Purpose To evaluate the accuracy of toric intraocular lens (IOL) calculation using estimated total corneal astigmatism based on the anterior-to-posterior corneal cylinder power ratio according to the axis orientation of anterior corneal astigmatism. Design Retrospective cross-sectional study. Methods Nine hundred twenty-eight eyes of 928 reference subjects and 20 cataract patients (20 eyes) implanted with a toric IOL were enrolled. Linear regression analysis parameters (β0 and β1) of relationship between the simulated keratometry cylinder (CylSimK) and posterior corneal cylinder power of reference subjects were used to calculate the estimated posterior corneal astigmatism (-[β1 × CylSimK + β0] @ 90). When regression analysis was not significant, estimated posterior corneal astigmatism was defined as the negative value of the mean posterior corneal cylinder power @ 90. Estimated total corneal astigmatism was defined as the vectorial sum of anterior corneal astigmatism and estimated posterior corneal astigmatism. Residual astigmatism error, predicted using SimK, was compared with that predicted using estimated total corneal astigmatism. Results Estimated posterior corneal astigmatism was determined to be -(0.15 × CylSimK + 0.22) @ 90 in eyes with with-the-rule astigmatism, -(0.05 × CylSimK + 0.27) @ 90 in oblique astigmatism, and -0.27 @ 90 in against-the-rule astigmatism. The median magnitude of the predicted residual astigmatism error calculated using estimated total corneal astigmatism (0.30 cylinder diopters) was significantly smaller than that calculated with SimK (0.50 cylinder diopters). Conclusions Toric IOL calculations using estimated total corneal astigmatism based on the anterior-to-posterior corneal cylinder power ratio provided a more appropriate toric IOL cylinder power than calculations using SimK astigmatism.
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