Lens Power Calculations Using Total Keratometry Better For Steep, Flat Corneas

Total keratometry (TK)-based intraocular lens (IOL) power calculations have better refractive outcomes than the standard keratometry (K)-based IOL power calculations when applied to patients with flat or steep corneas, a retrospective consecutive cross-sectional study published in the American Journal of Ophthalmology shows.

Patients (N=231) who underwent cataract phacoemulsification surgery without complications at the Zhongshan Ophthalmic Center in China between 2020 and 2021 were included in this study. Patients underwent swept-source optical coherence tomography (SS-OCT) to calculate ophthalmic features and the standard deviation (SD), mean absolute error (MAE), and median absolute error (MedAE) were calculated to assess refractive outcomes of TK- and K-based IOL power calculations.

The patients had flat K (≤42.0 D; n=55), normal K (42.0-<46.0 D, n=116), and steep K (≥46.0 D; n=60) eyes. In general, the patients with flat K had the longest axial lengths, lowest average K and TK, and greatest anterior chamber depth compared with the other groups.

Power formulas using K or TK data were ranked by SE. There was no significantly better formula using K data. For eyes with predicted error within ±0.5 D, the Hoffer Q (P =.014) and Holladay 1 (P =.046) formulas were preferred when using TK data.

For flat K eyes, significantly higher SD was observed for Holladay 1 (P =.013), Hoffer Q (P =.038), EVO (P =.042), and Haigis (P =.043) formulas when using K data compared with TK data. Overall, the EVO formula using TK data had the lowest SD (0.357) and MAE (0.28) and using K data, the Kane formula had the lowest MedAE (0.20).

In average K eyes, the EVO formula had the lowest SD (0.360) and MAE (0.28) when using K data and the Kane formula had the lowest MedAE when using TK data (0.21).

Among the steep K cohort, when using K data, the SRK/T (P =.029) and Hoffer Q (P =.036) formulas had significantly higher SD than when using TK data. Overall, the BUII formula had the lowest SD (0.431), MAE (0.32), and MedAE (0.26) when using TK data.

In general, compared with average K eyes, formula performance had a better trend on refractive outcomes when using TK data in the flat and steep K cohorts, in which there was a higher proportion of eyes within the ±0.5 D predictive error threshold.

In a subgroup analysis stratified by axial length, using TK data still led to lower SD, MAE, and MedAE values and higher refractive prediction accuracy than K data.

This study was limited by its retrospective design. These findings should be confirmed in prospective studies.

“The TK dataset showed a better trend of refractive outcomes in the flat keratometry and steep keratometry groups. The EVO (TK) and BUII TK formulas were suggested for eyes with K values lower than 42 D and K values higher than 46 D, respectively,” according to the study authors.