image
quality and spherical aberrations as a function of pupil diameter;
using a mathematical model describing the statistics of potential
lens misalignments and positioning errors induced by surgery or
wound healing processes; calculating the optical performance and
resulting aberrations employing said mathematical eye model
convoluted with the statistical model for lens displacements; and,
optically modeling an aspherical lens shape that replaces the
natural human crystalline lens in the eye that provides optical
power restoration while providing optical characteristics of the
human lens in order to cause the eye with the intraocular lens
having aspherical shape to have the same amount of spherical
aberrations and the same level of image quality than the
mathematical model eye as a function of the pupil diameter.
9. The aspheric intraocular lens of claim 8, wherein the lens
comprises an anterior and a posterior surface, whereby at least one
of the anterior and posterior surfaces is aspheric and wherein the
optical properties of these surfaces account for a spherical
aberration equal or close to the spherical aberration of the human
eye.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation application of
international patent application PCT/EP 2007/003674, filed Apr. 26,
2007, designating the United States and claiming priority from
German application 10 2006 021 521.4, filed May 5, 2006, and the
entire content of both applications is incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The invention relates to a novel intraocular lens (IOL) and
a method for improving such a lens in the field of opthalmology,
including modifications of the surface shape that differ from a
perfect spherical geometry.
BACKGROUND OF THE INVENTION
[0003] The treatment of cataract, as the world's most common cause
for blindness, is a well known process since the time of ancient
Rome (first and second century AD). Since that time, the complete
removal of the opaque human lens is still the best choice to
partially restore visual acuity of the patient. The achieved
results are unexpectedly poor because of the disregarded refractive
contributions of the natural human lens to the visual apparatus
which are not adequately compensated in this situation.
[0004] A breakthrough in cataract surgery was made in 1949 when the
English physician Harold Ridley successfully implanted the first
intraocular lens made of hard PMMA plastics. This lens was capable
to compensate for lost optical power of the natural human lens.
Since this time IOLs and surgical techniques were continuously
improved. Today cataract surgery is by far the most performed
surgery in opthalmology with more than 2.3 million patients per
year in the United States and approximately another 3 million
surgeries in Europe and Japan.
[0005] The capacity of the human eye as an optical system can only
be accomplished if the artificial lens is properly positioned and
focused. If this condition is satisfied, the incident rays from
distant object points form only minimally blurred spots at the
retina and provide sharp vision. The correct adaptation of an IOL
to the individual human eye remains difficult and the postoperative
visual acuity of the patient depends on several factors.
[0006] Inaccuracies during measurement of the various ocular
geometries, inaccuracies during surgery and postsurgical effects
(such as surgical trauma and wound healing processes) limit the
achievable visual acuity due to positioning errors of the implanted
IOL. Positioning errors with respect to the optical axis mainly
cause defocusing while tilt and decentration of the IOL will result
in induced astigmatism and coma errors. Higher-order optical
aberrations will appear as well.
[0007] Up to the present time, different IOL design approaches deal
with these problems and try to mitigate the problems with
particular emphasis on certain aspects.
[0008] A selection of prior art lens designs is described in brief
hereinafter.
[0009] The equi-convex lens design (example Bausch & Lomb
LI61U) is the most used intraocular lens design in clinical
practice. Both lens surfaces are spherical with equivalent radii of
curvature. As a consequence, these designs produce a significant
amount of spherical aberration. Due to the strong increase of
spherical aberration with increasing pupil diameter, the patients
will very likely suffer from blurry vision and contrast loss under
mesopic/scotopic conditions.
[0010] The biconvex or plano-convex lens (example AMO sensar AR40)
is another lens design. The additional degrees of freedom allow
designing a "best shaped IOL" that provides minimal spherical
aberration that is achievable with spherical surfaces. The amount
of spherical aberration is significantly reduced as compared with
the above lens. Since the amount of spherical aberration (SA) is
still higher than with the natural human lens, the patient will
very likely suffer from blurry vision and contrast loss under
mesopic/scotopic conditions due to spherical aberration.
[0011] A wavefront optimized IOL (example Pharmacia, TECNIS Z9000)
is described in U.S. Pat. No. 6,609,793 B2. The anterior surface is
aspherical. The deviations from the base sphere are expressed as a
sixth order polynomial expansion. The IOL design is based on
averaged wavefront aberrometry data obtained on a large patient
cohort. The objective of the aspherization is to compensate for the
positive spherical aberration as induced by the normal human
cornea. The IOL has to provide a certain amount of negative
spherical aberration to bring the entire optical apparatus to zero
spherical aberration. As viewed from a theoretical optics
perspective, this design should provide maximum optical performance
at the narrowest possible point spread function. The lens TECNIS
Z9000 provides a diffraction limited optical performance in the
axis-near region. This holds true even for large pupil diameters of
6 mm. Such lens design, however, has also some disadvantages. Due
to the intended significant negative spherical aberration of the
lens, the latter becomes very sensitive with respect to
decentration that is likely to occur during the implantation and
after implantation during capsular bag symphysis. The diffraction
limited performance of the lens vanishes immediately even if
slightly decentered.
[0012] The "aberration-free IOL" (example Bausch and Lomb, SofPort
A0 and Akreos A0) is disclosed in United States patent publication
US 2005/203619 A1 and WO 2004/090611 A3. Both surfaces of the IOL
are aspherical and the shape is defined by a conic constant.
Considering the specific optical conditions behind the cornea, the
IOL does not introduce any additional spherical aberration into the
optical system. In other words, the IOL is "transparent" for the
incoming aberrations. Systems that do not introduce spherical
aberrations do not introduce coma while decentered. Therefore,
these lenses can be significantly decentered without losing
contrast when compared to the perfectly centered state. Since the
spherical aberration of the cornea is not affected by the IOL, this
amount of spherical aberration is manifest and limits the optical
performance in the axial region. The "aberration-free IOL" does not
correspond to the physiological properties of the natural human
lens and therefore can lead to sub-optimal results. This lens can
be used for eyes after refractive surgery, eyes with keratoconus or
with atypical corneal spherical aberration.
[0013] There are several other patent publications directed to the
subject of increasing the spherical aberrations in order to provide
depth of field or achieve pseudoaccommodation.
[0014] In United States patent publication US 2004/0230299 (Nov.
18, 2005), an oscillating surface superimposed on a spherical
surface is provided to produce different focus points forward and
rearward of the best focus in order to obtain an increased depth of
focus.
[0015] Patent publication WO 2005/046527 (May 26, 2005) discloses a
multizone monofocal lens. Each zone presents a positive or negative
gradient of refractive power proceeding from the base power of the
lens in order to produce an extended depth of field.
[0016] U.S. Pat. No. 6,126,286 (Oct. 3, 2000) discloses a multizone
monofocal lens to produce an improved depth of field.
[0017] European patent publication 1 402 852 (Sep. 29, 2003)
discloses a monofocal aspherical lens which permits a
pseudoaccommodation by providing an improved depth of field (by
increasing the amount of spherical aberrations).
SUMMARY OF THE INVENTION
[0018] It is an object of the invention to overcome the
disadvantages of the prior art and to provide significantly
improved perceivable optical performance for patients who need an
IOL implant.
[0019] The invention provides a new aspheric intraocular lens and a
method for making such an IOL that results in obtaining an
intraocular lens which provides significantly improved perceivable
optical performance to IOL patients.
[0020] The aspherical intraocular lens according to the invention
has an anterior and a posterior surface and at least one of the two
surfaces is aspheric. The optical properties of these surfaces
account for a spherical aberration equal to or approaching the
spherical aberration of the human eye.
[0021] In another embodiment of the invention, the IOL can be made
of a material that has a varying refractive index so that a
spherical aberration results equal to or approaching the spherical
aberration of the human eye.
[0022] The invention also relates to a method for making an
intraocular lens which can be adjusted to the aberrations of the
eye to provide an optimal vision correction for the patient. The
method includes the following steps: providing a mathematical model
eye that describes the optical setup and the performance of the
natural human eye including at least one aspherical corneal
surface, a gradient index and/or an aspherical model of the natural
eye lens, a visual axis that is tilted with respect to the "optical
axis of symmetry" of the eye and a decentered iris diaphragm that
represents a decentered entrance pupil; determining the
performances of the mathematical model eye in terms of image
quality and spherical aberrations as a function of the pupil
diameter; using a mathematical model describing the statistics of
potential lens misalignments and positioning errors induced by
surgery or wound healing processes; calculating the optical
performance and resulting aberrations with the aid of the
mathematical eye model convoluted with the statistical model for
lens displacements; and, optically modeling an aspherical lens
shape that replaces the natural human lens in the eye and corrects
for spherical vision errors while preserving specific optical
properties of the human lens in order to cause the pseudophakic eye
to have the same amount of spherical aberrations as a function of
the pupil diameter and the same level of image quality as the
phakic model eye.
[0023] In such a method either of the anterior or the posterior or
both surfaces of the lens can be of aspherical shape.
[0024] It is advantageous that the radial distribution of
refractive optical power is divided into at least three functional
zones that account for photopic, mesopic and scotopic vision.
[0025] Preferably the optical optimization of the aspherical shape
is performed in order to minimize the sensitivity of the optical
performance parameters with respect to a potential lens tilt
induced by surgical effects or capsular bag symphysis.
[0026] Advantageously, the optical optimization of the aspherical
shape is performed in order to minimize the sensitivity of the
optical performance parameters with respect to a potential lens
decentration induced by surgery effects or wound healing
processes.
[0027] A preferred way of modeling and optimizing the lens shape
includes selecting the radii of base curvature of the anterior and
posterior surfaces as well as the central thickness, the edge
thickness and the refractive index.
[0028] In the method according to the invention, the amount of
spherical aberration of the artificial lens is maintained at the
same level as that of the natural human lens over a broad range of
pupil diameters.
[0029] Preferably the modified lens shape is defined in terms of a
linear combination of polynomials.
[0030] The modified lens shape can be defined by the equation:
z = cr 2 1 + 1 - ( 1 + Q ) c 2 r 2 + k 2 r 2 + k 4 r 4 + k 6 r 6 +
k 8 r 8 ##EQU00001##
wherein:
[0031] c=r.sub.curv.sup.1 (Curvature=1/base radius of
curvature);
[0032] r=independent variable, radius about the optical axis;
[0033] Q=conic constant; and,
[0034] k.sub.n=polynomial coefficient of order n.
[0035] In this way, the constant Q can be 0 or between -1 and 0.
The coefficient k.sub.2 can be equal to 0 and the coefficients
k.sub.n for n>6 can be equal to 0.
[0036] Advantageously, the modified lens shape is defined in terms
of a linear combination of polynomials or by splines or is
piecewise defined by linear combinations of polynomials.
[0037] The optical performance can be defined as contrast according
to contrast transfer function or definition brightness (Strehl
ratio) or wavefront aberration or in terms of image point spread
functions and encircled energy.
[0038] The aberrations of the entire optical system of the human
eye can be expressed in linear combinations of Zernike or Seidel
polynomials or as Fourier decomposition of the optical path length
differences of the wavefront.
[0039] An aspheric intraocular lens according to the invention can
be made of soft material or hydrophilic material (such as
hydrophilic acrylic polymer or copolymer) or a hydrophobic material
(such as hydrophobic acrylic or silicone).
[0040] The aspheric intraocular lens according to the invention can
also be made of monobloc material with hard and soft zones such as
described in European patent publication 1 003 446 or of hard
material such as polymethylmethacrylate also known as PMMA.
[0041] In addition to correcting spherical vision errors, the
surface modifications according to the invention allow the
restoration of the optical properties of the natural human lens as
they existed prior to extraction. Further, the intentional
balancing of the anterior and posterior surface modulations
provides a minimum sensitivity of the optical performance with
regard to mechanical positioning disturbances, such as decentration
and tilt of the IOL, which can be induced by surgery inaccuracy,
surgical trauma or capsular bag symphysis.
[0042] This is achieved by intentional adjustment of the optical
aberrations in a way to make them similar to the effects of the
natural human crystalline lens.
[0043] The image formation in the natural human eye is accomplished
by the combination of the ocular media and their boundary surfaces.
The main contribution in refractive power (-75%) is provided by the
cornea which forms the first air/media interface of the human eye.
Rays emitted by distant object points enter the cornea almost
parallel to the optical axis. The refraction of the cornea deflects
the rays toward the optical axis and a converging bundle results.
This bundle of rays passes the anterior chamber and enters the
human crystalline lens. If no crystalline lens would be in place,
the rays would converge to a single diffraction limited small spot
at the distance of the inverse corneal refractive power. The spot
size is determined by the diffraction effects at the periphery of
the entrance pupil and the wavelength.
[0044] The optical system of the human eye is not perfect in terms
of physics but it has developed and optimized over the ages. The
slight aspherical shape of the cornea acts in conjunction with the
non-linear Snell's law of refraction and prevents that all rays
emitted from a distant point source converge at a single spot. It
appears that rays from the outer regions of the pupil hit the
optical axis at a shorter distance than the axial rays do. This
effect is called spherical aberration (in the following abbreviated
SA) and is provided with a sign. If the pupil peripheral rays hit
the optical axis before the axial rays do, the SA is considered to
be "positive". Positive spherical lenses show this behavior. If the
pupil peripheral rays hit the optical axis at a more distant point
than the axial rays, the SA is considered to be "negative". This
behavior is found with planoparallel glass plates or negative
lenses.
[0045] Since the peripheral rays of the cornea hit the optical axis
before the axial rays do, the cornea adds positive SA to the
optical system. This effect prevents the formation of infinitely
sharp macular images. Instead, blurred spots result because there
is much light diffusion. The evolution of the human eye accounted
for this effect by developing a highly complex crystalline lens
design. The crystalline lens contributes the missing 25% of
refractive power to the optical system in order to adjust the focal
length exactly to the available axial length of the human eye. In
addition, the lens allows the accommodation to different viewing
distances by internal adjustment of the refractive lens power.
Beyond these obvious facts, the crystalline lens acts as an optical
correction means of the human eye in that it compensates for
optical errors introduced by the cornea. In order to avoid
excessive spot blurring induced by the corneal positive SA, the
crystalline lens provides a well adjusted amount of negative SA
that almost completely compensates for the amount induced by the
cornea. The optical performance of this combined optical system is
significantly better than that of its individual components. This
inherent compensation mechanism functions even for different
viewing distances and different pupil diameters due to changing
lighting conditions.
[0046] The main objective of the evolution of the human eye was not
to optimize the theoretical optical performance of the eye in terms
of image point spread functions or Strehl ratios as currently
widely believed. Rather, the optical apparatus should provide an
optical performance that accurately matches the requirements of the
cone and rod structure of the retina, their local density functions
and color perception properties. The mosaic of cones and rods
permits only to see images with a maximum spatial frequency of 75
cpd; a higher spatial frequency can produce aliasing and distortion
of the perceived image as described by Y. K. Nio et al in the
article "Spherical and irregular aberrations are important for the
optimal performance of the human eye", Ophthal. Physiol. Opt. 2002,
22, pages 103 to 112. The optical properties of the visual
apparatus, the configuration of the retina and the physiological
processing of the visual information in the visual cortex determine
the perceivable visual acuity of the patient.
[0047] From the above, the main objectives of a novel intraocular
lens can be derived. The inventors came to the conclusion that the
IOL according to the invention has to restore both the optical
power and the aberration characteristics of the natural human lens
in order to support the neuro-visual optical system with respect to
the best perceivable visual performance. For explanation, see, for
example, the article of P. Artal et al entitled "Neural
compensation for the eye's optical aberrations", Journal of Vision
(2004), 4, pages 281 to 287.
[0048] The design of the novel intraocular lens takes into account
the natural optical configuration of the human vision apparatus,
for example, the visual axis tilt and the pupil decentration. In
addition, the method accounts for potential positioning errors
caused by implantation and surgery effects.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] The invention will now be described with reference to the
drawings wherein:
[0050] FIG. 1 shows the Liou-Brennan model eye with pupil
decentration and visual axis tilt;
[0051] FIG. 2 shows the statistical distribution of IOL positioning
errors;
[0052] FIG. 3 shows the spherical aberration as a function of the
pupil diameter for different IOLs;
[0053] FIG. 4 shows the layout of the aspherical IOL according to
the invention;
[0054] FIG. 5 shows another embodiment of the aspherical IOL
according to the invention;
[0055] FIG. 6 shows the radial optical power and corresponding
zones for different IOLs;
[0056] FIG. 7 shows Strehl ratio as a function of pupil diameter
for different IOLs; and,
[0057] FIGS. 8 to 13 show modulation transfer functions for
different IOLs for different pupil diameters, decentrations and
tilt angles.
[0058] The aspheric IOL according to the invention is referred to
as "new aspherical IOL" in the graphs of FIGS. 3 and 6 to 13.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
[0059] In order to provide a design environment for an IOL, a
particular theoretical eye model needs to be applied. Many such
models are well known from the literature, for example, Gullstrand:
Helmholtz's Physiological Optics; Norrby et al: "Methods of
obtaining ophthalmic lenses providing the eye with reduced
aberrations"; U.S. Pat. No. 6,609,793; or Thibos et al: "A new
single surface eye that accurately predicts chromatic and spherical
aberrations in the human eye", Invest. Ophthal. Visual Sci. 34, 777
(1993).
[0060] All the above theoretical eye models as well as the majority
of published eye models rely on simplified ocular configurations of
the human eye. The cornea is reduced to a single surface element
and the visual axis is assumed to match exactly the axis of
symmetry of the eye. These reduced models try to duplicate the
optical system and aberrations of the human visual apparatus by the
use of single faced cornea models that apply some degree of
asphericity in order to reflect the measurable performance. The
authors proved that the above eye models comply with the measured
results according to the given assumptions. However, these eye
models disregard the specifics of the anatomy of the human eye in a
more or less systematic way. The most comprehensive eye model
currently available in literature was described by Liou and Brennan
in the article "Anatomically accurate, finite model eye for optical
modeling", J. Opt. Soc. Am. A, Vol. 14, No. 8, August 1997. The
Liou-Brennan eye, as shown in FIG. 1, represents the ocular anatomy
very closely and preserves the optical properties and aberration
characteristics of the human eye. This eye model includes an
aspherical cornea with anterior surface 1.1 and posterior surface
1.2 as well as an aspherical gradient-index lens model. The
anterior chamber is identified by reference numeral 8, the vitreous
body by 7 and the retina by 4. The model takes into account that,
for the majority of the population, the visual axis 5 is tilted by
5.degree. with respect to the axis of symmetry 9 of the eye in
order to focus in the macular region 4.1. In addition, the pupil 6
is slightly decentered by 0.5 mm in nasal direction 6.1 for the
majority of the population. The amount of spherical aberration (SA)
is balanced by an aspheric cornea that introduces positive
spherical aberration. An aspherical model of the natural lens,
including two gradient-index components with optical surfaces (2.1,
2.2, 2.3), provides a negative SA to compensate for the corneal
contribution. In total, the optical system provides a slight
positive spherical aberration which is equivalent to the measured
data and helps to increase the depth of focus. In contrast to other
model eyes, the Liou-Brennan eye is therefore not rotationally
symmetric.
[0061] The design of the novel artificial intraocular lenses uses
an eye model that is based on the Liou-Brennan eye model as
described by the listing of the surfaces:
TABLE-US-00001 Surface Comment Radius Thickness Glass Diameter
Conus OBJ -- Infinite Infinite -- 0 0 1 -- Infinite 1 -- 4.495 0 2
-- 0 -- -- 3 CORNEA_FRONT 7.77 0.5 CORNEA_LB 12 -0.18 4 CORNEA_BACK
6.4 3.16 WATER_LB 12 -0.6 5 PUPIL_DEC_1 -- 0 -- -- STO PUPIL
Infinite 0 WATER_LB 4 0 7 PUPIL_DEC_2 -- 0 -- -- 8 LENS_FRONT 12.4
1.59 -- 10 -0.94 9 LENS_CENTER Infinite 2.43 -- 10 0 10 LENS_BACK
-8.1 16.27 WATER_LB 10 0.96 IMA RETINA -18 WATER_LB 20 0
[0062] The invention is based on specific geometry and/or shape
modifications which are applied to the anterior or posterior
surface or to both surfaces of the novel intraocular lenses. The
modified IOL surfaces include rotationally symmetric deviations
from a spherical shape. This method is commonly understood as
aspherization of optical surfaces. Since aspherical surfaces are
already well known from the prior art, the following sections will
explain the characteristics and improvements and will furthermore
explain the differences to commonly known designs.
[0063] The new lens design is intended to improve the prior art in
such a way that it provides a measurable improvement of the optical
performance parameters that leads to a perceivable improvement of
visual acuity and contrast vision performances for the patient. In
order to do so, the disclosed lens design mimics the optical
properties of the natural human lens under the conditions as
described above in the Liou-Brennan eye model.
[0064] A substantial improvement of visual acuity is achieved by
taking the statistics of potential lens displacements into account
for the lens design. The shape of the IOL surfaces is optimized to
minimize the sensitivity of the optical performance with regard to
decentration and tilt of the implanted IOL. Different authors
(Taketani et al: "Influence of intraocular lens optical design on
higher-order aberrations", J. Cat. Refr. Surg., Vol. 31, May 2005)
report a mean decentration of 0.1 mm to 0.25 mm as the most likely
case with ranges up to 1 mm.
[0065] In addition, the new IOL design fulfills the boundary
condition of keeping the natural spherical aberration at the same
amount as the human crystalline lens for a broad range of pupil
diameters. This allows the neurovisual system to adapt quickly to
the new implant because the lifelong adaptation to the properties
of the natural human eye does not need to be changed.
[0066] FIG. 3 shows that the new aspherical IOL approach provides
the least deviation from the characteristics of the natural human
eye (Liou-Brennan). The orthonormal Zernike coefficients are
computed using the notation defined in R. Noll, "Zernike
polynomials and atmospheric turbulence", J. Opt. Soc. Am., Vol. 66,
No. 3, p. 207 (1976). This is also known as the
"Born-Wolf-notation" (Born, Wolf "Principles of Optics", Chapter
1). The amount of this particular aberration coefficient is
expressed in waves (546 nm). The reference group consisting of
several IOLs of the prior art (reference numeral 30) shows
significantly larger differences of SA in pupil zones up to 4.5 mm
(reference numeral 31) and above.
[0067] An aspherical shape that allows the above optical
performance and capabilities can be described by the equation:
z = cr 2 1 + 1 - ( 1 + Q ) c 2 r 2 + k 2 r 2 + k 4 r 4 + k 6 r 6 +
k 8 r 8 ( 1 ) ##EQU00002##
wherein:
[0068] c=r.sub.curv.sup.-1 (curvature=1/base radius of
curvature)
[0069] r=independent variable, radius about optical axis
[0070] Q=conic constant
[0071] k.sub.n=polynomial coefficient of order n.
[0072] Rotationally symmetric polynomial aspheric surfaces are
described by a polynomial expansion of the deviation from a
spherical surface (or an aspheric surface described by a conic
section). The even aspherical surface model uses only the even
powers of the radial coordinate to describe the asphericity. The
model uses the base radius of curvature and the conic constant.
[0073] The coefficients of the polynomial expansion as well as the
base radius are determined numerically in order to satisfy a least
square fit to a particular merit function. This merit function
accounts for the surgical statistics as described above and is
minimized for optical performance. The merit function is
represented by a set of different error and quality parameters that
describe the desired optical performance. By definition, the
optimal state of the optical system is reached at a global minimum
of the merit function. In order to optimize the IOL surface shape
to achieve the advantageous properties as disclosed, the merit
function is constructed using weighted wavefront aberration
operands, weighted MTF operands, localized optical power operands
as well as boundary constraints such as center thickness and edge
thickness.
[0074] The following set of coefficients describes a new aspherical
IOL at a base power of 22D (22 diopters).
TABLE-US-00002 surface r.sub.curv Q k2 k4 k6 k8 anterior 7.1497 0.0
0.0 0.0 0.0 0.0 posterior -36.3903 0.0 -6.8159E-003 1.0213E-003
-6.2142E-005 0.0
[0075] In accordance with equation (1), the required range of base
optical powers from 5D to 40D can be easily calculated by setting
the localized target power operands of the merit function to the
desired power values and minimizing the remaining errors
accordingly.
[0076] An example of a possible layout can be seen in FIG. 4. The
lens can be made of three parts but this is not a requirement.
Other preferred embodiments include 2-part configurations or single
part IOLs as presented in FIG. 5. Reference numeral 20 refers to
the IOL body or the bulk material of the IOL; 21 is the haptics
mechanism; and, 20.1 is the optically effective zone of the IOL. At
least one of the optical surfaces 22.1 and 22.2 is aspherical. In
the example described above, surface 22.2 is aspherical.
[0077] FIG. 6 shows the radial refractive power profile of the
modified IOL in comparison with other lens designs of the prior
art. The enhanced capabilities result from the particular
characteristic of the radial refractive power distribution as a
function of the radius normal to the optical axis. All IOLs start
at their paraxial refractive power of 22D (22 diopters) at a radius
of 0 mm. The refractive power of the symmetric biconvex lens
B&L LI61 increases continuously toward the lens edge. This
indicates a significant amount of SA that exceeds the naturally
given amount. In contrast, the optical power of the lens TECNIS
Z9000 decreases greatly with increasing radius to provide a
negative SA that compensates for the corneal contribution. The
drawback of this approach results from the high sensitivity of this
design with regard to a decentration of the IOL. The third example
of the prior art is the "aberration free IOL" B&L SofPort A0.
This lens assumes independence of the optical performance with
respect to decentration. This is accomplished by keeping the radial
power at a value equal to the paraxial power for all radii. In this
case the lens is free from an inherent SA. If this condition is
satisfied, a decentration does not cause coma errors which
compromise image quality dramatically in the presence of
decentration. Despite the mentioned advantages, this lens design
has a significant disadvantage. The natural compensation effect of
the human crystalline lens is completely ignored. The image quality
at the retina is therefore suboptimal for the patient since the
full amount of the corneal SA affects the visual acuity in a
negative way.
[0078] FIG. 6 shows how the new IOL resolves the problems of the
known IOL designs of the prior art. The distribution of the optical
power as a function of lens radius is selected in different zones
so that an optimal performance is achieved which is perceived by
the patient.
[0079] In zone I, the optical power decreases continuously and
smoothly in a pupil region starting from radius 0 mm through 2.0
mm. This pupil region is most active for photopic vision under
bright light conditions. The compensation for the corneal SA allows
a diffraction limited performance and an improved contrast vision.
In the zone II, a pupil region from r=2.0 mm through r=2.5 mm, the
optical power is less than that of the paraxial region in order to
compensate for corneal SA at large pupils under mesopic conditions.
The increase in optical power from r=2.5 mm to r=3 mm in zone III
reduces the sensitivity of the modulation transfer function with
respect to decentration and tilt.
[0080] FIG. 7 shows that the new lens design ensures a diffraction
limited performance up to a pupil size of 4 mm and equals the
performance (Strehl ratio as a function of pupil diameter) of the
natural crystalline lens for the entire pupil range.
[0081] Further, the new lens design equals the diffraction limited
optical performance (MTF) of the best prior art designs in case of
physical pupil diameters of 3 mm at no decentration (see FIG.
8).
[0082] FIG. 9 shows that the new IOL equals the optical performance
(MTF) of the natural human eye in case of physical pupil diameters
up to 4.5 mm at no decentration.
[0083] FIGS. 10 and 11 show a significantly reduced sensitivity of
the optical performance (MTF) with respect to decentration, while
FIGS. 12 and 13 show that the same is true with respect to
tilt.
[0084] It is understood that the foregoing description is that of
the preferred embodiments of the invention and that various changes
and modifications may be made thereto without departing from the
spirit and scope of the invention as defined in the appended
claims.
* * * * *