New Bi-Sign Aspheric IOL and Its Application

1040-5488/11/8901-0001/0 VOL. 89, NO. 1, PP. 1–●●●
OPTOMETRY AND VISION SCIENCE
Copyright © 2011 American Academy of Optometry
ORIGINAL ARTICLE
New Bi-Sign Aspheric IOL and Its Application
Valdemar Portney*
ABSTRACT
Purpose. To present a bi-sign aspheric intraocular lens (IOL) and make theoretical comparisons with spherical and
one-sign aspheric IOLs for retinal image quality in mesopic conditions and intermediate image capability in photopic
conditions.
Methods. Using ray-tracing in pseudophakic eye models, retinal image quality in terms of modulation transfer function
was analyzed between the bi-sign aspheric and three types of one-sign aspheric IOLs and the spherical IOL for the range
of corneal asphericities and different lens misalignments at 5 mm pupil diameter; comparison is made with the spherical
IOL at defocus conditions characterized by negative refractive errors at 3 mm pupil diameter.
Results. Retinal image quality of the bi-sign aspheric lens was equal to or exceeded the one-sign aspheric and spherical
IOLs in the majority of anterior cornea asphericities and lens misalignments within the spatial frequencies of 6 to 15 c/deg.
Intermediate image capability of the bi-sign aspheric exceeded the spherical IOL.
Conclusions. A new bi-sign aspheric lens produces regions of opposite spherical aberrations in the eye model with pupil
diameters above 3 mm to balance out aberrations within the lens itself in addition to compensating corneal aberration
as with a one-sign aspheric lens. This allows reduction in the variability of the retinal image quality at different anterior
cornea asphericities and lens misalignments, when compared with a one-sign aspheric and spherical lens. The bi-sign
aspheric produces positive longitudinal spherical aberration in the eye model within 3 mm pupil diameter that improves
intermediate image capability when compared with the spherical IOL.
(Optom Vis Sci 2012;89:1–●●●)
Key Words: intraocular lens, aspherical lens, spherical lens, retinal image
T
he term “aspheric” refers to the traditional view applied to
contact lenses that are designed to increase ocular aberrations to expand the depth-of-focus for treatment of presbyopia. Currently, the term “aspheric multifocal” is usually used for
and the term “aspheric” is applied to contact lenses and intraocular
lenses (IOLs) designed to reduce optical aberrations to improve
contrast sensitivity in the lens-centered condition and under large
pupil size occurring in mesopic and scotopic conditions. The latter
application of asphericity to the IOLs is the subject of this article,
where we introduce a new type of aspheric lens.
The aspheric lens has been a known entity for several centuries.
Moritz von Rohr, in his work for Carl Zeiss, patented the first
aspheric spectacle lens for aphakic applications in 1909, and soon
afterward, in 1912, eyeglasses were launched under the name Zeiss
Punktal.1
In U.S. Patent No. 4,504,982, Burk2 described an aspheric IOL
to correct spherical aberration after implantation in the eye. He
proposed an aspheric IOL with progressively longer radii toward
the outer zone of the lens. This monotonic nature of surface cur*PhD, FAAO
Vision Advancement LLC, Newport Coast, California.
vature change is referenced in this article as Burk design principle
of an aspheric IOL. The first aspheric IOL based on Burk invention was clinically tested in the mid 1980s by ORC,3 but the
aspheric lens became short lived when it was recognized that retinal
image quality depended on lens centration, which was difficult to
control under standard cataract surgery conditions at that time.
Atchison4 conducted an extensive theoretical optical analysis of
aspheric IOLs, which were based on Burk design principle. He
demonstrated that performance of a decentered aspheric lens could
be worse than the spherical lens of the same shape because of
astigmatism and coma. Dietze and Cox5 tested spherical aberration
correcting contact lenses. They found that the root mean square
of wave aberrations actually increased and visual acuity (VA)
remained unchanged. They also concluded that the increased
coma and uncorrected astigmatism limited possible benefits
from correction of ocular spherical aberration.
Two developments were primarily responsible for renewed interest in aspheric IOLs, resulting in the introduction of the Tecnis
IOL in the early 2000s that was designed to correct nominal corneal spherical aberration: (a) improvements in cataract surgery,
which produced better lens centration, and (b) corneal videokera-
Optometry and Vision Science, Vol. 89, No. 1, January 2012
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New Bi-Sign Aspheric IOL and Application—Portney
tometry for measuring anterior cornea shape, which allowed quantification of corneal asphericity.6 However, any new aspheric IOL
design that followed Burk original design principle of aspherization would require good centration in the capsular bag to provide
the potential for improved optical quality over spherical IOLs.7
Various aspheric IOL designs then followed (AcrySof IQ, SofPort AO, Acri. Smart, and others), having different levels of asphericity with varying corrections for corneal spherical aberration, in
TABLE 1.
Eye model specifications
Optical characteristics of cornea
Eye model
Refractive index
Anterior surface radius (mm)
1.377
7.8
Conic constant (asphericity Q)a
Sphere
Nominal
Spherical aberration corrected
Interim
Q⫽0
Q ⫽ ⫺0.26
Q ⫽ ⫺0.528
Q ⫽ ⫺0.4
Posterior surface radius (mm)
Central thickness (mm)
Aperture stop or pupil position from
posterior corneal surface (mm)
Aqueous refractive index
Vitreous refractive index
6.5
0.55
3.55
1.3374
1.336
a
Several investigators have shown that the mean Q value of the
anterior cornea over a 6 mm zone is ⫺0.26 (0.00 to ⫺0.50) with
a wide variance.9
an effort to reduce sensitivity to the aspheric IOL misalignment in
the eye and to find the best option for dealing with large variation
in corneal asphericity. All these aspheric designs were based on
Burk original design principle and relied on a compensation of the
corneal spherical aberration as follows: the Tecnis IOL was designed to compensate average corneal aberration with corneal asphericity Q ⫽ ⫺0.26; the AcrySof IQ-corneal asphericity Q ⬇
⫺0.4 and SofPort AO-ideal corneal asphericity Q ⫽ ⫺0.528 that
does not produce spherical aberration. Nevertheless, the consistent
performance of the aspheric IOLs that are based on Burk design
principle cannot be assured given the large variation in corneal
shapes and unforeseen lens misalignments after implantation. Aspheric IOL customization, based on measured corneal spherical
aberration, has been suggested as a way of addressing the variety of
corneal shapes.8 Aside from the logistic difficulties in selecting the
appropriate aspheric IOL, a customization would not be able to
address retinal image sensitivity to aspheric IOL misalignment.
We describe here a new type of aspheric IOL that is designed to
balance out aberrations within the lens itself in the presence of a
lens misalignment and at large pupil size as occurs in mesopic and
scotopic conditions and for a wide range of corneal shapes. Its
features offer important additions to the compensation of corneal
spherical aberration used in the designs of prior aspheric IOLs
(Portnoy V, patent pending). An additional benefit of the design
relates to a potential improvement in the intermediate image capability at photopic conditions.
Contrary to a monotonic surface curvature change, per Burk
design principle, the new IOL design incorporates a convex back
surface with a central region of flattening surface within about a 3
TABLE 2.
Specifications of one-sign aspheric IOLs
Optical characteristics
Assessed
Tecnis Z900a
Assessed
AcrySof IQb
SofPort AOc
Power (D)
Refractive index
Lens shape
Anterior surface
Radius (mm)
Conic constant
Fourth-order coefficient
Sixth-order coefficient
Posterior surface
Radius (mm)
Conic constant
Fourth-order coefficient
Sixth-order coefficient
Central thickness (mm)
22.0
1.458
Biconvex
Sixth-order aspheric
11.04
⫺1.03613
⫺0.0006748865
⫺0.0000025677027
Sphere
⫺11.043
0
0
0
1.164
22.0
1.550
Biconvex
Sphere
15.23
0
0
0
6th-order aspheric
⫺26.75
0
0.00033
⫺0.0000076
0.7
22.0
1.427
Biconvex
Conic asphere
7.29
⫺1.08566
0
0
Conic asphere
⫺9.470
⫺1.08566
0
0
1.208
a
Initial Tecnis Z900 specifications were taken per Altmann et al.10 It appeared that the referenced specifications manifested a
significant negative wave spherical aberration with the nominal anterior corneal aspherization Q ⫽ ⫺0.26. Optimization was
conducted on the aspheric coefficients to achieve zero spherical aberration with the anterior corneal asphericity Q ⫽ ⫺0.26, and the
corresponding IOL was referenced to as “Assessed Tecnis.”
b
No aspheric surface specifications of AcrySof IQ was found in the literature and the approximation was introduced based on the
reference that AcrySof IQ manifested about 74% of spherical aberration of the Tecnis Z900 (⫺0.20 ␮m of wave spherical aberration
vs. ⫺0.27 ␮m of Tecnis spherical aberration at 6 mm pupil).11 The corresponding IOL was referred to as “Assessed AcrySof IQ.”
c
SofPort AO specifications were taken per Altmann et al.10 The analysis of the eye model using correction for spherical aberration,
anterior cornea of Q ⫽ ⫺0.528,9 confirmed that SofPort AO specifications achieved spherical aberration correction only within the
SofPort AO itself.
Optometry and Vision Science, Vol. 89, No. 1, January 2012
New Bi-Sign Aspheric IOL and Application—Portney
mm diameter to produce a positive longitudinal spherical aberration (LSA) by the eye and a steepening surface at the peripheral
region, beyond the central region, to produce negative LSA by the
eye. In the case of a concave surface, the surface curvatures would
be reversed to produce equivalent LSAs per the same lens regions.
This allows a balancing of the aberrations produced by the central
and peripheral regions of the aspheric surface. The resulting aspheric optic is termed a bi-sign aspheric, and the designs with
monotonic surface curvature variation, per Burk design principle,
are termed one-sign aspheric in this article.
The article will focus on bi-sign asphericity applied to the IOL
but it can be applied to contact lenses and particularly to multifocal
contact lenses of diffractive design where an improvement in contrast at large pupil size is the premium. A diffractive optic design
allows control of light split between far and near foci independently of the pupil diameter, but the contrast is still reduced at
large pupil size and with lens decentration. The performance can
likely be improved by making the base surface of the diffractive
lens, or the opposite refractive surface, with an appropriate bi-sign
aspheric configuration.
We describe here retinal image qualities, in terms of the modulation transfer function (MTF), for the range of corneal asphericities with a mesopic pupil of 5 mm diameter that were produced by
the bi-sign aspheric lens, and compare findings with aspheric designs, derived from Burk design principle, for various magnitudes
of tilt and decentration from the optical axis. The impact on image
quality and intermediate vision capability with a photopic pupil of
3 mm diameter will also be addressed.
MATERIALS AND METHODS
Zemax optical software was used to construct a pseudophakic
eye model with the specifications listed in Table 1. The setting was
Vision Advancement, LLC., Newport Coast, CA. The term “pupil” in place of “aperture stop” of the eye optical system is used
throughout the text.
Specifications of the Tecnis and AcrySof types of aspheric IOLs
and the SofPort AO IOL are provided in Table 2, along with a
description of how these specifications were derived. All these aspheric IOLs are referred to as “one-sign aspherics.”
A spherical IOL was selected with a common shape and steeper
anterior, which was known to produce low spherical aberration.12
The posterior of the spherical optic was modified with a new aspheric design referred to as “bi-sign aspheric.” The similarity of the
optic shapes between spherical and bi-sign aspheric lenses allows
elimination of optic shape considerations in comparisons or evaluations between both lenses for retinal image quality and intermediate image capability.
The magnitude of the positive ray spherical aberration within a
3 mm aspheric surface diameter of the bi-sign aspheric was set in
the Zemax lens prescription to be similar to the absolute magnitude of the negative ray spherical aberration of the spherical lens
within the same 3 mm diameter. This was to target a similar retinal
image quality at 3 mm pupil size. The selection of the positive ray
spherical aberration within a 3 mm pupil diameter had been found
to be beneficial for depth-of-focus.13 The lens aspheric surface was
optimized by varying even aspheric coefficients for the best MTF at
multiple spatial frequencies within the range of 20 (6 c/deg) to 100
3
lp/mm (30 c/deg) at tilt and decentration conditions and 5 mm
diameter. The process involved optimizing the aspheric coefficients of the lens placed at 0.6 mm decentration in the eye model
for best modulations at three spatial frequencies and 5 mm pupil,
and independently optimizing aspheric coefficients of the lens
placed at 5.5° tilt in the eye model for best modulations at three
spatial frequencies and 5 mm pupil. The final bi-sign aspheric
configuration was selected that provided the average between best
MTFs for the described above two optimizations at the lens misalignments (Table 3).
The optical specifications of the IOLs were used for ray-tracing
analysis at 3 mm (photopic conditions) to establish retinal image
quality in terms of the MTF at defocus conditions and the best
focus position. The retinal image quality was then collected at 5
mm (mesopic condition) pupil diameter and optic misalignments
of 0.5 or 0.75 mm decentration and 4 or 7° tilt.
The MTF spatial frequency range used for analysis was 0 to 100
lp/mm (30 c/deg) corresponding to up to 20/20 VA for an average
eye dimension. This range was divided into a so called “contrast
range” between 10 and 50 lp/mm corresponding to the 6 to 15
c/deg typically used in contrast sensitivity testing and an “acuity
range” between 50 and 100 lp/mm (30 c/deg) corresponding to
20/40 to 20/20 VA. The emphasis of the MTF analysis in this
article is on the “contrast range” as the manifestation of the contrast quality measure.
Additional evaluation for intermediate image capability was
also conducted at a 3 mm pupil diameter for the spherical and
bi-sign aspheric at 0.5 mm decentered positions. The intermediate image capability was measured in terms of MTFs at different defocus positions toward intermediate foci characterized
by negative refractive errors: 0.0 D, ⫺0.125 D, ⫺0.25 D, and
⫺0.50 D.
The MTF is a 2-D function represented by two cross sections,
labeled the sagittal and tangential responses or vertical (calculated
for Y axis) and horizontal (calculated for X axis).14 The average
TABLE 3.
Specifications of spherical IOL and bi-sign aspheric IOL
Optical characteristics
Spherical
IOL
Bi-aspheric IOL
Power (D)
Refractive index
Lens shape
Anterior surface
Radius (mm)
Posterior surface
Radius (mm)
Conic constant
Fourth-order coefficient
Sixth-order coefficient
Eighth-order coefficient
10th-order coefficient
12th-order coefficient
14th-order coefficient
16th-order coefficient
Central thickness (mm)
22.0
1.489
Biconvex
Sphere
9.1
Sphere
⫺30.4
0
0
0
0
0
0
0
0
0.6
22.0
1.489
Biconvex
Sphere
9.1
14th-order aspheric
⫺27.5
0
0.0011
0.000007
⫺0.00004
0.0000033
0
⫺0.000000005
0.0000000005
0.6
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New Bi-Sign Aspheric IOL and Application—Portney
MTF between both cross sections was used for analysis to make the
retinal image evaluation manageable.
RESULTS
The description of the aspheric design used in this article was
based on terminology provided by Holladay9 who explained ray
and wave aberrations in general, and the LSA in particular. For
instance, marginal LSA of an optical system was defined as a difference between focus positions of the marginal (peripheral) ray
and paraxial (central) ray. “Ray aberration” has a physical meaning
and was used for aspheric design descriptions, contrary to “wave
aberration,” which is the mathematical abstraction.15
Fig. 1 demonstrates marginal LSA as a difference between focus
positions of the marginal (peripheral) ray 3 and paraxial (central)
ray 1. It is a negative number for a positive spherical lens. LSA can
be also represented as a function in the lens-centered position if all
focal positions between the marginal and paraxial rays are included. An LSA function is constructed by ray positions at the
pupil defined by the corresponding distances from the optical axis
of the ray’s contacts with the pupil as a function of the corresponding ray focus positions along the optical axis from a predefined best
focus position. In this eye model study, the best focus position is
defined at 3 mm pupil diameter as the focus position with the
highest MTF at 100 lp/mm (30 c/deg).
A term “Regional LSA” is introduced in this article, defined as
LSA for an annular region of the lens, i.e., a difference in focus
positions of the ray at the outside border of the region and ray at the
inside border of the region. For instance, the lens in Fig. 1 can be
divided into two regions located between rays 2 and 1 and rays 3
and 2, and the regional LSA21 and LSA32 are then defined for
these regions correspondingly. Regional LSA can also be represented as a function of the rays’ focus positions for the corresponding region.
Fig. 2 demonstrates LSA functions of the eye model with a
SofPort AO aspheric IOL and anterior cornea asphericity varying
from spherical shape (Q ⫽ 0) through nominal (Q ⫽ ⫺0.26) and
interim (Q ⫽ ⫺0.4), to fully corrected for spherical aberration
shape (Q ⫽ ⫺0.528).9 The zero position was set at best focus
position defined by the highest MTF at 100 lp/mm at 3 mm pupil
for corneal asphericity of Q ⫽ ⫺0.26. The LSA lines confirm that
the SofPort AO specifications correspond to the aspheric lens corrected for spherical aberration because the LSA line of the eye
model with anterior corneal asphericity Q ⫽ ⫺0.528 is represented by the vertical line, indicating the absence of the spherical
aberration for the combination of the spherical aberration corrected
cornea (Q ⫽ ⫺0.528) and SofPort AO. Fig. 2 also demonstrates the
significance of the corneal contribution to spherical aberration in the
eye models with different anterior corneal asphericities—the marginal
LSA at 5 mm pupil diameter approximately doubles from Q ⫽ ⫺0.4
to Q ⫽ ⫺0.26 and from Q ⫽ ⫺0.26 to Q ⫽ 0.
Fig. 3 demonstrates LSA functions of the lenses involved in the
analysis by this article. Any region of LSA functions of the eye
FIGURE 2.
“LSA functions” of eye models with SofPort AO at different anterior cornea
asphericities.
FIGURE 3.
FIGURE 1.
Marginal LSA and regional LSA.
“LSA functions” of eye models with spherical and different aspheric IOLs
at nominal anterior cornea asphericity Q ⫽ ⫺0.26.
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New Bi-Sign Aspheric IOL and Application—Portney
model with the assessed AcrySof IQ and SofPort AO lenses, drawn
from the corresponding best focus at 3 mm pupil diameter as the 0
coordinate, maintains the same sign of the spherical aberration,
negative ray aberration in this case. The eye model with the assessed Tecnis lens manifests 0 spherical aberration for the anterior
cornea Q ⫽ ⫺0.26 but would also demonstrate the same sign
spherical aberration at any other anterior cornea asphericity, more
negative toward lower asphericity (Q ⫽ 0, for instance) and positive toward higher anterior cornea asphericity (Q ⫽ ⫺0.528, for
instance). This is the basis for referring to the corresponding aspheric optics as “one-sign aspherics.”
The eye model with the bi-sign aspheric, however, incorporates
regions with opposite signs of ray spherical aberration. Again, the
zero coordinate corresponds to the best focus position defined at 3
mm pupil diameter. In this bi-sign aspheric design, the region of
the lens, up to about 3 mm diameter, manifests positive ray spherical aberration via the eye model, i.e., its regional LSA has a positive
FIGURE 4.
One-sign and bi-sign aspheric surface forms.
5
magnitude, and the region outside 3 mm diameter manifests negative ray spherical aberration via the eye model, i.e., its regional
LSA has a negative magnitude. This is the basis for referring to the
corresponding aspheric optic as a “bi-sign aspheric.” The optical
aberration of opposite signs comes into play to balance them out
within the bi-sign optic itself under conditions of increased pupil
diameter above 3 mm, in addition to a compensation for corneal
spherical aberration.
Fig. 4 demonstrates the physical difference in profiles of onesign aspheric and bi-sign aspheric surfaces. The one-sign aspheric
profile, as exemplified by the assessed Tecnis, continually flattens
toward the lens periphery, per Burk design principle, to result in
one-sign spherical aberration throughout any region of the LSA in
the eye model. The bi-sign aspheric profile, as a deviation from the
spherical surface of the equivalent power, manifests a more complex shape—it flattens at the internal region closer to the lens
center to provide positive ray aberration and then steepens toward
the lens periphery to provide negative ray aberration in the eye
model.
The bi-sign asphericity can be included with monofocal and
multifocal surfaces as shown in the example of the OptiVis multifocal IOL where the base curve of the diffractive surface includes
bi-sign asphericity. Fig. 4 demonstrates the OptiVis MIOL surface
with characteristic diffractive grooves, and the deviation from
sphere of the same effective far power having a characteristic bisign aspheric profile. The bi-sign aspheric IOLs in monofocal and
multifocal platforms are offered by Aaren Scientific.16
Fig. 5 demonstrates intermediate focus capabilities in terms of
MTFs at the defocus positions in the eye models with spherical (a)
and bi-sign aspheric (b) IOLs. The defocus values are characterized
by the refraction errors: 0.0, ⫺0.125 D, ⫺0.25 D, and ⫺0.50 D.
The 0 coordinate defines the best focus position for a distant object. Calculations were made for a 3 mm pupil diameter and the
lenses at 0.5 mm decentered positions.
Fig. 6 demonstrates retinal image qualities in terms of MTFs of
the eye models with nominal anterior cornea asphericity Q ⫽
⫺0.26 and different IOLs. The MTFs were calculated for the
lens-centered position (a); 0.5 mm (b) and 0.75 mm (c) decentered
FIGURE 5.
Intermediate vision capability in terms of MTFs at different defocus positions in the eye models with spherical (a) and bi-sign aspheric (b) IOLs: 0.0,
⫺0.125 D, ⫺0.25 D, and ⫺0.50 D. The calculations were conducted for 3 mm pupil diameter and the lenses at 0.5 mm decentered positions.
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New Bi-Sign Aspheric IOL and Application—Portney
FIGURE 6.
Retinal image qualities in terms of MTFs of the eye models with nominal anterior cornea asphericity Q ⫽ ⫺0.26, different IOLs and the lens-centered
position (a); 0.5 mm (b) and 0.75 mm (c) decentered positions; and 4° (d) and 7° (e) tilt positions.
positions; and 4° (d) and 7° (e) tilt positions. The best focus was
defined at 3 mm pupil diameter at each corresponding lens position. The rectangle placed within 10 to 50 lp/mm (6 to 15 c/deg)
spatial frequencies refers to the “contrast region” of the MTFs.
Fig. 7 demonstrates retinal image qualities in terms of MTFs of
the eye models with spherical anterior cornea (asphericity Q ⫽ 0)
and different IOLs. The MTFs were calculated for the lens-centered
position (a); 0.5 mm (b) and 0.75 mm (c) decentered positions; and 4°
(d) and 7° (e) tilt positions. The best focus was defined at 3 mm pupil
diameter at each corresponding lens position. The MTF of the spher-
ical IOL at 4° tilt with the nominal anterior cornea asphericity Q ⫽
⫺0.26 (Fig. 6d) was used as a control MTF. The rectangle placed
within 10 to 50 lp/mm (6 to 15 c/deg) spatial frequencies refers to the
“contrast region” of the MTFs.
Fig. 8 demonstrates retinal image qualities in terms of MTFs of
the eye models with spherical aberration corrected anterior cornea
(asphericity Q ⫽ ⫺0.528) and different IOLs. The MTFs were
calculated for the lens-centered position (a); 0.5 mm (b) and 0.75 mm
(c) decentered positions; and 4° (d) and 7° (e) tilt positions. The best
focus was defined at 3 mm pupil diameter at each corresponding lens
Optometry and Vision Science, Vol. 89, No. 1, January 2012
New Bi-Sign Aspheric IOL and Application—Portney
7
FIGURE 7.
Retinal image qualities in terms of MTFs of the eye models with spherical anterior cornea (Q ⫽ 0), different IOLs and the lens-centered position (a); 0.5
mm (b) and 0.75 mm (c) decentered positions; and 4° (d) and 7° (e) tilt positions.
position. The MTF of the spherical IOL at 4° tilt in the nominal
anterior cornea asphericity Q ⫽ ⫺0.26 (Fig. 6d) was used as a control
MTF. The rectangle placed within 10 to 50 lp/mm (6 to 15 c/deg)
spatial frequencies refers to the “contrast region” of the MTFs.
DISCUSSION
A bi-sign aspheric design is proposed that includes regions
of opposite signs of ray spherical aberration at diameters
above 3 mm to allow aberration balancing within the lens itself
with a large pupil. There is some compensation of corneal
spherical aberration but the aberration compensation within
the bi-sign aspheric lens itself is the dominant feature of the
design.
The bi-sign design provides positive ray spherical aberration within
a 3 mm pupil diameter to use the characteristic of axicon and improve
depth of focus in photopic conditions.14 Fig. 5 demonstrates that,
although the MTF of a spherical lens of the same shape is higher at the
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New Bi-Sign Aspheric IOL and Application—Portney
FIGURE 8.
Retinal image qualities in terms of MTFs of the eye models with spherical aberration corrected anterior cornea (Q ⫽ ⫺0.528); different IOLs and the
lens-centered position (a); 0.5 mm (b) and 0.75 mm (c) decentered positions; and 4° (d) and 7° (e) tilt positions.
ideal best focus position, even a small defocus slightly larger of ⫺0.125
D negates this advantage. They also show that, starting at about
⫺0.25 D defocus and larger, the retinal image quality provided by the
bi-sign aspheric actually exceeds the retinal image quality provided by
the spherical lens. These theoretical results indicate that the bi-sign
aspheric lens at least maintains, or may even improve, intermediate
image capability over the spherical optic, contrary to a one-sign aspheric where a reduction of total spherical aberration may degrade
distance-corrected near and intermediate VA.17,18
Contrary to a decentration, lens tilt is usually less controllable
because it requires special techniques for visualization such as Purkinje and Scheimpflung images.19 –21 On the basis of these most
recent references, the average tilt is about 2 to 3.5° and use of the
MTF of the spherical IOL at 4° tilt as a control MTF (Fig. 6d) is
justifiable. A tilt of 7° is most likely important for analysis if one
would consider a difference between the optical and visual axes of
about 5° and standard deviation of the tilt measurements being
about 1 to 1.5°.
Optometry and Vision Science, Vol. 89, No. 1, January 2012
New Bi-Sign Aspheric IOL and Application—Portney
By our analysis of the retinal image quality at various anterior
cornea asphericities and lens misalignments, certain conclusions,
described below, may be drawn regarding the potential for improving image contrast with a large pupil under mesopic conditions.
The MTFs were compared primarily in the “contrast range” of 6 to
15 c/deg of the spatial frequencies as a practical matter. Control
levels of MTF were referred to as a “safety” level provided by the
spherical IOL at 5 mm pupil diameter.
CONCLUSIONS
Optical performance of one-sign aspherics strongly depended
on the anterior cornea asphericity. Higher aspherized IOL designs
such as the assessed Tecnis and the assessed AcrySof IQ are effective
at corneal asphericity around the nominal value (Q ⫽ ⫺0.26).
Effectiveness is reduced with anterior corneal asphericity approaching a spherical shape (Q ⫽ 0) particularly with the lenses
tilted (Figs. 7d and 8e), but they still provide some advantage. The
design is somewhat effective when compared with the control
MTF with anterior corneal asphericity approaching the shape that
corrects for spherical aberration (Fig. 8), but the spherical IOL
becomes highly effective in this case and may be used instead if the
anterior cornea asphericity can be accurately measured before cataract surgery.
Lower aspherized IOLs such as the SofPort AO are more effective than the higher aspherized IOLs with a higher aspherized
anterior cornea (Fig. 8), but it is almost equivalent to the performance of the spherical IOL at least for the mid power 22 D used in
the analysis. (There is some advantage in the case of a high tilt, Fig.
6e.) It is possible that the SofPort AO is more effective for high
power IOLs that manifest a larger reduction in aberration from the
corresponding spherical lenses. As a whole, a spherical IOL would
be an excellent choice for higher aspherized anterior corneas, probably above Q ⫽ ⫺0.4.
An excellent review of clinical visual and optical quality comparisons between different aspheric IOLs (one-sign aspherics)
and spherical IOLs has been offered by Montés-Micó et al.22 in
an attempt to understand whether there was a quality benefit by
implanting one-sign aspheric IOLs over implanting spherical
IOLs. Results of the comparisons between different studies
were largely conflicting. Based on the analysis offered in our
study, this should not be surprising, because a fundamental
unpredictability was common for all one-sign aspheric IOLs
with their high variation with anterior cornea asphericity and
lens misalignment.
Lens misalignments (tilt and decentration) reduced the retinal
image quality of all lenses but particularly in the case of the higher
aspherized one-sign aspheric IOLs such as the assessed Tecnis and
the assessed AcrySof IQ. A reduction in retinal image quality with
higher aspherized lenses was particularly noticeable with nominal
anterior cornea asphericity (Fig. 6b to e) but still provided some
effectiveness over the spherical IOL in the “contrast range” within
6 to 15 c/deg when compared with the control MTF. Again, a
spherical lens is a good choice for a higher aspherized anterior
cornea if the anterior cornea asphericity can be measured accurately before surgery.
9
To target predictable vision quality, the bi-sign aspheric lens
provided significantly more consistent, as well as effective, retinal
image quality for the ranges of anterior cornea asphericities and
lens misalignments tested. Results with the bi-sign aspheric were
equivalent to or exceeded the higher aspherized one-sign lenses
such as the assessed Tecnis and the assessed AcrySof IQ within the
“contrast range” for all conditions of anterior cornea asphericities
and lens misalignment tested, except for ideal centration and small
decentration with a nominal anterior cornea asphericity (Fig. 6a,
b). Even with a higher aspherized cornea and tilt, the bi-sign aspheric lens was close to the performance of the spherical and SofPort AO lenses (Fig. 8e) and far exceeded the assessed Tecnis and
assessed AcrySof IQ IOLs.
ACKNOWLEDGMENTS
The author has a financial and proprietary interest in bi-sign aspheric design.
Received January 20, 2011; accepted August 18, 2011.
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Optometry and Vision Science, Vol. 89, No. 1, January 2012
Valdemar Portney
8 Via Ambra
Newport Coast, California 92657
e-mail: [email protected]