Modification of the Barrett Universal II formula by the combination of the actual total corneal power and virtual axial length.

Purpose: This study aims to investigate whether a combination of the total corneal power (TCP) and virtual axial length (AL) based on Gaussian optics makes the refractive prediction accuracy of the Barrett Universal II (BUII) formula better than the conventional anterior keratometry (K) and axial length.

Methods: The TCP and the virtual AL were calculated in two ways: the corneal index strategy and the TK index strategy. The former uses the corneal refractive index n as a variable, and the latter uses the TK index n as a variable. In a dataset of 225 eyes, the calculated TCP and the virtual AL were input into the BUII formula along with the anterior chamber depth, lens thickness, and white-to-white measured with the IOLMaster 700, and the refractive prediction accuracy was evaluated by the mean numerical prediction error (MNE), standard deviation (SD), mean absolute prediction error (MAE), median absolute prediction error (MedAE), percentages of eyes with prediction error (PE) within ± 0.50 diopter, and IOL formula performance index (FPI). The refractive prediction outcomes also underwent subgroup analyses and were compared with those of the anterior keratometry-based BUII-K of the IOLMaster 700.

Results: In the corneal index strategy, the FPI had the highest value at approximately n = 1.346. In the TK index strategy, the FPI had the highest value at approximately n = 1.3858. There was no tendency for the refractive prediction outcomes of the BUII-n = 1.346 and the BUII-n = 1.3858 to be inferior to those of the BUII-K, particularly in the medium range of subgroups.

Conclusion: The combination of the actual TCP and the virtual AL based on Gaussian optics may lead to a better refractive prediction accuracy of the BUII formula than that of BUII-K.