Optics: eye examining – vision testing and correcting – Spectacles and eyeglasses – Ophthalmic lenses or blanks
Reexamination Certificate
1997-03-28
2001-06-05
Sugarman, Scott J. (Department: 2873)
Optics: eye examining, vision testing and correcting
Spectacles and eyeglasses
Ophthalmic lenses or blanks
C351S16000R
Reexamination Certificate
active
06241355
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the design and fabrication of contact lenses, and in particular to a method using spline-based mathematical surfaces without restrictions of rotational symmetry.
BACKGROUND OF THE INVENTION
Besides the obvious cosmetic aspect, contact lenses generally offer improved visual acuity compared to spectacles. In some cases, the difference is dramatic. For example, in the case of the corneal pathology of keratoconus (a corneal condition in which there is local region of high curvature), contact lenses can often succeed in providing excellent visual acuity (perhaps 20/20 using the standard Snellen eye chart) whereas spectacles are not able to provide more than a minor improvement over the uncorrected vision. In addition to improved visual acuity, contact lenses are also indicated for other diverse purposes, such as a medicine delivery system or as a “bandage” for protection of the cornea after erosion, trauma, or surgery.
Current contact lenses have shapes formed from relatively simple geometries, mostly spherical of different radii, or conic sections, etc. Consequently, there are many limitations, including poor fit for corneas with complex shape (such as might be found in keratoconus or in post-surgical corneas), edges that are uncomfortable, limited optical correction, limited ballasting/stabilization designs (for orientation of non-rotationally symmetric lenses), etc.
The most complex designs of the anterior or posterior surface of a contact lens are based on two or three surface zones. They are usually spherical, or sometimes so-called “aspherical”. Although “aspherical” literally means “not a sphere”, this term is used more narrowly in the contact lens field to refer to what mathematicians call surfaces of revolution of conic sections or toric surfaces.
Most contact lenses are either lathed directly, or molded from molds that were produced from pins and inserts that were lathed or ground. The lathing and grinding technology that is commonplace in the contact lens industry produces rotationally-symmetric lenses (except for toric lenses for astigmatism which are generally produced by a rather ad-hoc “crimping” method) and the shapes are fairly simple geometrically. In order to realize more general shapes, more sophisticated fabrication techniques are necessary. Computer numerical control (CNC) machining moves a cutting tool along a path on a part according to a mathematical model. The concept is that the computer instructs the machine how to make the complex shape, and the machine is capable of making such a shape. Furthermore, such high accuracy is achieved that the usual requirements of polishing the scallops (ridges) is greatly reduced or even eliminated. Some CNC machines can produce contact lenses that are non-rotationally symmetric.
There is a widely-held precept that making custom shapes is impractical and costly—that “one size fits all” is the only economical method. The advent of CNC machining shatters some deeply-held beliefs about manufacturing. In the traditional manufacturing process, the notions of mass production and economies of scale are predicated on the assumption of producing many identical copies of a product; but these ideas date from the industrial revolution. “Any color as long as it is black” is a Model T concept. Nowadays, automobiles are manufactured efficiently despite the fact that each one that rolls out of the factory door has a unique permutation of a dizzying array of options. Mass customization can be realized by integrating computers into the manufacturing process such that each contact lens can be automatically produced to custom specifications; the computer simply uses the particular set of values of the parameters of the mathematical model for each unit. Concerns about minimizing the number of different stock keeping units (SKU's) could be a thing of the past by embracing concepts of just in time manufacturing.
Although CNC machines enable the fabrication of complex surfaces, and the variation from one unit to the next, they require that a powerful mathematical model be used. In the evolution from traditional manual machining to automated CNC technology, all details must be specified competely and precisely.
PRIOR ART
U.S. Pat. No. 5,452,031, Sep. 19, 1995, to Christopher A. Ducharme of Boston Eye Technology, Inc. perpetuates the classic concept of a surface of revolution of a curve and is restricted to rotationally symmetric surfaces. Ducharme uses the word “spline” in a narrow sense, which will be further explained later, of a piecewise cubic function with continuity of position and the first two derivatives, and that patent does not include any broader mathematical specifications. He describes the use of the spline only for the peripheral zone, not for the optic zone. Furthermore, Ducharme does not address the determination of the optics for the lens in the case of a spline-based design. Although he mentions thickness, he does not provide any information that would enable someone to design a lens to achieve desired optics. He describes only a spline that joins the optic zone with C
1
(first derivative) continuity. Ducharme's discussion is limited to C
1
continuity with the optic zone because it is simply implementing the so-called “clamped” spline, and provides no indication whatsoever of more sophisticated mathematics to transcend such limitations on the level of continuity. His discussion is limited to polynomials. Ducharme assumes that the lens must be lathe cut.
A related international application, PCT/US94/10354, published Mar. 16, 1995, of David M. Lieberman is restricted to contact lenses smaller than the diameter of the cornea. It also only addresses the peripheral portion of the posterior surface. Lieberman does not provide details of the shape. He discusses a simple “brute force” collection of point data with no higher-order mathematical structure developed. He refers to the use of 1500 points to describe the surface. He does not include any mathematical methodology. This lack of mathematical model results in a representation that is unnecessarily large and inefficient.
A related U.S. Pat. No. 5,114,628, May 19, 1992, to Peter Hofer, Peter Hagmann, Gunther Krieg, and Eberhard Vaas of Ciba-Geigy Corporation in Germany shows the manufacture of individually fitted contact lenses from corneal topography but does not provide details of the shape, and does not present any higher-order mathematical description. Like the Lieberman patent, the Hofer et al patent does not include any mathematical methodology.
Note that even a combination of these prior-art references does not provide the physical features of the present application.
OBJECTS AND ADVANTAGES OF THE INVENTION
Accordingly, several objects and advantages of this invention are a more powerful mathematical representation to enable more complex shape description, without restrictions of rotational symmetry, for contact lenses to improve fit, optics, patient comfort, and corneal health.
The very general spline formulations used in this method incorporate many features, including (but not limited to):
continuity of arbitrarily high order
geometric continuity as well as parametric continuity
shape parameters, if desired
elimination of rotational symmetry restriction
spline-based optical zone (not constrained to be spherical)
ability to embed exact spherical zones
eccentrically-located optical zone
complex-shaped tear layer gap (“mismatch”)
capability to have non-circular periphery
The novel techniques presented here enable the design and fabrication of contact lenses that transcend the state of the art. Based on the more powerful mathematical representation of splines, these contact lenses can have posterior surfaces that provide a good fit to corneas of complicated shapes. This enables the design of lenses (including soft lenses) with good optics for irregularly shaped corneas.
Further objects and advantages of this invention will become apparent from a consideration of the drawings and ensuing description.
REFE
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