Corneal Thickness Measurements WithContact and Noncontact Specular Microscopic

and Ultrasonic Pachymetry

LASZLO MODIS, JR., MD, PHD, ACHIM LANGENBUCHER, PHD, ANDBERTHOLD SEITZ, MD, EBOD

● PURPOSE: To evaluate the central corneal thicknessvalues in normal and postkeratoplasty corneas with thenew Topcon SP-2000P noncontact specular micro-scopic, contact specular microscopic, and the “commonstandard” ultrasonic pachymetry.● METHODS: Central corneal thickness was determinedin 119 eyes of 81 patients (73 normal eyes of 44 patientsand 46 eyes after penetrating keratoplasty) first with anoncontact specular microscopic (Topcon SP-2000P;Topcon Corporation, Tokyo, Japan), then an ultrasonic(AL-1000; Tomey, Erlangen, Germany), and finallywith a contact specular microscopic (EM-1000; Tomey,Erlangen, Germany) pachymetry two times each by thesame investigator.● RESULTS: Reliability of the central corneal measure-ments was equally high both in normal and in postkera-toplasty corneas with all of the instruments (Cronbachalpha � 0.99). Noncontact specular microscopic cornealthickness determination correlated significantly bothwith ultrasonic (r � .86, P < .0001) and contactspecular microscopic pachymetry (r � .62, P < .0001).The ultrasonic pachymetry correlated well with theTomey pachymetry (r � .69, P < .0001). The Topconnormal mean central corneal thickness value (542 � 46�m) was 28 � 4 �m lower (P < .0001) compared withthe ultrasonic data (570 � 42 �m), which was 68 � 1�m lower (P < .0001) compared with Tomey thickness(638 � 43 �m).

● CONCLUSIONS: Central corneal thickness measure-ments with noncontact specular microscopic, contactspecular microscopic, and ultrasonic pachymetry demon-strate that each of the instruments is reliable but cannotbe simply used interchangeably. (Am J Ophthalmol2001;132:517–521. © 2001 by Elsevier Science Inc. Allrights reserved.)

E VALUATION OF CORNEAL THICKNESS IS IMPORTANT

in a wide range of disorders, such as ectatic dystro-phies,1,2 contact-lens–related complications,3,4 glau-

coma,5–7 dry eye,8,9 and diabetes mellitus.10,11 The designand outcome of refractive surgical procedures also rely onthe accuracy of pachymetry measurements, especiallywhen the laser in situ keratomileusis procedure is per-formed.

Currently, slit-lamp mounted optical, ultrasound, andspecular microscopic pachymetry are widely used andaccepted methods to measure corneal thickness.12–14 Mea-surements can be obtained after reflection of light orultrasound from the anterior and posterior surface of thecornea.

Recently, a new, automated noncontact specular mi-croscopy has been introduced to evaluate the endothelialstatus and to determine corneal thickness. The TopconSP-2000P noncontact specular microscope (Topcon Cor-poration, Tokyo, Japan) is capable of specular microscopyand pachymetry at the same time. Focusing on the endo-thelium, this machine provides specular images and mea-sures the focal distance, which can be calculated as cornealthickness. This noncontact specular microscope recordsendothelial and thickness information on the central andon the midperipheral cornea in the superior, inferior,nasal, and temporal directions 3 mm from the center.

The purpose of this study was to determine centralcorneal thickness of normal human and postkeratoplastycorneas with three instruments. Contact and noncontactspecular thickness measurements were compared with the“common standard” of ultrasonic pachymetry.

Accepted for publication Jun 5, 2001.From the Department of Ophthalmology, Medical and Health Science

Centre, University of Debrecen (L.M.), Debrecen, Hungary, and theDepartment of Ophthalmology, University of Erlangen-Nurnberg, Erlan-gen, Germany.

This study was supported by the Hungarian Eotvos Fellowship, Minis-try of Education, Budapest, Hungary, and the Gertrude Kusen-Stiftung,Hamburg, Germany.

Requests for reprints to Laszlo Modis, MD, PhD, Department ofOphthalmology, Medical and Health Science Center, University ofDebrecen, H-4012 Debrecen, Nagyerdei krt 98, Hungary; fax: �36 (52)415816; e-mail: lmodis@dragon.klte.hu

© 2001 BY ELSEVIER SCIENCE INC. ALL RIGHTS RESERVED.0002-9394/01/$20.00 517PII S0002-9394(01)01109-6

METHODS

CENTRAL CORNEAL THICKNESS WAS DETERMINED IN 119

eyes of 81 patients (73 normal eyes of 44 patients, meanage 66 � 18 years and 46 penetrating corneal grafts of 37patients, mean age 52 � 18 years) as a part of routineclinical examination at the Department of Ophthalmol-ogy, University of Erlangen-Nurnberg, Erlangen, Ger-many. In the following sequence noncontact specularmicroscopic (Topcon SP-2000P; Topcon Corporation, To-kyo, Japan), ultrasonic (AL-1000; Tomey, Erlangen, Ger-many), and contact specular microscopic (EM-1000;Tomey, Erlangen, Germany) pachymetry were performedtwo times on each eye by the same investigator. A randomgroup of healthy eyes was selected in patients who hadnegative history and no sign of previous or present oculardisease and surgery. Contact lens wearers were excludedfrom the study. Postkeratoplasty corneas were clear onslit-lamp examination, and no graft rejections were re-corded. The indication for keratoplasty was as follows:keratoconus (n � 28), Fuchs dystrophy (n � 11), stromaldystrophy (n � 3), avascular scar after injury (n � 2), andpseudophakic bullous keratopathy (n � 2). The meanfollow-up time after surgery ranged from 1 to 360 monthswith a mean of 48 months (median, 24 months). Thisstudy was designed as an observational case series, andcases were collected prospectively over a 3-month periodat the beginning of year 2000.

For the Topcon SP-2000P machine, the patient’s headwas positioned similar to a slit lamp and the patient wasinstructed to look straight ahead into the built-in fixationtargets. Images can be taken with high and low flashintensity, in manual and in automated mode. In this studyauto mode low flash intensity pictures from the center ofthe cornea were taken. After the proper position of thealignment dot, circle, and bar on the screen, the picturewas captured and printed out with numerical data, includ-ing thickness. Patients were asked to remove their headfrom the chin holder, blink, and a second measurementwas performed.

For contact pachymetry, the cornea was anesthetizedwith topical proparacaine hydrochloride. The ultrasoundprobe and the specular cone were sterilized with alcoholafter each subject. For ultrasound pachymetry, the patientwas asked to look straight ahead to a fixation target, andthe probe was applied perpendicularly to the centralcorneal surface. After a blink, a second measurement wasobtained. The last thickness evaluation was performedwith contact specular microscopy. Patient position, fixa-tion, and the condition of the repeated snapshots weresimilar to the noncontact microscope. The endotheliumwas focused sharply, focus values indicating thickness wereread from the monitor, and then data were stored on thecomputer.15

Subjects were divided into total, normal, and postkera-toplasty subgroups. Descriptive statistical results were dis-

played as mean, median for follow-up, standard deviation,minimum, and maximum values. Wilcoxon paired andMann-Whitney U unpaired sample test were applied forcomparison between groups or variables. For bivariatecorrelation analysis, Pearson’s rank correlation coefficient(r) was used. The Cronbach-alpha test was performed tocompare the reliability (alpha) of measurements. Reliabil-ity refers to the values obtained in each of the two tests oneach patient with the same instrument. A P value of .05 orless was considered statistically significant.

RESULTS

RELIABILITY OF CENTRAL CORNEAL THICKNESS MEASURE-

ments was comparable in normal and in postkeratoplastycorneas with all of the tested instruments (alpha � 0.99).

Noncontact specular microscopic corneal thickness de-termination correlated significantly both with ultrasonic(r � .86, P � .0001; Figure 1) and contact specularmicroscopic pachymetry (r � .62, P � .0001; Figure 2).Likewise, the ultrasonic pachymetry correlated well withthe Tomey pachymetry (r � .69, P � .0001; Figure 3).

The mean central thickness data of the cornea asdetermined with different devices and different groups arelisted in Table 1. The normal mean central cornealthickness was 542 � 46 �m with the noncontact specularinstrument, and increasing values were obtained withultrasonic (570 � 42 �m) and contact specular micro-scopic pachymetry (638 � 43 �m). The noncontactspecular method resulted in 28 � 4 �m lower mean value(P � .0001), and the contact specular method resulted in

FIGURE 1. Correlation between noncontact specular micro-scopic and ultrasonic pachymetry measurements (r � .86, P <.0001; n � 238).

AMERICAN JOURNAL OF OPHTHALMOLOGY518 OCTOBER 2001

68 � 1 �m higher mean value than ultrasonic pachmetry(P � .0001) in normal subjects.

The tendency was similar in the postkeratoplasty eyes.Noncontact specular microscopic measurements disclosedthe lowest results (545 � 58 �m) followed by ultrasonic(573 � 53 �m) and contact specular pachymetry values

(635 � 43 �m). The ultrasonic mean value was 28 � 5�m thicker compared with the noncontact specular mi-croscopic (P � .0001) and was 62 � 10 �m thinnercompared with the mean contact specular microscopicthickness (P � .0001) in postkeratoplasty eyes.

There was no statistically significant correlation be-tween age and thickness with noncontact (r � .06, P �.69), ultrasonic (r � .04, P � .76), and contact specularmicroscopic pachymetry (r � .06, P � .69) in normalsubjects. Patients’ ages varied within a certain limit (66 �18 years); therefore, age-related thickness alterations couldnot be detected. In postkeratoplasty eyes corneal thicknessdid not correlate with the follow-up period (noncontact[r � .02, P � .85], ultrasound [r � .004, P � .97], andcontact specular microscope [r � .15, P � .30]).

DISCUSSION

EARLIER STUDIES PROVED THAT SLIT-LAMP MOUNTED OP-

tical thickness determination compared with specular mi-croscopic and ultrasonic pachymetry disclosed higherinterobserver variations, less reproducibility, and providedsubjective measurements in healthy subjects.16–18

Recently, a wide range of new and sophisticated instru-ments has been developed for the determination of cornealthickness, such as the different optical laser interferom-eters,19–21 confocal microscope,22,23 ultrasonic biomicro-scope,24 scanning slit pachymeter,25,26 and the TopconSP-2000P noncontact specular microscope.14

The present study evaluated central corneal thicknessvalues in normal and postkeratoplasty corneas with threeinstruments. Measurements were obtained with the newnoncontact specular device, the Tomey contact specularmicroscopic pachymetry, and were compared with ultra-sonic values as the “common standard.”

The different pachymeters used in this study providedreliable measurements (alpha � 0.99) within a similarrange of standard deviation. However, the results indicatedthat the different instruments did not result in comparablethickness values. The noncontact specular microscopicmeasurements were significantly smaller than the ultra-sound results, which were significantly smaller than thecontact specular microscopic values. This tendency waspresent in normal and in postkeratoplasty eyes.

Other studies also disclosed the reliability of differentultrasonic and specular microscopic instruments, but theydocumented significantly different results comparing thedifferent pachymetry devices.13,14,27,28 A comparative re-port by Wheeler13 demonstrated that ultrasonic thicknessdata were significantly higher compared with contactspecular microscopic values. In a recent paper ultrasonicpachymetry also resulted in a significant 32-�m thickervalue than those obtained by the Topcon SP-2000Pnoncontact specular microscope.14

The present observations are consistent with these

FIGURE 2. Correlation between contact and noncontact spec-ular microscopic pachymetry values (r � .62, P < .0001; n �238).

FIGURE 3. Correlation between contact specular microscopicand ultrasonic pachymetry values (r � .69, P < .0001; n �238).

SPECULAR MICROSCOPIC AND ULTRASONIC PACHYMETRYVOL. 132, NO. 4 519

findings and confirm that these devices are reliable in theirmeasurements but cannot simply be used interchangeably.The results further confirm the validation of pachymetrydevices but additionally point out the greatest weakness intoday’s pachymeters: good precision with uncertain accu-racy. Therefore, the main clinical relevance of the presentstudy is that for refractive procedures and for long-termpatient’s follow-up one certain instrument is recom-mended.

The differences between devices can result from theirdistinct operating principles. However, the basic operationtheory is common, the reflection of light or ultrasoundfrom anterior and posterior corneal surfaces. In cases ofultrasound pachymetry the exact posterior corneal reflec-tion point is not known; it may be located between theDescemet membrane and the anterior chamber.29 It wasalso proven, that the ultrasound probe can easily displacethe 7-�m to 40-�m-thick tear film away, and even theepithelium can be thinned with examination probe.29

However, recent studies have not confirmed this lattertheory.30

There are several advantages and disadvantages of thedifferent pachymeters. The specular microscopes alsoprovide extra information on corneal status, such asendothelial cell density and morphology. In cases ofsemitransparent or opaque corneas the image capture isdifficult or impossible, representing a limitation of bothspecular microscopes. Contact pachymetry devices requiretopical anesthetics and corneal contact with the possiblerisk of epithelial defects and iatrogenic infections. Centralthickness measurements can be evaluated quickly in mostcases with ultrasonic pachymetry. However, in certainpathologic conditions the speed of ultrasound may vary indifferent hydration levels of the cornea.16,29

In summary, the present study evaluated normal andpostkeratoplasty central corneal thickness measurementswith three different devices. The mean thickness valuesdiffered significantly, and increasing thickness was ob-served with the noncontact specular microscopic, ultra-sonic, and contact specular microscopic pachymetry inboth subgroups. The results indicate that these instrumentsare reliable but cannot be simply used interchangeably.

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TABLE 1. Central Corneal Thickness in Normal, Keratoplasty, and Total Number of Patients

Normal

(n � 146)

Keratoplasty

(n � 92)

Total

(n � 238)

Mean � SD (minimum/maximum) (�m)

Noncontact specular microscope 542 � 46 (436/667) 545 � 58 (411/705) 543 � 51 (411/705)

Ultrasound 570 � 42 (480/690) 573 � 53 (498/737) 571 � 47 (480/737)

Contact specular microscope 638 � 43 (530/720) 635 � 43 (560/770) 636 � 43 (530/770)

AMERICAN JOURNAL OF OPHTHALMOLOGY520 OCTOBER 2001

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The full-text of AJO is now available online at www.ajo.com. AuthorsInteractive�, currently available in limited form, is undergoing an upgrade.

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