ELP Estimation

The rationale of the “thin-lens” IOL power calculation formula and the definition of the estimated lens plane (ELP) are explained. Because the ELP is often back-calculated from the observed outcome the ELP is different from the physical position of the IOL. The exact manner by which the ELP is predicted varies between the various formulas, which may take the axial length, the K-reading, the anterior chamber depth, the lens thickness, and other parameters to increase the predictability. However, care should be taken when dealing with post-LASIK eyes, keratoconus, or other unusual eyes. Unlike the “thin-lens” ELP-formulas, the Olsen formula utilizes the C-constant concept, predicting the physical position of the IOL from the position and anatomy of the crystalline lens. Finally, an error propagation model is presented predicting the total IOL prediction error as a function of errors in the ELP prediction (largest source), the keratometry, the axial length reading, and other sources of error.