The purpose of this study was to assess the repeatability and comparability of the dual Scheimpflug-Placido topographer (Galilei G4 Dual) versus the light-emitting diode topographer (Cassini) in normal and post-refractive surgery eyes including subjects with a typical age of cataract patients. The mean age of our cohort was 55 ± 18.5 years. Both the Galilei G4 Dual and Cassini showed high intra-observer repeatability for all corneal power measurements in normal and post-refractive eyes. With regards to corneal astigmatism, both the Galilei G4 Dual and Cassini showed high intra-observer repeatability in normal and post-refractive eyes for simulated keratometric (simK) and total corneal astigmatism. Posterior corneal astigmatism, in normal and post-refractive eyes, however, showed low intra-observer repeatability using both the Galilei G4 Dual and Cassini.
Wang et al found that the dual Scheimpflug analyzer has high repeatability in measurements of corneal power, anterior chamber depth (ACD), corneal aberrations, and pachymetry. [18]. Cervino found moderate repeatability for wavefront aberrations and astigmatism and high repeatability for corneal power and central pachymetry [19]. Kim et al furthermore assessed the repeatability of the Scheimpflug analyzer in simK, total and posterior corneal curvature measurements in both normal and post-refractive surgery eyes and found high repeatability in all measurements, except for posterior corneal astigmatism which was moderately repeatable [10]. Lu et al. found high repeatability in both normal and post-refractive eyes for Cassini and Sirius (Placido-Scheimpflug) systems for simK, total corneal power and astigmatism recordings except for low repeatability in total corneal astigmatism in post-refractive eyes [20]. In comparing the ICC between Cassini and Galilei, Lee et al found high repeatability for simK power measurements in normal (0.991) and post-refractive surgery eyes (0.986), and poor repeatability in posterior corneal astigmatism measurements in both analyzers [11].
As is consistent with previous results, in our study, the Galilei G4 Dual and Cassini showed high reproducibility in all corneal power measurements (simK, total, and posterior) in both normal and post-refractive eyes [10, 11, 18-23]. For instance, Savini et al. found the ICCs of simK, total, and posterior corneal power to be 0.999, 0.999, and 0.994, respectively, in a group of both normal and post-refractive eyes. Kim et al recorded similar measurements in normal (0.998, 0.997, 0.992) and post-refractive surgery (0.998, 0.998, 0.990) eyes. This is similar to our values seen using the Galilei in normal (0.998, 0.996, 0.981) and post-refractive surgery (0.997, 0.997, 0.999) eyes and using the Cassini in normal (0.988, 0.989, 0.936) and post-refractive surgery (0.982, 0.988, 0.966) eyes.
In addition, in both normal and post-refractive eyes, repeatability was high for simK and total corneal astigmatism measurements but was low for posterior corneal astigmatism measurements with the Galilei and Cassini. The ICC for posterior corneal astigmatism magnitude measurements in Galilei were 0.616 in normal eyes and 0.669 in post-refractive eyes and with the Cassini 0.554 in normal eyes and 0.380 in post-refractive eyes. This was comparable to previously reported posterior corneal astigmatism magnitudes for both normal eyes 0.499 and post-refractive 0.183 under dual rotating Scheimpflug–Placido and color-LED corneal topography in healthy volunteers with median age of 32 ± 7 years and 32 ± 8 years respectively for normal and post-refractive groups. [11].
These results are in agreement with previous studies that noted either low or moderate repeatability of posterior astigmatism measurements using both the Scheimpflug and Cassini analyzer [10, 11, 19, 20, 24]. The low repeatability may be explained by the low magnitude of posterior corneal astigmatism (Galilei mean -0.31 ± 0.12 D and -0.31 ± 0.16 D for normal and post-refractive eyes, respectively; Cassini mean -0.34 ± 0.17 D and -0.34 ± 0.17 D for normal and post-refractive eyes, respectively) as described by Kobashi [25]. In addition, the 95% LOA for the Cassini vs Galilei posterior corneal astigmatism were between -0.38 to 0.3 in normal eyes and -0.44 to 0.32 in post-refractive eyes. Since the upper limit of the 95% LOA for the Cassini vs Galilei was -0.38 and -0.44 in normal and post-refractive eyes, respectively; there is only a 5 % probability that both analyzers would overestimate the posterior corneal astigmatism measurement by more than -0.38 and -0.44 D, respectively. Subtle differences in steep and flat meridian locations are difficult to identify in eyes with low astigmatism as compared to eyes with high astigmatism, contributing to the low repeatability.
For the Galilei the posterior keratometric parameter is derived in the 0.5 to 2.0mm zone where as for the Cassini posterior corneal measurement is calculated analyzing reflections of 7 white LEDs in approximately the 3.0 mm annular region. This difference may account for part of the statistically significant difference (p<0.01) comparing the mean measurements between the Cassini and Galilei G4 devices for total corneal power in normal eyes and for posterior corneal power in post-refractive eyes. As evidenced in the results of this study, the differences in both devices with respect to anterior astigmatism values in normal and post-refractive eyes were not significant, indicating comparable results between devices. However, with respect to total corneal power in normal eyes and posterior corneal power in post-refractive eyes, the machines were not interchangeable.
Most studies evaluating corneal topography repeatability enrolled only young, healthy patients, primarily between 18-40 years of age [11, 18, 19, 20, 22, 24, 26]. Our study enrolled patients with a mean age of 55 ± 18.5 years, and an age range of 21.5 and 91.5 years. The high repeatability of both the Galilei G4 Dual and Cassini analyzer measurements were comparable to prior studies of young volunteers, re-confirming good performance of both analyzers across a broad clinical setting.
Limitations of our study include: (1) We did not evaluate inter-observer and intersession repeatability, (2) patients with other corneal diseases were excluded and (3) We did not evaluate the repeatability of pachymetry or corneal wavefront aberrations.
In summary, our results indicate that the Galilei G4 Dual and Cassini analyzer both have high repeatability in recording simK, total, and posterior corneal power but low repeatability in posterior corneal astigmatism measurements in patients with and without a history of refractive surgery in the older cataract population.