Aaren's Bi-Sign Aspheric Monofocal and Multifocal Optics - FAQs
What is a one-sign aspheric lens?
All aspheric IOLs except for Aaren Scientific's PAL (monofocal) and OptiVis™ (multifocal) lenses are one-sign aspherics.
The operating principle of a one-sign aspheric is to compensate for a certain amount of corneal spherical aberration when the lens is centered. The amount of corneal aberration is defined by the corneal asphericity. An eye with a one-sign aspheric lens does not manifest spherical aberration at any pupil size if: the lens perfectly centered, and if the one-sign aspheric lens is combined with its optimum corneal shape. If both of these conditions are met, the result is excellent retinal image quality in this particular eye.
What is bi-sign aspheric design?
The operating principle of a bi-sign aspheric lens is to create compensation for spherical aberration of opposite signs from the central zone of the lens (responsible for vision at daytime conditions) and the peripheral zone of the lens that comes into play during nighttime conditions. The aberration compensation is controlled by the bi-sign aspheric lens itself and occurs for a wide range of corneal shapes.
An eye with a bi-sign aspheric lens manifests some amount of aberration at some pupil diameters, but the level of aberration is maintained significantly lower for a wide range of corneas. This results in excellent retinal image quality at large pupil sizes not only at a specific cornea, but for wide range of corneal shapes.
What is a nominal cornea?
The population's average cornea of elliptical shape defined by asphericity Q = -0.26 is called the Nominal cornea. It manifests 0.27 microns of wave spherical aberration and -0.9 mm ray spherical aberration at 6 mm diameter. Nominal cornea is commonly used as a reference cornea in comparing image quality produced by different IOLs.
What is the range of corneal shapes?
Corneal shape varies broadly among the population from a spherical shape of asphericity Q = 0 to a fully corrected corneal shape with asphericity Q = -0.528. The latter is also called an "ideal cornea" because it doesn't produce spherical aberration at any diameter.
What are the benefits and limitations of a spherical IOL for retinal image quality?
A typical spherical IOL provides excellent outcomes for pupil diameters common in daytime (photopic) conditions, i.e. pupils around 3 mm in diameter. At nighttime (mesopic or scotopic) conditions, the pupil diameter usually increases and the amount of aberration produced by both the cornea and a spherical lens also increases significantly enough to substantially reduce retinal image quality.
It is only for corneal shapes close to the ideal cornea, that the aberrations at large pupil sizes do not increase significantly. In these particular cases, high retinal image quality is maintained even with a spherical IOL.
Where does the term "one-sign aspheric" come from?
A cornea will manifest both positive sign wave spherical aberration and negative sign ray spherical aberration, at any diameter. Based upon the one-sign aspheric principle of operation, a one-sign aspheric optic is designed to manifest negative sign wave spherical aberration and positive sign ray spherical aberration at any lens diameter, to compensate for opposite sign corneal aberration. This is the reason to term the lens as a "one-sign aspheric" design.
What are examples of one-sign aspheric IOLs?
All aspheric IOLs except those produced by Aaren Scientific are one-sign aspherics. They are defined by the corneal shape optimum for spherical aberration compensation by the corresponding lens in a perfectly centered position.
For instance, the Tecnis® IOL is designed to compensate for spherical aberration of the nominal cornea, asphericity Q = -0.26; SofPort® is designed to compensate for the ideal cornea, Q = -0.528; while AcrySof® is designed to compensate for a corneal shape between nominal and ideal corneas, Q ≈ -0.4.
What is "residual spherical aberration", RSA?
A nominal cornea manifests a 0.27 micron wave spherical aberration at a 6 mm diameter. A one-sign aspheric lens is designed to manifest a negative sign wave aberration to compensate for specific levels of corneal spherical aberration. The difference between the wave spherical aberration of the nominal cornea, and the corresponding magnitude of wave spherical aberration of a one-sign aspheric lens at the same 6 mm diameter, is called "residual spherical aberration".
For instance, Tecnis® is designed to compensate for nominal cornea spherical aberration and therefore, its RSA is 0 microns. SofPort® is designed to work with the ideal cornea with a spherical aberration of zero, i.e. only the lens spherical aberration corrected to zero. Its RSA = 0.27 microns (0.27 - 0.0 = 0.27), i.e. the RSA of SofPort® is equal to a nominal cornea wave aberration at a 6 mm diameter. AcrySof® is designed to compensate for a corneal spherical aberration between nominal and ideal corneas of asphericity around Q ≈ -0.4. Its RSA ≈ 0.10 microns, meaning that the optimum corneal shape for this lens manifests 0.27 - 0.10 = 0.17 microns wave spherical aberration at 6 mm diameter.
In conclusion, "residual spherical aberration" is a value characterizing a one-sign aspheric lens. The wave aberration produced by the optimum corneal shape for a given one-sign aspheric lens at a 6 mm diameter is equal to the difference between the nominal cornea wave aberration of 0.27 microns and the "residual spherical aberration" corresponding to this one-sign aspheric lens.
What are the benefits and limitations of one-sign aspheric lenses?
A one-sign aspheric lens may provide excellent results in improving retinal image contrast at a large pupil size if used with the optimum corneal shape for this particular lens. For instance, the Tecnis® IOL with the nominal cornea (0.27), AcrySof® with the cornea of Q ≈ -0.4 and SofPort® with the ideal cornea. On the other hand, spherical IOLs combined with ideal corneas also provide excellent retinal image quality for large pupils.
Retinal image quality at large pupil sizes may be significantly reduced even below that of spherical IOLs if the one-sign aspheric is combined with a non-optimum corneal shape.
Retinal image quality with a one-sign aspheric is also sensitive to lens misalignment producing a coma aberration. The quality may be reduced below that of a spherical lens for the same misalignment. It is of lesser importance only for the SofPort® IOL which is only slightly aspherized. Its sensitivity to misalignment is similar to a spherical lens.
In the case of a one-sign aspheric combined with its optimum corneal shape, it will manifest the benefits of image contrast at large pupil sizes, but it also may manifest limitations in reducing intermediate vision in nighttime and daytime conditions due to the aberration compensation.
What are the benefits and limitations of one-sign aspheric design applied to a multifocal optic?
Asphericity can be applied to a diffractive multifocal optic, either to the base surface of the diffractive surface or to the opposite refractive surface.
Retinal image contrast is the primary goal for a multifocal optic and the benefits and limitations of one-sign aspheric monofocal lenses become even more pronounced when applied to a multifocal lens.
Retinal image contrast at large pupil sizes might be substantially improved if the one-sign aspheric multifocal lens is combined with its optimum corneal shape, but it might also be substantially reduced even below that of spherical multifocal design if the post-op corneal shape is not the optimum for that particular one-sign aspherization.
Another issue is even higher sensitivity to misalignment by a one-sign aspheric multifocal lens because the retinal image contrast is already compromised by the multifocality. A reduction in retinal image contrast at large pupil sizes can be even more pronounced with lens post-op decentration and tilt as compared with the corresponding one-sign aspheric monofocal design.
Except for Aaren Scientific's OptiVis™ Multifocal IOL, intermediate vision in all diffractive multifocal lenses is created by the overlap between Far and near Peaks. This is the reason why the Add power can be reduced in order to improve Intermediate. Intermediate image quality is more important for multifocal optics because of its ultimate goal of spectacles independence. However, Intermediate vision may be reduced even with an optimum corneal shape for a one-sign aspheric lens and even under photopic conditions, because the Near and Far peak overlap is reduced with the aberration compensation.
What are examples of bi-sign aspheric lenses?
Aaren Scientific produces bi-sign aspheric IOLs in monofocal form under its PAL brand. These lenses include flatter curvature within the central 3 mm diameter (the central zone), and steeper curvature outside this 3mm area (the peripheral zone).
The central zone produces spherical aberration of the eye that is the same magnitude as that produced by a spherical lens of the same power but of the opposite sign. Spherical lenses of average power produce about 0.10 microns of wave spherical aberration at a 3 mm pupil, or -0.2 mm of ray spherical aberration. By comparison, Aaren Scientific's bi-sign aspheric produces about -0.1 micron of wave spherical aberration or +0.2 mm of ray spherical aberration at a 3 mm pupil occurrring at photopic conditions. As the pupil increases in nighttime (mesopic or scotopic) conditions, the aberration of the opposite sign comes into play to compensate for the aberration formed at the 3 mm central zone of the lens. As a result, the aberrations do not increase at large pupil sizes as they do with a spherical IOL.
Aaren Scientific has also incorporated equivalent bi-sign asphericity into its OptiVis™ multifocal optic in order to improve retinal image contrast at nighttime conditions for wide range of corneal shapes, even with lens misalignment.
Where does the term "bi-sign aspheric" come from?
Based upon its principle of operation, bi-sign aspheric is designed to produce opposite sign spherical aberrations: one within the central zone (inner 3 mm diameter of the lens) and the other at the lens periphery (outside the 3 mm diameter) in order to compensate for spherical aberrations of opposite signs at large pupil sizes. This is the reason to term the lens as "bi-sign aspheric" design because the aberration of both signs come into play to produce an improvement in retinal image quality.
What is the "residual spherical aberration" of a bi-sign aspheric lens?
A nominal cornea manifests 0.27 microns of wave spherical aberration at 6 mm pupil diameter. The difference between this nominal cornea magnitude of wave spherical aberration, and the magnitude produced by an aspheric lens at the same diameter is called "residual spherical aberration".
The term serves the definition of the wave spherical aberration produced by the optimum corneal shape for this aspheric lens at a 6 mm pupil diameter. Optimum corneal shape means the shape that produces spherical aberration equal to the amount which the aspheric lens is designed to compensate. Therefore, the term "residual spherical aberration" is only applicable to one-sign aspherics.
Bi-sign aspheric lenses are not designed to compensate for spherical aberration of a particular cornea, but to operate for the range of corneal asphericity. The term "residual spherical aberration" is not applicable to bi-sign aspheric lenses.
What are the benefits and limitations of bi-sign aspheric lenses?
Aaren Scientific bi-sign aspheric lenses are designed to produce consistently high levels of retinal image quality for a wide range of corneal shapes, even with lens misalignment. Consequently, the key benefit of a bi-sign aspheric design is predictable image contrast.
A one-sign aspheric lens provides higher image quality than a bi-sign aspheric, when used with a cornea that is optimum for that particular one-sign aspheric lens. However, this is only a theoretical limitation of bi-sign aspheric design. Practically, Aaren Scientific bi-sign aspheric lenses provide substantially high levels of retinal image contrast at large pupil sizes that clinically, are very likely to be indistinguishable from that a one-sign aspheric, even with an optimum corneal shape (for that specific one-sign aspheric lens).
Another advantage of bi-sign aspheric lenses from Aaren Scientific is that the positive spherical aberration at photopic conditions (3 mm pupil), may improve image quality at intermediate viewing distances.
What are the benefits and limitations of bi-sign aspheric design as applied to a multifocal optic?
Asphericity can be applied to a diffractive multifocal optic, and Aaren Scientific has included bi-sign asphericity with its OptiVis™ multifocal design. The base surface of the diffractive zone, and refractive peripheral zone of the multifocal surface, are configured to provide bi-sign asphericity.
Retinal image contrast is the primary goal for a multifocal optic. The benefits and practical absence of limitations seen in a bi-sign aspheric monofocal lens, become even more pronounced when applied to a multifocal optic.
The inclusion of bi-sign asphericity in the OptiVis™ design produces consistent levels of retinal image quality for a wide range of corneal shapes - even with lens misalignment. Therefore, the key benefit of this mutlifocal design is predictable image contrast.
The OptiVis™ design includes a central refractive zone of progressive power that covers both Intermediate and Distance foci. The positive spherical aberration produced by the base surface of the diffractive zone up to about a 3 mm diameter pupil (photopic conditions), compliments the central refractive zone in further improving the image quality for intermediate viewing distances. This is another advantage of bi-sign aspheric application to the OptiVis™ design.