·Review Article·
Personalized
aspheric intraocular lens implantation based on corneal spherical aberration: a
review
Wei Du, Wei Lou, Qiang Wu
Department of Ophthalmology, Shanghai Jiao Tong
University Affiliated Sixth People’s Hospital, Shanghai 200233, China
Correspondence to: Qiang Wu. Department of Ophthalmology, Shanghai Jiao Tong
University Affiliated Sixth People’s Hospital, Shanghai 200233, China.
qiang.wu@shsmu.edu.cn
Received:
Abstract
With the evolution of cataract surgery from visual
rehabilitation to refractive surgery, aspheric intraocular lenses (IOLs) are
being increasingly used in the field of ophthalmology. This increased use can
be attributed to negative or zero spherical aberrations with unique optical
designs, which counteract some of the positive spherical aberrations of the
cornea. These alterations reduce the total spherical aberration of human eyes
and improve the visual acuity in patients with cataract postoperatively. At
present, various types of aspheric IOLs are used worldwide. Although the
implantation of aspheric IOL is beneficial to the patients who need correction
of spherical aberrations, much controversy is still associated with ocular
residual spherical aberrations that facilitate the best visual quality for
patients postoperatively. In order to provide reference for future clinical
work and scientific research, this report reviews the relationship between the
ocular residual spherical aberration of human eyes and visual quality.
KEYWORDS: corneal
spherical aberration; aspheric intraocular lens; visual quality
DOI:10.18240/ijo.2019.11.19
Citation: Du
W, Lou W, Wu Q. Personalized aspheric intraocular lens implantation based on
corneal spherical aberration: a review. hthalmol 2019;12(11):1788-1792
INTRODUCTION
Cataracts are the leading cause of preventable blindness
worldwide[1-2]. Epidemiological
surveys show that the number of people with blindness or visual impairment
caused by cataracts has declined annually during the period 1990-2010[3]. However, the total number of cataract surgeries has
increased more than three times worldwide and the frequency of surgeries is
also increasing owing to a reduction in surgical complications, especially in
Asia[4]. With the increase in surgical treatment,
the expectations of patients regarding postoperative outcomes is also increasing.
The 2016 clinical survey of the European Cataract and Refractive Surgery
Association reported that 43% of cataract surgery has a refractive target that
achieves monocular vision[5]. The ideal
intraocular lens (IOL) should be able to restore vision, with or without
complications, and without any complaints of distant, medium, or near vision[6]. In most cases, the use of different types of IOLs can achieve
an emmetropia status in patients after surgery. At present, the mainstream
artificial lenses include monofocal IOLs, multifocal IOLs, and
astigmatism-corrected IOLs.
The human eye is the most sophisticated optical
instrument; however, its optical performance does not always equate to that of
optical instruments[7]. In addition to defocus
aberrations and astigmatism, normal visual quality is also affected by ocular
spherical aberrations and coma aberrations. The demand for superior
postoperative visual quality among patients with cataract is increasing, as
they seek to achieve independence from spectacles and favorable visual outcomes
at both near and far distances to meet the needs of daily life and work[8]. Visual quality refers to the ability to distinguish
the details and sharpness of an object[9].
High-order aberrations of the human eye are the most
important factors that affect visual quality. With improved living standards,
the requirements for visual quality are also increasing. However, the reason
for the objective visual acuity of the patient reaching 20/20 vision, and the
subjective vision remaining blurred has not yet been determined. This requires
an evaluation of the visual quality of the human eye with more accurate and
comprehensive examination. The use of wavefront aberration technology in
cataract surgery and IOL design is a considerable milestone in the history of
IOL development. This technology can sensitively and objectively detect the
visual quality of patients with cataract postoperatively, and evaluate the
quality of cataract surgery[10].
DISTRIBUTION OF CORNEAL SPHERICAL ABERRATION IN THE NORMAL POPULATION AND ITS
RELATIONSHIP WITH VISUAL QUALITY
Although the shape of the cornea, as part of the
refractive medium of the eye, is aspheric, the spherical aberration can still
be measured by specific instruments. Wei et al[11]
measured the higher-order aberrations (HOAs) in the central 6-mm zone of the
cornea using an iTrace wavefront analyzer, and found that the corneal spherical
aberration ranged from 0.015 to 0.726 μm, with a median value of 0.258 μm. Nemeth et al[12]
investigated one healthy eye of 227 subjects using the Pentacam HR system on
automatic mode, and concluded that corneal root mean square of the HOA shows a
significant growth with increasing age. Among the HOAs, primary and secondary
spherical aberrations, vertical coma, and vertical trefoil are significantly
increased with age, whereas other HOAs show no correlation with aging. Beiko et
al[13] used the Oculus Easygraph (Oculus
Optikgeraete GmbH) to measure the corneal spherical aberration of 696 healthy
subjects. They found that the mean spherical aberration was 0.270±0.089 µm, and
the fluctuation range was between +0.041 and +0.632 µm. As a result of the
various principles applied, including ray tracing, Hartmann-Shack, Tscherning,
and automatic retinoscopy, wavefront aberration values may differ[14]. Furthermore, corneal spherical aberration is also
influenced by microfluctuations in accommodation, instability of the tear film,
and small eye movements. In addition to corneal spherical aberration, the study
of other corneal aberrations is also worthy of consideration. A previous study
reported that coma aberrations of the cornea are correlated with age, whereas
spherical aberrations show no significant correlation with age[15]. In contrast, another study has demonstrated a
negative correlation between axial length and corneal spherical aberration[16]. The reason for the disparity in results could be
attributed to the differences in sample size. Philip et al[14] studied the total ocular HOAs and corneal topography
of myopic, emmetropic, and hyperopic eyes of 675 adolescents (aged 16.9±0.7y)
and found no significant difference in anterior corneal spherical aberration.
Although it is widely accepted that the visual quality of the human eye is
age-related, the fundamental cause of its decline is an increase in the number
of age-related HOAs[17]. Oshika et al[18] found that the total number of corneal aberrations
are also increased with age (measured at a pupil diameter of
VISUAL QUALITY WITH ASPHERIC MONOFOCAL IOLS AND MULTIFOCAL INTRAOCULAR LENS IMPLANTATION
In the past, the focus of cataract surgery was on the
material of IOLs, distance visual acuity and prevention of posterior capsular
opacification. The main purpose of cataract surgery today is not only to focus
on distance vision of the naked eye but, more importantly, to pay attention to
the postoperative visual quality and patient comfort and satisfaction. The
initial function of IOLs was to compensate for changes in the lens diopter of
patients after cataract surgery[20]. However,
this is not sufficient to improve the postoperative visual quality of patients.
This can only be achieved by using accurate IOL calculations to compensate for
defocus and low-order aberrations to obtain the best corrected vision. Song et
al[21] concluded that in cases of aspheric
IOL implantation, ocular vertical coma may be a major HOA associated with
better near visual acuity. Although corneal aberrations are major determinants
of ocular aberrations after cataract surgery, aberrations from internal optics
may still play an important role in visual performance.
At present, mainly monofocal IOLs and multifocal IOLs are
available. Although multifocal IOLs can facilitate distance, medium, and near
vision for patients, some studies have shown that multifocal IOL implantation
can cause a 50% decline in contrast sensitivity[22].
Researchers investigated three types of IOLs (Tetraflex, ZMA00, and Akreos
Advanced Optics) in 128 eyes of 86 patients with age-related cataract who
underwent phacoemulsification, and found that all three types allowed greater
distance visual acuity. The only exception was observed in patients of the
ZMA00 group, who had more adverse effects, such as halos and glare, compared to
patients in the other two groups[23]. Moreover,
any disturbance in optical phenomena is considered the most severe problem
after multifocal IOL implantation[24].
Studies have shown that monofocal IOLs produce similar
effects of distance vision as multifocal IOL implantation postoperatively[25]. Shah et al[26]
compared the visual outcome between multifocal and monofocal IOLs, and showed
that the percentage of patients who achieved uncorrected distance visual acuity
of 20/40 or better at 6mo was 92% in the multifocal group and 97% in the
monofocal group.
Monofocal IOLs were also associated with better
patient-reported scores for glare[26]. Compared
with multifocal IOLs, monofocal IOLs are less affected by optical phenomena,
such as halo and flash[27]. Furthermore, the
probability of IOL replacement after monofocal IOL implantation is lower[28]. Implantation of aspheric monofocal IOLs changes the
overall spherical aberration of the whole eye after surgery by conferring zero
spherical or negative spherical aberration, and thereby improving the
postoperative visual quality of patients to a certain degree. Chen et al[29] used the Optical Quality Analysis System (OQAS) to
show that the objective visual quality of aspheric IOLs is better than that of
the spherical lens. Many studies have been conducted on the visual quality of
aspheric IOLs. Scialdone et al[30] studied
72 eyes in which either the SN6AT IOL or AT Torbi
Although the implantation of aspheric IOLs can reduce
spherical aberration, achieve superior functional vision, and shows widely
accepted optical advantages, visual quality after their implantation can also
be affected by pupil diameter, and IOL depth of focus. Researchers investigated
the difference in visual quality between spherical and aspheric IOLs with
various pupil diameters in patients with cataract, and showed that at pupil
diameter of
A study[33]
has shown that although the ocular spherical aberrations are significantly
lower in eyes with implanted aspheric IOLs, the quality of vision determined by
the MTF and point spread function (PSF) shows no significant differences in
subjective and objective parameters. Although the current indications for the
implantation of aspheric IOLs remain controversial[34],
the reduction in total ocular aberration can indeed lead to improved contrast
sensitivity and visual quality[35].
Thus, the implantation of aspheric
IOLs can improve the best corrected distance vision after surgery, and improve
the postoperative contrast sensitivity to some extent. Nevertheless, the
postoperative tilt and eccentricity of the lens, the size of the pupil after
surgery, corneal astigmatism, and high-order aberrations of the cornea caused
by surgery will, to a large extent, affect the postoperative visual quality.
Therefore, to achieve the maximum benefit of surgery, the state of the IOL
should be regularly evaluated after surgery.
RESIDUAL SPHERICAL ABERRATION AND OPTIMAL VISUAL QUALITY AFTER ASPHERIC INTRAOCULAR LENS IMPLANTATION
Various studies report different opinions concerning
residual spherical aberration following the implantation of aspheric IOLs.
However, the recent study[36] has revealed that
defocus and HOAs have an interactive relationship. Some studies have shown that
positive spherical aberration improves the visual quality of patients who have
both myopia and cataract, and negative spherical aberration improves the visual
quality of patients who have both hyperopia and cataract. Some researchers
believe that maintaining a certain spherical aberration after surgery can
improve the postoperative visual quality of all patients with cataract. Jia and
Li[37] found that customized selection of
aspheric IOL implants improved mesopic contrast sensitivities at high spatial
frequencies. Another study reported that the implantation of an aspheric
aberration-correcting monofocal IOL (Tecnis ZCB00, Abbott Medical Optics) after
cataract surgery resulted in very low residual HOA and normal straylight[38].
The existence of spherical aberration is not only related
to contrast sensitivity, but also the depth of focus. A recent study showed
that positive spherical aberrations introduce myopic shifts relative to the
best subjective focus for dark letters on a bright background, whereas negative
spherical aberrations introduce hyperopic shifts in optimal focus[39]. Studies have also been conducted on the implantation
of a monofocal spherical IOL, which resulted in an increased depth of focus
without significant degradation of distance visual acuity or contrast
sensitivity[40]. Gharaee et al[41] compared the depth of focus of Tecnis and Akreos AO
IOLs after implantation, and found that a specific residual spherical
aberration can increase the depth of focus after surgery. Mu et al[42] found that personalized aspheric IOL implantation
based on preoperative corneal spherical aberration is feasible. This technique
can reduce total ocular aberrations postoperatively and lead to aberrations
that are close to the predictive value.
Thus, the main challenge of how to balance the
postoperative total ocular aberration and postoperative depth of focus
following aspheric IOL implantation, to provide the best visual quality for
patients, remains the focus of current research in refractive surgery.
CONCLUSION
At present, the implantation of
personalized aspheric IOLs based on corneal spherical aberrations improves the
patient’s best corrected distance vision after surgery, preserves the spherical
aberration, and improves the contrast sensitivity. However, we know that the
HOAs of the cornea consist of not only spherical aberrations but also other
HOAs such as coma and trifoliate aberrations. Therefore, more precise
implantation of personalized IOLs based on HOAs of the cornea would remain the
focus of clinical and scientific research in the future.
ACKNOWLEDGEMENTS
Conflicts of Interest: Du W, None; Lou W, None; Wu Q, None.
REFERENCES