Posters

The problem of lost refraction in customised ray tracing for IOL calculation

Poster Details

First Author: H. Ginis GREECE

Co Author(s): C. Katsoulos   P. Zafirakis   G. Livir-Rallatos              

Abstract Details

Purpose:

To perform customised, exact ray tracing for the calculation of IOL power in cataract patients using one type of IOL (Alcon Acrysof SA60AT) and compare the accuracy with that obtained with the SRK/T and Hoffer formulas. Moreover to repeat raytracing using postoperative data (when IOL position and power are known) and analyse residual refractive error. This residual refractive error (the lost refraction) is the result of the combined measurement error, IOL true power and posterior corneal power.

Setting:

Athens Eye Hospital, Athens, Greece.

Methods:

Raytracing was performed retrospectively in 58 eyes of 43 patients that underwent cataract surgery and received the SA60AT IOL. Ray tracing was obtained using OpticStudio 16.5 (Zemax, LLC, Kirkland, WA, USA) raytracing software using biometry and keratometry data from the Lenstar optical biometer (Haag-Streit AG, SW). Preoperative prediction of IOL depth was made using an Olsen’s C-constant equal to 0.28, as determined in a previous study in the same setting. Postoperative raytracing with the actual lens position and updated keratometry revealed a residual error that was generally different than that measured clinically: The lost refraction.

Results:

The mean postoperative refraction using the SRK/T formula was -0.22D (SD= 0.58D). The outcomes, should one of the other methods had been used (Hoffer, Raytracing+Olsen's C) would not be have been different by a statistically significant dioptric bias. Postoperative raytracing revealed that the lost refraction (SD=0.48) can be as high as 1.25D in some eyes.

Conclusions:

In the particular data set , all three methods had similar results in the prediction of IOL power in terms of bias and standard deviation. It is hypothesised that the lost refraction is the combined error of measurement in keratometry and biometry, the true (vs nominal) IOL power and posterior corneal power. Calculations based on instument specifications and IOL specifications (ISO standard) suggest that the main source of error in modern IOL calculation is the uncertainty on the posterior corneal power.

Financial Disclosure:

None