J CATARACT REFRACT SURG - VOL 32, MARCH 2006

Comparison of central corneal thickness

measurements with a new optical device

and a standard ultrasonic pachymeter

Gabor Nemeth, MD, Alexis Tsorbatzoglou, MD, Katalin Kertesz, MD, Attila Vajas, MD,

Andras Berta, MD, PhD, DSci, Laszlo Modis Jr, MD, PhD

PURPOSE: To compare central corneal thickness (CCT) values obtained with ultrasonic pachymetryand a new optical method using partial coherence interferometry (PCI).

SETTING: Department of Ophthalmology, Medical Health and Science Center, University of Debrecen,Debrecen, Hungary.

METHODS: The study comprised 136 eyes of 70 patients whose spherical refractive error was notgreater than G6.0 diopters (D) and whose keratometric astigmatism was not greater than 2.0 D.Central corneal thickness was measured 5 times with a new optical device (ACMaster, Zeiss) andwith an ultrasonic pachymeter (AL-2000, Tomey). All measurements were obtained by the sameinvestigator.

RESULTS: Mean CCT was 531.2 mm G 3.9 (SD) with PCI and 547.8 G 36.0 mm with the ultrasonicdevice. The difference between groups was significant (P Z .001). There was no difference betweenCCT values measured in right and left eyes (P Z .55) with ultrasonography and PCI (P Z .67). Thecoefficient variation was 0.73% for PCI and 6.5% for ultrasonography. Correlation between the CCTmeasurements with both devices was strong and statistically significant (Spearman correlation Z .91,P Z .001).

CONCLUSIONS: Mean CCT values measured by the PCI method were significantly smaller than thosemeasured by the ultrasonic device. Central corneal thickness measured with PCI is more reproducibleand seems to be more reliable than that measured by ultrasonography.

J Cataract Refract Surg 2006; 32:460–463 Q 2006 ASCRS and ESCRS

Corneal thickness measurement has become an important

factor in most types of anterior segment surgery in the eval-

uation of corneal dehydrating function and for determining

intraocular pressure. To avoid complications in refractive

surgery such as keratectasia after laser in situ keratomileu-

sis or perforation in photorefractive keratotomy, accurate

corneal thickness measurements are also important.1–5

Accepted for publication August 5, 2005.

From the Department of Ophthalmology, Medical and HealthScience Center, University of Debrecen, Debrecen, Hungary.

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

Reprint requests to Gabor Nemeth, MD, Department of Ophthal-mology, University of Debrecen, Nagyerdei Blvd. 98, H-4012 De-brecen, Hungary. E-mail: nemeth222@yahoo.com.

Q 2006 ASCRS and ESCRS

Published by Elsevier Inc.

460

For decades, the standard technique of central corneal

thickness (CCT) measurement was the contact ultrasono-

graphic method; however, differences in corneal thickness

measurements have been reported between ultrasonic pa-

chymeter devices.6,7 The aim of this study was to evaluate

CCT by 2 different methods: a new device using noncontact

partial coherence interferometry (PCI) and the standardcontact ultrasound (US) method.

PATIENTS AND METHODS

The study comprised 136 eyes of 70 patients (70 right and 66left eyes). The mean age of the patients was 66.2 years G 11.3(SD) (range 36 to 86 years) with a male-to-female ratio of36:34. Central corneal thickness was measured with a new devicethat used the PCI method (ACMaster, Zeiss) and with an ultra-sonic pachymeter (AL-2000, Tomey). Patients with a history ofwearing contact lenses, a spherical refractive error greater thanG6.0 diopters (D), keratometric astigmatism greater than 2.0 D,

0886-3350/06/$-see front matterdoi:10.1016/j.jcrs.2005.12.138

CCT MEASUREMENTS BY PCI AND US

or any anterior segment abnormalities were excluded from thestudy.

Five separate, sequential measurements were performed by 1examiner, first with the noncontact PCI device and then with theUS device. The mechanism of the PCI method has been de-scribed.8–10 For the PCI method, the patient assumed a sittingposition, placing the chin on a chin rest and the forehead intoa headband. The patient was instructed to keep both eyes openand to focus the eye being tested on the built-in measurement tar-get of the device.

For the US method, the corneas were anesthetized with top-ical tetracaine hydrochloride. Patients were again asked to sit andlook straight at a target on the wall. The probe tip of the ultrasonicpachymeter was applied to the central cornea perpendicularly andthe surface of the cornea was slightly touched. The probe tip wassterilized with alcohol after use on each patient.

Statistical analysis was performed and CCT data were de-scribed in terms of means and standard deviations. The intraob-server variability was described with a coefficient of variation (CV).Differences in CCT values between the devices were recordedwith the paired test of Wilcoxon, and a P value of .01 was consid-ered the level of significance. The association between groups wasdescribed with the Spearman correlation.

RESULTS

The mean CCT was 531.2 G 3.9 mm when measured

by the PCI device and 547.8 G 36.0 mm when measured

by US pachymetry. The difference between the CCT values

was strongly significant (P Z.001). There was no significant

difference between the right and left eyes (PCI, P Z.67; US,

P Z.55) (Table 1). Figure 1 shows the measured CCTs.

Intraobserver variability was lower with the PCI

method than with the US technique (CV 0.73% and 6.5%,respectively). There were no differences in intraobserver

variability between right and left eyes with PCI (CV 0.8%

versus 0.72%) or US (CV 6.5% versus 6.58%). The correla-

tion between pachymetric devices was strong, as shown in

Figure 2 (r Z .91; P Z .001).

DISCUSSION

Ophthalmic biometry should provide rapid, objective,

and accurate measurements of different ocular parameters.

Change in corneal thickness is an important sign of wors-ening function of the corneal endothelium. In corneal re-

fractive surgery, a high degree of accuracy in corneal

thickness measurement is required.3–5

Table 1. Mean central corneal thickness values with PCI and with

ultrasonic pachymeter.

Eye PCI Ultrasonography

Right 533.6 G 4.3 mm 545.9 G 35.9 mmLeft 528.9 G 3.8 mm 549.5 G 36.2 mm

J CATARACT REFRACT SURG

Ultrasound pachymetry has been the standard methodof evaluating corneal thickness for the past few decades.

Numerous new pachymetric techniques are now available,

although ultrasonic pachymetry is still the common stan-

dard. Many studies have compared CCT measurement

using different methods and devices (eg, specular micro-

scope, optical coherence tomography, ultrasound biomicro-

scope, Orbscan (Bausch & Lomb), and devices based on

PCI).7,11–16

In this study, CCT was measured with a new noncon-

tact ophthalmic device developed for the anterior segment

of the eye using the PCI method, and the results were com-

pared with measurements obtained using the standard

ultrasonic pachymetry technique. The mean CCT values

obtained with the noncontact optical method were smaller

than those derived from the ultrasonic method. The differ-

ences in CCT values were statistically significant (P Z.001).The unquestionable advantage of the PCI technique is

the noncontact method. The noncontact (ie, noninvasive)

pachymetry method has important advantages besides its

short measuring time. There is no need for anesthesia

and no risk for corneal infection, which ensure a high de-

gree of patient comfort.

Giasson and Forthomme17 report significant left–right

differences in ultrasonic CCT measurements due to thehandedness of the examiner; ie, left corneal thickness was

measured significantly larger by a right-handed examiner.

This phenomenon has been reported in several studies

and has been attributed to measurements not being per-

formed perpendicularly to the corneal surface.11 In our

Figure 1. The CCT values measured with the ACMaster and AL-2000 ultra-

sonic pachymeters. The mean CCTs are presented with continuous (US)

and dotted lines (PCI).

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CCT MEASUREMENTS BY PCI AND US

Figure 2. Mean CCT values obtained with PCI and

ultrasonic pachymetry.

study, the investigator sat on the left side of the patient

while measuring CCT with the ultrasonic device. There-

fore, measuring CCTof the right eye was somewhat compli-

cated, but this factor did not generate large statistical

differences between right and left CCT values (P Z .55).One possible explanation for this finding is that our ultra-

sonic device measures the CCT in an automated mode only

when the probe tip is perpendicular to the corneal surface.

Some authors postulate additional reasons for the

larger corneal thickness values measured with ultrasonic

pachymeters, including ultrasonic probe tip tilting, epithe-

lial edema from local anesthesia, repeated pressure of the

cornea,18,19 and inadequate calibration of the devices.Others suspect that the speed of sound varies between

different layers of the cornea and between normal and

pathologic corneal tissues.6,20,21 Some authors recommend

using a modified refractive index for optical coherence

J CATARACT REFRACT SURG462

tomography (OCT) measurements (n Z 1.365) and a mod-

ified velocity of sound (1664 m/s) for ultrasonic CCT mea-

surements.11 We measured the CCT with ultrasonography

at the sound velocity of 1640 m/s.

Mean corneal thickness values obtained with opticalpachymeters (PCI method), OCT, and Orbscan were signif-

icantly less than values obtained with US pachymetry, sug-

gesting that US-based devices slightly overestimate corneal

thickness.11–14,16,22 These differences might be clinically

relevant. However, the reason for the significantly smaller

CCT values measured with the PCI method, OCT, and

Orbscan remains unknown. Other studies mention larger

CCTs obtained with specular microscopy, US biomicros-copy, and findings opposite the above-mentioned Orbscan

findings.13 Moreover, it is impossible to state whether the

optical or the US measurements are closer to the actual

corneal thickness. The measured CCTs depend on the

Table 2. Differences between CCT values measured with different methods and devices and recorded as extracted value of the 2. method by the 1. method.

Author* 1. Method 2. Method Micrometer Difference (2.-1. method)

Bechmann22 OCT Ultrasonography 50Modis15 Noncontact specular Ultrasonography 28Rainer18 PCI Ultrasonography 21.5Rainer18 Orbscan Ultrasonography 20Wheeler7 Specular Ultrasonography 20Fishman12 OCT Ultrasonography 12Wirbelauer24 OCT Ultrasonography 8Modis15 Ultrasonography Specular 68Fishman12 Ultrasonography Orbscan 40Tam14 Ultrasonography Specular 22Tam14 Ultrasonography Ultrasound biomicroscopy 5

Present study PCI Ultrasonography 16.6

OCT Z optical coherence tomography; PCI Z partial coherence interferometry

*First author

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CCT MEASUREMENTS BY PCI AND US

refractive index of the cornea and the US velocity in the cor-

nea. In our study, we compared the CCT values obtained

using a new optical device and an ultrasonic device. The

mean CCTwas 16.6 mm greater when measured with ultra-

sonography than when measured with the PCI method.

This is consistent with data derived from previous studies(Table 2).11–16,22

The intraobserver reproducibility represents the vari-

ability of values in a series of measurements (all of our mea-

surements were taken by the same investigator). The results

of our study show that the intraobserver variability with the

PCI method is almost one-tenth smaller than with the US

method (CV 0.73% versus 6.5%). This contradicts the liter-

ature in which these data are comparable. However, somedata obtained with the Orbscan system show even greater

variability. This fact confirms the accuracy and reproduc-

ibility of CCT values observed with the PCI method.13,23

A high positive correlation coefficient was obtained be-

tween the 2 methods of central corneal pachymetry (r Z0.91; P Z .001), suggesting that the CCT values of these

methods are count over to each other, but are not

interchangeable.In conclusion, we have shown that the CCT values

measured by the ultrasonic pachymeter are higher than

those obtained by the PCI method. The differences between

optical and ultrasonic pachymetry measurements were sta-

tistically significant. The accuracy of these pachymetric de-

vices is acceptable in measuring CCT. Further studies are

required to confirm these findings and to evaluate the accu-

racy, reproducibility, and independence of corneal pachy-metry research using the PCI method.

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