Influence of posterior corneal astigmatism on total corneal astigmatism in eyes with moderate to high astigmatism.
To evaluate the influence of posterior corneal astigmatism on total corneal astigmatism in patients with 1.00 diopter (D) or more of corneal astigmatism. Private practice, Bologna, Italy. Prospective case series. Corneal astigmatism was measured using a Scheimpflug camera combined with a corneal topographer (Sirius). Keratometric astigmatism, anterior corneal astigmatism, posterior corneal astigmatism, and total corneal astigmatism were evaluated. Vector analysis was performed according to the Næser method. One hundred fifty-seven eyes were enrolled. Keratometric astigmatism was with the rule (WTR), against the rule (ATR), and oblique in 84.0%, 11.5%, and 4.5% of eyes, respectively. Posterior corneal astigmatism exceeded 0.50 D and 1.00 D in 55.4% of eyes and 5.7% of eyes, respectively. The mean posterior corneal astigmatism was 0.54 D, inclined 91 degrees in relation to the steeper anterior corneal meridian. The steepest meridian was vertically aligned in 93.0% of cases. Compared with total corneal astigmatism, keratometric astigmatism overestimated WTR astigmatism by a mean of 0.22 D ± 0.32 (SD), underestimated ATR astigmatism by 0.21 ± 0.26 D, and overestimated oblique astigmatism by 0.13 ± 0.37 D. In the whole sample, a difference in astigmatism magnitude of 0.50 D or more was detected between keratometric astigmatism and total corneal astigmatism in 16.6% of cases and the difference in the location of the steep meridian was greater than 10 degrees in 3.8% of cases. In patients who are candidates for surgical correction of astigmatism, measuring only the anterior corneal curvature can lead to inaccurate evaluation of the total corneal astigmatism. Mr. Versaci and Mr. Vestri are employees of Costruzione Strumenti Oftalmici Srl. No author has a financial or proprietary interest in any material or method mentioned.