ARTICLE
Intrasubject repeatability of corneal power, thickness, and wavefront aberrations with a new version of a dual rotating Scheimpflug–Placido system Alejandro Cervi~ no, PhD, Alberto Dominguez-Vicent, MSc, Teresa Ferrer-Blasco, PhD, Santiago García-L azaro, PhD, Cesar Albarran-Diego, PhD
PURPOSE: To determine the intrasubject repeatability of a recently introduced dual-camera rotating Scheimpflug–Placido imaging system (Galilei G4) in determining corneal thickness, power, and wavefront aberrations in young healthy subjects. SETTING: University of Valencia, Valencia, Spain. DESIGN: Prospective evaluation of diagnostic technology. METHODS: The study comprised right eyes of 25 subjects aged 20 to 40 years with a spherical equivalent ranging from 4.25 to C1.00 diopters. The central corneal thickness, thinnest point value and location, anterior and posterior surface curvatures, total corneal power, and corneal wavefront aberrations were measured for distance vision using the dual Scheimpflug–Placido system. Three consecutive measurements were taken in each eye. RESULTS: Twenty-five eyes were evaluated. Repeated-measures analysis of variance showed the only statistically significant difference between the 3 repeated measures to be in trefoil aberration. Intraclass correlation coefficients (ICCs) were higher than 0.950 for all the parameters except the thinnest point chord distance to geometric corneal center (0.528) and chord angle (0.742), corneal astigmatism (0.811) and its vector components J0 (0.891) and J45 (0.724), and all wavefront aberrations. CONCLUSIONS: The new dual Scheimpflug–Placido system had high intraobserver repeatability for corneal power and thickness and moderate repeatability for corneal astigmatism and corneal wavefront aberrations in healthy corneas with low astigmatism. This iteration of the device performed better in young healthy corneas than preceding versions. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2015; 41:186–192 Q 2015 ASCRS and ESCRS
Accurate corneal power determination is particularly important for intraocular lens calculation and surgical planning of astigmatism correction. The corneal wavefront aberration profile can be used for custom ablation planning. In addition, determining the corneal thickness is crucial in several areas, including refractive surgery candidate assessment, surgical planning, corneal crosslinking, and intrastromal corneal ring segment. Over the years, many new imaging devices for anterior segment parameter determination have become 186
Q 2015 ASCRS and ESCRS Published by Elsevier Inc.
available to clinicians. Although these systems are accurate and repeatable, the significant differences between them limit the interchangeability of the data obtained. The greatest progress in the past few years has been in Scheimpflug imaging and optical coherence tomography (OCT). Every time a new device or an enhanced version of an existing device is launched, it is essential to determine its repeatability. Previous versions of the Galilei dual-camera rotating Scheimpflug imaging device (Ziemer Ophthalmic Systems AG) have been http://dx.doi.org/10.1016/j.jcrs.2014.04.037 0886-3350
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thoroughly assessed.1–6 The device has been shown to provide repeatable and precise measurements of anterior segment parameters and compared against other Scheimpflug devices and alternative imaging techniques.1,2,5–8 Although the latest version approved for clinical practice (Galilei G4) has been studied,9 there are no reports of the device's intrasubject repeatability. Thus, the aim of the present study was to determine the clinical performance of the dual Scheimpflug–Placido in measuring corneal thickness, power, and wavefront aberrations in healthy eyes. SUBJECTS AND METHODS The study comprised healthy right eyes of young subjects. The subjects were randomly included from the postgraduate and staff population at the University of Valencia. All subjects provided informed consent after receiving a verbal and written explanation of the nature and possible consequences of the study. The study followed the tenets of the Declaration of Helsinki.
to the Femto LDV laser (Ziemer Ophthalmic Systems AG) for exchange of patient data between devices. It also gives information about cone location, magnitude, and keratoconus likelihood and allows calculation of the equivalent defocus from Zernike coefficients. In the present study, the system was used to automatically obtain measurements of corneal power, thickness, and aberrations. Corneal power data were obtained using the system's “total corneal power by ray tracing” setting, which calculates the propagation of incoming parallel rays and uses the Snell law to refract these rays through the anterior and posterior corneal surfaces. The thinnest point location was analyzed by computing the chord distance from the geometric center and chord angle from the x–y offset coordinates given by the instrument. Analysis of wavefront aberrations was performed with a 6.0 mm pupil diameter. Corneal astigmatism was recorded as the dioptric value given by the instrument as well as by determining its vectorial components J0 and J45, as suggested by Thibos et al.,10 because vector analysis includes the information for both the power and the angle. The conversion to power vector notation was performed using the following equations, where C is the negative cylindrical power and a is the cylinder axis: J0 Z
C cos2a 2
J45 Z
C sin2a 2
Inclusion and Exclusion Criteria Inclusion criteria were age between 20 years and 40 years and a corrected visual acuity (CDVA) of 20/25 or better. Subjects with a CDVA worse than 20/25, ocular or systemic disease, history of ocular surgery, intraocular pressure above 21 mm Hg, or retinal or optic disc pathology were excluded from taking part in the study.
Scheimpflug–Placido System The Galilei G4 system is the latest version of the Galilei topographer. This noninvasive optical diagnostic device designed for the assessment of the eye's anterior segment is based on a rotating dual-Scheimpflug camera and a Placido topography system. The images recorded by this device include the cornea, iris, pupil, limbus, anterior chamber, and lens. For each standard 3-dimensional (3-D) scan, a 3D model of the anterior segment is generated by integrating the Placido image and the rotated dual-Scheimpflug images. The instrument has a red light–emitting diode near-to-far adjustable fixation target that can be moved from 20.00 to C20.00 D in steps of 0.25 diopter. According to the manufacturer, the new features include the ability to select fewer images per scan (range between 7 and 30) and connectivity
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Experimental Technique All eyes were measured under distance viewing conditions 3 consecutive times. The patient was repositioned and the device realigned after each measurement. The same experienced operator, who was not aware of the study goal, performed all the measurements.
Statistical Analysis Statistical analysis was performed using SPSS software (SPSS/Pc C 10.1 for Windows, SPSS, Inc.). Repeatedmeasures analysis of variance (ANOVA) was performed and the intraclass correlation coefficient (ICC) and coefficient of variation (CoV) were obtained for each parameter analyzed. The ICCs were classified using a system suggested by McGraw and Wong11 as follows: (1) less than 0.75 Z poor agreement; 0.75 to less than 0.90 Z moderate agreement; (3) 0.90 or greater Z high agreement. A P value less than 0.05 was considered statistically significant.
RESULTS
Submitted: January 12, 2014. Final revision submitted: March 24, 2014. Accepted: April 14, 2014. From the Optometry Research Group, Optics Department, University of Valencia, Valencia, Spain. Supported by an Atraccio de Talent research scholarship, Universidad de Valencia (Mr. Domınguez-Vicent). Corresponding author: Alejandro Cervin~o, PhD, Departamento de Optica–Universidad de Valencia, C/Dr. Moliner, 50–46100– Burjassot, Espa~na. E-mail: [email protected].
The study included 25 eyes of 25 subjects (12 men, 13 women). Table 1 shows the patients' demographics and the values of the measured parameters. Corneal Thickness Table 2 shows the repeatability of the corneal thickness measurements. Repeated-measures ANOVA did not show a statistically significant difference in the values between the 3 repeated measures for any corneal thickness analyzed; that is, corneal thickness
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Table 1. Patient demographics and characteristics. Parameter
Mean G SD
Age (y) Refractive error SE(D) Corneal thickness (mm) Average in central region CCT Thinnest point Thinnest point chord (mm) Thinnest point chord angle (radians) Corneal power (D) Total Flat Steep Astigmatism Astigmatic J0 Astigmatic J45 Central Midperipheral Peripheral Corneal aberrations (mm) Trefoil Z(3, 3) Coma Z(3, 1) SA Z(4,0) Coma Z(3,1) Trefoil Z(3,3)
30.36 G 7.32 0.80 G 2.33
Range 20, 40 4.25, C1.00
554.59 G 41.20 486.00, 611.67 547.17 G 41.01 478.67, 604.67 544.30 G 42.22 473.00, 599.67 0.91 G 0.20 0.43, 1.27 0.476 G 0.228 0.039, 0.932
42.59 G 1.58 42.25 G 1.61 42.93 G 1.57 0.68 G 0.43 0.21 G 0.31 0.03 G 0.13 42.62 G 1.55 42.97 G 1.79 44.57 G 2.12
39.32, 44.79 39.06, 44.51 39.55, 44.99 0.31, 2.01 0.52, 0.99 0.25, 0.24 39.41, 44.72 39.14, 45.74 39.87, 47.92
0.051 G 0.164 0.068 G 0.231 0.121 G 0.077 0.013 G 0.361 0.011 G 0.128
0.360, 0.283 0.337, 0.430 0.000, 0.280 0.440, 0.640 0.210, 0.190
CCT Z central corneal thickness; J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees; SA Z spherical aberration; SE Z spherical equivalent
in the central region, central corneal thickness (CCT), and thinnest point (all P O .05). The ICCs were greater than 0.950 in all cases, showing good agreement between the repeated measures. The thinnest point location was at the inferotemporal quadrant in all cases (Figure 1) and differences were statistically significant between the 3 consecutive measurements (P O .05). The ICCs, however, were low for the chord distance to the geometric corneal center (0.528) and to the chord angle (0.742), indicating
that the agreement in thinnest point location was low between the repeated measurements. The CoV for the chord distance and the angle for the thinnest point were 16.01% and 43.94%, respectively. Total Corneal Power Table 3 shows the repeated measurements of the total corneal power, flattest corneal power, steepest corneal power, corneal astigmatism magnitude and its J0 and J45 components, and the corneal power in the central, midperipheral, and peripheral corneal locations. The variation in the measurements obtained for total, flat, and steep power was less than 0.50 D in all cases (Figure 2). Repeated-measures ANOVA did not show significant differences in the values between the repeated measurements (all P O .05). The lowest agreement was in the corneal astigmatism magnitude (ICC, 0.811) particularly in the J45 component (ICC, 0.724). Figure 3 shows the distribution of corneal astigmatism as a function of its vectorial components. Corneal Wavefront Aberrations Table 4 shows the results of the corneal aberrations analysis. There were statistically significant differences in the trefoil component Z(3, 3) (P Z .013) with mild agreement between the repeated measurements. The ICC for the trefoil component Z(3,3) was the lowest (0.552). The CoV of the wavefront aberrations was not computed given the low magnitude of the parameter and that values can be positive or negative. DISCUSSION Over the past few years, the use of devices for imaging the anterior segment based on the Scheimpflug principle has resulted in expanded capabilities in anterior segment imaging and measurement. Because of the good results, these devices are becoming standard in clinical practice and are often used to plan refractive management.
Table 2. Analysis of corneal thickness measurements. Reliability
ANOVA Corneal Thickness (mm) Average in central region CCT Thinnest point Thinnest point chord (mm) Thinnest point chord angle (radians)
Mean Diff
Range
P Value
Sw
CoV (%)
ICC
95% CI
1.88 2.49 4.51 0.18 0.128
0.67, 4.67 0.67, 5.33 0.00, 60.67 0.07, 0.27 0.016, 0.446
.176 .675 .207 .835 .676
1.50 2.01 3.79 0.14 0.102
0.27 0.36 0.65 16.01 43.94
0.998 0.996 0.977 0.528 0.742
0.996, 0.999 0.992, 0.998 0.953, 0.990 0.276, 0.745 0.554, 0.872
ANOVA Z analysis of variance; CCT Z central corneal thickness; CI Z confidence interval; CoV Z coefficient of variation; Diff Z difference; ICC Z intraclass correlation coefficient; Sw Z within-subject standard deviation
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et al.4 assessed its repeatability in determining anterior and posterior corneal powers, corneal thickness, and corneal aberrations, among other parameters, obtaining high repeatability values for all. Wang et al.3 found almost identical reliability for corneal power and thickness measurements in 20 unoperated eyes. In a recent study, Aramberri et al.5 compared the measurements of the Galilei G2 device and the Pentacam HR single-camera system (Oculus Optikger€ ate GmbH) in 35 healthy subjects. They found good repeatability and reproducibility for curvature, power, thickness, and anterior chamber depth measurements and improved precision over previous versions of both devices. However, the Pentacam HR system performed better for curvature, astigmatism, and corneal aberrations while the Galilei G2 system performed better for pachymetry. More recently, Huang et al.7 compared 3 rotating Scheimpflug cameras, including the Galilei, and an OCT system for measuring corneal thickness in 66 healthy subjects. Although they found very good repeatability with all systems, the Galilei was the most precise. Al-Mohtaseb et al.6 found good repeatability of Galilei CCT measurements and moderate to excellent agreement with values obtained by ultrasound (US) in normal eyes and eyes that had refractive surgery. Similar agreement with US was reported by Yeter et al.12 Menassa et al.,1 also found good repeatability of Galilei corneal thickness measurements and good agreement with Orbscan II (Bausch & Lomb) values. In the present study, excellent agreement was found for repeated measures of mean central corneal thickness, CCT, and thinnest point thickness. These results agree with those of previous studies of previous versions of the Galilei device.1,3,4,6,7 The thinnest point was at the inferotemporal quadrant in all cases, agreeing with previous findings
Figure 1. The thinnest point location in the 3 consecutive measurements of each eye (SD Z standard deviation).
Although many studies have assessed the clinical performance of Scheimpflug devices in determining all parameters they measure, to date there are no reports of the intrasubject repeatability of the values obtained using the most recently introduced commercially available version of the Galilei dual camera Scheimpflug–Placido imaging system, the G4. A newer version of the device (Galilei G6) was recently launched but, to our knowledge, is not yet commercially available. The G6 model has 3 high-definition charge-coupled device cameras for image acquisition and a Placido illumination of 750 nm (760 nm in the G4). The present study aimed to determine the intrasubject repeatability of corneal thickness, power, and wavefront aberrations measurements obtained using the Galilei G4 system. Several studies have assessed the repeatability of automatic measurements of multiple parameters using previous versions of the Galilei system. Savini Table 3. Analysis of corneal power measurements.
Reliability
ANOVA Corneal Power (D) Total Flat Steep Astigmatism Astigmatic J0 Astigmatic J45 Central Midperipheral Peripheral
Mean Diff
Range
P Value
Sw
CoV (%)
ICC
95% CI
0.19 0.17 0.14 0.21 0.11 0.08 0.11 0.06 0.10
0.02, 0.36 0.01, 0.40 0.03, 0.38 0.07, 0.54 0.03, 0.32 0.02, 0.16 0.02, 0.44 0.01, 0.38 0.03, 0.24
.528 .618 .444 .706 .458 .647 .663 .266 0.442
0.08 0.13 0.11 0.17 0.09 0.06 0.08 0.05 0.08
0.20 0.30 0.25 28.16 ----0.20 0.11 0.17
0.996 0.992 0.993 0.811 0.891 0.724 0.995 0.998 0.998
0.991, 0.998 0.983, 0.996 0.986, 0.997 0.661, 0.909 0.794, 0.949 0.529, 0.863 0.990, 0.998 0.996, 0.999 0.996, 0.999
ANOVA Z analysis of variance; CI Z confidence interval; CoV Z coefficient of variation; Diff Z difference; ICC Z intraclass correlation coefficient; J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees; Sw Z within-subject standard deviation
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Figure 2. Bland-Altman plot representing the individual repeatability of corneal power as the individual standard deviation of 3 consecutive measurements plotted against the mean power value.
with other measuring devices.13–15 The agreement between repeated measurements was moderate for the location of the thinnest point, being low for the chord distance from the corneal center (0.528). Despite this low agreement, the difference in chord distance between repeated measurements ranged from 0.07 to 0.27 mm, which could be considered clinically insignificant. With regard to the meridian in which the thinnest point was located, it differed by a mean of 7.28 degrees (range 0.92 to 25.55 degrees). Repeatability of measurements was excellent for total, flat, and steep corneal powers as well as for central, midperipheral, and peripheral corneal powers. These results are in agreement with those in studies of previous versions of the device.3–5 Reliability was only moderate, however, for corneal astigmatism, with low agreement for the astigmatic component J45 (ICC, 0.724). A possible explanation for this is the low magnitude of astigmatism in the cohort analyzed (mean 0.68 G 0.43 D), as suggested by Kobashi et al.16 The results are, however, significantly better than those reported by Aramberri et al.5
Figure 3. Distribution of corneal astigmatism in the 3 consecutive measurements of each eye (J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees).
using the Galilei G2 version in healthy eyes with similar levels of astigmatism (mean 0.86 G 0.50 D); they obtained ICCs of 0.603, 0.700, and 0.622 for corneal astigmatism, J0, and J45, respectively. The variance in the location of the pupil center in repeated measurements adds noise to the repeatability of wavefront aberrations, contributing to the variance. The reliability analysis showed ICCs ranging from 0.552 to 0.915, with the lowest ICC for trefoil. In a study by Wang et al.,3 up to 40% of the variance in repeated measurements was accounted for by the variation in pupil location in less aberrated eyes. The value increased to approximately 80% in highly aberrated eyes. Bearing this in mind, and considering additional sources of variability such as tear-film fluctuations,17 the repeatability in the present study was high for coma component Z(3,1) only. It was moderate for spherical
Table 4. Analysis of corneal wavefront aberration measurements. Reliability
ANOVA Corneal Aberration (mm) Trefoil Z(3 3) Coma Z(3, 1) SA Z(4,0) Coma Z(3,1) Trefoil Z(3,3)
Mean Diff
Range
ICC
Sw
ICC
95% CI
0.089 0.096 0.028 0.119 0.097
0.007, 0.267 0.027, 0.293 0.007, 0.127 0.027, 0.347 0.027, 0.407
0.013* 0.710 0.377 0.687 0.302
0.070 0.076 0.022 0.093 0.076
0.811 0.849 0.857 0.915 0.552
0.661, 0.909 0.772, 0.929 0.736, 0.933 0.837, 0.961 0.304, 0.760
ANOVA Z analysis of variance; CI Z confidence interval; Diff Z difference; ICC Z intraclass correlation coefficient; SA Z spherical aberration; Sw Z withinsubject standard deviation *Statistically significant
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aberration, coma component Z(3, 1), and the trefoil component Z(3, 3) and low for trefoil component Z(3,3). A recent study by Bayhan et al.18 assessed the repeatability of aberrometric measurements obtained with a single-camera Scheimpflug device in normal eyes and keratoconic eyes; the ICC values were similar overall to those in the present study. However, our reliability results are slightly worse than those reported by others with the first version of the device.3,4 In the study by Aramberri et al.5 comparing the Galilei G2 (dual camera) and the Pentacam HR (single camera), the repeatability in corneal wavefront aberration measurements was better with the single-camera Scheimpflug system. However, the ICCs were better with the Galilei G2 system. The results in the study by Aramberri et al. show similar or slightly worse values than those in the present study. The limitations of the present study must be considered when interpreting the results reported because only normal healthy corneas were examined and a limited range of corneal thickness and power was assessed. Studies expanding these analyses to larger samples of eyes with different corneal conditions would complement these results. In conclusion, the Galilei G4 dual-camera Scheimpflug imaging–Placido system had high intraobserver repeatability for corneal power and thickness measurements, as in studies of previous versions of the device, and only moderate repeatability for corneal astigmatism and corneal wavefront aberrations in normal healthy corneas. The new version of the device performs better than the preceding versions in measurements of young healthy corneas. WHAT WAS KNOWN Agreement between repeated measurements of corneal wavefront aberrations with Scheimpflug devices is moderate. WHAT THIS PAPER ADDS As did previous models, the new version of the dual Scheimpflug–Placido imaging had excellent repeatability of corneal power and thickness measurements in healthy corneas. Compared with previous versions, the new system yielded higher repeatability of corneal astigmatism measurements. Agreement between repeated measurements of corneal wavefront aberrations was moderate.
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REFERENCES 1. Menassa N, Kaufmann C, Goggin M, Job OM, Bachmann LM, Thiel MA. Comparison and reproducibility of corneal thickness and curvature readings obtained by the Galilei and the Orbscan II analysis systems. J Cataract Refract Surg 2008; 34:1742–1747 2. Shirayama M, Wang L, Weikert MP, Koch DD. Comparison of corneal powers obtained from 4 different devices. Am J Ophthalmol 2009; 148:528–535 3. Wang L, Shirayama M, Koch DD. Repeatability of corneal power and wavefront aberration measurements with a dualScheimpflug Placido corneal topographer. J Cataract Refract Surg 2010; 36:425–430 4. Savini G, Carbonelli M, Barboni P, Hoffer KJ. Repeatability of automatic measurements performed by a dual Scheimpflug analyzer in unoperated and post-refractive surgery eyes. J Cataract Refract Surg 2011; 37:302–309 5. Aramberri J, Araiz L, Garcia A, Illarramendi I, Olmos J, Oyanarte I, Romay A, Vigara I. Dual versus single Scheimpflug camera for anterior segment analysis: precision and agreement. J Cataract Refract Surg 2012; 38:1934–1949 6. Al-Mohtaseb ZN, Wang L, Weikert MP. Repeatability and comparability of corneal thickness measurements obtained from Dual Scheimpflug Analyzer and from ultrasonic pachymetry. Graefes Arch Clin Exp Ophthalmol 2013; 251:1855–1860 7. Huang J, Ding X, Savini G, Pan C, Feng Y, Cheng D, Hua Y, Hu X, Wang Q. A Comparison between Scheimpflug imaging and optical coherence tomography in measuring corneal thickness. Ophthalmology 2013; 120:1951–1958 8. de Jong T, Sheehan MT, Dubbelman M, Koopmans SA, Jansonius NM. Shape of the anterior cornea: comparison of height data from 4 corneal topographers. J Cataract Refract Surg 2013; 39:1570–1580 lvez-Romın D, Pe rez-Vives C, 9. Domınguez-Vicent A, Monsa s-Mico R. Measurement of angle Ferrer-Blasco T, Monte kappa with Orbscan II and Galilei G4: effect of accommodation. Graefes Arch Clin Exp Ophthalmol 2014; 252:249–255 10. Thibos LN, Wheeler W, Horner D. Power vectors: an application of Fourier analysis to the description and statistical analysis of refractive error. Optom Vis Sci 1997; 74:367–375. Available at: http://journals.lww.com/optvissci/Abstract/1997/06000/Power_ Vectors__An_Application_of_Fourier_Analysis.19.aspx. Accessed September 3, 2014 11. McGraw KO, Wong SP. Forming inferences about some intraclass correlation coefficients. Psychol Meth 1996; 1:30–46 €nmez B, Beden U. Comparison of central corneal 12. Yeter V, So thickness measurements by Galilei Dual-Scheimpflug analyzerÒ and ultrasound pachymeter in myopic eyes. Ophthalmic Surg Lasers Imaging 2012; 43:128–134 13. Ashwin PT, Shah S, Pushpoth S, Wehbeh L, Ilango B. The relationship of central corneal thickness (CCT) to thinnest central cornea (TCC) in healthy adults. Cont Lens Anterior Eye 2009; 32:64–67 14. Keech A, Simpson T, Jones L. Repeatability of pachymetry and thinnest point localization using a Fourier-domain optical coherence tomographer. Optom Vis Sci 2010; 87:736–741. Available at: http://journals.lww.com/optvissci/Fulltext/2010/ 10000/Repeatability_of_Pachymetry_and_Thinnest_Point.6.aspx. Accessed September 3, 2014 15. Hashemi H, Asgari S, Mehravaran S, Emamian MH, Shariati M, Fotouhi A. The distribution of corneal thickness in a 40- to
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64-year-old population of Shahroud, Iran. Cornea 2011; 30:1409–1413 16. Kobashi H, Kamiya K, Igarashi A, Ishii R, Sato N, Wang G, Shimizu K. Comparison of corneal power, corneal astigmatism, and axis location in normal eyes obtained from an autokeratometer and a corneal topographer. J Cataract Refract Surg 2012; 38:648–654 s-Mico R, Alio JL, Mun ~ oz G, Charman WN. Temporal 17. Monte changes in optical quality of air-tear film interface at anterior cornea after blink. Invest Ophthalmol Vis Sci 2004; 45:1752– 1757. Available at: http://www.iovs.org/cgi/reprint/45/6/1752. Accessed September 3, 2014 18. Bayhan HA, Aslan Bayhan S, Muhafız E, Can _I. Repeatability of aberrometric measurements in normal and keratoconus eyes
using a new Scheimpflug-Placido topographer. J Cataract Refract Surg 2014; 40:269–275
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First author: Alejandro Cervi~ no, PhD Optometry Research Group, Optics Department, University of Valencia, Valencia, Spain
Intrasubject repeatability of corneal power, thickness, and wavefront aberrations with a new version of a dual rotating Scheimpflug–Placido system Alejandro Cervi~ no, PhD, Alberto Dominguez-Vicent, MSc, Teresa Ferrer-Blasco, PhD, Santiago García-L azaro, PhD, Cesar Albarran-Diego, PhD
PURPOSE: To determine the intrasubject repeatability of a recently introduced dual-camera rotating Scheimpflug–Placido imaging system (Galilei G4) in determining corneal thickness, power, and wavefront aberrations in young healthy subjects. SETTING: University of Valencia, Valencia, Spain. DESIGN: Prospective evaluation of diagnostic technology. METHODS: The study comprised right eyes of 25 subjects aged 20 to 40 years with a spherical equivalent ranging from 4.25 to C1.00 diopters. The central corneal thickness, thinnest point value and location, anterior and posterior surface curvatures, total corneal power, and corneal wavefront aberrations were measured for distance vision using the dual Scheimpflug–Placido system. Three consecutive measurements were taken in each eye. RESULTS: Twenty-five eyes were evaluated. Repeated-measures analysis of variance showed the only statistically significant difference between the 3 repeated measures to be in trefoil aberration. Intraclass correlation coefficients (ICCs) were higher than 0.950 for all the parameters except the thinnest point chord distance to geometric corneal center (0.528) and chord angle (0.742), corneal astigmatism (0.811) and its vector components J0 (0.891) and J45 (0.724), and all wavefront aberrations. CONCLUSIONS: The new dual Scheimpflug–Placido system had high intraobserver repeatability for corneal power and thickness and moderate repeatability for corneal astigmatism and corneal wavefront aberrations in healthy corneas with low astigmatism. This iteration of the device performed better in young healthy corneas than preceding versions. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2015; 41:186–192 Q 2015 ASCRS and ESCRS
Accurate corneal power determination is particularly important for intraocular lens calculation and surgical planning of astigmatism correction. The corneal wavefront aberration profile can be used for custom ablation planning. In addition, determining the corneal thickness is crucial in several areas, including refractive surgery candidate assessment, surgical planning, corneal crosslinking, and intrastromal corneal ring segment. Over the years, many new imaging devices for anterior segment parameter determination have become 186
Q 2015 ASCRS and ESCRS Published by Elsevier Inc.
available to clinicians. Although these systems are accurate and repeatable, the significant differences between them limit the interchangeability of the data obtained. The greatest progress in the past few years has been in Scheimpflug imaging and optical coherence tomography (OCT). Every time a new device or an enhanced version of an existing device is launched, it is essential to determine its repeatability. Previous versions of the Galilei dual-camera rotating Scheimpflug imaging device (Ziemer Ophthalmic Systems AG) have been http://dx.doi.org/10.1016/j.jcrs.2014.04.037 0886-3350
REPEATABILITY DUAL SCHEIMPFLUG–PLACIDO CORNEAL MEASUREMENTS
thoroughly assessed.1–6 The device has been shown to provide repeatable and precise measurements of anterior segment parameters and compared against other Scheimpflug devices and alternative imaging techniques.1,2,5–8 Although the latest version approved for clinical practice (Galilei G4) has been studied,9 there are no reports of the device's intrasubject repeatability. Thus, the aim of the present study was to determine the clinical performance of the dual Scheimpflug–Placido in measuring corneal thickness, power, and wavefront aberrations in healthy eyes. SUBJECTS AND METHODS The study comprised healthy right eyes of young subjects. The subjects were randomly included from the postgraduate and staff population at the University of Valencia. All subjects provided informed consent after receiving a verbal and written explanation of the nature and possible consequences of the study. The study followed the tenets of the Declaration of Helsinki.
to the Femto LDV laser (Ziemer Ophthalmic Systems AG) for exchange of patient data between devices. It also gives information about cone location, magnitude, and keratoconus likelihood and allows calculation of the equivalent defocus from Zernike coefficients. In the present study, the system was used to automatically obtain measurements of corneal power, thickness, and aberrations. Corneal power data were obtained using the system's “total corneal power by ray tracing” setting, which calculates the propagation of incoming parallel rays and uses the Snell law to refract these rays through the anterior and posterior corneal surfaces. The thinnest point location was analyzed by computing the chord distance from the geometric center and chord angle from the x–y offset coordinates given by the instrument. Analysis of wavefront aberrations was performed with a 6.0 mm pupil diameter. Corneal astigmatism was recorded as the dioptric value given by the instrument as well as by determining its vectorial components J0 and J45, as suggested by Thibos et al.,10 because vector analysis includes the information for both the power and the angle. The conversion to power vector notation was performed using the following equations, where C is the negative cylindrical power and a is the cylinder axis: J0 Z
C cos2a 2
J45 Z
C sin2a 2
Inclusion and Exclusion Criteria Inclusion criteria were age between 20 years and 40 years and a corrected visual acuity (CDVA) of 20/25 or better. Subjects with a CDVA worse than 20/25, ocular or systemic disease, history of ocular surgery, intraocular pressure above 21 mm Hg, or retinal or optic disc pathology were excluded from taking part in the study.
Scheimpflug–Placido System The Galilei G4 system is the latest version of the Galilei topographer. This noninvasive optical diagnostic device designed for the assessment of the eye's anterior segment is based on a rotating dual-Scheimpflug camera and a Placido topography system. The images recorded by this device include the cornea, iris, pupil, limbus, anterior chamber, and lens. For each standard 3-dimensional (3-D) scan, a 3D model of the anterior segment is generated by integrating the Placido image and the rotated dual-Scheimpflug images. The instrument has a red light–emitting diode near-to-far adjustable fixation target that can be moved from 20.00 to C20.00 D in steps of 0.25 diopter. According to the manufacturer, the new features include the ability to select fewer images per scan (range between 7 and 30) and connectivity
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Experimental Technique All eyes were measured under distance viewing conditions 3 consecutive times. The patient was repositioned and the device realigned after each measurement. The same experienced operator, who was not aware of the study goal, performed all the measurements.
Statistical Analysis Statistical analysis was performed using SPSS software (SPSS/Pc C 10.1 for Windows, SPSS, Inc.). Repeatedmeasures analysis of variance (ANOVA) was performed and the intraclass correlation coefficient (ICC) and coefficient of variation (CoV) were obtained for each parameter analyzed. The ICCs were classified using a system suggested by McGraw and Wong11 as follows: (1) less than 0.75 Z poor agreement; 0.75 to less than 0.90 Z moderate agreement; (3) 0.90 or greater Z high agreement. A P value less than 0.05 was considered statistically significant.
RESULTS
Submitted: January 12, 2014. Final revision submitted: March 24, 2014. Accepted: April 14, 2014. From the Optometry Research Group, Optics Department, University of Valencia, Valencia, Spain. Supported by an Atraccio de Talent research scholarship, Universidad de Valencia (Mr. Domınguez-Vicent). Corresponding author: Alejandro Cervin~o, PhD, Departamento de Optica–Universidad de Valencia, C/Dr. Moliner, 50–46100– Burjassot, Espa~na. E-mail: [email protected].
The study included 25 eyes of 25 subjects (12 men, 13 women). Table 1 shows the patients' demographics and the values of the measured parameters. Corneal Thickness Table 2 shows the repeatability of the corneal thickness measurements. Repeated-measures ANOVA did not show a statistically significant difference in the values between the 3 repeated measures for any corneal thickness analyzed; that is, corneal thickness
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Table 1. Patient demographics and characteristics. Parameter
Mean G SD
Age (y) Refractive error SE(D) Corneal thickness (mm) Average in central region CCT Thinnest point Thinnest point chord (mm) Thinnest point chord angle (radians) Corneal power (D) Total Flat Steep Astigmatism Astigmatic J0 Astigmatic J45 Central Midperipheral Peripheral Corneal aberrations (mm) Trefoil Z(3, 3) Coma Z(3, 1) SA Z(4,0) Coma Z(3,1) Trefoil Z(3,3)
30.36 G 7.32 0.80 G 2.33
Range 20, 40 4.25, C1.00
554.59 G 41.20 486.00, 611.67 547.17 G 41.01 478.67, 604.67 544.30 G 42.22 473.00, 599.67 0.91 G 0.20 0.43, 1.27 0.476 G 0.228 0.039, 0.932
42.59 G 1.58 42.25 G 1.61 42.93 G 1.57 0.68 G 0.43 0.21 G 0.31 0.03 G 0.13 42.62 G 1.55 42.97 G 1.79 44.57 G 2.12
39.32, 44.79 39.06, 44.51 39.55, 44.99 0.31, 2.01 0.52, 0.99 0.25, 0.24 39.41, 44.72 39.14, 45.74 39.87, 47.92
0.051 G 0.164 0.068 G 0.231 0.121 G 0.077 0.013 G 0.361 0.011 G 0.128
0.360, 0.283 0.337, 0.430 0.000, 0.280 0.440, 0.640 0.210, 0.190
CCT Z central corneal thickness; J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees; SA Z spherical aberration; SE Z spherical equivalent
in the central region, central corneal thickness (CCT), and thinnest point (all P O .05). The ICCs were greater than 0.950 in all cases, showing good agreement between the repeated measures. The thinnest point location was at the inferotemporal quadrant in all cases (Figure 1) and differences were statistically significant between the 3 consecutive measurements (P O .05). The ICCs, however, were low for the chord distance to the geometric corneal center (0.528) and to the chord angle (0.742), indicating
that the agreement in thinnest point location was low between the repeated measurements. The CoV for the chord distance and the angle for the thinnest point were 16.01% and 43.94%, respectively. Total Corneal Power Table 3 shows the repeated measurements of the total corneal power, flattest corneal power, steepest corneal power, corneal astigmatism magnitude and its J0 and J45 components, and the corneal power in the central, midperipheral, and peripheral corneal locations. The variation in the measurements obtained for total, flat, and steep power was less than 0.50 D in all cases (Figure 2). Repeated-measures ANOVA did not show significant differences in the values between the repeated measurements (all P O .05). The lowest agreement was in the corneal astigmatism magnitude (ICC, 0.811) particularly in the J45 component (ICC, 0.724). Figure 3 shows the distribution of corneal astigmatism as a function of its vectorial components. Corneal Wavefront Aberrations Table 4 shows the results of the corneal aberrations analysis. There were statistically significant differences in the trefoil component Z(3, 3) (P Z .013) with mild agreement between the repeated measurements. The ICC for the trefoil component Z(3,3) was the lowest (0.552). The CoV of the wavefront aberrations was not computed given the low magnitude of the parameter and that values can be positive or negative. DISCUSSION Over the past few years, the use of devices for imaging the anterior segment based on the Scheimpflug principle has resulted in expanded capabilities in anterior segment imaging and measurement. Because of the good results, these devices are becoming standard in clinical practice and are often used to plan refractive management.
Table 2. Analysis of corneal thickness measurements. Reliability
ANOVA Corneal Thickness (mm) Average in central region CCT Thinnest point Thinnest point chord (mm) Thinnest point chord angle (radians)
Mean Diff
Range
P Value
Sw
CoV (%)
ICC
95% CI
1.88 2.49 4.51 0.18 0.128
0.67, 4.67 0.67, 5.33 0.00, 60.67 0.07, 0.27 0.016, 0.446
.176 .675 .207 .835 .676
1.50 2.01 3.79 0.14 0.102
0.27 0.36 0.65 16.01 43.94
0.998 0.996 0.977 0.528 0.742
0.996, 0.999 0.992, 0.998 0.953, 0.990 0.276, 0.745 0.554, 0.872
ANOVA Z analysis of variance; CCT Z central corneal thickness; CI Z confidence interval; CoV Z coefficient of variation; Diff Z difference; ICC Z intraclass correlation coefficient; Sw Z within-subject standard deviation
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et al.4 assessed its repeatability in determining anterior and posterior corneal powers, corneal thickness, and corneal aberrations, among other parameters, obtaining high repeatability values for all. Wang et al.3 found almost identical reliability for corneal power and thickness measurements in 20 unoperated eyes. In a recent study, Aramberri et al.5 compared the measurements of the Galilei G2 device and the Pentacam HR single-camera system (Oculus Optikger€ ate GmbH) in 35 healthy subjects. They found good repeatability and reproducibility for curvature, power, thickness, and anterior chamber depth measurements and improved precision over previous versions of both devices. However, the Pentacam HR system performed better for curvature, astigmatism, and corneal aberrations while the Galilei G2 system performed better for pachymetry. More recently, Huang et al.7 compared 3 rotating Scheimpflug cameras, including the Galilei, and an OCT system for measuring corneal thickness in 66 healthy subjects. Although they found very good repeatability with all systems, the Galilei was the most precise. Al-Mohtaseb et al.6 found good repeatability of Galilei CCT measurements and moderate to excellent agreement with values obtained by ultrasound (US) in normal eyes and eyes that had refractive surgery. Similar agreement with US was reported by Yeter et al.12 Menassa et al.,1 also found good repeatability of Galilei corneal thickness measurements and good agreement with Orbscan II (Bausch & Lomb) values. In the present study, excellent agreement was found for repeated measures of mean central corneal thickness, CCT, and thinnest point thickness. These results agree with those of previous studies of previous versions of the Galilei device.1,3,4,6,7 The thinnest point was at the inferotemporal quadrant in all cases, agreeing with previous findings
Figure 1. The thinnest point location in the 3 consecutive measurements of each eye (SD Z standard deviation).
Although many studies have assessed the clinical performance of Scheimpflug devices in determining all parameters they measure, to date there are no reports of the intrasubject repeatability of the values obtained using the most recently introduced commercially available version of the Galilei dual camera Scheimpflug–Placido imaging system, the G4. A newer version of the device (Galilei G6) was recently launched but, to our knowledge, is not yet commercially available. The G6 model has 3 high-definition charge-coupled device cameras for image acquisition and a Placido illumination of 750 nm (760 nm in the G4). The present study aimed to determine the intrasubject repeatability of corneal thickness, power, and wavefront aberrations measurements obtained using the Galilei G4 system. Several studies have assessed the repeatability of automatic measurements of multiple parameters using previous versions of the Galilei system. Savini Table 3. Analysis of corneal power measurements.
Reliability
ANOVA Corneal Power (D) Total Flat Steep Astigmatism Astigmatic J0 Astigmatic J45 Central Midperipheral Peripheral
Mean Diff
Range
P Value
Sw
CoV (%)
ICC
95% CI
0.19 0.17 0.14 0.21 0.11 0.08 0.11 0.06 0.10
0.02, 0.36 0.01, 0.40 0.03, 0.38 0.07, 0.54 0.03, 0.32 0.02, 0.16 0.02, 0.44 0.01, 0.38 0.03, 0.24
.528 .618 .444 .706 .458 .647 .663 .266 0.442
0.08 0.13 0.11 0.17 0.09 0.06 0.08 0.05 0.08
0.20 0.30 0.25 28.16 ----0.20 0.11 0.17
0.996 0.992 0.993 0.811 0.891 0.724 0.995 0.998 0.998
0.991, 0.998 0.983, 0.996 0.986, 0.997 0.661, 0.909 0.794, 0.949 0.529, 0.863 0.990, 0.998 0.996, 0.999 0.996, 0.999
ANOVA Z analysis of variance; CI Z confidence interval; CoV Z coefficient of variation; Diff Z difference; ICC Z intraclass correlation coefficient; J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees; Sw Z within-subject standard deviation
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Figure 2. Bland-Altman plot representing the individual repeatability of corneal power as the individual standard deviation of 3 consecutive measurements plotted against the mean power value.
with other measuring devices.13–15 The agreement between repeated measurements was moderate for the location of the thinnest point, being low for the chord distance from the corneal center (0.528). Despite this low agreement, the difference in chord distance between repeated measurements ranged from 0.07 to 0.27 mm, which could be considered clinically insignificant. With regard to the meridian in which the thinnest point was located, it differed by a mean of 7.28 degrees (range 0.92 to 25.55 degrees). Repeatability of measurements was excellent for total, flat, and steep corneal powers as well as for central, midperipheral, and peripheral corneal powers. These results are in agreement with those in studies of previous versions of the device.3–5 Reliability was only moderate, however, for corneal astigmatism, with low agreement for the astigmatic component J45 (ICC, 0.724). A possible explanation for this is the low magnitude of astigmatism in the cohort analyzed (mean 0.68 G 0.43 D), as suggested by Kobashi et al.16 The results are, however, significantly better than those reported by Aramberri et al.5
Figure 3. Distribution of corneal astigmatism in the 3 consecutive measurements of each eye (J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees).
using the Galilei G2 version in healthy eyes with similar levels of astigmatism (mean 0.86 G 0.50 D); they obtained ICCs of 0.603, 0.700, and 0.622 for corneal astigmatism, J0, and J45, respectively. The variance in the location of the pupil center in repeated measurements adds noise to the repeatability of wavefront aberrations, contributing to the variance. The reliability analysis showed ICCs ranging from 0.552 to 0.915, with the lowest ICC for trefoil. In a study by Wang et al.,3 up to 40% of the variance in repeated measurements was accounted for by the variation in pupil location in less aberrated eyes. The value increased to approximately 80% in highly aberrated eyes. Bearing this in mind, and considering additional sources of variability such as tear-film fluctuations,17 the repeatability in the present study was high for coma component Z(3,1) only. It was moderate for spherical
Table 4. Analysis of corneal wavefront aberration measurements. Reliability
ANOVA Corneal Aberration (mm) Trefoil Z(3 3) Coma Z(3, 1) SA Z(4,0) Coma Z(3,1) Trefoil Z(3,3)
Mean Diff
Range
ICC
Sw
ICC
95% CI
0.089 0.096 0.028 0.119 0.097
0.007, 0.267 0.027, 0.293 0.007, 0.127 0.027, 0.347 0.027, 0.407
0.013* 0.710 0.377 0.687 0.302
0.070 0.076 0.022 0.093 0.076
0.811 0.849 0.857 0.915 0.552
0.661, 0.909 0.772, 0.929 0.736, 0.933 0.837, 0.961 0.304, 0.760
ANOVA Z analysis of variance; CI Z confidence interval; Diff Z difference; ICC Z intraclass correlation coefficient; SA Z spherical aberration; Sw Z withinsubject standard deviation *Statistically significant
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aberration, coma component Z(3, 1), and the trefoil component Z(3, 3) and low for trefoil component Z(3,3). A recent study by Bayhan et al.18 assessed the repeatability of aberrometric measurements obtained with a single-camera Scheimpflug device in normal eyes and keratoconic eyes; the ICC values were similar overall to those in the present study. However, our reliability results are slightly worse than those reported by others with the first version of the device.3,4 In the study by Aramberri et al.5 comparing the Galilei G2 (dual camera) and the Pentacam HR (single camera), the repeatability in corneal wavefront aberration measurements was better with the single-camera Scheimpflug system. However, the ICCs were better with the Galilei G2 system. The results in the study by Aramberri et al. show similar or slightly worse values than those in the present study. The limitations of the present study must be considered when interpreting the results reported because only normal healthy corneas were examined and a limited range of corneal thickness and power was assessed. Studies expanding these analyses to larger samples of eyes with different corneal conditions would complement these results. In conclusion, the Galilei G4 dual-camera Scheimpflug imaging–Placido system had high intraobserver repeatability for corneal power and thickness measurements, as in studies of previous versions of the device, and only moderate repeatability for corneal astigmatism and corneal wavefront aberrations in normal healthy corneas. The new version of the device performs better than the preceding versions in measurements of young healthy corneas. WHAT WAS KNOWN Agreement between repeated measurements of corneal wavefront aberrations with Scheimpflug devices is moderate. WHAT THIS PAPER ADDS As did previous models, the new version of the dual Scheimpflug–Placido imaging had excellent repeatability of corneal power and thickness measurements in healthy corneas. Compared with previous versions, the new system yielded higher repeatability of corneal astigmatism measurements. Agreement between repeated measurements of corneal wavefront aberrations was moderate.
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First author: Alejandro Cervi~ no, PhD Optometry Research Group, Optics Department, University of Valencia, Valencia, Spain
