Prism ballasted contact lens

A contact lens with a toric optical zone has its anterior and posterior optical zone surfaces tilted with respect to one another to forming prism in the optical zone. Thickness profiles of the peripheral region are independent of the tilt angle between the anterior and posterior optical zone surfaces.

BACKGROUND OF THE INVENTION

Contact lenses having a toric optical zone (commonly referred to as “toric contact lenses”) are used to correct refractive abnormalities of the eye associated with astigmatism. The toric optical zone provides cylindrical correction to compensate for the astigmatism. Since astigmatism requiring vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are generally prescribed also with a spherical correction to correct myopic astigmatism or hypermetropic astigmatism. The toric surface may be formed in either the posterior lens surface (back surface toric lens) or in the anterior lens surface (front surface toric lens).

Whereas spherical contact lenses may freely rotate on the eye, toric contact lenses have some type of ballast to inhibit rotation of the lens on the eye so that the cylindrical axis of the toric zone remains generally aligned with the axis of the astigmatism. Toric contact lenses are manufactured with a selected relationship (or offset) between the cylindrical axis of the toric optical zone and the orientation of the ballast. This relationship is expressed as the number of degrees (rotational angle) that the cylindrical axis is offset from the orientation axis of the ballast. Accordingly, toric contact lens prescriptions specify this offset, with toric lenses generally being offered in 5 or 10-degree increments ranging from 0° to 180°.

In summary, a prescription for a toric contact lens will typically specify spherical correction (spherical power), cylindrical correction (cylindrical power) and axes offset to define the optical correction, as well as lens diameter and base curve to define fitting parameters.

One type of ballast is prism ballast, which has proven effective for maintaining a toric contact lens in a desired rotational orientation on the eye. An example of prism ballasting is disclosed in U.S. Pat. No. 6,113,236. Prism may be attained by various manners including: decentering the optic zone of the lens vertically downwards, such that a “wedge” of thickness across the optic zone is achieved; or tilting the entire anterior surface with respect to the posterior surface, such that a “wedge” of thickness across the entire lens is achieved. Then, for either of these approaches, the lens periphery is designed to achieve better fitting and on-eye comfort. However, both of these techniques for introducing prism place limitations on the ability to control the peripheral region of the lens. The two primary levers for changing the peripheral thickness become the amount of decentration or angle of tilt put into the lens, and the lens center thickness (CT) to increase or decrease the overall thickness profile of the lens design. In addition, decentering the lens optic introduces the additional optical higher order aberration coma, which is an undesirable lens feature.

BRIEF SUMMARY OF THE INVENTION

In one aspect, this invention provides a contact lens comprising:

an optical zone and a peripheral region surrounding the optical zone;

a posterior surface comprising a posterior optical zone surface having a posterior center point, and a posterior peripheral zone; and

an anterior surface comprising an anterior optical zone surface having an anterior center point, and an anterior peripheral zone;

wherein one of the posterior optical zone and the anterior optical zone surfaces is a toric optical zone surface and the other of the posterior optical zone and the anterior optical zone surfaces is a non-toric optical zone surface;

wherein the posterior and the anterior center points are aligned with one another, such that a centerline intersecting said center points is perpendicular to the toric optical zone surface at its center point;

wherein the non-toric optical zone surface is non-perpendicular to the centerline at its center point, such that the anterior optical zone surface and the posterior optical zone surface are tilted with respect to one another to define a tilt angle therebetween, thereby forming prism in the optical zone;

wherein the posterior and anterior peripheral zones define the peripheral region of the contact lens, and thickness profiles of the peripheral region are independent of said tilt angle.

In one specific aspect, the posterior optical zone surface is a toric optical zone surface and the anterior optical zone surface is a non-toric optical zone surface. In another specific aspect, the anterior optical zone surface is a toric optical zone surface and the posterior optical zone surface is a non-toric optical zone surface.

According to other aspects, the tilt angle has a magnitude between 0.1 and 5 degrees, or a magnitude between 0.5 and 2 degrees, or a magnitude of about 1 degree.

According to another aspect, the thickness profiles of the peripheral region are defined radially, for example, at radial increments. There may be at least twelve radial increments, or at least 24 radial increments.

According to yet another aspect, maximum thicknesses of radial thickness profiles in an inferior section of the peripheral region are greater than radial thickness profiles in a superior section of the peripheral region.

According to other aspects, the non-toric optical zone surface may be spherical or a non-toric asphere, such as an asphere that imparts a predetermined amount of spherical aberration to a refractive correction of the optical zone.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2schematically illustrate a toric contact lens according to various aspects of this invention. Toric contact lens10includes an optical zone11and a peripheral region13which terminates at edge40. Toric contact lens10includes a posterior surface20and an opposed anterior surface30. Posterior surface20comprises a posterior optical zone surface21having a posterior center point22, and a posterior peripheral zone23. In the illustrated embodiment, the posterior optical zone surface21is toric. Anterior surface30comprises an anterior optical zone surface31having an anterior center point32, and an anterior peripheral zone33. In the illustrated embodiment, the anterior optical zone surface31is non-toric.

The posterior and the anterior center points22,32are aligned with one another, such that centerline12intersects center points22,32. Thus, the optical zones21,31are not decentered with respect to one another, as in some prior prism ballasted toric contact lenses.

Additionally, centerline12, an imaginary reference line, is perpendicular to the posterior optical zone surface21at its center point22. However, the anterior optical zone surface31is not perpendicular to centerline12at its center point32, such that the anterior optical zone is tilted with respect to the posterior optical zone. The tilt between these two surfaces may be defined by tilt angle14. This tilt between the two optical zone surfaces thereby forms prism in the optical zone of the contact lens. In this illustrated embodiment, the prism is restricted to the optical zone11.

FIG. 3schematically illustrates this tilt angle14, also designated by θ. In the illustrated portion of the optical zone, imaginary line19represents the anterior optic zone surface without tilt between the anterior and posterior optical zone surfaces31,21, whereby this portion of the optical zone would have a constant thickness x. However, with the imparted tilt between surfaces31,21, thickness y is greater than thickness x.

The tilt angle14may have a magnitude from 0.1 to 5 degrees, preferably from 0.5 to 2 degrees. In the illustrated embodiment, tilt angle14is about 1 degree.

According to an aspect of this invention, the thickness profiles of the peripheral region may be defined radially, i.e., along radii generating from the lens center. In theory, the peripheral thickness profiles could be defined along an infinite number of such radii, but in practice, one designing the toric contact lens will define the peripheral thickness profiles at a selected number of radial increments. This is illustrated inFIG. 1, showing the radial16spaced by interval15. Preferably, the peripheral thickness profiles are defined by at least twelve radial increments. As an example, the illustrated embodiment includes 24 radial increments, spaced at 15-degree intervals.

As mentioned, the thickness profiles of the peripheral region are independent of the tilt between the optical zone surfaces, since these tilted surfaces are restricted to the optical zone11. Prism is present only in the optical zone, thereby placing less restrictions on design of the peripheral region.

According to various aspects, the maximum thicknesses of the radial thickness profiles in an inferior section17of the peripheral region are greater than radial thickness profiles in a superior section18of the peripheral region. This is shown in the embodiment illustrated inFIG. 2. Line A-A inFIG. 1corresponds to what is commonly referred to as the vertical meridian of a contact lens. The upper half of the contact lens is commonly referred to as the superior section18, and the lower half is commonly referred to as the inferior section17. For reference purposes, the top of the vertical meridian is referenced as the 90-degree position, and the bottom of the vertical meridian is referenced as 270-degree position. For the illustrated embodiment, the halves of the contact lens lying on the two sides of the vertical meridian are mirror images.

A representative example of the peripheral region follows, to illustrate one manner of designing the peripheral regions. For the peripheral region between radii16corresponding to 225 degrees and 315 degrees, the radial thickness profiles along these radii16may have a fairly consistent maximum thickness, which also represents the maximum thickness present in the peripheral region13. For the peripheral region between radii16at 315 degrees and 0 degrees, the radial thickness profiles may then have maximum thicknesses that taper downwardly from 315 degrees to 0 degrees. Similarly, for the peripheral region between 225 degrees and 180 degrees, the radial thickness profiles may then have maximum thicknesses that taper downwardly from 225 degrees to 180 degrees.

Along each radii16, the thickness can taper downwardly along that radii, i.e., taper downwardly from the maximum thickness as the edge40of the contact lens is approached.

Of course, other peripheral region designs may be selected. The main considerations are to provide a peripheral region that provides a contact lens that is relatively easy for a practitioner to fit and that is not uncomfortable while worn. Additionally, the peripheral region, in conjunction with the prism in the optical zone, provides a prism ballasted contact lens. The prism ballast inhibits rotation of the contact lens while worn. Additionally, in the case where the contact lens does become rotationally misaligned while worn, the prism ballast returns the contact lens to its intended rotational alignment upon interaction with the eyelids during blinking.

As in conventional toric contact lenses, optical zone11provides the desired cylindrical and spherical refractive corrections. In other words, optical zone surfaces21,31, in conjunction with each other, provide the refractive corrections, with the toric optical zone surface ensuring the desired cylindrical correction. The non-toric optical zone surface may be spherical or may be a non-toric asphere. An example of a non-toric asphere is one which imparts a predetermined amount of spherical aberration to the refractive correction of the optical zone, as disclosed in U.S. Pat. No. 5,815,239, for example, the disclosure of which is incorporated by reference herein. The non-toric optic zone surface further may comprise a multifocal surface, such as where the non-toric optic zone surface includes a central optic zone providing a near vision correction and an outer optic zone providing a far vision correction. The non-toric optic zone surface may further include an intermediate optic zone, between a central optic zone and an outer optic zone, providing an intermediate vision correction. Multifocal optic zone surface designs which may be employed in the present invention include those disclosed in U.S. Pat. No. 5,754,270, for example, the disclosure of which is incorporated by reference herein.

In the discussion of the illustrated representative embodiment, the posterior optical zone surface21was sometimes described as toric with the anterior optical surface31being described as non-toric. However, this invention is applicable for back surface toric contact lenses and front surface toric contact lenses. Accordingly, this invention includes the posterior optical zone surface21being non-toric with the anterior optical surface31being toric.

Although certain illustrative embodiments have been described, the invention is not limited thereto and modifications and variations would be evident to a person of ordinary skill in the art.