Adjustment of Anterior Corneal Astigmatism Values to Incorporate the Likely Effect of Posterior Corneal Curvature for Toric Intraocular Lens Calculation

PURPOSE: To establish if average refractive overcorrection or undercorrection of corneal astigmatism based on the "rule" of the astigmatism occurs if toric intraocular lenses (IOLs) are calculated on the basis of anterior corneal measurements, and to calculate an adjustment for individual eyes to avoid this systematic error. METHODS: One hundred forty-three consecutive eyes of 115 patients underwent phacoemulsification with IOL powers calculated using anterior corneal curvature data alone. Eyes were grouped as either "with-the-rule" or "against-the-rule" on the basis of the steep anterior corneal meridian. Targeted versus achieved astigmatic outcomes were compared. Main outcome measure was residual refractive astigmatism following the insertion of a toric IOL due to the likely effect of posterior corneal astigmatism. RESULTS: Significant prediction errors in astigmatic outcome occurred only with IOL cylinders of 2 diopters or less. Overcorrection occurred by a factor of 1.38 in with-the-rule eyes and undercorrection occurred by a factor of 0.65 in against-the-rule eyes. CONCLUSIONS: A coefficient of adjustment of 0.75 for with-the-rule eyes and 1.41 for against-the-rule eyes can be applied to the corneal astigmatism power value to calculate a more appropriate IOL cylinder power than that calculated by using unadjusted anterior corneal curvature measurements. These adjustment coefficients apply only to those eyes that would have received IOLs with 2 diopters of cylinder or less and calculated with such unadjusted measurements. Greater IOL cylinder powers are sufficiently accurately calculated using unadjusted values.