Refractive Outcomes for Toric Intraocular Lenses

Two articles by Goggin et al^1- 2 give clinical results for toric intraocular lenses (IOLs). The toric IOLs implanted were of relatively low astigmatic power (1.5 diopters [D], 2.25 D, and 3 D at the IOL), and they reduced the overall astigmatism of the eyes by correcting for astigmatism at the cornea. The accuracy of estimating corneal plane cylinder power is explored in the articles, and an alternative calculation method to the one used in the Alcon online toric calculator is proposed. Unfortunately, the “vertex power” formula that is given is not appropriate for converting from the IOL toric power to the corneal toric power (it was also originally mistyped, and the correct equation is E = F /[1 + (d /1.336) F]). The basis for this equation is a calculation that can be used to estimate the refractive error at the spectacle plane if the refractive error of an eye is known at the corneal plane. The calculation assumes that collimated light is initially incident on the cornea when the refraction is determined and that collimated light is also incident at the spectacle plane when the alternative refraction is calculated. This equation does not correctly describe the imaging situation in the eye, where collimated light that is focused with high vergence by the cornea is no longer collimated at the IOL.