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Comparison of Corneal Tomography Measurements Using Galilei, Orbscan II, and Placido Disk–based Topographer Systems
Abstract
To evaluate agreement in keratometry readings and anterior and posterior elevation map measurements among the Galilei V4.01 (Ziemer), Orbscan IIz (Bausch & Lomb), and Corneal Map topographer (Costruzione Strumenti Oftalmici) systems.
This prospective comparative study comprised 184 eyes of 92 consecutive refractive surgery candidates who were simultaneously examined with the Galilei (dual Scheimpflug), Orbscan II (scanning-slit), and Corneal Map topographer (Placido disk–based) systems. Keratometry readings and anterior and posterior elevation map measurements were compared using analysis of variance and paired t test, respectively.
Mean keratometry reading was 44.30±1.49 diopters (D), 44.11±1.47 D, and 44.60±1.56 D with the Galilei, Orbscan, and Corneal Map topographer, respectively. Despite a significant difference in mean keratometry ( P<.001), the correlation among these three systems was strong. The maximum mean difference between two sets in simulated keratometry and astigmatism was <0.50 D. In the evaluation of anterior best-fit-sphere (BFS) and posterior BFS, the correlation between Galilei and Orbscan II was found to be 0.960 and 0.947, respectively. Maximum anterior central elevation measured by Orbscan II and Galilei was 9.2±5.1 µm and 3.2±1.8 µm, respectively. Maximum posterior central elevation by Orbscan II and Galilei was 33.8±9.3 µm and 6.8±3.8 µm, respectively.
Despite significant differences in mean keratometry readings and anterior and posterior elevation measurements among the three systems, the keratometry readings can be used interchangeably, as this difference is not clinically significant.
From the Ophthalmic Research Center (Karimian, Feizi, Faramarzi, Yaseri), Labbafinejad Medical Center (Karimian, Feizi, Doozandeh, Faramarzi), Shahid Beheshti University, Tehran, Iran.
This study was financially supported by the Ophthalmic Research Center of Shahid Beheshti University, Tehran, Iran.
The authors have no financial or proprietary interest in the materials presented herein.
The authors thank Hossein Kiani, MS; Saeed Ansari, MS; and Noushin Dadbin, BS, for performing the topography measurements.
AUTHOR CONTRIBUTIONS
Study concept and design (F.K., A.F.); data collection (A.D.); analysis and interpretation of data (F.K., S.F., M.Y.); drafting of the manuscript (F.K., S.F., A.D.); critical revision of the manuscript (F.K., A.F., M.Y.); statistical expertise (S.F., M.Y.); obtained funding (F.K., M.Y.); administrative, technical, or material support (F.K., S.F., A.F.); supervision (F.K.)
Correspondence: Farid Karimian, MD, Labbafinejad Medical Center, Dept of Ophthalmology, Boostan 9 St, Pasdaran Ave, 16666, Tehran, Iran. Tel/Fax: 9821 22825390; E-mail: karimianf@yahoo.com
In the absence of a definitive or genetic test to determine the risk for late complications of keratorefractive surgery, current diagnostic imaging devices enhance our ability to detect, suspect, and prevent one of its most devastating complications—iatrogenic keratectasia. 1,2 Patient eligibility for keratorefractive surgery depends on accurate measurement of corneal thickness and corneal curvature. Accurate assessment of the posterior corneal surface has been shown to be a better screening parameter for forme fruste keratoconus. 1 Therefore, imaging techniques have evolved from those that only evaluate the anterior corneal surface, such as manual keratometers and Placido disk–based topographers, to more sophisticated systems with the capability of depicting the posterior corneal curvature such as the Orbscan II (Bausch & Lomb Rochester, New York), Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany), and Galilei (Ziemer Ophthalmic Systems AG, Zurich, Switzerland) systems.
Placido disk–based topography has some limitations including inaccurate corneal periphery measurements as well as the inability to depict posterior corneal curvature. 3 The combination of Placido disk with scanning-slit topography used by the Orbscan II system has become popular over the past decade because of its ability to evaluate the posterior surface. 4 Recently introduced Scheimpflug imaging techniques have the potential to produce an elevation map across the entire cornea. 5 The Galilei, a dual Scheimpflug camera incorporating a Placido disk to analyze the anterior curvature, provides an analysis of the anterior and posterior corneal surfaces as well as corneal thickness and anterior chamber dimensions.
It is not entirely clear whether the results of the Galilei method are comparable to the Orbscan II and whether the two can be used interchangeably. The purpose of this study was to compare keratometry readings by the Galilei dual Scheimpflug system with those of the Orbscan II scanning-slit topography system and Corneal Map (model CM-1; Costruzione Strumenti Oftalmici, Florence, Italy) conventional Placido disk–based topography, as well as investigate agreement between the Galilei and Orbscan II in terms of radius of curvature of anterior and posterior best-fit-sphere (BFS) and maximum central elevation values.
Patients and Methods
This prospective comparative study comprised refractive surgery candidates aged between 18 and 45 years. No patient had ocular morbidity other than refractive error. Refractive error was first measured by subjective and manifest refraction. Subsequently, three drops of 0.5% tropicamide were instilled at 5-minute intervals and after 30 minutes cycloplegic refraction was performed. Any systemic disease such as diabetes mellitus and connective tissue disorder or previous intraocular or corneal surgery led to participant exclusion. The research center ethics committee approved the study and all participants signed a written informed consent after the purpose of the study was explained.
On a different day, between 3 pm and 7 pm, all study participants underwent sequential examinations using the dual Scheimpflug system (Galilei V4.01), scanning-slit topographer (Orbscan IIz), and Corneal Map topographer. Measurements were taken by three qualified technicians (H.K., S.A., N.D.) who were blind to the results obtained by the other methods. For each system, at least three measurements were taken and averaged after excluding outliers.
Galilei Dual Scheimpflug Analyzer
The Galilei is a noninvasive diagnostic system designed to analyze the anterior eye segment using a rotating dual Scheimpflug camera (1000×1000 pixel charged coupled device camera) integrated with a Placido disk–based topographer (20 monochrome rings, 200-mm diameter). The flash illumination is output from a 475-nm wavelength blue light emitting diode (ultraviolet free), and it measures more than 122 000 data points per complete scan. The dual Scheimpflug cameras are located 180° apart to compensate for error associated with scans at an oblique angle and take images of the anterior segment including the cornea, iris, pupil, anterior chamber, and lens to evaluate and analyze corneal shape, corneal thickness, pupil size, and anterior chamber (size, volume, and angle). It is also capable of performing lens densitometry for evaluation of crystalline lens opacities.
Orbscan II
The Orbscan II is a three-dimensional slit-scanning topography system for analysis of the anterior and posterior surface of the cornea. It uses a slit-scanning system to measure 9000 data points to produce multiple slit images of the anterior segment. It also measures corneal thickness and anterior chamber depth, and uses a Placido disk–based system to make necessary adjustments and hence produce topography data.
Corneal Map Topographer
The Corneal Map topographer uses an illuminated cone of Placido-type mires consisting of 12 concentric rings. A video camera records an image of the reflected mires and by automated digitization with electronic detection of the mires, a sub-pixel resolution and keratometric accuracy of <0.25 diopters (D) can be achieved.
Measurement Technique
For measurements with the Galilei or Orbscan II, patients were seated with their chin on a chinrest and forehead against the forehead strap while fixating on the target. In the Galilei, appropriate alignment of the scan center with the corneal apex was checked by using an initial Scheimpflug image formed on the monitor. In the Orbscan, the corneal image was brought into focus with the help of two reflected and inverted half circles.
For both techniques, a 9.0-mm diameter BFS was fitted in float and calculated manually. To measure the maximum anterior and posterior elevation, the most elevated point in a central 4-mm zone was determined manually.
Statistical Analysis
Statistical analysis was performed using SPSS version 15 (SPSS Inc, Chicago, Illinois) and P<.05 was considered statistically significant. Data were expressed as mean, standard deviation, and range, and normality was confirmed with the Kolmogorov-Smirnov test. Keratometry measurements obtained from the three different methods were compared using analysis of variance (ANOVA) with repeated measures and the Scheffé multiple-comparison test. A paired t test was used to compare anterior and posterior elevation map results between the Galilei and Orbscan systems. Pearson’s correlation coefficients and Bland-Altman plots with 95% limits of agreement (LoA) were used to assess the difference among measurements for each patient.
Results
One hundred eighty-four eyes of 92 refractive surgery candidates (38 men and 54 women) aged between 18 and 42 years (mean age: 25.7±5.5 years) were enrolled in this study. Mean spherical equivalent refraction was -3.87±2.15 D (range: -8.50 to +4.25 D) with a mean refractive astigmatism of 1.04±0.76 D (range: 0.0 to 4.00 D).
Mean keratometry reading was 44.30±1.49 D (range: 41.10 to 47.80 D) with the Galilei, 44.11±1.47 D (range: 40.80 to 48.10 D) with the Orbscan, and 44.60±1.56 D (range: 41.30 to 48.50 D) with the Corneal Map topographer ( P<.001). Table compares the results by any two of the three instruments and Figures – demonstrate agreement between each pair of instruments.
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Table 1: Mean Keratometry Measured With the Galilei, Orbscan, and Corneal Map Topographer |
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Figure 1. A) Scatterplot and B) Bland-Altman plot of the mean keratometry reading with the Galilei versus the Orbscan II. |
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Figure 3. A) Scatterplot and B) Bland-Altman plot of the mean keratometry reading with the Orbscan II versus the Corneal Map (CSO) topographer. |
Mean keratometric astigmatism was 1.27±0.78 D (range: 0.0 to 4.16 D) with the Galilei, 1.23±0.77 D (range: 0.20 to 4.10 D) with the Orbscan, and 1.19±0.78 D (range: 0.07 to 4.10 D) with the Corneal Map topographer (repeated measures ANOVA, P<.001). However, a strong association was noted among the keratometric astigmatism measured by the three instruments and a relatively narrow range of 95% LoA (-0.75 and +0.62 D) was observed (Table , Figs –).
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Table 2: Mean Keratometric Astigmatism Measured With the Galilei, Orbscan, and Corneal Map Topographer |
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Figure 4. A) Scatterplot and B) Bland-Altman plot of the mean keratometric astigmatism with the Galilei versus the Orbscan II. |
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Figure 6. A) Scatterplot and B) Bland-Altman plot of the mean keratometric astigmatism with the Orbscan II versus Corneal Map (CSO) topographer. |
Table compares the results by the Galilei and Orbscan in terms of anterior and posterior BFS and the maximum elevation. Despite being strongly correlated, the Orbscan yielded significantly higher values of anterior and posterior BFS. Anterior and posterior maximum elevations measured by the two devices were neither comparable nor strongly correlated.
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Table 3: Anterior and Posterior Best-Fit-Sphere and Anterior and Posterior Maximum Elevation Measured With the Galilei and Orbscan II |
Bland-Altman plots showed a narrower range of 95% LoA for anterior BFS (0.03 to 0.32) compared to posterior BFS (-0.11 to 0.23) (Fig ).
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Figure 7. Bland-Altman plots depict agreement between Galilei and Orbscan II in terms of A) anterior best-fit-sphere (BFS), B) posterior BFS, C) anterior maximum elevation, and D) posterior maximum elevation. |
Discussion
Despite a strong association, a significant difference was observed among the three instruments regarding mean keratometry readings and keratometric astigmatism in the present study. The Corneal Map yielded the steepest keratometry readings, whereas the keratometry results by the Orbscan were the flattest. The keratometric astigmatism measurements by the Galilei and Orbscan were statistically greater than those by the Corneal Map, whereas the difference between the Galilei and Orbscan did not reach statistical significance. However, these differences do not appear to be clinically important as the maximum mean difference in mean keratometry and keratometric astigmatism among the three instruments was <0.30 D and 0.10 D, respectively, far below the reported diurnal keratometric fluctuation of cornea. Comparing keratometry readings in the flattest and steepest meridians using the Orbscan and Galilei, Menassa et al 6 found that the Galilei provided flatter keratometry values in both meridians although the difference was not statistically significant. Similarly, they reported comparable keratometric astigmatism results by the two instruments, which are in accordance with our findings.
In the present study, the anterior and posterior elevation map measurements by the Galilei were significantly lower than those by the Orbscan. Anterior and posterior BFS and measured maximum elevation points had higher values by Orbscan. These differences can be explained by the measurement techniques used by each device: Orbscan uses scanning slit and triangulation whereas the Galilei uses Scheimpflug. The Galilei yielded lower anterior and posterior maximum elevation results by approximately 6.0 µm and 27.0 µm, respectively. Although anterior and posterior BFS readings by the two instruments were strongly correlated, such a correlation was not found regarding the maximum anterior and posterior elevation. However, the results for the anterior elevation map by the two instruments were closer than those for the posterior elevation map. Evaluating agreement between the Galilei and Pentacam, Salouti et al 7 reported that the anterior and posterior elevation measurements by the Galilei were significantly flatter than the Pentacam. In their study, the two imaging systems showed more agreement in anterior elevation measurements than in posterior elevation measurements. From these observations it can be concluded that the Galilei tends to yield flatter results, especially for posterior corneal surface, than other imaging systems with the capability of evaluating the posterior curvature such as the Orbscan and Pentacam.
Accurate measurements of corneal shape are vital for the detection of early stages of keratoconus to avoid complications of keratorefractive surgery. 1 As keratoconus may initially manifest in the posterior corneal surface, many imaging systems have been developed to accurately measure this surface to screen and exclude those patients who are at risk for iatrogenic keratectasia. 1 The Orbscan II uses a horizontally moving slit beam to produce slit images of the anterior segment and provide data for corneal topography, corneal thickness, and anterior chamber depth. 4 Conventional Placido disk–based topography is combined with scanning-slit topography in the Orbscan II system to assess the curvature of the anterior corneal surface. It measures corneal thickness by triangulation; hence calculating this parameter indirectly. 8
The recently introduced double-Scheimpflug Galilei system has two cameras situated 180° apart to overcome errors associated with scans at an oblique angle as well as compensate for micromovements during the examination. It evaluates corneal topography by measuring the anterior and posterior corneal surfaces using the dual Scheimpflug technique in which slit images are captured from opposite sides of the illuminated slit. The elevation data obtained from corresponding opposite slit images are then averaged. 5,9 It has been claimed that this feature can enhance its ability to evaluate the posterior corneal surface and hence detect the early stages of keratoconus. 7,9,10 However, the results of our study and the similar study by Salouti et al 7 reveal that the Galilei tends to underestimate the elevation map measurements of the anterior and, more significantly, the posterior surface of the cornea. If this imaging technique is used to screen candidates for refractive surgery, such an underestimation could result in the missed diagnosis of early keratoconus.
The current study has some limitations. First, because a standard reference is not available to compare the Galilei and Orbscan systems, we are not certain whether the Galilei underestimates or the Orbscan overestimates the elevation map readings. Second, this study was conducted in refractive surgery candidates without any corneal abnormalities. Therefore, it may not be possible to generalize these findings to patients with corneal pathologies such as keratoconus. To better evaluate accuracy of the Galilei system in detecting corneal ectatic disorders, a similar study should be conducted in patients with keratoconus of differing severity.
Acknowledgments
The authors thank Hossein Kiani, MS; Saeed Ansari, MS; and Noushin Dadbin, BS, for performing the topography measurements.
References
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- 2. Rabinowitz YS, Nesburn AB, McDonnell PJ. Videokeratography of the fellow eye in unilateral keratoconus. Ophthalmology. 1993;100(2):181–186.
- 3. Seitz B, Behrens A, Langenbucher A. Corneal topography. Curr Opin Ophthalmol. 1997;8(4):8–24. doi: 10.1097/00055735-199708000-00003 [CrossRef]
- 4. Cairns G, McGhee CN. Orbscan computerized topography: attributes, applications, and limitations. J Cataract Refract Surg. 2005;31(1):205–220. doi: 10.1016/j.jcrs.2004.09.047 [CrossRef]
- 5. Konstantopoulos A, Hossain P, Anderson DF. Recent advances in ophthalmic anterior segment imaging: a new era for ophthalmic diagnosis? Br J Ophthalmol. 2007;91(4):551–557. doi: 10.1136/bjo.2006.103408 [CrossRef]
- 6. 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(10):1742–1747. doi: 10.1016/j.jcrs.2008.06.024 [CrossRef]
- 7. Salouti R, Nowroozzadeh MH, Zamani M, Fard AH, Niknam S. Comparison of anterior and posterior elevation map measurements between 2 Scheimpflug imaging systems. J Cataract Refract Surg. 2009;35(5):856–862. doi: 10.1016/j.jcrs.2009.01.008 [CrossRef]
- 8. Doughty MJ, Jonuscheit S, The orbscan acoustic (correction) factor for central corneal thickness measures of normal human corneas. Eye Contact Lens. 2010;36(2):106–115. doi: 10.1097/ICL.0b013e3181d0b604 [CrossRef]
- 9. Shirayama M, Wang L, Weikert MP, Koch DD. Comparison of corneal powers obtained from 4 different devices. Am J Ophthalmol. 2009;148(4):528–535. doi: 10.1016/j.ajo.2009.04.028 [CrossRef]
- 10. Wolf A, Abdallat W, Kollias A, Frohlich SJ, Grueterich M, Lackerbauer CA. Mild topographic abnormalities that become more suspicious on Scheimpflug imaging. Eur J Ophthalmol. 2009;19(1):10–17.
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Figure 2. A) Scatterplot and B) Bland-Altman plot of the mean keratometry reading with the Galilei versus the Corneal Map (CSO) topographer. |
Mean Keratometry Measured With the Galilei, Orbscan, and Corneal Map Topographer
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| Galilei-Orbscan | 0.17±0.52 | <.001 | 0.94 | <.001 | -0.84 to 1.19 |
| Galilei-Corneal Map | -0.27±0.43 | <.001 | 0.96 | <.001 | -1.11 to 0.58 |
| Orbscan-Corneal Map | -0.47±0.69 | <.001 | 0.90 | <.001 | -1.83 to 0.89 |
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Figure 5. A) Scatterplot and B) Bland-Altman plot of the mean keratometric astigmatism with the Galilei versus the Corneal Map (CSO) topographer. |
Mean Keratometric Astigmatism Measured With the Galilei, Orbscan, and Corneal Map Topographer
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| Galilei-Orbscan | 0.03±0.34 | .21 | 0.91 | <.001 | -0.69 to +0.62 |
| Galilei-Corneal Map | 0.09±0.34 | .001 | 0.91 | <.001 | -0.75 to +0.58 |
| Orbscan-Corneal Map | 0.06±0.19 | <.001 | 0.97 | <.001 | -0.43 to +0.32 |
Anterior and Posterior Best-Fit-Sphere and Anterior and Posterior Maximum Elevation Measured With the Galilei and Orbscan II
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| Anterior BFS (mm) | 7.68±0.26 | 7.87±0.27 | <.001 | 0.96 | <.001 |
| Posterior BFS (mm) | 6.35±0.25 | 6.41±0.27 | <.001 | 0.95 | <.001 |
| Maximum elevation (µm) | |||||
| Anterior | 3.20±1.78 | 9.17±5.10 | .02 | 0.21 | .02 |
| Posterior | 6.81±3.78 | 33.84±9.33 | <.001 | 0.03 | .75 |