Method for determining the power of an intraocular lens

Abstract

A method for calculating the power of an intraocular lens including measuring an axial separation between the front surface of the cornea and the plane of the iris root; and determining the power of the intraocular lens using the measured axial separation together with other measured parameters and empirically determined lens constants.

Claims

1. A method for calculating a power of an intraocular lens comprising the steps of: measuring an axial separation (ACD') between a front surface of a cornea and a plane of an iris root; and determining the power of the intraocular lens using the measured axial separation together with other measured parameters and empirically determined lens constants.

2. The method of claim 1, wherein the plane of the iris root is determined using an optical coherence measurement at an infrared wavelength.

3. The method of claim 2, wherein the optical coherence measurement comprises one or more meridional scans.

4. The method of claim 3, wherein the location of the iris root is measured as outer end points of a strongly scattering layer in back of the iris.

5. The method claim 4, wherein the other measured parameters include an axial eye length and at least two central radii of the corneal front surface.

6. The method of claim 5, where the other measured parameters further include a lens thickness.

7. A method for calculating a power of an intraocular lens comprising steps of: measuring an axial separation (ACD) between a front surface of a cornea and a line connecting posterior edges of a ciliary body; and determining the power of the intraocular lens using the measured axial separation together with other measured parameters and empirically determined lens constants.

8. The method of claim 7, wherein the line connecting a posterior edge of a ciliary body is determined using an optical coherence measurement at an infrared wavelength.

9. The method of claim 8, wherein the optical coherence measurement comprises one or more meridional scans.

10. The method claim 7, wherein the other measured parameters include an axial eye length and at least two central radii of the corneal front surface.

11. The method of claim 7, where the other measured parameters further include a lens thickness.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross sectional view of the eye.

(2) FIG. 2 is a cross sectional view of the eye illustrating the axial distance (ACD) between the front surface of the cornea and the plane of the iris root.

DETAILED DESCRIPTION OF THE DRAWINGS

(3) As seen in the attached FIG. 1, in cross-sectional images of the anterior segment of the eye obtained by optical coherence tomography (OCT) at infrared wavelengths (e.g., 1310 nm) a strongly scattering layer 10 is visible near the back surface of the iris. This structure is commonly interpreted as the iris pigment epithelium. On the other hand, it has also been suggested that this scattering region may correspond to the iris dilator muscle.

(4) At the periphery, the absorbing layer ends at a well-defined radial position (location 12 in FIG. 1), which is anatomically close to, or co-located with the iris root. In a meridional cross sectional OCT scan of the cornea the two peripheral end points of the scattering layer in the iris can be used to uniquely identify two iris root points. A line connecting the two iris root points can be used to define a root-to-root line. This line is shown as item 20 in FIG. 2.

(5) The separation of the root-to-root line from the front of the cornea 16 can be used to define a modified anterior chamber depth parameter ACD (see FIG. 2). This can be defined in one of several different ways. Most simply, ACD can be defined as the longest perpendicular distance from the root-to-root line to the front surface of the cornea. If the patient fixates in a direction parallel to the direction of the OCT scan during measurement, the line of sight can be uniquely identified in the acquired cross sectional image. ACD can then alternatively be defined as the longest distance from the root-to-root line to the corneal front surface, measured parallel to the line of sight. Finally, the inner limits of the iris in the OCT scan can be used to mark the mid-point of the pupil along the root-to-root line. As a third alternative, the parameter ACD can be measured from this mid-point to the corneal front surface, in a direction parallel to the line of sight.

(6) The parameter ACD can be used to predict the post-op effective lens position. Like with presently used formulas this can be done by empirically determining regression coefficients for a set of parameters, such as ACD, AL, and/or LT. Other measured parameters can include the two central radii of the corneal front surface. The prediction of ELP thus obtained can be used in IOL calculation formulas together with empirical lens constants.

(7) Another possible approach relies on a measurement associated with a region located at the most anterior portion of a highly scattering layer posterior to the sclera (location 14 of FIG. 1). This layer is presumably a pigmented layer along the posterior boundary of the ciliary muscle. Similar to the approach shown in FIG. 2, a line connecting these two opposing points (14) can be drawn and the separation between this line and the front of the cornea (ACDnot shown) can be defined and used to predict the IOL position after surgery. The value for ACD can be used alone or in conjunction with the value for ACD.

(8) As is well known in the art, the determination of regression coefficients requires large data sets and produce formulas that have limited physical interpretation. The larger number of measurements to be included and the more complex the formula, the more data is required to develop those formulas. This can especially be a drawback in the modification of IOL calculation formulas for newly developed IOL's. The IOL calculation formula may instead take the form of regression formulas to calculate intermediate parameters such as the position of the IOL equator and the effective power of the lens. For example, the ELP is determined by a combination of anatomical features, such as the distance from the corneal vertex to the sulcus, by the design of the IOL and by surgical technique. Various surrogate measurements may be combined. For example the ACD characterizes the position of the iris root. A combination of the traditional ACD, LT, anterior radius of curvature of the crystalline lens, and possibly also posterior radius of curvature of the crystalline lens, characterize the crystalline lens equator. These and other surrogate measurements (including ACD) can be combined into a regression formula for predicting the position of the IOL equator. The ELP prediction can then be calculated as a combination of the optical power, derived from the radii of curvature and index of refraction, the predicted IOL equator. The resulting ELP estimate can be integrated into an IOL calculation formula.

(9) A particular embodiment of the inventive method consists in the following sequence: The axial length (AL) of a patient eye is measured using partial coherence interferometry (PCI), the modified anterior chamber depth ACD is determined using optical coherence tomography (OCT) and the corneal power is determined using a suitable keratometric setup. The keratometric setup can be a stand-alone keratometer or integrated into a combination device such as the IOLMaster. After obtaining these measurements, the values are processed together with the desired target refraction using the Haigis-Formula to determine the required power of an intraocular lens.

(10) In the Haigis-Formula

(11) $\mathrm{DL}=\frac{n}{L-d}-\frac{n}{n\text{/}z-d}\mathrm{with}z=\mathrm{DC}+\frac{\mathrm{ref}}{1-\mathrm{ref}\mathrm{dBC}}\mathrm{and}\mathrm{DC}=\frac{\mathrm{nC}-1}{\mathrm{RC}}$
where DL: IOL-refraction DC: cornea refraction RC: cornea radius nC: refractive index of the cornea ref: refraction to be obtained after surgery dBC: spectacle distance from cornea d: optical anterior chamber depth ACD L: Eye length n: refractive index of the eye (1.336) d is normally predicted using a function based on a multi-variable regression analysis from a large sample of surgeon and IOL-specific outcomes for a wide range of axial lengths (AL) and anterior chamber depths (ACD).

(12) In the preferred embodiment, the modified anterior chamber depths parameter ACD (or ACD) would be used in place of ACD in the regression fit.

(13) In other common IOL formulas an expression equivalent to d is used too as the table shows:

(14) TABLE-US-00001 SRK/T d = A-constant Hoffer Q d = pACD Holladay 1 d = Surgeon Factor Holladay 2 d = ACD

(15) Also in these formulas d may be substituted by the modified anterior chamber depth ACD (or ACD).

(16) The invention is not limited to the embodiments described, also other uses of the measured values ACD or ACD for IOL calculation fall within the scope of protection.