Download The Aberration

The Aberration-Free IOL:
Advanced Optical Performance Independent
of Patient Profile
Griffith E. Altmann, M.S., M.B.A.; Keith H. Edwards, BSc FCOptom Dip CLP FAAO, Bausch & Lomb
Some of these results were presented at the 2004 Symposium on Cataract, IOL and Refractive Surgery,
San Diego, Calif., May 2004.
Griffith Altmann is an optical engineer for Bausch & Lomb, Rochester, N.Y.
Keith Edwards, is a senior clinical research professional at Bausch & Lomb Rochester N.Y.
October 2004
Overview
Wavefront-based methods for measuring visual performance have
led to the design and marketing of aspheric intraocular lenses that
induce negative spherical aberration, including the Tecnis™
Z9000 and Z9001 (Advanced Medical Optics) and the
Acrysof™ SN60WF (Alcon Laboratories). As developed
originally for the Z9000, this technology uses a prolate anterior
surface of the IOL to compensate for the positive spherical
aberrations of an average cornea. Basic and clinical studies have
shown this can improve contrast sensitivity. However, common
levels of IOL decentration or tilt in vivo induce optical
aberrations that can compromise visual performance. 1, 2, 3, 4,
A new aberration-free IOL technology, with prolate anterior and
posterior lens surfaces and no inherent spherical aberration, is
being tested by Bausch & Lomb. Optical simulation testing and
computer modeling indicate that, even when decentered 1 mm,
this new SofPort™ Advanced Optics (SofPort AO) design for an
IOL performs better optically than does a well-centered
conventional IOL. Because the SofPort AO design does not
introduce spherical or monochromatic higher-order aberrations
(HOAs), it also avoids degradation of the retinal image in the
presence of mild to moderate decentration and moderate tilt.
Computer modeling show this gives the SofPort AO lens design
better optical performance overall in a wide range of situations
than a negative-spherical-aberration aspheric IOL. Consequently,
the SofPort AO technology offers the combination of
predictability and optimal optics for all pseudophakic patients,
in an easy-to-use, familiar IOL type that is designed to eliminate
post-operative surprises.
Background
In recent years, the adoption of ocular wavefront measurement in
clinical ophthalmology has moved the signposts of good vision
beyond 20/20 acuity. Studies of wavefront-based LASIK
demonstrate that limiting or reducing higher-order aberrations
(HOAs) gives patients better contrast sensitivity, and can correct
the light-scatter within the ocular system from aberrations such as
coma. 5, 6, 7 This improves the quality of the patient’s vision,
particularly at night. Conventional intraocular lenses also add
HOAs and spherical aberration to the visual system, limiting the
quality of vision possible with pseudophakia.4, 8, 9, 10, 11, 12, 13 This
knowledge has led to increasing speculation in the IOL world
about using wavefront-based methods to improve post-operative
contrast sensitivity with customized or wavefront-adjusted
IOLs. 14, 15 These new IOLs would need to correct the higherorder, non-axisymmetrical aberrations in the patient’s visual
system as well as the spherocylindrical refractive error.
However, ophthalmologists who want to offer their cataract
patients wavefront-optimized postoperative vision also must
consider the basic tenet of medicine – primum non nocere. – before
choosing a new IOL design for their patients. Basic optical
principles suggest that decentration or tilt of an aspheric IOL with
negative spherical aberration might result in a significantly lower
optical transfer function than equally decentered standard lens
implants, particularly at higher spatial frequencies. 1, 2, 3, 4, 16 This is
in part because of induced second- and third-order aberrations,
such as astigmatism and coma. Recent reports on negativespherical-aberration aspheric IOLs, detailed below, have confirmed
that this optical degradation can occur with this type of IOL
design.
Aspheric Designs
Artal et al. reported in 2002 that the HOAs of the cornea are
compensated for in young adults with negative spherical
aberration in the rest of the ocular system.17 Their wavefront
analysis in 17 people showed that younger subjects had more
corneal HOAs than total ocular aberrations, while the reverse
was true for the elderly.
They suggested that negative spherical aberration of internal
ocular surfaces declines with age. The researchers postulated that
the best retinal image for an elderly pseudophake would come
from an IOL that took this imbalance in corneal and internal
HOAs into account. This year, Amano et al. confirmed an increase
in positive spherical aberration in the visual system with age. 18
In 75 eyes of 75 patients, the researchers found that the corneal
spherical aberration did not increase with age, but the total ocular
spherical aberration did. They attributed the increase to
non-corneal, internal changes in the eye, which also increased
the patients’ ocular coma.
The Tecnis lens features a prolate anterior surface optimized to
neutralize the average spherical aberration found in the eyes of the
71 subjects in the design study. 1 Several studies of this aspheric
IOL have confirmed the expected increase in contrast sensitivity,
particularly in mesopic conditions, although this has not always
been clinically or statistically significant. 1, 2, 3, 4 As a result, other
negative-spherical-aberration aspheric IOLs are on the horizon for
clinical use.
Effects of Decentration
Perfect centration of an IOL is rare, and the degree of centration
can change over time as the capsule contracts after cataract
surgery. In patients without weak zonules or another predisposing
condition, the reasons for decentration include in-out of the bag
placement, incongruency between bag diameter and overall
diameter of lens, large capsulorhexis, asymmetrical capsular
coverage, lens placement in sulcus, capsular fibrosis, capsular
phimosis and radial bag tears. Decentration of a lens with either
positive or negative spherical aberration induces defocus,
astigmatism and coma, the magnitude of which depends on the
magnitude of the inherent spherical aberration, according to
theoretical studies of conventional IOLs. 3, 19, 20
IOL decentration appears to occur more often in silicone IOLs
than with those made of other materials. In a 2003 survey of
cataract surgeons, silicone IOLs were the most likely material to
require explantation, accounting for 27% of all explants. 21
Among 3-piece silicone IOLs, dislocation/decentration was
blamed for 34% of the explants of this IOL type.
The optical effects of decentration of an IOL can increase when
the lens has an aspheric design with negative spherical aberration.
In detailing the design of the first such IOL, Holladay et al.
noted that the lens’s optical performance degraded in the
presence of tilt and decentration, particularly the latter. 1 The
modulation transfer function (MTF) values for the 5 D to 20 D
powers of the IOL fell below those for a conventional IOL at
0.5 mm of decentration. The cutoff was above 0.5 mm for the 30
D Tecnis. With tilt, an angle of greater than 7 degrees reduced
the MTF to below that of a conventional IOL.
Even at lower levels of decentration, though, there was a
measurable increase in total ocular aberrations with the negativespherical-aberration aspheric design, German surgeon Martin
Baumeister, MD, has reported. 16 In a bilateral study of 10
patients, he and colleagues compared decentration and resulting
postoperative HOAs between the spherical AR40e (Advanced
Medical Optics) and the prolate Tecnis. The mean Tecnis
decentration 3 months after surgery was 0.3 +/- 0.2 mm,
equivalent to the degree of decentration seen in the AR40e.
However, only in the Tecnis did the extent of decentration
correlate with greater higher-order root mean square HOA values,
Dr. Baumeister reported. This was due to an increase in thirdorder aberrations (i.e., coma, which cannot be corrected with
spectacles 22). In one of the 10 eyes in which it was implanted,
the aspheric IOL was decentered by 1 mm. Mean tilt was 2.2
+/- 1.2 degrees, 25% lower than the mean for the AR40e.
More recently, Koch and Wang completed a computer simulation
confirming that coma would be the greatest aberration induced
by a decentered aspheric IOL. 23 They concluded that, with
perfect centration, only 10% to 44% of eyes implanted with a
wavefront-guided or negative-spherical-aberration aspheric IOL
would measure in the lowest 10th percentile of corneal HOAs.
For a 4 mm pupil, the decentration would have to be less than
0.34 mm to reduce total HOAs below the level of corneal HOAs
in 50% of the eyes, they calculated.
It is important to recognize that studies of HOAs from IOL
decentration generally are computer simulated with an idealized
model eye, in which the visual axis is centered in the pupil.
In vivo, however, the various components of the visual system
rarely are aligned either with each other or with the actual visual
axis, reducing the quality of the retinal image. 24 This would
suggest that, in clinical use, the optical performance of a negativespherical-aberration aspheric IOL might be impacted by smaller
levels of clinically observed decentration than simulations would
indicate.
Indeed, even a lens perfectly centered within the capsular bag may
be substantially decentered with respect to the visual axis. The
mean difference between the location of the achromatic axis and
the center of the pupil in young adults is 0.37 mm, with a
standard deviation of 0.24mm, according to Rynders et al. 25 In
addition, the visual axis can shift as a result of changes in pupil
size and shape under varying light levels.
A Different Approach:
SofPortTM Advanced Optics
A conventional IOL adds positive spherical aberration to that
which already exists in the cornea. Aspheric IOLs currently in use
impart negative spherical aberration, ideally neutralizing the
cornea’s average level of positive aberration. Rather than trying to
correct for this positive aberration, the SofPort™ Advanced
Optics (AO) method for reducing HOAs in the pseudophakic eye
avoids introducing any spherical aberration at all. SofPort AO
lenses have prolate anterior and posterior surfaces and no inherent
spherical aberration. The SofPort AO technology also does not
affect the other structural or mechanical characteristics of the
B&L lenses to which it is being added, so implanting a SofPort
AO-IOL will require no new techniques or equipment for
surgeons.
The effectiveness of the SofPort AO approach was recently
demonstrated using the same experimental model that was
originally used to validate and design the Tecnis. 1 In these
ray-tracing experiments, a SofPort AO version of Bausch &
Lomb’s LI61U silicone IOL was tested against a conventional
LI61U SE and an aspheric lens with negative spherical
aberration. 26, 27, 28
The simulations predicted, at a wide variety of pupil sizes and
IOL positions, that the SofPort AO design would produce better
vision for patients than an IOL with negative spherical aberrations
should. At 0.38 mm of decentration or higher, with a pupil
smaller than 5 mm, the SofPort AO’s modulation transfer
function (MTF) – and thus the quality of the retinal image –
exceeded that of the IOL with negative spherical aberration at all
spatial frequencies. In vivo, this should correlate with better
contrast sensitivity in low light conditions despite the IOL’s
decentration.
Spatial Frequency at the retina
Conclusion
Decentration at 3 mm
Decentration at 4 mm
Further analysis via Monte Carlo simulations showed the existing
negative spherical aberration aspheric IOL’s average performance
equaled that of the SofPort AO lens only at a pupil size of 5 mm.
In addition, the analysis demonstrated that the SofPort AO
design also represents an improvement over conventional IOLs.
This is because it does not introduce the spherical aberration that
is responsible for increased total HOAs induced by conventional
IOLs. Analysis showed that the SofPort AO lens would produce a
better quality retinal image than the conventional IOL in all
cases, even when it was decentered 1 mm and the LI61U SE was
perfectly centered. This is because the pseudophakic eye with a
SofPort AO lens has less spherical aberration, and because its
decentration does not induce the asymmetrical HOAs seen when
a conventional IOL decenters.
Bausch & Lomb’s method for designing apsheric intraocular lenses
overcomes the vision-limiting features not only of negativespherical-aberration aspheric IOLs, but also of conventionally
designed IOLs. SofPort AO lenses feature both anterior and
posterior prolate surfaces and have no inherent spherical
aberration. They require no special implantation procedures
beyond those already used with B&L IOLs. Computer modeling
has demonstrated the SofPort AO design can prevent the
degradation of image quality that occurs when a conventional
IOL or a negative spherical aberration aspheric IOL decenters. If
these optical advantages are confirmed in upcoming clinical tests,
the SofPort™ Advanced Optics method of IOL design could
improve ocular wavefront results for all pseudophakic patients,
without risking post-operative visual surprises arising from IOL
decentration and tilt.
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