Seminars in Ophthalmology, Early Online, 1–7, 2013! Informa Healthcare USA, Inc.

ISSN: 0882-0538 print / 1744-5205 online

DOI: 10.3109/08820538.2013.833264

ORIGINAL ARTICLE

Comparison of Central Corneal ThicknessMeasurements with a Rotating Scheimpflug Camera, a

Specular Microscope, Optical Low-CoherenceReflectometry, and Ultrasound Pachymetry in

Keratoconic Eyes

Yasin Cinar1, Abdullah Kursat Cingu1, Fatih Mehmet Turkcu1, Tuba Cinar2,Alparslan Sahin1, Harun Yuksel1, and Seyhmus Ari1

1Department of Ophthalmology, Faculty of Medicine, Dicle University, Diyarbakir, Turkey and2Diyarbakir Children Hospital, Ophthalmology Clinic, Diyarbakir, Turkey

ABSTRACT

Objective: To compare central corneal thickness (CCT) measurements with a rotating Scheimpflug camera (RSC),noncontact specular microscopy (SM), optical low-coherence reflectometry (OLCR), and ultrasonic pachymetry(UP) in keratoconus (KC) patients. Method: In this prospective study, four CCT measurements taken with anRSC, SM, OLCR, and UP were compared in 81 eyes of 44 consecutive KC patients. The KC patients weredivided into four subgroups according to Amsler-Krumeich’s KC classification. Results: The RSC and UPmeasurements of the CCT were not statistically significant in all the groups. Comparison of the SM vs. theOLCR measurements yielded statistically significant differences in all the KC patients and in all KC stages. Inall the KC patients, RSC and OLCR showed a high correlation coefficient factor (r = 0.87, p = 0.000). Conclusion:CCT measurements with RSC are comparable to those achieved with UP. Compared with the other devices,according to SM measurements, the central cornea is thicker in all keratoconic eyes and in all KC grades, and itis thinner according to OLCR. RSC, UP, SM, and OLCR should not be used interchangeably in keratoconic eyes.

Keywords: Central corneal thickness, keratoconus, pachymetry, scheimpflug camera, reflectometry

INTRODUCTION

Keratoconus (KC) is a bilateral noninflammatorydisorder characterized by progressive corneal thin-ning and steepening that results in induced myopiaand astigmatism. The diagnosis of KC in patients withclassical biomicroscopic findings or retinoscopic signsis not difficult. However, it is challenging in early KCor subclinical KC patients with good visual acuity anddelicate signs on examination. In progressive KC,early detection of corneal thinning might be importantfor patients who could be considered for collagencrosslinking treatment or intracorneal ring segmentimplantation.

Various techniques are available to measure thecentral corneal thickness (CCT), including ultrasonicpachymetry (UP), optical pachymetry,1 ultrasoundbiomicroscopy,2 slit scanning corneal topography,3–4

optical coherence tomography (OCT),5 confocalmicroscopy,6 optical low-coherence reflectometry(OLCR),7 contact or noncontact specular microscopy(SM),8 and dual-beam partial coherenceinterferometry.9

UP measures corneal thickness using the reflectionof sound waves, whereas SM and OLCR utilize lightreflection. OLCR measurements provide CCT, detailson the axial length, and keratometric readings. SMprovides information on the endothelium and CCT.

Correspondence: Yasin Cinar, Dicle Universitesi Tip Fakultesi, Goz Hastaliklari A.D.,21280 Kampus, Diyarbakir, Turkey. E-mail:dryasincinar@yahoo.com

Received 12 April 2013; accepted 26 July 2013; published online 19 September 2013

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The rotating Scheimpflug camera (RSC) system yieldsinformation on the anterior and posterior cornealtopography, corneal volume, corneal pachymetry, andanterior chamber depth.

The most widely used method of measuringcorneal thickness is UP, which is considered to bethe gold standard because of its ease of use, portabil-ity, and cost-effectiveness.10–11 As UP is a contactmethod, topical anesthesia is required. Touching thecornea during the procedure may cause patientsdiscomfort and lead to epithelial erosion and thetransmission of infections.12 Indentation of thecornea13 or tear film displacement by the probe14

may produce inaccurate (thinner) measurements.Inappropriate probe placement and tissue hydrationmay cause observer bias. Thus, UP measurementsrequire some experience.9,15

Many comparisons of different pachymetric tech-niques have been performed in healthy corneas,5,13,16

and there are few studies of keratoconic eyes.17–19 Inthis prospective study, we aimed to compare CCTmeasurements using RSC and SM, OLCR, and UP inkeratoconic eyes.

MATERIALS AND METHOD

Eighty-one eyes of 44 consecutive patients (24 femaleand 20 male) previously diagnosed as KC wereincluded in the study. Patients with KC with a clearcentral cornea were included in the study. Theexclusion criteria were as follows: use of contact lens,corneal scarring, previous ocular surgery (e.g.,cataract, pterygium, corneal collagen crosslinking,intracorneal ring segment implantation), systemicconnective tissue disease, atopy, and dry eye syn-drome. Informed consent was obtained from all thepatients in accordance with the Declaration of Helsinkibefore any intervention was performed, and the localethics committee approved this study. If the patientwas under 18 years, a consent form was also obtainedfrom his/her parents. A complete ophthalmic exam-ination was performed, including uncorrected and bestcorrected visual acuity, refraction for best correctedvisual acuity, KC classification, and slit lamp biomicro-scopy. A KC diagnosis was made if there was a scissorreflex on retinoscopy and central or paracentralsteepening of the cornea on corneal topography withone of the following biomicroscopic findings: central orparacentral steepening, hemosiderin deposition,Vogt’s stria, apical scarring, or breaks in theDescement’s membrane. The eyes were classified intofour groups according to the Amsler–Krumeich KCclassification,20 which is based on biomicroscopicfindings, keratometric readings, pachymetry at thethinnest area of the cornea, and refraction.

The corneal thickness measurements were per-formed in the order of RSC (Pentacam HR, Oculus,

Germany), SM (Konan Noncon Robo specular micro-scope, Konan Medical Inc., Hyogo, Japan), OLCR(LenStar LS 900, Haag-Streit AG, Koeniz,Switzerland), and UP (US 4000, Nidek, Japan) by thesame investigator on the same day. If the patientsmoved their eyes or blinked, the measurement wasrepeated to standardize the examinations. For eachsubject, the pachymetric measurements were per-formed on the same day, with a 10 min intervalbetween the measurements with the four devices.

During the measurement with the RSC, the patientswere seated and asked to place their chin on achinrest, with their forehead touching the headband.The patients were then instructed to look at the blackspot in the middle of the blue fixation lamp. On themonitor, the investigator evaluated the position of theimage of the eye and then focused the image.The image was then centralized in the aiming circleon the monitor. To eliminate operator-dependentvariables, the automatic release mode was used.Three successive measurements of both eyes of thepatients were taken over the course of a few minutes.Inappropriate measurements were indicated by a redcolor on the monitor and were deleted, as recom-mended by the manufacturer. The average of threepachymetric measurements of the center of the pupilwas used for the study.

For the SM, the patients were asked to look into thefixation target. The center of the cornea was adjustedby focusing on the center of the pupil on the screenuntil a clear image appeared on the monitor. When thealignment and focus were appropriate, the endothelialimage and the CCT were measured automaticallywith the device. Three successive measurements weretaken. The mean of these three measurements wasused for statistical analysis.

For the OLCR, the patients were seated and placedtheir chins on a chinrest with their foreheads on theheadrest of the device and were asked to look at thefixation light. According to the manufacturer’s rec-ommendations, both hands were used to adjust theposition of the device (for better alignment, one handwas placed on the joystick and the other one on thecross slide), and the device was not moved betweenthe measurements. Implausible measurementsmarked in a red color on the monitor were deleted,as recommended by the manufacturer. The softwarecalculated the mean of five consecutive measure-ments. Three successive measurements were obtainedfor both eyes and the mean of these measurementswas used for the analysis.

The UP measurements were performed lastbecause they require a contact method. After instilla-tion of one drop of 0.5% proparacaine hydrochlorideto the cornea for anesthesia, the patients were askedto look at a distant fixation point, and the UP probewas placed perpendicularly, touching the center of thecornea. Three consecutive measurements were

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performed for both eyes, and the mean of thesemeasurements was used for the analysis.

STATISTICAL ANALYSIS

The Statistical Package for the Social Sciences statis-tical software, version 11.5 (SPSS Inc., Chicago, IL,USA) was used for the statistical analysis. To analyzethe distribution of the data, a Kolmogorov-Smirnovtest was performed. The data were distributed nor-mally, and a paired t test was used to analyze thedifferences between the measurements of the devices.Pearson’s coefficient of correlation was used toexpress the strength of the relationship between themeasurements of the devices. A p value less than 0.05was considered significant.

RESULTS

Six patients had unilateral KC. One eye of a patientwith facial nerve paralysis accompanying mild expos-ure keratopathy was excluded. The mean age of thepatients was 19.0� 5.33 years (range 10–32 years). Themean keratometric value (K) was 50.5� 5.25 Diopter(D) according to the RSC. The mean manifest refrac-tion spherical equivalent (MRSE) was �5.13� 3.02 D.According to the Amsler-Krumeich classification,there were 25 stage 1, 35 stage 2, 13 stage 3, andeight stage 4 eyes. The mean MRSE was �3.2� 1.7 Din stage 1, �4.6� 1.8 D in stage 2, �7.3� 2.0 D in stage3, and �9.8� 4.4 D in stage 4 KC.

The CCT measurements could be performed in allthe patients using RSC and UP, whereas SM could notmeasure the CCT in seven eyes of six patients (twoeyes were stage 2, three eyes were stage 3, two eyes

were stage 4 KC). OLCR could not measure the CCTonly in one eye, which was stage 2 KC.

Comparison of the Mean CCTMeasurements

The KC grades and the mean CCT measurements ofall the patients obtained by the four devices are shownin Table 1. Table 1 also shows the p values of thebinary comparisons of the four different methodsused to measure the CCT. The CCT measurementsobtained by the RSC and UP were not significantlydifferent in any of the groups (Table 1). The SM vs. theOLCR measurements were significantly different inthe whole cohort and also in every KC stage. Therewere significant differences between the CCT meas-urements of RSC vs. SM, RSC vs. OLCR, SM vs.OLCR, SM vs. UP, and OLCR vs. UP in the wholecohort.

The RSC measures were, on average, 0.02� 26.7mmthinner than UP in all the KC patients. The CCTmeasurements with SM were significantly greaterthan those of the other devices in every stage of KC.

Correlations between the PachymetryMethods

Table 2 represents the correlation coefficients(r values) and p values of Pearson’s correlation test,showing the agreements between the CCT measure-ments with RSC, SM, OLCR, and UP in the study.A scatter plot of the CCT measurements is shown inFigure 1.

When all the patients were considered, the highestcorrelation coefficient was found between RSC and

TABLE 1. The mean CCT measurements of four instruments in the whole cohort and in the subgroupsaccording to Amsler-Krumeich classification; at the bottom p values of CCT measurements among fourdevices in all patients and subgroups.

RSC UP SM OLCR

All patients 458� 45.6 458� 46.5 474� 38.9 448� 44.6Grade 1 KC 478� 43.6 480� 40.2 487� 49.6 473� 36.1Grade 2 KC 468� 30.6 465� 27.8 468� 22.7 457� 23.2Grade 3 KC 441� 29.0 440� 38.5 456� 33.3 429� 24.8Grade 4 KC 379� 39.5 389� 69.2 474� 58.4 357� 38.7

*p Values RSC vs UP RSC vs SM RSC vs OLCR SM vs OLCR SM vs UP OLCR vs UPAll patients 0.993 0.004 50.001 50.001 0.009 0.002Grade 1 KC 0.674 0.062 0.274 0.013 0.106 0.126Grade 2 KC 0.307 0.535 50.001 0.001 0.394 0.005Grade 3 KC 0.902 0.025 0.018 0.018 0.183 0.167Grade 4 KC 0.658 0.078 0.277 0.010 0.087 0.273

UP: Ultrasonic Pachymetry; SM: Specular Microscopy; OLCR: Optical Low Coherence Reflectometer; RSC:Rotating Scheimpflug Camera; CCT: Central Corneal Thickness; KC: Keratoconus. p50.05 considered assignificant;*paired t test.

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OLCR (r = 0.87, p5.001). In the grade 1 KC group,there were high correlations between each binarycomparison of the four different methods, particularlywith RSC and UP (r = 0.9, p50.001). RSC vs. OLCRhad the highest correlation coefficient in grade 2 KC(r = 0.88, p50.001) and grade 3 KC (r = 0.85, p50.001)patients. In the grade 4 KC group, UP vs. SM had asignificant negative correlation coefficient (r =�0.89,p = 0.044) (Table 2).

DISCUSSION

The measurement of the CCT is crucial in thediagnosis, follow-up, and treatment of KC. In thecorneal collagen crosslinking procedure, the recom-mended corneal thickness is at least 400mm to avoiddamage to the corneal endothelium and deeperstructures of the eye, such as the lens and theretina.21 In corneal intrastromal ring segment implant-ation, the thickness at the implanted site should not beless than 400mm to prevent full thickness cornealincision or shallow placement of the ring segment.22

In recent years, new devices have been introducedto measure the CCT in a noncontact way. This studycompared CCT measurements obtained by four dif-ferent methods in keratoconic eyes: RSC, SM, OLCR,and UP. To the best of our knowledge, this is the firststudy to compare the use of these four devices tomeasure the CCT in KC patients.

Ucakhan et al. reported a comparison of CCT withRSC, SM, and UP measurements in normal andkeratoconic corneas.17 According to their results,RSC had better reproducibility than UP and SM inmoderate keratoconic eyes. They found a very highcorrelation between the RSC and UP measurements inmild and severe KC, whereas SM was unable tomeasure the CCT in any of the eyes with severe KCand 42.9% of the eyes with moderate KC in their studypopulation. They reported a correlation coefficient of0.92 between the RSC and UP measurements in thekeratoconic eyes. In the current study, the KC patientswere staged according to the Amsler–Krumeich KCclassification, and SM could not measure the CCT

only in seven eyes of six bilateral KC patients. Ofthese, two eyes were stage 2, three eyes were stage 3,and two eyes were stage 4 KC.

In the present study, when we compared the UPand RSC measurements in all the keratoconic patients,the measurements were not statistically significant(p = 0.993), and there was a significant correlation(r = 0.83, p50.001). The measurements with UP wereslightly thicker than those obtained with the RSC, afinding in agreement with some previous stu-dies13,17,23 in which UP measured thicker than RSC.The thinnest pachymetry point measured by the RSCand the slit scan corneal topography showed nosignificant difference in mean deviation.19 However,Dutta et al. demonstrated that UP yielded signifi-cantly higher CCT values than slit scan cornealtopography.18 This finding was attributed to theposterior reflection point of the sound waves of theUP not being identified. However, this point issupposed to be between Descement’s membrane andthe anterior chamber.17 Furthermore, compression ofthe epithelium and displacement of a 7 to 40 mm thicktear film by the UP probe may underestimate CCTmeasurements.24–25

Some studies have reported that the agreementbetween OLCR and US pachymetry in CCT measure-ments is very good.12,26 Spadea et al. reported that acomparison of OLCR and US pachymetry in themeasurement of the CCT before and after photore-fractive keratectomy had a smaller effect on theagreement between the two devices. They also notedthat these two devices could be used interchangeablyin normal eyes and in the planning of refractivesurgery.26 In the present study, the UP measurementswere thicker than those obtained with OLCR, inagreement with a previous study.27 When all the KCpatients were considered, the mean differencebetween the CCT measurements of these two deviceswas 10.5mm. The mean difference between the UP andOLCR measurements was greater in grade 4 KCcompared to grade 1 KC. The correlation coefficientbetween UP and OLCR was 0.79 (p50.001) in grade 1KC and 0.1 (p = 0.814) in grade 4 KC. These two devicesshould not be used interchangeably in KC patients.

TABLE 2. Degree correlation between the CCT measurements of the devices.

RSC vs UP RSC vs SM RSC vs OLCR SM vs OLCR SM vs UP OLCR vs UP

þr *p þr *p þr *p þr *p þr *p þr *p

All patients 0.83 50.001 0.59 50.001 0.87 50.001 0.59 50.001 0.44 50.001 0.79 50.001Grade 1 KC 0.9 50.001 0.87 50.001 0.88 50.001 0.86 50.001 0.89 50.001 0.87 50.001Grade 2 KC 0.83 50.001 0.6 50.001 0.88 50.001 0.63 50.001 0.49 0.004 0.8 50.001Grade 3 KC 0.68 0.011 0.8 0.005 0.85 50.001 0.57 0.083 0.68 0.03 0.75 0.003Grade 4 KC 0.46 0.251 �0.62 0.263 0.1 0.798 0.42 0.483 �0.89 0.044 0.1 0.814

UP: Ultrasonic Pachymetry; SM: Specular Microscopy; OLCR: Optical Low Coherence Reflectometer; RSC: Rotating ScheimpflugCamera; KC: Keratoconus; CCT: Central Corneal Thickness. þPearsons correlation coefficient; *paired t test.p50.05 considered as significant.

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Ucakhan and associates17 reported that the meandifferences between UP-SM, RSC-SM, and RSC-UPwere 18.8 mm, 12.7 mm, and �11.2 mm, respectively, inkeratoconic eyes. In the current study, the mean

differences between UP-SM, RSC-SM, and RSC-UPwere �14.4mm, �13 mm, and �0.02 mm, respectively.In contrast to the results of Ucakhan and associates,SM failed to measure the CCT in only two grade 4 KC

FIGURE 1. (a) Scatterplot of CCT measurements taken by UP and RSC; (b) scatterplot of CCT measurements taken by RSC and SM;(c) scatterplot of CCT measurements taken by RSC and OLCR; (d) scatterplot of CCT measurements taken by SM and OLCR;(e) scatterplot of CCT measurements taken by UP and SM; (f) scatterplot of CCT measurements taken by UP and OLCR.UP: Ultrasonic Pachymetry; SM: Specular Microscopy; OLCR: Optical Low Coherence Reflectometer; RSC: Rotating ScheimpflugCamera.

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eyes in the current study. This may be because theanterior corneal refractive power affects noncontactspecular images.28

Tai et al.29 reported CCT measurements in healthysubjects using RSC, SM, OLCR, and UP. They foundthat the mean CCT differences between the methodsin pairwise comparisons were statistically significantexcept for OLCR vs. UP and OLCR vs. RSC. Theyfound the greatest mean difference (30.57 mm) amongthe comparisons between the RSC and SM. Thesmallest mean difference was 3.54mm. This wasfound between OLCR and UP in the same study. Inthe current study, the greatest mean difference wasbetween SM and OLCR (21.8 mm), and the smallestmean difference was between UP and RCS (0.02 mm)in the whole cohort.

Barkana and associates13 reported that the RSCsystem and OLCR pachymeter are convenient andprovide excellent intraoperator repeatability andreproducibility in CCT measurements in healthysubjects. They found CCT values achieved by theRSC were similar to those achieved with both theOLCR pachymeter and the US pachymeter. Thecorrelation coefficient was 0.96 between the RSC andOLCR, according to their results. In the current study,the CCT measurements with the RSC and OLCR wereclose to each other, and the correlation coefficient was0.87 between these two devices. However, it was notstatistically significant in all the KC patients.

The current study has some limitations, such as thelack of a healthy control group, the reliability andreproducibility analyses of the devices, and the smallsample size of the study population. Nevertheless,this is the first study comparing CCT measurementsof SM and OLCR in keratoconic eyes. The study foundthat SM and OLCR measurements of the CCT aresignificantly different in keratoconic eyes.

In conclusion, CCT measurements with the RSCseem to be comparable to those achieved with UP.Compared with the other three devices, the CCT isthicker according to SM measures and thinner accord-ing to OLCR measures in all keratoconic eyes and inall KC grades. RSC, UP, SM, and OLCR should not beused interchangeably in keratoconic eyes.

DECLARATION OF INTEREST

The authors report no conflicts of interest. The authorsalone are responsible for the content and writing ofthe paper.

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