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Session Title: Imaging II
Session Date/Time: Monday 07/10/2013 | 16:30-18:00
Paper Time: 17:26
Venue: Main Lecture Hall (Ground Floor)
First Author: : G.Savini ITALY
Co Author(s): : K. Naeser
To evaluate the influence of posterior corneal astigmatism on total corneal astigmatism in patients with 1 diopter (D) of corneal astigmatism as measured by simulated keratometry (SimK).
All patients underwent corneal astigmatism measurement by means of a rotating Scheimpflug camera combined with a Placido disc based corneal topographer (Sirius, CSO). The following categories of corneal astigmatism were evaluated: CASimK (the difference between the power of the steepest and flattest corneal meridians on the basis of SimK, using the keratometric index =1.3375), CAPost (the difference between the posterior corneal surface power of the steepest and flattest meridians inside a 3mm circular zone centered on the vertex), and CART (the total corneal astigmatism measured through ray-tracing, inside a 3mm circular zone centered on the vertex).
Among 157 eyes, simulated keratometry showed with-the-rule (WTR) astigmatism in 132 eyes (84.0%), against-the-rule (ATR) astigmatism in 18 eyes (11.5%) and oblique astimatism in 7 eyes (4.5%). Posterior corneal astigmatism exceeded 0.50 D and 1.00 D in 55.4% and 5.7% of cases, respectively. The steepest meridian was vertically aligned in 93% of cases. In eyes with WTR astigmatism, the steepest posterior meridian was always vertical and a significant correlation was found between the magnitude (p<0.0001, r=0.7969, fig. 2) and axis (p<0.0001, r=0.8453) of anterior and posterior cylinder. In eyes with ATR astigmatism, the steepest meridian on the posterior corneal surface was vertical in 55.6% of cases, and no correlation was found between the magnitude and axis of anterior and posterior cylinder. In comparison to CART, on average CASimK overestimated WTR astigmatism by 0.23@92 D and underestimated ATR astigmatism by 0.27@172 D. However, in about 20% of eyes CASimK underestimated WTR astigmatism and overestimated ATR astigmatism. In the whole sample, a difference in astigmatism magnitude ?0.5 D was detected in 26.8% of cases and the difference in the location of the steep meridian between CASimK and CART was higher than 10° in 3.8% of cases.
In patients candidates to surgical correction of astigmatism, measuring only the curvature of the anterior corneal surface and neglecting that of the posterior corneal surface can lead to inaccurate evaluation of total corneal astigmatism.
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