U.S. patent application number 13/006082 was filed with the patent office on 2011-06-09 for excimer laser unit and relative control method for performing cornea ablation to reduce presbyopia.
Invention is credited to Franco Bartoli.
Application Number | 20110137301 13/006082 |
Document ID | / |
Family ID | 32500578 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110137301 |
Kind Code |
A1 |
Bartoli; Franco |
June 9, 2011 |
EXCIMER LASER UNIT AND RELATIVE CONTROL METHOD FOR PERFORMING
CORNEA ABLATION TO REDUCE PRESBYOPIA
Abstract
An excimer laser unit and a method of controlling the unit to
perform cornea ablation to reduce presbyopia, wherein the excimer
laser unit is controlled to form on the cornea a photoablative
pattern inducing a fourth-order ocular aberration, in particular a
positive spherical aberration. An aberrometric map of the eye is
first acquired indicating the visual defects of the eye, which
include second-order visual defects such as hypermetropia,
astigmatism, and myopia, and higher-order visual defects such as
spherical aberration; if the detected spherical aberration is
negative, it is reduced by numerically increasing its absolute
value to obtain an overcorrect photoablative inducing positive
spherical aberration; conversely, if the detected spherical
aberration is positive, its sign is changed and its absolute value
increased numerically to obtain an overcorrect photoablative
pattern inducing positive spherical aberration; and the
photoablative pattern so generated is supplied to the excimer laser
unit for implementation on the cornea.
Inventors: |
Bartoli; Franco; (Torino,
IT) |
Family ID: |
32500578 |
Appl. No.: |
13/006082 |
Filed: |
January 13, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10535617 |
Feb 6, 2006 |
7887531 |
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PCT/IT2003/000747 |
Nov 18, 2003 |
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13006082 |
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Current U.S.
Class: |
606/5 |
Current CPC
Class: |
A61F 9/00808 20130101;
A61F 9/008 20130101; A61F 2009/00895 20130101; A61F 2009/0088
20130101; A61F 2009/00872 20130101 |
Class at
Publication: |
606/5 |
International
Class: |
A61F 9/008 20060101
A61F009/008 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 19, 2002 |
IT |
TO 2002A001007 |
Claims
1. A method of reducing presbyopia, comprising: forming on the
cornea a photoablative pattern inducing a fourth-order ocular
aberration, resulting in a fourth-order ocular aberration after
treatment.
2. The method as claimed in claim 1; wherein said fourth-order
aberration is a spherical aberration.
3. The method as claimed in claim 2; wherein said spherical
aberration is a positive spherical aberration.
4. An excimer laser unit for performing cornea ablation to reduce
presbyopia, comprising: control means configured to control said
excimer laser unit to form on the cornea a photoablative pattern
inducing a fourth-order ocular aberration, resulting in a
fourth-order ocular aberration after treatment.
5. The excimer laser unit as claimed in claim 4; wherein said
induced fourth-order aberration is a spherical aberration.
6. The excimer laser unit as claimed in claim 5; wherein said
induced spherical aberration is a positive spherical
aberration.
7. A method for treating presbyopia of a patient having an eye, the
eye having a pupil, the method comprising: inducing a
presbyopia-mitigating quantity of fourth-order spherical aberration
in the eye, resulting in the eye having an induced spherical
aberration.
8. The method as claimed in claim 7; wherein said spherical
aberration is a positive spherical aberration.
9. A method for treating presbyopia of a patient having an eye, the
eye having a pupil, the method comprising: inducing a
presbyopia-mitigating quantity of high-order spherical aberration
in the eye, resulting in the eye having an induced spherical
aberration extending across the pupil.
10. The method as claimed in claim 9; wherein said spherical
aberration is a positive spherical aberration.
Description
[0001] The present application is a continuation of U.S. patent
application Ser. No. 10/535,617 filed Feb. 6, 2006, which claims
priority of International Application No. PCT/IT 2003/000747, filed
Nov. 18, 2003 and Italian Application No. TO 2002A001007, filed
Nov. 19, 2002, the complete disclosures of which are hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an excimer laser unit and
relative control method for performing cornea ablation to reduce
presbyopia.
[0004] 2. Description of Related Art
[0005] As is known, the human eye can be likened to a camera, in
which the lens is defined by two lenses in turn defined by the
cornea and the crystalline lens, the diaphragm by the pupil, and
the film by the retina.
[0006] The lens focuses the rays from the outside world on the
retina; the diaphragm expands and contracts to allow enough light
into the eye to permit optimum operation of the retina with no
glare phenomena; and the photosensitive film, defined by the
retina, converts the light energy impressed on it into a visual
message which is transmitted to the cortical centres for
interpretation.
[0007] A basic characteristic of the eye as an optical system is
its ability to accommodate, i.e. to adjust its characteristics to
the distance of the object, so that the image is always formed on
the retina.
[0008] The lens of the human eye, as stated, is a converging system
formed by the association of various diopters, i.e. slightly curved
spherical surfaces separating two mediums of different refraction
indexes.
[0009] FIG. 1 shows a human eye; and FIG. 2 the human eye
represented as an optical system, in which A indicates the cornea,
B the aqueous humour, C the crystalline lens, D the vitreous body,
and E the retina.
[0010] More specifically: [0011] the first diopter is defined by
the anterior surface of the cornea, which has a converging power of
about 48 diopters (a diopter is the inverse of the focal distance
expressed in metres); [0012] the second diopter is defined by the
posterior surface of the cornea, which has a diverging effect of
about 5 diopters; [0013] the third diopter is defined by the
crystalline lens, which may be likened to a biconvex lens, in which
the radius of curvature of the anterior surface is 10 mm, and that
of the posterior surface 6 mm; the converging power of the lens
various from about 19 to 33 diopters, depending on the curvature of
the anterior surface of the crystalline lens; [0014] alternating
with the ocular diopters and the retina are the aqueous humour and
the vitreous body, which have a refraction index of about 1.33.
[0015] Of the surface of the cornea as a whole, only the central
area, known as the optical area and of about 4 mm in diameter, is
normally used, and is defined by the opening of the pupil
diaphragm.
[0016] Length is one of the three basic elements of the optical
system of the eye, together with vertex power and the refraction
index of the mediums.
[0017] In the emmetropic, i.e. normal, eye, the light rays of
distant objects are focused exactly on the retina.
[0018] Myopia, astigmatism, and hypermetropia are defects of the
optical system which result in the image not being focused
correctly on the retina.
[0019] Refraction defects can be determined by various methods, and
research and analysis of them has developed in recent years thanks
to the use of aberrometry and advanced optical aberration measuring
equipment known in medical circles as aberrometers.
[0020] In simple terms, aberration of a wavefront is a deviation of
the analysed wavefront form from a geometrically perfect reference
form.
[0021] Wavefronts are affected by the composition of the medium
through which light travels, in that different mediums, e.g. glass,
air, water or fabric, produce different light speeds. In mediums
with a lower light speed (higher refraction index), the wavelength
is lower, on account of the wavefront travelling more slowly.
[0022] FIG. 3a shows what happens to a spherical wavefront
travelling through a perfect focusing lens with the focal point
coincident with the excitation centre of the wave.
[0023] More specifically, once past the lens, the spherical
wavefront flattens out; whereas any imperfection of the lens
produces deviations in the flat wavefront behind the lens, as shown
in FIG. 3b.
[0024] Aberrations of the eye are thought to be deviations of the
wavefront issuing from the eye with respect to a flat wavefront.
The light diffused at a given point on the retina acts as a point
light source and produces a spherical wavefront. The situation is
very similar to the one shown in FIGS. 3a and 3b. The cornea,
crystalline lens, and vitreous body act as a focusing lens; and if
the optical system of the eye were perfect (i.e. functioned like a
perfect lens), the wavefront issuing from the eye would be
flat.
[0025] Aberrations within the eye are caused by various factors,
e.g. variations in density within various optical subsystems of the
eye, irregular or deformed shape of the interfaces between
different parts of the eye, etc., which produce local changes in
the wavefront form with respect to a given optimum form.
[0026] Depending on the extent of it, aberration of the human eye
may result in considerable loss of visual acuity, as shown by way
of example in FIGS. 4a and 4b. More specifically, FIG. 4a shows the
image actually observed by a patient; and FIG. 4b what the patient
actually sees without any correction.
[0027] In ophthalmology, aberration is commonly measured using
Zernike's polynomials, which give a mathematical presentation of
the aberrant wavefront as the sum of coefficient-weighted
elementary functions, i.e. geometrical figures expressed as
polynomials as a function of (x, y).
[0028] The reason for this choice lies in Zernike's polynomials
being commonly used to describe aberrations in optical systems.
[0029] Using the coefficients of Zernike's polynomials, the
wavefront on the pupil can be represented as the following sum:
WR ( x , y ) = n = 0 m = 0 c nm Z nm ( x , y ) ##EQU00001##
where Z.sub.nm are Zernike's polynomials, and c.sub.nm are the
respective reconstruction coefficients weighting each specific
Zernike term. The coefficients are expressed in .mu.m, and numbers
n and m characterize each polynomial.
[0030] The extent to which the reconstructed wavefront WR(x,y)
approximates the real wavefront increases alongside an increase in
the order n considered in the series.
[0031] FIG. 5 shows the geometrical figures defined by Zernike's
polynomials up to the fourth order.
[0032] The table in FIG. 6 shows the mathematical description of
Zernike's polynomials up to the fourth order, and in particular
shows, for each polynomial, the identification symbol ("term"), the
order, the polar and cartesian form, and a description of the type
of aberration.
[0033] The Zernike terms in the table are shown in the notation
commonly used in ophthalmology, i.e. Z.sub.n.sup.-v, which shows
the contributing frequencies directly. The superscript is
correlated simply with n and m by v=2m-n.
[0034] Aberrometry provides for measuring the two basic values used
in ophthalmology to measure second-order refractive defects; the
sphere S and the cylinder C, which are expressed in diopters. The
calculation shown below, using second-order Zernike coefficients,
is now the method most commonly used in optics, even though the
values may differ slightly from those measured using other (e.g.
refractometric) methods, which calculate an average of second-order
aberrations (the most important) and higher-order aberrations
(third, fourth and higher); [0035] S=+2c.sub.2.sup.0 therefore
corresponds to an aberration defined by the coefficient of
Zernike's polynomial Z.sub.2.sup.0 (second order, symmetrical);
[0036] C=.+-. {square root over
((c.sub.2.sup.-2).sup.2+(c.sub.2.sup.2).sup.2)}{square root over
((c.sub.2.sup.-2).sup.2+(c.sub.2.sup.2).sup.2)} therefore
corresponds to the root mean square of the coefficients of the two
asymmetrical second-order Zernike polynomials.
[0037] One criterion by which to assess the total extent of
aberration is the RMS (Root Mean Square) value, which provides for
quantifying, and so comparing, aberrations obtained from different
measurements and patients.
[0038] The RMS value calculation indicates how the reconstructed
wavefront differs from a flat wave, with reference to the
two-dimensional variance of the wavefront .sigma..sup.2.
[0039] Variance of the wavefront is given by:
.sigma. 2 = .intg. .intg. ( WR - WR _ ) 2 x y .pi. ##EQU00002##
with integration to be performed in unit disk D
(x.sup.2+y.sup.2.ltoreq.1), and where WR represents the average
wavefront determined, and IVR the specimen wavefront.
[0040] Visual performance of the eye is currently acquired and
diagnosed using a wavefront analyser, which performs a complete
analysis of the refractive path of light inside the eye using a
technique based on the Shack-Hartmann wavefront sensor to analyse
the wavefront.
[0041] By way of example, FIG. 7 shows a wavefront analyser known
in medical circles as a WASCA manufactured by Carl Zeiss Meditec
AG.
[0042] As shown schematically in FIG. 8, when point light is
directed onto the retina, the WASCA wavefront analyser breaks up
the reflected wavefront to obtain highly accurate, practically
instantaneous ocular aberration measurements.
[0043] The WASCA wavefront analyser was designed to simplify ocular
aberration examination. The patient's eye is aligned directly in
front of the examination window with the aid of a television camera
image of the iris displayed on the screen; at which point, the
measurement can be made using the aberrometer. Once the point is
created on the retina, a light beam emerges from the eye, and
travels through the optic train of the unit and directly onto the
Shack-Hartmann sensor. This comprises an array of small lenses
connected to a CCD television camera, and is sensitive to
alterations in the slope of the wavefront; and the CCD image is
sent to a computer for data acquisition, storage and
processing.
[0044] Data is displayed in the form of coloured three- or
two-dimensional wavefront aberration maps, i.e. a "height map" in
.mu.m.
[0045] Data acquisition using a Shack-Hartmann sensor takes only 13
msec, which safeguards against the slightest eye movement in the
process.
[0046] FIG. 9 shows a three-dimensional example of a wavefront and
ocular aberration measurement. More specifically, FIG. 9 shows the
wavefront as it emerges from the cornea, and reconstructed by WASCA
wavefront analyser data. The radial measurement of this wavefront
is normalized with respect to the pupil radius, so that this
wavefront corresponds with the size of the pupil.
[0047] The WASCA wavefront analyser employs two-dimensional
wavefront representation with colour-coded height indications
(green=same as the reference level; warm colours=reading; cold
colours=dip), and supplies: [0048] the coefficients of Zernike's
polynomials to the fourth order; [0049] the equivalent sphere and
cylinder parameters; and [0050] the root means square (RMS)
value.
[0051] By way of example, FIG. 10 shows a two-dimensional
representation of the wavefront, and the coefficients of Zernike's
polynomials to the fourth order, as supplied by the WASCA wavefront
analyser.
[0052] As shown in FIG. 10, the wavefront shows marked third- and
fourth-order aberration, which could not be measured using
conventional instruments.
[0053] FIG. 11 shows the aberration table supplied by a WASCA
wavefront analyser, and which contains the following values
calculated for each aberration (the numbers below correspond with
those in the aberration table):
1. Second-order aberrations, i.e. sphere and cylinder, expressed in
diopters. 2. Pupil diameter in mm, as measured by the wavefront
analyser. 3. Analysis diameter in mm, for analysing wavefront data;
may vary, with a maximum diameter limited by the pupil diameter. 4.
Third- and fourth-order, so-called higher-order, aberrations. 5.
Numbers describing aberrations of the eye: [0054] PV OPD:
peak-valley optical path difference of the measured wavefront
(original data) or of the wavefront reconstructed by all the
Zernike terms up to the fourth order (database importation); [0055]
RMS OPD: root mean square value of OPD (on the basis of Zernike's
polynomials up to the fourth order); [0056] HO only indicates the
respective values for higher-order aberrations; the selected
correction terms arc subtracted from the global wavefront before
calculating the PV and RMS values (corresponding to the High-Order
Aberration map in the wavefront section). 6. x, y coordinates of
the centre of the pupil with respect to the centre of the wavefront
sensor.
[0057] The parameters to be corrected so as to also simulate
residual post-ablation aberration can be selected in the aberration
table (middle column).
[0058] The aberrometric analysis is shown on the computer screen
connected to the aberrometer in various ways, one of the most
common of which combines the aberration table and the
two-dimensional colour graphic shown in FIG. 10.
[0059] Refractive defects of the eye are corrected, or at least
reduced, by subjecting the cornea to ablation by an excimer laser
unit.
[0060] FIG. 12 shows, by way of example, a MEL 70 G-SCAN excimer
laser unit manufactured by Carl Zeiss Meditec AG, which can be
connected directly to the WASCA aberrometer of the same make.
[0061] The main commands for manual ablation control are entered
via the keyboard and monitor with which the excimer laser unit is
equipped.
[0062] The excimer laser and cornea tissue interact by the
high-energy photons in the ultraviolet light of the laser breaking
the intermolecular bonds. The uniqueness of excimer laser cornea
ablation lies in individual photons having sufficient energy to
break individual molecular bonds. The energy of a 193 millimicron
laser light photon is much higher than that required to break
molecular bonds, and the surplus energy serves to excite the
fragments, and contributes in providing the kinetic energy to expel
them from the surface. When energy intensity exceeds the ablation
threshold, each laser light pulse removes a precise quantity of
cornea tissue of uniform depth. Ablation depth depends on the
amount of energy striking the cornea. The most effective energy
intensity in ablation terms is 120-180 mJ/cm.sup.2, and each spot
removes 0.25.mu. per pulse.
[0063] In excimer laser ablation, it is essential to obtain a
smooth, uniform surface. Smoothness and uniformity are essential to
maintain transparency of the cornea, and depend on two major
factors: constant hydration of the stromal tissue, and homogeneity
of the laser beam.
[0064] An excimer laser beam has two main characteristics: fluence
and homogeneity, by which are meant, respectively, the amount of
energy applied to the ablation area, and the energy distribution
pattern within the ablation area.
[0065] More specifically, fluence is expressed in mJ/cm.sup.2, and
ranges from 100 to 230 mJ/cm.sup.2, depending on the laser.
Theoretically, an increase in fluence improves the quality of the
beam, but also increases the heat effect and acoustic shock, and
produces more rapid wear of the optical components of the
laser.
[0066] The ablation rate (cut rate) is the amount of tissue removed
per pulse, and depends on the characteristics of the tissue being
treated. At cornea level, each layer of tissue has a different
ablation rate, the average being calculated as .about.0.25.mu..
[0067] Each excimer laser unit has a definite beam shape or energy
profile, which may be homogenous (top hat) or Gaussian, as shown in
FIGS. 13a and 13b. The homogeneous profile has an equal energy
distribution density, and is therefore square in shape, whereas the
(bell-shaped) Gaussian profile has a higher density in the middle
than at the periphery.
[0068] The profile of the laser beam issuing from the resonant
cavity, in fact, is rectangular, and never homogeneous, i.e. has
energy peaks of different intensity, so each excimer laser unit has
a computer program (delivery system) for imparting a given profile
and obtaining a homogenous laser beam.
[0069] The importance of the beam profile lies in radiation
reproducing the shape of its energy profile directly on the cornea.
In other words, the laser beam striking the cornea reproduces the
shape of its profile as an impression on the cornea.
[0070] A non-homogeneous laser beam profile results in non-uniform
ablation. So, to obtain a homogeneous profile, the laser beam is
remodelled using lenses, mirrors, attenuators, prisms, and a
prismatic integrator with a telescopic zoom.
[0071] More specifically, a homogeneous-profile (top hat) beam,
with equal amounts of energy at the centre and periphery, removes a
homogeneous amount of tissue, whereas a Gaussian-profile beam
removes more tissue at the centre than at the periphery of the
impact area.
[0072] Correction of refractive defects of the eye, be they
spherical and/or cylindrical, call for specific photoablative
patterns: [0073] central flattening of the cornea for myopia:
circular central ablation area; [0074] central curving of the
cornea for hypermetropia: peripheral circular corona ablation;
[0075] flattening and curving along only one meridian for
astigmatism; [0076] a combination of different photoablative
patterns to correct spherical-cylindrical defects; and [0077]
customized photoablative patterns to correct asymmetrical or
irregular or higher-order defects.
[0078] Geometric ablation figures must therefore be constructed on
the cornea tissue, so as to only modify its refractive power in
axial defects, and to modify its refractive power by rounding the
surface in cylindrical defects. In asymmetrical or irregular
defects, the photoablative pattern is guided by topography.
[0079] Using an excimer laser unit, sub-micron portions of cornea
tissue can be removed extremely accurately to alter the curvature,
and hence refractive power, of the cornea.
[0080] In 1988, Munnerlyn devised an algorithm relating ablation
diameter and depth to required dioptric variance, and which allows
control of an excimer laser unit on the basis of optical parameters
(diopters) as opposed to geometrical parameters, thus greatly
simplifying operation of the unit.
[0081] The laser beam generated by an excimer laser unit may be:
[0082] broad and circular (broad beam): this removes cornea tissue
in concentric layers of varying diameter, and is suitable for
constructing geometrically simple photoablative patterns (myopia);
[0083] slitted, in which the laser beam is diaphragmed to obtain a
rectangular beam of variable size, which is distributed over the
cornea by a linear or rotation system: this provides for
constructing medium-simple photoablative patterns (myopia and
myopic astigmatism); [0084] a flying spot, in which a very small
laser beam (1-2 mm) is used, and which removes a small patch of
tissue at each spot. Correction is achieved by the laser spot
scanning the cornea, and being passed several times where more
material is to be removed. This system provides for constructing
any photoablative pattern (geometric photoablative figure) and so
correcting any ametropia of the eye.
[0085] The MEL 70 G-SCAN excimer laser unit mentioned above, for
example, generates a 1.8 mm flying spot laser beam with a Gaussian
profile for random circular scanning or random spot scanning
ablation.
[0086] The following is a detailed analysis of each refractive
defect of the eye and how it is corrected.
[0087] Hypermetropia is an extremely common refractive defect, so
much so that, statistically, 53-56% of eyes are hypermetropic by
0.5 of a diopter or more.
[0088] In this defect, rays from an infinite distance are focused
behind the retina, on account of the poor vertex power of the eye
with respect to its length.
[0089] As opposed to a point image, a larger, blurred image is
therefore formed on the retina, as shown in FIG. 14.
[0090] The defect is measured by the "sphere" parameter value,
which is positive. To bring vision back to normal, the vertex power
must be increased, which may be done partly by accommodation or
totally in artificial manner with the aid of positive spherical
lenses. The degree of hypermetropia is normally expressed by the
power of the positive lens which, placed in front of the eye,
focuses rays from an infinite distance on the retina.
[0091] The object in correcting a hypermetropic defect is to
increase the vertex power of the cornea. Hypermetropic ablation
aims at increasing the curvature of the central optical area of the
cornea. Unlike myopic photoablation, the central portion of the
cornea is practically left untreated, and is curved by removing the
periphery.
[0092] A fairly large (5 mm) centrally curved optical area must
therefore be obtained to also ensure good night vision.
Fortunately, hypermetropes have a sufficiently small-diameter
pupil. The circular corona treated area is located six to nine
millimetres from the centre of the pupil. It is therefore an
extensive excavation, with both central and peripheral transitions,
to avoid sharp changes in curvature, which induce severe scarring
processes.
[0093] It is essential not to induce a post-treatment increase in
central curvature of over 50 diopters, which would result in a
central keratoconus, with associated visual and central
reepithelialization problems. Correctable hypermetropia is
therefore limited (4-5 diopters): the flatter the original cornea,
the greater the extent to which hypermetropia is correctable.
[0094] Myopia, on the other hand, is a refractive defect in which
the relationship between eyeball length and vertex power is so
altered that the vertex power is too great for the length of the
eyeball, with the result that parallel rays striking the surface of
the cornea are focused in front of the retina, as shown in FIG.
15.
[0095] In myopia, for the image of an object to be focused on the
retina, the object must be placed at a finite distance, so that the
rays from it diverge onto the surface of the cornea.
[0096] The defect is measured by the "sphere" parameter value,
which is negative.
[0097] Myopia may be caused by: [0098] a longer than normal eyeball
(the most common cause); [0099] greater than normal curvature of
the cornea; [0100] a greater than normal curvature of the anterior
surface of the crystalline lens (as in accommodation spasm); [0101]
a crystalline lens too close to the cornea, i.e. a lower than
normal anterior chamber; [0102] a higher than normal refraction
index of the crystalline lens core (as in initial cataract
stages).
[0103] The object in correcting myopia is to reduce the vertex
power of the cornea, which means reducing the curvature of i.e.
flattening, the central optical area of the cornea.
[0104] This is done by circular tissue ablation, which gets deeper
as it gets larger.
[0105] The ablation area must be as large as possible, at least
larger than projection of the pupil on the cornea, with a very
gradual connection to the periphery, with no sharp variations in
curvature; must maintain the original prolate profile (curving more
at the centre than at the periphery); and must be as regular and
smooth as possible. All these are necessary for acceptance and
homogeneous coverage of the new surface of the cornea by the
epithelium.
[0106] Finally, astigmatism is a refractive defect in which the
diopter of the eye does not have the same refractive power in all
meridians. Given a point source and a converging lens which does
not have the same power in all meridians, a point image can never
be formed. Instead, when the screen is moved back and forth, two
lines, one perpendicular to the other and in different planes, will
be focussed, as shown in FIG. 16.
[0107] The defect is measured by the "cylinder" parameter value,
which is other than 0, and comes in two forms: regular astigmatism,
in which curvature differs between one meridian and another, but is
always the same along the same meridian; and irregular astigmatism,
in which curvature differs at different points in the same
meridian.
[0108] Ophthalmometric analysis of astigmatism gives the mean
dioptric value of the two main cornea meridians in a 3 mm central
area, thus characterizing astigmatism quantitatively (diopters) and
qualitatively (regular or irregular) within the central area.
[0109] Topographical analysis, with a point by point evaluation of
the radii of curvature over an extensive surface, enables
morphological evaluation of the cornea from the refractive
standpoint, and shows the cornea to be, not spherical, but
aspherical: curving more at the centre, and flatter at the
periphery.
[0110] The photoablative technique for correcting astigmatism, be
it positive or negative, is based on applying a hypermetropic or
myopic pattern to only one meridian, i.e. only one meridian is
curved or flattened. The current tendency is for ablation in two
planes of symmetry to modify above all the flatter meridian, and to
remove tissue from the flatter meridian to bring it to the same
curvature as the more curved meridian.
[0111] The widely varying morphology of astigmatism explains the
difficulty encountered, in the early years of photoablation, in
correcting it using excimer laser equipment with rigid ablation
patterns. Only in recent years, in fact, has it been possible to
adapt photoablation to topographical data (topographical link).
[0112] Correction of myopia, hypermetropia and astigmatism is based
on laser ablation techniques employing photoablative patterns
designed to eliminate the cylinder and sphere, i.e. to eliminate
second-order aberrations.
[0113] Ablations can also be combined to eliminate in one pass both
the sphere defect (myopia or hypermetropia) and the cylinder defect
(astigmatism).
[0114] Higher-order aberrations are normally left unchanged. More
specifically, third-order aberrations are normally associated with
"coma" visual defects, while fourth-order aberrations, and
particularly the spherical aberration measured by the coefficient
of Zernike's polynomial Z.sub.4.sup.0, are partly related to
transient accommodation phenomena.
[0115] By way of example, FIG. 17 shows the breakdown of an
aberration into its second-order, i.e. cylinder and sphere, and
higher-order components.
[0116] The WASCA aberrometer is able to isolate these aberrations
and produce a particular photoablative pattern to specifically
eliminate higher-order aberrations.
[0117] The photoablative pattern is generated electronically and
sent directly to the excimer laser unit. The aberrometer can be
adjusted to modify the coefficients of Zernike's polynomials to
obtain special ablative patterns.
[0118] Presbyopia, on the other hand, is a visual defect which
consists in diminished accommodation power of the eye to focus on
near objects, is mainly encountered in adults, and is due to a loss
of elasticity of the crystalline lens. Unlike myopia, hypermetropia
and astigmatism, presbyopia is therefore not a refractive defect
and, unlike the cases described above, is not easy to solve using
photoablative techniques.
SUMMARY OF THE INVENTION
[0119] It is a primary object of the present invention to provide
an excimer laser unit and relative control method for performing
cornea ablation to reduce presbyopia.
[0120] According to the present invention, there is provided a
method of controlling an excimer laser unit to perform cornea
ablation to reduce presbyopia.
[0121] The present invention also relates to an excimer laser unit
for performing cornea ablation to reduce presbyopia.
BRIEF DESCRIPTION OF THE DRAWINGS
[0122] A non-limiting embodiment of the present invention will be
described by way of example with reference to the accompanying
drawings, in which:
[0123] FIG. 1 shows a human eye;
[0124] FIG. 2 shows the human eye as an optical system;
[0125] FIGS. 3a and 3b show what happens to a spherical wavefront
travelling through a perfect focusing lens and an imperfect
focusing lens respectively;
[0126] FIGS. 4a and 4b show, respectively, an image observed by a
patient with aberration of the eye, and what the patient actually
sees with no correction;
[0127] FIG. 5 shows the geometrical figures defined by Zernike's
polynomials to the fourth order;
[0128] FIG. 6 shows a table containing a mathematical description
of Zernike's polynomials to the fourth order;
[0129] FIG. 7 shows a WASCA wavefront analyser;
[0130] FIG. 8 shows, schematically, the operating principle of a
WASCA wavefront analyser;
[0131] FIG. 9 shows a three-dimensional wavefront as it emerges
from the cornea;
[0132] FIG. 10 shows a two-dimensional wavefront and the
coefficients of Zernike's polynomials to the fourth order, as
supplied by a WASCA wavefront analyser;
[0133] FIG. 11 shows the aberration table supplied by a WASCA
wavefront analyser;
[0134] FIG. 12 shows an excimer laser unit;
[0135] FIGS. 13a and 13b show a homogenous profile and a Gaussian
profile respectively of a laser beam emitted by an excimer laser
unit;
[0136] FIG. 14 shows the image formed on the retina of a
hypermetropic eye;
[0137] FIG. 15 shows the image formed on the retina of a myopic
eye;
[0138] FIG. 16 shows the image formed on the retina of an
astigmatic eye;
[0139] FIG. 17 shows the breakdown of a generic aberration into
second- and higher-order components; and
[0140] FIG. 18 shows a flow chart of the operations performed
according to the method of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0141] It is to be understood that the figures and descriptions of
the present invention have been simplified to illustrate elements
that are relevant for a clear understanding of the present
invention, while eliminating, for purposes of clarity, many other
elements which are conventional in this art. Those of ordinary
skill in the art will recognize that other elements are desirable
for implementing the present invention. However, because such
elements are well known in the art, and because they do not
facilitate a better understanding of the present invention, a
discussion of such elements is not provided herein.
[0142] The present invention will now be described in detail on the
basis of exemplary embodiments.
[0143] The present invention is the result of research conducted by
the Applicant, and which has revealed that, unlike myopia,
hypermetropia and astigmatism correction, in which laser ablation
is aimed at eliminating second-order, i.e. cylinder and sphere,
aberrations, presbyopia can be reduced by laser ablation of the
cornea using a photoablative pattern which induces fourth-order, in
particular positive spherical, ocular aberration.
[0144] Research by the Applicant, in fact, has revealed a
relationship between the accommodation power of the eye and
fourth-order, in particular spherical, aberrations. More
specifically, research has shown that, during accommodation of the
eye, a variation, in particular an increase, occurs in spherical
aberration, which, from neutral or slightly negative, becomes
positive.
[0145] As a result, it has now been determined that, in losing
accommodation power with age, presbyopes lose the ability to induce
spherical aberration.
[0146] The obvious conclusion to be drawn here is therefore the
possibility of partly compensating for the loss of accommodation
power of the presbyope by inducing an increase in spherical
aberration.
[0147] Moreover, research has shown that correction of higher-order
aberrations increases the visual capacity of the eye in the
presbyope, and that combining an increase in spherical aberration
with high-order aberration treatment therefore produces a
significant improvement in near vision.
[0148] Spherical aberration can be induced using the same excimer
laser unit as for myopia, hypermetropia and astigmatism correction,
providing it is so controlled as to produce photoablative patterns
specifically designed for the purpose.
[0149] The way in which the excimer laser unit is controlled to
produce specific photoablative patterns to reduce presbyopia will
be described below in detail with reference to the FIG. 18 flow
chart.
[0150] As shown in FIG. 18, the first step is to acquire an
aberrornetric map of the eye, e.g. using the WASCA wavefront
analyser described above (block 10).
[0151] The second step is to separate the low-order, i.e. sphere
and cylinder, defects from the higher-order defects (block 20).
[0152] The third step is to isolate the detected spherical
aberration (block 30).
[0153] The fourth step is to determine whether the detected
spherical aberration is positive or negative (block 40).
[0154] If the detected spherical aberration is negative (NO output
of block 40), it is reduced by numerically increasing its absolute
value to obtain an overcorrect photoablative pattern and so induce
positive spherical aberration (block 50).
[0155] Conversely, if the detected spherical aberration is positive
(YES output of block 40), its sign is changed, and it is increased
numerically in absolute value to obtain an overcorrect
photoablative pattern and so induce positive spherical aberration
(block 60).
[0156] Since, as stated, combining an increase in spherical
aberration with high-order aberration treatment produces a
significant improvement in near vision, the photoablative pattern
so generated is further modified to also take corrections of higher
than second-order aberrations into account (block 70).
[0157] The photoablative pattern is then sent to the excimer laser
unit for the operation, and is implemented by the unit on the
cornea in known manner not described in detail (block 80).
[0158] More specifically, as regards positive spherical aberration
induction, the excimer laser unit must be controlled in a
particular way, depending on the refractive defect associated with
the presbyopia, as described below.
[0159] The starting point I the decision table shown below, which,
depending on the second-order refractive defect associated with the
presbyopia, indicates the ablative treatment required:
TABLE-US-00001 Sphere Cylinder Defect Treatment 1 + + Hypermetropia
and Type A + P positive astigmatism 2 + - Hypermetropia and Type A
+ P negative astigmatism 3 + = Hypermetropia Type P 4 - + Myopia
and Type A + M positive astigmatism 5 - - Myopia and Type A + M
negative astigmatism 6 - = Myopia Type M 7 = = Emmetropia To be
assessed 8 = - Negative astigmatism To be assessed 9 = + Positive
astigmatism To be assessed where: + indicates a dioptric value of
over 0.5; - indicates a dioptric value of below 0.5; = indicates a
dioptric value of -0.5 to +0.5.
[0160] More specifically, as regards treatment in the first six
cases in the above decision table: [0161] P type treatment is
performed by controlling the excimer laser unit to perform the
following operations: [0162] P.1) ablation of a circular corona of
maximum 6 mm inside diameter, maximum 9 mm outside diameter, and of
such a depth as to compensate the spherical defect; [0163] P.2)
ablation with a customized ablative pattern to eliminate higher
than second-order defects, with reference to aberrometric data
acquired prior to operation P.1; and [0164] P.3) if the above
operations fail to achieve a coefficient of Zernike's polynomial
Z40 ranging between 0.1 and 1.0, ablation with a customized
ablative pattern to obtain even greater spherical aberration;
[0165] M type treatment is performed by controlling the excimer
laser unit to perform the following operations: [0166] M.1)
ablation to such a depth as to compensate the spherical defect; and
[0167] M.2) ablation with a customized ablative pattern to induce
positive spherical aberration with a coefficient of Zernike's
polynomial Z40 ranging between 0.1 and 1.0; and [0168] A type
treatment is performed by controlling the excimer laser unit to
perform the following operation: [0169] A.1) cylindrical ablation,
with the excimer laser unit set solely to the cylindrical defect,
to bring the cylindrical defect close to zero.
[0170] As regards treatment in the last three cases in the above
decision table: [0171] Case 7: if vision of the eye improves with a
positive lens, P type treatment is performed; conversely, if it
improves with a negative lens, M type treatment is performed; and
[0172] Cases 8 and 9: type A treatment is performed to achieve
emmetropia, followed by treatment as in Case 7.
[0173] In cases (1), (2), (4) and (5), both treatments may be
combined into one, if the excimer laser unit can be so
programmed.
[0174] In all cases, after the operation, a check must be made to
determine the coefficient of Zernike's polynomial Z.sub.4.sup.0 is
within the 0.1-1.0 range. The fact that the RMS value has even
increased with respect to the pre-operation value is of no
importance.
[0175] The advantages of the present invention will be clear from
the foregoing description.
[0176] In particular, the present invention provides for correcting
presbyopia using, with appropriate control steps, the same excimer
laser unit formerly only used to correct refractive defects of the
human eye, such as myopia, hypermetropia, and astigmatism.
[0177] While this invention has been described in conjunction with
the specific embodiments outlined above, it is evident that many
alternatives, modifications, and variations will be apparent to
those skilled in the art. Accordingly, the preferred embodiments of
the invention as set forth above are intended to be illustrative,
not limiting. Various changes may be made without departing from
the spirit and scope of the inventions as defined in the following
claims.
* * * * *