DE102012022059A1 - Method for determining total refracting power of cornea of e.g. astigmatic eye, involves determining refracting power of cornea front side, and determining central thickness of cornea based on total refracting power of cornea - Google Patents

Abstract

The method involves measuring radius of front side and rear side of cornea. Refracting power of the cornea front side is determined. Central thickness of the cornea is determined based on total refracting power of the cornea. The refracting power of the cornea front side, the refracting power of the cornea back side, the radius of the cornea front side, radius of the cornea back side, refraction index of air, refraction index of the cornea, refraction index of chamber water and central thickness of the cornea are defined. Radii of the refracting powers of the cornea are indicated in diopters.

Description

The present invention relates to a method for determining the total refractive power of the cornea of an eye and is based on a combination of keratometric or topographic and OCT-based measurements. The total refractive power of the cornea is particularly important with regard to the calculation and selection of intraocular lenses.

According to the known state of the art numerous solutions are known.

For high-precision measurement, solutions based on optical coherence tomography (OCT), partial coherence interferometry (PCI) or the like have been established in the prior art.

The basic principle of the OCT method is based on white-light interferometry and compares the propagation time of a signal using an interferometer (usually Michelson interferometer). The arm with known optical path length (= reference arm) is used as a reference to the measuring arm. The interference of the signals from both arms gives a pattern from which one can read the relative optical path length within an A-scan (single depth signal). In the one-dimensional scanning methods, the beam is then transversely guided in one or two directions analogously to the ultrasound technique, with which a planar B-scan or a three-dimensional tomogram (C-scan) can be recorded. The amplitude values of the individual A-scans are typically displayed in logarithmized gray-scale or false-color values.

However, if additional measured variables are required, they can be determined, for example, from keratometric or topographical image recordings by the eye.

Although these other measured variables and the OCT measured values can be measured by different devices, the integration of the measurement of both the OCT and the other measured variables in one device allows easier handling, for example. B. only one-time alignment of the device to the patient and an improved lateral registration of the OCT measured values with the other measured values.

In a first group of solutions, the different shots are sequentially, i. H. taken in succession.

An example shows the US 2005/0203422 A1 which shows a combination system of keratometer and OCT tomography. In order to separate the two modalities, a time separation is also proposed here.

Another example is the IOLMaster from Carl Zeiss. This is a combination device that determines the keratometry, the axial length via PCI (partial coherence interferometry) and the anterior chamber depth via slit illumination and image detection, as well as other parameters of the eye such as the so-called "Weis-to-Weis-Abstand".

In all of these sequential measurements, the time required for the measurements is higher. Another disadvantage is that the different measurements of OCT and ultrasound or keratometry could be due to possible eye movements at slightly different locations. In general, therefore, a repeatability of the measurement is correspondingly difficult to realize.

In a second group of solutions, the different images are recorded simultaneously, for which the measuring systems must have a corresponding optical separation.

As another example is in US 2005/0018137 A1 a combination system of keratometer and axial length measurement by means of PCI. The separation of both modalities is realized by beam splitting by polarization separation.

The already mentioned above US 2005/0203422 A1 mentions, as an alternative to the sequential measurement of the modalities (by means of OCT and keratometry) also a separation thereof by a dichroic beam splitter.

In all of these examples, optical separation of the different measurement systems occurs either through the use of different wavelengths or through additional optical elements that prevent the measurement systems from interfering with each other.

The disadvantage of the techniques described above is that the classical methods of keratometry and topography measure only the radius R _a of the anterior surface of the cornea and from this approximately determine the total refractive power K _{Ker of} the cornea, including the optical effect of the back side. According to the literature [7], the total refractive power K _Ker is calculated as follows: K _Ker = nK - 1 / Ra (0)

The assumptions about corneal thickness and front / back side ratio as well as their refractive indices are implicitly included in the value n _K.

Other approaches measure the total corneal refractive power without assuming front / back aspect ratio and / or corneal thickness using OCT or a combination of OCT and topography. Good repeatability of OCT measurement of corneal radii is difficult to achieve. In particular, due to possible eye movement during the measurement, high scan speeds and resolutions are required. Correct measurement of the radii requires exact calibration of the OCT (including scanner). This also applies in combination with topography.

In order to select an intraocular lens (IOL) by means of an IOL calculation formula, the empirical knowledge of the ULIB database is frequently used in practice. For this purpose, it is necessary that the measured total corneal refractive power in the population for normal eyes corresponds to the measurements of the keratometer of the IOL master.

This is not the case or at least not proven for existing approaches for determining the total refractive power of the cornea of an eye.

literature

• [1] M. Tang, Y. Li, M. Avila, and D. Huang, "Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography," J Cataract Refract Surg, vol. 32, no. 11, pp. 1843-1850, Nov. 2006 ,
• [2] Tang, A. Chen, Y. Li, and D. Huang, "Corneal power measurement with Fourier domain optical coherence tomography," J Cataract Refract Surg, vol. 36, no. 12, pp. 2115-2122, Dec. 2010 ,
• [3] L. Wang, AM Mahmoud, BL Anderson, DD Koch, and CJ Roberts, "Total corneal power estimation: ray tracing method versus gaussian optics formula," Invest. Ophthalmol. Vis. Sci, vol. 52, no. 3, pp. 1716-1722, Mar. 2011 ,
• [4] T. Kawarnorita, H. Uozato, K. Kamiya, L. Sax, K. Tsutsui, D. Aizawa, and K. Shimizu, "Repeatability, Reproducibility, and Agreement characteristics of rotating Scheimpflug photography and scanning-slit corneal topography for corneal power measurement, "Journal of Cataract & Refractive Surgery, vol. 35, no. 1, pp. 127-133, Jan. 2009 ,
• [5] M. Tang, L. Wang, DD Koch, Y. Li, and D. Huang, "Intraocular lens power calculation after myopic and hyperopic laser vision correction using optical coherence tomography," Saudi Journal of Ophthalmology, vol. 26, no. 1, pp. 19-24, Jan. 2012 ,
• [6] M. Tang, Y. Li, M. Avila, and D. Huang, "Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography," Journal of Cataract & Refractive Surgery, vol. 32, no. 11, pp. 1843-1850, 2006 ,
• [7] T. Olsen, "On the calculation of power from the curvature of the cornea," Br J Ophthalmol, vol. 70, no. 2, pp. 152-154, Feb. 1986 ,
• [8th] LN Thibos and D. Horner, "Power Vector Analysis of the Optical Outcome of Refractive Surgery," Journal of Cataract & Refractive Surgery, vol. 27, no. 1, pp. 80-85, Jan. 2001 ,
• [9] R. Navarra, L. Gonzalez, and JL Hernandez, "Optics of the average normal cornea from general and canonical representations of its surface topography," J Opt Soc Am A Opt Image Sci Vis, vol. 23, no. 2, pp. 219-232, Feb. 2006 ,

The present invention has for its object to develop a method for determining the total refractive power of the cornea of an eye, which waives the assumption of normal values and in particular for the determination of the total refractive power of the laser-treated cornea of an eye is applicable.

• a) der Radius R_a der Hornhautvorderseite keratometrisch oder topographisch gemessen und daraus die Brechkraft K_a der Hornhautvorderseite wie folgt bestimmt wird: Ka = n 1 – n0 / Ra (1)
• b) der Radius R_p der Hornhautrückseite OCT-basiert gemessen und daraus die Brechkraft K_p der Hornhautrückseite wie folgt bestimmt wird: Kp = n2 – n1 / Rp (2)
• c) die zentrale Dicke d der Hornhaut OCT-basiert gemessen und danach die Gesamtbrechkraft K der Hornhaut wie folgt bestimmt wird: K = Ka + Kp – d / n1KaKp (3) in denen

K

die Gesamtbrechkraft der Hornhaut,

K_a

die Brechkraft Hornhautvorderseite,

K_p

die Brechkraft Hornhautrückseite,

R_a

den Radius Hornhautvorderseite,

R_p

den Radius Hornhautrückseite,

n₀

den Brechungsindex von Luft,

n₁

den Brechungsindex der Hornhaut,

n₂

den Brechungsindex des Kammerwasser und

d

die zentrale Dicke der Hornhaut

definieren, wobei die Radien R_a, R_p und die Dicke d in Metern und die Brechkräfte K, K_a und K_p in Dioptrien angegeben sind.This object is achieved by the method according to the invention for determining the total refractive power of the cornea of an eye based on the measured values of a keratometric or topographic and at least one OCT-based measurement, according to the following method steps, in that:

• a) the radius R _{a of} the anterior corneal surface is measured keratometrically or topographically and from this the refractive power K _{a of} the anterior corneal surface is determined as follows: Ka = n 1 -n 0 / Ra (1)
• b) the radius R _{p of} the back of the cornea is measured OCT-based and from this the refractive power K _{p of} the back of the cornea is determined as follows: Kp = n2 - n1 / Rp (2)
• c) the central thickness d of the cornea is measured OCT-based and then the total refractive power K of the cornea is determined as follows: K = Ka + Kp - d / n1 KaKp (3) in which

K

the total refractive power of the cornea,

K _a

the refractive power of the front of the cornea,

K _p

the refractive power of the back of the cornea,

R _a

the radius cornea front,

R _p

the radius cornea back,

n ₀

the refractive index of air,

n ₁

the refractive index of the cornea,

n ₂

the refractive index of aqueous humor and

d

the central thickness of the cornea

define the radii R _a , R _p and the thickness d in meters and the refractive powers K, K _a and K _{p are given} in diopters.

According to the invention the object is solved by the features of the independent claims. Preferred developments and refinements are the subject of the dependent claims.

The method according to the invention serves to determine the total refractive power of the cornea of an eye and is based on a combination of keratometric or topographic and OCT-based measurements. The method is applicable to both normal-sighted and astigmatic eyes.

The invention will be described in more detail below with reference to exemplary embodiments.

• d) der Radius R_a der Hornhautvorderseite keratometrisch oder topographisch gemessen und daraus die Brechkraft K_a der Hornhautvorderseite wie folgt bestimmt: Ka = n1 – n0 / Ra (1)
• e) der Radius R_p der Hornhautrückseite OCT-basiert gemessen und daraus die Brechkraft K_p der Hornhautrückseite wie folgt bestimmt: Kp = n2 – n1 / Rp (2)
• f) die zentrale Dicke d der Hornhaut OCT-basiert gemessen und danach die Gesamtbrechkraft K der Hornhaut wie folgt bestimmt: K = Ka + Kp – d / n1KaKp (3) in denen

K

die Gesamtbrechkraft der Hornhaut,

K_a

die Brechkraft Hornhautvorderseite,

K_p

die Brechkraft Hornhautrückseite,

R_a

den Radius Hornhautvorderseite,

R_p

den Radius Hornhautrückseite,

n₀

den Brechungsindex von Luft,

n₁

den Brechungsindex der Hornhaut,

n₂

den Brechungsindex des Kammerwasser und

d

die zentrale Dicke der Hornhaut

definieren, wobei die Radien R_a, R_p und die Dicke d in Metern und die Brechkräfte K, K_a und K_p in Dioptrien angegeben sind.The method according to the invention for determining the total refractive power of the cornea of an eye is based on the measured values of a keratometric or topographic as well as at least one OCT-based measurement. Specifically, according to the following process steps:

• d) the radius R _{a of} the anterior corneal surface is measured keratometrically or topographically and from this the refractive power K _{a of} the front surface of the cornea is determined as follows: Ka = n1 - n0 / Ra (1)
• e) the radius R _{p of} the back of the cornea is measured OCT-based and from this the refractive power K _{p of} the back of the cornea is determined as follows: Kp = n2 - n1 / Rp (2)
• f) the central thickness d of the cornea is measured OCT-based and then the total refractive power K of the cornea is determined as follows: K = Ka + Kp - d / n1 KaKp (3) in which

K

the total refractive power of the cornea,

K _a

the refractive power of the front of the cornea,

K _p

the refractive power of the back of the cornea,

R _a

the radius cornea front,

R _p

the radius cornea back,

n ₀

the refractive index of air,

n ₁

the refractive index of the cornea,

n ₂

the refractive index of aqueous humor and

d

the central thickness of the cornea

define the radii R _a , R _p and the thickness d in meters and the refractive powers K, K _a and K _{p are given} in diopters.

According to the statements made by T. Olsen in [7] on the determination of the refractive power of the cornea, the description of the total refractive power K of the cornea also takes place here by means of paraxial optics (also called "thick lens").

• • Brechungsindex Luft n₀ = 1,
• • Brechungsindex Hornhaut n₁ = 1,376 und
• • Brechungsindex Kammerwasser n₂ = 1,336.

The following typical values serve as a prerequisite for the refractive indices:

• Refractive index air n ₀ = 1,
• • refractive index cornea n ₁ = 1.376 and
• • refractive index aqueous humor n ₂ = 1.336.

In contrast to the solutions of the prior art, which are based on different classical measuring methods with corresponding assumptions, in the method proposed here, in addition to the radius of the corneal front side R _a , the radius of the corneal reverse side R _{p is also} measured.

While the radius of the anterior corneal surface R _{a is} measured keratometrically or topographically, the determination of the other measured values, such as the radius of the back of the cornea R ' _p and the central thickness d of the cornea is OCT-based.

• g) bestimmte Vorder-/Rückseitenverhältnis c: R_p = c·R_a (4) mit c = R'p / R'a

indem, die Radien R'_a und R'_p OCT-basiert gemessen werden und R_p einen resultierenden Messwert aus keratometrischer oder topographischer und OCT-basierter Messung darstellt. Bei der Berechnung der Gesamtbrechkraft K wird gemäß der Formel (4) das Vorder-/Rückseitenverhältnis c berücksichtigt.According to the invention, the OCT-based measured radius R ' _{p of} the back of the cornea is not directly included as a measured value in the calculation of the total refractive power K according to method step c), but takes into account the previously in an additional step:

• g) certain front / back ratio c: R _p = c · R _a (4) With c = R'p / R'a

in that the radii R ' _a and R' _p are measured OCT-based and R _{p is} a resulting measurement from keratometric or topographic and OCT-based measurement. In the calculation of the total refractive power K, according to the formula (4), the front / back ratio c is taken into consideration.

This procedure has the advantage that the requirements for a correct behavior of the OCT scanner and for an exact optical adjustment can be significantly reduced. In addition, a measurement of the actual scanner behavior can be dispensed with.

Errors in the OCT-based measurements of the radii can be expressed as follows: R ' _a = x _a · R _a and R' _p = x _p · R _p with x _a , x _p ≠ 0 (5)

If the factors x _a and x are the same _p (x _a = x _p), the front-based OCT determined / back ratio c is correct in every case, cancel out as the factors of formula (4). Such errors, which may arise for example due to an incorrect width of the OCT-B scans due to incorrect behavior of the scanners, thus have no influence on the determination of the total refractive power K.

Although other types of errors for which x _a ≈ x _p apply, affect both the front / back ratio c and the total refractive power K of the cornea, but the resulting error is low, since only the ratio x _p / x _a ≈ 1 enters c and K. This is particularly true for linear errors in the OCT-based measured radii, which can be described as follows: R ' _a = x _a * R _a + y and R' _p = x _p * R _p + y with | y | << | x _a · R _a | (6)

In a further advantageous embodiment of the method according to the invention, the OCT-based measurement takes place in the form of at least one B-scan, along at least one meridian.

Although this procedure should be sufficient for eyes with normal vision, the accuracy of the determination of the total refractive power of the cornea of an eye can be additionally increased if the OCT-based measurement in the form of several B-scans preferably takes place along several meridians.

If more than one measured value is thus present for R ' _a , R' _p and / or d, the OCT-based measured values are arithmetically averaged after previous removal of "outliers" and then go into the calculation of the total refractive power K according to method step c). one.

According to the invention, the OCT-based measurement of the radii of the cornea is carried out by applying image processing algorithms to the B-scans. For example, functions on the front and back of the cornea can be fitted. Preferably, in each case a circle is attached to the front and back of the cornea, which is particularly preferably symmetrical to the vertex.

In this case, it is preferable to fit a zone symmetrically around the vertex, which is either 6 mm wide or corresponds to the zone in which the radius R _{a of} the front surface of the cornea is measured.

In this context it should be noted that the fitting of polynomials and the determination of the radius of curvature at the vertex generally leads to systematically deviating measurements of the radii and also the front / back ratio.

In a particularly advantageous embodiment, the method according to the invention is used for determining the total refractive power of the cornea of an astigmatic eye, from the measured values of a keratometric or topographic and OCT-based measurements in the form of two B-scans, along two meridians corresponding to the previously determined main sections. used.

For the selection of a toric IOL for the compensation of an astigmatic eye defect, the measurement of the corneal refractive powers K ₁ and K ₂ along the two main sections and the associated axial position α is required. According to the solutions of the prior art, established methods for this measure the two radii R _a1 and R _a2 and the angle α _{a of} the front surface of the cornea and calculate the total refractive powers K ₁ and K ₂ under certain assumptions.

According to the invention, however, it is also desirable here to take into account the amounts of the corneal reverse side and thickness by express measurement. The assumption here is that the main cuts on the front of the cornea also apply to the back or the cornea as a whole.

• a) die Radien R_a1 und R_a2 der Hauptschnitte der Hornhautvorderseite keratometrisch oder topographisch gemessen und daraus die Brechkraft K_a der Hornhautvorderseite wie folgt bestimmt wird: Ka1 = n1 – n0 / Ra1 Ka2 = n1 – n0 / Ra2 (1)
• b) die Radien R_p1 und R_p2 der Hauptschnitte der Hornhautrückseite OCT-basiert gemessen und daraus die Brechkräfte K_p1 und K_p2 der Hornhautrückseite wie folgt bestimmt wird: Kp1 = n2 – n1 / Rp1 K₂ = n2 – n1 / Rp2 (2)
• c) die zentrale Dicke d der Hornhaut OCT-basiert gemessen und danach die Gesamtbrechkräfte K₁ und K₂ der Hauptschnitte der Hornhaut wie folgt bestimmt wird: K1 = Ka1 + Kp1 – d / n1Ka1Kp1 K2 = Ka2 + Kp2 – d / n1Ka2Kp2(3)

in denen

K_1,2

die Gesamtbrechkraft der Hornhaut,

K_a1,2

die Brechkraft Hornhautvorderseite,

K_p1,2

die Brechkraft Hornhautrückseite,

R_a1,2

den Radius Hornhautvorderseite,

R_p1,2

den Radius Hornhautrückseite,

n₀

den Brechungsindex von Luft,

n₁

den Brechungsindex der Hornhaut,

n₂

den Brechungsindex des Kammerwasser und

d

die zentrale Dicke der Hornhaut

definieren, wobei die Radien R_a1,2, R_p1,2 und die Dicke d in Metern und die Brechkräfte K_a1,2, K_p1,2 und K_1,2 in Dioptrien angegeben sind.To determine the total refractive power of the cornea of an astigmatic eye, in detail according to the following method steps:

• a) the radii R _a1 and R _{a2 of} the main sections of the anterior corneal surface are measured keratometrically or topographically and from this the refractive power K _{a of} the anterior surface of the cornea is determined as follows: Ka1 = n1 - n0 / Ra1 Ka2 = n1 - n0 / Ra2 (1)
• b) the radii R _p1 and R _{p2 of} the main _{sections of} the back of the cornea are measured OCT-based and from this the refractive powers K _p1 and K _{p2 of} the back of the cornea are determined as follows: Kp1 = n2 - n1 / Rp1 K ₂ = n2 - n1 / Rp2 (2)
• c) the central thickness d of the cornea is measured OCT-based and then the total refractive powers K ₁ and K _{2 of} the main sections of the cornea are determined as follows: K1 = Ka1 + Kp1 - d / n1Ka1Kp1 K2 = Ka2 + Kp2 - d / n1Ka2Kp2 (3)

in which

K _1,2

the total refractive power of the cornea,

K _a1,2

the refractive power of the front of the cornea,

K _p1,2

the refractive power of the back of the cornea,

R _a1,2

the radius cornea front,

R _p1,2

the radius cornea back,

n ₀

the refractive index of air,

n ₁

the refractive index of the cornea,

n ₂

the refractive index of aqueous humor and

d

the central thickness of the cornea

define the radii R _a1,2 , R _p1,2 and the thickness d in meters and the refractive powers K _a1,2 , K _p1,2 and K _{1,2 are given} in diopters.

• • Brechungsindex Luft n₀ = 1,
• • Brechungsindex Hornhaut n₁ = 1,376 und
• • Brechungsindex Kammerwasser n₂ = 1,336.

Again, the prerequisite for the refractive indices are the aforementioned typical values:

• Refractive index air n ₀ = 1,
• • refractive index cornea n ₁ = 1.376 and
• • refractive index aqueous humor n ₂ = 1.336.

In contrast to the solutions of the prior art, which are based on various classical measuring methods with corresponding assumptions, the radii of the corneal back side R ₁ and R _{2 are} measured in the method proposed here in addition to the radii of the cornea front side R _a1 and R _a2 .

While the radii of the anterior corneal surface R _a1 and R _{a2 are} measured keratometrically or topographically, the determination of the other measured values, such as the radii of the back of the cornea R ' _p1 and R' _p2 and the central thickness d of the cornea are OCT-based.

According to the invention, the OCT-based measured radii R ' _p1 and R' _{p2 of} the back of the cornea are not directly included here as measured values in the calculation of the total refractive powers K ₁ and K ₂ according to method step c), but are also considered in an additional step

d) certain front / back ratios c ₁ and C ₂ : R _p1 = c ₁ · R _a1 R _p2 = c ₂ · R _a2 (4) With c1 = R'p1 / R'a1 c2 = R'p2 / R'a2 in that the radii R ' _a and R' _p are measured OCT-based and R _{p is} a resulting measurement from keratometric or topographic and OCT-based measurement. In the calculation of the total refractive power K, according to the formula (4), the front / back ratio c is taken into consideration.

For the calculation of the total refractive powers K ₁ and K ₂ , the radii R _p1 and R _{p2 of} the back of the cornea are determined from the front / rear ratios c ₁ and c ₂ by changing the formulas (4).

With regard to the requirements for a correct behavior of the OCT scanner and to an exact optical adjustment, as well as occurring errors in the OCT-based measured values, the previously described applies accordingly.

Since there is necessarily more than one measured value for R _a1 , R _a2 , R _p1 , R _p2 and / or d, the OCT-based measured values are also arithmetically averaged here after previous removal of "outliers" and then go into the calculation of Total refractive powers K _1.2 a.

According to the invention, the OCT-based measurement of the radii of the cornea also takes place here in that image processing algorithms are applied to the B-scans. For example, functions on the front and back of the cornea can be fitted. Preferably, in each case a circle is fitted, which is preferably symmetrical to the vertex.

According to a further embodiment of the method according to the invention, it is possible not only to realize B-scans along the previously determined main sections, but also along several meridians and to construct a height map of the back of the cornea therefrom. This is possible either purely OCT-based or by combination of keratometry / topography and pachymetry (from OCT). A detailed description can be found in the literature [6].

The measurement of asphericity, d. H. The so-called "Shape Factor" is possible with topographs as well as kerateters with more than one measuring zone and is also common. If conclusions about the optical properties of the cornea, such as the spherical aberrations, are to be drawn from this shape parameter, a measurement of the contribution of the back of the cornea is desirable. Accordingly, the one described previously for the toric measurement can also be used correspondingly for the asphericity. Instead of a toric, an aspherical view should be used for both surfaces. The combination of the optical effects of the two corneal surfaces is correspondingly generalized by means of ray tracing, for example.

With the solution according to the invention, a method is made available with the total refractive power of the cornea of an eye can be accurately determined.

The method dispenses with the assumption of normal values and is also applicable for the determination of the total refractive power of the laser-treated cornea of an eye.

According to the invention, the OCT-based measured radius R _{p of} the back of the cornea is not directly included as measured values in the calculation of the total refractive powers K, but rather from the previously determined front / back ratio c.

This procedure has the advantage that the requirements for a correct behavior of the OCT scanner and for an exact optical adjustment can be significantly reduced. In addition, a measurement of the actual scanner behavior can be dispensed with.

In addition, errors in the OCT-based measured values have little or no influence on the determination of the total refractive power K. Thus, the accuracy requirements of certain aspects of data acquisition and evaluation are significantly lower than those known from the prior art Solutions.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• US 2005/0203422 A1 [0008, 0013]
• US 2005/0018137 A1 [0012]

Claims (12)

Method for determining the total refractive power of the cornea of an eye, from the measured values of a keratometric or topographic and at least one OCT-based measurement, characterized in that a) the radius R _{a of} the anterior corneal surface is measured keratometrically or topographically and from this the refractive power K _{a of} the anterior corneal surface following is determined: K _a = n1 - n0 / Ra (1) b) the radius R _{p of} the back of the cornea is measured OCT-based and from this the refractive power K _{p of} the back of the cornea is determined as follows: Kp = n2 - n1 / Rp (2) c) the central thickness d of the cornea is measured OCT-based and then the total refractive power K of the cornea is determined as follows: K = Ka + Kp - d / n1 KaKp (3) where K is the overall refractive power of the cornea, K _a is the refractive power of the anterior corneal surface, K _{p is} the refractive power of the cornea back, R _{a is} the radius of the anterior corneal surface, R _p the radius of the cornea back, n ₀ is the refractive index of air, n ₁ is the refractive index of the cornea, ₂ the refractive index n of the aqueous humor and d define the central thickness of the cornea, the radii Ra, Rp and the thickness d in meters and the powers K, Ka and Kp in diopters are given. A method according to claim 1, characterized in that the OCT-based measured radius R ' _p the cornea back not directly as a measured value in the calculation of the total refractive power K after step c) received, but the previously determined in an additional step d) front / back ratio c considered: R _p = c · R _a (4) With c = R'p / R'a in that the radii R ' _a and R' _p are measured OCT-based and R _{p is} a resulting measurement from keratometric or topographic and OCT-based measurement. A method according to claim 1 and 2, characterized in that the OCT-based measurement takes place in the form of at least one B-scan, along at least one meridian. Method according to at least one of the preceding claims, characterized in that the OCT-based measurement in the form of several B-scans is preferably carried out along several meridians. Method according to at least one of the preceding claims, characterized in that the OCT-based determined measured values are averaged arithmetically after prior removal of "outliers" and then entered into the calculation of the total refractive power K according to method step c). Method according to at least one of the preceding claims, characterized in that the OCT-based measurement of the radii of the cornea is performed by image processing algorithms applied to the B-scans, for example by fitting functions on the front and back of the cornea. Method according to at least one of the preceding claims, characterized in that on the front and back of the cornea in each case a circle is fitted, which is preferably symmetrical to the vertex. Method for determining the total refractive power of the cornea of an astigmatic eye, from the measured values of a keratometric or topographic and OCT-based measurements in the form of two B-scans, along two meridians corresponding to the previously determined main sections, characterized in that a) the radii R _a1 and R _{a2 of} the main sections of the anterior corneal surface are measured keratometrically or topographically and from this the refractive power K _{a of} the anterior surface of the cornea is determined as follows: Ka1 = n1 - n0 / Ra1 Ka2 = n1 - n0 / Ra2 (1) b) the radii R _p1 and R _{p2 of} the main _{sections of} the back of the cornea are measured OCT-based and from this the refractive powers K _p1 and K _{p2 of} the back of the cornea are determined as follows: Kp1 = n2 - n1 / Rp1 Kp2 = n2 - n1 / Rp2 (2) c) the central thickness d of the cornea is measured OCT-based and then the total refractive powers K ₁ and K _{2 of} the main sections of the cornea are determined as follows: K1 = Ka1 + Kp1 - d / n1Ka1Kp1 K2 = Ka2 + Kp2 - d / n1Ka2Kp2 (3) in which K _1,2 the total refractive power of the cornea, K _a1,2 the refractive power of the anterior corneal surface, K _p1,2 the refractive power of the cornea back, R _a1,2 the radius of the anterior corneal surface, R _p1,2 the radius of corneal back, n ₀ is the refractive index of air, n ₁ define the refractive index of the cornea, n ₂ the refractive index of the aqueous humor and d the central thickness of the cornea, the radii R _a , R _p1,2 and the thickness d in meters and the refractive powers K _1,2 , Ka _a1,2 and K _{p1,2 are given} in diopters. A method according to claim 8, characterized in that the OCT-based measured radii R ' _p1 and R' _{p2 of} the corneal back not directly as a measured value in the calculation of the total refractive power K ₁ and K ₂ after step c) received, but previously in an additional step d) consider certain front / rear ratios c ₁ and c ₂ : R _p1 = c ₁ · R _a1 R _p2 = c ₂ · R _a2 (4) With c1 = R'p1 / R'p2 c2 = R'p2 / R'a2 in that the radii R ' _a and R' _p are measured OCT-based and R _{p is} a resulting measurement from keratometric or topographic and OCT-based measurement. Method according to claim 8 and 9, characterized in that the OCT-based determined measured values are averaged arithmetically after previous removal of "outliers" and then entered into the calculation of the total refractive powers K ₁ and K ₂ according to method step c). A method according to claims 8 to 10, characterized in that the OCT-based measurement of the radii of the cornea is performed by image processing algorithms applied to the B-scans, for example by fitting functions on the front and back of the cornea. A method according to claims 8 to 11, characterized in that at the front and back of the cornea in each case a circle is fitted, which is preferably symmetrical to the vertex.