Posterior Corneal Astigmatism in Toric IOL Power Calculation: The New Kid on the Block

Sumit (Sam) Garg, MD; Elizabeth Shen, MD


July 20, 2018

Correcting for Posterior Corneal Astigmatism

With refractive outcomes for cataract surgery becoming increasingly precise, expectations for astigmatism correction are rising in equal measure. Although toric intraocular lens (IOL) implants have revolutionized the correction of astigmatism, toric IOL calculation is still evolving.

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.[1] Thus, before any accounting was done for PCA, patients with WTR astigmatism were often overcorrected and those with ATR astigmatism were undercorrected.

A New Online Algorithm

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. The authors tested the accuracy of the formula retrospectively on 274 eyes implanted with a toric IOL by comparing results from the traditional Tecnis toric calculator with the PCA algorithm-incorporated toric calculator. The main outcome was the error in prediction—the difference between the predicted refractive astigmatism and the actual refractive astigmatism measured 6 months after surgery.

The authors found that the PCA algorithm significantly reduced the centroid error in predicted refractive astigmatism. Patients with WTR, ATR, or oblique anterior corneal astigmatism all had more accurate predicted calculations for astigmatism with the PCA algorithm.

Overall, the centroid error was reduced from 0.50 @ 1 (without PCA algorithm) to 0.19 @ 3 (with PCA algorithm) when preoperative keratometry (K) values were used. For ATR astigmatism (n = 126), the centroid error was 0.22 @ 3 when preoperative K values were used; for WTR (n = 118), the error was 0.23 @ 0.

Furthermore, lower values for absolute error were achieved in a greater proportion of patients for whom the PCA algorithm was used. For example, 93% of patients were within 1.00 diopters of absolute error when the PCA algorithm was used, compared with 82% of patients when the algorithm was not used. One limitation of the study was the small sample size for oblique astigmatism (n = 30); an acceptably low error was observed but was indeterminate overall.

The authors concluded that the addition of this PCA correction factor increases the predictability of astigmatism correction with toric IOLs regardless of the orientation of anterior corneal astigmatism. However, predictability depends on other factors as well, such as the accuracy of one's surgically induced astigmatism and the effective lens position.

The curvature of the posterior cornea has always been a challenge for refractive outcomes in cataract surgery. Previously, this was an unknown. However, now that we understand the importance of PCA, we can strive for improved toric IOL outcomes. This new formula may be the next stride toward improving outcomes in refractive cataract surgery.


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