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J. R. Lewis, A. M. Mahmoud, R. F. Beran, C. J. Roberts; Comparison Between Single- and Double-Scheimpflug Keratometry and Pachymetry and the Effect of Misalignment. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1021. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To compare single- and dual-Scheimpflug devices in measuring keratometry, pachymetry and the effect of a 1-mm horizontal misalignment.
Eight eyes of four subjects, including three normal and one post-radial keratotomy, were imaged twice by single-Scheimpflug (Pentacam, Oculus) and dual-Scheimpflug (Galilei, Ziemer Group) devices. Anterior keratometry (BFS, SimK1, SimK2) and thinnest pachymetry was recorded. Measurements were repeated with a 1-mm horizontal misalignment using manual-mode acquisition. The direction and magnitude of misalignment was equal for both eyes. Placido keratometry (Keratron Scout, Optikon) was performed in duplicate on each eye to reference the aligned SimKs. Two-tailed T-Tests (SAS, p<.05) were performed to compare the means and variances between the devices for both aligned and misaligned conditions.
Comparing aligned and misaligned keratometry, both Pentacam and Galilei BFS values are not different (p>.05). The Keratron K1 is not statistically different to the aligned Pentacam (p=.92) or Galilei (p=.09). The Keratron K2 is statistically different to the Galilei (p=.006) and not the Pentacam (p=.15), however the difference is clinically insignificant (-0.22 ± 0.15 D). Across all eyes the thinnest pachymetry is different between the aligned-misaligned Galilei (p=.04) and not the Pentacam (p=.19), however the Galilei magnitude and variability (1.66 ± 1.92 um) are very small and within clinically acceptable levels versus the Pentacam (6.38 ± 12.28 µm). The Galilei aligned-misaligned OD pachymetry is not statistically different (p=.51), however the Galilei aligned-misaligned OS pachymetry is statistically different (p=.009) but at a clinically insignificant level with low variability (2.3 ± 0.76 µm). The Pentacam aligned-misaligned OD pachymetry is not significantly different (p=.29) and has a predominantly negative error (-3.9 ± 6.1 µm), whereas the OS pachymetry is significantly different (p=.01) and has a predominantly positive error (16.6 ± 5.9 µm).
Both single- and dual-Scheimpflug devices have comparable keratometry relative to the Keratron Scout except for a clinically insignificant difference in the mean Galilei K2. The Galilei has low pachymetry error and variability under conditions of misalignment. The Pentacam pachymetry error and variability is relatively larger under conditions of misalignment and the error changes sign between the left and right eyes.
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