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Effect of Chromatic Aberration on Contrast Sensitivity in Pseudophakic Eyes
Kazuno Negishi, MD;
Kazuhiko Ohnuma, PhD;
Norio Hirayama, MS;
Toru Noda, MD;
for the Policy-Based Medical Services Network Study Group for Intraocular
Lens and Refractive Surgery
Arch Ophthalmol. 2001;119:1154-1158.
ABSTRACT
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Objective To evaluate the effect of chromatic aberrations in pseudophakic eyes
with various types of intraocular lenses (IOLs).
Patients and Methods The study included 51 eyes of 33 patients who underwent cataract surgery.
The eyes were divided into 3 groups according to the material from which their
IOL was made: group 1, polymethyl methacrylate; group 2, silicone; and group
3, an acrylate/methacrylate copolymer. Ten normal phakic control eyes (group
4) underwent the same examination. Best-corrected distance visual acuity and
contrast sensitivity were measured under white light and monochromatic light
with wavelengths of 470 nm, 549 nm, and 630 nm, with the best correction under
white light.
Results There were no significant differences in best-corrected visual acuity
and contrast sensitivity under the 549-nm monochromatic light in any group.
However, under both white multichromatic light and 470- and 630-nm monochromatic
light, the mean contrast sensitivity in group 3 tended to be lower, sometimes
significantly, than in the other IOL groups.
Conclusions Our results showed that longitudinal chromatic aberrations of some IOLs
may degrade the quality of the retinal image. Attention must be paid to the
detailed optical performance of IOL materials to achieve good visual function.
INTRODUCTION
THE REFRACTIVE indexes of the ocular media are wavelength dependent,
and consequently the refractive power of the human eye varies across the visible
spectrum. For example, when multichromatic light such as white light passes
through the lens, different wavelengths of the light focus on different points
of the ocular media. This chromatic dispersion results in chromatic aberrations,
and the retinal image is blurred, reducing contrast. Chromatic aberrations,
especially longitudinal chromatic aberrations (LCAs), are also thought to
be related to various physiologic functions, such as accommodative control1 and resolution and depth of focus.2
Therefore, it is important for the physiologic function of the optical system
to remain as close as possible to that of the normal eye. In the normal human
eye, the cornea and lens are primarily responsible for chromatic aberrations;
the contribution of the crystalline lens to chromatic aberrations is reported
to be about 28.5% of the entire ocular media.3
If the chromatic aberration becomes larger and exceeds the normal range, it
could degrade the quality of the retinal image and affect visual function.
The chromatic aberrations of intraocular lenses (IOLs) depend on the
Abbe number of the optical material. The smaller the Abbe number, the larger
the chromatic aberration and the lower the quality of the retinal image. In
recent years, small-incision surgery followed by IOL implantation has become
the primary technique of cataract surgery. Different types of IOL optical
materials have been developed to enable IOLs to be implanted through a smaller
incision. Consequently, some materials are being used with Abbe numbers that
are far different from that of the human lens. Regarding IOL materials presently
available, the Abbe number is smaller in the acrylate/methacrylate copolymer
and high refractive index silicone than in polymethyl methacrylate (PMMA)
or normal human lenses. Therefore, more degradation of the retinal image can
occur in pseudophakic eyes that have been implanted with IOLs made from these
materials.
In this study, we investigated the effect of LCAs on contrast sensitivity
in pseudophakic eyes with various kinds of IOLs.
PATIENTS AND METHODS
This study was conducted at the National Tokyo Medical Center in Tokyo,
Japan. Subjects were recruited retrospectively from among patients who underwent
phacoemulsification and aspiration followed by IOL implantation at the hospital
from December 1997 through December 1999. Patients with either preoperative
systemic or ocular, intraoperative, or postoperative complications were excluded.
Informed consent was obtained from each patient by one of the investigators
(T.N.) before enrollment in the study.
Continuous curvilinear capsulorrhexis was performed in all cases, and
IOLs were implanted in the capsular bag. The implanted IOLs were well centered
and were not tilted postoperatively. The examination was performed at least
1 month after surgery. Fifty-one eyes of 33 patients were included in this
study, and the mean ± SD follow-up period was 4.9 ± 6.9 months
(range, 1-39 months).
The patients were divided into 3 groups according to the model of IOL
implanted: group 1, 811C (Pharmacia & Upjohn, Tokyo); group 2, AQ110NV
(CanonStaar Inc, Tokyo); and group 3, AcrySof MA60BM (Alcon Laboratories Inc,
Fort Worth, Tex). All IOLs had a biconvex optical design, but they differed
in the materials from which they were made: group 1, PMMA; group 2, silicone;
and group 3, an acrylate/methacrylate copolymer. Ten phakic eyes of 7 patients
who had neither systemic nor ocular complications, including cataract, underwent
the same examination and served as controls (group 4).
The mean patient ages and the mean refractive powers of the implanted
IOLs are presented in Table 1.
There were no significant differences among the 3 IOL groups. However, the
mean age of the phakic group (group 4) was significantly lower than that of
the other groups because patients with cataract were excluded from group 4.
The Abbe number of the optical materials was obtained from the IOL manufacturers.
All patients underwent ocular examinations at least 1 month after surgery.
They were evaluated for best-corrected distance visual acuity under white
light. Contrast sensitivity was measured under white light and monochromatic
light with wavelengths of 470 nm, 549 nm, and 630 nm, using a contrast vision
tester modified from the MCT 8000 (Vistech Co, Dayton, Ohio), with the best
correction under white light. In all cases, pupil size was about 3 mm. In
group 4, to eliminate the effect of accommodation, all examinations were performed
through a 3-mm-diameter artificial pupil placed in a trial frame and centered
subjectively with respect to the subject's pupil after the administration
of cycloplegics.
The main components of the modified contrast vision tester are shown
in Figure 1. The target for distance
vision was presented under white light or monochromatic light (wavelengths,
470 nm, 549 nm, and 630 nm, each with 20-nm bandwidths) with the same luminance
of 18 candelas/m2 adjusted by an external light source (MegaLight
100; Hoya-Schott Inc, Tokyo) and neutral density filter (Fuji Filter ND 0.6;
Fuji Photo Film Co, Tokyo).
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Figure 1. Schematic representation of the
contrast sensitivity measurement system. Wavelengths for monochromatic light
were 470 nm, 549 nm, and 630 nm.
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We also distributed a questionnaire to patients to determine whether
they had disturbances in color perception in their daily lives. Data analysis
was performed using Statview (Abacus Concepts Inc, Berkeley, Calif). The mean
age and the mean refractive power of the implanted IOLs were compared using
1-factor analysis of variance (ANOVA) and the Fisher protected least significant
difference (PLSD) test. The best-corrected visual acuities and contrast sensitivities
were analyzed using ANOVA and the Fisher PLSD test after converting the logarithmic
values.
RESULTS
In all cases, the best-corrected visual acuity was 20/20 or better.
The mean best-corrected visual acuity levels were 20/17 in all groups, and
no significant differences were found among the groups (P>.05). The range of the refractive power for best correction was -
4.5 to +1.5 diopters (D) in sphere and 0 to - 2.25 D in cylinder. The
mean contrast sensitivity levels are shown in Figure 2, Figure 3, Figure 4, and Figure 5. There were no significant differences in contrast sensitivity
among the 3 IOL groups (groups 1 to 3) at any spatial frequency under the
549-nm monochromatic light (Figure 4).
However, under the white light and the 470-nm and 630-nm monochromatic light,
the mean contrast sensitivity in group 3 tended to be lower than in groups
1 and 2, and there were significant differences at some spatial frequencies
(Figure 2, Figure 3, and Figure 5).
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Figure 2. Mean contrast sensitivity under
white light. A single asterisk indicates that
P<.01 when groups 1 and 3 were compared. A double asterisk indicates that
P<.05 when groups 2 and 3 were compared.
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Figure 3. Mean contrast sensitivity under
the 470-nm monochromatic light. A single asterisk indicates that P<.05 when groups 2 and 3 were compared. A double asterisk indicates
that P<.05 when groups 1 and 3 were compared.
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Figure 4. Mean contrast sensitivity under
549-nm monochromatic light. There were no significant differences among the
intraocular lens groups (groups 1 to 3) at any spatial frequency.
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Figure 5. Mean contrast sensitivity under
630-nm monochromatic light. A single asterisk indicates that P<.05 when groups 1 and 3 were compared. A double asterisk indicates
that P<.01 when groups 1 and 3 were compared.
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The mean contrast sensitivity in the control group (group 4) was significantly
higher than in all IOL groups under all conditions. No patients reported disturbances
in color perception in their daily lives.
COMMENT
Before reviewing the data, we considered that the mean age of the patients
in group 4 was significantly lower than for the other groups. Because contrast
sensitivity is affected by age,4 the mean contrast
sensitivity of group 4 was naturally higher than that of other groups under
all conditions. Therefore, the data from group 4 should be treated as a reference
and cannot be compared statistically with those from the other groups. However,
because there were no significant differences in the profiles of the other
3 groups, including mean age, mean refractive power of the implanted IOLs,
pupil diameter, and complications, the data from these groups can be compared
statistically according to the different models of implanted IOLs.
Our results showed no significant differences in mean best-corrected
visual acuity, but there were significant differences in mean contrast sensitivity
among the IOL groups (groups 1 to 3) under normal conditions (white light).
In addition to the patients' profiles, several factors (primarily the cornea,
IOL, and corrective lenses) can affect the contrast sensitivity or modulation
transfer function in the optical system of the pseudophakic eye. Spherical
aberration, irregular and asymmetric monochromatic aberrations, chromatic
aberration of these components, and inaccurate focus all influence the modulation
transfer function of the eye.5-8
The main aberrations affecting visual function are spherical and chromatic.
Spherical aberrations of the corrective lenses can be ignored because the
powers of these lenses are small (within ±5 D) and do not differ among
cases. Spherical aberrations of IOLs can also be ignored because they are
sufficiently small compared with the chromatic aberrations under the pupil
diameter (approximately 3 mm)9 of the patients
in this study. Regarding corneal spherical and chromatic aberrations, the
effect on the differences of the mean contrast sensitivity can be disregarded
because the data from each group were averages of patients with the same profiles.
As a result, the main factor that caused the difference in contrast sensitivity
was the chromatic aberration of the IOLs.
Chromatic aberration can be divided into 3 primary types: (1) LCA; (2)
chromatic difference of magnification (CDM); and (3) chromatic difference
of position (CDP).10 Chromatic aberration exists
in normal phakic eyes but seems to cause no major problem under normal conditions.
The LCA is higher than 2 D across the visible spectrum in normal phakic
eyes.10 In clinical terms, 2 D of uncorrected
refractive error causes a serious visual handicap. However, previous reports
showed that the effect of LCA on visual performance is not a major problem
either theoretically11 or experimentally.12 If the individual accommodates to focus a wavelength
in the middle of the spectrum, the range of refractive error is effectively
halved to ±1.0 D. Because the extremes of the visible spectrum have
less luminosity, the greatest amount of defocus is produced by those wavelengths
that are least visible. Moreover, in clinical cases, such large LCAs do not
become a major problem because chromatic blur is always masked by the luminance
component.13
According to a previous report, when the peak of the luminance spectrum
is in focus, most of the light is less than 0.25 D out of focus.11
Therefore, when all the images of different wavelengths are superimposed on
the retina, the visibility of the target is dominated by wavelengths that
are only slightly defocused. Campbell and Gubisch12
also reported less than a 0.2-log-unit difference in contrast sensitivity
for white and monochromatic lights across a 10 to 40 cycles per degree (cpd)
range of spatial frequencies. The fact that we usually do not feel the effect
of LCA supports these studies.
In pseudophakic eyes, the situation is different. The calculated LCAs
of pseudophakic eyes were reported to be 0.64 D with a PMMA IOL, 0.98 D with
an AcrySof IOL, and 0.74 D in the normal phakic eye (the Gullstrand thematic
eye) between the wavelengths of 500 nm and 640 nm.14
The calculated LCA of an eye implanted with an AcrySof lens is greater
than that of the normal eye because the Abbe number of AcrySof is smaller
than that of the normal eye. Because there is no accommodation in a pseudophakic
eye, the LCA might affect visual function, especially in a pseudophakic eye
with an IOL that has a small Abbe number. Our results seem to support this
hypothesis.
Our results showed that contrast sensitivity was not significantly different
among the 3 IOL groups at 549 nm. However, at other wavelengths, the contrast
sensitivity of group 3 (AcrySof) tended to be lower than that of the other
groups, and there were significant differences at some spatial frequencies.
In this study, refractions were typically performed with broad-spectrum white
lights, which possibly have luminance centroids near 555 nm.3
Humans tend to adjust their accommodation to minimize blur irrespective of
stimulus wavelength, so they are likely to accommodate to a wavelength near
555 nm when viewing a white stimulus. Because the 549-nm wavelength was used
for one measurement and is close to 555 nm, the refraction under the 549-nm
monochromatic light might approximate the best correction in all groups. Therefore,
contrast sensitivity at 549 nm was not significantly different in the 3 groups.
However, the contrast sensitivity in group 3 (AcrySof) was significantly lower
than that of the other IOL groups in 470- and 630-nm light because the LCA
of group 3 was greater than that of the other 2 groups. Our results support
the hypothesis that defocusing by the larger chromatic aberration is a major
factor causing degradation of contrast sensitivity in a pseudophakic eye with
an IOL that has a small Abbe number.
Reportedly, when limiting the discussion to relatively low spatial frequencies
(3 cpd) and moderate pupil sizes (3 mm), diffraction, off-axis imagery (angle  ),
transverse chromatic aberration, and even spherical aberration have a minimal
effect on contrast, and the principal cause of retinal blur is LCA.6-7 Sensitivity to the effects of LCA is
prominent at 3 cpd.15 These studies support
our results that the effect of LCA on the mean contrast sensitivity may be
most apparent at 3 cpd.
Chromatic difference of magnification is such a small change in angular
magnification that it is generally thought to be insignificant for vision,11 and has been measured only experimentally.16 Its effects appear minor when compared with those
of LCA17; in our results, the effect of the
transverse aberration seems less important. However, because the magnitude
of the effect depends on the axial distance from the entrance pupil to the
nodal point, CDM becomes an increasingly important factor when looking through
an artificial pupil or any optical or clinical instrument, which moves the
limiting aperture outside the eye.18-19
In the pseudophakic eye with an AcrySof IOL, visual function might be affected
by CDM when using an artificial pupil such as a microscope.
The effect of CDP depends on the angle of incidence of the ray bundle
or chief ray, which depends in turn on the transverse position of the pupil
and the eccentricity of the object.20 Wavelength
differences in image position for fixated targets depend on the relative position
of the pupil and visual axis. Because the natural pupil of the normal eye
is well centered on the visual axis, foveal CDP is typically less than 1 minute
of arc across the entire spectrum. However, CDP is not this small in eyes
with decentered pupils and can exceed 25 minutes of arc between 400 and 700
nm if the pupil is displaced by 4 mm, even in phakic eyes.10
In pseudophakic eyes implanted with IOLs with a small Abbe number, if the
postoperative pupil is eccentric, the CDP can affect visual function more
than in phakic eyes.
The results from the questionnaires indicated that patients did not
perceive a disturbance of color perception in their daily lives. However,
Nagata et al14 reported the potential for a
color vision disturbance caused by LCA under low luminance. If we use an IOL
with a small Abbe number, such as AcrySof, the color perception might be affected
by LCA under special conditions such as low luminance. Although the biocompatibility,
manufacturing, and maneuverability of new IOL materials have been previously
studied, our results indicate that the LCA of some IOLs may degrade the retinal
image quality under special conditions. We must give attention to the detailed
optical performance of the material to achieve optimum visual function after
IOL implantation.
AUTHOR INFORMATION
The Policy-Based Medical Services Network
Study Group for Intraocular Lens and Refractive Surgery
Coordinating Center
National Tokyo Medical Center, Department of Ophthalmology,Tokyo,
Japan: Toru Noda, MD, principal clinical investigator; Kazuno Negishi,
MD, coprincipal clinical investigator.
Clinical Centers
National Sendai Hospital, Department of Ophthalmology,
Miyagi, Japan: Shigemi Okuyama, MD. National Kanazawa
Hospital, Department of Ophthalmology, Ishikawa, Japan: Takashi Yanagida,
MD. National Chiba Hospital, Department of Ophthalmology,
Chiba, Japan: Norio Takeda, MD. National Nagoya Hospital,
Department of Ophthalmology, Aichi, Japan: Fumitaka Ando, MD. National Osaka Hospital, Department of Ophthalmology, Osaka, Japan: Ichirou Ishimoto, MD. National Okayama Hospital,
Department of Ophthalmology, Okayama, Japan: Kouichi Ohshima, MD. National Zentsuji Hospital, Department of Ophthalmology, Kagawa,
Japan: Masahiro Kogiso, MD. National Kyoto Hospital,
Department of Ophthalmology, Kyoto, Japan: Mitsuko Sunagawa, MD. National Kyushu Medical Center, Department of Ophthalmology, Fukuoka,
Japan: Ikue Takagi, MD. National Nagasaki Medical
Center, Department of Ophthalmology, Nagasaki, Japan: Toshiaki Kubota,
MD. National Kumamoto Hospital, Department of Ophthalmology,
Kumamoto, Japan: Eiko Ando, MD.
Accepted for publication January 10, 2001.
This study was supported by Health Sciences Research Grant 12120201
from the Ministry of Health and Welfare, Tokyo, Japan.
Presented as a poster at the American Society of Cataract and Refractive
Surgery, Boston, Mass, May 20-24, 2000.
Corresponding author and reprints: Kazuno Negishi, MD, Department
of Ophthalmology, Toden Hospital, 9-2 Shinanomachi, Shinjyuku-ku, Tokyo 160-0016,
Japan (e-mail: fwic7788{at}mb.infoweb.ne.jp).
From the Department of Ophthalmology, Toden Hospital (Dr Negishi),
Tokyo, Japan; the Japan Graduate School of Science and Technology, Chiba University
(Dr Ohnuma), Chiba, Japan; HOYA Healthcare Corporation (Mr Hirayama); and
the Department of Ophthalmology, National Tokyo Medical Center (Dr Noda),
Tokyo, Japan. The authors have no financial interests and have not received
payments as consultants, reviewers, or evaluators. The authors have no commercial
or proprietary interest in any products or companies used in this study.
REFERENCES
| |
1. Kruger PB, Pola J. Stimuli for accommodation: blur chromatic aberration and size. Vision Res. 1986;26:957-971.
FULL TEXT
|
ISI
| PUBMED
2. Kruger PB, Mathews S, Aggarwala KR, Sanchez N. Chromatic aberration and ocular focus: Fincham revisited. Vision Res. 1993;33:1397-1411.
FULL TEXT
|
ISI
| PUBMED
3. Uosato H, Hirai H, Fukuhara J, Matsuura T. Fundamentals of Visual Optics [in Japanese]. Tokyo, Japan: Kanahara & Co; 1990.
4. Derefeldt G, Lennerstrand G, Lundh B. Age variations in normal human contrast sensitivity. Acta Ophthalmol (Copenh). 1979;57:679-690.
5. Bour Lo J. MTF of the defocused optical system of the human eye for incoherent
monochromatic light. J Opt Soc Am. 1980;70:321-328.
PUBMED
6. Van Meeteren A. Calculations on the optical modulation transfer function of the human
eye for white light. Opt Acta (Lond). 1974;21:395-412.
7. Charman WN. The retinal image in the human eye. Prog Retinal Res. 1983;2:1-50.
8. Thibos LN. Calculation of the influence of lateral chromatic aberration on image
quality across the visual field. J Opt Soc Am A. 1987;4:1673-1680.
PUBMED
9. Carretero L, Fuentes R, Fimia A. Measurement of spherical aberration of intraocular lenses with the
Ronci test. Optom Vis Sci. 1992;69:190-192.
PUBMED
10. Bradley A, Glenn A. Fry Award Lecture 1991: perceptual manifestations of imperfect optics
in the human eye: attempts to correct for ocular chromatic aberration. Optom Vis Sci. 1992;69:515-521.
FULL TEXT
|
ISI
| PUBMED
11. Thibos LN, Bradley A, Zhang X. Effect of ocular chromatic aberration on monocular visual performance. Optom Vis Sci. 1991;68:599-607.
ISI
| PUBMED
12. Campbell FW, Gubisch RW. The effect of chromatic aberration on visual acuity. J Physiol (Lond). 1967;192:345-358.
FREE FULL TEXT
13. Isono H, Sakata H, Kusaka H. Subjective evaluation of apparent reduction of chromatic blur depending
on luminance signals. IEEE Trans Broadcasting. 1978;24:107-111.
14. Nagata T, Kubota S, Watanabe I, Aoshima S. Chromatic aberration in pseudophakic eyes. Nippon Ganka Gakkai Zasshi. 1999;103:237-242.
PUBMED
15. Bradley A, Zhang X, Thibos LN. Experimental estimation of the chromatic difference of magnification
of the human eye [abstract]. Invest Ophthalmol Vis Sci. 1990;31(suppl):493. Abstract 2423.
16. Kruger PB, Stevens D, Mathews S. Accommodation and chromatic aberration: effect of spatial frequency
[abstract]. Invest Ophthalmol Vis Sci. 1989;30(suppl):134. Abstract 49.
17. Simonet P, Campbell MCW. The optical transverse chromatic aberration on the fovea of the human
eye. Vision Res. 1990;30:187-206.
PUBMED
18. Bradley A, Zhang X, Thibos LN. Achromatizing the human eye. Optom Vis Sci. 1991;68:608-616. Review.
PUBMED
19. Zhang X, Thibos LN, Bradley A. Relation between the chromatic difference of refraction and the chromatic
difference of magnification for the reduced eye. Optom Vis Sci. 1991;68:456-458.
PUBMED
20. Thibos LN, Bradley A, Still DL, Zhang X, Howarth PA. Theory and measurement of ocular chromatic aberration. Vision Res. 1990;30:33-49.
FULL TEXT
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ISI
| PUBMED
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