Comparison of intraocular lens decentration and tilt measurements using 2 Purkinje meter systems

Abstract

Purpose
To evaluate the difference in intraocular lens tilt and decentration measurements with 2 Purkinje meters.

Setting
Vienna Institute for Research in Ocular Surgery, Hanusch Hospital, Vienna, Austria.

Design
Prospective evaluation of diagnostic test.

Methods
This single-center study included pseudophakic patients in 2 substudies in which 3 consecutive measurements were performed with 2 Purkinje meters (Spanish and German). In substudy 1, an inexperienced examiner performed all measurements after a short learning period. In substudy 2, all measurements were taken by experienced examiners under direct supervision of the inventors of the devices.

Results
Substudy 1 included 53 pseudophakic eyes in which all 53 scans were successful with the Spanish device; however, only 35 measurements (66%) were successful with the German Purkinje meter. The mean tilt measured with the Spanish Purkinje meter was 4.35 degrees ± 2.50 (SD) and 9.20 ± 6.96 degrees with the German Purkinje meter. The mean decentration was 0.44 ± 0.19 mm and 0.74 ± 0.91 mm, (P = .44), respectively. In substudy 2 (29 pseudophakic eyes), the number of successful scans was 29 (100%) and 18 (62%) for the Spanish meter and German Purkinje meter, respectively. The mean horizontal and vertical tilt difference vector between the 2 systems was 4.89 ± 3.24 degrees and 7.57 ± 3.82 degrees, respectively.

Conclusions
Concerning clinical feasibility, the Spanish Purkinje meter had a greater percentage of successful scans than the German device. In addition, this device measured significantly higher tilt values than the Spanish Purkinje meter.

Full text

ARTICLE
Comparison of intraocular lens
decentration and tilt measurements using
2 Purkinje meter systems
Sophie Maedel, MD, Nino Hirnschall, MD, PhD, Natascha Bayer, MSc, Sabine Markovic, MD,
Juan Tabernero, PhD, Pablo Artal, PhD, Frank Schaeffel, PhD, Oliver Findl, MD, MBA
Purpose: To evaluate the difference in intraocular lens tilt and de-
centration measurements with 2 Purkinje meters.
Setting: Vienna Institute for Research in Ocular Surgery, Hanusch
Hospital, Vienna, Austria.
Design: Prospective evaluation of diagnostic test.
Methods: This single-center study included pseudophakic patients
in 2 substudies in which 3 consecutive measurements were p erformed
with 2 Purkinje meters (Spanish and German). In substudy 1, an
inexperienced examiner performed all measurements after a short
learning period. In substudy 2, all measurements were taken by
experienced examiners under direct supervision of the inventors of
the devices.
Results: Substudy 1 included 53 pseudophakic eyes in which
all 53 scans were successful with the Spanish device; however,
only 35 measurements (66%) were successful with the German
Purkinje meter. The mean tilt measured with the Spanish Pur-
kinje meter was 4.35 degrees G2.50 (SD) and 9.20 G6.96
degrees with the German Purkinje meter. The mean decentra-
tion was 0.44 G0.19 mm and 0.74 G0.91 mm, (PZ.44),
respectively. In substudy 2 (29 pseudophakic eyes), the number
of successful scans was 29 (100%) and 18 (62%) for the
Spanish meter and German Purkinje meter, respectively. The
mean horizontal and vertical tilt difference vector between the
2 systems was 4.89 G3.24 degrees and 7.57 G3.82 degrees,
respectively.
Conclusions: Concerning clinical feasibility, the Spanish Purkinje
meter had a greater percentage of successful scans than the
German device. In addition, this device measured significantly high-
er tilt values than the Spanish Purkinje meter.
J Cataract Refract Surg 2017; 43:648–655 Q2017 ASCRS and ESCRS
The increasing number of implantations of aspheric,
toric, and multifocal intraocular lenses (IOLs) dur-
ing cataract surgery has improved postoperative vi-
sual quality of cataract patients.
1,2
However, exact
alignment of these premium IOLs is mandatory because
decentration, tilt, or rotation (in the case of toric IOLs)
could result in reduced visual quality. Holladay et al.
3
showed in an eye model that decentration more than
0.4 mm or tilt more than 7 degrees result in a loss of the
beneficial effect of an aspheric IOL compared with a
spherical IOL. Piers et al.
4
found these critical values to
be more tolerable with 0.8 mm of decentration and
10 degrees of tilt.
Several techniques have been used to measure IOL
decentration and tilt, such as slitlamp examination, retroil-
lumination photography,
5
rotating Scheimpflug imaging,
6–8
optical coherence tomography,
9–11
and the analysis of Pur-
kinje reflexes.
12–17
Slitlamp examination is a subjective
method that allows approximate decentration measure-
ments but no quantitative tilt measurements. For this mea-
surement, the pupil has to be dilated.
For Scheimpflug imaging, the pupil must be sufficiently
dilated to make the optic edge and the posterior surface
of the IOL visible.
The concept of Purkinje reflexes analysis dates back to
the 19th century, when candles were used to generate
Submitted: June 27, 2016 |Final revision submitted: December 22, 2016 |Accepted: January 22, 2017
From the Vienna Institute for Research in Ocular Surgery (Maedel, Hirnschall, Markovic, Findl), A Karl Landsteiner Institute, Hanusch Hospital, and Laser and Optics in
Applied Life Sciences (Bayer), University of Applied Sciences Technikum Wien, Vienna, Austria; Laboratorio de

Optica (Tabernero, Artal), Departamento de F
õsica,
Universidad de Murcia, Murcia, Spain; Vision & Eye Research Unit (Tabernero), Postgraduate Medical Institute, Anglia Ruskin University, Cambridge, and Moorfields Eye
Hospital NHS Foundation Trust (Findl), London, United Kingdom; the Section of Neurobiology of the Eye (Schaeffel), Institute for Ophthalmic Research, T€
ubingen,
Germany.
Presented in part at the XXIX Congress of the European Society of Cataract and Refractive Surgeons, Vienna, Austria, September 2011.
Corresponding author: Oliver Findl, MD, MBA, Hanusch Hospital, Department of Ophthalmology, Heinrich-Collin-Straße 30, 1140 Vienna, Austria. E-mail:
oliver@findl.at.
Q2017 ASCRS and ESCRS
Published by Elsevier Inc.
0886-3350/$ - see frontmatter
http://dx.doi.org/10.1016/j.jcrs.2017.01.022
648

reflections of ocular optic surfaces.
18
In the 1990s, light-
emitting diodes (LEDs) were used to analyze the alignment
of Purkinje reflexes III and IV.
19,20
The analysis of Purkinje
reflexes has been shown to be more accurate than using
Scheimpflug images.
21
A Purkinje meter developed by
Tabernero et al.
22
(called the Spanish Purkinje meter in
this study) at the University of Murcia, Spain, was shown
to be highly reproducible in assessing IOL tilt and
decentration
14
and detecting differences in capsular bag
performance of different IOL designs.
23
Another Purkinje
meter device developed by Schaeffel
24
(called the German
Purkinje meter in this study) has a setup similar to the
construction of Tabernero et al.
22
As a potential
advancement over the Spanish meter, this device is portable
and includes a gaze tracker. It has been shown to measure
crystalline lens tilt and decentration in healthy eyes even
without pupil dilation.
25
Recently, it has been shown to
have high reliability and reproducibility in pseudophakic
eyes.
26,27
The aim of this study was to evaluate the difference in
IOL tilt and decentration measurements with 2 clinical
Purkinje meter systems.
PATIENTS AND METHODS
This prospective single-center study included pseudophakic pa-
tients who had uneventful standard small-incision cataract sur-
gery 3 to 12 months before recruitment and had otherwise
healthy eyes. Exclusion criteria were complications during cataract
surgery, severe opacities of the cornea or the IOL, posterior capsule
opacification, or any pathologies preventing the patient from ac-
curate fixation, such as significant macula pathologies or ambly-
opia. All the research and measurements followed the tenets of
the Declaration of Helsinki.
This study consisted of 2 subgroups, for which only 1 visit was
necessary. During this visit, pupils were dilated before examina-
tions with tropicamide 1.0% (Mydriaticum) and phenylephrine
10.0% (Neosynephrin–POS 10.0%). In both substudies, 3 consec-
utive measurements were performed with the 2 Purkinje meters
(1 from Spain [Figure 1] and 1 from Germany [Figure 2]). The de-
vice to be used for measurements first was chosen randomly in
both substudies. Randomization was performed using an online
random generator.
A
In substudy 1, an inexperienced examiner performed all mea-
surements after a short learning period of 20 measurements
with both Purkinje meters. Measurements were performed on a
continuous cohort.
Figure 1. Left: Setup of the Spanish Purkinje meter. Right: Pupillary
margin (red) and Purkinje reflexes I, III, and IV after manual marking
with the meter’s software (P ZPurkinje reflex).
Figure 3. Pseudophakic eye modelled according to the Liou and
Brennan eye model
28
(IOL Zintraocular lens).
Figure 2. Top: Setup of German Purkinje meter. Bottom: Appear-
ance of the 3 Purkinje images in the pupil (LED Zlight-emitting
diode; P ZPurkinje reflex).
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In substudy 2, pseudophakic patients were invited to participate
in the study in which tilt and decentration measurements would be
taken with both devices. All measurements were taken by experi-
enced examiners under the supervision of the inventors of the de-
vices (J.T. and F.S.). Between the consecutive 3 measurements, the
patient was asked to sit back, after which the chinrest and the po-
sition of the Purkinje meter were changed slightly to allow system
realignment. All measurements of substudy 2 were performed on
the same day.
Spanish Purkinje Meter
The technical details of the Spanish Purkinje meter system have
been described.
22,23
In short, a semicircular array of LEDs was
projected on the patient’s eye as the patient fixated on a light.
The 3 reflexes (Purkinje I, III, and IV), reflected by the optical
surface of the eye, were captured with the digital camera of
the device. Then, the semicircles were marked manually by the
operator. Afterward, the IOL tilt in relation to the pupillary
axis (tilt
pup
) and to the line of sight (tilt
LOS
) as well as decentra-
tion were calculated automatically by the dedicated software ac-
cording to misalignment of the Purkinje images. Figure 1 shows
an image of the Purkinje reflections and the manual marking
method.
German Purkinje Meter
The setting of this device has been described.
24
The patient’s head
was placed on a chinrest, and the device was positioned in front of
him or her. Because this system is movable, focus is coded as sound
with variable frequency to allow positioning the camera in the op-
timum focus to obtain a sharp image of the eye. The patient was
asked to fixate on an LED target, and images of the Purkinje re-
flexes were captured. Subsequently, the observer manually marked
the pupil margin and the reflexes. To stimulate changes in gaze po-
sition in consecutive measurements, the patient was asked to fixate
on numbers printed on a plastic ring that was attached to the cam-
era lens. Three images were captured. Then, the program per-
formed regression analysis for the distance of Purkinje III and
Purkinje IV in the x-direction and the y-direction versus the
angular direction of the fixation axis in the x-direction and y-di-
rection. The regression lines were then displayed automatically.
In cases in which the significance of regression was too low
(regression coefficient R!0.95), an error message was displayed
and image capture was repeated.
Statistical Analysis
For statistical analysis, Excel for Mac software (2011, Microsoft
Corp.) and SPSS for Mac software (version 23.0, International
Business Machines Corp.) were used. Tilt and decentration values
are listed as vectors, if not stated otherwise. Descriptive data are
shown as 95% confidence interval, means GSD, and range. After
the distribution of data was shown to be not normally distributed,
a nonparametric test (Wilcoxon signed-rank test) was used for sig-
nificance testing, if not stated otherwise.
Computational Model
To show the effect of tilt and decentration of an aspheric IOL on
optical lower-order aberrations (LOAs) and higher-order aberra-
tions (HOAs), a ray-tracing simulation (Optic Studio 5, Zemax
LLC) was calculated within an eye model. The tilt and decentra-
tion sensitivity of the MC6125AS aspheric IOL (Humanoptics
AG) was tested using specifications of the Liou and Brennan sche-
matic eye.
28
Within the model, a pupil decentration of 0.5 mm and
a tilted visual axis of 5 degrees were simulated. Figure 3 shows the
modeled pseudophakic eye. The natural lens was replaced with an
aspheric IOL to simulate the influence of the IOL’s position on the
Zernike coefficients up to the 4th order.
During modeling, the IOL was positioned so that the ante-
rior principal plane of the artificial implant coincided with
the anterior principal plane of the natural lens. The study
used a 3.0 mm pupil diameter. The wavelength used for simu-
lation was 546 nm.
Figure 4 shows the displacements that were evaluated. The IOL
tilt around the y-axis and around the x-axis was tested. Decentra-
tion of the IOL was determined along the Cx-axis (nasally) and
the Cy-axis (superiorly).
Figure 4. Cross-sectionand coordinatesystem of a pseudophakic hu-
man eye and the total wavefront map obtained by an aberrometer.
Figure 5. Differences between tilt measurements with both systems
in substudy 1 and substudy 2. Positive values indicate that tilt
measured with the Spanish meter is higher than with the German
meter. Asterisks represent extreme outliers.
650 IOL DECENTRATION AND TILT MEASUREMENTS WITH 2 PURKINJE METER SYSTEMS
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RESULTS
Substudy 1 comprised 53 pseudophakic eyes of 53 patients.
All measured eyes had a 1-piece acrylic IOL with open-loop
haptics implanted (Tecnis ZCB00, Abbott Medical Optics,
Inc.; Superflex 620H, Rayner Intraocular Lenses Ltd.; or
Idea Xcelens, Croma Pharma). After excluding the unsuc-
cessful scans, the measurements of 35 eyes were included
for further analysis.
The mean tilt
LOS
measure was 4.35 G2.50 degrees (range
1.15 to 11.36 degrees) with the Spanish meter and
9.20 G6.96 degrees (range 2.23 to 37.80 degrees) with
the German meter. Both tilt
pup
and tilt
LOS
vectors were
lower with the Spanish meter than with the German meter
(P!.01). Figure 5 shows the differences between tilt vec-
tors of the devices. Decentration values were similar for
both devices with mean 0.44 G0.19 mm (range 0.08 to
0.85 mm) and 0.74 G0.91 mm (range 0.07 to 4.31 mm),
respectively (PZ.44) (Figure 6).
In substudy 2, 32 patients were invited to have measure-
ments; however, 3 did not answer the letter of invitation,
thus 29 patients attended on the day of measurements.
Concerning clinical feasibility, the measurements in all
29 eyes of 29 patients (100%) were successful with the
Spanish meter, whereas only 18 measurements (62%)
with the German meter could be used for analysis.
The mean horizontal and vertical tilt difference vector be-
tween the 2 systems was 4.89 G3.24 degrees (Spanish me-
ter) and 7.57 G3.82 degrees (German meter). The
mean difference vectors for horizontal and vertical
decentration measurements were 0.30 G0.28 mm
and 0.35 G0.15 mm, respectively (Figures 7 and 8).
Concerning the direction of horizontal and vertical IOL
tilt, the measurements with the 2 systems were in agreement
Figure 7. Vectors between tilt measurements (top) and decentration
measurements (bottom) for both devices in substudy 2. Asterisk
represents extreme outlier.
Figure 6. Differences between decentration measurements with
both systems for substudy 1 and substudy 2. Positive values indi-
cate that the decentration measured with the Spanish meter was
higher than with the German meter. Circles represent outliers, and
asterisks represent extreme outliers.
651IOL DECENTRATION AND TILT MEASUREMENTS WITH 2 PURKINJE METER SYSTEMS
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in 14 cases (78%) and 17 cases (94%), respectively. For hor-
izontal and vertical IOL decentration, direction of the mea-
surements was congruent in 10 cases (56%) and 13 cases
(72%), respectively.
Computational Model
Table 1 shows the results of the simulation using the model
eye. Figure 9 shows the simulated LOAs and HOAs.
DISCUSSION
The aim of this study was to assess the feasibility and short-
term reproducibility of 2 Purkinje meters. Concerning
feasibility, the Spanish meter performed significantly better
than the German meter. For the German meter, only 66% of
measurements were successful in substudy 1. To ascertain
that the number of unsuccessful measurements was not
caused by improper use of the Purkinje meter systems,
Figure 8. Bland-Altman plots for
horizontal tilt, vertical tilt, horizon-
tal decentration, and vertical de-
centration in substudy 2.
Table 1. The results of ocular wavefront simulation using the model eye by Liou and Brennan.
28
Image*y-Tilt ()x-Tilt ()x-Shift (mm) y-Shift (mm)
Simulated Aberrations (mm)
Z(1,1) Z(L1,1) Z(0,2) Z(2,2)
a1.15 ddd0.208 0.000 0.005 0.112
b2.23 ddd0.254 0.000 0.047 0.153
c4.35 ddd0.348 0.000 0.167 0.250
d9.2 ddd0.581 0.000 0.543 0.561
e11.36 ddd0.696 0.000 0.762 0.746
f37.8 ddd3.827 0.000 8.554 8.325
g4.89 ddd0.373 0.000 0.202 0.279
hd7.57 dd0.161 0.321 0.089 0.045
i4.89 7.57 dd0.381 0.331 0.351 0.160
jdd0.07 d0.150 0.000 0.053 0.079
kdd0.08 d0.148 0.000 0.053 0.079
ldd0.44 d0.118 0.000 0.067 0.099
mdd0.74 d0.120 0.000 0.091 0.110
ndd0.85 d0.127 0.000 0.100 0.115
odd4.31 ddddd
pdd0.30 d0.126 0.000 0.059 0.093
qddd0.35 0.162 0.056 0.069 0.084
rdd0.30 0.35 0.131 0.049 0.073 0.104
ZZZernike
*Letters correspond to letters of images in Figure 9
652 IOL DECENTRATION AND TILT MEASUREMENTS WITH 2 PURKINJE METER SYSTEMS
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the inventers of both systems attended all measurements in
substudy 2. The inventors of both systems were asked to
observe the examiners and guide them, if necessary. In a
few cases, they also repeated the measurements themselves
if the examiners did not obtain a successful measurement.
However, despite this change in measurement setup in sub-
study 2, 38% of eyes were still not measurable with the
German meter, whereas all eyes could be measured in
both substudies with the Spanish meter.
Using Purkinje reflexes for the analysis of IOL alignment
has advantages. It is a noncontact technique, not dependent
on pupil dilation, clinically feasible, and achieves fast results.
Nishi et al.
14
showed that the Spanish meter is a reliable
instrument to measure IOL tilt and decentration even in
extreme cases. Intraexaminer and interexaminer reproduc-
ibility of IOL decentration and tilt measurements was high
with the Spanish meter. Even in cases with insufficient pupil
dilation or clinically manifest IOL decentration and tilt, the
reproducibility of measurements was good.
As mentioned earlier, the German meter has been shown
to measure tilt of the crystalline lens in healthy eyes
24,25
and
pseudophakic eyes,
26,27
and measurements were shown to
be reproducible over time. In a clinical study by Mester
et al.,
27
IOL tilt and decentration measured with the
German meter were lower than the same values measured
with the German meter in our study and similar to the
values measured with the Spanish meter; however, Mester
et al. did not mention problems with the clinical feasibility
of the German meter.
We compared the systems and found differences con-
cerning using them in a clinical setting. The Spanish meter
has a fixed chinrest and an LED fixation target to allow
for repeatable measurements. The system is aligned in all
3 planes with a joystick, equivalent to most models used
for many ophthalmic imaging devices. A possible advantage
of the German meter is that the device consists of a compact
video camera and a laptop computer; this makes the system
portable. It might be why, in our hands, the German meter
system was more challenging to align because the recording
camera was mounted on a rotatable stick, which could be
varied in multiple directions. In addition, this system uses
only a single LED, whereas the Spanish meter uses a semi-
circular array of LEDs. The single-spot Purkinje reflexes
induced by the German meter make estimation of the opti-
mum focusing of the camera difficult. Another problem is
that the single-spot reflexes are small and symmetrical,
which in some cases made identification of the Purkinje re-
flexes I, III, and IV almost impossible, in particular when
the reflexes were superimposed on each other. This might
be an explanation for some of the unsuccessful measure-
ments with the German meter. However, one third of the
measurements with this device could not be used for anal-
ysis, even when the measurements were performed by the
inventor (F.S.).
To our knowledge, there are no actual data concerning the
comparability of the 2 Purkinje meters used in this study.
However, the clinical feasibility results were similar in the
2 substudies. Despite the presence of the inventor of the
Purkinje German meter during the measurements in
substudy 2, the number of successful scans did not increase.
Both inventors were asked to analyze the data obtained
with their respective device during our measurements,
which could be a source of error. Although the inventor
of the German meter did analyze the image data, the num-
ber of successful scans did not increase, which might indi-
cate that the influence of the experience of the examiner
during image acquisition and evaluation might not be as
relevant as one might think.
Table 1. (Cont.)
Simulated Aberrations (mm)
Z(L2,2) Z(1,3) Z(L1,3) Z(3,3) Z(L3,3) Z(0,4) Z(2,4) Z(L2,4) Z(4,4) Z(L4,4)
0.000 0.076 0.000 0.000 0.000 0.040 0.001 0.000 0.000 0.000
0.000 0.093 0.000 0.001 0.000 0.041 0.001 0.000 0.000 0.000
0.000 0.126 0.000 0.002 0.000 0.041 0.002 0.000 0.000 0.000
0.000 0.210 0.000 0.008 0.000 0.043 0.003 0.000 0.000 0.000
0.000 0.252 0.000 0.012 0.000 0.044 0.004 0.000 0.000 0.000
0.000 1.419 0.000 0.493 0.000 0.128 0.098 0.000 0.041 0.000
0.000 0.135 0.000 0.002 0.000 0.041 0.002 0.000 0.000 0.000
0.236 0.059 0.115 0.002 0.000 0.041 0.000 0.001 0.000 0.000
0.400 0.138 0.119 0.003 0.004 0.042 0.001 0.002 0.000 0.000
0.000 0.056 0.000 0.001 0.000 0.041 0.001 0.000 0.000 0.000
0.000 0.055 0.000 0.001 0.000 0.041 0.001 0.000 0.000 0.000
0.000 0.046 0.000 0.004 0.000 0.046 0.004 0.000 0.000 0.000
0.000 0.048 0.000 0.010 0.000 0.051 0.008 0.000 0.000 0.000
0.000 0.051 0.000 0.013 0.000 0.053 0.009 0.000 0.000 0.000
dddddddddd
0.000 0.049 0.000 0.003 0.000 0.044 0.003 0.000 0.000 0.000
0.029 0.060 0.018 0.001 0.001 0.041 0.000 0.002 0.000 0.000
0.014 0.050 0.015 0.001 0.004 0.045 0.002 0.003 0.000 0.000
653IOL DECENTRATION AND TILT MEASUREMENTS WITH 2 PURKINJE METER SYSTEMS
Volume 43 Issue 5 May 2017

Concerning IOL tilt measurements, the German meter
measured significantly higher tilt values than the Spanish
meter. The congruence between the devices concerning the
direction of IOL tilt was better for the vertical measurements
than for the horizontal measurements. The IOL decentration
values tended to be higher with the German meter, although
the differences were not statistically significant. Despite these
results in our pseudophakic patients, the German meter has
been shown to measure lens tilt and decentration reliably
and with good repeatabilty.
24,25
Repeatability with the Span-
ish meter had a relatively low influence on HOAs. In
contrast, repeatability of the German meter would have re-
sulted in significant HOAs, resulting in reduced visual qual-
ity. However, because 7 degrees of tilt and 0.4 mm of
decentration can have a significant influence,
3
we would
consider a repeatability of 1.4 degrees and 0.08 mm (20%
of the relevant influence) to be acceptable.
In conclusion, the Spanish meter had better clinical feasi-
bility in pseudophakic eyes than the German meter. Despite
the supervision of a highly experienced specialist during
measurements, the percentage of unsuccessful scans with
the German meter was relatively high. In addition, the
German device measured significantly higher tilt values
than the Spanish meter. Data published with these Purkinje
meters should not be directly compared because of the dif-
ferences found in this study.
WHAT WAS KNOWN
Several techniques to assess IOL tilt and decentration lack
accuracy and objectivity.
Using the Purkinje reflexes to measure IOL alignment yields
repeatable and reproducible results.
WHAT THIS PAPER ADDS
The clinical feasibility of the Spanish Purkinje meter system
was better than that of the German Purkinje meter.
In pseudophakic eyes, the German Purkinje meter
measured significantly higher IOL tilt and decentration values
than the Spanish Purkinje meter.
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Disclosures: Drs. Tabernero and Artal are patent assignees for the
Spanish Purkinje meter system. Dr. Schaeffel is patent assignee for the
German Purkinje meter. None of the other authors has a financial or
proprietary interest in any material or method mentioned.
First author:
Sophie Maedel, MD
Vienna Institute for Research in Ocular
Surgery, A Karl Landsteiner Institute,
Hanusch Hospital, Vienna, Austria
655IOL DECENTRATION AND TILT MEASUREMENTS WITH 2 PURKINJE METER SYSTEMS
Volume 43 Issue 5 May 2017