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Editor,—We read with interest the article by Sarhanet al 1 on the effect of disagreement between refractive, keratometric, and topographic determination of astigmatic axis on suture removal after penetrating keratoplasty.
The authors make some fundamental errors in their use of vectors for the calculation of mean astigmatism and have failed to refer to the dependence of astigmatism on the overall refractive power. As clearly discussed by several authors,2-4 it is inappropriate to analyse astigmatism without analysing the overall change in refractive power. The authors state that the two groups (agreement and disagreement) were comparable before suture removal in the preoperative vector of astigmatism but do not present the mean presuture removal vector. The authors appear to calculate a mean of the scalar component of astigmatism. Vectors have both direction and magnitude and cannot be averaged in this …