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ARTICLE
Higher-order aberration measurements: Comparison between Scheimpflug and dual Scheimpflug–Placido technology in normal eyes Andre L. Piccinini, MD, Oren Golan, MD, Farhad Hafezi, MD, PhD, J. Bradley Randleman, MD
Purpose: To compare higher order aberrations (HOAs) in normal eyes between a Scheimpflug imaging system (Pentacam HR) and dual Scheimpflug–Placido imaging system (Galilei G4). Setting: Emory University, Atlanta, Georgia, and the University of Southern California, Los Angeles, USA. Design: Retrospective case series. Methods: Eyes screened for refractive surgery were evaluated sequentially using a Scheimpflug device and a dual Scheimpflug– Placido device. Differences, correlations, and agreement between values for total root mean square (RMS), trefoil, coma, and spherical aberration were analyzed, and Bland-Altman plots were generated.
aberration values were not. There was moderate correlation between devices for trefoil (r Z 0.475 to 0.652), coma (r Z 0.574 to 0.651), and spherical aberration (r Z 0.483) and a strong correlation for total cornea RMS (r Z 0.817). There was no directional bias between groups. The 95% limits of agreement for absolute values was 0.039 mm for trefoil at 30 degrees, 0.405 mm for trefoil at 0 degree, 0.553 mm for horizontal coma, 0.545 mm for vertical coma, 0.318 mm for spherical aberration, and 0.617 mm for RMS.
Results: One hundred five eyes of 105 patients (44 men, 61
Conclusions: A Scheimpflug imaging device and dual Scheimpflug–Placido imaging device generated statistically different values for total cornea HOAs; however, the correlation between devices was moderate to strong and there was reasonable agreement in all measures for normal eyes. Based on these findings, the devices appear functionally equivalent for clinical use, although caution is warranted for outcomes-based research protocols that report HOAs.
women) were evaluated. Total RMS, coma, and trefoil were significantly different between groups (all P < .001), while spherical
J Cataract Refract Surg 2018; -:-–- Q 2018 ASCRS and ESCRS
T
he evolution of wavefront technology analysis has contributed significantly to the understanding and management of the eye’s aberrations, in particular for planning the correction of refractive errors with the excimer laser and cataract surgery.1–3 Conventional excimer laser ablation patterns induce significant higherorder aberrations (HOAs), including an increased root-mean-square (RMS) error, increased total HOAs, coma, and spherical aberrations.4–9 Wavefrontoptimized, wavefront-guided, and topography-guided ablation profiles minimize or reduce various HOAs, in particular in highly aberrated eyes.4,5,10–12 Small-incision lenticule extraction has been found to induce variable
degrees of coma postoperatively, frequently greater than for typical laser in situ keratomileusis procedures, depending on lenticule centration.13–15 In addition to tracking outcomes after corneal refractive surgery, HOAs have been used in keratoconus screening and tracking outcomes after corneal crosslinking and play an important role in intraocular lens selection for cataract surgery because of the differing aberration profiles between spherical and aspheric intraocular lens designs.16,17 At present, several technologies to assess wavefront aberrations are available, the most common of which use Hartmann-Shack, Tscherning, or laser ray tracing, in which optical aberrations are measured from the light emitted by
Submitted: August 27, 2018 | Final revision submitted: October 25, 2018 | Accepted: November 19, 2018 From the Keck School of Medicine, University of Southern California (Piccinini, Golan, Hafezi, Randleman), and USC Roski Eye Institute (Hafezi, Randleman), Los Angeles, USA; the Sadalla Amin Ghanem Eye Hospital (Piccinini), Joinville, SC, Brazil; the Department of Ophthalmology (Golan), Tel Aviv Souraski Medical Center, and the Tel Aviv University (Golan), Tel Aviv, Israel; ELZA Institute (Hafezi), Dietikon/Zurich, and the Ocular Cell Biology Group (Hafezi), University of Zurich, Zurich, Switzerland; University of Wenzhou (Hafezi), Wenzhou, China. Supported in part by an unrestricted departmental grant to the USC Roski Eye Institute from Research to Prevent Blindness, Inc., New York, New York, USA. Corresponding author: J. Bradley Randleman, MD, 1450 San Pablo St, Los Angeles, CA 90033, USA. Email:
[email protected]. Q 2018 ASCRS and ESCRS Published by Elsevier Inc.
0886-3350/$ - see frontmatter https://doi.org/10.1016/j.jcrs.2018.11.015
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the aberrometer as it is reflected by the retina.18,19 Two common devices in clinical use today for corneal tomographic evaluation use Scheimpflug technology (Pentacam HR, Oculus Surgical, Inc.) and dual Scheimpflug technology in combination with Placido-based reflection imaging (Galilei, Ziemer Ophthalmic Systems AG). Both systems also evaluate HOAs using corneal elevation data to construct corneal aberration data. Numerous studies have been performed to evaluate agreement between Scheimpflug and dual Scheimpflug imaging systems for multiple variables20–23; however, there is a paucity of data on the agreement of HOA measurements between these devices.24 The purpose of this study was to compare the agreement, equivalence, and interchangeability of HOAs measurements obtained with a Scheimpflug imaging device and a dual Scheimpflug–Placido imaging device in normal eyes. PATIENTS AND METHODS This retrospective between-devices comparative study was approved separately by the Institutional Review Boards of Emory University, Atlanta, Georgia, USA, and the University of Southern California, Los Angeles, USA. All patients in this study presented for refractive surgery evaluation at Emory Vision, Atlanta, Georgia, between 2015 and 2016. Patients included in this study had both eyes evaluated by the Scheimpflug imaging device and the dual Scheimpflug imaging device at the same visit. The evaluations were performed sequentially by the same technician and were followed by a complete clinical examination to determine whether the patient was a candidate for refractive surgery. All patients had normal evaluations and were deemed suitable candidates for laser in situ keratomileusis by an ophthalmologist (J.B.R.). Data output of the right eye of all patients were chosen for wavefront analysis from devices to standardize the data-collection process. Data collected included total cornea HOAs, including the RMS of total cornea HOAs, trefoil at 30 degrees and 0 degree, vertical and horizontal coma, and spherical aberration. Scheimpflug Imaging The Pentacam HR device uses a rotating Scheimpflug camera that takes up to 100 images with 500 measurement points on the anterior and posterior corneal surfaces over a 180-degree rotation. The elevation data from the images are combined to form a 3-dimensional reconstruction of the corneal structure. After processing the information, the corneal elevation profile is automatically
converted into corneal wavefront data. Figure 1, A, shows an example of the Zernike polynomial map from this Scheimpflug device. Dual Scheimpflug–Placido Imaging The Galilei G4 device uses 2 rotating Scheimpflug cameras integrated with a Placido topographer. Higher-order aberrations data are assessed based on elevation data and then described as a Zernike polynomial map. Figure 1, B, shows an example of the Zernike polynomial map from this dual Scheimpflug–Placido device. Statistical Analysis Paired t tests and Pearson r correlations were performed using SPSS for Windows software (version 24.0, IBM Corp.) to compare the RMS of total corneal HOAs, coma divided in vertical and horizontal, trefoil at 30 degrees and 0 degree, and spherical aberration. A P value less than 0.05 was considered statistically significant. Agreement between devices was analyzed using Bland-Altman plots generated by plotting the differences of measurements between devices against the average of the 2 measurements. The mean difference (bias) and 95% limits of agreement (LoA) were calculated. The LoA were computed as the mean difference G 1.96 SD, and they represent the limits of the range for the 95% of differences between the 2 instruments.
RESULTS One hundred five eyes of 105 patients were analyzed. The mean age of the 44 men and 61 women was 42 years G 19 (SD). Table 1 shows the summary data, with a comparison of wavefront measurements from both devices. Figure 2 shows the comparative differences in each HOA between device measurements; the mean differences were higher for all aberrations measured with the Scheimpflug device except for spherical aberration, which was not significantly different between devices. There was a moderate positive correlation between devices for total trefoil, total coma, and spherical aberration and a strong positive correlation for total RMS (Table 1). Bland-Altman analysis of the clinical agreement of total ocular HOAs measurements did not find any directional bias between the 2 devices (Figure 3). The 95% LoA for absolute values were 0.039 mm for trefoil at 30 degrees, 0.405 mm for trefoil at 0 degree, 0.553 mm for horizontal
Figure 1. Output of wavefront analysis showing Scheimpflug imaging from the Pentacam HR device (A) and dual Scheimpflug imaging from the Galilei G4 device (B).
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Table 1. HOA measurements for Scheimpflug and dual Scheimpflug–Placido devices. Mean (mm) ± SD
HOA
Scheimpflug
Dual Scheimpflug– Placido
Trefoil 3rd, 30-degrees Trefoil 3rd, 0-degree Coma horizontal Coma vertical Spherical RMS
0.042 G 0.108 0.010 G 0.081 0.025 G 0.126 0.060 G 0.159 0.224 G 0.083 1.510 G 0.687
0.008 G 0.130 0.062 G 0.113 0.177 G 0.168 0.005 G 0.172 0.212 G 0.076 1.238 G 0.677
Difference 0.034 G 0.101 0.052 G 0.103 0.152 G 0.141 0.055 G 0.139 0.012 G 0.081 0.272 G 0.413
95% LoA 0.233, C0.166 0.150, C0.254 0.125, C0.427 0.328, C0.217 0.159, C0.159 0.536, C1.081
P Value*
r Value†
.001 !.001 !.001 !.001 .125 !.001
0.652 0.475 0.574 0.651 0.483 0.817
HOA Z higher-order aberration; LoA Z limits of agreement; RMS Z root mean square *Significant at P ! .05, paired t test † Pearson correlation significant at 0.01 level
coma, 0.545 mm for vertical coma, 0.318 mm for spherical aberration, and 1.617 mm for RMS.
DISCUSSION In this study, the Pentacam HR Scheimpflug device and the Galilei G4 dual Scheimpflug–Placido device produced significantly different values for all HOAs evaluated except spherical aberration; however, correlations between devices were moderate to strong and there was reasonable agreement in all values. These findings indicate that HOA values appear reasonably equivalent for clinical use for normal eyes. The differences and lack of optimum agreement between devices could be significant in research reports that use HOAs as endpoint measures for diagnostic or therapeutic purposes.
In previous studies, Scheimpflug devices and dual Scheimpflug–Placido devices have shown varying degrees of comparability for other measures. Fahd et al.20 found a good correlation between the Pentacam device and the Galilei device in pachymetry, anterior and posterior elevation, and curvature measurements in normal eyes.21–23 Salouti et al.25 found significant differences in corneal anterior and posterior elevations and concluded that device interchangeability was limited. Jahadi Hosseini et al.26 also reported statistically significant differences central and thinnest corneal thickness measurements between the Pentacam HR device and the Galilei device, but with good correlation and agreement. Other studies18,19,27,28 evaluated the agreement in HOA measurements between devices based on different technologies and found they are not interchangeable but are often reasonably well correlated.
Figure 2. Boxplot of the comparative distribution of higher-order aberrations for Scheimpflug imaging and dual Scheimpflug–Placido imaging for trefoil at 30 degrees, trefoil at 0 degree, spherical aberration, horizontal coma, vertical coma, and total cornea root mean square (RMS). Volume - Issue - - 2019
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Figure 3. Bland-Altman analysis of the agreement between Scheimpflug imaging and dual Scheimpflug imaging for trefoil at 30 degrees, trefoil at 0 degree, spherical aberration, horizontal coma, vertical coma, and total cornea root mean square (RMS).
Few studies in the literature have directly compared aberrometers. Cade et al.18 performed an analysis of 4 aberrometers based on different software (HartmannShack, ray-tracing, and Tscherning) and found good agreement but significant differences in measurements of total HOAs. Xu et al.19 compared the agreement and interchangeability between the KR-1 W Hartmann-Shack aberrometer (Topcon) and the iTrace ray-tracing aberrometer (Tracey Technologies Corp.) and found significant differences in total HOA measurements. Hao et al.27 performed a study with the same design and same devices and found statistical differences in total ocular and internal HOAs but not in corneal HOAs, with the mean values measured by iTrace device being higher than those measured with the KR-1 W device. Burakgazi et al.28 compared and assessed the repeatability between the OPD-Scan (Nidek) device and 2 Hartmann-Shack aberrometer models; they found good repeatability and significant differences. One explanation for the discrepancy between measurements might be the differences in the algorithm used to locate the chief ray of each lenslet image or the pupil center. As a consequence, any disparity in the mathematic calculation used by each device can produce slightly different results.29 In our study, the agreement in total ocular HOA measurements between the Scheimpflug device and dual Scheimpflug–Placido device was similar to the results in previous studies20–23,26 that evaluated other parameters, such as anterior and posterior corneal elevation and pachymetry values.
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There are limitations to this study. Our data were based on retrospective analysis; however, we do not believe this affected our findings because all measures were performed on the same day by the same well-trained technician in a standardized fashion. We did not perform multiple measurements for each device but relied on a single quality examination from each device for comparison, similar to what is done in routine clinical practice. A previous study30 found that the Scheimpflug device and dual Scheimpflug–Placido device had reasonably good repeatability and reproducibility for overall HOAs. Nevertheless, a future prospective analysis could better determine the validity of these findings. Only normal eyes were evaluated in this study. It is therefore uncertain what the agreement between the 2 devices would be in other situations, such as in ectatic corneas, corneas with irregular astigmatism from other mechanisms, or in eyes that had corneal refractive surgery, in which the variations between devices might be higher. Finally, the lack of a gold standard method to measure ocular aberrations limits our ability to determine the superiority of one device over the other when differences arose.18 In summary, there were significant differences in HOA measurements obtained by the Pentacam HR Scheimpflug device and the Galilei G4 dual Scheimpflug–Placido device but reasonable correlation and LoA between devices. These findings suggest that HOA measurements from these devices can be considered reasonably equivalent for clinical use, although caution is warranted when using HOAs
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measured obtained by different technologies for diagnostic or therapeutic outcomes–based research.
WHAT WAS KNOWN A variety of systems are available to measure higher order aberrations (HOAs). Scheimpflug and dual ScheimpflugPlacido devices show moderate agreement for multiple elevation and pachymetric metrics. Less is known regarding agreement in HOA measurements between these devices.
WHAT THIS PAPER ADDS Measured HOAs (total root mean square, coma, trefoil, and spherical aberration) were significantly different between the 2 devices; however, values were reasonably correlated and limits of agreement were sufficiently close to deem measurements from each device reasonably equivalent for clinical use but not for outcomes-based research reporting.
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Disclosures: None of the authors has a financial or proprietary interest in any material or method mentioned.
First author: Andre L. Piccinini, MD Keck School of Medicine of the University of Southern California, Los Angeles, USA
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