Aims: To optimize the relationship between the actual and predicted refractive outcomes following phacoemulsification and axial length of the eye. Four IOL power calculation formulae were compared: SKR-T, Holladay I, Hoffer-Q and Haigis. Optical axial length transformation based on the original calibration of the IOLMaster was tested to improve the predictability of outcomes.
Methods: The refractive outcomes of 155 eyes of 120 patients using the IOLMaster were reviewed following phacoemulsification. After optimizing the IOL constants, the prediction error (=achieved spherical equivalent - calculated spherical equivalent) of each of the formulae was calculated for each eye and then correlated to the axial length of the eye with and without the optical transformation. Keratometry measurements of the IOLMaster were tested by transforming to that of standard auto-keratometry. This was validated in a 2nd group of 90 eyes.
Results: The prediction errors of all but the Haigis triple optimized formula were found to be positively correlated to the axial length. The optical and keratometric transformation statistically significantly reduced this correlation, leading to lower prediction errors at all axial lengths. Prediction error per mm axial length was 0.06D/mm (SRK-T), 0.16D/mm (Hoffer-Q), 0.14D/mm (Holladay I), 0.11D/mm (Haigis single) and -0.02D/mm (Haigis triple). Following transformation, best case prediction error was reduced to -0.01D/mm (SRK-T), 0.08D/mm (Hoffer-Q), 0.01D/mm (Holladay I), 0.01D/mm (Haigis single) and -0.01D/mm (Haigis triple).
Conclusion: Transforming the IOLMaster measurement into the optical axial length while compensating for its keratometry measurement was able to improve the predictability of outcomes at extreme axial lengths without further modification to the standard IOL formulae.