ARTICLE
Central corneal thic
kness measurementsafter myopic photorefractive keratectomyusing Scheimpflug imaging, scanning-slittopography, and ultrasonic pachymetryAmir Faramarzi, MD, Farid Karimian, MD, Mohammad Reza Jafarinasab, MD,Mohammad Hossein Jabbarpoor Bonyadi, MD, Mehdi Yaseri, MS
Q 2010 A
Published
SCRS an
by Elsev
PURPOSE: To compare central corneal thickness (CCT) measurements obtained with a dualScheimpflug camera system (Galilei), a scanning-slit topographer (Orbscan II), and anultrasound (US) pachymeter (SP-2000) after photorefractive keratectomy (PRK) for myopia.
SETTING: Ophthalmic Research Center, Labbafinejad Medical Center, Tehran, Iran.
METHODS: This prospective study enrolled patients who had PRK to correct myopia or myopicastigmatism and a postoperative follow-up of at least 5 months. The CCT was measured ina single session using Scheimpflug imaging, scanning-slit topography, and US pachymetry. Datawere analyzed with paired t tests, linear regression, and Bland-Altman plots.
RESULTS: The mean CCT in the 100 eyes (50 patients) was 524.06 mm G 38.56 (SD) with Scheimp-flug imaging, 505.92 G 47.35 mm with scanning-slit topography, and 496.97 G 42.74 mm with USpachymetry. The lower to upper 95% limits of agreement (LoA) with US pachymetry were 2.4 to51.8 mm for Scheimpflug imaging and �22.2 to 40.1 mm for scanning-slit topography. Afterapplication of a correction factor (0.95), the mean corrected Scheimpflug CCT measurement was497.33 G 36.59 mm and the lower to upper 95% LoA with US pachymetry were �25.9 to 25.5 mm.The acoustic factor for scanning-slit topography was 0.94.
CONCLUSIONS: The CCT measurements in eyes that had PRK were thicker with Scheimpflugimaging than with US pachymetry or scanning-slit topography in the late postoperative period.With application of a correction factor, the Scheimpflug measurements were closer to USpachymetry values and had better agreement than scanning-slit topography.
Financial Disclosure: No author has a financial or proprietary interest in any material or methodmentioned.
J Cataract Refract Surg 2010; 36:1543–1549 Q 2010 ASCRS and ESCRS
The use of corneal thickness measurements in ophthal-mology clinics is increasing. The function of corneal en-dothelial cells can be monitored by the central cornealthickness (CCT), and Goldmann applanation tonome-try is dependent on CCT.1,2 In excimer laser refractivesurgery, the remaining stromal corneal bed is one ofthe most important factors determining the safety ofthe procedure.3,4 Furthermore, we can diagnose sub-clinical keratoconus and abandon risky ablative refrac-tive surgery in these cases by precisely measuringcorneal thickness in different parts of the cornea.
Several instruments using different technologies formeasuring corneal thickness are available. Ultrasound
d ESCRS
ier Inc.
(US) pachymetry is the most commonly used and isthe gold standard for CCT measurements. It is porta-ble, is relatively inexpensive, and is easy of use. How-ever, because it is a contact method, there is a risk forepithelial damage and transmission of infection. Inaddition, because the repeatability and validity ofCCT measurements depend on accurate axial place-ment of the probe relative to the center of the cornea,US pachymetry is an operator-dependent method.
The Orbscan II scanning-slit topography system(Bausch & Lomb) measures corneal thickness by ana-lyzing reflecting images from the anterior and poste-rior surfaces of the cornea. The system also creates
0886-3350/$dsee front matter 1543doi:10.1016/j.jcrs.2010.03.042
1544 PACHYMETRY BY 3 METHODS AFTER MYOPIC PRK
elevation-based and Placido disk topographic maps ofthe cornea and measures corneal thickness at anygiven point of the cornea. There is no need for anesthe-sia because scanning-slit topography is an opticalnoncontact method. However, the ideal focal distancecannot be achieved for both the anterior and posteriorcorneal surfaces by the system’s camera because theilluminated slits are positioned differently. In hazycorneas, the quality of the posterior surface imagemay decrease because the light rays may be deviatedas they cross the cornea and the clarity of the posterioredge profile may diminish. Also, several studies5–8
have found that the system overestimates the CCT invirgin corneas compared with US pachymetrymethods. Therefore, an acoustic correction factor wasintroduced in the scanning-slit topography device toreduce the measured CCT values.
Another method of CCT measurement uses dual-rotating Scheimpflug cameras. One system using thistechnology is the recently introduced Galilei (ZeimerGroup), which also incorporates Placido disk topogra-phy. According to the manufacturer of the system, thedual Scheimpflug cameras measure corneal thicknessmore precisely than the Pentacam system (Oculus, Inc.),which has 1 Scheimpflug camera rather than 2 cameras.
This studywas performed to evaluate the accuracy ofCCT measurements using ultrasound pachymetry,scanning-slit topography, and dual Scheimpflug imag-ing after photorefractive keratectomy (PRK) performedat least 5 months previously. To our knowledge, this isthe first study to compare CCT measurements of the 3methods in eyes that have had PRK.
PATIENTS AND METHODS
This prospective study enrolled patients who had bilateralPRK to correct myopia or myopic astigmatism. All patientshad a postoperative follow-up of at least 5 months and nor-mal eyeswith no history of ocular surgery other than PRK. In
Submitted: August 6, 2009.Final revision submitted: March 23, 2010.Accepted: March 29, 2010.
From the Ophthalmic Research Center and Department of Ophthal-mology (Faramarzi, Karimian, Jafarinasab, Bonyadi), LabbafinejadMedical Center, Shaheed Beheshti Medical University, and the De-partment of Epidemiology and Biostatics (Yaseri), School of PublicHealth, Tehran University of Medical Sciences, Tehran, Iran.
Supported by the Ophthalmic Research Center, Shaheed BeheshtiMedical University, Tehran, Iran.
Corresponding author: Amir Faramarzi, MD, Ophthalmic ResearchCenter, Ophthalmology Department, Labbafinejad Medical Center,Pasdaran Avenue, Boostan 9 Street, Tehran 1666694516, Iran.E-mail: [email protected].
J CATARACT REFRACT SURG - V
eyes in which the ablation was greater than 50 mm,mitomycin-C was applied for up to 20 seconds. Slitlamp ex-amination of the corneawas performed to evaluate post-PRKhaziness according to the grading system of Hanna et al.9
Central Corneal Thickness Measurements
First, the same technicianmeasured the CCTwith the non-contact instruments (ie, Scheimpflug and scanning-slitdevices) according to the respective manufacturer’s guide-lines. Measurements were taken 3 times with each deviceand the data averaged. Then, after 1 drop of tetracaine wasinstilled in both eyes, another technician performed US pa-chymetry. The pachymeter probe was placed lightly on thecenter of the cornea 2 times. With each contact, the CCTwas automatically measured 10 times (auto mode) and themean of the measurements was automatically calculated.The interval between each measurement did not exceed 15minutes. Based on previous results in normal eyes (unpub-lished data), an acoustic correction factor of 0.96 was appliedto all scanning-slit topography measurements to achieveequivalence with US pachymetry measurements.
Preoperative scanning-slit topography and ablative sur-gery parameters were available. Therefore, it was possibleto compare the preoperative scanning-slit CCT measure-ments with the postoperative scanning-slit measurementsas well as with the theoretical ablation depth estimated bythe software of the excimer laser (EC-5000, Nidek, Inc.).
Instrumentation
Dual-Scheimpflug imaging was performed using the Gal-ilei analyzer. The 2 revolving Scheimpflug cameras of the de-vice are 180 degrees apart, and an integrated Placidotopographer provides data for anterior curvature analysis.In Scheimpflug imaging, the object plane, lens plane, and im-age plane are not parallel to each other but rather intersect ina common straight line; the geometry yields a wide depth offocus. The system captures slit images from opposite sides ofthe illuminated slit and averages the elevation data obtainedfrom corresponding opposite-slit images. The techniquemeasures posterior corneal surface parameters as well aspachymetry across the entire cornea. The 2 cameras recordsimultaneously to compensate for microsaccade eye move-ment during image capture.
Scanning-slit topography was performed using the Orbs-can II system. This combined Placido scanning-slit cornealtopography and pachymetry device uses a calibrated videoand slit-beam system to measure 3 space points on the frontand back corneal surfaces. It catches reflected light beamsprojected on the cornea from a 45-degree angle; 1 beam isfrom the left side and the other from the right side of theeye. The reflections are analyzed in 40 images at the levelsof the anterior and posterior corneal surfaces, the anteriorand posterior lens surfaces, and the anterior iris surface.Corneal thickness and pachymetry data are constructs ofthe difference between the elevations of the anterior surfaceand the posterior surface of the cornea.10
Ultrasound pachymetry was performed using the SP-2000pachymeter (Tomey Corp.). With US pachymetry, cornealthickness is calculated from the measured time-of-flight be-tween reflections from the anterior and posterior cornealsurfaces and the speed of sound in the cornea (1636 to1640 m/s11).
OL 36, SEPTEMBER 2010
Table 1. Central corneal thickness measurements by method.
Central Corneal Thickness (mm)
Method Number Mean SD Maximum Minimum
Ultrasound 100 496.97 42.738 605 381Scanning slit 100 505.92 47.348 601 385Scheimpflug 100 524.06 38.559 628 416CorrectedScheimpflug
100 497.28 36.589 596 395
1545PACHYMETRY BY 3 METHODS AFTER MYOPIC PRK
Statistical Analysis
Descriptive statistical results are presented as the mean GSD (range). Paired differences between measuring methodsare presented as mean differences, 95% confidence interval(CI), and 95% limits of agreement (LoA). The 95% LoAwere calculated as the mean G1.96 SD of the differences.Paired t tests were used to analyze the differences betweeneach pair of measurements obtained with 2 systems. A Pvalue less than 0.05 was considered statistically significant.
Regression analysis was used to assess the linear relation-ship between the 3methods.According to this analysis, a cor-rection factor was calculated for deriving CCT accuratevalues from the Scheimpflug system.
Bland-Altman and folded cumulative distribution plotswere used to evaluate the agreement in CCT measurementsbetween instruments. In the Bland-Altman plots, for eachpair of measurements obtained with 2 systems, the differ-ence of the values was plotted over the mean of the values.Folded cumulative distribution plots were created to showthe distribution of the differences between 2 methods.
RESULTS
The study evaluated 100 eyes of 50 patients. The meanage of the 26men (52%) and 24women (48%)was 28.84G 5.32 years (range 21 to 45 years). Mitomycin-C wasused in 60 eyes (60%). The mean ablation depth was61.45 G 21.61 mm (range 22 to 114 mm). The mean pre-operative spherical equivalent was �3.99 G 1.50diopters (D) (range �1.00 to �7.00 D). The mean timebetween refractive surgery and CCT measurementswas 9.72 G 2 .49 months (range 5 to 13 months). Noeye had corneal haze higher than grade 0.5 at thetime of measurement.
Table 2. Comparison of CCT data between the 3 methods.
Comparison
Central Corneal Thicknes
Mean SD Median Min M
Scanning slit � US 8.9 15.9 9.0 �31.0 5Scheimpflug � US 27.1 12.6 26.0 �2.0 6Corrected Scheimpflug � US �0.2 13.1 0.3 �29.3 3
CI Z confidence interval; LoA Z limits of agreement; US Z ultrasound*Paired t test†Pearson correlation
J CATARACT REFRACT SURG - V
Table 1 shows the mean CCT measurements. TheCCT measurement was the thickest with Scheimpflugimaging. Linear regression analysis to assess thecorrelation between the 3 instruments showed thatwith the application of a 0.949 correction factor, theScheimpflug readings approached the US pachymet-ric readings. Table 2 shows the correlations betweenthe US method and the 2 optical methods. Themean difference between US values and scanning-slit values was smaller than between US values andScheimpflug values. In addition, the 95% CI waslower between US pachymetry and slit-scanningtopography than between US pachymetry andScheimpflug imaging.
Figure 1 shows the scatterplots of Scheimpflug andscanning-slit measurements versus US pachymetrymeasurements. Figure 2 shows the scatterplot of cor-rected Scheimpflug and scanning-slit measurementsversus US pachymetry measurements. After applica-tion of the correction factor, the line of correctedScheimpflug values approached the 45-degree line,representing better correlation with US pachymetryreadings. Figure 3 shows the Bland-Altman plots ofscanning-slit, Scheimpflug, and corrected Scheimpflugmeasurements versus US pachymetry measurements.The mean difference was smallest between the cor-rected Scheimpflug values and the US pachymetryvalues. Figure 4 shows the folded cumulative distri-bution plots of these findings. The median differencebetween corrected Scheimpflug values and US pachy-metry values was closer to zero, indicating the2 methods had the best agreement.
The theoretical ablation depth estimated by theexcimer laser was 61.88 G 21.99 mm and the depthcalculated by subtracting the postoperative frompreoperative scanning slit readings was 60.34 G25.51 mm; the difference was not significant (P Z .369).
DISCUSSION
In ophthalmology, repeatability and accuracy are 2important aspects of any measuring technique.
s (mm)
P Value*%95 LoA
(Lower to Upper) r Value†ax 95% CI
4.0 5.8 to 12.1 !.001 �22.2 to 40.1 0.9433.0 24.6 to 29.6 !.001 2.4 to 51.8 0.9574.5 �2.8 to 2.4 .866 �25.9 to 25.5 0.957
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Figure 1. Scatterplot of Scheimpflug and scanning-slit CCTmeasure-ments versus US measurements. The dotted line represents the lineof equivalence (CCT Z central corneal thickness; US Z ultrasound).
Figure 2. Scatterplot of scanning-slit and corrected Scheimpflug CCTmeasurements versus US measurements. The dotted line representsthe line of equivalence (CCT Z central corneal thickness; US Zultrasound).
1546 PACHYMETRY BY 3 METHODS AFTER MYOPIC PRK
Repeatability indicates the ability to obtain the samevalues with repeated measurement under similar con-ditions. Accuracy or validitymeans howwell the read-ing agree with the results of the well-establishedacceptable method. In this study, we compared the ac-curacy of 2 noncontact optical methodsdScheimpflugimaging and scanning-slit topographydwith that ofUS pachymetry in measuring CCT in eyes that hadprevious PRK.
Several studies5–8 report that the scanning-slitsystem we used overestimated CCT in virgin eyescompared with US pachymetry; however, when theacoustic correction factor was applied, the methodswere in better agreement.12 On the other hand, otherstudies13–16 report that the scanning-slit system withthe acoustic correction factor underestimated CCTcompared with US pachymetry in eyes that had laserin situ keratomileusis or PRK. We did not observethis trend in our study; compared with US pachyme-try, the scanning-slit system with the application ofthe 0.96 correction factor overestimated the CCT by9 mm in eyes that had PRK. Before we began the study,we adjusted the scanning-slit device to minimize thedifference between its measurements and thoseobtained with US pachymetry in a group of normaleyes. The overestimation may have occurred becauseby 6 months postoperatively, most early-onset hazehad resolved; at the time of measurement no eye inour study had a corneal haze grade higher than 0.5
J CATARACT REFRACT SURG - V
according to the Hanna scoring system. Bosciaet al.17 found that although the scanning-slit systemunderestimated measurements in corneas with haze,it did not do so after the haze was treated with photo-therapeutic keratectomy. Furthermore, Fakhry et al.18
found a statistically significant correlation between thegrade of haze and the decrease in slit-scanning topo-graphy readings in eyes after PRK.
In our study, dual Scheimpflug imaging gave signifi-cantly thicker CCTmeasurements than US pachymetryand scanning-slit topography (27mmand18mm, respec-tively). In a study byMenassa et al.,19 the dual Scheimp-flug device did not overestimate central thickness invirgin corneas compared with US pachymetry andscanning-slit topography. To our knowledge, there areno other published studies comparing the dualScheimpflug device and other instruments in CCTmea-surements in healthy eyes or in ablated eyes. In severalstudies of healthy eyes,20–22 the CCT measured by thePentacam device, which has 1 Scheimpflug camera,was thinner than in US pachymetry readings. On theother hand, other studies,16,23 of healthy corneas foundthe single-camera Scheimpflug system overestimatedmeasured CCT compared with US pachymetry read-ings. The single-camera Scheimpflug systemwas foundto underestimate the CCT in ablated corneas comparedwith the US pachymetry and scanning-slit topographymeasurements22,24; in contrast, other studies16,25 founda good correlation.
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Figure 3. Bland-Altman plots of scanning-slit, Scheimpflug, and corrected Scheimpflug CCTmeasurements versus US measurements. The mid-dle line represents the mean, and the lines on either side represent the upper and lower 95% LoA (US Z ultrasound).
Figure 4. Folded cumulative distribution plots of the percentiles of thedifferences of scanning-slit, Scheimpflug, and corrected ScheimpflugCCT measurements versus US measurements (US Z ultrasound).
1547PACHYMETRY BY 3 METHODS AFTER MYOPIC PRK
Apart from the tendency to overestimate CCT, thewidth of 95% LoA for the agreement between thedual Scheimpflug and US pachymetry measurementswas narrower than the agreement between scanning-slit topography and US pachymetry (LoA: 2.4 to 51.8versus �22.2 to 40.1). Therefore, the dual Scheimpflugresults were less accurate but more reliable than thescanning-slit topography results. For increasingvalidity, the dual Scheimpflug results were multipliedby a correction factor of 0.949. This shifted theScheimpflug CCT readings closer to the US pachyme-try readings. When the correction factor was appliedto the Scheimpflug measurements, the mean of differ-ence approached zero, which means better agreementwith the US pachymetry measurements. Therefore,corrected dual Scheimpflug measurements can beused with less uncertainty. For example, the upperrange of 95% LoA was 22.6 mm, which means thechance of overestimating CCT by more than 22.6 mmwould be 5% with dual Scheimpflug technology. Thevalue for scanning-slit topography was almost40 mm; therefore, in 5% of individuals, the scanning-slit might overestimate CCT by up to 40 mm. Toprevent bias from using the same data to calculatethe correction factor and evaluate corrected dualScheimpflug results, we randomly chose 70% of sam-ples and then calculated the correction factor, whichwas 0.95 (95% CI, 0.943-0.956). There was no signifi-cant difference between this result and the result usingthe other correction factor (0.949). Figure 5 shows theBland-Altman plot of the remaining 30% of partici-pants using the new correction factor.
The cause of the disagreement between the dualScheimpflug device and the US pachymeter is notknown. One theory is that the noncontact methodsused by the dual Scheimpflug and scanning-slitdevices measure the hydrated mucous gel coveringthe corneal surface, which is reported to be up to40 mm thick.26 On the other hand, repeated contactwith a US probe can slightly reduce corneal thick-ness.27 Because of these factors, the manufacturer of
J CATARACT REFRACT SURG - VOL 36, SEPTEMBER 2010
Figure 5. Bland-Altman plot of the difference between correctedScheimpflug CCT measurements and US CCT measurements in30% of cases. The correction factor was based on 70% of randomlyselected eyes (US Z ultrasound).
1548 PACHYMETRY BY 3 METHODS AFTER MYOPIC PRK
the scanning-slit device proposes using an acousticcorrection factor for its products. We also recommenda correction factor of 0.95 for the dual Scheimpflug de-vice to compensate for the discrepancy in CCTmeasurement between it and US pachymetry, at leastin post-PRK corneas.
In summary, our study showed that in eyes havingPRK and without significant postoperative haze, thedual Scheimpflug system gave thicker CCT valuesthan the US pachymeter and scanning-slit system.However, the dual Scheimpflug system was more reli-able than the scanning-slit system in measuring CCTin these eyes. Introducing a correction factor of 0.95to measurements by the Scheimpflug device canincrease the similarity in CCT measurements betweenthe 3 instruments. The validity of dual Scheimpflugsystem CCT measurements in the normal cornearemains under investigation.
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36, SEPTEMBER 2010First author:Amir Faramarzi, MD
Ophthalmic Research Center, Ophthal-mology Department, LabbafinejadMedical Center, Tehran, Iran
