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Repeatability of corneal power and wavefront aberration measurements with a dual-Scheimpflug Placido corneal topographer

Wang, Li MD, PhD; Shirayama, Mariko MD; Koch, Douglas D. MD

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Journal of Cataract & Refractive Surgery: March 2010 - Volume 36 - Issue 3 - p 425-430
doi: 10.1016/j.jcrs.2009.09.034
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Abstract

In recent years, Scheimpflug photographic devices have become commercially available for anterior segment measurements. The Pentacam system (Oculus, Inc.) uses a single Scheimpflug camera to acquire multiple photographs of the anterior segment of the eye.1–6 The Galilei dual-Scheimpflug analyzer (Ziemer Group) uses dual-channel Scheimpflug cameras and combines a Placido disk with the goal of improving accuracy of corneal power and pachymetric measurements.7

Knowledge of repeatability of new devices is essential. In a previous study,8 we assessed the repeatability of the simulated keratometry values measured with the dual-Scheimpflug analyzer and compared them with values measured using the Humphrey Atlas corneal topographer (Carl Zeiss), IOLMaster partial coherence interferometry (PCI) biometer (Carl Zeiss), and a manual keratometer (Bausch & Lomb, Inc.).8 Menassa et al.9 evaluated the reproducibility and comparability of central corneal thickness (CCT) and keratometry readings using the dual-Scheimpflug device, the Orbscan II scanning-slit topographer (Bausch & Lomb, Inc.), and a 50 MHz pachymeter (Sonogage).

The purpose of this study was to evaluate the repeatability of the dual-Scheimpflug analyzer in measuring the total corneal power at the central, paracentral, and peripheral zones; posterior corneal power; corneal wavefront aberrations; corneal pachymetry at the central, paracentral, and peripheral zones; and anterior chamber depth (ACD).

SUBJECTS AND METHODS

Subjects

This study prospectively enrolled subjects who met the following inclusion criteria: no previous ocular surgery or trauma, no corneal or other ocular diseases, and not a contact lens wearer. One eye of each subject was selected; right eyes were chosen from the first 10 subjects and left eyes from the second 10 subjects. Institutional Review Board approval was obtained for the study, and all subjects provided informed consent.

Measurements

Measurements in this study were performed using the Galilei system, which has a dual-channel Scheimpflug camera and a Placido disk to measure anterior and posterior corneal surfaces. The device performs anterior corneal measurements by a proprietary method of merging the Placido and Scheimpflug data. The posterior corneal surface is measured using the Scheimpflug data. Using the device's software (version 3.0), the same examiner (M.S.) performed 3 sets of measurements. Subjects were asked to sit back after each measurement, and the device was realigned before each measurement. Subjects were instructed to blink completely just before each measurement. Four categories of parameters were evaluated.

  1. Corneal power. The total corneal power displayed on the dual-Scheimpflug device is calculated by ray tracing through the anterior and posterior corneal surfaces using the Snell law. The total corneal powers at the central zone (0.0 mm to 4.0 mm), paracentral zone (4.0 to 7.0 mm), and peripheral zone (7.0 to 8.0 mm) were evaluated. For the posterior corneal surface, the mean of the posterior curvatures at the steep meridian and flat meridian (Kavg; 1.0 to 4.0 mm) as well as the magnitude of astigmatism were calculated. To assess posterior corneal astigmatism, vector analysis was performed as proposed by Thibos et al.10 The vector components include M (spherical equivalent of refractive error), J0 (cylinder at 0-degree meridian), and J45 (cylinder at 45-degree meridian). In this study, M was equivalent to Kavg; therefore, only values for J0 and J45 are shown.
  2. Total corneal higher-order wavefront aberrations (3rd to 6th order). The dual Scheimpflug system displays the total corneal wavefront aberrations calculated from the front surface and back surface, centered on the pupil. The following values were recorded with a 6.0 mm pupil: individual Zernike coefficient for terms in the 3rd and 4th orders; root-mean-square (RMS) for the 3rd-, 4th-, 5th-, and 6th-orders; and total higher-order RMS. In this analysis, the normalized polar Zernike coefficients that combine the paired terms in the same order to give a single value were also calculated. For example, the 3rd-order coma terms of vertical coma and horizontal coma were combined to obtain 3rd-order coma Z(3,1).
  3. Corneal pachymetry. The mean corneal pachymetry at the central zone (0.0 to 4.0 mm), paracentral zone (4.0 to 7.0 mm), and peripheral zone (7.0 to 8.0 mm) was evaluated.
  4. Anterior chamber depth. The ACD is the distance between the corneal endothelium and the anterior surface of the crystalline lens. The direction of the distance measurement is perpendicular to the lens surface. In this study, the ACD value was the mean of all measured Scheimpflug scans.

Statistical Analysis

Statistical analysis was performed using SPSS software (version 15.0, SPSS, Inc.). Intraobserver repeatability was assessed using 5 parameters. The first parameter was the within-subject standard deviation (SD), which is also known as the SD of repeated measurements (Sw).11 The second parameter was precision; for 95% of observations, the difference between a subject's measurement and the true value would be expected to be less than 1.96 Sw.11 The third parameter was repeatability; for 95% of pairs of observation, the difference between 2 measurements for the same subject is expected to be less than 2.77 Sw.11 The fourth parameter was the coefficient of variation (COV), which is defined as the ratio of the SD of the repeated measurements to the mean. A lower COV value indicates higher repeatability. The advantage of COV values is that they can be compared between data sets with different units or widely different means. The disadvantage is that when the mean value is near zero, the COV is sensitive to small changes in the mean, limiting its usefulness. For wavefront data, because the mean values for individual Zernike coefficients were close to zero for most terms, the COV values were determined for normalized polar Zernike coefficients and RMS values. The fifth parameter was the intraclass correlation coefficient (ICC), which is a measure of correlation or consistency for data sets of repeated measurements. The ICC values range from 0 to 1, with 1 indicating perfect agreement.

RESULTS

Twenty eyes (10 right, 10 left) of 20 subjects (6 men, 14 women) were studied. The mean age of the subjects was 36 years ± 12.5 (SD) (range 23 to 62 years).

Corneal Power

The mean total corneal power (TCP) increased from center to periphery by a mean of 0.94 D (Table 1). The within-subject SD was less than 0.2 D for all regions for TCP and 0.03 D for the central 1.0–4 mm posterior region. The COV was ≤ 0.34% for the anterior zones, 0.35% for the 1.0–4 mm posterior region, and 17% for astigmatism due to the small mean value (in the denominator).

Table 1
Table 1:
Intraobserver repeatability for total corneal power and posterior corneal power measurements.

Total Corneal Higher-Order Wavefront Aberrations

For all 3rd- and 4th-order Zernike terms, the within-subject SDs for polar coefficients were 0.12 μm or less and the COV ranged from 7% for spherical aberration to 53% for tetrafoil (Table 2). From center to periphery of the Zernike tree, the ICC values decreased from 0.966 to 0.667 for 3rd-order terms and from 0.981 to 0.162 for 4th-order terms.

Table 2
Table 2:
Intraobserver repeatability for total corneal wavefront measurements (6.0 mm pupil).

Corneal Pachymetry and Anterior Chamber Depth

The mean corneal thickness increased from the center to the periphery. Within-subject SD values were less than 3 μm, repeatability values were less than 8 μm, the maximum COV value was 0.37%, and all ICC values were 0.999 (Table 3).

Table 3
Table 3:
Intraobserver repeatability for corneal pachymetry and anterior chamber depth.

For ACD, the within-subject SD was 0.04 mm, repeatability was 0.12 mm, COV was 0.69% and the ICC was 0.996 (Table 3).

DISCUSSION

The present study was designed to evaluate the repeatability of measurements of total corneal power and posterior corneal power, corneal wavefront aberrations, corneal pachymetry, and ACD measured with the Galilei dual-Scheimpflug Placido corneal topographer. In a previous study,8 we found that the keratometry measurements with the dual-Scheimpflug system, a corneal topographer, a PCI biometer, and a manual keratometer were highly reproducible, comparable, and correlated. Menassa et al.9 report that keratometry and central corneal pachymetry readings with the dual-Scheimpflug analyzer and a scanning-slit topographer showed good concordance and high reproducibility.

In the present study, the dual-Scheimpflug system had excellent reproducibility in measuring total corneal power at the central, paracentral, and peripheral zones and meridional values of posterior corneal power. For corneal pachymetry, the within-subject SD was less than 3.0 μm at the central, paracentral, and peripheral regions, similar to the central corneal pachymetry readings reported by Menassa et al.9 The repeatability of ACD measurements also showed high reproducibility.

Table 4 shows the coefficient of repeatability (COR) of corneal power, corneal thickness, and wavefront aberrations measurements using the Pentacam single-Scheimpflug system1–6; the COR was 1.96 times the SD of differences between 2 repeated measurements. For comparison, we arbitrarily selected the first and the second measurements in our study to calculate the COR. This showed that the dual-Scheimpflug system had a slightly smaller COR than the single-Scheimpflug system for corneal power measurements. For corneal pachymetric measurements using the single-Scheimpflug system, Shankar et al.5 found that CCT showed good repeatability, whereas the peripheral pachymetry repeatability was poor. We found the dual-Scheimpflug system had excellent repeatability for both central and paracentral corneal pachymetry (Table 4). This is perhaps due to the dual-channel Scheimpflug cameras implemented by the system. A rotating Scheimpflug camera observes the apparent slit image or thickness of the cornea. 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 2 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.7 Lewis et al. compared the response to misalignment in pachymetry measurement between single- and dual-Scheimpflug devices and found that the dual-Scheimpflug device was relatively insensitive to misalignment (J.R. Lewis, MD, 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).

Table 4
Table 4:
Coefficient of repeatability reported in previous studies of the single-Scheimpflug system and in the current study of the dual-Scheimpflug system.

For the total corneal wavefront measurements, we found that the repeatability tended to be better for Zernike terms at the center of the Zernike tree than for terms along the periphery of the Zernike pyramid (Figure 1). The repeatability for measurement of the spherical aberration was excellent (0.05 μm), indicating that for 95% of pairs of observation, the difference in spherical aberration between 2 measurements for the same subject is expected to be less than 0.05 μm. Compared with the COR in measuring corneal first-surface wavefront aberrations using the single-Scheimpflug system reported by Shanker et al.,6 the dual-Scheimpflug system had approximately one third less variability (Table 4). In a study of posterior corneal wavefront aberrations measured with the single-Scheimpflug system, Piñero et al.12 reported SD and ICC values similar to our values with the dual-Scheimpflug system.

Figure 1
Figure 1:
Within-subject SDs for 3rd-order Zernike terms and 4th-order Zernike terms (SD = standard deviation).

Various factors may add noise to the repeated measurements of wavefront aberrations. Variation in pupil center location in repeated measurements contributes to the variance in repeated wavefront measurements. Applegate et al. examined the impact of uncertainty of pupil center location on the variance of Hartmann-Shack wavefront sensing measurements of higher-order aberrations (HOAs) (R.A. Applegate, MD, et al., “Pupil Center Location Uncertainty Is a Major Source of Instrument Noise in WFE Measurements,” presented at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, USA, May 2009). Assuming that the pupil center of a 6.0 mm diameter pupil could vary randomly by up to 200 μm in any direction, 1000 new pupil centers were sampled for each eye. The authors found that in less aberrated eyes, depending on Zernike terms, up to 40% of variance in repeated measures was accounted for by pupil location variation. In eyes with higher aberrations, up to 80% of the variance in repeated measures was caused by the pupil center jitter. Studies13–15 have also shown that fluctuations in the tear film can cause increased irregular astigmatism and deteriorated optical quality. It has been reported that dynamic changes in HOAs after blinking showed variations, even in clinically normal subjects.16

Limitations of this study include that the repeatability of the dual-Scheimpflug system was evaluated in normal corneas only, interobserver and intersession repeatability of the system was not studied, and the repeatability of the system in measuring corneal wavefront aberrations was not compared with that of other topographers.

In conclusion, we found the dual-Scheimpflug system had excellent repeatability in measuring corneal power, corneal pachymetry, and ACD. For corneal wavefront aberrations, acceptable precision and repeatability were found for the Zernike terms at the center of the Zernike tree (coma and spherical aberration), with greater variability for terms along the peripheral of the Zernike pyramid (trefoil and tetrafoil). When evaluating the corneal wavefront aberrations, serial measurements may be useful.

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