Experience with toric IOLs has shown that posterior corneal astigmatism (PCA), if ignored, can throw off the predicted astigmatism correction. Patients with with-the-rule (WTR) astigmatism tend to be affected more by PCA, whereas patients with against-the-rule (ATR) astigmatism have less PCA. That can be explained by the observation that the refractive effect of PCA is against-the-rule in most eyes.

To address this problem, the Baylor nomogram was developed from population-based data to account for the PCA effect.[2] This nomogram recommends raising the threshold for considering a toric IOL for WTR astigmatism and lowering the threshold for ATR astigmatism.Recently, Canovas and colleagues[3]reported on a new algorithm that incorporates PCA into the Tecnis toric IOL calculator.

Patients with WTR, ATR, or oblique anterior corneal astigmatism all had more accurate predicted calculations for astigmatism with the PCA algorithm.

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