Evaluation of corneal power from an anterior segment OCT a thick lens model and raytracing – how reliable is the keratometer index?

Langenbucher, Achim; Hoffmann, Peter; Cayless, Alan; Wendelstein, Jascha and Szentmáry, Nóra (2023). Evaluation of corneal power from an anterior segment OCT a thick lens model and raytracing – how reliable is the keratometer index? Journal of Cataract and Refractive Surgery (Early access).

DOI: https://doi.org/10.1097/j.jcrs.0000000000001363

Abstract

Purpose: To investigate and compare different strategies of corneal power calculations using keratometry, paraxial thick lens calculations and raytracing.
Setting: Tertiary Care Center.
Design: Retrospective single centre consecutive case series.
Methods: Using a dataset with 9,780 eyes of 9,780 patients from a cataractous population the corneal front (Ra / Qa) and back (Rp / Qp) surface radius / asphericity, corneal thickness (CCT), and entrance pupil size (PUP) were recorded using the Casia 2 tomographer. Beside keratometry with the Zeiss (PKZ) and Javal (PKJ) keratometer index, a thick lens paraxial formula (PG) and raytracing (PR) was implemented to extract corneal power for pupil sizes from 2 mm to 5 mm in steps of 1 mm and PUP.
Results: With PUP PKZ / PKJ overestimates the paraxial corneal power PG in around 97% / 99% of cases and PR in around 80-85% / 99%. PR is around 1/6 or 5/6 dioptre lower compared to PKZ or PKJ For a 2 mm pupil PR is around 0.20 / 0.91 dioptres lower compared to PKZ / PKJ and for a 5 mm pupil PR is comparable to PKZ (around 0.03 dpt lower) but around 0.70-0.75 dioptres lower than PKJ.
Conclusions: ‘True’ values of corneal power are mostly required in lens power calculations before cataract surgery, and overestimation of corneal power could induce trend errors in refractive outcome with axial length and lens power if compensated with the effective lens position.

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