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ARCH SOC ESP OFTALMOL 2006; 81: 575-580
ORIGINAL ARTICLE
LASER RAY TRACING TO MEASURE OPTICAL
ABERRATIONS IN EYES WITH CONTACT LENSES
ABERRACIONES ÓPTICAS EN OJOS CON LENTES DE CONTACTO
MEDIDAS CON EL SISTEMA DE TRAZADO DE RAYOS
BLÁZQUEZ-SÁNCHEZ V1, MENDOZA-PÉREZ MA2, MERAYO-LLOVES JM3, NAVARRO-BELSUÉ R4
ABSTRACT
RESUMEN
Purpose: To evaluate the impact of disposable soft
contact lenses upon visual performance by comparing the optical aberrations of myopic eyes with and
without the use of contact lenses.
Method: The optical aberrations of a set of 18 eyes
were measured by a laser ray tracing system, an
objective measurement method, enabling comparison of the findings of the naked eye, the eye plus a
test lens, and the eye plus a disposable contact lens.
Results: There was a large variability among subjects, but the general tendency was that test lenses
did not modify aberrations, whereas contact lenses
tended to increase the ocular aberrations for large
pupils by an average of 40%. There was a lower
increase for smaller pupils.
Conclusion: This study predicts there will be no
significant adverse effects on the quality of vision
from the use of disposable contact lenses worn
under daylight conditions, but a small decline in
night vision (Arch Soc Esp Oftalmol 2006; 81: 575580).
Objetivo: Evaluar el impacto de las lentes de contacto desechables en la calidad visual, comparando
aberraciones ópticas, en ojos miopes, con y sin lentes de contacto.
Método: Las aberraciones ópticas se midieron
mediante el sistema del Trazado de Rayos Láser, el
cuál es un método de medida objetivo, sobre un
conjunto de 18 ojos, en 3 condiciones diferentes: a
ojo desnudo, ojo más lentes de prueba, ojo con lentes de contacto desechables.
Resultados: Existe una alta variabilidad entre sujetos, pero la tendencia general es que las lentes de
prueba no modifican las aberraciones, mientras que
las lentes de contacto tienden a incrementar las aberraciones oculares para pupilas grandes cerca de un
40% en promedio. Sin embargo tal incremento es
mucho menor para pupilas más pequeñas.
Conclusión: Este estudio revela que no se produce
efectos significativos en la calidad de visión en
pacientes que usan lentes de contacto desechables
bajo condiciones de luz diurna, así como un pequeño descenso en visión nocturna.
Key words: Aberrations, soft contact lenses, Laser
Ray Tracing.
Palabras clave: Aberraciones, lentes de contacto
blandas, método de trazado de Rayos.
Received: Sept. 13, 2004. Accepted: Oct. 23, 2006.
Instituto de Óptica «Daza de Valdés», Consejo Superior de Investigaciones Científicas (C.S.I.C.). Madrid. Spain.
CEO «Centro de Especialidades Oftalmológicas». Madrid. Spain.
1 Graduate in Optics and Optometry. Centre of Ophthalmological Specialties of Madrid (CEOS). Madrid.
2 Ph.D. in Physical Sciences. Instituto de Óptica «Daza de Valdés», Consejo Superior de Investigaciones Científicas (C.S.I.C.). Madrid.
3 Ph.D. in Medicine. CEOS and Institute of Applied Ophthalmobiology (IOBA). University of Valladolid.
4 Ph.D. in Physical Sciences. Instituto de Óptica «Daza de Valdés», Consejo Superior de Investigaciones Científicas (C.S.I.C.).
Correspondence:
Vanesa Blázquez Sánchez
C/. Antonio López, 149, 1.º B
28026 Madrid
Spain
E-mail: ltqb347@ifa.cetef.csic.es
BLÁZQUEZ SÁNCHEZ V, et al.
INTRODUCTION
Today, ametropia may be corrected using glasses,
contact lenses (CL) or refractive surgery. Doctors
must inform their patients about the existing pros
and cons of all three methods, so that patients may
be capable of making the most adequate choice
based on their specific needs. Since all of the above
methods correct ametropia (out-of-focus vision
and/or astigmatism) directly, it is important to
follow certain criteria in order to choose the best
solution for each particular individual.
In cases of moderate ametropia, conventional
glasses will not alter optical aberrations significantly due to its relatively low numerical aperture
(low aberrations); and, since they do not impact the
eye directly, neither do they alter its geometry.
Nevertheless, refractive surgery techniques seem to
increase optical aberrations by 2 (1) and even by 4
(2). As for CLs, recent studies have pointed out the
presence of several factors affecting visual quality
such as tear film, dynamics of the lens itself, time,
age, lens hydration and flexibility, as well as the
lens manufacturing process and changes in ocular
aberrations due to adjustment (3,4).
Few studies have analysed the optical quality of
the system made up by the eye plus the contact lens
using objective parameters based on the Modulation Transfer Function (5,6), which is found by
using a double-step method (7,8). Generally, previously published studies assessed optical compensation via CL exclusively focusing on subjective
techniques such as visual acuity (VA) or the Contrast Sensitivity Function (CSF). Furthermore, most
studies only focused on rigid gas permeable (RGP)
contact lenses. There is high variability as far as the
findings published in literature goes, where the differences observed were usually not significant.
Some of these studies found that RGP contact lenses improved visual quality by reducing corneal
aberrations, whereas a few studies have reported
slight improvements as a result of using soft contact
lenses. Other authors stated that improvement of
visual quality through the use of CLs varied and
depended to a certain extent on their own nature (919).
The department in charge of Image and Vision at
the Instituto de Óptica (CSIC, Madrid, Spain) has
received worldwide recognition for its contribution
in the field of physiological optics and vision. In
both basic and clinical studies, several methods
576
devised by this laboratory were successfully
applied to measure optical quality of the eye in
vivo. One of those methods, known as Laser Ray
Tracing (LRT), measures in an objective manner
optical aberrations (9) in the eye and was the method chosen for the present study.
SUBJECTS, MATERIAL AND
METHODOLOGY
The present research complied with the postulates contained in the Declaration of Helsinki and
thus all subjects signed the corresponding informed
consent forms after being briefed on the nature and
possible consequences of the study.
Material
Disposable CLs were chosen for the present
study; weekly CLs in case of prolonged use and biweekly contact lenses in case of daily use. CLs
were hydrophilic, with 58% humidity, radius ranging from 8.30 to 8.70 mm.
Subjects
The subjects were 10 myopic volunteers (18
eyes) free of ocular disorders susceptible of triggering intolerance to contact lenses. Six (6) females
and 4 men, ages ranging from 22 and 27 years.
Refractive errors went from –.75 and –7.00 D.
Table I summarizes the data concerning these subjects in ascending order based on myopia values.
Methodology
Eye Exam
All subjects underwent optometric and ophthalmic examinations which entailed measuring ametropia, adjustment to contact lenses, and a study of ocular health (measuring eye pressure, performing a slit
lamp study, measuring the iridocorneal angle, studying the eye fundus whenever necessary, reviewing
the patient’s history, high ametropia, etc). At least
two days were allowed between the time of adjustment to contact lenses and that of the experiment.
ARCH SOC ESP OFTALMOL 2006; 81: 575-580
Aberrations with contact lenses
Measuring Objective Ocular Aberration
Optical quality in each eye was measured via LRT,
the method devised by the Instituto de Óptica (CSIC)
to objectively measure optical aberrations (9). A
narrow laser beam (ray) was directed at several spots
on the retina; the positioning of the light spots formed by each beam was then recorded; differences
between theoretical locations and those actually
found yielded an estimated aberration revealed by
the light’s wave-front when traversing the eye (1,14).
Measurements were performed when at least two
of the following conditions were detected in each eye:
1. No correction (naked eye), the exception being
those subjects with more than 4 diopters, since higher myopia values could exceed the LTR’s measurement range.
2. Test lenses helped estimating the optical quality achieved with glasses. Due to the different
models available, test lenses were chosen in order
to standardize results. Measurements were performed in most cases, though only when the first case
proved hopeless.
3. In all cases, the CL to be assessed. In each
case, measurements were repeated 5 times.
The subjects did not wear CLs for at least 48
hours prior to measurement. Soft CLs were worn at
least 30 minutes prior to measurement in order to
allow for full eye stabilization.
FINDINGS
Figure 1 shows the RMS for wave-front error, not
taking into account out-of-focus vision and astigmatism (that is, only 3rd order aberrations or higher), for 18 eyes and 6.5 mm pupilas: uncorrected
eyes (SC, white bars); eyes wearing test lenses (LP,
black bars) and CL users (CL, bars with diagonal
grey lines). The data were classified based on myopia values (in ascending order). Eyes 1 to 16 showed spherical myopic errors ranging from .75 to
3.50 D, whereas eyes 17 and 18 exhibited errors up
to 7 D, as seen in Table 1 above.
As for the aberrations measured, results varied
greatly from one eye to the other. On average, test
lenses tended to slightly reduce optical aberrations
and to increase optical quality.
For CLs, several groups were observed: Eyes (18) with refractive errors less than or equal to 2.00 D;
CLs increased aberrations in all cases. On the other
hand, 70% of eyes with refractive errors greater
than 2.50 D (eyes 9-18) improved their optical quality using CLs. When applying the Paired-Samples
T-Test, the p-value obtained was .0092 when comparing distributions for RE and LC, and pvalue=.0229 for distributions of RE and LC. Both
were greater than α=.05 (95% confidence), so
results were statistically significant.
Table II shows average results for different aberrations.
Data Analysis
Wave aberration may be characterized as a 7th
order Zernike polynomial expansion (35 coefficients). The average set of Zernike coefficients was
estimated through adjustment of the data yielded by
all five measurements taken for each condition; therefore, these coefficients described the optical aberrations found. The root-mean-square (RMS) for
these coefficients (RMS for the wave-front error)
was used to measure the eye’s global optical quality, or that of the optical system formed by eye plus
lens (be it contact or test lens).
Measurements were performed on a 6.5 mm
pupi-la obtained through dilation with cyclopegic
drugs, although Zernike coefficients were again
recalculated based on a 3 mm pupil.
In order to assess whether these results were statistically significant or not, a Paired-Samples T-Test
with a 95% confidence level (_=.05) was performed.
Fig. 1: Third order aberrations or higher: wave-front
error RMS for 3rd order aberrations or higher, a pupila
6.5 mm in diameter, under 3 conditions: uncorrected
vision (SC, white), eyes wearing test lenses (LP, black)
and CL users (LC, diagonal grey lines). Data are shown
in ascending order based on myopia values.
ARCH SOC ESP OFTALMOL 2006; 81: 575-580
577
BLÁZQUEZ SÁNCHEZ V, et al.
Table I. Optometric Data for the Eyes
Eye
Refraction (D)
Keratometry (mm)
Biomicroscopy
Comfort wearing contact lenses
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
–.75
–.75
–1.50
–1.50
–1.50
–1.50
–1.50
–2.00
–2.50
–2.50
–2.75
–2.75
–2.75
–2.75
–2.75
–3.50
–5.25
–7.00
8.16 × 8.01
8.14 × 8.00
8.04 × 7.87
7.96 × 7.76
7.94 × .73
7.72
7.82
8.17 × 8.03
7.55 × 7.43
8.25 × 8.09
7.81 × 7.73
7.70 × 7.60
7.76 × 7.71
7.76 × 7.71
7.70 × 7.6
7.60 × 7.52
7.53 × 7.44
7.59 × 7.35
Reddening
Reddening
OK
OK
OK
OK
OK
Reddening
Reddening
Reddening
OK
OK
OK
OK
OK
Reddening
OK
OK
Slight discomfort
Slight discomfort
Comfortable
Comfortable
Comfortable
Comfortable
Comfortable
Comfortable
Slight discomfort
Slight discomfort
Comfortable
Comfortable
Slight discomfort
Slight discomfort
Comfortable
Comfortable
Comfortable
Comfortable
As expected, test lenses did not significantly alter
eye aberrations, though results revealed a slight
improvement. On the other hand, on average, CLs
increased optical aberrations in 40% of cases.
Nevertheless, it is worth mentioning that standard
deviations were always significantly greater than
the average, indicating high variability and different
behaviour for each individual eye (20). There was
also a strong dependency on the size of the pupila:
while the increase of optical aberrations was potentially greater for large pupilas (6.5 mm), values
found for smaller pupilas (3 mm) were much lower.
DISCUSSION
Results obtained using the LTR method predicted
an effect on visual quality which was negligible in
the case of disposable CLs under conditions of
natural daylight (the pupila’s diameter being not
much greater than 3 mm), and yet a moderate decrease in visual quality during night vision or with low
light levels (approximately 6.5 mm pupilas), while
test lenses did not alter visual quality. Nevertheless,
it is important to point at the high variability of
results found for different eyes (20). The present
study should be considered as a pilot experiment
and further studies will be needed to determine the
causes of such big differences, including a greater
number of patients and different brands of CLs as
well as additional parameters in order to perform
more accurate adjustments.
ACKNOWLEDGEMENTS
The present study was partially funded by the
CICYT (Ministry of Education and Science) under
TIC 98-0925-CO2-01. The authors wish to express
their gratitude to the Unidad Asociada de Investigación IOBA (University of Valladolid), Instituto
de Óptica (CSIC), and Unidad de Contactología del
Centro de Especialidades Oftalmológicas de
Madrid for their contribution; and to Ms. Lourdes
Llorente (Instituto de Óptica, CSIC) for her technical assistance.
Table II. Average Increase of RMS in Uncorrected Eyes
Pupila 6.5 mm
Pupila 3 mm
RMS Variation (mm)
Global (3rd to 7th order aberrations)
Coma (3rd order)
4th order
578
Test Lenses
Contact Lens
Test Lenses
Contact Lens
–.06 SD .21
–.08 SD .21
.00 SD .10
.20 SD .43
.05 SD .28
.14 SD .30
–.010 SD .033
–.010 SD .032
–.000 SD .007
.02 SD .12
.02 SD .11
.004 SD .053
ARCH SOC ESP OFTALMOL 2006; 81: 575-580
Aberrations with contact lenses
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