Impact of posterior corneal astigmatism on deviation in predicted residual astigmatism for toric IOL calculations in keratoconic eyes

PURPOSE: The objective of the study was to evaluate the error in predicted residual astigmatism (PRA) using measurements of corneal astigmatism obtained with IOLMaster-700 and Pentacam for toric intraocular lens (IOL) calculation in keratoconic eyes.
METHODS: For toric IOL calculations, we used keratometric astigmatism obtained by IOLMaster-700 and total corneal re-fractive power (TCRP) values determined by Pentacam Scheimpflug system. Using an online toric IOL calculator, PRA for keratometric astigmatism and TCRP with a toric IOL model suggested for keratometric astigmatism values was recorded. We also calculated the error in PRA as the difference between PRA with keratometric astigmatism and TCRP. For all calculations, vector analysis was used.
RESULTS: In our sample of 70 keratoconic eyes of 70 patients, the mean difference in PRA using TCRP instead of keratometric astigmatism measurements was −1.21±0.93 with a centroid of 0.85 at 25. The error in PRA was ≤1.0D in 36 eyes, between 1.0D and 3.0D in 26 eyes, and between 3.0D and 4.0D in eight eyes. Whereas 80% of eyes with with-the-rule astigmatism showed decreased cylindrical IOL power, 88.9% of eyes with against-the-rule astigmatism showed increased IOL power with TCRP instead of keratometric astigmatism.
CONCLUSION: Using TCRP measurements instead of keratometric astigmatism in toric IOL calculations caused a considerable deviation in eyes with keratoconus, most probably due to the posterior corneal astigmatism.