Patent Application
20080004698
The present invention generally relates to methods for designing an ocular implant, e.g., an intraocular lens (IOL), for surgical implantation in a patient's eye by taking into account ocular aberrations that can be induced during surgery, e.g., due to incision of the cornea. While the embodiments discussed below are generally directed to methods of designing an IOL for implantation in a patient's eye, the teachings of the invention can be equally applied to other ocular implants, such as intercorneal implants. Further, the term intraocular lens and its abbreviation “IOL” are used herein interchangeably to describe lenses that can be implanted into the interior of an eye to either replace the eye's natural crystalline lens or to otherwise augment vision regardless of whether or not the natural lens is removed.
During a cataract surgery, a small incision is made in the cornea, e.g., by utilizing a diamond blade. An instrument is then inserted through the corneal incision to cut a portion of the anterior lens capsule, typically in a circular fashion, to provide access to the opacified natural lens. An ultrasound or a laser probe is then employed to break up the lens, and the resulting lens fragments are aspirated. A foldable IOL can then be inserted in the capsular bag, e.g., by employing an injector. Once inside the eye, the IOL unfolds to replace the natural lens. The corneal incision is typically sufficiently small such that it heals without the need for sutures. However, in many cases, the incision—though healed—can induce corneal aberrations including astigmatism or modify pre-existing corneal aberrations including astigmatism. In the following embodiments, methods of designing an IOL are disclosed that allow the IOL to compensate for such surgically-induced corneal astigmatism, e.g., on a patient-by-patient basis. In some embodiments, the design methods allow customizing an IOL for a patient based on predicted surgically induced aberrations including astigmatism for that patient.
With reference to a flow chart of
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Typically, a cataract surgical incision can induce an astigmatism in a range of about ½ D to about 1 D. In some cases, such a surgically-induced astigmatism can modify a pre-existing astigmatism, e.g., worsen or ameliorate the pre-existing astigmatism. Modeling of the effect of the corneal incision in introducing or modifying astigmatic aberrations of the eye can take into account the incision type. By way of example, the effects of a temporal, a superior corneal incision, sub-conjunctival or other corneal incisions (e.g., a 3-mm incision) can be modeled. In many embodiments, other factors that can affect the surgically induced astigmatism (SIA), such as suturing method, presence of suture, incision type, the type of operation and incision width can be also taken into account when modeling SIA. By way of example, these factors are discussed in the following publication, which is herein incorporated by reference: “Optimal Incision Sites to Obtain Astigmatic-Free Cornea After Cataract Surgery With 3.2 mm Sutureless Incision,” by Matsusmoto et al. published in JCRS of Materials Science Letters 27, pp. 1841-1851 (2003).
Subsequently, a toricity for at least one optical surface of an ocular implant (e.g., an IOL) can be determined so as to enable the implant to provide compensation for the corneal astigmatism, including the modeled surgically-induced contribution. By way of example, a model eye having a cornea exhibiting the corneal astigmatic aberration of the patient, including the modeled surgically-induced contribution, can be established. A desired toricity for compensating the astigmatic aberration can then be determined by incorporating a hypothetical ocular implant (e.g., an IOL) in the model eye and varying a toricity of at least one of the implant's surfaces so as to optimize the optical performance of the model eye. In many embodiments, in establishing the model eye for a particular patient, not only the astigmatic aberrations, but also other visual defects of that patient (e.g., myopia, hyperopia) are taken into account.
In some embodiments, the optical performance of the implant can be evaluated by calculating a modulation transfer function (MTF) at the retinal plane of the model eye. As known in the art, an MTF provides a quantitative measure of image contrast exhibited by an optical system, e.g., a model eye incorporating an implant. More specifically, the MTF of an imaging system can 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. The human visual system utilizes most spatial frequencies resolvable by neural sampling. Thus, in some embodiments, the MTF values ranging from low (e.g., 10 line pairs (lp)/mm, corresponding to about 20/200 visual acuity) to high (e.g., 100 lp/mm, corresponding to about 20/20 visual acuity) can be averaged to obtain a measure of the optical performance of an implanted IOL. In some embodiments, the toricity of the surface can be varied until a maximal optical performance is obtained.
In some embodiments, the determined toricity of the surface can be mathematically defined, e.g., as a toric surface that can be represented as follows in an XYZ coordinate system (the positive Z-axis is assumed to be the optical axis):
where, rv is the radius of the circle and rh is the radius of the outer vertex of the toroid.
Once a desired toricity is established, an IOL having an optical surface exhibiting that toricity can be fabricated by utilizing a variety of techniques. For example, with reference to
In some other embodiments, the surfaces of the optical blank 10 can be shaped by utilizing an ablative laser beam. By way of example, an excimer laser, e.g., an argon-fluoride laser operating at a wavelength of 193 nm, can generate the laser beam. For example, in some cases, a mask having different transparencies at different portions thereof can be disposed between the laser beam and an optical surface of the blank so as to provide differential ablation of different surface portions so as to impart a desired shape to that surface. For example, at least one optical surface of the blank can be shaped so as to have a desired degree of toricity. Further details regarding the use of such ablation methods for fabricating IOLs can be found in U.S. Pat. No. 4,842,782, which is herein incorporated by reference.
In some embodiments, a machining method, herein referred to as Fast Tool Servo (FTS), is employed for imparting a toric profile to at least one surface of an optical blank. As shown schematically in
In some embodiments, the anterior and/or posterior surfaces of an optical blank, such as the above blank 10, can be shaped by employing the FTS machining method. For example, an optical blank formed of a soft acrylic material (cross-linked copolymer of 2-phenylethyl acrylate and 2-phenyl methacrylate) commonly known as Acrysof can be mounted in an FTS system such that a surface thereof faces the system's diamond blade. The motion of the blade can be programmed so as to cut a desired profile, e.g., a toric profile, into the blank's surface. In alternative embodiments, the FTS method can be employed to form optical pins, which can, in turn, be utilized to form the IOL from a desired material. Once the cylindrical axis of the toric profile is defined, it can be marked with axis mark on an optical pin or a lens. Then, when forming a haptic, it can be formed to be aligned with the cylindrical axis mark.
The above methods of designing an IOL advantageously allow custom-making an IOL for an individual patient. For example, prior to performing a cataract surgery on a patient, the patient's corneal topography can be determined, e.g., by utilizing wavefront aberration measurements. By way of example, an ophthalmologist (or other qualified personnel) can perform these measurements. These measurements can then be transmitted to an IOL design and manufacturing facility, which can employ them, together with a predicted surgically-induced astigmatism, to model an IOL suitable for the patient. An IOL can then be fabricated for that patient, which compensates for the astigmatic aberrations, and also corrects other vision defects of that patient.
Those having ordinary skill in the art will appreciate that various changes can be made to the above embodiments without departing from the scope of the invention.
