Central corneal thickness measurementsusing Orbscan II, Visante, ultrasound,and Pentacam pachymetry after laserin situ keratomileusis for myopia

Thomas Ho, MRCOphth, Arthur C.K. Cheng, MCRS, FCOphth(HK), Srinivas K. Rao, FRCS,Silvania Lau, Chris K.S. Leung, MRCS, Dennis S.C. Lam, FRCS, FRCOphth

PURPOSE: To compare corneal pachymetry assessment using 4 measurement methods in eyes af-ter laser in situ keratomileusis (LASIK) for myopia.

SETTING: Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong,Hong Kong Eye Hospital, Hong Kong SAR.

METHODS: Fifty-two consecutive patients (103 eyes) who had LASIK for the correction of myopiahad Orbscan II (Bausch & Lomb), Visante (Carl Zeiss Meditec), Pentacam (Oculus, Inc.), and ultra-sound (US) pachymetry (Sonomed, 200P) 6 months after surgery. Data were analyzed using thepaired sample t test, Bland-Altman plots, and linear regression.

RESULTS: The mean postoperative pachymetry measured by US, Orbscan (0.89 acoustic factor),Pentacam, and Visante pachymetry were 438.2 mm G 41.18 (SD), 435.17 G 49.63 mm, 430.66G 40.23 mm, and 426.56 G 41.6 mm, respectively. Compared with the US measurement, Pentacamand Visante measurements significantly underestimated corneal thickness by a mean of 7.54 G15.06 mm (P<.01) and 11.64 G 12.87 mm (P<.01), respectively. There was no statistically signif-icant difference between US and Orbscan measurements.

CONCLUSION: Pentacam and Visante measurements of corneal thickness 6 months after LASIKwere significantly less than those obtained using Orbscan and US pachymetry, although all 4 mea-surement methods showed a high correlation with each other.

J Cataract Refract Surg 2007; 33:1177–1182 Q 2007 ASCRS and ESCRS

ARTICLE

Accurate measurement of corneal thickness is impor-tant in corneal refractive procedures, especially laserin situ keratomileusis (LASIK), which is currently themost popular approach for the correction of refractiveerrors. This measurement allows determination of theextent of safe stromal ablation possible because it isnow believed that iatrogenic keratectasia can resultfrom excessive tissue removal in the stromal bed.1

This may be particularly important in patients whohad laser refractive surgerywith suboptimal outcomesand are being considered for an enhancementprocedure.

The current gold standard for corneal pachymetry isapplanation ultrasound (US) pachymetry, althougherrors caused by the indentation of the cornea havebeen reported.2 Concerns about the possibility of pa-tient discomfort, epithelial damage, and spread of in-fections with contact methods also exist.

Q 2007 ASCRS and ESCRS

Published by Elsevier Inc.

Today, several noncontact devices that allow assess-ment of corneal thickness are available. The Orbscan(Orbtek, Bausch & Lomb) corneal topography systemmeasures corneal thickness by analyzing images ofthe anterior and posterior corneal reflecting surfacesbased on slit-scanning technology and videokeratog-raphy. Using an acoustic adjustment factor, whichcan be customized for each unit, the second versionof Orbscan (Orbscan II) gives results comparable tothose of US pachymetry in pre-LASIK patients.3

However, it has been reported that in post-LASIKpatients, Orbscan measurements underestimate cor-neal thickness despite the use of a customized acousticfactor.3–5

The Visante device (Carl Zeiss Meditec) uses high-resolution, noncontact optical coherence tomography(OCT), customized for anterior segment evaluation.It allows assessment of corneal thickness across the

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1178 COMPARISON OF PACHYMETRY AFTER MYOPIC LASIK WITH 4 MEASUREMENT METHODS

entire corneal surface without direct contact. Theimage-acquisition system provides a video image ofthe examined zone and stores the last 7 images ata rate of 8 frames per second. At the end of the exam-ination, the software interprets the selected image andthe image is reconstructed to provide pachymetryinformation.

The Pentacam device (Oculus, Inc.) uses theScheimpflug principle to acquire cross-sectional im-ages of the cornea and lens. It has been used in theassessment of cataract6–8 and for measuring cornealcurvature and thickness.9 It is a rotating camera thatoffers a noninvasive assessment of the anterior seg-ment of the eye. Data on topographic corneal thick-ness, curvature, anterior chamber angle, volume, andheight are calculated from up to 25 000 data points.

In this study, we compared the accuracy of centralcorneal pachymetry using the Visante, Orbscan, andPentacam to the accuracy of US pachymetry in pa-tients who had LASIK for myopia.

PATIENTS AND METHODS

The case records of 52 patients (103 eyes) who had myopicLASIK in a university-based eye clinic from November 1 toNovember 31, 2005, were retrospectively analyzed. No pa-tient had ocular disease other than refractive error. Preoper-atively, all patients had manifest refraction and centralcorneal thickness (CCT) measurement by a US pachymeter(Sonomed, 200P) and Orbscan II as part of the routinepre-LASIK assessment. They had CCT assessment with theUS pachymeter, Orbscan II, Visante, and Pentacam 6monthsafter surgery.

Orbscan measurements were obtained as per the manu-facturer’s instructions. Because these measurements overes-timate US measurements, possibly because the tear film isalso included, an acoustic factor is used to make the 2 mea-surements comparable. Although the manufacturer-sug-gested default acoustic factor is 0.92, the use of a customacoustic factor specific to the study site is recommended.Based on previous experience (unpublished data) andstudy,10 the Orbscan (version 3.12.57) used in this study

Accepted for publication March 12, 2007.

From the Department of Ophthalmology & Visual Sciences, TheChinese University of Hong Kong, University Eye Center, HongKong Eye Hospital, Hong Kong, China.

No author has a financial or proprietary interest in any material ormethod mentioned.

Supported in part by the Action for Vision Eye Foundation, HongKong, China.

Corresponding author: Arthur Cheng, Associate Professor, Depart-ment of Ophthalmology & Visual Sciences, The Chinese Universityof Hong Kong, University Eye Center, 3/F., Hong Kong Eye Hospital,147K Argyle Street, Kowloon, Hong Kong, China. E-mail: artcheng@netvigator.com.

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requires an acoustic factor of 0.89 to obtain readings compat-ible with those of US in unoperated eyes.

For Visante measurements, patients were asked to fixateon the optical target in the system and the center of measure-ment was aligned with the corneal apex. A pachymetry scanprotocol was chosen for the assessment. The system auto-matically processes 8 line scans and presents a map of thepachymetry values. The average reading displayed in thecenter represents the central 2.0 mm and was used for anal-ysis in the study.

The Pentacam system consists of the Pentacam cameraand a computer. The software is almost fully automated. Af-ter the patient’s data are entered, the program changes to im-agingmode. The patient sits in front of the camerawith his orher chin on the chin rest and is asked to fixate on a target inthe center of the camera. The examiner sees a real-time imageof the patient’s eye on the computer screen. The image is thenfocused and centered. As soon as the image is perfectlyaligned, the patient is asked to keep still and keep his orher eye open, after which the scanning process is started.The rotating camera takes multiple (25 or 50, depending onuser setting) slit images of the anterior segment over approx-imately 2 seconds. In this study, the ‘‘25 images per scan’’ op-tionwas chosen. The center of the cornea is measured in eachof the single images of the scan, allowing the CCT to becalculated.

The noncontact assessmentsdOrbscan, Pentacam, andVisantedwere always performed first with no order of

Figure 1. Mean and 95% CI of CCT measurements by US, Orbscan,Pentacam, and Visante

Table 1. Descriptive statistics of CCT measurements by US,Orbscan, Pentacam, and Visante methods.

DeviceMean CCT(mm) G SD 95% CI Skewness Kurtosis

Ultrasound 438.2 G 41.18 446.25-430.16 0.82 0.42Orbscan 435.17 G 49.63 444.87-425.46 0.64 0.48Pentacam 430.66 G 40.23 438.52-422.8 0.83 0.62Visante 426.56 G 41.6 434.69-418.43 0.73 0.08

CCT Z central corneal thickness; CI Z confidence interval

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Table 2. Paired t test and correlation values between 6 pairs of devices.

Pearson Correlation

Device PairingsDifference of MeanValues (mm) G SD P Value r Value P Value*

Ultrasound–Orbscan 3.03 G 19.71 .121 0.922 !.001Ultrasound–Pentacam 7.54 G 15.06 !.001 0.932 !.001Ultrasound–Visante 11.64 G 12.87 !.001 0.952 !.001Orbscan–Pentacam 4.51 G 17.46 .010 0.946 !.001Orbscan–Visante 8.61 G 18.56 !.001 0.932 !.001Pentacam–Visante 4.10 G 10.65 !.001 0.967 !.001

*2 tailed

preference. After the measurements, the cornea was anesthe-tized with topical proparacaine hydrochloride 1% (Alcaine).The calibrated US probe was used to obtain 5 measurementsfrom the central cornea. The highest and the lowest valueswere excluded, and the mean of the remaining 3 was usedfor analysis. Optometrists from the university eye clinic per-formed all pachymetry measurements.

The same surgeon (A.C.) performed all LASIK proceduresusing the Allegretto Wave Excimer Laser STD platform(WaveLight Laser Technologie AG). Flaps were createdwith a Zyoptix XP microkeratome (Bausch & Lomb); the in-tended flap thickness was 120 mm.

All data were analyzed using the SPSS computer programfor Windows (version 11.0, SPSS, Inc.). The results are pre-sented as mean G SD. All data were tested for normalityusing skewness (acceptable range for normality is between�1 and 1) and kurtosis (acceptable range for normality is be-tween �1 and 1). If the data were normal, paired t tests wereused to analyze the association between postoperative CCTby US pachymetry, Orbscan, Pentacam, and Visante. Linearregression was used to assess the correlation between the 4pachymetry measurement methods. A P value less than0.05 was considered statistically significant; 95% limits ofagreement (LoA) were defined as the mean G 1.96 SD.Bland-Altman plots were also used to compare the measure-ment methods.

RESULTS

The preoperative manifest spherical equivalent refrac-tion in the 103 eyes was �8.88 G 3.15 diopters. Therewas no significant difference in the mean preoperativeUS pachymetric measurement (553.27 G 29 mm) andOrbscanpachymetricmeasurement (552.27G 26.65mm)(P Z .3).

Figure 1 and Table 1 show the mean postoperativeCCT values and 95% confidence intervals (CIs) mea-sured by US, Orbscan, Pentacam, and Visante. Alldata were tested to be normally distributed, andpaired t tests were used for data analysis. The Penta-cam and Visante methods significantly underesti-mated the US corneal thickness measurements bya mean of 7.54 G 15.06 mm (P!.01) and 11.64 G12.87 mm (P!.01), respectively. The Pentacam andVisante also significantly underestimated CCT

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compared with Orbscan measurements by a meanof 4.51 G 17.47 mm and 8.61 G 18.56 mm, respec-tively. There was a difference of 4.1 mm between Pen-tacam and Visante measurements, which wasstatistically significant (P!.01). There was no statisti-cally significant difference between Orbscan and USCCT measurements (P Z .121) (Table 2).

Linear regression showed that all 4 modalities ofCCT measurements correlated closely with each other(Figure 2). Figure 3 shows the Bland-Altman plots forthe 4 methods with LoA set at G 1.96 SD.

No patient had complications during the studyperiod.

DISCUSSION

Accurate assessment of corneal thickness is importantto minimize the risk for serious post-LASIK complica-tions such as keratectasia. Although US is the mostcommonly used method of pachymetry assessment,possible disadvantages include the need to anesthetizethe cornea, corneal indentation during measurement,and corneal surface disturbance, which can interferewith other investigations such as topographic assess-ment and wavefront acquisition. There is also a riskfor corneal epithelial damage and transmission ofinfections.

Although noncontact devices such as the Orbscan,Pentacam, and Visante overcome these possible disad-vantages, their reliability and accuracy in measuringcorneal thickness remain unclear. The use of an acous-tic correction factor for Orbscan measurements allowsmeasurements of corneal thickness similar to thosemeasured with US, the gold standard in normaleyes; however, studies report that Orbscan measure-ments underestimate corneal thickness in post-LASIKpatients, especially in the earlypostoperative period.3–5

In this study, the Orbscan II underestimated CCT by3.03 mm, which was insignificant.

By means of OCT, Visante allows assessment of thecorneal thickness across the entire corneal surface

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Figure 2. Scatterplots comparing CCT values between 4 modalities. A: Correlation between US and Orbscan. B: Correlation between US andPentacam.C: Correlation betweenUS andVisante.D: Correlation between Pentacam andOrbscan. E: Correlation betweenVisante andOrbscan.F: Correlation between Visante and Pentacam.

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Figure 3. Bland-Altman plots comparing the 4 modalities. A: Comparison between Visante and Pentacam. B: Comparison between Visante andOrbscan.C: Comparison between Visante andUS.D: Comparison betweenUS andOrbscan. E: Comparison betweenUS and Pentacam. F: Com-parison between Orbscan and Pentacam.

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without direct contact. Our results showed that Vi-sante measurements underestimated the cornea thick-ness by 11.64 mm compared with US measurements.The difference was 7.54 mm with the Pentacam. De-spite the statistically significant difference betweenPentacam and Visante measurements and US andOrbscan measurements, there was a close correlationbetween all 4 modalities.

Lackner et al.11 found that Pentacam can produce re-sults comparable to those of US pachymetry in normaleyes, with a slight underestimation (9.8 mm). In an-other study of normal eyes,12 Pentacammeasurementswere shown to have good correlation with US mea-surements, with a correlation coefficient of 0.883. Toour knowledge, this is the first study to investigatethe accuracy of the Pentacam in post-LASIK eyesand compare the results with those of 3 othermeasure-ment methods.

Another currently available device is the ACMaster(ZeissMeditec). Buehl et al.13 compared the CCT usingACMaster, Orbscan, and Pentacam and found thatACMaster measurements underestimated CCT by7.5 mm and 7.1 mm when compared with Orbscanand Pentacam measurements, respectively. Again, be-cause the results were obtained in normal eyes, the ac-curacy of the assessment of CCT in post-LASIKpatients requires further evaluation. In terms of re-peatability, several studies report high repeatabilitywith all 4 modalities in measuring central thicknessin normal corneas.12,14–16

In this study, we showed that different types of non-contact methods underestimate CCT when comparedwith US; it is possible that US overestimates the truevalue of the CCT. In one study, Gao et al.17 foundthat the use of analgesia eyedrops significantly in-creased the measured corneal thickness in normaleyes by 23.36 mm. The use of analgesic eyedrops beforeUS pachymetry may have caused epithelial edemaand resulted in higher estimated values.

One reason there was no statistical difference be-tween US and Orbscan II in this study may be becausean adjusted acoustic factor was introduced into theOrbscan to reduce the magnitude of underestimationof CCT.10 Although Visante and Pentacam underesti-mated CCT when compared with Orbscan and US pa-chymetry, future studies are warranted to furtherevaluate the reason for the underestimation.

REFERENCES1. Binder PS. Ectasia after laser in situ keratomileusis. J Cataract

Refract Surg 2003; 29:2419–2429

J CATARACT REFRACT S

2. Solomon OD. Corneal indentation during ultrasonic pachometry.

Cornea 1999; 18:214–215

3. Cheng ACK, Rao SK, Tang E, Lam DSC. Pachymetry assess-

ment with Orbscan II in postoperative patients with myopic LA-

SIK. J Refract Surg 2006; 22:363–366

4. Prisant O, Calderon N, Chastang P, et al. Reliability of pachy-

metric measurements using Orbscan after excimer refractive

surgery. Ophthalmology 2003; 110:511–515

5. Yaylali V, Kaufman SC, Thompson HW. Corneal thickness

measurements with the Orbscan Topography System and ul-

trasonic pachymetry. J Cataract Refract Surg 1997; 23:1345–

1350

6. Harper CL, Boulton ME, Bennett D, et al. Diurnal variations in hu-

man corneal thickness. Br J Ophthalmol 1996; 80:1068–1072;

erratum 1997; 81:175

7. Hockwin O, Lerman S, Ohrloff C. Investigations on lens

transparency and its disturbances by microdensitometric

analyses of Scheimpflug photographs. Curr Eye Res 1984; 3:

15–22

8. Wegener A, Hockwin O, Laser H, Strack C. Comparison of the

Nidek EAS 1000 system and the Topcon SL-45 in clinical appli-

cation. Ophthalmic Res 1992; 24(suppl 1):55–62

9. Morgan AJ, Harper J, Hosking SL, Gilmartin B. The effect of cor-

neal thickness and corneal curvature on pneumatonometer

measurements. Curr Eye Res 2002; 25:107–112

10. Li EY, Mohamed S, Leung CK, et al. Agreement among 3

methods to measure corneal thickness: ultrasound pachymetry,

Orbscan II and Visante anterior segment optical coherence to-

mography. In press, Ophthalmology 2007

11. Lackner B, Schmidinger G, Pieh S, et al. Repeatability and re-

producibility of central corneal thickness measurement with

Pentacam, Orbscan, and ultrasound. Optom Vis Sci 2005;

82:892–899

12. Barkana Y, Gerber Y, Elbaz U, et al. Central corneal thick-

ness measurement with the Pentacam Scheimpflug system,

optical low-coherence reflectometry pachymeter, and ultra-

sound pachymetry. J Cataract Refract Surg 2005; 31:1729–

1735

13. Buehl W, Stojanac D, Sacu S, et al. Comparison of three

methods of measuring corneal thickness and anterior chamber

depth. Am J Ophthalmol 2006; 141:7–12

14. Fishman GR, Pons ME, Seedor JA, et al. Assessment of central

corneal thickness using optical coherence tomography. J Cata-

ract Refract Surg 2005; 31:707–711

15. Marsich MM, Bullimore MA. The repeatability of corneal thick-

ness measures. Cornea 2000; 19:792–795

16. Suzuki S, Oshika T, Oki K, et al. Corneal thickness measure-

ments: scanning-slit corneal topography and noncontact specu-

lar microscopy versus ultrasonic pachymetry. J Cataract Refract

Surg 2003; 29:1313–1318

17. Gao L, Fan H, Cheng ACK, et al. The effects of eye drops

on corneal thickness in adult myopia. Cornea 2006; 25:404–407

First author:Thomas Ho, MRCOphth

Department of Ophthalmology & VisualSciences, The Chinese University of HongKong, University Eye Center, Hong KongEye Hospital, Hong Kong, China

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