Patent Application
20060116763
The present invention generally relates to intraocular lenses (IOL) and, more particularly, to such lenses that provide enhanced vision for large pupil sizes.
An Intraocular lens is routinely implanted in a patient's eye during cataract surgery to compensate for the lost optical power when the natural lens is removed. In other applications, an intraocular lens can be implanted in a patient's eye, which retains its natural lens, so as to provide an optical power for correcting a refractive error of the natural eye. The aberrations of the eye, and in particular those of the cornea, are typically ignored in designing conventional intraocular lenses. Hence, patients having such lenses can suffer from a degraded image quality, especially at low light levels and large pupil sizes.
Intraocular lenses that compensate for corneal aberrations are also known. Typically, such aspheric intraocular lenses are designed to counteract the asphericity of the patient's cornea by greatly reducing, or eliminating all together, the overall aberrations of the eye. Although intraocular lenses fabricated based on these techniques may provide a better image contrast, they generally result in a decrease in a patient's depth of field.
The present invention is generally directed to intraocular lenses that can provide a balance between image contrast and depth of field, upon implantation in a patient's eye, so as to afford the patient improved vision, especially under conditions where the pupil of the eye is large. More particularly, an intraocular lens of the invention can exhibit a selected degree of asphericity at one or more refractive surfaces such that the aberrations of the lens combine with those of the eye in a manner that would provide the patient not only with a useful image contrast, but also with a depth of field within an acceptable range, especially for large pupil sizes.
In one aspect, the present invention provides an intraocular lens (IOL) that includes an optic having an anterior refractive surface and a posterior refractive surface, which cooperatively provide a selected optical power, e.g., an optical power in a range of about zero to about 40 Diopters (D) or more, typically in a range of about 18 to about 26 Diopters. One or both of these surfaces are characterized by an aspheric profile for controlling the aberrations of an eye in which the IOL is implanted so as to provide the patient with an image contrast, as characterized by a peak modulation transfer function (MTF), of at least about 0.25 at a spatial frequency of about 50 line pairs per millimeter (1 p/mm) and a depth of field of at least about 0.75 Diopters (D). For example, the implanted lens can provide the patient with an MTF in a range of about 0.25 to about 0.4 and a depth of field in a range of about 0.75 to about 1.5 Diopters. The aspherical lenses of the present invention can control the aberrations of the eye of a pseudophakic patient, i.e., a patient having the IOL as a replacement for a natural lens. Alternatively, such lenses can control the aberrations of the eye of a phakic patient, i.e., a patient having the IOL in addition to the natural lens.
As is known to those skilled in the ophthalmic art, a modulation transfer function (MTF) provides a quantitative measure of image contrast exhibited by an optical system, e.g., a system formed of an IOL and the cornea or an optical system formed of an IOL, the cornea and the natural lens, as discussed in more detail below. Further, the terms “depth of field” and “depth of focus,” which are herein used interchangeably, are well known in the context of a lens and readily understood by those skilled in the art. To the extent that a quantitative measurement is needed to describe the present invention, the term “depth of field” or “depth of focus” as used herein, can be calculated and/or measured by an amount of defocus associated with the optical system at which a through-focus modulation transfer finction (MTF) of the system calculated and/or measured with an aperture, e.g., a pupil size, of about 4.5 mm and monochromatic green light, e.g., light having a wavelength of about 550 nm, exhibits a contrast of at least about 0.05 at a spatial frequency of about 50 line pairs per millimeter (1 p/mm).
In a related aspect, the aspheric profile of the anterior surface or the posterior surface, or both, can control the aberrations of an eye in which the IOL is implanted such that the combined lens and cornea would exhibit a peak modulation transfer function contrast of at least about 0.25 at a spatial frequency of about 50 1 p/mm and a depth of field of at least about 0.75 Diopters for pupil diameters in a range of about 4.5 mm to about 5 mm and for monochromatic light at a wavelength of about 550 nm. For example, the peak modulation transfer finction can be calculated in a model eye, as discussed in more detail below.
An IOL according to this invention can be fabricated preferably by employing a deformable biocompatible material, such as acrylic, silicone, or hydrogel polymeric materials and the like that allow the lens body to be folded for insertion into the eye. For example, the optic can be formed of a copolymer of acrylate and methacrylate. For illustrative examples of such copolymer compositions, see for example, U.S. Pat. No. 5,922,821 entitled “Ophthalmic Lens Polymers” issued to Lebouef et al. on Jul. 13, 1999 and U.S. Pat. No. 6,353,069 entitled “High Refractive Index Ophthalmic Device Materials” issued to Freeman et al. on Mar. 5, 2002, the teachings of both of which are hereby incorporated by reference. In other embodiments, rigid biocompatible materials, such as polymethyl methacrylate (PMMA) can be employed.
In some embodiments, the aspherical profile of one of the surfaces can exhibit a selected deviation from a putative spherical profile having a radius of curvature of R1 that substantially coincides with the aspherical profile at small radial distance from an optical axis of the lens, while the other surface can have a spherical profile having a radius of curvature R2. Alternatively, the other surface can also have an aspherical profile exhibiting deviations from a respective putative spherical profile having a radius of curvature of R2. The radii R1 and R2 are selected such that the lens would exhibit a desired optical power. In addition, if needed, R1 and R2 can be chosen to impart a selected shape factor (X) to the lens, which is generally defined by the following relation:
In a related aspect, at least one refractive surface of the IOL has an aspheric portion for controlling average aberrations exhibited by the eyes of a selected patient group such that upon implantation of the lens in a patient's eye the combined lens and cornea would exhibit a peak modulation transfer function (MTF) contrast of at least about 0.25 for monochromatic light having a wavelength of 550 nm and depth of field of at least about 0.75 Diopters. The MTF and the depth of field can be calculated or measured, for example, for a spatial frequency of about 50 line pairs per millimeter and for a pupil size of about 4.5 mm.
In another aspect, the profile of the aspheric surface can be characterized by the following relation:
wherein
z denotes a sag of the surface parallel to an axis (z) perpendicular to the surface,
C denotes a curvature at the vertex of the surface,
Q denotes a conic coefficient,
R denotes a radial position on the surface,
A denotes a fourth order deformation coefficient, and
B denotes a sixth order deformation coefficient. Distance units are given herein in millimeters. For example, the curvature constant is given in units of inverse millimeter, while A is given in units of
and B is given in units of
The curvature constant C can be chosen based on a desired optical power of the lens, and the aspherical coefficients Q, A, and B, as well as the higher order terms where applicable, can be chosen so as to impart a selected degree of asphericity to the surface. As discussed in more detail below, the choice of the aspherical coefficients can generally depend on the material from which the lens is fabricated, the shape factor of the lens, and the aberrations of the eye for which the lens is intended. For example, the conic constant for a biconvex lens of average power (e.g., 21 Diopters) formed of an acrylic polymer can be in a range of about 0 (zero) to about −100 (minus 100), or in a range of −10 to about −50, or in a range of about −15 to about −25 and the higher order deformation coefficients A and B can be, respectively, in a range of about −1×10−3 (minus 0.001) to about 1×10−3 (plus 0.001) and in a range of about −1×10−4 (minus 0.0001) to about 1×10−4 (plus 0.0001). Further, in many embodiments, the curvature coefficient (C) can be in a range of about 0.0125 to about 0.12, or in a range of about 0.025 to about 0.1 (the curvature can be positive or negative corresponding to convex or concave surfaces, respectively).
In another aspect, the invention provides a method of designing an intraocular lens having an anterior and a posterior refractive surface that includes deriving a model average of aberrations of the eye based on wavefront measurements of aberrations exhibited by the eyes of a selected patient population (alternatively the aberrations of an individual patient for whom the lens is intended can be employed), and adjusting asphericity of at least one of the refractive surfaces for controlling the average aberrations such that a patient in which the lens is implanted would exhibit an image contrast characterized by a peak modulation transfer function (MTF) contrast of at least about 0.25 and a depth of field of at least about 0.75 D.
Referring again to
As known to those having ordinary skill in the art, a quantitative measure of image contrast provided by a lens can be obtained by calculating and/or measuring a modulation transfer function (MTF) associated with that lens. In general, a contrast or modulation associated with an optical signal, e.g., a two-dimensional pattern of light intensity distribution emanated from or reflected by an object to be imaged or associated with the image of such an object, can be defined in accordance with the following relation:
wherein Imax and Imin indicate, respectively, a maximum or a minimum intensity associated with the signal. Such a contrast can be calculated or measured for each spatial frequency present in the optical signal. An MTF of an imaging optical system, such as the combined IOL and the cornea, can then be defined as a ratio of a contrast associated with an image of an object formed by the optical system relative to a contrast associated with the object. As is known, the MTF associated with an optical system is not only dependent on the spatial frequencies of the intensity distribution of the light illuminating the system, but it can also be affected by other factors, such as the size of an illumination aperture as well as the wavelength of the illuminating light.
Although in many embodiments an IOL according to the invention is utilized to enhance a patient's image contrast, in some embodiments, it can be employed to primarily enhance a patient's depth of field with a potential moderate decrease in the image contrast. For example, a patient whose cornea exhibits a highly aspherical flattening can benefit from an aspherical IOL according to one embodiment of the invention that can partially compensate for the severe flattening to enhance the patient's depth of filed albeit with a potential small decrease in the image contrast.
In some embodiments, the aspherical profile of the anterior surface 16 of the IOL 10 as a function of radial distance (R) from the optical axis 20, or that of the posterior surface or both in other embodiments, can be characterized by the following relation:
wherein
z denotes a sag of the surface parallel to an axis (z), e.g., the optical axis, perpendicular to the surface,
C denotes a curvature at the vertex of the surface,
Q denotes a conic coefficient,
R denotes a radial position on the surface,
A denotes a fourth order deformation coefficient, and
B denotes a sixth order deformation coefficient.
In many embodiments, the conic constant Q alone can be adjusted to obtain a desired deviation from sphericity with the higher order aspherical constants A and B, and others set to zero. In other embodiments, one or both of the higher order constants A and B, in addition to, or instead of, the conic constant Q, can be adjusted to provide a selected aspherical profile for one or both refractive surfaces of an IOL. The higher order aspherical constants can be particularly useful for tailoring the profile of the peripheral portions of the lens surface, i.e., portions far from the optical axis.
The choice of the aspherical constants can depend, for example, on the aberrations of the eye in which the IOL is implanted, the material from which the IOL is fabricated, and the optical power provided by the IOL. In general, these constants are selected such that the IOL can provide a balance between a patient's image contrast and depth of field, e.g., enhancing the image contrast while substantially preserving the depth of field. For example, in some embodiments in which the IOL is fabricated from an acrylic polymeric material for implantation in an eye exhibiting a corneal asphericity characterized by a corneal conic constant in the range of zero (associated with severe spherical aberration) to about −0.5 (associated with a high level of aspherical flattening) the conic constant Q of the lens in the above relation can be in a range of about 0 to about −50, while the deformation coefficients A and B can be, respectively, in a range of about −1×10−3 to about 1×10−3 and in a range of about −1×10−4 to about 1×10−4.
As noted above, the choice of the degree of sphericity of one or both surfaces of the IOL can depend, at least in part, on the lens's shape factor (X). For examples, in some embodiments in which the IOL exhibits a shape factor in a range of about 0 to about +1, the conic constant can be in range of about −50 to about 0.
An IOL according to the teachings of the invention can find a variety of different applications. By way of example, in phakic patients, the IOL 10 can be implanted in a patient's eye while retaining the eye's natural lens by inserting the optic 12 in the eye's anterior chamber 26 with the distal ends of the fixation members 14 in contact with an angle 28 of the iris 30, as shown in
In another application, in pseudophakic patients, the IOL 10 can be implanted in a patient's eye after removal of the patient's natural lens during cataract surgery. The aspherical profile of the IOL 10 can control the aberrations exhibited by the cornea so as to enhance the image contrast while substantially preserving the depth of field. In some cases, the cornea is substantially spherical, characterized, for example, by a vanishing conic constant, while in other cases the cornea itself can show a certain degree of asphericity. The aspherical profile of the IOL 10 can be adjusted accordingly to provide the desired degree of image contrast, or depth of field, enhancement. For example, in some cases, the aspherical profile of IOL can be characterized by curve that is flatter than that of a putative spherical profile, while in other cases it is steeper.
A variety of techniques can be employed to determine the requisite degree of asphericity for the IOL 10. For example, in one approach, aberrations exhibited by a patient's eye, or by a group of patients, are measured preoperatively by employing known topographical methods and systems. For phakic eyes, the measured aberrations can correspond primarily to the combined aberrations of the natural lens and the cornea while for the pseudophakic patients, they can correspond to those of the cornea. In one such method, wavefront aberrations of the eye can be measured at a selected measurement plane, e.g., the entrance pupil of the patient's eye, and at a selected wavelength, e.g., at a wavelength of about 830 nm. Further details regarding measurements of aberrations of the eye can be found in U.S. Pat. No. 6,786,603 and U.S. patent application Ser. No. 2002/0105617, both of which are herein incorporated by reference in their entirety.
Wavefront measurements can be employed to determine a requisite level of asphericity of the IOL needed for controlling the aberrations of the eye. For example, an aspherical profile of the IOL can be designed to reduce spherical aberrations of the cornea inferred from the wavefront measurements. One or more aspherical parameters of the IOL can be obtained theoretically or experimentally, or both. For example, a ray tracing program, such as OSLO, marketed by Lambda Research Corporation of Littleton, Mass., U.S.A, can be employed to model the eye and its aberrations inferred from the wavefront measurements as well as an IOL having one or more aspherical surfaces. The asphericity of the IOL can then be adjusted, e.g., via adjusting the conic constant and possibly the higher order deformation constants, to obtain a desired MTF and depth of field. In some cases, average aberrations exhibited by the eyes of a selected group of patients are considered for designing an IOL suitable for controlling on average such aberrations.
The aspherical parameters of an IOL according to the teachings of the invention can also be determined experimentally. For example, an IOL can be inserted in a model eye exhibiting aberrations corresponding to those inferred from the wavefront measurements, e.g., a corneal aberration characterized by a conic constant. Subsequently, modulation transfer functions obtained by a combined system of the model eye and intraocular lenses having different aspherical profiles are measured to select a suitable aspherical profile.
As noted above, in some cases, the aberrations of a population of patients are measured to design a lens suitable for controlling average aberrations exhibited by patients. For example, in some cases, two or more types of intraocular lenses, each type designed to control average aberrations exhibited by the eyes of select group of patients, can be provided.
To demonstrate the efficacy of intraocular lenses according to the teachings of the invention to provide a more useful balance between the image contrast and the depth of field, theoretical ray tracing calculations were performed to determine modulation transfer functions exhibited by a combined system of an IOL according to the teachings of the invention having an aspherical profile and an eye modeled to have a selected corneal aberration in a range typically exhibited by patients in the general population. More specifically, for each theoretically modeled lens, the modulation transfer function at 50 line pairs per millimeter (1 p/mm) and at a wavelength of about 550 nm, as well as a depth of focus, were calculated for the combined lens and cornea, as discussed below. In addition, corresponding control calculations of MTF and depth of focus were performed for a substantially identical IOL having a spherical profile, i.e., an IOL exhibiting a vanishing conic constant. The depth of field was determined as the amount of defocus about a nominal focus corresponding to the peak MTF at which the MTF value drops to about 0.05.
As another example,
An average patient may have a cornea with an asphericity characterized by a conic constant of −0.26. Although no calculation was performed for such a cornea with an aspherical IOL,
To summarize the data discussed above for the exemplary conditions A-E,
In another embodiment, an intraocular lens (IOL) of the invention can have one or two toric refractive surfaces that exhibit two different optical powers along two orthogonal surface directions. Such toric IOLs can be employed, for example, to correct astigmatism. In some embodiments, one surface is toric and the other non-toric. A selected degree of asphericity can be imparted to the toric surface, to the non-toric surface or to both. Alternatively, both lens surfaces can be toric with at least one exhibiting asphericity. For example, at least one of the toric surfaces can exhibit an asphericity along one or both of the two surface orthogonal directions, each associated with an optical power different than the power along the other direction, such that a combination of the lens and the eye in which the lens is implanted provides not only a useful image contrast, but also a depth of field within an acceptable range, such as those discussed above in connection with the other embodiments. For example, with reference to
Those having ordinary skill in the art will appreciate that various modifications can be made to the above embodiments without departing from the scope of the invention.
