June 2021
Volume 62, Issue 8
Open Access
ARVO Annual Meeting Abstract  |   June 2021
Preclinical Estimation of Constants for Intraocular Lens Power Calculations
Author Affiliations & Notes
  • Michael Simpson
    Simpson Optics LLC, Arlington, Texas, United States
  • Footnotes
    Commercial Relationships   Michael Simpson, Simpson Optics LLC (E)
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Investigative Ophthalmology & Visual Science June 2021, Vol.62, 514. doi:
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      Michael Simpson; Preclinical Estimation of Constants for Intraocular Lens Power Calculations. Invest. Ophthalmol. Vis. Sci. 2021;62(8):514.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose : Intraocular lenses (IOLs) are labeled with a lens power, but a corresponding “lens constant” is also needed for IOL power calculations. This is typically determined by back-calculation from earlier postoperative results, and initial patients may have less accurate refractive outcomes. To address this, a method for estimating the SRK/T A-Constant before any IOLs have been implanted is described. Parameters that have a strong effect on the A-Constant are also discussed, such as the shape factor, the asphericity, and the methods used for IOL testing.

Methods : The final axial location of the IOL is the greatest unknown, but this is primarily determined by the lens capsule, which contracts around the lens and haptics, while also being stretched flat by the zonules. A new IOL can be physically compared to an existing single-piece IOL to estimate the location, and image distances can be calculated or measured using the ISO2 model eye to estimate the optical effects (Fig. 1a). An average eye is assumed, and a central range of IOL powers (eg 19D–23D)). Zemax calculations demonstrate the method here for several conceptual high index (1.55) IOL styles that have different shape factors and spherical aberration values (Fig. 1b). The haptic location was used to position the lenses.

Results : The “best focus” image distances for a 3 mm pupil diameter are plotted against the labeled power values in Fig 2, along with estimated power changes relative to the equiconvex spherical lens. The calculations illustrate how changes to the shape factor and the asphericity can each change the effective power of the IOL by 0.5 D, with more than 2 D difference across this extreme range of potential styles. Differences can be used to adjust the A-Constant.

Conclusions : The focusing effect of IOLs in an average laboratory eye can be used to estimate the SRK/T “A constant” through comparison to an existing lens. The IOL axial location is envisaged to be constrained primarily by the capsule, and this is estimated by comparing design features (in this case, simply the haptic location). This estimation is the weakest part of the analysis. Calculations are useful, but the best results would come from actual lens measurements at 35 C in saline, to be close to actual eye conditions. The theoretical designs would then not be needed, and any unexpected effects would also be addressed, with the use of several powers averaging out any fabrication variations.

This is a 2021 ARVO Annual Meeting abstract.

 

 

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