ARTICLE

Comparison of ocular biometry and intraocularlens power using a new biometer and a standard
biometer
Sabong Srivannaboon, MD, Chareenun Chirapapaisan, MD, Pratuangsri Chonpimai, BS,Sunisa Koodkaew, BS

PURPOSE: To compare the repeatability and reproducibility of ocular biometry and intraocularlens (IOL) power obtained with a new optical biometer (AL-Scan) and a standard optical biometer(IOLMaster 500).
SETTING: Siriraj Hospital, Mahidol University, Bangkok, Thailand.
DESIGN: Prospective comparative study.
METHODS: Two independent operators measured eyes with cataract using both biometers. The
keratometry values, axial length, anterior chamber depth, white-to-white (WTW) corneal
diameter, and IOL power calculated using the Holladay 1 formula obtained with each device wererecorded. Intraoperator repeatability and interoperator reproducibility of both devices wereanalyzed using the intraclass correlation coefficient (ICC). The agreement in ocular biometry and
IOL power between the 2 devices was evaluated by the Bland-Altman method.
RESULTS: The study recruited 137 eyes of 81 patients. The repeatability and reproducibility of bothdevices were high for all ocular biometry measurements (ICC, 0.87-1.00). Except for the WTW
corneal diameter (ICC, 0.44), the agreement between the biometers was also high (ICC, 0.980.99). The IOL powers calculated by the Holladay 1 formula were similar between the 2 biometers.
CONCLUSION: The new optical biometer provided excellent repeatability and reproducibility for all
ocular biometry. Agreement with the standard optical biometer was good except for the WTW
corneal diameter.
Financial Disclosure: No author has a financial or proprietary interest in any material or method
mentioned.
J Cataract Refract Surg 2014; 40:709–715 Q 2014 ASCRS and ESCRS

Ocular biometry is essential for intraocular lens (IOL)
power calculation in cataract surgery. The biometricvariables that are used for IOL calculation depend on
the chosen IOL formula. Current third-generationand fourth-generation IOL formulas require the basic
variables of keratometry (K) values and axial length(AL).1,2 Some formulas may require additional variables, such as anterior chamber depth (ACD) or horizontal white-to-white (WTW) corneal diameter.3
However, the accurate measurement of variables is
crucial.
Unlike ultrasound (US) biometry, modern opticalbiometry devices measure several variables. Moreover, the built-in software in these devices provides
more accurate IOL power calculation and multiple

Q 2014 ASCRS and ESCRS
Published by Elsevier Inc.

choices of IOL formulas.4,5 The IOLMaster (Carl Zeiss
Meditec AG) is considered to be one of the firststandard modern optical biometry devices.6 It uses
the principle of partial coherence interferometry(PCI) to obtain the AL with high precision.7–9 The
device also measures the K values, ACD, and WTW
diameter.10 Several reports have shown the accuracyof the the newest version of IOLMaster (IOLMaster500) for IOL calculation in routine and complicatedcataract cases.8,11–13
Several new optical biometry devices can also
perform ocular biometry and IOL power calculation.14
Among them, the AL-Scan (Nidek Co., Ltd.) is one of
the most recently released on the market. The device
measures 6 variables, including the K value, AL,

0886-3350/$ - see front matter
http://dx.doi.org/10.1016/j.jcrs.2013.09.020

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IOL POWER USING OPTICAL BIOMETRY AND PCI

ACD, WTW diameter, pupil size, and central corneal
thickness (CCT). The IOL power is generated byseveral formulas built into the device. To our knowledge, no study has evaluated the repeatability, reproducibility, and accuracy of this device.
The purpose of this study was to evaluate therepeatability, reproducibility of variables (eg, K, AL,
ACD, horizontal WTW corneal diameter), and IOL
power measured using the AL-Scan device andcompare the results with those obtained with the IOLMaster 500 device.
PATIENTS AND METHODSThis prospective comparative study recruited cataract
patients from Siriraj Eye Clinic, Bangkok, Thailand. Institutional ethic committee approval and patient informed consent were obtained. Eyes with ocular disease other thancataract were excluded.
The AL-Scan optical biometer (new biometer) measures K
values using double-mire rings projected onto the cornea atthe 2.4 mm zone and 3.3 mm zone. The Scheimpflug imaging
technique is applied for CCT and ACD measurements. The
AL is acquired by PCI.
The IOLMaster 500 optical biometer (standard biometer)
measures K values using 6 spots of light projected onto thecornea at the central 2.5 mm zone. The ACD is measured
by lateral slit illumination, and the AL is acquired by the
PCI method.
The first operator took 3 consecutive measurements witheach device (intraoperator repeatability). The patients wereasked to close their eyes between each measurement. Then,the second operator took another measurement (interoperator reproducibility). Four major parameters (K value, AL,
ACD, WTW) were recorded. The IOL power by the Holladay
1 formula from both devices was calculated using the IOL
constant provided by the User Group for Laser Interference
Biometry (ULIB) web site.A
Statistical analysis was performed using SPSS software(version 19.0, SPSS, Inc.). The data normality was checkedwith the Shapiro-Wilk test. The intraoperator repeatability
was calculated using the 3 sets of the measurements takenby the first operator. For interoperator reproducibility, thefirst measurement of the first operator was selected andcompared with the measurement of the second operator.
The agreement between the 2 devices was also evaluatedusing the first measurement of the first operator for each device. An intraclass correlation coefficient (ICC) of more than
0.8 was acceptable as a high value.15 The Pearson correlation
was used to determine the relationship between all

Submitted: May 6, 2013.
Final revision submitted: September 30, 2013.
Accepted: September 30, 2013.

parameters. A statistically significant difference was defined
as P!.05. Bland-Altman plots were used to assess the agreement of measurements between the 2 devices.

RESULTS
The study recruited 137 eyes of 81 patients. The mean
age of patients was 65.28 years G 10.56 (SD). Table 1
shows the values of 4 ocular parameters (K, AL,
ACD, WTW) and the calculated IOL power obtainedwith the new biometer and the standard biometer.
The mean value of all measurements, except theWTW distance, and the calculated IOL power showed
a good correlation between the 2 devices.
Repeatability
The intraoperator repeatability of both devices wasvery good. The ICC of the new biometer and the standard biometer ranged from 0.94 to 1.00 (Table 2). For
ACD, the new biometer produced a much lower
mean difference with a small standard deviation(SD) than the standard biometer. In contrast, theWTW measurement of the new biometer had a slightly
lower ICC and wider SD of the mean difference than
the standard biometer.
Reproducibility
The interoperator reproducibility of both devices
was good, which yielded a high ICC value for all parameters (Table 3). The new biometer produced
slightly higher ICC values than the standard biometerfor most parameters, especially for the ACD measurement. Moreover, the mean difference and SD of the
ACD measurement by the new biometer was muchsmaller than by the standard biometer. However, thenew biometer provided a lower ICC value than thestandard biometer for WTW.
Agreement
The agreement of the ocular parameters derived
from both biometry devices was high except for the
WTW distance (Table 4). The mean difference andSD of the WTW parameter had the highest values ofall parameters. However, the Bland-Altman plots
showed that the limits of agreement (LoA) between
the 2 biometers remained narrow over all parameters(Figures 1 to 5).
DISCUSSION

From the Faculty of Medicine, Siriraj Hospital, Mahidol University,
Bangkok, Thailand.
Corresponding author: Chareenun Chirapapaisan, MD, Departmentof Ophthalmology, Siriraj Hospital, Mahidol University, 2 Prannok
Road, Bangkoknoi, Bangkok 10700, Thailand. E-mail:chareenun@gmail.com.

This study found that the AL-Scan biometer (new biometer) and the IOLMaster 500 biometer (standard
biometer) had high repeatability and reproducibility
for all tested parameters. The agreement between
them was also very high except for the WTW parameter. The WTW diameter measured with the new

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Table 1. Parameters obtained by the 2 optical biometers.
Parameter
K 2.4 mm (D)Mean G SD
Range
K 3.3 mm (D)Mean G SD
Range
AL (mm)Mean G SD
Range
ACD (mm)
Mean G SD
Range
WTW (mm)
Mean G SD
Range
IOL power (D)Mean G SD
Range

New BiometerStandard Biometer

P Value*r Value

44.31 G 1.63
40.42, 47.31

44.22 G 1.60
40.52, 47.67

!.05

0.994

44.30 G 1.64
40.26, 47.12

NA

!.05

0.993

23.51 G 1.02
21.18, 25.97

23.50 G 1.02
21.17, 25.94

.09

0.999

3.11 G 0.44
2.18, 4.06

3.14 G 0.43
2.35, 3.94

!.05

0.970

11.51 G 0.75
8.70, 12.60

12.03 G 0.44
10.90, 12.90

!.05

0.445

20.33 G 2.57
12.73, 25.51

20.60 G 2.60
13.15, 26.25

!.05

0.995

ACD Z anterior chamber depth; AL Z axial length; IOL Z intraocular lens; K Z keratometry; NA Z not applicable; r Value Z Pearson correlation coefficient;WTW Z white-to-white corneal diameter*Statistically significant when P!.05

Table 2. Repeatability.
Intraoperator Reproducibility
LoA
Parameter
K 2.4 mm (D)New biometerStandard biometer
K 3.3 mm (D)New biometerStandard biometer
AL (mm)New biometerStandard biometer
ACD (mm)New biometerStandard biometer
WTW (mm)New biometerStandard biometer
IOL power (D)New biometerStandard biometer

CL

Mean Difference* G SD

Lower

Upper

ICC

Lower

Upper

0.000 G 0.136
0.009 G 0.091

0.271
0.174

0.271
0.191

0.999
0.999

0.998
0.999

0.999
0.999

0.022 G 0.180
NA

0.338
NA

0.382
NA

0.998
NA

0.998
NA

0.999
NA

0.005 G 0.017
0.005 G 0.016

0.040
0.037

0.030
0.026

1.000
1.000

0.999
0.999

1.000
1.000

0.003 G 0.014
0.029 G 0.106

0.025
0.240

0.032
0.182

0.999
0.993

0.999
0.988

0.999
0.995

0.042 G 0.584
0.087 G 0.179

1.125
0.272

1.209
0.445

0.945
0.974

0.914
0.959

0.967
0.984

0.020 G 0.190
0.001 G 0.108

0.361
0.214

0.400
0.216

0.999
0.999

0.999
0.999

0.999
0.999

ACD Z anterior chamber depth; AL Z axial length; CL Z confidence limit; ICC Z intraclass correlation coefficient; IOL Z intraocular lens; K Z keratometry;
LoA Z 95% limits of agreement; NA Z not applicable; WTW Z white-to-white corneal diameter*Mean difference between 2 measurements for each subject by same operator

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Table 3. Reproducibility.
Interoperator Reproducibility
LoA
Parameter
K 2.4 mm (D)New biometerStandard biometer
K 3.3 mm (D)New biometerStandard biometer
AL (mm)New biometerStandard biometer
ACD (mm)New biometerStandard biometer
WTW (mm)New biometerStandard biometer
IOL power (D)New biometerStandard biometer

CL

Mean Difference* G SD

Lower

Upper

ICC

Lower

Upper

0.02 G 0.13
0.00 G 0.13

0.24
0.25

0.29
0.26

0.998
0.998

0.997
0.997

0.998
0.999

0.04 G 0.16
NA

0.27
NA

0.35
NA

0.997
NA

0.996
NA

0.998
NA

0.001 G 0.05
0.006 G 0.05

0.10
0.10

0.10
0.11

0.999
0.996

0.999
0.994

0.999
0.997

0.004 G 0.01
0.022 G 0.10

0.03
0.21

0.03
0.17

0.999
0.987

0.999
0.980

0.999
0.991

0.06 G 0.50
0.03 G 0.20

0.94
0.44

1.06
0.37

0.873
0.946

0.804
0.917

0.918
0.965

0.04 G 0.19
0.003 G 0.15

0.42
0.31

0.34
0.30

0.998
0.991

0.997
0.998

0.999
0.999

ACD Z anterior chamber depth; AL Z axial length; CL Z confidence limit; ICC Z intraclass correlation coefficient; IOL Z intraocular lens; K Z keratometry;
LoA Z 95% limits of agreement; NA Z not applicable; WTW Z white-to-white corneal diameter*Mean difference between 2 measurements for each subject by 2 operators

biometer was significantly smaller than that measured
with the standard biometer, and there was low correlation between the measurements.
Today, ophthalmologists are gaining more accessibility to optical biometry devices. In a study of cataract
surgery practice patterns in the United States Veterans
Health Administration,16 83% of the respondents said
they used PCI devices for preoperative biometry. At
present, several optical biometry devices are used inclinical practice. However, different types of opticalbiometry devices may yield different IOL power results. Most IOL power calculations based on PCI technology use the IOL constant provided by the ULIB
web site.A The web site currently provides the optimized IOL constant for 2 optical biometers only; thatis, the IOLMaster and the Lenstar (Haag-Streit AG).
A new device that uses the ULIB constant must be verified against the standard biometer. Therefore, weTable 4. Agreement of each parameter between the new biometer and standard biometer.
Agreement
LoA
Parameter
K 2.4 mm (D)K 3.3 mm (D)AL (mm)ACD (mm)
WTW (mm)
IOL power (D)CL

Mean Difference* G SD

Lower

Upper

ICC

Lower

Upper

0.09 G 0.18
0.08 G 0.20
0.01 G 0.02
0.02 G 0.11
0.51 G 0.68
0.26 G 0.26

0.26
0.32
0.04
0.24
1.87
0.74

0.44
0.48
0.05
0.19
0.84
0.22

0.996
0.995
0.999
0.983
0.448
0.994

0.991
0.991
0.998
0.974
0.106
0.947

0.998
0.997
0.999
0.989
0.701
0.998

ACD Z anterior chamber depth; AL Z axial length; CL Z confidence limit; ICC Z intraclass correlation coefficient; IOL Z intraocular lens; K Z keratometry;
LoA Z 95% limits of agreement; WTW Z white-to-white corneal diameter*Mean difference in each parameter measurement between new biometer and standard biometer

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713

Figure 1. Agreement of K measurements between biometers. The
solid lines show the mean difference and 95% LoA for K at the 2.4
mm zone. The dashed lines show the mean difference and 95% LoA
for K at the 3.3 mm zone (K Z keratometry).

Figure 2. Agreement of AL measurements between biometers. Thelines show the mean difference and 95% LoA (AL Z axial length).

chose to validate the AL-Scan biometer using the
IOLMaster 500 biometer. The parameters in this study(K, AL, ACD, WTW) were chosen for comparison
because they are required by current thirdgeneration and fourth-generation IOL formulas.2,3,17
The new biometer measures the K value at 2 zone diameters (2.4 mm and 3.3 mm), while the standard biometer measures it at 2.5 mm only. The otherparameters are measured similarly by the 2 devices.
In this study, all parameter measurements, except
the WTW, were comparable between devices. TheWTW from the IOLMaster device has been shown tobe precise.18,19 There had been no study of the WTWdiameter measured by the AL-Scan device. The
mean difference in the WTW measured by the 2 devices was 0.51 mm. The agreement and correlation between them was also low (ICC Z 0.448 and r Z 0.445,
respectively). The explanation for this finding could be
the difference in the algorithm for edge detection

around iris image between the 2 devices. Furthermore,
the dissimilarity in the light source for image acquisition between the devices could have a role. The ALScan device uses a green light source (wavelength
525 nm) to measure the WTW, whereas the IOLMasterdevice uses an infrared light source (wavelength
880 nm).B,C The infrared light source has been usedin ophthalmic devices for eye tracking with highaccuracy for decades.20–22 The infrared light might bea better choice for the WTW measurement. Even
though the repeatability and reproducibility of theWTW measured from the AL-Scan device were quite
high (ICC 0.945 and ICC 0.873, respectively), they
were lower than those with the IOLMaster 500 device(ICC 0.974 and ICC 0.946, respectively). Moreover, theSDs of the measurement difference in repeatability
and reproducibility of the WTW measurement werehigher for the AL-Scan device (0.58 and 0.50, respectively) than for the IOLMaster 500 device (0.17 andFigure 3. Agreement of ACD measurements between biometers. Thelines show the mean difference and 95% LoA (ACD Z anteriorchamber depth).

Figure 4. Agreement of WTW corneal diameter measurements between biometers. The lines show the mean difference and 95%
LoA (WTW Z white-to-white corneal diameter).

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Figure 5. Agreement of IOL power measurements between
biometers. The lines show the mean difference and 95% LoA (IOL
Z intraocular lens).

0.20, respectively). Therefore, the WTW measurementfrom the AL-Scan device should be used with caution.
The ACD measurement by both devices in thisstudy was similar. However, the new biometer had
slightly better repeatability and reproducibility. The
mean difference and its SD were lower for the new biometer than for the standard biometer (repeatability
0.003 G 0.014 versus 0.029 G 0.106; reproducibility
0.004 G 0.01 versus 0.022 G 0.10). The LoA range
was also lower for the new biometer. This finding
could be the result of the Scheimpflug image principle
used for the ACD measurement by the new biometer;
the standard biometer uses a scanning-slit image. The
measurement from the Scheimpflug image has beenshown to have better repeatability than slit imaging,
US biomicroscopy, and magnetic resonance imaging.23,24 However, the ACD measured using a
different method was considerably below a clinicallysignificant level.
The IOL power (Holladay 1 formula) derived byboth devices was quite similar, with good agreement
and correlation. The mean difference was only 0.26
diopter (D), less than the increment in the IOL power
step (0.50 D). Although both devices did not have aclinically significant impact on IOL power, the chosen
IOL formula should be considered. If the WTW measurement is required in a formula, such as in the Holladay 2, the result might be different.
In the Bland-Altman plots, a few cases were notwithin the LoA. But in most cases, the difference between the 2 devices was considered not clinically significant. The maximum difference in K value was
0.48 D, in AL was 0.07 mm, in ACD was 0.3 mm, in
WTW was 2.3 mm, and in IOL power was 0.78 D. In
cases in which the difference in IOL power was morethan 0.50 D, it was mainly the result of the differencein the K value.

This study was not intended to compare penetration
through a dense cataract between the 2 devices. However, the 2 devices performed very similarly. In eyes inwhich the AL cannot be obtained by the IOLMaster
500 device, it cannot be obtained by the AL-Scandevice either.
A limitation of this study might be both eyes of thesome patients were enrolled. In this study, we recruited both eyes of 56 patients (112 eyes) and 1 eyeof 25 patients. A previous study25 discussed the bias
effects of collecting the data from both eyes andcomparing them with data from 1 eye. However, theaim of our study was to compare the performance ofthe 2 devices; thus, the data obtained from 1 eye or 2eyes should not significantly affect the accuracy ofeither biometer.
In conclusion, the AL-Scan biometer performed
very similarly to the IOLMaster 500 biometer. Theyboth showed very high repeatability and reproducibility and were in good agreement with each other.
The WTW measurement from the AL-Scan biometershould be further verified.
WHAT WAS KNOWN
 The standard device in the study is one of the mostaccurate PCI optical biometers and had high repeatability
and reproducibility.
WHAT THIS PAPER ADDS The repeatability and reproducibility of the new PCI opticalbiometer was high.
 The accuracy of the new biometer was comparable to thatof the standard biometer. In most cases, the mean ocularparameter values, repeatability, and reproducibility weresimilar between the 2 biometers.
 The WTW measurements were different between the 2biometers. The results have to be verified. Therefore,
the WTW distance derived from the new biometer should
be used with caution.

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J CATARACT REFRACT SURG - VOL 40, MAY 2014

First author:
Sabong Srivannaboon, MD
Faculty of Medicine, Siriraj Hospital,
Mahidol University, Bangkok, Thailand