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

PURPOSE: To quantify the total eye astigmatism that is not accounted for by measurement of anterior corneal astigma- tism and posterior corneal astigmatism and knowledge of intraocular lens (IOL) astigmatism and assess whether it is correlated with candidate sources of or correlates with left- over 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 cor- neal astigmatism (P < .001).CONCLUSIONS: Leftover astigmatism is clinically substan- tial. 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 (previ- ously described as adjustments for "posterior corneal astig- matism") are more successful than adjustment on the basis of measured posterior corneal astigmatism.