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

Measurement of anterior corneal astigmatism and posterior corneal astigmatism and knowledge of IOL astigmatism does not account for total eye astigmatism.  The purpose was to quantify the astigmatism not accounted for and assess if it is correlated with some biometric variables

The Queen Elizabeth Hospital, Adelaide and the University of Adelaide

Vector subtraction of anterior corneal, posterior corneal and IOL astigmatism from total ocular astigmatism as represented by spectacle astigmatism to yield a value of “leftover” astigmatism which 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 some biometric variables.

In 103 pseudophakic eyes with known IOL toricity, mean leftover astigmatism was 0.71D (SD 0.43).  This is significantly correlated with against-the-rule anterior corneal astigmatism (p,0.001).

In toric IOL cylinder power calculation, population-based adjustments to anterior corneal astigmatism previously described as adjustments for “posterior corneal astigmatism” are based on observed outcome, e.g. the Baylor Nomogram, the Goggin Nomogram, and are successful. They are more successful than adjustment on the basis of measured posterior corneal astigmatism. This is because they are correcting for posterior corneal astigmatism in combination with this leftover astigmatism.  These kinds of adjustments may have to be renamed since they correct for something in addition to posterior corneal astigmatism.