To directly measure internal astigmatism and evaluate its agreement with posterior corneal astigmatism in pseudophakic eyes.

This prospective study enrolled 32 eyes that underwent phacoemulsification with implantation of a non-toric monofocal intraocular lens (IOL). Two months postoperatively, posterior corneal astigmatism was measured using a Pentacam Scheimpflug analyzer. Manifest refractive astigmatism was measured after fitting a spherical hard contact lens. This refractive astigmatism that was vertexed to the corneal plane was considered internal astigmatism (the element of total ocular astigmatism that is not caused by the anterior corneal surface). The magnitudes of internal astigmatism and posterior corneal astigmatism were compared using the Wilcoxon signed-rank test. The relationship and agreement between these two astigmatisms were investigated using the Spearman correlation coefficient and Bland-Altman plots.

The mean patient age was 56.3±9.6 years. IOL decentration or tilt, or posterior segment abnormalities were not encountered in any cases postoperatively. The mean refractive astigmatism was -0.810.56 D. Internal astigmatism (-0.17±0.21 D) was significantly different from posterior corneal astigmatism (-0.30±0.15 D, P=0.046). Regression analysis demonstrated a significant association between internal astigmatism and posterior corneal astigmatism (r2=0.22, P=0.013). Bland-Altman plots produced 95% limits of agreement for these two astigmatisms from -0.49 to 0.75 D.

Posterior corneal astigmatism explained only 22% of the internal astigmatism in the pseudophakic eyes. This result indicates that the precision of the astigmatism correction after toric intraocular lens implantation may not be optimal, even if the magnitude of posterior corneal astigmatism is incorporated into the toric intraocular lens power calculation.