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Stijn Klijn, Nicolaas J. Reus, Charlotte M. van der Sommen, Victor Arni D.P. Sicam; Accuracy of a novel specular reflection technique for measurement of total corneal astigmatism. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1903.
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© ARVO (1962-2015); The Authors (2016-present)
Measurements of corneal astigmatism have traditionally been based on information of only the anterior corneal surface. Accurate planning of toric intraocular lens (IOL) implantation requires knowledge on total corneal astigmatism, which includes the posterior corneal surface. We prospectively evaluated the accuracy of total corneal astigmatism measurements with a novel specular reflection technique.
Refractive astigmatism was measured with autorefraction (ARK-530A, Nidek) in 24 eyes of 24 patients with a monofocal, non-toric IOL (Acrysof SA60AT, Alcon). Alignment errors of the IOL were assumed to be negligible; thus, refractive astigmatism was assumed to consist entirely of anterior and posterior corneal astigmatism. Anterior and posterior corneal curvatures were measured with a specular reflection technique (Cassini, i-Optics). Subsequently, corneal astigmatism was calculated based on only the anterior curvatures as well as based on both anterior and posterior curvatures. Measurement error of either approach was defined as the absolute difference with autorefraction. Reduction of measurement error if the calculation included posterior corneal curvatures was tested for statistical significance using the paired-samples T test.
The mean measurement error of cylinder magnitude was 0.29 ± 0.21 D and 0.27 ± 0.20 D if the measurement excluded and included posterior curvatures, respectively. The mean measurement error of cylinder axis was 26.6 ± 38.7 degrees and 22.3 ± 38.5 degrees if the measurement excluded and included posterior curvatures, respectively. The reduction in measurement error of cylinder magnitude was not statistically significant (P=0.60). However, the reduction in measurement error of cylinder axis was statistically significant (P<0.01).
We presented a novel specular reflection technique that enables measurement of total corneal astigmatism. Compared to measurements of only anterior corneal curvatures, the measurement error of cylinder axis was statistically significantly reduced by 4.3 degrees if posterior curvatures were included. This may be particularly advantageous in the planning of toric IOL implantation.
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