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A physiological effective corneal refractive index for IOL power calculation

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Session Details

Session Title: Pseudophakic IOLs: Depth of Focus. Cataract Surgery Equipment

Session Date/Time: Monday 07/09/2015 | 16:30-18:30

Paper Time: 18:14

Venue: Room 16

First Author: : K.Næser DENMARK

Co Author(s): :    J. Bregnhoj   G. Savini                 

Abstract Details


To measure and compare various expressions for total corneal power from Scheimpflug camera assessment of corneal surfaces. To calculate a physiologic effective corneal refractive index for IOL power calculation.


GB Bietti Foundation IRCCS, Rome, Italy. Department of Ophthalmology, Randers Regional Hospital, Denmark


A rotating Scheimpflug camera (Pentacam, Oculus, Germany) was used to measure the corneal thickness and the powers of the anterior and posterior surfaces in 951 normal eyes. The following expressions for spherical equivalent corneal power were recorded: Simulated keratometry with an effective refractive index of 1.3375 (SimK1.3375), Total Refractive Corneal Power (TRCP) based on Snell´s law ray tracing, and the Equivalent Power (EP), which is the thin lens with the same optical qualities as the thick cornea. A new effective corneal refractive index – nCalc - was calculated as the ratio between EP and the anterior corneal curvature.


The spherical equivalent corneal power expressed as SimK1.3375, TRCP, and EP averaged 43.42 (±1.49) D, 42.78 (±1.51) D, and 42.26 (±1.46) D. These mean values all differed statistically from each other (p<0.001; ANOVA). The mean calculated refractive index nCalc was 1.3284 (±0.0009). Multiplying this refractive index with the anterior corneal curvature produced the calculated corneal power (PCalc). The difference PCalc – EP was close to zero with a standard deviation of ±0.12 D.


SimK1.3375,TRCP, and EP gave different values for corneal spherical equivalent power, and these expressions can therefore not be interchanged. SimK with the historical refractive index of 1.3375 yields much larger spherical equivalent power than the 2 methods including the posterior corneal power. Employment of the 1.3284 effective corneal refractive index with SimK produced spherical equivalent powers similar to the E.

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