optical performance of 3 intraocular lens designs in the

Optical performance of 3 intraocular lensdesigns in the presence of decentration

Griffith E. Altmann, MS, MBA, Louis D. Nichamin, MD, Stephen S. Lane, MD,Jay S. Pepose, MD, PhD

Purpose: To study the theoretical optical performance of 3 intraocular lens (IOL)designs in the presence of IOL decentration.

Setting: Optics Center, Bausch & Lomb, Rochester, New York, USA.

Methods: A ray-tracing program was used to evaluate the effect of IOL decentrationon the optical performance of 3 silicone IOLs (LI61U, Bausch & Lomb; Tecnis Z9000,Advanced Medical Optics; and a new aberration-free IOL [SofPort AO, Bausch &Lomb]) in an experimental model eye. The study was done using pupil diameters of3.0 mm, 4.0 mm, and 5.0 mm and IOL decentrations of 0 mm, 0.25 mm, 0.50 mm,0.75 mm, and 1.00 mm. The modulation transfer functions were computed andplotted. A Monte Carlo simulation analysis with 1000 trials with IOL decentrationrandomly varying for each pupil size was performed.

Results: Decentration of LI61U and Tecnis Z9000 IOLs led to asymmetricalhigher-order aberrations that adversely affected the optical performance of themodel eye; performance was not affected with the aberration-free IOL because itlacks inherent spherical aberration. Optical performance with the aberration-freeIOL was better than with the LI61U IOL as the former has less spherical aberrationand did not introduce other aberrations when decentered. Performance with theaberration-free IOL was better than with the Tecnis Z9000 IOL for 3.0 mm, 4.0 mm,and 5.0 mm pupils when decentration exceeded 0.15 mm, 0.30 mm, and 0.38 mm,respectively. Performance with the LI61U IOL was better than with the Tecnis Z9000IOL for 3.0 mm, 4.0 mm, and 5.0 mm pupils when decentration exceeded 0.3 mm,0.5 mm, and 0.5 mm, respectively. Monte Carlo simulations showed the expectedpostoperative results of the LI61U IOL and aberration-free IOL would be repeatableand predictable, whereas the outcomeswith the Tecnis Z9000 IOLwould vary widely.

Conclusions: The optical performance of the model eye was not affected bydecentration of an aspheric IOL designed to have no inherent spherical aberration.With decentration, the performance with the new IOL was better than witha conventional spherical IOL and an aspheric IOL designed to offset the sphericalaberration of an average cornea.

J Cataract Refract Surg 2005; 31:574–585 ª 2005 ASCRS and ESCRS

The recent incorporation of wavefront aberrometry

into clinical ophthalmology practice has acceler-

ated the convergence of refractive and cataract surgery.

Standard intraocular lenses (IOLs) generally do not

correct the corneal spherical aberration of eyes having

cataract surgery, resulting in a suboptimal optical

transfer function. This has created renewed interest in

aspheric IOLs and the approval of the first IOL (Tecnis

Z9000, AdvancedMedical Optics) designed to offset an

� 2005 ASCRS and ESCRS

Published by Elsevier Inc.

average amount of corneal spherical aberration in

patients who have cataract surgery.

A basic tenet of medicine is primum non nocere.Whereas well-centered IOLs with an aspheric design

have the potential to improve optical performance and

contrast sensitivity by reducing postoperative spherical

aberration, numerous factors can cause IOL decentra-

tion with respect to the visual axis. Basic principles of

optics suggest that IOLs designed to correct corneal

0886-3350/05/$-see front matter

doi:10.1016/j.jcrs.2004.09.024

LABORATORY SCIENCE: EFFECT OF DECENTRATION ON IOL OPTICAL PERFORMANCE

Table 1. Optical characteristics of the theoretical pseudophakic model eye with a 22.0D LI61U IOL.

Surface Radius (mm) Conic Constant Thickness (mm)RefractiveIndex

Object — — Infinity 1.0

Cornea 7.575 �0.14135 3.6 1.3375

Iris — — 0.9 1.336

Anterior lens surface 8.234 0 1.202 1.427

Posterior lens surface �8.234 0 16.996 1.336

Retina — — — —

spherical aberration (eg, Tecnis) might result in a

significantly lower optical transfer function than with

equally decentered standard IOLs, particularly at higher

spatial frequencies, in part because of induced 2nd- and

3rd-order aberrations such as astigmatism and coma. If

this were the case, degradation in the quality of visual

function may necessitate explantation or surgical re-

positioning of decentered aspheric IOLs.

We describe an aspheric silicone IOL (SofPort AO,

Bausch & Lomb) that maintains an excellent optical

transfer function, is neutral to the induction of positive

or negative spherical aberration, and does not induce

significant higher-order aberrations (HOAs), even when

the lens is decentered. We compared the theoretical

optical performance of this IOL with that of 2 other

IOL models when various amounts of decentration

from the visual axis were simulated.

Materials and MethodsThe optical performance of 3 IOLs was evaluated in

a theoretical model of a pseudophakic eye using a commer-

Accepted for publication July 9, 2004.

From the Optics Center, Bausch & Lomb (Altmann), Rochester,New York; the Laurel Eye Clinic (Nichamin), Brookville, Pennsylvania;and the Pepose Vision Institute (Pepose) and the Department ofOpthalmology & Visual Sciences, Washington University School ofMedicine (Pepose), St. Louis, Missouri, USA.

Supported in part by Bausch & Lomb, Inc., and the Midwest CorneaResearch Foundation, St. Louis, Missouri, USA.

Mr. Altmann is an employee of Bausch & Lomb. Dr. Nichamin isa medical monitor for Bausch & Lomb. Dr. Lane has no financial orproprietary interest in any material or method presented. Dr. Peposehas received research support from Bausch & Lomb.

Reprint requests to Griffith E. Altmann, Bausch & Lomb, A54W,1400 North Goodman Street, Rochester, New York 14609, USA.E-mail: [email protected].

J CATARACT REFRACT SUR

cially available ray-tracing program (Zemax, Focus Software).The theoretical model eye1 was the model used to develop theTecnis Z9000 silicone IOL with 1 exception: A Gaussianapodization filter was placed in the entrance pupil to simulatethe Stiles-Crawford effect.2 According to van Meeteren,3 thetransmission, T, as a function of normalized radial position inthe entrance pupil of such a filter, was calculated by theequation

TðrÞ ¼ expð�ar2Þ

where r is the normalized radial pupil coordinate anda equals 0.054. Although the benefits of the Stiles-Crawfordeffect on spatial visual performance are small,4 the effect wasincluded for completeness.

The theoretical model eye was used in this study for 3reasons. First, the positive spherical aberration of the single-surface model cornea matches the average value measured inrecent clinical studies.5–12 Using the recently standardizedZernike polynomials and double-index format for thecoefficients,13 the Z4

0 Zernike coefficient for sphericalaberration of the average cornea is approximately 0.28 mmover a 6.0 mm central zone. Second, the anterior chamberdepth of 4.5 mm matches the measurements of IOL axialpositioning in pseudophakic eyes.14–17 Third, this is the best-case scenario for evaluation of the Tecnis Z9000 IOL. Table 1shows the optical characteristics of the model eye.

Three 22.0 diopter (D) silicone IOLs with differentoptical designs were studied: a conventional IOL withspherical anterior and posterior surfaces (LI61U, Bausch &Lomb), an advanced IOL with a prolate anterior surface anda spherical posterior surface (Tecnis Z9000), and a newaberration-free IOL with aspheric anterior and posteriorsurfaces (SofPort AO). Each IOL model was assumed to haveanterior and posterior optical zone diameters of 6.0 mm. TheLI61U IOL, like any IOL with spherical surfaces, has positiveinherent spherical aberration. The Tecnis IOL has negativespherical aberration designed to offset the positive sphericalaberration of the average cornea. The aberration-free IOL hasno spherical aberration. With the exceptions of asphericanterior and posterior surfaces and a 360-degree square edge,the aberration-free IOL is similar to the LI61U IOL. The Z4

0

575G—VOL 31, MARCH 2005

mailto:[email protected]


Table 2. Optical characteristics and technical specification of the 3 IOL models.

Optical Characteristic LI61U Tecnis Z9000 SofPort AO

Power (D) 22.0 22.0 22.0

Optic material Silicone Silicone Silicone

Refractive index 1.427 1.458 1.427

Lens shape Equiconvex Equiconvex Biconvex

Anterior surface Sphere 6th-order asphere Conic asphere

Radius (mm) 8.234 11.043 7.285

Conic constant 0 �1.03613 �1.085657

4th-order constant 0 �0.000944 0

6th-order constant 0 �0.0000137 0

Posterior surface Sphere Sphere Conic asphere

Radius (mm) �8.234 �11.043 �9.470

Conic constant 0 0 �1.085657

Center thickness (mm) 1.202 1.164 1.206

A-constant 118.0 119.0 118.0

Optic size (mm) 6.0 6.0 6.0

Overall length (mm) 13.0 12.0 13.0

Haptic material PMMA PVDF PMMA

Haptic angulation (degrees) 5 6 5

PMMA Z poly(methyl methacrylate); PVDF Z polyvinylidene fluoride

coefficient for the inherent spherical aberration of the 22.0 DLI61U, Tecnis, and aberration-free IOLs over a 6.0 mm zoneis 0.32 mm, �0.72 mm, and 0 mm, respectively. Table 2shows the optical characteristics and technical specificationsof the 3 IOLs. One of the authors (G.E.A.) provided theoptical characteristics of the LI61U and aberration-free IOLs.The optical characteristics of the 22.0 D Tecnis Z9000 IOLwere taken directly from the U.S. patent.1

Each IOL was evaluated by centering it in the theoreticalmodel eye so that the anterior surface of the IOL was 0.9 mmbehind the iris. For each combination of IOL model andpupil diameter, the distance between the posterior surface ofthe IOL and the retina was optimized to attain the best opticalperformance for an on-axis object located at infinity ata wavelength of 555 nm. When an IOL is perfectly centered,only axial aberrations (eg, spherical aberrations) of the modelcornea and the IOL itself degrade the image on the modelretina. The IOL was then successively decentered in the tan-gential plane by 0.25mm, 0.50mm, 0.75mm, and 1.00mm.The optical performance of the model eye was evaluated foreach combination of IOL model, pupil diameter, and IOLdecentration. The distance from the IOL to the retina was notreoptimized for each iteration of decentration. The cornea,pupil, and retina were always centered on the optical axisof the theoretical model eye. Pupil diameters of 3.0 mm,4.0 mm, and 5.0 mm were studied. An array of 512 � 512

576 J CATARACT REFRACT SUR

(262 144) rays was traced, and the modulation transferfunction (MTF) was computed for each simulation. Theresulting tangential and sagittal MTF curves over a spatialfrequency range of 0 to 60 cycles per degree (cpd) wereplotted for each simulation.

For each combination of IOLmodel and pupil diameter,a Monte Carlo simulation of 1000 trials was performed. Inthe analysis, the amount of IOL decentration was randomlyselected from a probability distribution, which was derivedfrom a compilation of several recent IOL decentrationstudies.18–24 Descriptive statistics showed the mean of theprobability distribution used in the Monte Carlo analysis was0.36 mm 6 0.22 (SD).

ResultsThe performance of each IOL model in the

presence of decentration was analyzed for 3 pupil

diameters. Figures 1 to 3 show a total of 9 MTF plots

(3 pupil diameters � 3 IOL decentrations).

3.0 mm PupilFor a 3.0 mm pupil, the adverse effects of the

spherical aberration of the cornea and the IOL were

G—VOL 31, MARCH 2005


Figure 1. The MTF curves for a theoretical pseudo-

phakic model eye with a 3.0 mm pupil. A: The IOLs are

perfectly centered, and the adverse effects of corneal

and IOL spherical aberration are minimal. B: The IOLs are

decentered 0.5 mm. Induction of asymmetrical HOAs

degraded the performances of LI61U and Tecnis IOLs,

causing the MTF curves to droop and separate. C: The

IOLs are decentered 1.0 mm, further degrading perfor-

mance of the LI61U and Tecnis IOLs but not the

aberration-free IOL (S Z sagittal, T Z tangential).

577J CATARACT REFRACT SURG—VOL 31, MARCH 2005



phakic model eye with a 4.0 mm pupil. A: Although the

IOLs are perfectly centered, corneal spherical aberration

decreases the performance of the LI61U and aberration-

free IOLs. B: The IOLs are decentered 0.5 mm. Induced

asymmetrical HOAs cause the MTF curves of the LI61U

and Tecnis IOLs to droop and separate but negligibly

affect the curves of the aberration-free IOL. C: The IOLs

are decentered 1.0 mm. Performance with the LI61U IOL

and Tecnis IOL further degrades; the aberration-free IOL is

unaffected (S Z sagittal, T Z tangential).

578 J CATARACT REFRACT SURG—VOL 31, MARCH 2005



phakic model eye with a 5.0 mm pupil. A: The IOLs are

perfectly centered. The adverse effects of corneal

spherical aberration cause poorer performance with

LI61U and aberration-free IOLs. B: The IOLs are

decentered 0.5 mm. Induced asymmetrical HOAs

significantly degrade performance of the Tecnis IOL,

causing its MTF curves to droop and separate sub-

stantially. Degradation with the LI61U IOL is less pro-

nounced because spherical aberration dominates the

induced aberrations. The MTF curves for the aberration-

free IOL are negligibly affected. C: The IOLs are

decentered 1.0 mm (S Z sagittal, T Z tangential).



small. The Z40 coefficient for corneal spherical

aberration was 0.016 mm. The performance of the

model eye with no decentration was near diffraction-

limited with all 3 IOL designs (Figure 1, A). As the IOLs

decentered (Figure 1, B and C ), the performance

degraded with the LI61U and Tecnis IOLs, but not with

the aberration-free lens. The inherent spherical aberra-

tion of the LI61U and the Tecnis IOLs created

asymmetrical HOAs when the IOLs were decentered.

Astigmatism and coma were the primary induced

asymmetrical aberrations. The aberration-free IOL,

which does not have inherent spherical aberration, did

not create HOAs when it decentered.

The model eye performed better with the aberra-

tion-free IOL than with the LI61U IOL at all spatial

frequencies and all IOL decentrations and than with the

Tecnis IOL when decentration exceeded 0.15 mm.

Performance was better with an aberration-free IOL

decentered by 1.0 mm than with a perfectly centered

LI61U IOL at all spatial frequencies and a Tecnis IOL

decentered by 0.15 mm. The performance was better

with the Tecnis IOL than with the LI61U IOL when

decentration was less than 0.3 mm.

4.0 mm PupilFor a 4.0 mm pupil, the adverse effects of spherical

aberration of the cornea and the IOL were more

problematic. The Z40 coefficient for corneal spherical

aberration was 0.051 mm. When the IOLs were

perfectly centered (Figure 2, A), the performance of

the model eye with the Tecnis IOL was diffraction-

limited by design. The performance with the aberra-

tion-free IOL was reduced by the spherical aberration of

the cornea, and the performance with the LI61U IOL

was further reduced by the lens’ inherent positive

spherical aberration. As the IOLs decentered (Figure 2,

B and C ), the performance of the model eyes degraded

with the LI61U and Tecnis IOLs but not with the

aberration-free IOL.



frequencies and all IOL decentrations and than with

the Tecnis IOL when decentration exceeded 0.30 mm.



LI61U IOL at all spatial frequencies and than a Tecnis

IOL decentered by 0.30 mm. Overall, the performance


with the Tecnis IOL was better than with the LI61U

IOL when decentration was less than 0.50 mm. With

IOL decentration of 0.50 mm, the performance with

the Tecnis IOL was better than with the LI61U IOL

at spatial frequencies less than 20 cpd.

5.0 mm PupilThe adverse effects of spherical aberration of the

cornea and the IOL were most significant for a 5.0 mm

pupil. The Z40 coefficient for corneal spherical

aberration was 0.130 mm. When the IOLs were

perfectly centered (Figure 3, A), the performance of

the model eye with the Tecnis lens was diffraction-

limited by design. The performance of the model eye

with the aberration-free IOL was reduced by spherical

aberration of the cornea, and the performance with the

LI61U IOL was further reduced by its inherent

spherical aberration. As the lenses decentered (Figure 3,

B and C ), the performance of the model eye degraded

with the LI61U and Tecnis IOLs but not with the

aberration-free IOL.



frequencies and all IOL decentrations and than with

the Tecnis IOL when decentration exceeded 0.38 mm.



LI61U IOL and a Tecnis IOL decentered by 0.38 mm.

Overall, the performance with the Tecnis IOL was

better than with the LI61U IOL when decentration was

less than 0.50 mm. With decentration of 0.50 mm, the

performance with the Tecnis IOL was better than with

the LI61U IOL at spatial frequencies less than 15 cpd.

Monte Carlo AnalysisThe averages of the tangential and sagittal MTF

curves for a 3.0 mm, 4.0 mm, and 5.0 mm pupil

diameter are shown in Figures 4, 5, and 6, respectively.

The MTF curves for the worst 10% of cases, the best

10% of cases, and the median cases for each IOL model

are shown. Because the performance with the aberra-

tion-free IOL is independent of IOL decentration, the

worst 10%, best 10%, andmedianMTF curves lie upon

one another. Because the LI61U andTecnis IOL designs

have inherent spherical aberration, their performances

are dependent on IOL decentration; thus, the worst

10%, best 10%, and median MTF curves are separated.



Figure 4. From a Monte Carlo analysis, the best 10%,

median, and worst 10% MTF curves for a theoretical

pseudophakic model eye with a 3.0 mm pupil, showing

the results with the LI61U and aberration-free IOLs are

expected to be excellent and repeatable. Excellent

results would also be expected with the Tecnis IOL in

all but 10% of cases.


median, and worst 10% MTF curves for a theoretical


the results with the LI61U IOL are expected to be good

and repeatable and the results with the aberration-free

IOL, even better. The results with the Tecnis IOL are

expected to vary widely; the average result is expected

to be better than with the LI61U IOL but worse than with

the aberration-free IOL.


median, and worst 10% MTF curves for a the theoretical


the results with the LI61U IOL are be expected to be good

and repeatable and the results with the aberration-free

IOL, even better. The results with the Tecnis IOL are

expected to vary widely, with exceptional results in 10%

of the cases. Average low spatial frequency (contrast

sensitivity) results with the Tecnis IOL are to be better

than with the LI61U and aberration-free IOL; however,

better mid and high spatial frequency (visual acuity)

results are expected with the LI61U and aberration-free

IOLs.



Greater separation between the worst 10% and best 10%

MTF curves indicates less repeatability and predictabil-

ity in postoperative outcomes. Thus, in general, repeat-

able and predictable postoperative results would be

expected with the LI61U and aberration-free IOLs and

widely varying results with the Tecnis IOL.

For a 3.0 mm pupil (Figure 4), all MTF curves for

the aberration-free IOL lie above the MTF curve for

a perfectly centered LI61U IOL and nearly coincidewith

the best 10% MTF curve for the Tecnis IOL. Thus,

optical performance with the aberration-free IOL was

better than with the LI61U IOL in 100% of cases and

than with the Tecnis IOL in slightly less than 90% of

cases. The median MTF curves of the LI61U IOL

and Tecnis IOL lie almost upon one another; thus,

the median performance with the 2 lenses was nearly

equivalent.


the aberration-free IOL lie above the MTF curve for

a perfectly centered LI61U IOL and the median MTF

curve for the Tecnis IOL. Thus, optical performance

with the aberration-free IOL was better than with the

LI61U IOL in 100% of cases and than with the Tecnis

IOL in most cases. Similar to the aberration-free IOL,

the worst 10%, best 10%, and median MTF curves for

the LI61U IOL are similar to one another, showing that

IOL decentration only slightly affects optical perfor-

mance. In other words, for a pupil diameter of 4.0 mm

or greater, the spherical aberration of the cornea is the

dominant aberration. The median MTF curve of the

Tecnis IOL lies above the MTF curves of the LI61U

IOL, showing performance with Tecnis IOL was better

than with the LI61U IOL in most cases.


the aberration-free IOL lie above the MTF curve for a

perfectly centered LI61U IOL; thus, performance with

the aberration-free IOL was better than with the LI61U

IOL in 100% of cases. In most cases, the performance

with the aberration-free IOL was better than with the

Tecnis IOL at spatial frequencies greater than 17 cpd,

and vice versa. The performance with the Tecnis IOL

was better than with the LI61U IOL in the most cases,

especially at spatial frequencies lower than 23 cpd.

DiscussionThe introduction of the Tecnis Z9000 silicone IOL

has generated much interest in lenses with aspheric


surfaces. The Tecnis IOL is designed to provide

improved retinal image quality by compensating for

the average corneal spherical aberration of the pop-

ulation. A recent theoretical study25 primarily evaluated

the on-axis performance of the Tecnis IOL and

downplayed the effects of IOL decentration and tilt.

However, perfect centration is rare for many reasons

including in–out of the bag placement, incongruency

between bag diameter and overall IOL diameter, a large

capsulorhexis, asymmetrical capsule coverage, IOL

placement in sulcus, capsule fibrosis, capsule phimosis,

and radial bag tears.26 Even if the IOL is perfectly

centered, the other optical components of a human eye

are rarely, if ever, centered on the visual axis or on any

common axis.

The ray-tracing results show that IOL decentra-

tion adversely affects the optical transfer function

in pseudophakic eyes with IOLs that have inherent

spherical aberration. Theoretical studies of conventional

lenses with spherical surfaces show IOL decentration

induces defocus, astigmatism, and coma and the

magnitude of the aberrations depends on the magnitude

of the inherent spherical aberration.27–30 Because it is

uncorrectable with spectacles, coma may be the pre-

dominant monochromatic source of retinal image

degradation31 and may be related to the glare and halos

that have been observed with decentered IOLs.

In our eye model, performance with the aberration-

free IOL was always better than with conventional

IOLs with spherical surfaces for 2 reasons. First, the

aberration-free IOL has no spherical aberration, so

a pseudophakic eye with the aberration-free IOL would

be expected to have less spherical aberration. Second,

the aberration-free IOL does not induce other aberra-

tions when decentered, so a pseudophakic eye with this

IOL would be expected to have less induced aberration.

Even when it was decentered 1.0 mm or more, the

optical performance with the aberration-free IOL was

better than with a perfectly centered conventional IOL.

In the Monte Carlo analyses, the aberration-free IOL

always performed better than the LI61U IOL, regardless

of IOL decentration or pupil diameter.

Performance with the aberration-free IOL was

better than with the Tecnis IOL for 3.0 mm, 4.0 mm,

and 5.0 mm pupils when IOL decentration exceeded

0.12 mm, 0.30 mm, and 0.38 mm, respectively. Based

on the Monte Carlo analysis, for patients with a



postoperative pupil diameter smaller than 5.0 mm, the

aggregate visual performance with the aberration-free

IOL would be expected to be better than that with the

Tecnis IOL. For a 5.0 mm pupil, the average perform-

ances would be similar.

Our results agreewith those ofHolladay et al.,25 who

report the theoretical performance of the Tecnis IOLwas

better than that of a conventional IOLwhen decentration

was less than roughly 0.4 to 0.5 mm. In particular, for

3.0 mm, 4.0 mm, and 5.0 mm pupils, the performance

with the Tecnis IOL was better than with the LI61U

IOL when decentration was less than 0.3 mm, 0.5 mm,

and0.5mm, respectively.Our results also agreewith clin-

ical observations32–34 that show eyes with a Tecnis IOL

have better contrast sensitivity than eyes with various

conventional lenses, although visual acuity is similar

between the 2 lens types. For moderate IOL decentration

and pupil diameters, the MTF curves of the Tecnis IOL

exceeded those of the LI61U IOL at spatial frequencies

less than 20 cpd but were nearly equivalent at higher

spatial frequencies. Contrast sensitivity is typically

measured at spatial frequencies lower than 20 cpd,

usually at 1.5, 3, 6, 12, and 18 cpd.35 For visual acuity

measurement, spatial frequencies greater than 20 cpd are

more important when acuity exceeds 20/40.

According to Holladay (J. Holladay, MD, ‘‘De-

fining and Measuring Quality of Vision and the

Theoretical Basis for the Tecnis Modified Prolate

IOL,’’ presented at the XX1st Congress of the European

Society of Cataract & Refractive Surgeons, Munich,

Germany, September 2003), there are contraindications

to Tecnis Z9000 IOL implantation because of the lens’

optical design. These include difficulty centering the

IOL, a small pupil, and a superprolate cornea. Another

contraindication to Tecnis IOL implantation is a decen-

tered visual axis, which can occur in patients with

keratoconus, keratoglobus, pellucid marginal degen-

eration, and corectopia and in those who have had

decentered corneal refractive surgery. There are no con-

traindications to the new aberration-free IOL resulting

from its optical design.

Optical performance should be measured along the

visual axis, not along the axis that passes through the

center of the pupil. Studies typically measure IOL

decentration relative to the center of the pupil.

However, the visual axis is generally offset from the

center of the pupil. Rynders and coauthors36 report that


the mean displacement between the achromatic axis

and the center of the pupil in a series of young adult eyes

was 0.376 0.24 mm. Thus, the true decentration of an

IOL is probably greater than the values measured in the

aforementioned recent studies. Even an IOL perfectly

centered in the capsular bag may be significantly

decentered with respect to the visual axis. Furthermore,

the visual axis may shift given changes in pupil size and

shape under photopic,mesopic, and scotopic conditions.

Important questions remain unanswered. For

example, is correcting the average corneal spherical

aberration of the population the optimal target for an

IOL? It is common for cataract surgeons to target

emmetropia, and positive spherical aberration may help

patients with hyperopic postoperative refractions.

Modest amounts of spherical aberration may mitigate

the adverse effects of chromatic aberration37 and higher-

order monochromatic aberrations. Spherical aberration

can be compensated for with spectacle correction;

however, asymmetrical aberrations such as coma

cannot. Is the tradeoff of less spherical aberration for

more asymmetrical aberrations worthwhile? Because

most eyes, including those with exceptional vision, have

positive spherical aberration, the brain appears adept at

interpreting retinal images with positive spherical

aberration. With the Tecnis IOL, negative postopera-

tive spherical aberration is expected in 50% of patients.

Finally, left–right mirror symmetry has been observed

in corneal and total ocular aberrations. Because the

Tecnis IOL will likely introduce asymmetrical aberra-

tions arbitrarily, mirror symmetry of retinal aberrations

may be disrupted. Will the brain be as adept at

interpreting the retinal images with negative spherical

aberration and less mirror symmetry?

Other factors that affect the optical performance of

any pseudophakic eye were not assessed in the study.

These include residual postoperative refractive error,

corneal aberrations, IOL tilt, chromatic aberration, IOL

manufacturing imperfections, ocular pathology, and

scatter of optical media. Clinical evaluation of the

aberration-free IOL is required to verify theoretical

predictions of improved quality of vision and fewer

complications after cataract surgery.

References1. Norrby S, Artal P, Piers PA, et al, inventors; Pharmacia

Groningen BV, assignee. Methods of obtaining

583G—VOL 31, MARCH 2005


ophthalmic lenses providing the eye with reduced aber-rations. US patent 6 609 793. August 26, 2003

2. Stiles WS, Crawford BH. The luminous efficiency of raysentering the eye pupil at different points. Proc R SocLond [Biol] 1933; 112:428–450

3. van Meeteren A. Calculations on the optical modulationtransfer function of the human eye for white light. OpticaActa 1974; 21:395–412

4. Atchison DA, Joblin A, Smith G. Influence of Stiles-Crawford effect apodization on spatial visual perfor-mance. J Opt Soc Am A Opt Image Sci Vis 1998; 15:2545–2551

5. Guirao A, Redondo M, Artal P. Optical aberrations ofthe human cornea as a function of age. J Opt Soc Am AOpt Image Sci Vis 2000; 17:1697–1702

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