Evaluation of corneal power from an AS-OCT thick lens model and ray tracing: reliability of the keratometer index

Purpose: To investigate and compare different strategies of corneal power calculations using keratometry, paraxial thick lens calculations and ray tracing. Setting: Tertiary care center. Design: Retrospective single-center consecutive case series. Methods: Using a dataset with 9780 eyes of 9780 patients from a cataractous population the corneal front (Ra/Qa) and back (Rp/Qp) surface radius/asphericity, central corneal thickness (CCT), and entrance pupil size (PUP) were recorded using the Casia 2 tomographer. Beside keratometry with the Zeiss (PK Z ) and Javal (PK J ) keratometer index, a thick lens paraxial formula (PG) and ray tracing (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 PK Z /PK J overestimates the paraxial corneal power PG in around 97%/99% of cases and PR in around 80% to 85%/99%. PR is around 1/6 or 5/6 diopters (D) lower compared with PK Z or PK J . For a 2 mm pupil PR is around 0.20/0.91 D lower compared with PK Z /PK J and for a 5 mm pupil PR is comparable with PK Z (around 0.03 D lower) but around 0.70 to 0.75 D lower than PK J . 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.