Patients who have had LASIK or PRK have altered corneal shapes that traditional formulas often miscalculate.
Once measurements are taken, they are plugged into mathematical formulas to predict the necessary lens power. These formulas have evolved through "generations" based on how they estimate the —the final resting place of the IOL in the eye. Biometry & IOL Power Calculations - PDF
Biometry is the measurement of the eye's physical dimensions. To calculate the correct IOL power, surgeons need two primary measurements: Patients who have had LASIK or PRK have
Used a simple regression equation; often inaccurate for very long or short eyes. Biometry & IOL Power Calculations - PDF