Leftover Astigmatism: The Missing Link Between Measured and Calculated Posterior Corneal Astigmatism

Abstract

PURPOSE: To quantify the total eye astigmatism that is not accounted for by measurement of anterior corneal astigmatism and posterior corneal astigmatism and knowledge of intraocular lens (IOL) astigmatism and assess whether it is correlated with candidate sources of or correlates with leftover astigmatism. METHODS: Vector subtraction of anterior corneal, posterior corneal, and IOL astigmatism from total eye astigmatism as represented by spectacle astigmatism to yield a value of “leftover” astigmatism that is neither corneal nor lenticular. This value was derived in a series of eyes following cataract surgery. This novel entity was examined for correlation with candidate sources of or correlates with leftover astigmatism. RESULTS: In 103 pseudophakic eyes with known IOL toricity, mean leftover astigmatism was 0.71 ± 0.43 diopters. This was significantly correlated with against-the-rule anterior corneal astigmatism ( P < .001). CONCLUSIONS: Leftover astigmatism is clinically substantial. Because it is included in IOL cylinder power calculations based on refractive outcome, it may explain why methods of IOL cylinder power calculation using refractive outcome–based adjustments to anterior corneal astigmatism (previously described as adjustments for “posterior corneal astigmatism”) are more successful than adjustment on the basis of measured posterior corneal astigmatism. [ J Refract Surg. 2022;38(9):559–564.]