Vector analysis after toric intraocular lens implantation to correct astigmatism

We read with interest the paper by Krall et al.1 We found that the methods they followed to evaluate astigmatism correction by toric intraocular lenses (IOLs) might be optimized to obtain a more complete analysis of their results. First, the authors state that vectorial analysis was performed to determine the refractive astigmatic changes. The target induced astigmatism, surgically induced astigmatism, and difference vector were investigated using the Alpins method. It seems, therefore, that neither corneal astigmatism nor IOL toricity was taken into consideration. We believe that in such a study, this kind of information should be provided and analyzed. Considerable discrepancy might exist, in fact, between refractive astigmatism and corneal astigmatism. Moreover, when toric IOLs are implanted, calculations are based on corneal astigmatism and not on refractive astigmatism. Second, in the results section, the authors report that they did not find a statistically significant change in the corneal topographic or keratometric flat or steep meridian between the preoperative and postoperative measurements. However, it would have been useful to report the mean surgically induced corneal astigmatism. In this regard, we were surprised to read, “The incisions were placed at the 12 o’clock position to ensure that no surgically induced corneal astigmatism would influence the preexisting corneal astigmatism.” To our knowledge, there is no scientific demonstration that a superior incision does not induce any corneal astigmatism. Third, given that keratometric astigmatism has been shown to lead to cylinder overcorrection in eyes with with-the-rule (WTR) astigmatism and undercorrection in eyes with against-the-rule (ATR) astigmatism,2 it would have been interesting to separately analyze eyes with WTR and ATR astigmatism. Because many factors play a role in the refractive outcome of toric IOL implantation (eg, induced corneal astigmatism, corneal astigmatism, IOL toricity at the IOL plane and at the corneal plane), we believe that a more comprehensive analysis, like the one recently described,2 would be able to provide a more complete understanding of the surgical results.