Trapezoidal astigmatic keratotomy. Quantification in human cadaver eyes.

The mechanism by which surgical procedures correct astigmatism are incompletely understood and, therefore, the results are often unpredictable. In this study, a Terry keratometer was used to analyze the effect of each component part of a 3-, 4-, and 5-mm trapezoidal astigmatic keratotomy. Semiradial incisions along the horizontal meridian produced corneal flattening along the horizontal meridian twice that of the vertical meridian, therefore inducing positive cylinder astigmatism at 90 degrees. A single pair of tangential incisions placed 5 mm apart, when added to the semiradial incisions, accounted for 100%, 87%, and 78% of the maximal effect from the 3-, 4-, and 5-mm completed trapezoidal astigmatic keratotomies, respectively. This study suggests that maximal correction of astigmatism may be attained with a single set of tangential incisions placed 5 mm apart between two sets of semiradial incisions.