ARTICLE

Repeatability, reprodu

cibility, and agreementof corneal power measurements obtained

with a new corneal topographerXinjie Mao, MD, Giacomo Savini, MD, Zuopao Zhuo, MD, Yifan Feng, MD, Jian Zhang, MD,

Qinmei Wang, MD, Hao Chen, MD, Jinhai Huang, MD

Q 2013 A

Published

SCRS an

by Elsev

PURPOSE: To study the repeatability and reproducibility of corneal power measurements ob-tained with a new corneal topographer (Keratograph 4) and assess their agreement with thoseobtained by a rotating Scheimpflug camera (Pentacam HR) and an automated keratometer(IOLMaster).

SETTING: Eye Hospital, Wenzhou Medical University, Wenzhou, China.

DESIGN: Observational cross-sectional study.

METHODS: One eye of normal subjects was measured 3 times using all devices. Another oper-ator performed an additional 3 consecutive scans using the corneal topographer. One weeklater, the first operator repeated the corneal power measurements using the corneal topogra-pher. Parameters were flat meridian power, steep meridian power, average power, J0, andJ45. The repeatability and reproducibility of measurements were assessed by the within-subject standard deviation (Sw), repeatability (2.77 Sw), coefficient of variation (CoV), andintraclass correlation coefficient (ICC). Agreement between devices was assessed using 95%limits of agreement (LoA).

RESULTS: Intraoperator repeatability and interoperator and intersession reproducibility of all mea-sured parameters showed a CoV of less than 0.3%, a 2.77 Sw of 0.32 diopter or less, and an ICC ofmore than 0.97. No significant differences in the parameters were noted between the corneal topog-rapher and Scheimpflug camera. Statistically significant differences existed between the parametersanalyzed by the corneal topographer and the automated keratometer, except J45. The mean differ-ences between the corneal topographer and the other 2 devices were small, and the 95% LoA werenarrow for all measurements.

CONCLUSION: The new corneal topographer had excellent reliability and high agreement with theother 2 devices in corneal power measurements in normal subjects.

Financial Disclosure: No author has a financial or proprietary interest in any material or methodmentioned.

J Cataract Refract Surg 2013; 39:1561–1569 Q 2013 ASCRS and ESCRS

Measurements of corneal power are useful in severalapplications, such as contact lens fitting,1 monitoringcorneal shape after ocular surgery,2–4 and diagnosingand monitoring keratoconus.5–7 Several technologiesfor measuring corneal power are available. Theyinclude manual and automated keratometry, comput-erized videokeratography, raster stereogrammetry,slit-scanning tomography, rotating Scheimpflugtomography, and optical coherence tomography. Vid-eokeratoscopic systems provide corneal topographiesby capturing and analyzing the Placido disk images

d ESCRS

ier Inc.

reflected from the corneal surface within a diameterof 8.0 to 9.0 mm.

The Keratograph 4 (Oculus Optikger€ate GmbH) isa new Placido disk–based corneal topographer. In ad-dition to the capability to perform topographic mea-surements, this instrument has other functions, suchas pupillometry, tear-film assessment using noninva-sive methods, contact lens back-side measurement,lid-angle measurement, and imaging measurement.One study8 found that the new corneal topographerprovides highly repeatable measurements of corneal

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1562 CORNEAL POWER MEASUREMENTS WITH A NEW TOPOGRAPHER

power; however, the intersession and interoperator re-producibility of this device have not been assessed. Inaddition, there are no data describing the agreementbetween this corneal topographer and 2 validatedand commonly used instruments; that is, the Pentacam(Oculus Optikger€ate GmbH) and the IOLMaster (CarlZeiss Meditec AG), which measure corneal power byScheimpflug imaging and automated keratometry, re-spectively.9–15

The purpose of this study was to prospectivelyevaluate the intraoperator repeatability and the inter-operator and intersession reproducibility of the Kera-tograph 4 and assess the agreement of its cornealpower measurements with those provided by the Pen-tacam HR and IOLMaster.

SUBJECTS AND METHODS

Normal healthy volunteers were prospectively recruited atthe Wenzhou Medical University, China. One eye of eachsubject was randomly selected using a predeterminedcomputer-generated randomization schedule. The exclusioncriteria were a history of ocular pathology, a history of cor-neal/intraocular surgery and trauma, contact lens use, anddry eye.

Before being enrolled, each subject was informed of thepurpose of the study and gavewritten consent to participate.The study methods adhered to the tenets of the Declarationof Helsinki for the use of human participants in biomedicalresearch and were approved by the Research Review Board,Wenzhou Medical University.

Instruments

The Keratograph 4 is a new Placido disk–based cornealtopographer that projects 22 rings onto the cornea. In the

Submitted: November 30, 2012.Final revision submitted: March 27, 2013.Accepted: April 3, 2013.

From the School of Optometry and Ophthalmology and EyeHospital (Mao, Zhuo, Feng, Wang, Chen, Huang), Wenzhou MedicalUniversity, the Key Laboratory of Vision Science (Mao, Zhuo,Wang, Chen, Huang), Ministry of Health P.R. China, and theDepartment of Ophthalmology (Zhang), First Affiliated Hospital ofWenzhou Medical University, Wenzhou, China; the G.B. BiettiFoundation IRCCS (Savini), Rome, Italy.

Supported in part by the Foundation of Wenzhou City Science &Technology Bureau (Y20110045, Y20120176), the innovativeproject of the School of Optometry and Ophthalmology and EyeHospital, Wenzhou Medical University (YNCX201101, YNKT201101), the Health Bureau of Zhejiang Province (2012KYB135),and the Scientific Research Fund of Zhejiang Provincial EducationDepartment (Y201223147).

Corresponding author: Jinhai Huang, MD, Eye Hospital of WenzhouMedical University, 270 West Xueyuan Road, Wenzhou, Zhejiang,325027 China. E-mail: viphjh@hotmail.com.

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present study, software version 1.74 was used. By analyz-ing the images reflected from the corneal surface, the soft-ware constructs mathematic representations of the anteriorcorneal surface topography and measures corneal curva-ture at 22 000 points. Image acquisition is fully automatedgiven that the corneal topographer determines the dis-tance from the corneal apex to the instrument’s cameraand automatically captures images. The instrument calcu-lates the simulated keratometry (K) values by entering thevalue of the anterior corneal curvature radius (in meters)into the thin-lens formula for paraxial imagery, whichconsiders the cornea as a single refractive sphere and isgiven by

Corneal powerZ ðn� 1Þ=corneal radiuswhere n is the keratometric index of refraction (1.3375) and 1is the refractive index of air. The mean simulated K value iscalculated as the average between the power of the flattestmeridian at the 3.0mmdiameter and the power of themerid-ian 90 degrees away from it.

The PentacamHR instrument uses a rotating Scheimpflugcamera to image the anterior segment of the eye. The instru-ment’s digital camera and slit illumination system rotatearound the corneal apex to capture 50 cross-sectionalScheimpflug images of the anterior eye, each separated by3.6 degrees. It captures 138 000 true elevation points usinga high-resolution 1.45 megapixel camera. In the presentstudy, the automatic release mode was used to reduce thenumber of operator-dependent variables. Only scans withan examination quality specification of “OK” were chosenfor analysis. Software version 1.17was used. Among the sev-eral corneal powers calculated by the rotating Scheimpflugcamera, this study assessed the average K value, which isthe arithmetic mean of the power of the pair of meridians90 degrees apart (K1 and K2) with the greatest differencein axial power within the central 3.0 mm andwhich is equiv-alent to the simulated K of traditional corneal topographersbecause it is calculated by using the a keratometric index of1.3375.

The IOLMaster instrument measures corneal power byautomated keratometry and uses data from a hexagonal ar-ray of 6 points reflected off the surface of the cornea at a diam-eter of approximately 2.5 mm, depending on the cornealcurvature.14,16 Software version 5.4 was used. A keratomet-ric index of 1.3375 was used to convert the measured radiiinto diopters (D).

Measurement Technique

In the present study, reproducibility, repeatability, andagreement were defined according to the British StandardsInstitute and the International Organization for Standard-ization.17–19 During the first session, the first well-trainedoperator obtained 3 consecutive corneal curvature scansrandomly from each subject using each device to deter-mine agreement between them. Three additional consecu-tive scans were acquired by a second well-trained operatorusing the new corneal topographer to evaluate interopera-tor repeatability. The order of the 2 operators wasrandomly chosen. Subjects were asked to perform a com-plete blink just before each measurement to spread anoptically smooth tear film over the cornea. The devicewas moved backward and realigned for the next scan toeliminate interdependence of successive measurements.The time spent by the operator between repeated scans

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1563CORNEAL POWER MEASUREMENTS WITH A NEW TOPOGRAPHER

was the minimum possible, and the measurements takenwith the 3 devices were continuous without significanttime intervals while changing from 1 instrument to theother. One week later, intersession reproducibility was as-sessed by 3 additional consecutive corneal power scansperformed by the first operator at almost the same timeas the first session using the same protocol. All measure-ments were taken between 10 AM and 5 PM to minimizethe effect of diurnal variation on corneal power.

Statistical Analysis

Statistical analysis was performed using SPSS for Win-dows software (version 13, SPSS Inc.) and Microsoft OfficeExcel (Microsoft Corp.). A P value less than .05 was consid-ered statistically significant. The distributions of the datasetswere checked for normality using Kolmogorov-Smirnovtests. The results indicated that the data were normally dis-tributed (PO.05). For eachmeasurement, the flat K and steepK corneal power values, the average power of flat K andsteep K, and the axes of flat K and steep K were noted. Cor-neal astigmatism was converted into a vector representationof Jackson (J)0 (cylinder at 0-degree meridian) and J45 (cylin-der at 45-degree meridian), which were calculated accordingto the following formulas20: J0 Z (�cylinder/2) cos(2 �axis); J45 Z (�cylinder/2) sin(2 � axis). These values werecalculated for 3 measurements taken during each sessionand then averaged to determine the reproducibility andcomparability.

Intraoperator Repeatability, Interoperatorand Intersession Reproducibility

To determine the intraoperator repeatability of each de-vice, the within-subject standard deviation (Sw), test–retestrepeatability, coefficient of variation (CoV), and intraclasscorrelation coefficients (ICCs) were calculated for the 3 re-peated measurements obtained by the first operator andsecond operator.21 The test–retest repeatability was de-fined as 2.77 Sw, which means an interval within which95% of the differences between measurements are ex-pected to lie. The CoV was calculated as the ratio of theSw to the overall mean. A lower CoV is associated withhigher repeatability.

The advantage of CoV values is that they can be comparedbetween data sets with different units or widely differentmeans. The disadvantage is that when the mean value isnear zero, the CoV is sensitive to small changes in themean, limiting its usefulness. The ICCs (ranging from 0 to1) measure the consistency for datasets of repeated measure-ments. The closer the ICC is to 1, the better the measurementconsistency. To assess interoperator and intersession repro-ducibility, the mean of the 3 readings from each operatorand session was first calculated for each device. Then, the in-teroperator and intersession Sw, 2.77 Sw, CoV, and ICCswerecalculated.

Comparison of Devices

Repeated-measures analysis of variance (ANOVA) withthe Bonferroni correction was used to identify pairs thatwere significantly different. Bland-Altman analysis wasperformed to evaluate the agreement between devices.This involved the use of the 95% limits of agreement(LoA) as the mean difference G 1.96 standard deviation

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(SD). Narrower 95% LoA indicate higher agreement be-tween techniques.19

RESULTS

The mean age of the 77 enrolled subjects was 25.52years G 2.17 (SD) (range 22 to 33 years). The meanspherical equivalent refraction was �3.33 G 2.20 D(range C0.25 to �9.00 D). All 77 subjects participatedin the study to assess the agreement of corneal mea-surements between the 3 devices, whereas 44 sub-jects were included in the study to determine therepeatability and reproducibility of the new cornealtopographer.

Intraoperator Repeatability

Table 1 shows the mean values, Sw, 2.77 Sw, CoV,and ICC values for the flat K, steep K, mean K, andpower vectors J0 and J45 for the 3 repeated measure-ments. The CoV values of flat K, steep K, and meanK were within 0.3%, and the ICCs were more than0.97 for all parameters. Thus, the intraoperator repeat-ability of corneal powermeasurements by the new cor-neal topographer was excellent.

Interoperator and Intersession Reproducibility

Table 2 shows the mean values, Sw, 2.77 Sw, andCOV of the flat K, steep K, mean K, J0 and J45 forthe assessment of interoperator reproducibility. TheCoV values of the flat K, steep K, and mean K valueswere within 0.15%, the Sw values were within 0.06 D,and the 2.77 Sw values were within 0.17 D. The Swand 2.77 Sw J0 and J45 values were within 0.02 Dand 0.06 D, respectively. These results indicate the ex-cellent interoperator reproducibility of the new cor-neal topographer.

The intersession reproducibility of the new cornealtopographer was also excellent (Table 3). The CoV offlat K, steep K, and mean K values for the sessionswere within 0.21%; the Sw values were within 0.09 D;and the 2.77 Sw values were within 0.25 D. The Swand 2.77 Sw of the J0 and J45 values were within 0.04 Dand 0.11 D, respectively.

Comparison of Devices

Table 4 shows the mean corneal power measure-ments by the Scheimpflug camera and automatedkeratometer. Repeated-measures ANOVA showedstatistically significant differences in all parametersmeasured by the 3 instruments (P!.05). Accordingto the Bonferroni post hoc test, there were no statisti-cally significant differences between corneal powermeasurements obtained by the new corneal topogra-pher and the rotating Scheimpflug camera. The meandifference between the measurements was very small

VOL 39, OCTOBER 2013

Table 1. Intraoperator repeatability of readings by the new corneal topographer.

Parameter/Session Mean (D) G SD Sw (D) 2.77 Sw (D) CoV (%) ICC

Kf1st 42.51 G 1.16 0.09 0.26 0.22 0.9942nd 42.51 G 1.15 0.11 0.32 0.27 0.990

Ks1st 43.53 G 1.50 0.10 0.28 0.23 0.9952nd 43.53 G 1.49 0.11 0.31 0.26 0.994

Km1st 43.02 G 1.30 0.09 0.25 0.21 0.9952nd 43.02 G 1.29 0.11 0.30 0.25 0.993

J01st �0.46 G 0.35 0.03 0.09 d 0.9912nd �0.46 G 0.35 0.03 0.08 d 0.993

J451st 0.07 G 0.17 0.03 0.08 d 0.9732nd 0.07 G 0.17 0.02 0.07 d 0.981

CoVZwithin-subject coefficient of variation; ICCZ intraclass correlation coefficient; J0Z cylinder at 0-degree meridian; J45Z cylinder at 45-degreemeridian;Kf Z flat keratometry; Km Z mean keratometry; Ks Z steep keratometry; Sw Z within-subject standard deviation

1564 CORNEAL POWER MEASUREMENTS WITH A NEW TOPOGRAPHER

(approximately 0.01 D), and the 95% LoA between thenew corneal topographer and the rotating Scheimp-flug camera were narrow, showing high agreementbetween the 2 devices (Figures 1 to 5). In contrast, sta-tistically significant differences were found in all pa-rameters between the new corneal topographer andthe automated keratometer, except J45. Although theautomated keratometer produced higher cornealpower readings than the new corneal topographer,the difference was extremely small and could not beconsidered clinically significant. Figures 1 to 5 alsoshow the narrow 95% LoA and good agreement be-tween the 2 instruments.

DISCUSSION

When a new instrument is introduced, it is importantto evaluate the repeatability and reproducibility ofits measurements and assess the agreement of its

Table 2. Intraoperator reproducibility of readings by the newcorneal topographer.

Parameter Mean (D) G SD Sw (D) 2.77 Sw (D) CoV (%)

Kf 42.51 G 1.15 0.06 0.17 0.15Ks 43.53 G 1.49 0.06 0.17 0.14Km 43.02 G 1.29 0.06 0.16 0.13J0 �0.46 G 0.35 0.02 0.06 d

J45 0.07 G 0.17 0.02 0.04 d

CoV Z within-subject coefficient of variation; J0 Z cylinder at 0-degreemeridian; J45 Z cylinder at 45-degree meridian; Kf Z flat keratometry;Km Z mean keratometry; Ks Z steep keratometry; Sw Z within-subject standard deviation

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readings with those of the commonly used devices.The Pentacam and IOLMaster are validated instru-ments for themeasurement of corneal power. Previousstudies9,10,13,14 have shown that they have good re-peatability and accuracy.

The reliability of the Keratograph 4, a new cornealtopographer, was recently tested by Best et al.,8 whofound that the device can provide highly repeatablemeasurements of corneal power. In our study, weconfirmed the high intraoperator repeatability of cen-tral corneal power measurements obtained by thenew corneal topographer, as evidenced by the lowSw (!0.12 D) and high ICC (O0.97) values. In addi-tion, we showed for the first time the high interoper-ator reproducibility and intersession reproducibilityof corneal power measurements obtained by theKeratograph 4.

Our results are comparable to the reliability re-ported for other Placido-based corneal topographers

Table 3. Intersession reproducibility of readings by the new cor-neal topographer.

Parameter Mean (D) G SD Sw (D) 2.77 Sw (D) CoV (%)

Kf 42.51 G 1.17 0.07 0.21 0.18Ks 43.53 G 1.50 0.09 0.25 0.21Km 43.02 G 1.31 0.08 0.21 0.17J0 �0.46 G 0.35 0.04 0.11 d

J45 0.06 G 0.17 0.02 0.07 d

CoV Z within-subject coefficient of variation; J0 Z cylinder at 0-degreemeridian; J45 Z cylinder at 45-degree meridian; Kf Z flat keratometry;Km Z mean keratometry; Ks Z steep keratometry; Sw Z within-subject standard deviation

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Table 4. Readings by the rotating Scheimpflug camera and theautomated keratometer.

Parameter

Mean (D) G SD

Scheimpflug Camera Automated Keratometer

Kf 42.82 G 1.19 42.89 G 1.21Ks 43.85 G 1.42 44.02 G 1.42Km 43.33 G 1.28 43.46 G 1.29J0 �0.46 G 0.30 �0.51 G 0.31J45 0.07 G 0.20 0.04 G 0.19

J0Z cylinder at 0-degree meridian; J45Z cylinder at 45-degree meridian;KfZ flat keratometry; KmZmean keratometry; KsZ steep keratometry

1565CORNEAL POWER MEASUREMENTS WITH A NEW TOPOGRAPHER

used in common clinical practice. For example, Readet al.13 found excellent repeatability of the Medmonttopographer (Medmont Pty. Ltd.), with the ICCs be-ing more than 0.99 and the 2.77 Sw values being lessthan 0.19 D. In a study to assess the performance ofthe Eyesys 2000 topographer (Eyesys, Inc.) in cali-brated steel balls within a wide range of radii of cur-vature, Gonz�alez P�erez et al.22 found goodreliability. Jeandervin and Barr23 also showed thatthe Eyesys 2000 can provide measurements of theperipheral corneal power with high repeatability,as obtained in the study of the portable Eyesys Vistainstrument. More recently, Wang et al.24 found thatthe Topolyzer (Wavelight Technologie AG) andEyesys Vista Placido disk-based videokeratoscopeshad excellent intraoperator repeatability, witha 2.77 Sw of less than 0.35 D and less than 0.36 D, re-spectively, and an ICC of more than 0.99 and morethan 0.98, respectively, for flat K, steep K, andmean K. The Medmont topographer showed better

Figure 1. Bland-Altman plots showing the means plotted against the diffetopographer and Scheimpflug camera (A) and between the new corneal topmean difference (bias). The upper and lower lines represent the 95% LoA.

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repeatability, with a 2.77 Sw of less than 0.23 D.Compared with similar devices based on Placidodisk, the Keratograph 4 and Topolyzer deviceshave higher resolution because they feature 22 ringsand measure 22 000 data points. The Eyesys Vista in-strument contains 26 Placido rings and measures9360 points, and the Medmont instrument has 32rings and measures 9600 data points. A new device,the OPD Scan III (Nidek Co. Ltd.) with 33 rings, hasnot yet been tested for its repeatability. It is likelythat more rings and data points improve the reliabil-ity of the measurements; however, further studiesare needed to confirm this hypothesis.

In the present study, the reliability vectors J0 and J45decreased; however, they were still shown to be rea-sonably good. The J0 and J45 decomposition compo-nents denote a Jackson cross-cylinder, with the axesat 90 degrees/180 degrees and at 45 degrees/135 de-grees, respectively, based on Fourier transformation.20

It was shown that these parameters were sensitive tothe magnitudes and meridians of the variability inastigmatism. This study included few subjects withastigmatism meridians at 45 degrees/135 degrees.Some subjects had astigmatism of a small magnitude,which was also true in the study by Chen and Lam.25

Chen and Lam report the variability of vector compo-nents J0 and J45was greater than that of flat K, steep K,and mean K; however, all parameters had reasonableclinical reliability.

Factors that contribute to the variability of cornealtopography measurements are investigator trainingand compliance with the test protocol, diseaseseverity, corneal irregularity, corneal steepness,examination interval, and patient experience andcooperativeness.12,26,27 Therefore, to reduce the

rences in values of flat K for a comparison between the new cornealographer and automated keratometer (B). The solid line indicates the

VOL 39, OCTOBER 2013

Figure 2. Bland-Altman plots showing the means plotted against the differences in values of steep K for a comparison between the new cornealtopographer and Scheimpflug camera (A) and between the new corneal topographer and automated keratometer (B). The solid line indicates themean difference (bias). The upper and lower lines represent the 95% LoA.

1566 CORNEAL POWER MEASUREMENTS WITH A NEW TOPOGRAPHER

variability related to the 2 operators in the presentstudy, the operators were first trained to follow verysimilar procedures and make similar judgments dur-ing measurements.

Differences in the measurements can be attributednot only to the instrument and operator but also tochanges that occur in the eye. According toGonz�alez-M�eijome et al.,27 in healthy corneas, themain patient-related factors that could adverselyaffect measurements are fixation instability, facial fea-tures that limit the focus and centration, and tear-filminstability. Some misalignment of the eye during im-age acquisition was unavoidable in our study andmight have influenced the results. In fact, according

Figure 3. Bland-Altman plots showing the means plotted against the differtopographer and Scheimpflug camera (A) and between the new corneal topmean difference (bias). The upper and lower lines represent the 95% LoA.

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to Hick et al.,28 rotational misalignment and transla-tional misalignment influence topography. Moreover,the Keratograph 4 topographer is based on a Placidodisk and its performance can be highly affected bytear-film instability. For this reason, in our study,subjects were asked to blink just before measure-ments to spread an optically smooth tear film overthe cornea.

In the current study, there was high agreement incorneal power measurements obtained by the Kerato-graph 4 and Pentacam HR instruments, suggestingthat the corneal power measurements obtained fromthe 2 devices can be used interchangeably. Althoughwe have not seen a study that compared the corneal

ences in values of mean K for a comparison between the new cornealographer and automated keratometer (B). The solid line indicates the

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Figure 4. Bland-Altman plots showing themeans plotted against the differences in values of vector J0 for a comparison between the new cornealtopographer and Scheimpflug camera (A) and between the new corneal topographer and automated keratometer (B). The solid line indicates themean difference (bias). The upper and lower lines represent the 95% LoA.

1567CORNEAL POWER MEASUREMENTS WITH A NEW TOPOGRAPHER

power measurements of the 2 instruments, previousstudies have compared the Pentacam HR systemwith other instruments that are based on a Placidodisk. Savini et al.15 did not find a statistically signifi-cant difference between the Pentacam system andthe 2 corneal topographers; that is, the TMS-2 (TomeyCorp.) and the Keratron Scout (Optikon 2000 SpA).However, they found low agreement between the 2Placido disk–based instruments and the rotatingScheimpflug camera, suggesting that caution shouldbe taken before considering the measurements pro-vided by the 2 technologies as interchangeable. Thedifference between our results and those reported bySavini et al.15 may be because the Keratograph 4 andPentacam instruments are built by the same

Figure 5. Bland-Altman plots showing themeans plotted against the differetopographer and Scheimpflug camera (A) and between the new corneal topmean difference (bias). The upper and lower lines represent the 95% LoA.

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manufacturer, whereas the 2 corneal topographers inthe Savini et al. study are not built by the same manu-facturer as that of the Scheimpflug camera. Readet al.13 assessed the agreement of corneal power mea-surements between the Pentacam system and a Placidodisk–based videokeratoscope; they found reasonableagreement between the 2 devices. However, their re-sults cannot be compared with ours because they didnot report the K values.

We also compared the corneal power measure-ments provided by the Keratograph 4 corneal topog-rapher and IOLMaster automated keratometer andfound them to be statistically different, with the auto-mated keratometer yielding slightly higher values.However, the difference in the readings was small

nces in values of vector J45 for a comparison between the new cornealographer and automated keratometer (B). The solid line indicates the

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1568 CORNEAL POWER MEASUREMENTS WITH A NEW TOPOGRAPHER

and there was good agreement between the 2 de-vices. Thus, the mean corneal power measurementsobtained from the 2 devices may be consideredinterchangeable. Similar results have been reportedby Savini et al.,15 who found that the IOLMaster au-tomated keratometer provided higher corneal powermeasurements (mean 43.97 G 1.44 D) than the TMS-2Placido disk-based corneal topographer (mean 43.85G 1.49 D).

The possible causes of the difference in measure-ments between these devices may be attributed todifferences in the measurement principle, measure-ment accuracy, reconstruction algorithms, andcalibration. The diameter of the cornea imaged byeach device likely plays a major role. The Kerato-graph 4 and IOLMaster instruments obtain kerato-metric measurements within a 3.0 mm diameterand a 2.5 mm diameter, respectively. The peripheralcorneal curvature is flatter than the central cornealcurvature.

Our study has limitations. We enrolled only youngadults with healthy eyes; thus, the capabilities of theKeratograph 4 corneal topographer were tested ona large array of normal corneal shapes. Our study ex-cluded subjects with corneal abnormalities or a his-tory of corneal surgery; therefore, further researchis required to comprehensively assess the perfor-mance of the Keratograph 4 topographer in cornealpower measurements of irregular or postsurgical cor-neas. Moreover, Best et al.8 found that the Kerato-graph topographer consistently measured flattercorneal power than the Tonoref II autorefractor kera-tometer (Nidek Co. Ltd.) and suggested the Kerato-graph topographer requires a calibration offset tobe comparable to other keratometry devices.Although we found very small difference and 95%LoA between the Keratograph topographer and thePentacam HR device and between the Keratographtopographer and the IOLMaster device, we suggestoptimizing the constants of intraocular lens powercalculation formulas when changing from other de-vices (eg, the Pentacam, IOLMaster, and Tonoref II)to the Keratograph 4 topographer. Last, the Kerato-graph 4 corneal topographer can perform additionalfunctions, and further research is required to assessthe reliability of these.

In summary, corneal powermeasurements obtainedusing the Keratograph 4 corneal topographer showednot only excellent intraoperator repeatability but alsogood interoperator and intersession reproducibility.Agreement of corneal power measurements betweenthe PentacamHR Scheimpflug camera and IOLMasterautomated keratometer was good, and these measure-ments may be considered interchangeable for mostclinical applications.

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WHAT WAS KNOWN

� Accurate and precise determination of corneal power isfundamental to many clinical and research applicationsin ophthalmology. Only 1 previous study has shown thata new corneal topographer, the Keratograph 4, providesmeasurements of intraoperator corneal power that arehighly repeatable.

WHAT THIS PAPER ADDS

� This new corneal topographer provided corneal powermeasurements in normal eyes with excellent intraopera-tor repeatability and intersession and interoperatorreproducibility.

� The Keratograph 4 showed high agreement with 2 vali-dated and commonly used instruments, the PentacamHR and the IOLMaster, for corneal power measurements.

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