Comparison of Simulated and True Keratometry Measurements Using Swept-Source Optical Coherence Tomography and Dual Scheimpflug–Placido Imaging

Abstract

Purpose. To compare simulated and total keratometry and corneal astigmatism values between the IOLMaster 700 and Galilei G4 devices. Methods. A retrospective chart review was conducted for all patients undergoing phacoemulsification by a single surgeon (RTP) from March through September 2020 and who underwent imaging with both the IOLMaster 700 and Galilei G4. Exclusion criteria were prior corneal surgery, keratectatic diseases and inability to obtain a reliable image during image acquisition. Mean, flat, and steep keratometry values as well as astigmatism magnitude were compared. Results. A total of 200 eyes of 100 patients were included. Intraclass correlation coefficients (ICC) were moderate or high for all variables. Mean difference ± SD in SimK and TrueK between devices (G4-IOLM) was 0.05 ± 0.318 diopters and −1.1156 ± 0.438 diopters, respectively ( p < 0.05 for both). The IOLM measured steeper TrueK value than the G4. For SimK, there was a statistically significant difference between devices only for mean keratometry (K), whereas for TrueK, there were significant differences in flat K, steep K, and mean K. Astigmatism analysis revealed a difference in mean (±SD) SimK of 0.07 (±0.57) D at 94 degrees and in mean TrueK of 0.04 (±0.85) D at 108 degrees. Conclusion. Though there is overall good correlation between the IOLMaster 700 and Galilei G4 in SimK and astigmatism measurements, there is a significant difference in TrueK measurements, with the IOLM measuring steeper values by about 1.0 diopter as compared to the G4.

Comparison of keratometry using SS-OCT and dual Scheimpflug-Placido imaging in postmyopic corneal refractive surgery eyes

“…These results are consistent with prior studies on normal eyes that showed highly correlated and interchangeable SimK values between the IOLM and G4 but with a consistently steeper True-K value by approximately 1.0 D on the IOLM vs the G4. 3,5 Lu et al demonstrated that posterior K values (mean, flat, and steep) were moderately correlated and not interchangeable between devices, with the IOLM measuring flatter posterior corneal curvatures, thus resulting in steeper True-K values with the IOLM. 5…”

“…These results are consistent with prior studies on normal eyes that showed highly correlated and interchangeable SimK values between the IOLM and G4 but with a consistently steeper True-K value by approximately 1.0 D on the IOLM vs the G4. 3,5 Lu et al demonstrated that posterior K values (mean, flat, and steep) were moderately correlated and not interchangeable between devices, with the IOLM measuring flatter posterior corneal curvatures, thus resulting in steeper True-K values with the IOLM. 5 Various reasons have been attributed to explain the difference in posterior corneal measurements and therefore true corneal power between SS-OCT and Scheimpflug corneal tomographers, such as the different hardware and technologies of the devices, as well as the software including different densities and locations of posterior corneal points measured between the devices.…”

“…4 When comparing SimK and True-K between IOLM and G4 devices in normal corneas, there is a good overall correlation in SimK and astigmatic measurements. 3,5 IOLM has been shown to measure steeper True-K values compared with G4 in normal corneas. 3,5 To the authors' knowledge, no prior study has compared keratometry between IOLM and G4 in postmyopic refractive corneas.…”

“…3 Using ray tracing, total corneal power (TCP IOL ) is obtained. 3 TCP IOL represents the overall power of the cornea, including both the central and peripheral regions of the cornea, using the actual indices of refraction for air (1.000), cornea (1.376), and aqueous (1.336).…”

“…2 The Galilei G4 Dual Scheimpflug Analyzer (G4; Ziemer Ophthalmic Systems AG) not only measures the anterior corneal curvature using Placido ring images to determine SimK but also measures anterior and posterior corneal curvatures and corneal pachymetry with dual Scheimpflug imaging using infrared light at a wavelength of 810 nm. 3 Using ray tracing, total corneal power (TCP IOL ) is obtained. 3 TCP IOL represents the overall power of the cornea, including both the central and peripheral regions of the cornea, using the actual indices of refraction for air (1.000), cornea (1.376), and aqueous (1.336).…”