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Posterior corneal curvature has little effect on some toric IOL powers
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Anterior keratometric curvature data lead to some amount of overcorrection or undercorrection with toric IOLs if information on posterior corneal astigmatism is not incorporated. However, the effect of posterior corneal astigmatism is not clinically significant when the IOL cylinder power is greater than 2.5 D, according to a study.
Retrospective data were collected for 113 eyes of 101 patients who underwent lens surgery with toric IOL implantation. The required cylinder power was 2.5 D or greater, and eyes with both with-the-rule (WTR) and against-the-rule (ATR) astigmatism were included.
Cylinder power calculated on the basis of anterior corneal astigmatism alone resulted in overcorrection and undercorrection in eyes with WTR and ATR astigmatism, respectively, leading to a mean prediction error of +0.16 ± 0.57 for WTR eyes and –0.14 ± 0.53 for ATR eyes.
Despite being statistically significant, this amount of error does not have clinically significant effects on vision and is therefore not worth addressing, according to the authors.
“These values are small in clinical terms, and any adjustment based on these values to attempt to decrease the overcorrection and undercorrection would be small,” they noted.
This study indicated that the posterior corneal astigmatism effect on corneal astigmatism is proportionally lower with increases in anterior corneal astigmatism. While eyes that require a toric IOL with cylinder power of 2 D or less may benefit from adjustment of anterior keratometry values via online calculators, no adjustment was recommended by the authors with higher power IOLs.
“Standard practice is to adjust all eyes regardless of the magnitude of their anterior corneal astigmatism. However, we have shown that IOL cylinder powers are sufficiently accurately calculated using unadjusted anterior keratometry values in eyes with relatively high corneal astigmatism,” Benjamin R. LaHood, MBChB, PGDipOphth, first author of the study, told Healio.com/OSN. “We have been using unadjusted anterior keratometry values in eyes requiring toric IOLs of cylinder power 2.5 D or greater for some time now with excellent visual results. It was great to see our biologically plausible hypothesis proved correct. Until we are able to measure posterior corneal astigmatism accurately, we would encourage all surgeons to use unadjusted keratometry in planning cataract surgery when eyes require an IOL cylinder value of 2.5 D or greater.” – by Michela Cimberle
Disclosures: The authors report no relevant financial disclosures.
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