The disclosure of Japanese Patent application No. 2001-081556 filed on Mar. 21, 2001, and No. 2002-013081 filed on Jan. 22, 2002 including the specifications, drawings and abstracts is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates generally to contact lenses adapted to be worn on a surface of an eyeball and including a generally circular central zone serving for providing an optical zone and an annular peripheral zone disposed around the central zone. More particularly, the present invention is concerned with such a contact lens that is novel in construction and that permits an enhanced lens wearing comfort as felt by a lens wearer.
2. Description of the Related Art
There is known a contact lens that is worn on a surface of a cornea of a human eye for treating myopia, hyperopia, astigmatism, presbyopia and the like. Since the contact lens is worn on the cornea accompanied with contact thereof with very sensitive surfaces of the cornea and eyelids of a lens wearer, it is material to ensure an excellent wearing comfort as felt by the lens wearer.
In general, the contact lens includes a generally circular central zone serving for providing an optical zone that transmits light to a pupil, and an annular peripheral zone integrally disposed radially outward of the central zone and serving for placing the contact lens on a predetermined position of the cornea. The peripheral zone is designed to permit a good circulation of tear fluid existing between the cornea and the contact lens. The contact lens has a back surface whose radius of curvature is substantially equal to that of the surface of the cornea of the lens wearer so that the back surface of the contact lens may suitably conform to the shape of the cornea of the lens wearer. Generally, this radius of curvature of the back surface of the contact lens is referred to as a base curve.
However, such a conventional contact lens as described above gives rise to a problem of discomfort as felt by the contact lens wearer when he or she wear the lens, although degrees of the discomfort may vary among individual wearers. In particular, a conventional contact lens of hard or rigid type gives the lens wearer a relatively large wearing discomfort. Thus, the conventional contact lens is insufficient to assure a good wearing comfort thereof.
In addition, the conventional hard contact lens is likely to suffer from a problem that dusts or other floating objects in the atmosphere are introduced into the tear fluid existing between the contact lens and the cornea. This results in significant deterioration of the lens wearing comfort as felt by the lens wearer, and sometimes gives rise to a burden on the lens wearer to remove the contact lens to wash it.
It is therefore an object of this invention to provide a contact lens which is novel in structure, and which is capable of eliminating or minimizing a discomfort as felt by a contact lens wearer when he or she wear the contact lens, and assuring an improved wearing condition of the lens, while preventing an entrance of the dust or the like into a tear fluid layer existing between the lens and the cornea.
The above and/or other objects of this invention may be attained according to at least one of the following modes of the invention. Each of these modes of the invention is numbered like the appended claims and depends from the other mode or modes, where appropriate, to indicate possible combinations of elements or technical features of the invention. It is to be understood that the principle of the invention is not limited to those modes of the invention and combinations of the technical features, but may otherwise be recognized based on the thought of the present invention that disclosed in the whose specification and drawings or that may be recognized by those skilled in the art in the light of the disclosure in the whole specification and drawings.
(1) A contact lens having a back surface adapted to face a cornea, wherein the back surface including (a) a generally circular central zone and (b) an peripheral zone disposed around the central zone, the central zone having a radius of curvature gradually decreasing from a center thereof toward a periphery thereof.
In the contact lens according to this mode of the invention, the radius of curvature of the back surface gradually decreases with an increase in a radial or lateral distance from the center of the central zone that serves for providing an optical zone. In this arrangement, the back surface of the central zone has a curve that is made gradually steeper from its center toward its periphery, so that a protrusion protruding backward of the contact lens, namely, protruding toward the side of the cornea is formed on the periphery of the central zone. The protrusion extends in a circumferential direction of the contact lens so as to surround the central zone. With the contact lens worn on a cornea of a lens wearer, this protrusion protrudes toward the cornea from the back surface of the contact lens, and is located closely adjacent to a surface of the cornea. In the presence of the protrusion, a tear fluid layer existing between the back surface of the contact lens and the cornea is narrowed or constricted at the peripheral portion of the central zone where the protrusion protrudes toward the cornea. Thus, the back surface of the contact lens is located closely adjacent to the surface of the cornea at this protrusion.
Accordingly, the contact lens of this mode of the invention permits an improved lens wearing comfort as felt by the lens wearer. In addition, since the protrusion partially narrows the tear fluid layer at the outer circumferential portion of the contact lens, unfavorable entrance of dusts or other floating objects in the atmosphere into the tear fluid layer is effectively limited or minimized, thus assuring an enhanced usability of the contact lens. It is presumed that these advantages of the present contact lens, namely, the improved lens wearing comfort and the limited entrance of the dusts into the tear fluid layer, stem from the following technical basis or reasons. Namely, the narrowed portion of the tear fluid layer located at the peripheral portion of the central zone may function as a barrier to the unfavorable entrance of the dusts into the tear fluid. In addition, the protrusion disposed at the periphery of the central zone and adapted to be located closely adjacent to the surface of the cornea may lead the contact to be desirably positioned relative to the cornea. That is, the presence of the protrusion may limit an amount of displacement of the contact lens relative to the cornea, when the lens wearer blinks, for example. Therefore, the contact lens of this mode of the invention enables to decrease or moderate discomfort as felt by the lens wearer due to friction between the contact lens and eyelid and/or cornea caused by the displacement of the contact lens relative to the cornea. Further, since a clearance between the back surface of the contact lens and the cornea is narrowed at a radially outward portion of the central zone, unfavorable entrance of the dust or contaminants accompanied with a circulation of the tear fluid may be prevented or limited.
With the contact lens of this mode of the invention worn on the cornea, the protrusion formed at the periphery of the central zone is pressed onto the surface of the cornea upon application of an external pressing force on a front surface of the contact lens. Such an external pressing force is applied on the front surface of the contact lens, by the eyelids of the lens wearer, when the lens wearer blinks, for example. As a result, the back surface of the contact lens is located adjacent to the surface of the cornea as close as possible, making it possible to restrict the displacement of the contact lens relative to the cornea. In this respect, the presence of the protrusion does not adversely effect on optical characteristics of the contact lens, since the optical zone is provided on a central portion of the center zone of the contact lens.
This mode of the present invention is preferably applicable to a rigid gas permeable lens (RGPL) so that a sufficient amount of oxygen permeated through the contact lens is applied to the cornea, even if an amount of circulation of the tear fluid existing between the cornea and the contact lens is decreased. The principle of this mode of the invention is also applicable to a soft contact lens and a hard contact lens, which are made of polymethyl metacrylate (PMMA). Further, the principle of the present invention may be applicable to various kinds of known contact lens to be worn on the cornea for treating myopia, hyperopia, astigmatism, or the other disability or disease of a person""s eye.
(2) A contact lens according to the above-indicated mode (1), wherein the central zone has an outer circumferential portion adapted to cooperate with a surface of the cornea to form an annular tear fluid reserving portion therebetween, and a central portion adapted to apply pressure on the surface of the cornea in order to reshape the cornea. In this mode of the invention, the central portion of the central zone is adapted to apply a pressing force on a central portion of the surface of the cornea, to thereby reshape the cornea. Since a refraction power of the cornea itself is desirably changed due to the reshaping of the cornea, the contact lens of this mode is capable of exhibiting a myopia correction effect according to a so-called xe2x80x9corthokeratologyxe2x80x9d treatment. As disclosed in U.S. Pat. No. 4,952,045, for example, operations, effects or usage of the contact lens in the orthokeratology treatment are well known in the arts. For this reason, no detail description of this matter is provided for the sake of simplicity of the description.
(3) A contact lens according to the above-indicated mode (1) or (2), wherein the central zone has an ellipsoidal shape in diametrical cross section of the contact lens, whose minor axis extends substantially parallel to a center axis of the contact lens. This mode of the invention is effective to provide a contact lens having the central zone formed with a smooth surface whose radius of curvature gradually decreases from its center toward its periphery in the radially or laterally outward direction thereof. In particular, when the principle of this mode of the invention is applied to the contact lens according to the above-indicated mode (2) of the invention, the central zone of the back surface of the contact lens has a shape in the form of a smooth ellipsoidal surface of revolution in its entirety, which shape is to be transfer on the surface of the cornea. Since a minor axis of the ellipsoidal surface of revolution conforms to an axis of revolution, the peripheral portion of the central zone is located closely adjacent to the surface of the cornea. Therefore, the contact lens of this mode of the invention assures a desired corneal reshaping effect, while enhancing a positioning stability thereof relative to the cornea owing to a bearing effect of the contact lens on the surface of the cornea. Thus, the contact lens of the present mode of the invention is capable of reshaping the surface of the cornea into a desired shape with high stability.
(4) A contact lens according to any one of the above-indicated modes (1)-(3), wherein the central zone has a surface expressed by a quadric of revolution having a conic coefficient xe2x80x9ckxe2x80x9d of larger than 0. In this mode of the invention, the conic coefficient xe2x80x9ckxe2x80x9d is used to determine a configuration of a conic surface as well know in the art. That is, the surface of the central zone is expressed by the following known equation that is used to define a surface of revolution about a Z-axis in a three dimensional coordinate where X, Y, and Z axes are mutually orthogonal:             h      2        =                  x        2            +              y        2                  z    =                  (                              ch            2                    /                      (                          1              +                                                (                                      1                    -                                                                  (                                                  k                          +                          1                                                )                                            ⁢                                              c                        2                                            ⁢                                              h                        2                                                                              )                                                  1                  /                  2                                                      )                          )            +              A        ⁢                  xe2x80x83                ⁢                  h          4                    +              B        ⁢                  xe2x80x83                ⁢                  h          6                    +              C        ⁢                  xe2x80x83                ⁢                  h          8                    +              D        ⁢                  xe2x80x83                ⁢                  h          10                    
where,
c is a center curvature; and
A, B, C, D are constants.
That is, the contact lens of the present mode of the invention has the central zone whose surface is arranged to satisfy the above indicated equation in which the conic coefficient: k, i.e., an aspherical coefficient in a term for expressing a second-order surface or a quadric of revolution is made larger than xe2x80x9c0xe2x80x9d (k greater than 0). This makes it possible to effectively form the central zone having an ellipsoidal surface whose minor axis conforms to the center axis of the contact lens. Preferably, the value of the conic coefficient: k is held within a range of 1-9 (0xe2x89xa6kxe2x89xa69). The central zone having such a specific ellipsoidal surface permits that the contact lens exhibits a further improved positioning stability thereof on the surface of the cornea. Thus, the contact lens of the present mode of the invention is effectively usable for the aforementioned orthokeratology treatment. Moreover, this arrangement of the present mode of the invention makes it possible to design the entire surface of the central zone of the contact lens according to a single equation, thus moderating difficulty in designing and calculating the shape of the back surface of the contact lens, while permitting the central zone to have a smooth surface over its entire area. Therefore, the contact lens according to the present mode of the invention eliminates or reduces a need for cutting and polishing the surface of the contact lens.
In one preferred form of the contact lens according to any one of the above-indicated modes (1)-(4) of the invention, the contact lens according to the mode (4) of the invention, the central zone of the back surface has a vertex radius of curvature that is made larger than a vertex radius of curvature of the surface of the cornea on which the contact lens is worn, while the peripheral portion of the central zone of the back surface has the radius of curvature that is made smaller than the radius of curvature of the corresponding peripheral portion of the surface of the cornea. Namely, the contact lens having the central zone dimensioned as described is capable of applying a pressing force on the vertex of the surface of the cornea with high stability, while effectively forming the annular tear reserving portion disposed around the vertex of the surface of the cornea. Therefore, the contact lens according to this preferred form of the invention can exhibit an excellent effect of the orthokeratology treatment for correcting myopia.
In another preferred form of the contact lens according to any one of the above-indicated modes (1)-(4) of the invention, the central zone of the back surface of the contact lens has the vertex radius of curvature that is made larger than the vertex radius of curvature of the surface of the cornea that is opposed to the back surface of the contact lens, while the peripheral portion of the central zone has the radius of curvature that is made smaller than the radius of curvature of the peripheral portion of the surface of the cornea that is opposed to the peripheral portion of the central zone of the back surface of the contact lens. This arrangement permits a further improved effect of the orthokeratology treatment.
In still another preferred form of the contact lens according to any one of the above-indicated modes (1)-(4) of the invention, the peripheral zone disposed around the central zone has a radius of curvature that is made larger than the radius of curvature of the peripheral portion of the central zone. This arrangement makes it possible to prevent the peripheral zone of the contact lens from being undesirably pressed onto the surface of the cornea or the sclera, and to effectively narrow or constrict the tear fluid layer at the peripheral portion of the central zone, while assuring a good wearing comfort as felt by the lens wearer.
In yet another preferred form of the contact lens according to any one of the above-indicated modes (1)-(4) of the invention, the central zone is connected at an peripheral portion thereof to the peripheral zone with a continuous surface which joins to the central zone along knots lying on tangents common to curves of the central zone and the continuous surface, and joins to the peripheral zone along knots lying on tangents common to curves of the peripheral zone and the continuous surface. In this arrangement, the back surface of the contact lens is smoothly and closely located adjacent to the surface of the cornea at the peripheral portion of the central zone where the clearance between the contact lens and the cornea is narrowed or minimized. As a result, the contact lens can be worn on the cornea with no or minimized adverse influence due to the contact of the back surface of the contact lens with the cornea, thus assuring a further improved wearing comfort as felt by the lens wearer. It should be appreciated that the continuous surface may be constituted by common tangent points in which the optical and the intermediate zones are directly connected to each other and lie on tangents common to both curves of the back surfaces of the central and peripheral zones, or alternatively may be constituted by a curved surface interposed between the central zone and the intermediate zone and lies on tangents whose slopes are continuously changed over its entire width including connecting points to the central zone and the peripheral zone as seen in the diametrical cross section of the lens.
In a further preferred form of the contact lens according to any one the above-indicated modes (1)-(4) of the invention, the peripheral zone has an outer diameter within a range of 8-16 mm. While contact lenses generally have a diameter smaller than 11 mm, the contact lens of the present invention is able to have a relatively large diameter. This arrangement permits that the eyelids of the lens wearer apply a relatively large pressing force on the upper surface of the contact lens, making it possible to position the contact lens on the cornea with further improved stability, and to further enhance lens wearing comfort as felt by the lens wearer, or alternatively making it possible to apply the contact lens to the orthokeratology treatment for further facilitating reshaping of the cornea with enlarged abutting pressure of the contact lens on the cornea. As is understood from the aforementioned description with respect to this preferred form of the invention, the contact lens of this preferred form is applicable to a sclera lens as well as a corneal contact lens.
In a still further preferred form of the contact lens according to any one of the above-indicated modes (1)-(4) of the invention, the central zone has a radial width within a range of 50-90% of a radius of the contact lens. This arrangement permits an optical zone formed in a central portion of the central zone to exhibit excellent optical characteristics in terms of acuity of viewing and visibility, while permitting the peripheral portion of the central zone to function for effectively positioning the contact lens on the surface of the cornea.
The contact lens constructed according to present invention may be manufactured according to various kinds of method of producing hard-type or soft-type contact lenses. For instance, the contact lens of the present invention may be manufactured by molding with a mold having a mold cavity whose profile corresponding to a desired profile of the contact lens. The contact lens of the invention may also be manufactured by injection molding, or alternatively by cutting in which a desirably shaped contact lens is cut out from a lens blank by utilizing techniques of turning. Further, a polishing process may be performed on the present contact lens as needed, like in the general process of manufacturing contact lenses.