Contact Lens & Anterior Eye 37 (2014) 331–336
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Contact Lens & Anterior Eye journal homepage: www.elsevier.com/locate/clae
Repeatability and reproducibility of Galilei measurements in normal keratoconic and postrefractive corneas Emre Güler a,∗ , Ramazan Ya˘gcı b , Mesut Akyol c , Zeynel Arslanyılmaz d , Mehmet Balcı e , I˙ brahim F. Heps¸en f a
Turgut Özal University, Medical School, Department of Ophthalmology, Ankara, Turkey Pamukkale University, Medical School, Department of Ophthalmology, Denizli, Turkey c Yıldırım Beyazıt University, Department of Biostatistics, Ankara, Turkey d Adıyaman University Training and Research Hospital, Department of Ophthalmology, Adıyaman, Turkey e Abdurrahman Yurtaslan Oncology Training and Research Hospital, Department of Ophthalmology, Ankara, Turkey f Gazi University, Medical School, Department of Ophthalmology, Ankara, Turkey b
a r t i c l e
i n f o
Article history: Received 6 August 2013 Received in revised form 23 April 2014 Accepted 26 April 2014 Keywords: Galilei Keratoconus Repeatability Reproducibility
a b s t r a c t Objective: To assess the repeatability and reproducibility of the anterior segment measurements performed with a dual Scheimpflug analyzer (Galilei) in normal, keratoconic and post-refractive surgery corneas. Methods: To evaluate the repeatability, two additional measurements were performed by the first examiner. To assess reproducibility, this was later followed by a single reading by the second examiner. The following parameters were recorded and evaluated in this study; central corneal thickness (CCT), thinnest corneal thickness (TCT), mean total corneal power (TCP) in central (0–4 mm), mean posterior corneal power (PCP) in central (0.5–2 mm), anterior and posterior elevation (best fit sphere [BFS]) in central 8 mm anterior and posterior eccentricity (ε2 ) in central 8 mm. Repeatability and reproducibility for each corneal parameter was assessed using the Bland–Altman analysis. Results: Each of the three groups was consisted of 20 subjects (totally 60 patients, 30 men and 30 women). The 95% LoA for repeatability was very small, indicating small discrepancies between measurements related to CCT. Acceptable repeatability was also achieved for the other parameters in each group. However, the 95% LoA for value TCP was larger in keratoconic eyes. The 95% LoA for reproducibility was also very small, and acceptable for all measured parameters in each group. In addition, the 95% LoA was larger for the measurement of CCT and TCT for postrefractive corneas. Conclusions: The anterior segment measurements provided by Galilei showed good repeatability and reproducibility for normal, keratoconic and postrefractive corneas. © 2014 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
1. Introduction In the past decade, Scheimpflug video photographic devices have become popular for objective evaluation of cornea [1]. The Pentacam (Oculus, Germany) was the first Scheimpflug analysis system which uses a single Scheimpflug camera to perform multiple photographs of the anterior segment of the eye [2]. In 2007, a new topographer, Galilei (Ziemer Ophthalmology Co.) was marketed. This device uses a double Scheimpflug system
∗ Corresponding author at: Alparslan Türkes¸ Cad. No. 57 06510 Emek, Ankara, Turkey. Tel.: +90 0312 336 09 09; fax: +90 312 336 34 39. E-mail address: [email protected] (E. Güler).
combined with a Placido disk. While the Pentacam derives keratometry data of the surface from the Scheimpflug images only; the Galilei system uses the Placido disk to analyze the anterior curvature. Additionally, this dual system provides accurate pachymetry not only of the central cornea but also the peripheral cornea, even in cases of micromovements [3]. Low intraobserver and interobserver alteration is required to perform correct individual measurements in the diagnosis and long-term follow up of corneal diseases [4–6]. Few studies have evaluated the repeatability of the Galilei [1,3,7], and none of them included eyes that had keratoconus. This study evaluates the repeatability and reproducibility of the anterior segment parameters measured with Galilei dualScheimpflug analyzer in normal, keratoconic, and postrefractive corneas.
http://dx.doi.org/10.1016/j.clae.2014.04.004 1367-0484/© 2014 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
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2. Materials and methods In this prospective study, three groups of subjects were assessed: Subjects who had previous myopic photorefractive keratectomy (PRK) or LASIK (at least 3 months elapsed after surgery), subjects with a diagnosis of keratoconus, and subjects with normal corneas. The initial diagnosis of keratoconus was based on clinical slit-lamp findings and associated characteristic Placido-based topographic patterns [8]. The mean simulated keratometric values obtained with Placido-based topography ranged from 45.65 to 59.00 diopters (D). Keratoconic corneas with previous acute corneal hydrops or a history of corneal surgery were excluded from the study. Normal corneas met the following criteria’s; no history of ocular surgery, trauma, contact lens usage, or disease except for myopia, hyperopia, and/or astigmatic ametropia. All subjects had best corrected visual acuity (with glasses and rigid contact lenses) of at least 20/40. One eye was randomly selected in each subject. The study was conducted in accordance with the ethical standards stated in the 1964 Declaration of Helsinki. Informed and written consent was obtained from all subjects. The study was approved by the Local Ethics Committee of Turgut Özal University. Measurements were performed using the Galilei system, which performs anterior corneal measurements by using the Placido and Scheimpflug data. However, Galilei measures the posterior corneal surface by the Scheimpflug data. The device’s software (version 5.2.1) was used to perform measurements. Measurements with the dual Scheimpflug analyzer were performed according to the manufacturer’s guidelines. The device was brought into focus and the subject’s eye was aligned along the visual axis by a central fixation light. The subjects were asked to sit back after each measurement, and the device was realigned before each measurement. Subjects were asked to sit back after each measurement, and to blink completely just before each measurement. All measurements were taken between 10 AM and 3 PM with non-dilated pupil in identical lighting conditions. All subjects had two repeated readings taken by the examiner 1 (repeatability). This was later followed by a single reading by the examiner 2 who was blind to the results of examiner 1 (reproducibility). The following parameters were recorded and evaluated in this study; 1) Average and thinnest central corneal thickness (CCT): The dual Scheimpflug analyzer used in this study provides corneal thickness measurements for the central 9.0 mm. Data are automatically reported in concentric circles (with a diameter of 1.0 mm, 3.0 mm, and 4.0 mm), although corneal thickness can be measured at any point by manually placing the cursor at that point. 2) Mean total corneal power: This is a specific feature of the dual Scheimpflug analyzer used in this study. Measurements of the power of the anterior and posterior corneal surfaces are obtained through ray tracing rather than the Gaussian optics formula, as in the case of the true net power provided by the Pentacam device. For each point on the map, the angle of incidence is calculated relative to the anterior surface normal for incoming parallel rays. The angle of refraction is calculated using the Snell law with air refractive index (Z) 1.0 and cornea Z 1.376. This angle of refraction is used to determine the nonparallel direction of incoming rays relative to the posterior surface normal and is used to calculate the angle of incidence for the posterior surface. A new angle of refraction is calculated for the posterior surface using the Snell law with cornea Z 1.376 and aqueous Z 1.336. This final angle of refraction is used to calculate the intersection of the ray along the (0.0) axis and the resultant focal length that is used to determine total power for that point on the map. 3) Mean posterior corneal power: This value (derived from the posterior axial curvature map) is the arithmetic mean of the pair
of meridians 90◦ apart with the greatest difference in average power, from a 0.5 to 2.0 mm distance from the center. The power of the steep and flat meridian is calculated using the cornea (1.376) and aqueous humor (1.336) refractive indexes. 4) Anterior and posterior elevation (best-fit sphere [BFS]) maps: These are calculated for an analysis area of 8.0 mm. The corresponding radius of curvature was evaluated in the present study. 5) Eccentricity (ε2 ): Eccentricity ε is reported as its square ε2 . This term is one of four parameters by which the shape of a conic section can be described: Q (asphericity), p value and E (corneal shape factor) are the others. Galilei calculates the eccentricity ε2 of the surface within a central diameter of 8 mm averaged over all meridians. This is done for the anterior and posterior surface. 2.1. Statistical analysis The definitions of coefficient of repeatability and reproducibility were based on those adopted by the International Organization for Standardization and a previous report [8,9] the coefficient of repeatability was defined as 2 SDs of the differences between the measurements obtained for the same subject at the different sessions by the same observer. The coefficient of reproducibility was defined as 2 SDs of the differences between the measurements obtained for the same participant at the same visit by different observers. Because the distribution of all the data obtained was not significantly different from normal (Kolmogorov–Smirnov tests, p > 0.05), parametric tests were used for analysis of differences. Repeatability/reproducibility of the data obtained was determined only when there was no statistically significant difference between measurements. Plots of the within-examiner (repeatability) or between-examiner (reproducibility) differences against their means and the 95% limits of agreement (LoA) (mean difference ± 1.96 SD) were determined as suggested by Bland and Altman [10] where appropriate. Statistical analysis was performed using the SPSS for Windows Version 11.0 (SPPS Inc., Chicago, IL, USA). p values less than 0.05 were considered statistically significant. 3. Results Each of the three groups was consisted of 20 subjects (totally 60 subjects, 30 men and 30 women). The mean age of the postrefractive, keratoconic and normal subjects were 36.0 ± 11.8, 25.0 ± 12.5, and 23.0 ± 11.5 years, respectively. 3.1. Repeatability Table 1 shows the repeatability assessment of examiner 1 for each measured parameters by groups. There were no significant within-examiner differences for any of the parameters tested (p > 0.05). In all groups, measurements related to CCT showed excellent repeatability. The 95% LoA for CCT was very small, indicating small discrepancies between measurements. For the repeatability of CCT, the Galilei gave the 95% LoA values from +5.72 to −9.22 m for normal corneas, +9.82 to −9.02 m for keratoconic corneas, and +12.45 to −9.45 m for postrefractive corneas. Fig. 1 shows the plots of within-examiner differences in CCT for all groups. Based on the 95% LoA, good repeatability was also achieved for the other parameters in each group. The 95% LoA for TCP was larger in keratoconic eyes. For repeatability of TCP, the 95% LoA values ranged from +1.61 to −1.34 D for normal corneas, +1.98 to −2.09 D for keratoconic corneas, and +0.44 to −0.49 D for postrefractive corneas. Fig. 2 shows the plots of within-examiner differences of TCP for all groups.
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Table 1 Summary of repeatability results for each measured parameter by group. Patients
Parameter
Mean ± SD First examination
t
p
Mean difference
Second examination
95% LoA Upper limit
Lower limit
Normal
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
550.85 540.7 41.7 6.2 7.9 6.6 0.3 0.3
± ± ± ± ± ± ± ±
37.0 36.7 1.9 0.3 0.3 0.3 0.2 0.2
552.2 541.0 41.6 6.2 7.9 6.6 0.3 0.2
± ± ± ± ± ± ± ±
37.9 36.4 1.6 0.3 0.3 0.3 0.1 0.2
2.054 0.280 0.817 1.126 1.059 0.318 0.300 1.996
0.054 0.783 0.424 0.274 0.303 0.754 0.768 0.060
−1.75 −0.25 0.14 0.02 −0.01 0.00 0.01 0.06
± ± ± ± ± ± ± ±
3.81 4.0 0.75 0.09 0.06 0.05 0.17 0.13
5.72 7.59 1.61 0.21 0.11 0.09 0.35 0.32
−9.22 −8.09 −1.34 −0.16 −0.13 −0.10 −0.32 −0.20
Keratoconus
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
498.2 473.4 46.4 7.4 7.4 6.0 1.2 1.5
± ± ± ± ± ± ± ±
36.1 38.4 3.6 0.8 0.4 0.3 0.6 0.7
497.8 473.0 46.5 7.4 7.3 6.0 1.2 1.5
± ± ± ± ± ± ± ±
34.8 38.1 3.6 0.8 0.4 0.4 0.5 0.5
0.372 0.389 −0.224 −0.111 1.037 1.146 0.786 0.202
0.714 0.701 0.825 0.913 0.313 0.266 0.442 0.842
0.40 0.40 −0.05 −0.01 0.03 0.02 0.04 0.01
± ± ± ± ± ± ± ±
4.81 4.59 1.04 0.22 0.12 0.08 0.25 0.32
9.82 9.40 1.98 0.43 0.26 0.18 0.53 0.64
−9.02 −8.60 −2.09 −0.44 −0.21 −0.14 −0.45 −0.61
Postrefractive
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
493.0 472.3 40.6 6.5 8.2 6.5 0.5 0.4
± ± ± ± ± ± ± ±
41.6 43.2 2.2 0.5 0.4 0.3 0.3 0.5
491.5 472.0 40.6 6.4 8.2 6.5 0.5 0.4
± ± ± ± ± ± ± ±
39.2 43.2 2.2 0.7 0.4 0.3 0.4 0.5
1.201 1.364 0.084 0.099 0.014 0.003 0.06 0.01
0.245 0.562 0.611 0.310 0.653 0.149 0.818 0.113
1.5 0.30 −0.03 −0.10 0.00 −0.01 −0.01 −004
± ± ± ± ± ± ± ±
5.59 2.27 0.24 0.42 0.02 0.03 0.14 0.11
12.45 4.76 0.44 0.73 0.05 0.04 0.27 0.17
−9.45 −4.16 −0.49 −0.93 −0.05 −0.06 −0.29 −0.25
CCT: central corneal thickness; TCT: thinnest corneal thickness; TCP: total corneal power, PCP: posterior corneal power, BFS: best fit sphere. ε2 : eccentricity; SD: standard deviation; LoA: limits of agreement; p < 0.05 was considered statistically significant.
Fig. 1. Plots of repeatability differences for CCT in (a) normal, (b) keratoconic and (c) postrefractive corneas. The middle line represents the mean. The lines on the side represent the upper and lower 95% confidence interval limits of agreement.
Fig. 2. Plots of repeatability differences for TCP in (a) normal, (b) keratoconic and (c) postrefractive corneas. The middle line represents the mean. The lines on the side represent the upper and lower 95% confidence interval limits of agreement.
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3.2. Reproducibility Table 2 shows the reproducibility results for the measured parameters by both examiners for all groups. Measurement results were similar for each groups and there were no statistically significant differences between two examiners (p > 0.05). The 95% LoA for reproducibility was also very small, and acceptable for all measured parameters in each group. The 95% LoA was larger for the measurement of CCT and TCT in postrefractive corneas. The 95% LoA for reproducibility was ranged from +15.43 to −11.83 m for CCT, and +13.07 to −11.17 m for TCT. Figs. 3 and 4 show the plots of between-examiner differences of CCT and TCT in all groups. 4. Discussion Reliability studies of diagnostic devices are necessary to ensure that the error involved in measurement is small enough to detect actual changes in what is being measured [11]. This present study was designed to evaluate the repeatability and reproducibility of the anterior segment measurements provided by the Galilei dual Scheimpflug Placido corneal topographer in normal, postrefractive and keratoconic corneas. The repeatability of corneal pachymetric measurements using the Galilei Scheimpflug system, Menassa et al. [3] showed high repeatability for central average corneal thickness and thinnest corneal thickness in normal corneas. Wang et al. [1] evaluated 20 unoperated eyes, and reported high repeatability for corneal thickness. Savini et al. [7] also reported excellent repeatability for measurements of central and thinnest corneal thickness in normal and postrefractive corneas. The repeatability for keratoconic corneas was not assessed in these previous studies. de Sanctis et al. [12] found more reproducible and repeatable measurements of central thickness by Pentacam than those obtained with ultrasound pachymetry in keratoconic corneas. In this study we found excellent repeatability results for pachymetry readings in normal, postrefractive and keratoconic corneas.
In a previous study, Menassa et al. [3] reported that keratometry and central corneal pachymetry readings with the Galilei and a scanning-slit topographer (Orbscan II, Bausch & Lomb) showed high reproducibility in normal corneas. However, Menassa et al. [3] evaluated the reproducibility of the dual Scheimpflug system only in normal corneas. In our study we included keratoconic corneas, and found excellent reproducibility for pachymetry readings. For corneal pachymetric measurements using the single Scheimpflug system (Sirius, Costruzione Strumenti Oftalmici), Shankar et al. [2] reported good repeatability for CCT, whereas the peripheral pachymetry repeatability was poor. We found that the dual-Scheimpflug system showed excellent repeatability for both central and paracentral corneal pachymetry, similarly to Wang et al. [1]. This is perhaps due to the dual-channel Scheimpflug cameras implemented by the system. If the alignment varies between measurements, different thicknesses will be detected depending on the camera location and magnitude of decentration. In contrast, measurement values obtained by averaging data from two cameras should minimize the problems caused by altered corneal position because such a shift will produce a thinner measurement by 1 camera and a correspondingly thicker measurement by the other camera [13]. In this study, all corneal power and anterior–posterior best-fit sphere measurements yielded high repeatability results in normal and postrefractive corneas. Our results were similar to those previously reported by Savini and Wang [1,7]. To our knowledge, there are no studies evaluating the repeatability of corneal power and anterior–posterior best-fit sphere measurements by the Galilei device in keratoconic eyes. In keratoconic corneas, similar repeatability was found in the measurement PCP, anterior-posterior BFS, and TCP. The 95% LoA for TCP was larger when compared with normal and postrefractive corneas. However, the 95% LoA of TCP ranged between 1.98 and −2.09, therefore Galilei can be accepted to perform reliable TCP measurements in keratoconic corneas. In a previous study Wang et al. [1] reported the dualScheimpflug system (Galilei) had excellent reproducibility in measuring total corneal power at the central, paracentral, and
Table 2 Summary of reproducibility results for each measured parameter by group. Mean ± SD
Patients
Parameter
Normal
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
551.73 540.78 41.65 6.19 7.91 6.58 0.33 0.26
± ± ± ± ± ± ± ±
37.4 36.46 1.7 0.3 0.31 0.3 0.12 0.19
551.75 541.7 41.82 6.2 7.88 6.58 0.37 0.29
± ± ± ± ± ± ± ±
37.73 37.5 2.28 0.33 0.33 0.31 0.34 0.21
Keratoconus
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
498.0 473.15 46.46 7.39 7.34 6.0 1.18 1.47
± ± ± ± ± ± ± ±
35.35 38.20 3.55 0.78 0.40 0.35 0.52 0.56
496.95 473.45 46.43 7.42 7.33 6.0 1.18 1.49
± ± ± ± ± ± ± ±
Postrefractive
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
492.25 472.15 40.59 6.40 8.24 6.48 0.49 0.41
± ± ± ± ± ± ± ±
40.28 43.15 2.19 0.58 0.44 0.33 0.35 0.50
490.45 471.20 40.59 6.36 8.24 6.47 0.48 0.48
± ± ± ± ± ± ± ±
Examiner 1
t
p
Mean difference
0.051 1.347 0.834 0.362 1.148 0.229 0.648 0.647
0.960 0.194 0.415 0.721 0.265 0.821 0.525 0.525
−0.02 −0.93 −0.17 −0.01 0.02 0.00 −0.04 −0.02
± ± ± ± ± ± ± ±
2.19 3.07 0.90 0.08 0.07 0.05 0.27 0.17
4.27 5.09 1.60 0.16 0.16 0.11 0.49 0.30
−4.32 −6.94 −1.94 −0.17 −0.12 −0.10 −0.56 −0.35
33.73 36.80 3.15 0.73 0.38 0.33 0.45 0.55
1.288 −0.422 0.162 −0.783 0.768 0.100 −0.176 −0.865
0.213 0.678 0.873 0.443 0.452 0.921 0.862 0.398
1.05 −0.30 0.03 −0.03 0.01 0.00 −0.01 −0.03
± ± ± ± ± ± ± ±
3.65 3.18 0.77 0.17 0.06 0.06 0.20 0.13
8.19 5.93 1.54 0.30 0.13 0.11 0.39 0.23
−6.09 −6.53 −1.49 −0.36 −0.11 −0.11 −0.41 −0.28
39.44 43.58 2.21 0.54 0.44 0.33 0.34 0.55
1.158 0.687 0.108 0.751 0.439 0.475 0.383 2.119
0.261 0.500 0.915 0.462 0.665 0.640 0.706 0.055
1.80 0.95 0.00 0.03 0.01 0.01 0.01 −0.07
± ± ± ± ± ± ± ±
6.95 6.18 0.18 0.20 0.07 0.07 0.12 0.14
15.43 13.07 0.34 0.43 0.15 0.14 0.24 0.21
−11.83 −11.17 −0.35 −0.37 −0.13 −0.12 −0.22 −0.34
Examiner 2
95% LoA Upper limit
Lower limit
CCT: central corneal thickness; TCT: thinnest corneal thickness; TCP: total corneal power, PCP: posterior corneal power, BFS: best fit sphere; ε2 : eccentricity; SD: standard deviation; LoA: limits of agreement; p < 0.05 was considered statistically significant.
E. Güler et al. / Contact Lens & Anterior Eye 37 (2014) 331–336
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Fig. 3. Plots of reproducibility differences for CCT in (a) normal, (b) keratoconic and (c) postrefractive corneas. The middle line represents the mean. The lines on the side represent the upper and lower 95% confidence interval limits of agreement.
Fig. 4. Plots of reproducibility differences for TCT in (a) normal, (b) keratoconic and (c) postrefractive corneas. The middle line represents the mean. The lines on the side represent the upper and lower 95% confidence interval limits of agreement.
peripheral zones and meridional values of posterior corneal power in normal corneas. In the present study, we also assessed the reproducibility of corneal power measurements and anterior-posterior best-fit sphere in postrefractive and keratoconic eyes. All corneal power measurements had excellent reproducibility for all groups. Dao et al. [14] suggested that eccentricity can be helpful to diagnose keratoconus at an early stage. Previous reports found that the anterior corneal ε2 value ranges from −0.01 to −0.80 in normal corneas. In this study we found similar ε2 results ranging −0.1 to −0.9 in normal corneas. In the current study, the 95% LoA for results for ε2 showed small variability between measurements in all groups. Therefore in clinical practice, the ε2 measurements of Galilei may be accepted reliable in all study groups, in particular to keratoconic eyes. To our knowledge this is the first study to show; (1) reproducibility of the measurements of Galilei in keratoconic and postrefractive corneas; (2) repeatability of the measurements of Galilei in keratoconic corneas. Our data shows that this instrument has high repeatability for all measured parameters; however the measurement of TCP in keratoconic corneas had larger 95% LoA. The reproducibility was excellent in normal, keratoconic and postrefractive corneas. The 95% LoA for reproducibility of CCT and TCT in postrefractive corneas was larger than normal and keratoconic corneas. In summary the measurements of the Galilei rotating Scheimpflug system, showed a remarkably low intraobserver and interobserver variation, therefore the instrument can be confidently used for routine clinical use and research purposes.
Conflict of interest There is no proprietary interest or research funding. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.clae.2014.04.004. In all supplemental figures, ‘a’ refers to normal; ‘b’ refers to keratoconic; ‘c’ refers to postrefractive corneas. References [1] Wang L, Shirayama M, Koch DD. Repeatability of corneal power and waveform aberration measurements with a dual-Scheimpflug Placido corneal topographer. J Cataract Refract Surg 2010;36(3):425–30. [2] Shankar H, Taranath D, Santhirathelagan CT, Pesudovs K. Anterior segment biometry with the Pentacam: comprehensive assessment of repeatability of automated measurements. J Cataract Refract Surg 2008;34(1):103–13. [3] 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):1742–7. [4] O’ Donnell C, Maldonado-Codina C. Agreement and repeatability of central thickness measurement in normal corneas using ultrasound pachymetry and the OCULUS Pentacam. Cornea 2005;24(8):920–4. [5] Marsich MW, Bullimore MA. The repeatability of corneal thickness measures. Cornea 2000;19(6):792–5. [6] Cho P, Lam AK, Mountford J, Ng L. The performance of four different corneal topographers on normal human corneas and its impact on orthokeratology lens fitting. Optom Vis Sci 2002;79(3):175–83.
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[7] 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(2):302–9. [8] International Organization for Standardization. Accuracy 272 (Trueness and Precision) of Measurement Methods and Results, Part 1. General Principles and Definitions (ISO 5725-1). Geneva, Switzerland: ISO; 1994. [9] International Organization for Standardization. Accuracy (Trueness and Precision) of Measurement Methods and Results, Part 2. Basic Methods for the Determination of Repeatability and Reproducibility of a Standard Measurement Method (ISO 5725-2). Geneva, Switzerland: ISO; 1994. [10] Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;1(8476):307–10.
[11] Rankin G, Stokes M. Reliability of assessment tools in rehabilitation: an illustration of appropriate statistical analysis. Clin Rehabil 1998;12(3):187–99. [12] de Sanctis U, Missolungi A, Mutani B, Richiardi L, Grignolo FM. Reproducibility and repeatability of central corneal thickness measurement in keratoconus using the rotating Scheimpflug camera and ultrasound pachymetry. Am J Ophthalmol 2007;144(5):712–8. [13] Lewis JR, et al. Comparison of Response to Misalignment in Pachymetry Measurement between Single- and Dual-Scheimpflug Devices. Presented at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, San Francisco, California, USA, April; 2009. [14] Dao CL, Kok JH, Brinkman CJ, van Mil CJ. Corneal eccentricity as a tool for the diagnosis of keratoconus. Cornea 1994;13(4):339–44.
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Repeatability and reproducibility of Galilei measurements in normal keratoconic and postrefractive corneas Emre Güler a,∗ , Ramazan Ya˘gcı b , Mesut Akyol c , Zeynel Arslanyılmaz d , Mehmet Balcı e , I˙ brahim F. Heps¸en f a
Turgut Özal University, Medical School, Department of Ophthalmology, Ankara, Turkey Pamukkale University, Medical School, Department of Ophthalmology, Denizli, Turkey c Yıldırım Beyazıt University, Department of Biostatistics, Ankara, Turkey d Adıyaman University Training and Research Hospital, Department of Ophthalmology, Adıyaman, Turkey e Abdurrahman Yurtaslan Oncology Training and Research Hospital, Department of Ophthalmology, Ankara, Turkey f Gazi University, Medical School, Department of Ophthalmology, Ankara, Turkey b
a r t i c l e
i n f o
Article history: Received 6 August 2013 Received in revised form 23 April 2014 Accepted 26 April 2014 Keywords: Galilei Keratoconus Repeatability Reproducibility
a b s t r a c t Objective: To assess the repeatability and reproducibility of the anterior segment measurements performed with a dual Scheimpflug analyzer (Galilei) in normal, keratoconic and post-refractive surgery corneas. Methods: To evaluate the repeatability, two additional measurements were performed by the first examiner. To assess reproducibility, this was later followed by a single reading by the second examiner. The following parameters were recorded and evaluated in this study; central corneal thickness (CCT), thinnest corneal thickness (TCT), mean total corneal power (TCP) in central (0–4 mm), mean posterior corneal power (PCP) in central (0.5–2 mm), anterior and posterior elevation (best fit sphere [BFS]) in central 8 mm anterior and posterior eccentricity (ε2 ) in central 8 mm. Repeatability and reproducibility for each corneal parameter was assessed using the Bland–Altman analysis. Results: Each of the three groups was consisted of 20 subjects (totally 60 patients, 30 men and 30 women). The 95% LoA for repeatability was very small, indicating small discrepancies between measurements related to CCT. Acceptable repeatability was also achieved for the other parameters in each group. However, the 95% LoA for value TCP was larger in keratoconic eyes. The 95% LoA for reproducibility was also very small, and acceptable for all measured parameters in each group. In addition, the 95% LoA was larger for the measurement of CCT and TCT for postrefractive corneas. Conclusions: The anterior segment measurements provided by Galilei showed good repeatability and reproducibility for normal, keratoconic and postrefractive corneas. © 2014 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
1. Introduction In the past decade, Scheimpflug video photographic devices have become popular for objective evaluation of cornea [1]. The Pentacam (Oculus, Germany) was the first Scheimpflug analysis system which uses a single Scheimpflug camera to perform multiple photographs of the anterior segment of the eye [2]. In 2007, a new topographer, Galilei (Ziemer Ophthalmology Co.) was marketed. This device uses a double Scheimpflug system
∗ Corresponding author at: Alparslan Türkes¸ Cad. No. 57 06510 Emek, Ankara, Turkey. Tel.: +90 0312 336 09 09; fax: +90 312 336 34 39. E-mail address: [email protected] (E. Güler).
combined with a Placido disk. While the Pentacam derives keratometry data of the surface from the Scheimpflug images only; the Galilei system uses the Placido disk to analyze the anterior curvature. Additionally, this dual system provides accurate pachymetry not only of the central cornea but also the peripheral cornea, even in cases of micromovements [3]. Low intraobserver and interobserver alteration is required to perform correct individual measurements in the diagnosis and long-term follow up of corneal diseases [4–6]. Few studies have evaluated the repeatability of the Galilei [1,3,7], and none of them included eyes that had keratoconus. This study evaluates the repeatability and reproducibility of the anterior segment parameters measured with Galilei dualScheimpflug analyzer in normal, keratoconic, and postrefractive corneas.
http://dx.doi.org/10.1016/j.clae.2014.04.004 1367-0484/© 2014 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
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2. Materials and methods In this prospective study, three groups of subjects were assessed: Subjects who had previous myopic photorefractive keratectomy (PRK) or LASIK (at least 3 months elapsed after surgery), subjects with a diagnosis of keratoconus, and subjects with normal corneas. The initial diagnosis of keratoconus was based on clinical slit-lamp findings and associated characteristic Placido-based topographic patterns [8]. The mean simulated keratometric values obtained with Placido-based topography ranged from 45.65 to 59.00 diopters (D). Keratoconic corneas with previous acute corneal hydrops or a history of corneal surgery were excluded from the study. Normal corneas met the following criteria’s; no history of ocular surgery, trauma, contact lens usage, or disease except for myopia, hyperopia, and/or astigmatic ametropia. All subjects had best corrected visual acuity (with glasses and rigid contact lenses) of at least 20/40. One eye was randomly selected in each subject. The study was conducted in accordance with the ethical standards stated in the 1964 Declaration of Helsinki. Informed and written consent was obtained from all subjects. The study was approved by the Local Ethics Committee of Turgut Özal University. Measurements were performed using the Galilei system, which performs anterior corneal measurements by using the Placido and Scheimpflug data. However, Galilei measures the posterior corneal surface by the Scheimpflug data. The device’s software (version 5.2.1) was used to perform measurements. Measurements with the dual Scheimpflug analyzer were performed according to the manufacturer’s guidelines. The device was brought into focus and the subject’s eye was aligned along the visual axis by a central fixation light. The subjects were asked to sit back after each measurement, and the device was realigned before each measurement. Subjects were asked to sit back after each measurement, and to blink completely just before each measurement. All measurements were taken between 10 AM and 3 PM with non-dilated pupil in identical lighting conditions. All subjects had two repeated readings taken by the examiner 1 (repeatability). This was later followed by a single reading by the examiner 2 who was blind to the results of examiner 1 (reproducibility). The following parameters were recorded and evaluated in this study; 1) Average and thinnest central corneal thickness (CCT): The dual Scheimpflug analyzer used in this study provides corneal thickness measurements for the central 9.0 mm. Data are automatically reported in concentric circles (with a diameter of 1.0 mm, 3.0 mm, and 4.0 mm), although corneal thickness can be measured at any point by manually placing the cursor at that point. 2) Mean total corneal power: This is a specific feature of the dual Scheimpflug analyzer used in this study. Measurements of the power of the anterior and posterior corneal surfaces are obtained through ray tracing rather than the Gaussian optics formula, as in the case of the true net power provided by the Pentacam device. For each point on the map, the angle of incidence is calculated relative to the anterior surface normal for incoming parallel rays. The angle of refraction is calculated using the Snell law with air refractive index (Z) 1.0 and cornea Z 1.376. This angle of refraction is used to determine the nonparallel direction of incoming rays relative to the posterior surface normal and is used to calculate the angle of incidence for the posterior surface. A new angle of refraction is calculated for the posterior surface using the Snell law with cornea Z 1.376 and aqueous Z 1.336. This final angle of refraction is used to calculate the intersection of the ray along the (0.0) axis and the resultant focal length that is used to determine total power for that point on the map. 3) Mean posterior corneal power: This value (derived from the posterior axial curvature map) is the arithmetic mean of the pair
of meridians 90◦ apart with the greatest difference in average power, from a 0.5 to 2.0 mm distance from the center. The power of the steep and flat meridian is calculated using the cornea (1.376) and aqueous humor (1.336) refractive indexes. 4) Anterior and posterior elevation (best-fit sphere [BFS]) maps: These are calculated for an analysis area of 8.0 mm. The corresponding radius of curvature was evaluated in the present study. 5) Eccentricity (ε2 ): Eccentricity ε is reported as its square ε2 . This term is one of four parameters by which the shape of a conic section can be described: Q (asphericity), p value and E (corneal shape factor) are the others. Galilei calculates the eccentricity ε2 of the surface within a central diameter of 8 mm averaged over all meridians. This is done for the anterior and posterior surface. 2.1. Statistical analysis The definitions of coefficient of repeatability and reproducibility were based on those adopted by the International Organization for Standardization and a previous report [8,9] the coefficient of repeatability was defined as 2 SDs of the differences between the measurements obtained for the same subject at the different sessions by the same observer. The coefficient of reproducibility was defined as 2 SDs of the differences between the measurements obtained for the same participant at the same visit by different observers. Because the distribution of all the data obtained was not significantly different from normal (Kolmogorov–Smirnov tests, p > 0.05), parametric tests were used for analysis of differences. Repeatability/reproducibility of the data obtained was determined only when there was no statistically significant difference between measurements. Plots of the within-examiner (repeatability) or between-examiner (reproducibility) differences against their means and the 95% limits of agreement (LoA) (mean difference ± 1.96 SD) were determined as suggested by Bland and Altman [10] where appropriate. Statistical analysis was performed using the SPSS for Windows Version 11.0 (SPPS Inc., Chicago, IL, USA). p values less than 0.05 were considered statistically significant. 3. Results Each of the three groups was consisted of 20 subjects (totally 60 subjects, 30 men and 30 women). The mean age of the postrefractive, keratoconic and normal subjects were 36.0 ± 11.8, 25.0 ± 12.5, and 23.0 ± 11.5 years, respectively. 3.1. Repeatability Table 1 shows the repeatability assessment of examiner 1 for each measured parameters by groups. There were no significant within-examiner differences for any of the parameters tested (p > 0.05). In all groups, measurements related to CCT showed excellent repeatability. The 95% LoA for CCT was very small, indicating small discrepancies between measurements. For the repeatability of CCT, the Galilei gave the 95% LoA values from +5.72 to −9.22 m for normal corneas, +9.82 to −9.02 m for keratoconic corneas, and +12.45 to −9.45 m for postrefractive corneas. Fig. 1 shows the plots of within-examiner differences in CCT for all groups. Based on the 95% LoA, good repeatability was also achieved for the other parameters in each group. The 95% LoA for TCP was larger in keratoconic eyes. For repeatability of TCP, the 95% LoA values ranged from +1.61 to −1.34 D for normal corneas, +1.98 to −2.09 D for keratoconic corneas, and +0.44 to −0.49 D for postrefractive corneas. Fig. 2 shows the plots of within-examiner differences of TCP for all groups.
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Table 1 Summary of repeatability results for each measured parameter by group. Patients
Parameter
Mean ± SD First examination
t
p
Mean difference
Second examination
95% LoA Upper limit
Lower limit
Normal
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
550.85 540.7 41.7 6.2 7.9 6.6 0.3 0.3
± ± ± ± ± ± ± ±
37.0 36.7 1.9 0.3 0.3 0.3 0.2 0.2
552.2 541.0 41.6 6.2 7.9 6.6 0.3 0.2
± ± ± ± ± ± ± ±
37.9 36.4 1.6 0.3 0.3 0.3 0.1 0.2
2.054 0.280 0.817 1.126 1.059 0.318 0.300 1.996
0.054 0.783 0.424 0.274 0.303 0.754 0.768 0.060
−1.75 −0.25 0.14 0.02 −0.01 0.00 0.01 0.06
± ± ± ± ± ± ± ±
3.81 4.0 0.75 0.09 0.06 0.05 0.17 0.13
5.72 7.59 1.61 0.21 0.11 0.09 0.35 0.32
−9.22 −8.09 −1.34 −0.16 −0.13 −0.10 −0.32 −0.20
Keratoconus
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
498.2 473.4 46.4 7.4 7.4 6.0 1.2 1.5
± ± ± ± ± ± ± ±
36.1 38.4 3.6 0.8 0.4 0.3 0.6 0.7
497.8 473.0 46.5 7.4 7.3 6.0 1.2 1.5
± ± ± ± ± ± ± ±
34.8 38.1 3.6 0.8 0.4 0.4 0.5 0.5
0.372 0.389 −0.224 −0.111 1.037 1.146 0.786 0.202
0.714 0.701 0.825 0.913 0.313 0.266 0.442 0.842
0.40 0.40 −0.05 −0.01 0.03 0.02 0.04 0.01
± ± ± ± ± ± ± ±
4.81 4.59 1.04 0.22 0.12 0.08 0.25 0.32
9.82 9.40 1.98 0.43 0.26 0.18 0.53 0.64
−9.02 −8.60 −2.09 −0.44 −0.21 −0.14 −0.45 −0.61
Postrefractive
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
493.0 472.3 40.6 6.5 8.2 6.5 0.5 0.4
± ± ± ± ± ± ± ±
41.6 43.2 2.2 0.5 0.4 0.3 0.3 0.5
491.5 472.0 40.6 6.4 8.2 6.5 0.5 0.4
± ± ± ± ± ± ± ±
39.2 43.2 2.2 0.7 0.4 0.3 0.4 0.5
1.201 1.364 0.084 0.099 0.014 0.003 0.06 0.01
0.245 0.562 0.611 0.310 0.653 0.149 0.818 0.113
1.5 0.30 −0.03 −0.10 0.00 −0.01 −0.01 −004
± ± ± ± ± ± ± ±
5.59 2.27 0.24 0.42 0.02 0.03 0.14 0.11
12.45 4.76 0.44 0.73 0.05 0.04 0.27 0.17
−9.45 −4.16 −0.49 −0.93 −0.05 −0.06 −0.29 −0.25
CCT: central corneal thickness; TCT: thinnest corneal thickness; TCP: total corneal power, PCP: posterior corneal power, BFS: best fit sphere. ε2 : eccentricity; SD: standard deviation; LoA: limits of agreement; p < 0.05 was considered statistically significant.
Fig. 1. Plots of repeatability differences for CCT in (a) normal, (b) keratoconic and (c) postrefractive corneas. The middle line represents the mean. The lines on the side represent the upper and lower 95% confidence interval limits of agreement.
Fig. 2. Plots of repeatability differences for TCP in (a) normal, (b) keratoconic and (c) postrefractive corneas. The middle line represents the mean. The lines on the side represent the upper and lower 95% confidence interval limits of agreement.
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3.2. Reproducibility Table 2 shows the reproducibility results for the measured parameters by both examiners for all groups. Measurement results were similar for each groups and there were no statistically significant differences between two examiners (p > 0.05). The 95% LoA for reproducibility was also very small, and acceptable for all measured parameters in each group. The 95% LoA was larger for the measurement of CCT and TCT in postrefractive corneas. The 95% LoA for reproducibility was ranged from +15.43 to −11.83 m for CCT, and +13.07 to −11.17 m for TCT. Figs. 3 and 4 show the plots of between-examiner differences of CCT and TCT in all groups. 4. Discussion Reliability studies of diagnostic devices are necessary to ensure that the error involved in measurement is small enough to detect actual changes in what is being measured [11]. This present study was designed to evaluate the repeatability and reproducibility of the anterior segment measurements provided by the Galilei dual Scheimpflug Placido corneal topographer in normal, postrefractive and keratoconic corneas. The repeatability of corneal pachymetric measurements using the Galilei Scheimpflug system, Menassa et al. [3] showed high repeatability for central average corneal thickness and thinnest corneal thickness in normal corneas. Wang et al. [1] evaluated 20 unoperated eyes, and reported high repeatability for corneal thickness. Savini et al. [7] also reported excellent repeatability for measurements of central and thinnest corneal thickness in normal and postrefractive corneas. The repeatability for keratoconic corneas was not assessed in these previous studies. de Sanctis et al. [12] found more reproducible and repeatable measurements of central thickness by Pentacam than those obtained with ultrasound pachymetry in keratoconic corneas. In this study we found excellent repeatability results for pachymetry readings in normal, postrefractive and keratoconic corneas.
In a previous study, Menassa et al. [3] reported that keratometry and central corneal pachymetry readings with the Galilei and a scanning-slit topographer (Orbscan II, Bausch & Lomb) showed high reproducibility in normal corneas. However, Menassa et al. [3] evaluated the reproducibility of the dual Scheimpflug system only in normal corneas. In our study we included keratoconic corneas, and found excellent reproducibility for pachymetry readings. For corneal pachymetric measurements using the single Scheimpflug system (Sirius, Costruzione Strumenti Oftalmici), Shankar et al. [2] reported good repeatability for CCT, whereas the peripheral pachymetry repeatability was poor. We found that the dual-Scheimpflug system showed excellent repeatability for both central and paracentral corneal pachymetry, similarly to Wang et al. [1]. This is perhaps due to the dual-channel Scheimpflug cameras implemented by the system. If the alignment varies between measurements, different thicknesses will be detected depending on the camera location and magnitude of decentration. In contrast, measurement values obtained by averaging data from two cameras should minimize the problems caused by altered corneal position because such a shift will produce a thinner measurement by 1 camera and a correspondingly thicker measurement by the other camera [13]. In this study, all corneal power and anterior–posterior best-fit sphere measurements yielded high repeatability results in normal and postrefractive corneas. Our results were similar to those previously reported by Savini and Wang [1,7]. To our knowledge, there are no studies evaluating the repeatability of corneal power and anterior–posterior best-fit sphere measurements by the Galilei device in keratoconic eyes. In keratoconic corneas, similar repeatability was found in the measurement PCP, anterior-posterior BFS, and TCP. The 95% LoA for TCP was larger when compared with normal and postrefractive corneas. However, the 95% LoA of TCP ranged between 1.98 and −2.09, therefore Galilei can be accepted to perform reliable TCP measurements in keratoconic corneas. In a previous study Wang et al. [1] reported the dualScheimpflug system (Galilei) had excellent reproducibility in measuring total corneal power at the central, paracentral, and
Table 2 Summary of reproducibility results for each measured parameter by group. Mean ± SD
Patients
Parameter
Normal
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
551.73 540.78 41.65 6.19 7.91 6.58 0.33 0.26
± ± ± ± ± ± ± ±
37.4 36.46 1.7 0.3 0.31 0.3 0.12 0.19
551.75 541.7 41.82 6.2 7.88 6.58 0.37 0.29
± ± ± ± ± ± ± ±
37.73 37.5 2.28 0.33 0.33 0.31 0.34 0.21
Keratoconus
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
498.0 473.15 46.46 7.39 7.34 6.0 1.18 1.47
± ± ± ± ± ± ± ±
35.35 38.20 3.55 0.78 0.40 0.35 0.52 0.56
496.95 473.45 46.43 7.42 7.33 6.0 1.18 1.49
± ± ± ± ± ± ± ±
Postrefractive
CCT TCT TCP PCP BFS anterior BFS posterior ε2 anterior ε2 posterior
492.25 472.15 40.59 6.40 8.24 6.48 0.49 0.41
± ± ± ± ± ± ± ±
40.28 43.15 2.19 0.58 0.44 0.33 0.35 0.50
490.45 471.20 40.59 6.36 8.24 6.47 0.48 0.48
± ± ± ± ± ± ± ±
Examiner 1
t
p
Mean difference
0.051 1.347 0.834 0.362 1.148 0.229 0.648 0.647
0.960 0.194 0.415 0.721 0.265 0.821 0.525 0.525
−0.02 −0.93 −0.17 −0.01 0.02 0.00 −0.04 −0.02
± ± ± ± ± ± ± ±
2.19 3.07 0.90 0.08 0.07 0.05 0.27 0.17
4.27 5.09 1.60 0.16 0.16 0.11 0.49 0.30
−4.32 −6.94 −1.94 −0.17 −0.12 −0.10 −0.56 −0.35
33.73 36.80 3.15 0.73 0.38 0.33 0.45 0.55
1.288 −0.422 0.162 −0.783 0.768 0.100 −0.176 −0.865
0.213 0.678 0.873 0.443 0.452 0.921 0.862 0.398
1.05 −0.30 0.03 −0.03 0.01 0.00 −0.01 −0.03
± ± ± ± ± ± ± ±
3.65 3.18 0.77 0.17 0.06 0.06 0.20 0.13
8.19 5.93 1.54 0.30 0.13 0.11 0.39 0.23
−6.09 −6.53 −1.49 −0.36 −0.11 −0.11 −0.41 −0.28
39.44 43.58 2.21 0.54 0.44 0.33 0.34 0.55
1.158 0.687 0.108 0.751 0.439 0.475 0.383 2.119
0.261 0.500 0.915 0.462 0.665 0.640 0.706 0.055
1.80 0.95 0.00 0.03 0.01 0.01 0.01 −0.07
± ± ± ± ± ± ± ±
6.95 6.18 0.18 0.20 0.07 0.07 0.12 0.14
15.43 13.07 0.34 0.43 0.15 0.14 0.24 0.21
−11.83 −11.17 −0.35 −0.37 −0.13 −0.12 −0.22 −0.34
Examiner 2
95% LoA Upper limit
Lower limit
CCT: central corneal thickness; TCT: thinnest corneal thickness; TCP: total corneal power, PCP: posterior corneal power, BFS: best fit sphere; ε2 : eccentricity; SD: standard deviation; LoA: limits of agreement; p < 0.05 was considered statistically significant.
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Fig. 3. Plots of reproducibility differences for CCT in (a) normal, (b) keratoconic and (c) postrefractive corneas. The middle line represents the mean. The lines on the side represent the upper and lower 95% confidence interval limits of agreement.
Fig. 4. Plots of reproducibility differences for TCT in (a) normal, (b) keratoconic and (c) postrefractive corneas. The middle line represents the mean. The lines on the side represent the upper and lower 95% confidence interval limits of agreement.
peripheral zones and meridional values of posterior corneal power in normal corneas. In the present study, we also assessed the reproducibility of corneal power measurements and anterior-posterior best-fit sphere in postrefractive and keratoconic eyes. All corneal power measurements had excellent reproducibility for all groups. Dao et al. [14] suggested that eccentricity can be helpful to diagnose keratoconus at an early stage. Previous reports found that the anterior corneal ε2 value ranges from −0.01 to −0.80 in normal corneas. In this study we found similar ε2 results ranging −0.1 to −0.9 in normal corneas. In the current study, the 95% LoA for results for ε2 showed small variability between measurements in all groups. Therefore in clinical practice, the ε2 measurements of Galilei may be accepted reliable in all study groups, in particular to keratoconic eyes. To our knowledge this is the first study to show; (1) reproducibility of the measurements of Galilei in keratoconic and postrefractive corneas; (2) repeatability of the measurements of Galilei in keratoconic corneas. Our data shows that this instrument has high repeatability for all measured parameters; however the measurement of TCP in keratoconic corneas had larger 95% LoA. The reproducibility was excellent in normal, keratoconic and postrefractive corneas. The 95% LoA for reproducibility of CCT and TCT in postrefractive corneas was larger than normal and keratoconic corneas. In summary the measurements of the Galilei rotating Scheimpflug system, showed a remarkably low intraobserver and interobserver variation, therefore the instrument can be confidently used for routine clinical use and research purposes.
Conflict of interest There is no proprietary interest or research funding. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.clae.2014.04.004. In all supplemental figures, ‘a’ refers to normal; ‘b’ refers to keratoconic; ‘c’ refers to postrefractive corneas. References [1] Wang L, Shirayama M, Koch DD. Repeatability of corneal power and waveform aberration measurements with a dual-Scheimpflug Placido corneal topographer. J Cataract Refract Surg 2010;36(3):425–30. [2] Shankar H, Taranath D, Santhirathelagan CT, Pesudovs K. Anterior segment biometry with the Pentacam: comprehensive assessment of repeatability of automated measurements. J Cataract Refract Surg 2008;34(1):103–13. [3] 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):1742–7. [4] O’ Donnell C, Maldonado-Codina C. Agreement and repeatability of central thickness measurement in normal corneas using ultrasound pachymetry and the OCULUS Pentacam. Cornea 2005;24(8):920–4. [5] Marsich MW, Bullimore MA. The repeatability of corneal thickness measures. Cornea 2000;19(6):792–5. [6] Cho P, Lam AK, Mountford J, Ng L. The performance of four different corneal topographers on normal human corneas and its impact on orthokeratology lens fitting. Optom Vis Sci 2002;79(3):175–83.
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[7] 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(2):302–9. [8] International Organization for Standardization. Accuracy 272 (Trueness and Precision) of Measurement Methods and Results, Part 1. General Principles and Definitions (ISO 5725-1). Geneva, Switzerland: ISO; 1994. [9] International Organization for Standardization. Accuracy (Trueness and Precision) of Measurement Methods and Results, Part 2. Basic Methods for the Determination of Repeatability and Reproducibility of a Standard Measurement Method (ISO 5725-2). Geneva, Switzerland: ISO; 1994. [10] Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;1(8476):307–10.
[11] Rankin G, Stokes M. Reliability of assessment tools in rehabilitation: an illustration of appropriate statistical analysis. Clin Rehabil 1998;12(3):187–99. [12] de Sanctis U, Missolungi A, Mutani B, Richiardi L, Grignolo FM. Reproducibility and repeatability of central corneal thickness measurement in keratoconus using the rotating Scheimpflug camera and ultrasound pachymetry. Am J Ophthalmol 2007;144(5):712–8. [13] Lewis JR, et al. Comparison of Response to Misalignment in Pachymetry Measurement between Single- and Dual-Scheimpflug Devices. Presented at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, San Francisco, California, USA, April; 2009. [14] Dao CL, Kok JH, Brinkman CJ, van Mil CJ. Corneal eccentricity as a tool for the diagnosis of keratoconus. Cornea 1994;13(4):339–44.
