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July 2012

Toric Intraocular Lens Calculations

Author Affiliations

Author Affiliations: G. B. Bietti Foundation–Istituto di Ricerca a Carattere Clinico e Scientifico, Rome, Italy (Dr Savini); and Jules Stein Eye Institute, University of California, Los Angeles, and St Mary's Eye Center, Santa Monica (Dr Hoffer).

Arch Ophthalmol. 2012;130(7):947-949. doi:10.1001/archophthalmol.2011.2747

Goggin et al1 should be commended for their interesting analysis of the conversion from the IOL to the corneal plane of the cylinder of toric lenses, as to our knowledge no one else has focused the attention on such a relevant topic. However, some important issues must be addressed.

When using the example of the SN60T3 IOL in the introduction, the authors confused “thin lens” and “thick lens” formulas. They calculated the equivalent cylinder at the corneal plane “using the manufacturer's effective lens position of 5.2 mm.” By means of the thick lens vertex power formula, they obtained values of 1.32 D and 1.22 D, respectively, for the 17.0-D and 28.0-D IOLs, compared with the nominal 1.03-D value provided by the manufacturer. These calculations are not correct because the 5.2-mm ELP given by the manufacturer (and adopted by the authors) is valid only for thin lens formulas. In fact, it describes the principal plane of the thin lens and does not correspond to the physical distance of any thick lens, which is usually posterior to the actual position of the anterior surface of the IOL in the capsular bag.2,3 As a consequence, the ELP cannot be entered into any thick lens formula, like the authors have done.