The present application claims priority under 35 U.S.C. §119(e) to provisional application No. 61/418,234 filed on Nov. 30, 2010 under the same title, which is incorporated herein by reference in its entirety. Full Paris Convention priority is hereby expressly reserved.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to ophthalmic lenses and laser vision correction, and more particularly, to a method for designing, evaluating and optimizing ophthalmic lenses and laser vision correction in order to optimally manage issues resulting from, or related with, halos.
2. Description of the Background
Ophthalmic lenses, such as intraocular lenses (IOLs), phakic IOLs, piggy-back IOLs, spectacle lenses, contact lenses, and corneal implants may be used to enhance or correct vision. For example, IOLs are routinely used to replace the crystalline lens of an eye during cataract surgery.
Ophthalmic lenses, such as IOLs may be monofocal or multifocal. A monofocal IOL provides a single focal point, whereas a multifocal IOL provides multiple focal points for correcting vision at different distances. For example, a bifocal IOL provides two different focal points, routinely one for near vision and one for distant vision.
Ophthalmic lenses, such as the aforementioned multifocal IOLs, may be refractive, diffractive, or both refractive and diffractive. Multifocal refractive IOLs may be comprised of several concentric annular optical zones with each zone providing for a near or a far focus. A diffractive multifocal IOL is generally divided into a plurality of annular zones, or echelettes, that are offset parallel to the optical axis by predetermined diffractive step heights in order to provide a specific phase relationship between the annular zones. A diffractive multifocal IOL may divide incident light into two diffractive orders to provide near and distant vision.
Although multifocal lenses are effective for vision correction, further enhancements would be advantageous. One problem associated with multifocal/bifocal IOLs, in part due to the typically bifocal configuration of the refractive/diffractive zones, is dysphotopsia, and in particular halos under low light conditions. Halos may arise when light from the unused focal image creates an out-of-focus image that is superimposed on the used focal image. For example, if light from a distant point source is imaged onto the retina by the distant focus of a bifocal IOL, the near focus of the IOL will simultaneously superimpose a defocused image on top of the image formed by the distant focus. This defocused image may manifest itself in the form of a ring of light surrounding the in-focus image, and is referred to as a halo. In addition to multifocality, add power and light distribution may also contribute to dysphotopsia.
Discomfort, visual disturbance or nuisance from dysphotopsia may be tied to personal attributes or habits. For example, a patient's psychological profile may play an important role; more critical patients may be more affected by halos than those less critical. In addition, habitual circumstances may influence discomfort, e.g. truck drivers are typically more affected by halos due to night driving.
Aberrations of the cornea and in particular higher order corneal aberrations have a direct impact on halos. Corneal topographic analysis using photokeratoscopic or videokeratographic methods provides objective measures of corneal topography. Current measurement devices typically employ several concentric rings or multiple discrete light sources to reflect a luminous object of known dimension from the cornea. The size of the cornea-reflected images of this object is then measured with photographic or electro-optical recording methods to compare the shape of the cornea with a theoretical spherical shape. If the cornea is spherical, for example, the reflected images of the ring-shaped objects will be equally spaced, continuous, concentric ring-shaped patterns. If the cornea has surface defects, or is not precisely spherical, the resultant ring images will be less equally spaced or will have a different shape, such as an elliptical shape.
Corneal topography can thus be used to determine the optical aberrations of the cornea. Such aberrations in conjunction with the designs, methods, and systems disclosed herein may be used to manage halos. And, based on the aforementioned, a need exists for a lens design and, more particularly, to an apparatus, system and method for designing, evaluating and optimizing ophthalmic lenses for such management.
SUMMARY OF THE INVENTION
The present invention is and includes an apparatus, system and method to design, evaluate and optimize ophthalmic lenses, such as IOLs. In addition, the apparatus, system and methods can be used to optimize a laser vision correction nomogram.
A method of optimizing, evaluating and/or designing an ophthalmic lens involves initially measuring the preoperative corneal aberrations of a patient. Then, a simulated halo image, with a multifocal IOL incorporated, may be calculated for the corneal aberrations; the simulated image determining the halo size, shape and intensity. A reference halo which demonstrates acceptable dysphotopsia may then be compared with the simulated halo. Based on the comparison, a decision may be made whether to implant the multifocal IOL.
Another preferred embodiment, involves the following steps: measuring the preoperative corneal aberrations of a patient (or group of patients); calculating a simulated halo image for these aberrations, with the multifocal IOL; determining the halo size, shape and intensity; having a reference halo which demonstrates acceptable dysphotopsia; optimizing the IOL aberration profile so as to result in minimal halo, specifically when combined with the patient's (or group of patients') corneal aberration profile; and implanting the custom multifocal IOL.
It is understood that an important aspect of certain embodiments of this invention includes the characterization of the corneal aberrations of a selected group of patients or population for expressing an average corneal aberration. Average corneal aberration terms of the population expressed, for example, as an average linear combination of polynomials can then be calculated and used in the lens design method.
In another preferred method, after a multifocal IOL is implanted, the corneal aberrations of a patient are measured. Then, a simulated halo image is calculated for these aberrations with the multifocal IOL; the simulated image revealing the halo size, shape and intensity. A reference halo which demonstrates acceptable dysphotopsia is then compared to the simulated halo. Based on the comparison a determination is made whether the halo is predominantly caused by the corneal aberrations. If it is, the corneal aberrations may be modified by laser vision correction to minimize the halos, and with that, minimize the discomfort caused by halos.
Another preferred embodiment, involves the following steps: measuring the preoperative corneal aberrations of the multifocal IOL patient; calculating a simulated halo image for these aberrations, with the multifocal IOL; determining the halo size, shape and intensity; having a reference halo which demonstrates acceptable dysphotopsia; optimizing the laser vision correction so as to result in minimal halo; applying the laser vision correction to the patient's cornea.
Another preferred embodiment, involves the following steps: measuring the preoperative corneal aberrations of the multifocal IOL patient; using a vision simulator, measure the patient's visual performance (e.g. halo size, shape and intensity; discomfort, contrast vision, visual acuity), while varying the patient's corneal aberration; based on the test, determining the optimal corneal aberration as to optimize the visual performance; applying a laser vision correction to generate the optimal corneal aberration onto the patient's cornea.
Another preferred embodiment, involves the following steps: optimizing a corneal correction (e.g. presby-lasik), the simulated halo image being one of the optimization parameters; applying the presby-laser vision correction to the patient's cornea. Prior to optimizing a corneal correction, one may measure the corneal aberrations of a patient suffering discomfort or reduced visual performance.
An exemplary ophthalmic lens would include an anterior surface and an opposing posterior surface wherein at least one of the surfaces of the ophthalmic lens is characterized by an equation including a first coefficient configured to shape the halo and intensity profile in order to minimize bother from the halo.
A preferred embodiment includes an ophthalmic lens wherein at least one of the surfaces is characterized by a phase profile configured to modify the wavefront aberration in order to shape the halo and intensity profile in order to minimize bother from the halo. The phase profile may modify spherical aberration, coma, trefoil, and/or the product of any combination.
Thus, the present invention provides a method for designing, evaluating and optimizing ophthalmic lenses and laser vision correction.
BRIEF DESCRIPTION OF THE DRAWINGS
Understanding of the disclosure will be facilitated by consideration of the following detailed description of the embodiments, taken in conjunction with the accompanying drawings, in which like numerals refer to like parts and in which:
FIG. 1 is an illustration of an eye in the natural state;
FIG. 2 is an illustration of an eye having an intraocular lens;
FIG. 3 is a flow diagram illustrating a method for optimizing an ophthalmic lens in accordance with the present invention;
FIG. 4 illustrates the halo image of 46 physiological eyes as further detailed below;
FIG. 5 is a flow diagram illustrating a method for optimizing an ophthalmic lens in accordance with the present invention;
FIGS. 6a and 6b are examples of halo size, shape and intensity in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
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 the purposes of clarity, many other elements found in typical optical and optical simulation apparatuses, systems and methods. Those of ordinary skill in the art will recognize that other elements are desirable and/or required in order to implement 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.
FIG. 1 is an illustration of an eye 10 in the natural state. The eye 10 includes a retina 12 for receiving an image, produced by the cornea 14 and the natural lens 16, from light incident upon the eye. The natural lens 16 is disposed within a capsular bag 20. The iris 26 separates the anterior and posterior chambers of the eye and may operate to change the aperture, i.e. pupil size of the eye. More specifically, the diameter of the incoming light beam is controlled by the iris 26, which forms the aperture stop of the eye.
The capsular bag 20 is a resilient material that changes the shape and/or location of natural lens in response to ocular forces produced when the ciliary muscles 22 contract and stretch the natural lens 16 via the zonular fibers 24 disposed about an equatorial region of the capsular bag 20. This shape change may flatten the natural lens 16, thereby producing a relatively low optical power for providing distant vision in an emmetropic eye. To produce intermediate and/or near vision, the ciliary muscles 22 contract, thereby relieving tension on the zonular fibers 24. The resiliency of the capsular bag 16 thus provides an ocular force to reshape the natural lens 16 to modify the curvature to provide an optical power suitable for required vision. This change, or “accommodation,” is achieved by changing the shape of the crystalline lens. Accommodation, as used herein, includes changing the focus of the eye for different distances.
FIG. 2 illustrates an eye 10 having a natural lens replaced with an IOL 102. The natural lens may require removal due to a refractive lens exchange, or due to a disease such as cataracts, for example. Once removed, the natural lens may be replaced by an IOL 102 to provide improved vision in the eye. The IOL 102 may include an optic and haptics 104 or support structure for centering the optic. The haptics 104 may center the optic, and may transfer ocular forces from the ciliary muscle 22, zonules 24, and/or capsular bag 20 to the optic to change the shape, power, and/or axial location of the optic relative to the retina 12.
FIGS. 3 is a flow diagram illustrating methods of optimizing an ophthalmic lens, such as, for example, the IOL illustrated in FIG. 2, in accordance with the present invention. In the illustrated method, an ophthalmic lens may be designed and/or provided for modeling and for clinical application. With reference to FIG. 3, a method of optimizing, evaluating and/or designing an ophthalmic lens is comprised of measuring the preoperative corneal aberrations of the patient according to well-known topographical measurement methods. This may be done by taking the difference in optical path between the chief ray and a marginal ray over the pupil which yields the wavefront aberration for the cornea. (Guirao, A., & Artal, P. (2000). Corneal wave aberration from videokeratography: accuracy and limitations of the procedure. J Opt Soc Am A, 17 (6), 955-965.). Alternatively, ray tracing can be performed, e.g. using general purpose optical design software (e.g. Code V, Zemax, OSLO).
Preferably, at least the front corneal surface is measured and more preferably both the front and rear corneal surfaces are measured and characterized together in resulting wavefront aberration terms, such as a linear combination of polynomials which represent the total corneal wavefront aberrations. In the art of optics, topographical processes may include mathematically modeling a surface of the cornea using polynomial expansion series techniques, e.g. Seidel or Zernike polynomials, or the wavefront aberration can be calculated over a grid of points over the pupil.
For normal healthy corneas, 5th order Zernike expansion is typically sufficient to describe the corneal aberrations. The aberrations include both lower order terms, such as defocus and astigmatism, along with higher order terms, such as spherical aberration, coma, trefoil, etc., up to pentafoil. However, for non-uniform corneas, like post-LASIK corneas, more terms may be needed. For non-uniform corneas, it may be necessary to describe the corneal aberrations at discrete points on a grid filling the pupil.
The correlation between corneal aberrations and halo shape and intensity is demonstrated in the following example which encompasses the use of a set of 46 physiological eye models. The eye model (computer models) are based on the eyes of 46 cataract patients, and are described in further detail in the following which are incorporated herein by reference: Weeber, H. A., Featherstone, K. A., & Piers, P. A. (2010). Population-based visual acuity in the presence of defocus well predicted by classical theory. J. Biomedical Optics, 15 (4), 040509/040501-040509/040503; Weeber, H. A., & Piers, P. A. (2010). Optical and Visual Performance of Patient Populations Implanted With Monofocal and Multifocal IOLs in the Presence of Defocus. Invest. Ophthalmol. Vis. Sci., 51: E-Abstract 5751; Weeber, H. A., & Piers, P. A. (2011). Theoretical Performance of Intraocular Lenses correcting both Spherical and Chromatic Aberration. J. Refr. Surg., DOI: 10.3928/1081597X-20111103-01
The corneas of these eye models are described by 5th order Zernike sag surfaces, and the eye models have spectacle lenses in front of them. For this analysis, the eye models were ‘implanted’ with a diffractive multifocal IOL, having a pupil-independent diffractive profile across the entire optic, and a 50%:50% light distribution between far and near focus. However, it should be appreciated by those skilled in the art that any other multifocal lens would generate comparable analysis.
In addition to the set of physiological eyes, diffraction limited eyes were generated having the same corneal power, but generating no wavefront aberrations. These eye models served as reference models, being ‘perfect’ eyes.
All eye models have a physical pupil diameter of 4 mm which represents the pupil diameter of an average cataract patient under mesopic lighting conditions.
For both sets—the physiological eye models, and the corresponding diffraction limited eye models—polychromatic point spread functions (PSF) were calculated.
Then, an extended source, representing a headlight of 15 cm diameter at a 100 m distance was convoluted with the PSF. This resulted in the retinal image of the headlight, furtheron referenced as ‘halo images’. For optimal display and print, the pictures were processed using a gamma correction of 0.4. FIG. 4 shows the halo images of the 46 physiological eye which clearly demonstrates that the halo appearance differs considerably between the different eye models.
An assessment of the patient inconvenience from halos can be done in a variety of ways. One way is to determine the retinal image of halos in patients, and then assess the nuisance perceived by these patients (e.g. by a questionnaire). Alternatively (and as done in this example), the patient inconvenience from halo images can be estimated by assessment of the shape, area, brightness and contrast of the halo images. There are two additional ways to assess the halo image: by evaluating the halo image of the physiological eye, and by evaluating the difference between the halo image of the physiological eye and that of the diffraction limited eye. In the latter case, the halo image of the diffraction limited eye is subtracted from the halo image of the physiological eye. The resulting image (‘delta image’) highlights the halo only, without the central headlight.
These aspects are consolidated in the following metrics for halo patient inconvenience and corneal aberrations, the correlation between which are illustrated in Tables 1 and 2 below:
H1. (Image-) Correlation between the halo image of the physiological eye and the halo image of the diffraction limited eye. As the halo image of the diffraction limited is rotationally symmetric, with a uniform brightness of the halo, deviations denote changes in shape and brightness of the image.
H2. Area of the halo image
H3. Brightness of the halo image
H4. Brightness of the delta image
H5. RMS Contrast of the halo image
H6. RMS Contrast of the delta image
The optics of the cornea can be expressed in a variety of ways, including as follows:
O1. Wavefront aberration: RMS of the higher order aberrations, based on the Zernike coefficients over a 4-mm pupil (HOA)
O2. Wavefront aberration: RMS of the higher order aberrations, including the astigmatism terms, based on the Zernike coefficients over a 4-mm pupil (HOAA)
O3. Wavefront aberration: RMS of the asymmetrical higher order aberrations, including the astigmatism terms, based on the Zernike coefficients over a 4-mm pupil (AHOAA)
O4. Wavefront aberration: Coma, based on the Zernike coefficients over a 4-mm pupil (HOA)
O5. Wavefront aberration: Astigmatism, based on the Zernike coefficients over a 4-mm pupil (HOA)
O6. Wavefront aberration: Coma multiplied by Astigmatism, based on the Zernike coefficients over a 4-mm pupil (HOA)
O7. MTF Volume
O8. Area under the radial MTF curve
It should be understood that metrics have been derived from the halo images, and many other metrics can be derived, as known by those skilled in the art. The central theme is that the metrics are based on a retinal image of an extended object. In this example, the extended object is the headlight of a car.
Similarly, it should be understood that metrics have been derived from the optics of the eye, and many other metrics can be derived, as known by those skilled in the art. The central theme is that the metrics are based on the optics of the eye. In this example, the optics are described by corneal wavefront aberrations. The optics may further include aberrations caused by the internal optics of the eye, including those caused by the posterior cornea, and IOL misalignments.
Tables 1 and 2 show the results of single variable linear regression between the metrics of the bother of halo images and the metrics of the optics of the eye. Table 1 shows the P-values of least squares linear regression between the H- and O metrics. Table 2 shows the regression coefficient R2. These results show that the patient inconvenience from halo images is significantly correlated with the optical characteristics of the eye.