ORIGINAL ARTICLE

Orbscan Global Pachymetry: Analysis ofRepeated Measures

HAN-BOR FAM, FRCSE, KOOI-LING LIM, BOptom, and DAN Z. REINSTEIN, FRCSC

The Eye Institute, Tan Tock Seng Hospital, Singapore, (HBF), iLaser Centre Island Hospital, Penang, Malaysia, (KLL), and LondonVision Clinic, London, United Kingdom (DZR)

ABSTRACT: Purpose. The Orbscan II is a hybrid slit-scanning and Placido disc corneal topographer capable mappingglobal pachymetry over a 10-mm corneal diameter. In this study, the repeatability of the Orbscan global pachymetrywas determined. Methods. Five consecutive Orbscan examinations were performed on one eye of 20 healthy volunteersby one examiner in one session. Mean, standard deviation, range of readings, and coefficient of variance werecalculated for the central, thinnest point and global pachymetry. Results. Orbscan repeatability for the centralpachymetry was 3.62 �m (95% confidence interval [CI], 0.57–6.66) with a coefficient of variance of 0.67% (95% CI,0.09–1.25) and intraclass correlation coefficient (ICC) of 0.978 (95% CI, 0.959–0.990). Thinnest point pachymetryhad a repeatability of 4.25 �m (95% CI, 1.19–7.31) with a coefficient of variance of 0.80% (95% CI, 0.19–1.42) andICC of 0.973 (95% CI, 0.950–0.988). Global pachymetry had a coefficient of variance <1.5% in the central 4 mm and<2.5% across the entire cornea. Global pachymetry showed an area of greatest repeatability (<�5 �m) within acentral 3.0-mm horizontal and 4.0-mm vertical diameter. Discussion. Orbscan is capable of giving repeatable pachym-etry readings that are comparable to the ultrasound pachymeter within the central and thinnest point of the cornea.There is a gradual loss of repeatability toward the peripheral cornea possibly as a result of a lack of overlapping datapoints. (Optom Vis Sci 2005;82:1047–1053)

Key Words: Orbscan corneal topography, global pachymetry, repeatability, clinical performance

Corneal thickness is an indication of the health of the cor-nea. Changes in corneal thickness may be physiological,1,2

pathologic,3,4 and in the present age of refractive surgery,iatrogenic.5 Measuring central corneal thickness is currently thestandard practice in refractive surgery. However, global pachym-etry of the entire cornea is certainly more informative because thethinnest point of the cornea is usually not centered and not at afixed location on the cornea.6 We assume that the thinnest point ofthe cornea is located in the geometric center because it is com-monly the only point measured during ultrasound pachymetry.

Presently, several devices are available for measuring central cor-neal thickness. These include optical pachymetry,7 ultrasonicpachymetry,8,9 optical coherence tomography,10,11 laser Dopplerinterferometry,12 very high-frequency ultrasound pachymetry,13

specular microscopy,14 optical low coherence interferometry,15

and online optical coherence pachymetry.16 However, many ofthese methods generally measure only discrete points on the corneaand do not provide a convenient method of measuring pachymetryof the entire cornea. Furthermore, single measurement devicessuch as optical pachymetry, ultrasound pachymetry, and opticallow coherence interferometry require that the probe be aligned

perpendicularly to the corneal surface, and repeatability dependson the examiner’s precision in placing the probe at the cornealcenter. A significant disadvantage of many of these methods ofmeasuring corneal pachymetry is that they do not allow the exam-iner to determine the magnitude or the location of the thinnestpoint of the cornea. Early users of the ultrasound pachymeterlamented the fact that it was (and still is) unable to accuratelydetect the location of corneal thinning in abnormalities such askeratoconus.17

Orbscan II (Bausch & Lomb, Rochester, NY) acquires data byhybrid slit-scan and Placido ring technology. An average of 40 slitsof light are projected sequentially onto the corneal surface wherethe anterior and posterior edges of the projected slits are capturedfor analysis. Two hundred forty points along each slit edge areanalyzed to generate over 9000 data points over a maximum10-mm corneal diameter. These images are processed into rawdata, which is represented using color-coded topographic maps.Placido disc reflections are used to supplement slit-scan data togenerate curvature-based maps such as axial and tangential kerato-metric maps. In addition, the concurrent measurement of bothanterior and posterior surface depth of the cornea is used to deter-

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mine the thickness of the cornea. The availability of data from thewhole cornea enables global pachymetry involving all points inwhich both anterior and posterior slits are available for processing.This allows measurement of central, peripheral, and the thinnestpoint of the cornea simultaneously. In this article, we define thismeasurement of multiple points simultaneously as the globalpachymetry.

With an early version of the Orbscan, Yaylali et al18 foundsignificant systematic differences between the Orbscan-measuredpachymetry and ultrasound pachymetry with the Orbscan overes-timating central pachymetry by 23 to 28 �m in normal eyes.Optical pachymeters like Orbscan measures the reflected lightbeams through corneal tissue, whereas ultrasound pachymetry re-lies on the reflectance of sound waves from the anterior and pos-terior corneal surfaces. In addition, Liu et al6 hypothesized that theOrbscan measures the mucous layer on the corneal surface in ad-dition to the actual corneal thickness leading to an overestimationof Orbscan pachymetry over ultrasound pachymetry. The manu-facturer subsequently recalibrated the Orbscan to the ultrasoundpachymeter using a transformation factor known as the acousticequivalent. The default value of the acoustic equivalent of theOrbcan is 0.92. This relationship, however, is restricted to healthyvirgin eyes because the optical and ultrasound reflectance proper-ties of the cornea may change in the presence of disturbances suchas edema11 and refractive surgery.19,20

Although the repeatability and precision of Orbscan centralpachymetry has been extensively reported,18,21–29 the repeatabilityof global pachymetry is not well known.30 Using the manufactur-er’s recommended acoustic equivalent settings, the central cornealpachymetry measured with the Orbscan has compared favorablywith that measured using the ultrasound pachymeter with no sta-tistically significant differences between Orbscan and ultrasoundpachymetry in normal eyes.21,22,24-26,28

The use of global pachymetry is increasingly relevant in this ageof customized corneal ablation

31,32

and conductive keratoplas-ty.33,34 In this study, we look beyond the central corneal pachym-etry and examine the repeatability and variability of Orbscanglobal pachymetry throughout the measured cornea.

MATERIALS AND METHODS

The Orbscan II Topography System (version 3.0d) was used tomeasure the corneal thickness of all subjects. All measurements ofeach subject were done using the same unit of the Orbscan deviceby the same examiner. The right eye of 20 healthy volunteers wasstudied. All subjects were volunteers recruited from the staff of this

institution. The Institutional Review Board (IRB) of Island Hos-pital allowed expedited IRB approval for the experimental protocolof this study because it posed minimal risk to the research subjects.Criteria for subjects included good ocular health, good generalhealth, no previous ocular surgery or disease, no contact lens wear,and finally, good best-corrected visual acuity (20/20 or better).Each eye was measured five times sequentially at the same sitting.After each acquisition, the subject was asked to sit back and thenrepositioned on the Orbscan for realignment and reacquisition.Total time for acquiring all measurements did not exceed 10 min-utes for each subject. This was to minimize the effect of diurnalvariation of pachymetry.2

Data processing was set at the default auto edit setting. Exami-nations that were rejected by the auto edit feature were repeated,but otherwise no other editing was done. The default acousticequivalent setting of 0.92 was used for this study. On completionof data processing, the pachymetry map was displayed. The centralpoint and the thinnest point were read off the pachymetry map.Pachymetry data (in Cartesian coordinates) from the entire mapwas exported to Microsoft Excel (Microsoft Corp., Redmond,WA) for further analysis. Only corresponding data that was re-corded in all five maps of every individual eye were used for theanalysis. Data at the fringes where an incomplete set of less thanfive readings were excluded to avoid bias.

The range of pachymetry readings was determined by the dif-ference between the maximum and minimum recorded values ateach point. Repeatability of the Orbscan pachymetry was definedas the standard deviation of pachymetry values at each particularpoint. The coefficient of variance (CV) was calculated by dividingthe standard deviation by mean value.

Contour plots of the mean, repeatability, and coefficient of vari-ance of the global pachymetry were generated using DeltaGraph5.0 (SPSS Inc., Chicago, IL). Intraclass correlation coefficient(ICC) was calculated based on the repeated-measures analysis ofvariance35 using SPSS 11.0 (SPSS Inc.). p values of � 0.05 wereconsidered statistically significant.

RESULTS

Central and thinnest point values are summarized in Table 1.Mean pachymetry found in our subjects was normally distrib-

uted with Kolmogorov-Smirnov Z � 0.68 (p � 0.75) for centralpachymetry and Z � 0.74 (p � 0.64) for thinnest point pachym-etry. Both central and thinnest point measures showed high repeat-ability and concordance within the repeated measures of this groupof subjects. Global pachymetry contour plots of mean pachymetry,

TABLE 1.Central and thinnest point pachymetry values

Central thickness (Mm) Mean 543.0Standard deviation (95% confidence interval) 3.62 (0.57–6.66)Coefficient of variation (%) (95% confidence interval) 0.67 (0.09–1.25)Range (95% confidence interval) 8.60 (1.23–11.30)

Thinnest point (�m) Mean 534.3Standard deviation (95% confidence interval) 4.25 (1.19–7.31)Coefficient of variation (%) (95% confidence interval) 0.80 (0.19–1.42)Range (95% confidence interval) 10.30 (2.60–18.00)

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repeatability of pachymetry, and coefficient of variance are shownin Figures 1, 2, and 3.

Intraclass correlation coefficients were 0.98 (p � 0.01) (95%confidence interval [CI], 0.96–0.99) for central pachymetry and0.97 (p � 0.01) (95% CI, 0.95–0.99) for thinnest point pachym-etry. Repeated-measures analysis of variance showed no significantdifferences between the five measures of central (p � 0.592, F �0.70) and thinnest point (p � 0.96, F � 0.15) pachymetry. How-ever, the localization of the thinnest point pachymetry showedmarked variability in some subjects. To illustrate the differenttypes of variability, the scatter plots from four subjects are shown inFigure 4.

The minimum pachymetry can be found at x � 0.1, y � -0.2,the inferonasal quadrant of the cornea.

The repeatability of pachymetry was greatest in the center of thecornea, decreasing toward the peripheral areas. The vertical merid-ian of the central cornea showed greater repeatability of pachym-etry compared with the corresponding horizontal area. Overall, therepeatability of pachymetry was not worse than 15 �m throughoutthe measured cornea.

The variability of pachymetry was smallest within the centralcornea (�1%) and gradually increased toward the peripheral areas.The coefficient of variability did not exceed 1.5% within the cen-tral 4 mm of the cornea and 2.5% across the entire cornea. As seenwith the repeatability of pachymetry, vertical measurementsshowed less variability compared with horizontal measurements.

The repeatability of Orbscan pachymetry was not dependent onthe measured pachymetry (Fig. 5) as determined using regressionanalysis. This was true for both central (R2 � 0.29, p � 0.47) andthinnest point (R2 � 0.10, p � 0.17) measurements.

DISCUSSION

From a review of available literature, the intraobserver repeat-ability of the Orbscan and other devices for measuring centralpachymetry is shown in Table 2. The intraobserver repeatability

for central pachymetry in our study was also similar to that re-ported in other studies of the Orbscan25 and ultrasoundpachymeter.25,36,37 In addition, the Orbscan repeatability foundin this study is comparable if not better than that of some otherdevices such as the laser Doppler interferometer12 and the opticalcoherence tomographer.10,11 Because the purpose of this study wasto determine the intraobserver repeatability of the Orbscan and notto confirm the accuracy of the device, we did not compare thepachymetry of each subject obtained by the Orbscan with that ofthe ultrasound pachymeter.

However, the repeatability of the location of the thinnest pointshowed variability within all eyes (Figure 4). It is to be noted thatthe variability lies in the determination of the location of the thin-nest point and not the actual value of the thinnest point. We haveobserved that this variability in the localization of the thinnestpoint may also be influenced by the pachymetric distribution of

FIGURE 1.Mean pachymetry. Color versions of figures 1–3 are available online atwww.optvissci.com.

FIGURE 2.Repeatability (standard deviation) of pachymetry.

FIGURE 3.Coefficient of variation (%) of pachymetry.

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each eye. Eyes with a very distinct thinnest point (steep rate ofpachymetric change, example Figure 4, subjects [A] and [B]) tendto have less positional variability compared to eyes that have grad-ual thinning (slow rate of pachymetric change, example Figure 4,

subjects [C] and [D]). Examples of these showing the pachymetricdistribution are illustrated in Figure 6.

We also speculate that some degree of variability could be causedby slight movements of the eye during the Orbscan examinationacquisition or local changes in the tear film thickness during thefew seconds it takes to project all the slits during an examination.

Earlier reports by Cho et al28 and Gonzalez-Meijome et al23

noted that there was also a loss of pachymetric precision of theOrbscan in the peripheral cornea. From our data, the globalpachymetry of the Orbscan also showed a progressive loss of re-peatability toward the paracentral and peripheral areas of the cor-nea similar to that reported by Jonsson.38 Based on our own clin-ical experience and previous reported values shown in Table 2, wechose 5 �m as the clinical repeatability of the ultrasound and usedthis as a comparison with the Orbscan repeatability. When this wascompared with the global Orbscan pachymetry found in thisstudy, the Orbscan was found to give highly repeatable pachymetryreadings comparable to that of the ultrasound within the central3.0-mm diameter of the cornea horizontally and up to a 4-mmdiameter vertically (Figure 2). This also corresponds to the areawith approximately 1% coefficient of variance (in which the stan-

FIGURE 4.Location of thinnest point of pachymetry. Subjects (A) and (B) showed high consistency in the location of the thinnest point, but subjects (C) (right eye)and (D) (right eye) had marked variability.

FIGURE 5.The distribution of Orbscan repeatability according to pachymetry.

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dard deviation is not more than 1% of the mean pachymetry),indicating small variance and high repeatability of measurements(Figure 3).

What is notable about the Orbscan global pachymetry is thatthere is greater repeatability in the vertical meridian compared with

the horizontal meridian. The discrepancy between the horizontaland vertical resolution of the Orbscan can be explained by the factthat the Orbscan projects vertical measurement slits onto the cor-nea. This allows the device to record more data points vertically (upto 240 data points along each slit, according to the manufacturer)compared with horizontally (in which only 40 data points arerecorded) (Figure 7). Thus, more interpolation of data will berequired along the horizontal meridian as compared with the ver-tical meridian. In addition, the repeatability is higher within thecentral corneal region because there is an overlapping of projectedslits in this region that increases the resolution of the measurement.Because there are fewer slits projected onto the peripheral areasduring acquisition, this explains the loss of repeatability in theperipheral cornea with the Orbscan. Note that the arc-like zone ofleast variability conforms to the zone with the overlapping slits(Figure 7). Consequently, the reduced repeatability of the Orbscanhorizontally could hypothetically be increased by using horizontalslits during data acquisition as well.

TABLE 2.The repeatability of various measures of central corneal thickness

Author Method Repeatability (Mm)

Wirbelauer et al10 Ultrasound 4.0Optical coherence tomography 5.8

Bechmann et al11 Ultrasound 4.90Optical coherence tomography 4.96

Gordon et al9 Ultrasound 6Tam14 Specular microscopy 7.82

Utrasound 4.14Lattimore2 Orbscan 5Yaylali et al18 Orbscan 8.42

Ultrasound 5.84Hitzenberger et al12 Laser Doppler interferometry 7Reinstein et al13 Very high-frequency ultrasound 0.74Wirbelauer et al16 Online optical coherence tomography 4.3Bohnke15 Optical low coherence interferometry 1

FIGURE 6.Differences in pachymetric distribution showing steep thinning (top) andgradual thinning (bottom). Color versions of figures 6-7 are availableonline at www.optvissci.com.

FIGURE 7.Orbscan acquisition slits as projected on the eye.

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Furthermore, because corneal elevation and power maps gen-erated by the Orbscan are calculated using the same acquireddata as the pachymetry, the information gleaned from this studycould be applied to the repeatability of these other measure-ments as well.

We conclude that the Orbscan is capable of high repeatabilityand clinically consistent pachymetry measurements in normaleyes. However, because there is variability in the pachymetry mea-sured by every device, it would be prudent to for the clinician toperform repeated measurements, especially if only one type ofpachymeter is used.

Received April 10, 2005; accepted September 2, 2005.

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Lim Kooi-LingClinical Director & Optometrist

iLaser Centre, Island Hospital308 Macalister Road

10450 Penang, Malaysiae-mail: kooiling@ilasercentre.com

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