Intraobserver and interobserver repeatability of curvature and aberrometric measurements of the posterior corneal surface in normal eyes using Scheimpflug photography

ARTICLE

Intraobserver and interobserver repeatability of curvature and aberrometric measurements of the posterior corneal surface in normal eyes using Scheimpflug photography David P. Pin˜ero, PhD, Cristina Saenz Gonza´lez, OD, Jorge L. Alio´, MD, PhD

PURPOSE: To assess the intraobserver and interobserver repeatability of curvature and aberrometric measurements of the posterior corneal surface provided by a Scheimpflug photography system in normal eyes. SETTING: Vissum-Instituto de Oftalmolo´gico de Alicante, Alicante, Spain. METHODS: All eyes received a comprehensive ophthalmologic examination including corneal topographic analysis with a Scheimpflug photography system (Pentacam). Three repeated consecutive measurements were taken by 2 independent experienced examiners to assess intraobserver and interobserver repeatability for posterior corneal surface measurements. Keratometry, astigmatism, best-fit sphere (BFS), asphericity (Q), and aberrometry (6.0 mm pupil diameter) were analyzed. Precision, repeatability, and intraclass correlation coefficients were calculated for evaluating intraobserver repeatability. Bland-Altman analysis was used for assessing interobserver repeatability. RESULTS: Twenty eyes (20 patients) were included. Patients ranged in age from 21 to 50 years. The best intraobserver precision values were for BFS and Q in the central 8.0 mm (0.037 mm and 0.05, respectively). For both observers, intraobserver precision and repeatability achieved an acceptable level for keratometric readings (precision <0.09 diopter). The most limited intraobserver precision was for Q in the central 6.0 mm (0.117 observer 1; 0.099 observer 2). Interobserver ranges of agreement were not clinically relevant for any parameter except Q in the central 6.0 mm (0.116). Regarding aberrometry, intraobserver repeatability and interobserver repeatability were acceptable only for primary and secondary spherical aberration. CONCLUSION: The Pentacam system provided reliable measurements of corneal posterior keratometry, astigmatism, and asphericity for large diameters of analysis. However, poor repeatability was observed for aberrometric measurements. J Cataract Refract Surg 2009; 35:113–120 Q 2009 ASCRS and ESCRS

The analysis of posterior corneal surface has gained importance in step with the advances in ophthalmic diagnostic technology.1 The optical contribution of this refractive surface to the total corneal power, and then to the total ocular power, is very limited due to the small difference in the refractive index between cornea and aqueous.2,3 However, the study of the shape of this surface has become increasingly important. Posterior corneal elevation and curvature have been considered as screening factors in refractive surgery for detecting subclinical ectatic disorders.4–7 In addition, they have been used as control parameters of corneal stability during the follow-up of patients who have had laser in situ keratomileusis (LASIK).8–15 In any case, there Q 2009 ASCRS and ESCRS Published by Elsevier Inc.

is controversy regarding posterior corneal surface changes after keratorefractive procedures with the excimer laser because of concerns over the accuracy of the available measuring systems.8,12,16–18 Two technologies for measuring the posterior corneal surface have been described: combined scanning-slit and Placido-disk technology19 (Orbscan system, Bausch & Lomb) and Scheimpflug photography20 (Pentacam system, Oculus Optikgera¨te). However, not all the posterior corneal surface parameters that are estimated by these systems have been validated. Indeed, it has been shown that the Orbscan system does not provide reliable measurements of posterior corneal asphericity and of any posterior 0886-3350/09/$dsee front matter doi:10.1016/j.jcrs.2008.10.010

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corneal measurement in the early postoperative LASIK period.20 On the other hand, only intraobserver repeatability for keratometric and best-fit sphere (BFS) data have been reported for the Pentacam system.21,22 Therefore, only the parameters whose repeatability has been tested previously should be used for evaluation of posterior corneal changes. Alternatively, additional studies of repeatability should be performed. Dioptric and geometric measurements are important for a complete corneal analysis. In addition, corneal aberrometry is a valuable tool for corneal evaluation.23–25 The level of coma-like aberrations of the anterior corneal surface has been defined as a criterion for determining the severity of corneal ectatic disorders.26,27 Posterior corneal aberrations have not been widely studied because the available technology did not supply this information. The Pentacam was the first commercially available system that provided wavefront aberration analysis for the posterior corneal surface. Its use has not been tested as a screening factor, and its precision has not been evaluated. The aim of the present study was to evaluate the intraobserver and interobserver repeatability of the Pentacam Scheimpflug imaging–based system for posterior corneal measurements, including wavefront aberration analysis. PATIENTS AND METHODS Patients This study comprised candidates for corneal refractive surgery who were screened at the Vissum-Instituto de Oftalmolo´gico de Alicante. Patients and eyes were selected according to a random number sequence (dichotomy sequence, 0 and 1). Only eyes with active ocular pathology or previous ocular surgery were excluded. All patients were informed about

Submitted: June 17, 2008. Final revision submitted: September 29, 2008. Accepted: October 2, 2008. From the Vissum-Instituto de Oftalmolo´gico de Alicante (Pin˜ero, Saenz Gonza´lez, Alio´), and Departamento de O´ptica (Pin˜ero), Farmacologı´a y Anatomı´a, Universidad de Alicante, and the Division of Ophthalmology (Alio´), Universidad Miguel Herna´ndez, Alicante, Spain. No author has a financial or proprietary interest in any material or method mentioned. Supported in part by a grant from the Spanish Ministry of Health, Instituto Carlos III, Red Tema´tica de Investigacio´n Cooperativa en Salud Patologı´a Ocular del Envejecimiento, Calidad Visual y Calidad de Vida, Subproyecto de Calidad Visual (RD07/0062) Corresponding author: David P. Pin˜ero, PhD, Vissum-Instituto de Oftalmolo´gico de Alicante, Avenida de Denia s/n, Edificio Vissum, 03016 Alicante, Spain E-mail: [email protected].

inclusion in this study and signed an informed consent in accordance with the Helsinki Declaration.

Measurement Parameters All eyes had a comprehensive ophthalmologic examination that included corneal topographic analysis with the Pentacam system. Three repeated consecutive measurements were taken by 2 independent experienced examiners (D.P.L., C.S.G.) to assess intraobserver and interobserver repeatability. The starting examiner was assigned randomly in each case. For all measurements, the option of 100 scans in 2 seconds and the automatic release mode (Cornea Fine) were used. The following posterior corneal surface parameters were recorded and analyzed at each examination: corneal dioptric power in the flattest meridian for the 3.0 mm central zone (K1); corneal dioptric power in the steepest meridian for the 3.0 mm central zone (K2); BFS for an analysis area of 8.0 mm diameter; mean asphericity for a corneal area of 6.0 mm diameter (Q6); mean asphericity for a corneal area of 7.0 mm diameter (Q7); mean asphericity for a corneal area of 8.0 mm diameter (Q8); 3rd- and 4th-order Zernike coefficients Z(3,1), Z(3, 1), Z(3,3), Z(3, 3), Z(4,0), Z(4,2), Z(4, 2), Z(4,4), and Z(4, 4); and Zernike coefficients for secondary coma Z(5,1), Z(5, 1) and secondary spherical aberration Z(6,0). All Zernike coefficients were calculated for a pupil diameter of 6.0 mm. Intraobserver and interobserver repeatability were evaluated for all parameters.

Scheimpflug Camera System Pentacam is a noninvasive system for measuring and characterizing the anterior segment using Scheimpflug photography as a basis.22 With this system, a rotating Scheimpflug camera takes 100 images with 500 measurement points on the anterior and posterior corneal surfaces over a 180-degree rotation. The elevation data from all these images are combined to form a 3-dimensional reconstruction of the corneal structure. After all this information is processed, the internal software provides a large number of different calculations. In this study, the Pentacam software version 6.02r10 was used. The same measurement procedure was used in all cases. The patient was asked to blink twice and then look at the fixation device before each measurement. The examiner adjusted the joystick until perfect alignment was shown. Then, the system automatically took 100 images of the cornea within a 2-second period. Acceptable maps had at least 10.0 mm of corneal coverage with no extrapolated data in the central 9.0 mm zone. Scans not meeting acceptable criteria (blinks during the scan or other artifacts) according to the Pentacam software indications were repeated. Only data provided for posterior corneal surface were extracted and analyzed in this study. All scans were centered on the pupil center.

Statistical Analysis Statistical analysis was performed using the SPSS for Windows software (version 11.0, SPSS, Inc.). Normality of all data distributions was confirmed by the Kolmogorov-Smirnov test. Then, parametric statistics were applied. The unpaired Student t test was used for analyzing the comparison between examiners for each corneal parameter. All tests were 2 tailed; P values less than 0.05 were considered statistically significant.

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Table 1. Summary of intraobserver repeatability results for the curvature parameters of the posterior corneal surface. Observer 1 Parameter K1 (D) K2 (D) BFS (mm) Q6 Q7 Q8

Overall Mean G SD (Range) 6.14 G 0.30 ( 6.70 to 5.63) 6.49 G 0.30 ( 7.17 to 5.90) 6.47 G 0.31 (5.90 to 7.04) 0.10 G 0.26 ( 0.72 to 0.45) 0.22 G 0.25 ( 0.90 to 0.24) 0.33 G 0.24 ( 0.99 to 0.05)

Observer 2

Sw

Pr

Rep

ICC (Range)

Overall Mean G SD (Range)

0.029

0.057

0.080

0.980

0.038

0.074

0.104

0.986

0.019

0.037

0.052

0.994

0.060

0.117

0.165

0.925

0.036

0.070

0.099

0.972

0.022

0.044

0.062

0.990

6.14 G 0.30 ( 6.67 to 5.60) 6.48 G 0.41 ( 7.13 to 5.93) 6.47 G 0.31 (5.91 to 7.04) 0.09 G 0.27 ( 0.77 to 0.42) 0.21 G 0.27 ( 0.96 to 0.22) 0.32 G 0.25 ( 1.01 to 0.04)

Sw

Pr

Rep

ICC (Range)

0.028

0.055

0.078

0.980

0.042

0.082

0.116

0.983

0.019

0.038

0.054

0.996

0.051

0.099

0.140

0.952

0.036

0.070

0.099

0.976

0.028

0.054

0.077

0.984

BFS Z best-fit sphere for an analysis area of 8.0 mm; ICC Z intraclass correlation coefficient; K1 Z corneal dioptric power in the flattest meridian for the 3.0 mm central zone; K2 Z corneal dioptric power in the steepest meridian for the 3.0 mm central zone; Pr Z precision; Q6 Z mean asphericity over a corneal area of 6.0 mm; Q7 Z mean asphericity over a corneal area of 7.0 mm; Q8 Z mean asphericity over a corneal area of 8.0 mm; Rep Z repeatability; Sw Z within-subject standard deviation

Intraobserver repeatability for each corneal parameter was assessed using the following statistical parameters: the within-subject standard deviation (Sw) of the 3 consecutive measurements, intraobserver precision, intraobserver repeatability, and the intraclass correlation coefficient (ICC). The within-subject standard deviation is a simple way of estimating the size of the measurement error. The intraobserver precision was defined as (G1.96  Sw)28; this parameter indicates how large the range of error of the repeated measurements for 95% of observations is. The repeatability was computed as (2.77  Sw); this is another useful way of presenting the range of measurement error.29 The ICC is an analysis-of-variance type correlation that measures the relative homogeneity within groups (between the repeated measurements) in ratio to the total variation.30 The ICC will approach 1.0 when there is no variance within repeated measurements, indicating that the total variation in measurements is due solely to variability in the parameter being measured. Interobserver repeatability was evaluated by Bland-Altman analysis.28 This method uses graphing to assess whether there is agreement between 2 clinical procedures.28 In the current study, agreement between mean measurements of several topographic and aberrometric parameters obtained by the 2 observers with the Pentacam system was evaluated. Graphs of the differences between measurements obtained by each observer against means were plotted (Bland-Altman plots). The limits of agreement (LoA) were calculated as the mean difference in measurements obtained by each observer G1.96 SD of the differences.28 This standard deviation is by definition the interobserver range of agreement (1.96 times), with lower values indicating higher repeatability. The interobserver repeatability is not acceptable if the range of agreement is clinically relevant (error with significant implication in the clinical practice), indicating that the evaluated clinical methodology does not provide repeatable measurements. The number of patients included in the study was chosen according to the results from sample-size calculations.

Twenty eyes provided a statistical power for all comparison tests higher than 90%.

RESULTS Patients Twenty eyes of 20 candidates for corneal refractive surgery (age range 21 to 50 years) were screened. The mean sphere was 1.26 diopters (D) G 2.54 (SD) (range 6.00 to C4.00 D). The mean astigmatism was 0.54 G 1.00 D (range 0.00 to 3.50 D). All eyes had a best spectacle-corrected visual acuity of 1.0 (Snellen decimal notation) or higher. Intraobserver Repeatability Table 1 shows the intraobserver repeatability results for the curvature parameters. For both observers, the best intraobserver precision values were for BFS and Q8; there were no statistically significant differences in these parameters between examiners (all PO.07, unpaired Student t test). Regarding keratometric readings (K1 and K2), precision and repeatability were acceptable for both observers, with no significant differences between them (K1, P Z .94; K2, P Z .70; unpaired Student t test). In addition, for both observers, very limited precision was found for the parameter Q6 (0.117 observer 1; 0.099 observer 2). The magnitude of the intraobserver variability for this parameter was larger than the mean value. Table 2 shows the intraobserver repeatability results for the aberrometric parameters of the posterior corneal surface. Zernike coefficients corresponding to

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Table 2. Summary of intraobserver repeatability results for the aberrometric parameters of the posterior corneal surface. The Zernike coefficients corresponding to different optical defects are shown. Observer 1 Parameter (mm) 3rd order Trefoil Z(3,3) Z(3, 3) Primary coma Z(3,1) Z(3, 1) 4th order Primary SA Z(4,0) Secondary astigmatism Z(4,2) Z(4, 2) Quadrafoil Z(4,4) Z(4, 4) 5th order Secondary coma Z(5,1) Z(5, 1) 6th order Secondary SA Z(6,0)

Observer 2

Sw

Pr

Rep

ICC (Range)

Overall Mean G SD (Range)

0.121

0.238

0.336

0.583

0.178

0.350

0.494

0.559

0.089 G 0.155 ( 0.116 to 0.482) 0.010 G 0.208 ( 0.297 to 0.326)

0.081

0.158

0.223

0.967

0.049

0.096

0.135

0.953

1.067 G 0.343 (0.492 to 1.895)

0.072

0.140

0.198

0.943

0.018 G 0.200 ( 0.462 to 0.346) 0.045 G 0.232 ( 0.415 to 0.694)

0.059

0.116

0.164

0.897

0.071

0.139

0.197

0.887

0.099

0.194

0.274

0.778

0.101

0.199

0.281

0.803

0.055

0.108

0.152

0.921

0.035

0.069

0.098

0.918

0.028

0.054

0.077

0.827

Overall Mean G SD (Range)

0.089 G 0.193 ( 0.119 to 0.582) 0.048 G 0.277 ( 0.515 to 0.440) 0.216 G 0.519 ( 1.031 to 0.962) 0.046 G 0.267 ( 0.584 to 0.639)

0.362 G 0.231 ( 0.057 to 0.774) 0.023 G 0.238 ( 0.344 to 0.419)

0.109 G 0.222 ( 0.715 to 0.239) 0.051 G 0.137 ( 0.194 to 0.379)

0.047 G 0.073 ( 0.216 to 0.048)

Sw

Pr

Rep

ICC (Range)

0.132

0.259

0.365

0.393

0.178

0.348

0.492

0.407

0.053

0.104

0.148

0.981

0.057

0.111

0.157

0.949

1.090 G 0.353 (0.418 to 1.862)

0.054

0.105

0.148

0.969

0.014 G 0.182 ( 0.470 to 0.358) 0.055 G 0.253 ( 0.610 to 0.676)

0.072

0.141

0.200

0.819

0.043

0.085

0.120

0.960

0.379 G 0.272 ( 0.143 to 0.789) 0.046 G 0.254 ( 0.318 to 0.441)

0.127

0.249

0.351

0.783

0.109

0.213

0.301

0.773

0.053

0.105

0.148

0.936

0.037

0.073

0.104

0.943

0.021

0.041

0.058

0.888

0.204 G 0.466 ( 0.905 to 0.924) 0.029 G 0.279 ( 0.721 to 0.523)

0.098 G 0.226 ( 0.750 to 0.293) 0.068 G 0.170 ( 0.240 to 0.553)

0.058 G 0.072 ( 0.221 to 0.053)

ICC Z intraclass correlation coefficient; Pr Z precision; Rep Z repeatability; SA Z spherical aberration; Sw Z within-subject standard deviation

the extremes of the 3rd order and 4th order (trefoil and tetrafoil components) showed the worst intraobserver repeatability for both examiners. Acceptable precision and repeatability values were found for Z(3,1) (primary coma component) and for primary and secondary spherical aberration (central components of Zernike pyramidal graphical representation). No statistically significant differences were found in repeatability parameters between observers (all PO.07, unpaired Student t test) except for Z(4, 2) (P Z .02, unpaired Student t test).

Interobserver Repeatability Table 3 shows the interobserver repeatability results for curvature parameters. Range and LoA were not clinically relevant for any parameter except Q6. The range of error for Q6 was higher than the mean value, which indicates very relevant variability. Regarding aberrometric coefficients (Table 4), the LoA were acceptable only for primary spherical aberration Z(4,0) and secondary spherical aberration Z(6,0). For the other aberrometric coefficients, the LoA were large compared with the magnitude of the mean.

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Table 3. Summary of interobserver repeatability results for the curvature parameters of the posterior corneal surface.

Parameter

Range of Agreement*

K1 (D) K2 (D) BFS (mm) Q6 Q7 Q8

0.054 0.112 0.033 0.116 0.077 0.059

Mean Difference 0.002 0.008 0.001 0.012 0.008 0.011

LoA* 0.052, 0.056 0.104, 0.120 0.034, 0.032 0.104, 0.129 0.069, 0.084 0.049, 0.070

BFS Z best-fit sphere for an analysis area of 8.0 mm; K1 Z corneal dioptric power in the flattest meridian for the 3.0 mm central zone; K2 Z corneal dioptric power in the steepest meridian for the 3.0 mm central zone; LoA Z limits of agreement; Q6 Z mean asphericity over a corneal area of 6.0 mm; Q7 Z mean asphericity over a corneal area of 7.0 mm; Q8 Z mean asphericity over a corneal area of 8.0 mm * Bland-Altman analysis

Bland-Altman plots for the Zernike coefficients with the smallest ranges of agreement, Z(4,0) and Z(6,0), and the largest ranges of agreement, Z(3, 3) and Z(4,4), are shown in Figures 1 and 2. The ranges of agreement were large for the trefoil and tetrafoil components. DISCUSSION In normal eyes, the posterior corneal surface accounts for a very small part of the total refractive power of the cornea.3 In the current study, the mean value of the posterior corneal refractive power (in the 3.0 mm central area) was approximately 6.30 D, which agrees with findings in previous studies using Scheimpflug photography.21,22,31 In addition, we found a mean BFS for the posterior corneal surface of 6.47 mm (8.0 mm diameter area), which also agrees with previous findings.21 In the present study, posterior curvature in the 3.0 mm central area and BFS were repeatable when measured with the Pentacam system in normal eyes. Intraobserver and interobserver repeatability analysis showed excellent results, indicating the consistency of curvature data provided by this Scheimpflug-based technology. These outcomes agree with those in other studies.21,22 Therefore, the Pentacam system is a useful and valid tool for evaluating the curvature of the posterior corneal surface. In addition, we believe that this study is the first report evaluating the repeatability of the Pentacam system for posterior corneal asphericity measurements. We found that the larger the analysis area, the better the intraobserver repeatability. Indeed, asphericity calculated for a 6.0 mm diameter area was associated with very limited intraobserver precision. This tendency was also found for the interobserver range of

Table 4. Summary of interobserver repeatability results for the aberrometric parameters of the posterior corneal surface.

Parameter 3rd order Trefoil Z(3,3) Z(3, 3) Primary coma Z(3,1) Z(3, 1) 4th order Primary SA Z(4,0) Secondary astigmatism Z(4,2) Z(4, 2) Quadrafoil Z(4,4) Z(4, 4) 5th order Secondary coma Z(5,1) Z(5, 1) 6th order Secondary SA Z(6,0)

Range of Mean Agreement* Difference

LoA*

0.291 0.334

0.012 0.171

0.218, 0.195 0.314, 0.028

0.207 0.143

0.0003 0.057

0.291, 0.292 0.277, 0.392

0.132

0.022

0.110, 0.154

0.149 0.157

0.004 0.010

0.153, 0.146 0.147, 0.168

0.361 0.156

0.017 0.023

0.343, 0.378 0.133, 0.179

0.127 0.113

0.012 0.017

0.116, 0.139 0.097, 0.130

0.058

0.010

0.068, 0.048

LoA Z limits of agreement; SA Z spherical aberration * Bland-Altman analysis

agreement. Asphericity calculated for 7.0 mm diameter areas or larger with the Pentacam system is a repeatable and valid parameter for use in topographic diagnosis. This same tendency toward increasing repeatability with largest diameters of analysis was observed by Gonza´lez-Me´ijome et al.32 in their calculations of anterior corneal asphericity using the VOL-CT software (Sarver & Associates, Inc) and the data extracted from a Placido-based topography system. It is likely that the use of a constant Q-value model is not adequate for defining the posterior corneal contour. It has been proved that a conic is a poor estimator of the peripheral shape of the anterior corneal surface because in this area there is a more significant flattening and less astigmatism than in the central area.33 The same behavior is probably also present in the posterior corneal surface; however, this must be studied comprehensively in future investigations because to our knowledge, the issue has not been reported. In addition, meridional variations of asphericity, as in the anterior surface,33 may be characteristic of the posterior corneal surface. The use of nasal, temporal, superior, and inferior Q values provided by the Pentacam software could be better

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Figure 1. Bland-Altman plot showing the differences in Z(4,0) (A) and in Z(6,0) (B) between observer 1 and observer 2 plotted against the mean value for both. The upper line and the lower line represent the LoA, calculated as mean G1.96 SD (N Z 20 eyes).

descriptors. On the other hand, use of the largest areas of analysis implies the consideration of a greater number of points for defining the Q-value with less probable variability in the adjustment. We believe that this is the first report that shows a complete description of the Zernike aberrometric coefficients of the posterior corneal surface in normal eyes. There are few reports of aberrometry of the posterior corneal surface,2,34 and none used the Zernike polynomial decomposition for defining the wavefront error. We studied 3rd- and 4th-order Zernike components as well as the central components of the 5th and 6th order because these errors have a more negative impact on visual acuity.35 The largest Zernike coefficient values were for primary coma Z(3,1), primary spherical aberration Z(4,0), and tetrafoil Z(4,4); the values were near zero for the other wavefront error terms. Regarding repeatability of aberrometry, the central coefficients of the Zernike pyramidal diagram had

better intraobserver and interobserver repeatability. Acceptable precision values were found for only the primary and secondary spherical aberration coefficients. For primary and secondary coma, as well as for secondary astigmatism, good ICCs were obtained; however, the range of error (precision) or variability associated with the repeated measurements was high enough. This variability has clinical relevance because it implies measurement errors that can lead the clinician to a wrong diagnosis. Recently, a problem of repeatability in anterior corneal wavefront analysis with the Pentacam system was reported in normal eyes and keratoconic eyes (also with a 6.0 mm pupil),36 indicating that the Zernike polynomial decomposition used for the reconstruction of the wavefront error is not useful. In the current study, a problem of repeatability with the same system was also found for the posterior corneal wavefront analysis using the Zernike polynomial expansion. Thus, Zernike polynomial analysis is probably not a good option for

Figure 2. Bland-Altman plot showing the differences in Z3 3 (A) and in Z44 (B) between observer 1 and observer 2 plotted against the mean value for both. The upper line and the lower line represent the LoA, calculated as mean G1.96 SD (N Z 20 eyes).

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characterizing the posterior corneal aberrations and new approaches are necessary. More research of the characterization of posterior corneal aberration must be performed to find the best mathematical descriptors. It has been proved that Zernike polynomial decomposition is not the best option for the description of anterior corneal aberrations, especially when abnormal shapes are present, such as in keratoconus or in post-keratorefractive surgery corneas.37 It has been suggested that the number of Zernike polynomials should not be a fixed number (as in the Pentacam system) but rather a number based on surface-specific properties.38 Another possible cause of the lack of repeatability of the posterior corneal wavefront analysis is misalignment or movement during scanning. The acquisition time is not long, only 2 to 3 seconds; however, this is enough time for very subtle movements that could affect wavefront-analysis repeatability. In conclusion, caution must be taken when posterior corneal surface changes are studied because not all parameters provided by topography systems are valid and repeatable. With the Pentacam system, keratometric readings in the 3 .0 mm central area and corneal asphericity for large diameters of analysis were reliable. Thus, these parameters can be used in the characterization of the posterior corneal surface. However, wavefront aberration analysis provided by the Pentacam system for the posterior corneal surface was not reliable, with a significant variability, especially for the modes in the extremes of the Zernike pyramidal decomposition. The algorithm used for the wavefront error calculation by the Pentacam system is not adequate and does not provide a good mathematical description. Therefore, this type of analysis should not be used in studies of the cornea until the software is modified to address this issue. Future mathematical models should be developed for providing a better description of the wavefront errors of the posterior corneal surface. REFERENCES

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First author: David P. Pin˜ero, PhD ´ ptica, Farmacologı´a Departamento de O y Anatomı´a, Universidad de Alicante, Alicante, Spain