Central corneal thickness measurementwith Pentacam, Orbscan II, and ultrasounddevices before and after laser refractive

surgery for myopiaHassan Hashemi, MD, Shiva Mehravaran, MD

PURPOSE: To determine the agreement in central corneal thickness (CCT) measurements betweenthe gold standard method of ultrasound (US) pachymetry (UP-1000, Nidek) and 2 noncontact sys-tems based on Scheimpflug imaging (Pentacam, Oculus) and scanning-slit topography (Orbscan II,Bausch & Lomb) in myopic eyes before and after laser refractive surgery.

SETTING: Noor Vision Correction Center, Tehran, Iran.

METHODS: In this prospective study, 30 consecutive patients having refractive surgery for myopiawere enrolled. All 60 eyes were examined with the 3 devices preoperatively and 6 weeks after sur-gery; the US measurements were performed last. The Pentacam and Orbscan II CCT readings werecompared with the US readings. Both the original and corrected Orbscan II readings were used inthe analyses.

RESULTS: The mean CCT readings with US, Pentacam, and Orbscan II were, respectively, 555 mm,548 mm, and 580 mm before surgery and 478 mm, 468 mm, and 474 mm after surgery. Preopera-tively, the 95% limits of agreement (LoA) with US were �31 mm and C19 mm for the Pentacamdevice and �5 mm and C57 mm for the Orbscan II device. Postoperatively, the LoA were �40 mmand C19 mm and �51 mm and C50 mm, respectively. Corrected Orbscan II measurements gave95% LoA of �48 mm and C6 mm before surgery and �85 mm and C5 mm after surgery.

CONCLUSIONS: Refractive surgery had a modest effect on the agreement between Pentacam read-ings and US measurements. With Orbscan II, the 95% LoA width nearly doubled after surgery.Although the Pentacam seems to show better agreement than Orbscan II, especially after refractivesurgery, it is not advisable to use the 3 devices interchangeably in every clinical situation.

J Cataract Refract Surg 2007; 33:1701–1707 Q 2007 ASCRS and ESCRS

ARTICLE

Accuratemeasurement of corneal thickness is an essen-tial component of an advanced ophthalmologic exam-ination. This measurement helps detect and managecorneal pathology associated with corneal thinningand discriminate between keratoconus and contactlens–induced corneal thinning.1 In contact lens re-search, determining the level of corneal swelling is nec-essary to ensure the appropriateness of extended-wearcontact lens use.2 Moreover, to avoid complicationsthat may result from refractive surgical procedures,knowledge of corneal thickness is a prerequisite. It en-ables the surgeon to compute the depth of residual cor-neal tissue and determine the safety limit of a givenprocedure.3

The devices available for measuring corneal thick-ness are based on several technologies.4,5 The

Q 2007 ASCRS and ESCRS

Published by Elsevier Inc.

conventional ultrasound (US) method of pachymetryis the most frequently and widely used and is recog-nized as the gold standard.6,7 Some disadvantages ofthis method may limit its accuracy and clinical value;they include the need for topical anesthesia; contactbetween a probe and the cornea, with the associatedrisk for infection; the potential for inaccurate probepositioning due to lack of fixation and gaze control;estimation of the thickness of a single point with eachcontact; variation in sound speed with changes intissue hydration; and difficulty performing scanningpachymetry to detect abnormally thin areas of thecornea.4,7,8

Unlike US devices, the Orbscan (Bausch & Lomb)and Pentacam (Oculus) can perform a thoroughscan of the anterior segment in a noncontact fashion

0886-3350/07/$dsee front matter 1701doi:10.1016/j.jcrs.2007.05.040

1702 MYOPIC LASER CORRECTION AND PACHYMETRY WITH 3 DEVICES

and generate full pachymetry maps. These mapsdemonstrate the thickness of the cornea at any givenpoint by use of color scales or numeric labels. How-ever, studies show that measurement accuracy withthe Orbscan system can be affected by changes inthe optical quality or a loss of transparency in thecornea9,10 and not many studies of the Pentacamsystem have been published.

The goal of this study was to compare central cor-neal thickness (CCT) measurements obtained by thePentacam and Orbscan II devices and a US pachy-meter in a group of myopic patients before and afterlaser refractive surgery and to determine the effect ofsurgery on interdevice agreement.

PATIENTS AND METHODS

In this prospective observational case series performed atNoor Vision Correction Center, 30 consecutive patientsscheduled to have laser refractive surgery for myopia wereenrolled. All patients were informed of the nature of thestudy and consented to have additional examinations beforesurgery and 6 weeks after surgery.

To avoid the effects of diurnal variation in corneal thick-ness,11 participants were examined at least 3 hours after theirwake-up time. The examinations of each patient were per-formed on the same day, starting with the Pentacam andOrbscan II noncontact methods followed by the NidekUP-1000 US pachymeter. Subsequently, corneal examinationwith the slitlamp was performed to ensure no damage wascaused by the procedure.

The CCT value generated by each examination was re-corded. For the Orbscan II, all readings were crude and noacoustic equivalent correction equation was used. To makecomparisons with similar studies, the manufacturer’s sug-gested acoustic factor for Orbscan pachymetry readingswas applied in a subsequent analysis.

In the analyses, the differences between the devices wereassessed by paired t tests. The Pearson correlations were de-termined to show the correlations between compared data.The method described by Bland and Altman12 was used tostudy the agreement between the 2 devices in the studiedvariables, and the 95% limits of agreement (LoA) were

Accepted for publication May 30, 2007.

From the Farabi Eye Hospital (Hashemi), Department of Ophthal-mology, School of Medicine, Tehran University of Medical Sci-ences, and the Noor Ophthalmology Research Center (Hashemi,Mehravaran), Noor Eye Hospital, Tehran, Iran.

Neither author has a financial or proprietary interest in any materialor method mentioned.

Presented at the XXIV Congress of the European Society of Cataract &Refractive Surgeons, London, United Kingdom, September 2006.

Corresponding author: Shiva Mehravaran, MD, Noor Ophthalmol-ogy Research Center, Noor Eye Hospital, Number 106 EsfandiarBoulevard, Vali’asr Avenue, Tehran, 19686-53111, Iran. E-mail:smehravaran@noorvision.com.

J CATARACT REFRACT SURG

calculated. Data are presented as mean G standard devia-tion. A P value less than 0.05 was considered significant.

RESULTS

Sixty eyes of 30 patients were studied. Themean age ofthe patients was 28.4G 6.1 years (range 22 to 41 years),and 22 patients (73%) were women. The mean mani-fest refraction was �3.74 diopters (D) of sphere (range�1.25 to �8.00 D) and �0.88 D of cylinder (range 0 to�4.25 D). The surgical procedure for myopic correc-tion was laser in situ keratomileusis (LASIK) in 38eyes (63%), photorefractive keratectomy (PRK) in 14eyes (23%), and PRK plus mitomycin-C in 8 eyes.

Table 1 shows the preoperative and postoperativeCCT measurements obtained with each instrument.Preoperatively, the mean CCT measurements wasthe largest with the Orbscan II device. Postoperatively,the highest mean was found with the US pachymeter.ThemeanOrbscan II measurements correctedwith themanufacturer’s suggested acoustic factor of 0.92 werelowest preoperatively and postoperatively. In bothgroups, especially in operated eyes, the ranges of mea-surements were wider with the Orbscan II device(Figure 1).

Table 2 shows the results of comparisons betweenthe gold standard US method and the noncontactmethods. The mean difference with US readings wassmallest with the Pentacam device in preoperativeeyes (�6 mm versus C26 mm with Orbscan II and�21 mm with corrected Orbscan II) and with theOrbscan II device in operated eyes (�1 mm versus�9 mm with Pentacam and �40 mm with corrected

Table 1. Preoperative and 6-week postoperative CCT measure-ments with the 3 systems. The last 2 rows show the differencesbetween the preoperative and postoperative measurements,indicating the amount of ablation.

CCT Measurement (mm)

Exam Ultrasound PentacamOrbscan

IIOrbscanII�.92

PreopMean G SD 555 G 30 548 G 32 580 G 40 533 G 37Range 489–618 483–606 470–658 432–605

PostopMean G SD 478 G 51 468 G 48 474 G 67 436 G 62Range 334–581 333–556 335–609 308–560

Preop � postop(ablation)Mean G SD 78 G 33 81 G 28 104 G 40 96 G 37Range 21–167 32–164 40–189 37–174

CCT Z central corneal thickness; Orbscan II�.92 Z corrected with themanufacturer’s suggested acoustic factor of 0.92

- VOL 33, OCTOBER 2007

1703MYOPIC LASER CORRECTION AND PACHYMETRY WITH 3 DEVICES

Orbscan II). Figure 2 shows scatterplots of Pentacamand Orbscan II measurements against US readings.

In terms of 95% LoA, better agreement with US wasfound for the Pentacam device in preoperative eyesand postoperative eyes (Table 2). Figure 3 shows the95% LoA for each data set and how the width in-creased in postoperative measurements. This increasewas 8 mm for Pentacam readings, 39 mm for Orbscan IIreadings, and 35 mm for corrected Orbscan II readings.

Table 1 also shows the amount of ablation in eacheye, determined by subtracting the preoperative read-ings from postoperative measurements generated by

Figure 1. Boxplots of US, Pentacam, Orbscan II, and corrected Orbs-can II CCT readings in all eyes before and 6 weeks after laser refrac-tive surgery for myopia.

Table 2. Comparison between Pentacam and Orbscan II mea-surements and us measurements.

Exam P � US OII � US OII�.92 � US

PreoperativeCorrelation 0.908 0.925 0.925D mean G SD (mm) �6 G 13 C26 G 16 �21 G 14D range (mm) �40 to C18 �29 to C49 �67 to C2P value .001 !.0001 !.000195% CI �10 to �3 C21 to C30 �25 to �1795% LoA �31 to C19 �5 to C57 �48 to C6

PostoperativeCorrelation 0.951 0.930 0.930D mean G SD (mm) �9 G 15 �1 G 26 �40 G 23D range (mm) �57 to C21 �53 to C53 �89 to C9P value !.0001 .687 !.000195% CI �14 to �6 �8 to C6 �46 to �3395% LoA �39 to C19 �52 to C50 �85 to C5

D Z interdevice difference; CI Z confidence interval; Correlation ZPearson correlation; LOA Z limits of agreement; OII Z Orbscan II;OII�.92 Z corrected with the manufacturer’s suggested acoustic factorof 0.92; P Z Pentacam; US Z ultrasound

J CATARACT REFRACT SURG

each device. The mean reduction in CCT was smallestwith the US pachymeter and largest with the OrbscanII device.

DISCUSSION

To realize the replacement of conventional pachyme-try and topography devices with a single noncontactsystem such as Orbscan II or Pentacam, it is importantto determine the devices’ repeatability and validity inperforming different measurements. The repeatabilityor precision of a measurement device indicates its abil-ity to provide similar values on different occasions.

The validity indicates how well measurementsagree with a standard or acceptable measuring device.In this study, we sought to determine the validity ofCCT measurements by the Orbscan II and Pentacamsystems compared with measurements by a gold stan-dard US device and to assess the effect of myopic abla-tive surgery on interdevice agreements by repeatingthe tests in the same patients 6 weeks after surgery.

Many studies have addressed the repeatability ofcorneal thickness measurements with the Orbscan de-vice, which has been studied independently11,13–15 andin comparison to other pachymeters including opticaland US devices.16–21 The studies agree that despitebeing acceptable for repeated CCT measurements,the repeatability decreases for peripheral pachyme-try.14,15,17,21 For example, Marsich and Bullimore17

compared the repeatability of Orbscan, US, and opticalpachymetry and found Orbscan was the most repeat-able, with 95% LoA of �12 and C10 mm. Nicholset al.19 assessed Orbscan test–retest reliability and re-ported 95% LoA of �13 and C13 mm for CCT mea-surements and an intraclass correlation coefficient of0.97. Sanhermelando et al.13 assessed the interoperatorrepeatability of Orbscan II CCT measurements andfound the device was highly repeatable.

Published studies of the Pentacam system are lim-ited. Barkana et al.22 report excellent repeatabilityand interoperator CCT reproducibility for the Penta-cam system, with 95% LoA of �23 mm and C35 mm.O’Donnell and Maldonado-Codina23 performed mea-surements 48 hours apart and calculated 95% LoA of�24 mm and C21 mm, slightly worse than the repeat-ability of the Allergan-Humphrey US pachymeterused in their study. The repeatability reported by Uca-khan et al.24 was similar to that of the US pachymeterin their study but better than that of the specularmicroscope.

Lackner et al.25 compared the repeatability of thePentacam and Orbscan systems and SP-2000 US pa-chymeter (Tomey) in measuring CCT. The Pentacamdevice had the best between-observer repeatability.

- VOL 33, OCTOBER 2007

1704 MYOPIC LASER CORRECTION AND PACHYMETRY WITH 3 DEVICES

Figure 2. Scatterplots of Pentacam,Orbscan II, and corrected OrbscanII CCT readings against US mea-surements in all eyes before and 6weeks after laser refractive surgeryfor myopia (Table 2). The fit linesare dotted, and the solid lines repre-sent the 45-degree angle.

Similarly, several studies compared the validity ofthe Orbscan method and US pachymetry. In some ofthe studies, the earlier version of the Orbscan wasused7,16,18,26,27 and in others, the Orbscan II.4,6,18,20,28–34

One modification in the more advanced versions ofthe system is the inclusion of an acoustic equivalentcorrection factor for pachymetry to compensate forthe corneal thickness overestimation shown in initialstudies. This correction factor may minimize themean interdevice difference in normal eyes but has lit-tle effect on the variability or thewidth of the 95%LoA.Furthermore, when the same correction factor wasused in the evaluation of post-LASIK or keratoconiceyes, significant underestimation was found.35–38 Ina study by Lackner et al.,25 Pentacam, Orbscan II, andUS measurements in normal eyes were compared.The authors report that in healthy corneas, uncorrected

J CATARACT REFRACT SUR

Orbscan measurements yielded larger CCT valueswhile corrected Orbscan and Pentacammeasurementstended to give more conservative estimates than USpachymetry. As they found Pentacam CCT valuescloser to US measurements, they concluded that thePentacam device produces valid CCT measurements.In the present study, we found the same results in pre-operative eyes; that is, overestimated uncorrectedOrbscan II readings, underestimated corrected Orbs-can II values, and underestimated but closer to USreadings with the Pentacam device. However, to ourknowledge, there are no peer-reviewed publishedstudies comparing the effect of refractive surgery onthe validity of the Pentacam and Orbscan II systems.

In operated eyes, although the uncorrected valuesshowed a mean difference of �1 G 26 mm, the rangeof the differences increased from 78 mm (range �29

G - VOL 33, OCTOBER 2007

1705MYOPIC LASER CORRECTION AND PACHYMETRY WITH 3 DEVICES

Figure 3. Bland-Altman plots ofPentacam, Orbscan II, and cor-rected Orbscan II CCT readingsagainst US measurements in alleyes before and 6 weeks after laserrefractive surgery for myopia. Themiddle line is the mean and thelines on the side represent the up-per and lower 95% LoA.

to C49 mm) to 106 mm (range �53 to C53 mm). As ex-pected, we found an increase in the mean differencebetween corrected Orbscan II measurements and USmeasurements (�21 G 14 mm versus �40 G 23 mm).Apart from the tendency to underestimate CCT mea-surements, postoperative Orbscan II readings showeda wider range of values and 95% LoA; the LoA widthsof uncorrected readings and corrected readings were62 mm and 54 mm, respectively, preoperatively, whichincreased to 101 mmand 90 mm, respectively, postoper-atively. These observations indicate lower interdeviceagreement measurements of operated eyes. In addi-tion, the scatterplots in Figure 2 hint that the OrbscanII device is not only affected by corneal surgical proce-dures but also tends to underestimate CCT in thinnercorneas and overestimate measurements in thicker

J CATARACT REFRACT SUR

ones. Because the participants in this study were laserrefractive surgery candidates, there were no thin cor-neas preoperatively; thus, this tendency is best seenin the postoperative readings, in which the CCT rangewas wider. This may also explain the decreased agree-ment or wider 95% LoA postoperatively. A study ofthe general population and all CCT ranges can furtherclarify this issue.

With the Pentacam device, underestimations wereseen preoperatively and in operated eyes, with a slightincrease in themean difference (�6 G 13 mmversus�9G 15 mm), range of differences (�40 to C18 mm versus�57 to C21 mm), and width of 95% LoA (51 mm versus59 mm). The 95% LoA for the agreement between theUS and Pentacam methods were comparable to thosefound in the repeatability studies mentioned above.

G - VOL 33, OCTOBER 2007

1706 MYOPIC LASER CORRECTION AND PACHYMETRY WITH 3 DEVICES

Compared with Orbscan II readings, the ranges ofmeasurements with the Pentacam device were closerto those of US pachymetry and refractive surgeryhad a much smaller effect on the interdevice agree-ment. However, the small number of cases of eachtype of surgery was a limitation that did not allowus to study the effect on interdevice agreementsseparately.

Overall, one could use the Pentacam device withless skepticism. For example, the upper range of 95%LoA for PentacammeasurementswasC19 mm regard-less of the condition of the eye; thus, there is only a 5%probability that the Pentacam would overestimate theCCT by more than 19 mm, just as a repeated measure-ment might.22–24 However, Orbscan II readings de-pend on the acoustic equivalent correction factor onwhich it is set. Without a correction factor, 5% of casesmight be overestimatedmore than 57 mm (50 mm in op-erated eyes). When the manufacturer-suggested 0.92correction factor is used, these figures could be assmall as 5 or 6 mm, but there is a great chance of under-estimation of up to 48 mm and 85 mm in 95% of preop-erative eyes and operated eyes, respectively.

In some clinical situations, a small measurementerror may result in big differences. For example, inscreening for refractive surgery, corneal thickness un-derestimation may lead to the exclusion of potentiallyeligible cases and overestimation can result in compli-cations ensuing from excessive corneal thinning. Be-cause underestimations and overestimations wereseen with both devices, it seems unlikely that a single,simple, linear transformation can lead us to preciseand interchangeable data and one must always bearsuch possibilities in mind. Further studies are neededto determine the cause of such discrepancies, includ-ing the effect of corneal thickness itself.

REFERENCES1. Pflugfelder SC, Liu Z, Feuer W, Verm A. Corneal thickness indi-

ces discriminate between keratoconus and contact lens-induced

corneal thinning. Ophthalmology 2002; 109:2336–2341

2. Moezzi AM, Fonn D, Simpson TL, Sorbara L. Contact lens-in-

duced corneal swelling and surface changes measured with

the Orbscan II corneal topographer. Optom Vis Sci 2004;

81:189–193

3. Price FW Jr, Koller DL, Price MO. Central corneal pachymetry in

patients undergoing laser in situ keratomileusis. Ophthalmology

1999; 106:2216–2220

4. Javaloy J, Vidal MT, Villada JR, et al. Comparison of four corneal

pachymetry techniques in corneal refractive surgery. J Refract

Surg 2004; 20:29–34

5. Swartz T, Liu Z, Yang X, et al. Topographic technologies. In:

Wang M, ed, Corneal Topography in the Wavefront Era; a Guide

for Clinical Application. Thorofare, NJ, Slack, 2006; 31–39

6. Iskander NG, Anderson Penno E, Peters NT, et al. Accuracy of

Orbscan pachymetry measurements and DHG ultrasound pa-

chymetry in primary laser in situ keratomileusis and LASIK

J CATARACT REFRACT SU

enhancement procedures. J Cataract Refract Surg 2001;

27:681–685

7. Chakrabarti HS, Craig JP, Brahma A, et al. Comparison of cor-

neal thickness measurements using ultrasound and Orbscan

slit-scanning topography in normal and post-LASIK eyes. J Cat-

aract Refract Surg 2001; 27:1823–1828

8. Sallet G. Comparison of optical and ultrasound central corneal

pachymetry. Bull Soc Belge Ophtalmol 2001; 281:35–38; Avail-

able at: www.ophthalmologia.be/download.php?dof_idZ115;

Accessed June 13, 2007

9. Giessler S, Duncker GIW. Orbscan pachymetry after LASIK is

not reliable [letter]. J Refract Surg 2001; 17:385–387

10. Boscia F, La Tegola MG, Alessio G, Sborgia C. Accuracy of

Orbscan optical pachymetry in corneas with haze. J Cataract

Refract Surg 2002; 28:253–258

11. Lattimore MR Jr, Kaupp S, Schallhorn S, Lewis R IV. Orbscan

pachymetry; implications of a repeated measures and diurnal

variation analysis. Ophthalmology 1999; 106:977–981

12. Bland JM, Altman DG. Statistical methods for assessing agree-

ment between two methods of clinical measurement. Lancet

1986; 1:307–310

13. Sanhermelando MV, Lleo A, Alonso L, et al. Repeatability of cen-

tral corneal thickness and ocular anterior chamber depth mea-

surements with the Orbscan topography system. Eur J Anat

2002; 6:59–64; Available at: http://www.sociedadanatomica.es/

EuropeanJournalofAnatomy/SUMMARIES/4022D99E-E846-

4D7E-89AC-15D5636F8343_files/EJA_V6_N2_01.pdf. Accessed

June 13, 2007

14. Fam H-B, Lim K-L, Reinstein DZ. Orbscan global pachymetry: ana-

lysis of repeated measures. Optom Vis Sci 2005; 82:1047–1053

15. Rabsilber TM, Becker KA, Auffarth GU. Reliability of Orbscan II

topography measurements in relation to refractive status. J Cat-

aract Refract Surg 2005; 31:1607–1613

16. Yaylali V, Kaufman SC, Thompson HW. Corneal thickness mea-

surements with the Orbscan Topography System and ultrasonic

pachymetry. J Cataract Refract Surg 1997; 23:1345–1350

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

ness measures. Cornea 2000; 19:792–795

18. Wong AC-M, Wong C-C, Yuen NS-Y, Hui S-P. Correlational

study of central corneal thickness measurements on Hong

Kong Chinese using optical coherence tomography, Orbscan

and ultrasound pachymetry. Eye 2002; 16:715–721

19. Nichols JJ, Kosunick GM, Bullimore MA. Reliability of corneal

thickness and endothelial cell density measures. J Refract

Surg 2003; 19:344–352

20. 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

21. Jonuscheit S, Doughty MJ. Regional repeatability measures of

corneal thickness: Orbscan II and ultrasound. Optom Vis Sci

2007; 84:52–58

22. Barkana Y, Gerber Y, Elbaz U, et al. Central corneal thickness

measurement with the Pentacam Scheimpflug system, optical

low-coherence reflectometry pachymeter, and ultrasound

pachymetry. J Cataract Refract Surg 2005; 31:1729–1735

23. O’Donnell C, Maldonado-Codina C. Agreement and repeatabil-

ity of central thickness measurement in normal corneas using ul-

trasound pachymetry and the OCULUS Pentacam. Cornea

2005; 24:920–924

24. Ucakhan OO, Ozkan M, Kanpolat A. Corneal thickness mea-

surements in normal and keratoconic eyes: Pentacam compre-

hensive eye scanner versus noncontact specular microscopy

and ultrasound pachymetry. J Cataract Refract Surg 2006;

32:970–977

RG - VOL 33, OCTOBER 2007

1707MYOPIC LASER CORRECTION AND PACHYMETRY WITH 3 DEVICES

25. Lackner B, Schmidinger G, Pieh S, et al. Repeatability and

reproducibility of central corneal thickness measurement with

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

82:892–899

26. Modis L Jr, Langenbucher A, Seitz B. Scanning-slit and specular

microscopic pachymetry in comparison with ultrasonic determi-

nation of corneal thickness. Cornea 2001; 20:711–714

27. Rainer G, Findl O, Petternel V, et al. Central corneal thickness

measurements with partial coherence interferometry, ultra-

sound, and the Orbscan system. Ophthalmology 2004;

111:875–879

28. Fakhry MA, Artola A, Belda JI, et al. Comparison of corneal

pachymetry using ultrasound and Orbscan II. J Cataract Refract

Surg 2002; 28:248–252

29. Gonzalez-Meijome JM, Cervino A, Yebra-Pimentel E,

Parafita MA. Central and peripheral corneal thickness measure-

ment with Orbscan II and topographical ultrasound pachymetry.

J Cataract Refract Surg 2003; 29:125–132

30. McLaren JW, Nau CB, Erie JC, Bourne WM. Corneal thickness

measurement by confocal microscopy, ultrasound, and scan-

ning slit methods. Am J Ophthalmol 2004; 137:1011–1020

31. Gherghel D, Hosking SL, Mantry S, et al. Corneal pachymetry in

normal and keratoconic eyes: Orbscan II versus ultrasound.

J Cataract Refract Surg 2004; 30:1272–1277

32. Hashemi H, Yazdani K, Mehravaran S, Fotouhi A. Central cor-

neal thickness measurement with Orbscan II and an ultrasound

pachymeter. J Iran Soc Ophthalmol 2004; 4(17):1–9

33. Basmak H, Sahin A, Yildirim N. The reliability of central corneal

thickness measurements by ultrasound and by Orbscan system

in schoolchildren. Curr Eye Res 2006; 31:569–575

J CATARACT REFRACT SURG

34. Thomas J, Wang J, Rollins AM, Sturm J. Comparison of corneal

thickness measured with optical coherence tomography, ultra-

sonic pachymetry, and a scanning slit method. J Refract Surg

2006; 22:671–678

35. Kawana K, Tokunaga T, Miyata K, et al. Comparison of corneal

thickness measurements using Orbscan II, non-contact specu-

lar microscopy, and ultrasonic pachymetry in eyes after laser

in situ keratomileusis. Br J Ophthalmol 2004; 88:466–468

36. Kawana K, Miyata K, Tokunaga T, et al. Central corneal thick-

ness measurements using Orbscan II scanning slit topography,

noncontact specular microscopy, and ultrasonic pachymetry in

eyes with keratoconus. Cornea 2005; 24:967–971

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

ment with Orbscan II in postoperative patients with myopic

LASIK. J Refract Surg 2006; 22:363–366

38. Haque S, Simpson T, Jones L. Corneal and epithelial thickness

in keratoconus: a comparison of ultrasonic pachymetry, Orbs-

can II, and optical coherence tomography. J Refract Surg

2006; 22:486–493

First author:Hassan Hashemi, MD

Farabi Eye Hospital, Departmentof Ophthalmology, School of Medicine,Tehran University of Medical Sciences,and Noor Ophthalmology Research Center,Noor Eye Hospital, Tehran, Iran

- VOL 33, OCTOBER 2007