ES2253951B1 - SYSTEM FOR THE DESIGN OF PERSONALIZED CORNEAL PHOTO-ABLATION PATTERNS, OBTAINED PATTERN AND LASER SURGERY SYSTEM FOR CORNEA CARVING. - Google Patents

Abstract

System for the design of custom corneal photo-ablation patterns, obtained pattern and laser surgery system for carving the cornea. System (100) for the design of a custom corneal photo-ablation pattern for an eye (1) under study, comprising a data subsystem (110) and a processing subsystem (120) connected to each other. The data subsystem comprises a corneal topography module (111), which generates a first data output; an aberrometry module (112), which generates a second data output; a biometrics module (113), which generates a fourth data output, said first, second and third data outputs being transmitted to the processing subsystem. This comprises an optical modeling module (121), which generates a fourth data output consisting of an optical model of the eye, and a module for generating an ablation pattern (122), which receives the fourth data output and generates a ablation pattern, so that said ablation pattern is capable of simultaneously correcting second and upper order aberrations of said eye.

Description

System for the design of patterns custom corneal photo-ablation, pattern obtained and laser surgery system for carving the cornea.

Field of the Invention

The invention is essentially located in the sectors of optometry and ophthalmology, and more specifically in the application of the system of the invention in refractive surgery of cornea

Background of the invention

Traditionally there was only the treatment for low order refractive errors, also called ametropias Corneal refractive surgery consists of carving or corneal tissue ablation to correct these ametropias. New systems have recently emerged to make patterns of custom ablation to simultaneously correct the error refractive, blur and astigmatism and order aberrations superior, although at the moment the results are not entirely satisfactory

If the eye were an ideal optical system all the rays that start from a specific object located in the line of look would focus on a common point located in the retina. Without However, this is not the case and each ray tends to deviate from its ideal trajectory. These deviations from ideal behavior are They call optical aberrations. How minor are these aberrations, the better the optical quality and therefore the quality visual. By jointly analyzing the aberrations of all rays that pass through the pupil, different modes appear that are Sort according to your order. Thus the classical ametropias (myopia, hyperopia and astigmatism) are second-order aberrations that can be corrected by conventional lenses or by also conventional surgical techniques. The aberrations of third order and higher had no treatment until very dates recent, since they cannot be corrected by procedures conventional, and require customized treatments adapted to Correct each mode (comma, spherical aberration, etc.).

Aberrations of order greater than two are present in most subjects and can degrade the quality of vision when they exceed certain limits. One of the challenges to which is currently facing physiological optics and surgery Refractive is to determine what is the impact of these aberrations of superior order in visual quality and of course try to correct them

Corneal refractive surgery aims to correct the refractive error of the eye by modifying the geometry of the anterior aspect of the cornea. However, the ablation patterns used in refractive surgery have been based on shaping the cornea as suggested by the more traditional modes of refractive correction, that is, glasses and contact lenses. Thus spherical patterns are implanted to correct spherical and cylindrical refractive error to correct astigmatism. The most widespread techniques are called PRK (Photorefractive Keratectomy) and LASIK (Laser In Situ Keratomileusis), both consisting of photo-ablation using corneal tissue excimer laser. Ablation patterns used in conventional refractive surgery are calculated to correct only ametropias and also assume that the cornea is spherical in shape. These treatments not only do not correct aberrations of order three and higher, but generally increase them markedly, resulting in a worsening of the optical quality in the patient in certain circumstances such as night vision. Recently, various techniques have been developed to produce "personalized" treatments in which attempts are made to correct not only classical ametropias but also higher aberrations.
order.

From the optical point of view, the human eye 1 (see figure 1) can be considered a system consisting of four surfaces, two corresponding to front face 2 and face posterior 3 of the cornea and two others corresponding to the face anterior 4 and posterior face 5 of lens 6, all separated they by means of different index of refraction. The amount of light that reaches the retina 7 is regulated by a membrane of variable diameter, iris 8, located near the anterior face of the lens. The retina is a photosensitive tissue that collects the light that falls on it and transforms it into small signals that The optic nerve sends to the brain for processing.

The eye is also an offset optical system, that is, the centers of curvature of these four surfaces, which would define the optical axis 9 of the eye, they are not aligned, no they rest on the same line, and in addition, the pupil is slightly offset transversely with respect to this axis, this that is, its center is not a point on the optical axis. There is, by agreement, a better approach to the optical axis of the eye that is given by the direction that produces the superposition of the first and fourth Purkinje images, these being the mirror images corresponding to each of the four surfaces of the eye. In addition the vision in the human eye is done through the line of gaze 10, defined as the direction that joins the point of fixation 11 that the eye observes with the center 12 of the pupil. This direction does not match the optical axis, in fact it is separated by a value that can oscillate, depending on the eye, between 1.4º and 90º.

As you can see in figure 2, it has been defined thus the keratometric axis 13, imaginary line that joins the point of fixation 11 with the center of curvature 14 of the cornea, relative to the which corneal topography maps are centered.

The point of intersection of the optical axis with the anterior aspect of the cornea is called apex, indicated in the Figures 1 and 2 as A; the axis intersection point keratometric with the anterior aspect of the cornea is called vertex, designated as V; the point of intersection of the line of look with the anterior aspect of the cornea is called the pole, indicated as P.

To successfully carry out an intervention of Refractive surgery is necessary for the surgeon to know the eye behavior as a complete optical system, that is, you need to establish from the information they provide clinical diagnostic equipment, eye behavior and Image quality that forms on the retina. It is what I know called an optical model, a functional representation of the eye as an optical system From the results of this model and the aspect of your cornea the ablation profiles that subsequently they are introduced into the laser that practices the carving of the cornea Once this carving is done, the cornea adopts a new form and modifies its optical power, getting the image is formed in the retina and is also free of aberrations that cause poor vision.

The main information required for to elaborate this optical model would be, in order of importance, the appearance of the surfaces, the distance that separates them and the index of refraction of internal media. As for the first, the current technology unfortunately only allows to know the appearance of the surfaces that make up the cornea, anterior faces and later. The information is obtained through some equipment called corneal surveyors or videokeratoscopes, which provide a map of corneal elevation relative to a plane reference. These corneal topography maps are presented in numerical form offering for each X and Y coordinate of the space a elevation value Z. The appearance and appearance are appreciated in them. deformations that the cornea presents.

When a patient is subjected to an examination to obtain his corneal topography, he must fix his eyes on a light that is placed in front of the device. The measuring instrument, which observes the eye in the same direction in which this light strikes, takes the image through which the topography map is generated just when it observes that the intensity of the reflection produced by this light on the anterior face of The cornea is maximum. There are in the market corneal surveyors who perform several consecutive examinations to the patient with the objective of generating from them an average map that reduces the measurement error. The procedure they use to average is to take as Z elevation for an X and Y coordinate point the arithmetic mean of the Z1, Z2, ... ZN elevations of the individual maps for the same X and Y coordinates. This procedure leads to error since the individual maps are referenced with respect to the keratometric axis and this axis changes from one measure to another. It is evident that this axis depends on the position of the fixation light and so if the patient repeats the exam, even if it is a few seconds after the first one is finished, the keratometric axis will have varied by doing so the relative position of the eye with respect to the point of fixation, producing a topography map centered on another point slightly offset from the previous one. Therefore this conventional technique
to obtain the averaged map of the topography, it is not satisfactory since it implies a degree of error.

It also happens with ophthalmic aberrometry, technique that allows to know the degree of aberrations of the eye. In this case the instrument is aligned with the line of sight 10 (direction of the beam that joins the fixing point with the center of the pupil) and the aberrations are obtained with respect to this axis and centered with the pupil. In the topography the data is obtained with respect to the keratometric axis.

In these equations or algorithms the surgeon enters the refractive data of the patient (how much myopia, farsightedness or astigmatism he has) and in some, in addition, his corneal topography map. Carving algorithms have been based on classic equations available in the literature, the so-called equations of Munnerlyn, Munnerlyn, CR, SJ Koons, et al . (1988). " Photorefractive keratectomy: A technique for laser refractive surgery " Journal of Catarat Refractive Surgery 14: Appendix D, but the new LASIK surgery techniques use algorithms based on these equations and on corneal topography maps.

To know a priori what the anterior aspect of the cornea should take to correct vision defects, it is necessary to previously obtain an optical model of the eye with which to simulate its behavior. This optical model allows to predict what the quality of vision will be once the cornea has been carved, and for this it is necessary that the model faithfully reproduces the behavior of the eye before the operation. The classic optical models are approximate since the parameters that define them are obtained as an average value among a large population, so for example the cornea is considered a spherical surface with a radius of curvature of 7.8 millimeters. They do not contemplate the particularity of each case. They served in their day, and currently serve, to know general parameters of the eye such as the position of the cardinal points or their paraxial behavior.

Description of the invention

It is an objective of the present invention provide a system for designing patterns of Custom corneal photo-ablation for one eye on study, in order to minimize defects in the vision and whose design is governed by two principles fundamental: minimum reduction of corneal tissue and maximum quality of the retinal image. This way you get a maximum vision quality (equal to or greater than normal) and at the same time minimize post-surgical problems that could lead to excessive mechanical weakening of the cornea.

It is an objective of the present invention provide a laser surgery system for carving the cornea that is not influenced by the previously commented problematic, in which ablation and centering patterns are used optimal.

It is a system composed of different modules responsible for collecting patient information and process it properly to obtain by optical simulation and numerical optimization, the optimal corneal ablation pattern for Custom correction, both of conventional ametropias, as of higher order aberrations, simultaneously minimizing the volume of corneal tissue to ablate. This information is transmits to a laser system responsible for carrying out the process of photo-ablation on the cornea.

The present invention relates to a system for the design of a photo-ablation pattern custom corneal for an eye under study, which includes:

- a data subsystem and

- a processing subsystem,

that are connected to each other by means of Connection.

Said connection means may consist of example, in a serial or parallel data transmission.

The data subsystem comprises:

- a corneal topography module, configured to acquire a topography map of the front face of the cornea of said eye, and that generates a first data output,

- an aberrometry module, configured to get the aberrations of that eye, and that generates a second data output, and

transmitting said first and second data outputs from the data subsystem to the subsystem of processing through the means of Connection.

In turn, the processing subsystem understands:

- an optical modeling module, which receives said first and second data outputs, and generates a third data output consisting of an optical model of said eye, and

- a module for generating a pattern of ablation, which receives the third data output from the optical modeling module, and generates an ablation pattern,

so that said pattern of Ablation generated by this system is able to correct simultaneously second order and higher aberrations of said eye.

Said first data output preferably includes the elevation dimensions of each surface point cornea) with respect to a reference plane.

Also preferably, said second outlet of data includes aberrometric data of said eye, described in Zernike coefficient terms.

Preferably, the data subsystem understands

- a biometrics module, configured to get the biometrics data from that eye, and that generates a fourth data output,

said fourth output being transmitted of data to the optical modeling module through the means of Connection.

Said fourth data output preferably includes thickness data from anterior chamber, posterior chamber, lens and cornea of said eye.

Preferably, the modeling module optics comprises:

- means to replace in a schematic model generic of any eye, first elements of said model with the data coming from said first output of data,

- means for adjusting said model about second elements to reproduce the optical behavior of first order, using subjective refraction data manifest obtained by conventional methods,

- means for obtaining aberration data from said model in which they have modified said first and second elements.

- means of comparison of these aberrations with the second data output generated by the module aberrometry

In case the data subsystem understands the biometrics module, preferably the modeling module optics comprises

- means to replace in a schematic model generic of any one eye, a first third elements of said model with the data coming from said third output of data, and

- means for obtaining aberration data from model in which they have modified said first, second and third elements.

According to a preferred embodiment of the system of the invention, the optical modeling module understands

- means for determining the orientation of the optical axis of the cornea from said topography map of the anterior face of the cornea of said eye.

Also, the pattern generation module of ablation preferably comprises

- means of mathematical parameterization of the anterior cornea.

The invention also relates to a pattern of custom corneal photo-ablation for one eye on study generated by a system as described previously.

The invention also relates to a system of laser surgery for corneal carving that includes

- a system for the design of a pattern of custom corneal photo-ablation for one eye on study as described above, and

- a carving module connected to the module generation of an ablation pattern, which receives as input said custom ablation pattern, and that includes a laser unit configured to implement said ablation pattern in said eye.

Brief description of the drawings

Then it goes on to describe very brief a series of drawings that help to better understand the invention and that expressly relate to an embodiment of said invention presented as a non-limiting example of is.

Figure 1 shows an outline of an eye human.

Figure 2 shows the geometry of a map of corneal topography.

Figure 3 shows a block diagram of the subsystems that make up the surgery system of the invention, according to a preferred embodiment thereof.

Figure 4 shows the depth of ablation at the vertex according to the criteria K1 or K2 is used for the pattern generation module optimization procedure of ablation.

Figure 5 shows a flow chart of the Pattern generation module procedure.

Figure 6 shows the volume of ablation custom limited to the optical zone, not including the zone transition.

Figure 7 shows the volume of ablation after the application of transition zones.

Description of a preferred embodiment of the invention

The laser surgery system 200 object of the invention consists of a system 100 for the design of a pattern of custom corneal photo-ablation for 1 eye in  study, and a carving module 210.

The system 100 in turn consists of two Subsystems: 110 data subsystem and processing subsystem 120. Data subsystem 110, in turn consists of three modules, the first one called corneal topography module 111 is the responsible for acquiring a topography map of the front face of the cornea This module consists of an acquisition system and information processing generates an ASCII file as output which contains the elevation dimensions of each surface point corneal with respect to a reference plane. The spacing between points may be selected by the operator by establishing as default values a diameter of 10 mm for the surveyed area with a resolution of 0.1 mm. This will generate an output matrix of 101 rows and 101 columns, where the columns will indicate horizontal coordinates (X in Cartesian coordinates) and rows vertical coordinates (and in Cartesian coordinates). This information is transmitted to the optical modeling module 121 included in processing subsystem 120 either by direct serial or parallel connection either via support units of information such as a flexible or compact disk. He aberrometry module 112 of the data subsystem consists of a aberrometer with which the total aberrations of the eye are obtained of the patient. The aberrometer can be based on any of the techniques available in the market for this purpose, method of Hartmann-Shack (HS) or Laser Ray Tracing (LRT). In In either case the module will generate an output, again a ASCII file, which will contain the aberrometric data of eye 1 of the patient described in terms of Zernike coefficients. Be indicates in the file the data corresponding to the size of the pupil with whom the test was performed, manifest refraction and accommodative status (in this case indicate if the accommodation with some specific substance). The polynomial base of Zernike used to express aberrations should follow the format recommended by the Optical Society of America ("Standards for Reporting the Optical Aberrations of Eyes "). Otherwise, the format followed must be indicated and perform the transformation in the processing subsystem necessary. The third module of this first subsystem is the module biometric 113 for providing the biometrics data of the patient. These data correspond to the thicknesses of the cornea, chamber anterior (distance between posterior aspect of cornea and anterior of the lens), rear camera (distance between the back of lens and retina) and lens thickness. This is available of an ultrasound biometer that takes ten measurements and performs on them a statistic to transmit to the module of processing a value with its corresponding error of these magnitudes The output of this module will be an ASCII file that contain the thicknesses of the anterior chamber, posterior chamber, crystalline and cornea.

The information collected by the three modules of the first subsystem is transferred to processing subsystem 120 which takes this data as input and generates as output a file ASCII with the optimal ablation volume for the low patient study. This subsystem is divided into two modules with different Functionality: the first is called the optical modeling module 121 and aims to make an optical model of the eye to be treated that allows you to simulate your geometry as closely as possible, Performance and optical quality. The output of this module will be a optical model of the patient's eye, this is the set of spokes, thicknesses, refractive indexes of the elements that make up the particularized eye for the patient, system parameters such as the fixation point, pupil runout, spatial orientation of the cornea, accommodative state, pupil size, and simulation of its visual quality by generating simulated maps of aberrations, impact diagrams, or any other function that gives  Optical quality account. This exit, which will serve as input for the ablation pattern generation module 122, part of the Same processing subsystem. The ablation pattern custom is obtained after an optimization process based on two fundamental principles: minimization of the volume of ablation (maximum corneal savings) and image quality optimization retinal (minimum dispersion error in the diagram of impacts).

The following are exhaustively described. the procedures that govern the operation of the two modules of the processing subsystem:

Optical modeling module 121

Unlike classic eye models schematic, in which the eye was represented by parameters obtained on average among a large population, the New clinical diagnostic techniques allow models to be developed custom where the parameters that define the eye as optical instrument correspond to the actual data of the patient. The steps to follow to develop an optical model Custom are:

a) Starting from a generic schematic model, "not personalized", from the eye.

b) Enter the data from the modules of the data subsystem corresponding to topography corneal and biometrics.

c) Adjust generic elements to reproduce the first order optical behavior, that is the refraction manifest.

d) Verify that the optical quality, that is the aberrations predicted by the model conform sufficiently to the real aberrometric measurements, comparing them with the output of the aberrometry module of the first subsystem.

For the ablation pattern design to have Success is necessary from the best possible optical model. By this, only if the adjustment is within predetermined tolerances the model is accepted, otherwise it must be rejected and adopt a  strategy based exclusively on corneal topography.

The elevation map of the anterior face of the cornea, provided by the corneal topography module, will be Essential to customize the model. The anterior aspect of the cornea It is the element of the eye with the greatest refracting power. Of the sixties Total power diopter of the eye provides about forty-eight, so it is decisive to know your topography to simulate faithfully eye behavior The elevation map is a surface z = z (x, y) where z is taken from a reference plane, which is introduced into the model as an array of discrete samples z_ {i, j} where the indices correspond to equally spaced positions in the X and Y Cartesian axes. This information is contained in the file from the corneal topography module of the first subsystem.

As indicated above, it is built the model replacing the elements of the generic model with all the personal data of the patient available, maintaining the generic data when those are not available. The optical modeling module will place each element in the proper position and orientation and will make the adjustments that are described below:

1.- Obtaining the optical axis : When a patient is subjected to an examination to obtain his corneal topography, he must fix his sight on a light that is placed in front of the apparatus. The measuring instrument, which observes the eye in the same direction in which this light strikes, takes the image through which the topography map is generated just when it observes that the intensity of the reflection produced by this light on the anterior face of The cornea is maximum. Regarding the keratometric axis, the corneal topography maps are centered. Thus, if the topography map is exported as a data matrix, the central element that marks the origin of coordinates will correspond to vertex V, or point of intersection of the keratometric axis and the anterior face of the cornea. If this data is entered into an optical design tool, where all the angular references refer to the optical axis of the system, it is necessary to rotate the topography data to a magnitude that reproduces the actual behavior. The method detailed below is intended to determine the orientation of the optical axis of the cornea from the data of its topography. The method is equally applicable to any optical surface of which topography data of its entire length or simply of an area thereof are available and its optical axis is to be known from them. It is a method applicable to both optometry and optical metrology.

The determination of the optical axis of the surface includes the transformation of the topography data of the surface to fit an adapted parametric representation to the shape of the surface, and from this representation determine its optical axis defined by coordinates X0, Y0, Z0 and, corresponding to the cut-off point of the optical axis with said surface and angles?,?,? that determine its Spatial Orientation.

The parametric representation consists of a generic surface model defined by a regular surface second grade, that is to say a general quadric, plus a arbitrary deformation of said expressed regular surface through a polynomial expansion. The cut-off point of the optical axis with the surface and orientation of the optical axis, they are obtained applying a transformation that reduces the equation of the second degree surface to its canonical form, whereby move and rotate the surface so that its optical axis matches with the coordinate axis (Z), thus aligning the surface and optically centered, i.e. referenced with respect to the axis optical.

Arbitrary deformation can be described. as a development in a polynomial base. For surfaces with circular shape you can use the base of orthonormal polynomials from Zernike, or second-class Chebyshev polynomials for rectangular surfaces The surface S (x, y, z) can decompose therefore on a second degree surface g (x, y, z) and a set of polynomials that represent the deformation of S with respect to g,

S (x, y, z) = g (x, y, z) + \ sum \ limits_ {n} c_ {n} P_ {n}

where P_ {n} is the polynomial of order n of the base of orthonormal polynomials and c_ {n} its coefficient correspondent.

The second degree surface g (x, y, z) in \ Re3 is an expression of the form

g (x, y, z) = a_ {11} x2 + a_ {22} y2 {+} + a_ {33} z2 + a_ {12} xy + a_ {13} xz + a_ {23} yz + a_ {1} x + a_ {2} and + a_ {3} z + a = 0

clearing z you can express the previous equation how

z = f (x, Y)

The least squares method is used to adjust the set of data derived from the topography to the previous equation and thus obtain the value of the coefficients a_ {ij}.

If the original matrix has m x n elements, try to find the set of coefficients a_ {ij} that minimize the mean square error function, RMS.

RMS = \ sqrt {\ frac {\ sum \ limits_ {m} \ sum \ limits_ {n} (T (m, n) - f (x (m, n), y (m, n)) 2} {m \ cdot n}}

where the functions x (m, n) y and (m, n) relate the Cartesian coordinates of a point with its coordinates Matrixes

The terms of order two of the general equation determine a quadratic form A, in \ Re3, whose matrix is

one

If you write X = (x, y, z) and it is called L (X) = a_ {1} x + a_ {2} and + a_ {3} z to the linear part, can be reduced the general equation to

X \ cdot A \ cdotX '+ L (X) + a = 0

Since X \ cdot A \ cdot X 'is a form quadratic in the Euclidean space, \ Re3 can be reduced, by an orthonormal base {\ vec {u} _ {1}, \ vec {u} _ {2}, \ vec {u} _ {}} a a sum of squares

λ 1 x 2 1 + λ 2 and 2 1 + λ 3 z 2 1

where \ lambda_ {1}, \ lambda_ {2}, \ lambda_ {3} are the eigenvalues of the matrix A (\ lambda_ {1} \ in \ Re). The matrix of the base change it is orthogonal since it transforms the canonical base of \ Re3 into an orthonormal basis. The unit vectors {\ vec {u} 1, \ vec {u} _ {2}, \ vec {u} _ {}} contain the addresses of the axes in the new base, and one of them corresponding to the optical axis of the surface. In this new base the general equation is reduced to:

λ 1 x 2 1 + λ 2 and 2 1 + λ 3 z 2 1 + b 1 x 1 + b_ {2} y_ {1} + b_ {z} {1} + b = 0

The most general case, in which all \ lambda_ {1} are nonzero, it is obtained by completing squares:

2

Applying the translation

\hskip1cm

y_{2} = y_{1} + \frac{b_{2}}{2\lambda_{2}};

\hskip1cm

z_{2} = z_{1} + \frac{b_{3}}{2\lambda_{3}} x_ {2} = x_ {1} + \ frac {b_ {1}} {2 \ lambda_ {1}};

 \ hskip1cm 

y_ {2} = y_ {1} + \ frac {b_ {2}} {2 \ lambda_ {2}};

 \ hskip1cm 

z_ {2} = z_ {1} + \ frac {b_ {3}} {2 \ lambda_ {3}}

conical is reduced to the shape λ 1 x 2 2 + λ 2 and 2 2 + λ 3 z 2 2 = C

This is the canonical shape of the surface with center, where the center is defined as that point C_ {0} = (x_ {0}, y_ {0}, z_ {0}) such that (x_ {0} + a_ {0}, y_ {0} + b_ {0}, z_ {0} + c_ {0}) is a point on the surface, its symmetric about C_ {0}, that is, (x_ {0} - a_ {0}, y_ {0} - b_ {0}, z_ {0} - c_ {0}), is also on the surface.

The surface axes are:

\hskip1cm

b = \sqrt{\frac{C}{|\lambda_{2}|}},

\hskip1cm

c = \sqrt{\frac{C}{|\lambda_{3}|}},a = \ sqrt {\ frac {C} {| \ lambda_ {1} |}},

 \ hskip1cm 

b = \ sqrt {\ frac {C} {| \ lambda_ {2} |}},

 \ hskip1cm 

c = \ sqrt {\ frac {C} {| \ lambda_ {3} |}},

The optical modeling module allows generating averaged topographies of the anterior aspect of the cornea from of at least two individual topography maps of the surface, once aligned and optically focused on space, that is referenced with respect to the optical axis obtained by the method previously described. The average topographic map of the surface is obtained as arithmetic mean of the various individual topographies. In this case the topography module corneal must generate as many ASCII files as maps of topography have been performed on the patient's eye.

2.- Adjustment of the refractive state of the patient . The model adjusts the refractive state of the patient using the data of subjective subjective refraction, sphere and cylinder obtained by conventional methods. The procedure consists in simulating exactly the emmetropization process used by the optometrist by introducing a corrective lens in the optical model that corrects this manifest refraction. Since the eye system plus corrective lens should give a focused image and without astigmatism, the radius of curvature of the posterior face of the lens is modified until the image is focused on the retina and minimizing astigmatism. This last process will eliminate the refractive error in the model due to blurring and astigmatism but will not have any corrective effect on higher order aberrations that remain unchanged.

3.- Adjustment of the aberrations and validation of the model . The last step in the modeling stage is to compare the optical aberrations of the model with the aberrometry data from the aberrometry module of the first subsystem. Various strategies fit here, depending on the degree of complexity you want to address. In any case, it is necessary to establish a metric and tolerances that allow to assess the fidelity achieved with the model. The procedure will consist in calculating the mean square error (rms) between the wavefront predicted by the model and the one measured in an aberrometer. Since both measurement and modeling errors may exist in the pupil position, it is convenient to record both wave fronts, moving one over the other until the minimum rms is obtained. The tolerance to validate the model will be established in terms of the relative rms error of the error between the predicted and measured wave fronts, versus the rms measured in the patient in so many percent (for example 10%). The model will be more faithful the smaller this relative error. If the discrepancy or rms error between the predicted and measured wavefront is greater than the tolerance, the model must be discarded.

Ablation Pattern Generation Module 122

The ablation pattern generation module Contains the procedure for optimized pattern design custom ablation, based on the previous optical model of the eye to treat The optimum ablation pattern shall be understood as the one that gives place to a cornea that in turn produces minimum values of both ametropics (myopia, farsightedness and astigmatism) as of the higher order aberrations, with a minimum volume of ablation (minimal damage to corneal tissue).

This procedure consists of a process of optimization, and as such it is important to reduce as much as possible the dimensionality of the problem, limiting the number of parameters Free to optimize. For this, the first step is to parameterize the face  anterior cornea leading to a realistic description with the minimum number of parameters: this is achieved by breaking down the surface in a regular component, a conical of revolution around the optical axis, Z, and a component that accounts for the possible irregularities, a series of polynomials of Zernike:

z = \ frac {c (x2 + y2)} {1 + \ sqrt {1 - (1 + \ delta) \ c2} \ (x2 + y2)} + \ sum \ limits_ {n} c_ {n} Z_ {n}

where z represents the elevation of the real cornea with respect to the reference plane, c is the curvature in the vertex, δ the conicity constant, and c_ {n} and Z_ {n} Zernike coefficients and polynomials respectively.

With the parameterized cornea and the optical model developed, it is intended to find a new face shape anterior cornea z '(x, y) that produces minimal errors refractive, both low (ametropia) and high (aberrations) order. Since the surgical procedure will consist of ablation, that is, in the removal of corneal tissue, the ablation must necessarily be fulfilled positive:

Δz (x, y) = z (x, y) - z '(x, y) \ geq 0

The first step in optimization is the classic in optical design. A series of variable parameters is chosen in the model, and proceed to modify them to find the minimum of the merit function In this case the variables will be the parameters that define the shape of the cornea: the curvature c, the conicity δ and the Zernike coefficients c_ {n}. It is also important in this case consider the final thickness of the cornea. The merit function must present a zero value for an eye without visual defect The optimization process will lead to reduce the value of this merit function until its minimum value is found. How this function is defined based on the parameters designated as variables and taking into account criteria of optimization to be described, the result is a new aspect of the cornea that satisfies those requirements. The output is presented in ASCII file form as elevation map of the front face of the cornea, that is, a matrix whose elements indicate the dimension elevation of a point of the cornea with respect to a plane of reference.

Keep in mind that optimization optics can produce a theoretical eye free of defects, but there are to consider that the volume of ablation should be positive and what as small as possible Therefore you have to find a compromise between the optimization criteria that apply:

K1.

Minimize the size of the image , that is, find the set of variables that produce an impact diagram with a mean square deviation (rms) with respect to its center as small as possible. That is to say, the rays coming from a luminous point source disperse as little as possible when hitting the retina.

K2

Minimize the volume of ablation . Among the possible solutions, choose the one in which the difference between the volume of the original cornea and the final cornea is minimal. This criterion will lead to a solution that rewards the saving of corneal tissue, as established by the principles of refractive surgery.

These two criteria can be contradictory, so it is necessary to reach a compromise as illustrated in Figure 5. But here it is necessary to add another factor that is decisive and can modify, at least in part, the solution found when applying The two previous criteria. It is about the transition zone :

In refractive surgery it is necessary to establish the optical zone on which the ablation will be performed. It is generally a circular zone that is defined by its diameter. Optical zones between 6 and 7 mm are common and they are limits the photo-ablation process purely corrective. Now, although this is the area that has been determined as optically active, an area must be ablated transition between the limit of the optical zone and a diameter exterior located between 9 and 10 mm to avoid discontinuity that occurs between the ablated region and the outside of the cornea. The transition zone can become optically zone active when the pupil dilates and exceeds the optical zone. In in this case the rays that pass through it will suffer a great dispersion regarding those that cross the center of the cornea and will contribute notably to raise high order aberrations and get worse The quality of vision. To avoid this risk the surgeon could define an optical zone of greater diameter, generating by counter greater depth of ablation at the vertex and therefore higher volume of ablation. It is evident therefore that there is a compromise solution between the size of the optical zone and the depth of ablation at the vertex.

Once the zone diameters have been specified optical and transition, the ablation depth is varied at the vertex \ Deltaz_ {0}, obtaining for each value of \ Deltaz_ {0} the rest of the pattern \ Deltaz (x, y) of ablation that gives the minimum rms error in the image. This way you get the graph of figure 4 where you can see how the two lines k1 and k2, which correspond to criteria K1 and K2, respectively, they cross at one point. In it the surgeon observes how vision quality (rms) improves as the depth of ablation, until reaching a constant value, region in which the ablation exceeds the optical zone. In this figure 4 it also shows how the volume of ablation increases with the ablation depth as well as the classical ablation line A_ {c}. This classical ablation (according to Munnerlyn's expression) always generates patterns with a vertex ablation depth higher than suggested by custom patterns.

In Figure 4, the shaded area shows the Suggested region to set the ablation pattern. The best solution will correspond to the point of minimum ablation depth in the vertex that produces the least RMS error of the wavefront. In this  sense it is convenient to establish a criterion of visual quality in RMS terms that allow you to associate it with an emmetropic eye. He operator can set this value called threshold and that by default takes the value of 0.5 microns.

Once the optimal ablation pattern is established the module calculates the transition that guarantees a behavior smooth between the end of the optical zone and an outside radius of the Cornea that is set to 9.5 mm by default.

Figures 6 and 7 show the volume of custom ablation limited to the optical zone, not including the transition zone and ablation volume after application of the transition zones, respectively.

The optimization procedure that follows the pattern generation module is shown schematically in the flow chart shown in figure 5.

With the fourth data output of the optical modeling module as input (Input) and the front face of the parameterized cornea (step s1), an optical zone (step s2) is established on which the ablation is to be performed; As shown in step s3, the corresponding d_ {i} data of the cornea thickness is modified. Through the optimization process a new cornea is established (step s4), with the corresponding modified parameters (R ', K' and \ sumC_ {j} '). A transition zone (step s5) is established between the limit of the optical zone ZO and an external radius Rext that avoids the discontinuity that occurs between the ablated region and the exterior of the cornea. The new cornea will be established taking into account both the ablated zone and the transition zone (step s6), and the corresponding mean square deviation (rms) (criterion K1), the ablation volume (criterion K2) is calculated (step s7 ). . Among the possible solutions, one is chosen in which the difference between the volume of the original cornea and the final cornea is minimal, and a point of the commitment curve of the photo-ablation pattern is established (step s8). Next, it is evaluated if more points are necessary (step s9); if so, the optimization procedure is repeated until there are enough points to establish the custom corneal photo-ablation pattern to the eye under study. The set of the cornea ablated with its transition region for the selected ablation depth constitutes the output of the generation module of the ablation pattern 122. The output can be given in an ASCII file in the same way as the elevation maps of the cornea, but this time indicating depth of ablation for each point instead of elevation with respect to a reference plane. The resolution of this ablation volume or the spacing between points whose ablation is indicated is a configurable parameter that the operator will select based on the size of the laser shot. This information is transferred to the carving module 210.

This carving module 210 is composed of the laser unit responsible for implementing the ablation pattern obtained in the previous module. This unit takes as input the numerical file containing the data corresponding to the volume of ablation at each coordinate point (x, y) of the cornea of the patient incorporating it into his carving algorithms.

Claims (9)

1. System (100) for the design of a pattern of custom corneal photo-ablation for one eye (1) under study, which includes: - a data subsystem (110) and - a processing subsystem (120), connected to each other by means of Connection, characterized in that the data subsystem comprises: - a corneal topography module (111), configured to acquire a topography map of the front face 2 of the cornea of said eye, and that generates a first output of data, - an aberrometry module (112), configured to obtain the aberrations of said eye, and that generates a second data output, - a biometrics module (113), configured to get the biometrics data from that eye, and that generates a fourth data output, and transmitting said first, second and third data outputs from the data subsystem to processing subsystem through the means of Connection, and because the processing subsystem understands: - an optical modeling module (121), which receives said first, second and third data outputs, and generates a fourth data output consisting of an optical model of said eye, and - a module for generating a pattern of ablation (122), which receives the fourth data output from the optical modeling module, and generates an ablation pattern, so that said pattern of Ablation generated by this system is able to correct simultaneously second order and higher aberrations of said eye. 2. System according to claim 1, characterized in that the optical modeling module (121) comprises: - means to replace in a schematic model generic of any eye, first elements of said model with the data coming from said first and third output of data, - means for adjusting said model about second elements to reproduce the optical behavior of first order, using subjective refractive data manifest obtained by conventional methods, - means for obtaining aberration data from said model in which they have modified said first and second elements, and - means of comparison of these aberrations with the second data output generated by the module aberrometry (113). 3. System according to any of the preceding claims, characterized in that said first data output includes the elevation dimensions of each point of the corneal surface with respect to a reference plane. System according to any of the preceding claims, characterized in that said second data output includes aberrometric data of said eye, described in terms of Zernike coefficients. 5. System according to any of the preceding claims, characterized in that said fourth data output includes data of anterior chamber, posterior chamber, crystalline and cornea thicknesses of said eye. 6. System according to any of the preceding claims, characterized in that the optical modeling module (121) comprises: - means for determining the orientation of the optical axis (9) of the cornea from said topography map of the anterior aspect of the cornea of said eye.

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System according to any of the preceding claims, characterized in that the module for generating an ablation pattern (122) comprises - means of mathematical parameterization of the anterior cornea. 8. Photo-ablation pattern custom corneal for an eye under study generated by a system according to any of the claims 1-7 9. Laser surgery system (200) for carved cornea that includes - a system (100) according to any of the claims 1-7 - a carving module (210) connected to the module generating an ablation pattern, which receives as input said custom ablation pattern, and that includes a unit laser configured to implement said ablation pattern in said eye