Part A: Biometry and Intraocular Lens Power Calculation

Formulas For Intraocular lens Power calculatIon Modern cataract surgery replaces the natural lens of the eye with an intraocular lens (IOL). After surgery, the optical system of the eye will consist of two major refractive elements, the cornea and the newly inserted IOL. This two-lens system is at the basis of IOL power calculation. To facilitate calculations, both the cornea and the IOL are considered thin lenses. Although there are multiple IOL power formulas, most are similar in nature. They all require measurements of the axial length and of the corneal power. However, all these formulas require knowledge of the distance separating the two thin lenses, also known as the estimated lens position (ELP). P = n – n L – c (n/K) c In this vergence formula, P is the power of the IOL needed for emmetropia in diopters, L is the axial length in meters (all formulas adjust the equation to allow the axial length to be entered in millimeters), K is the corneal power in diopters, c is the ELP, and n is the index of refraction of the aqueous and vitreous, which is known to be 1.336. All formulas will require measurement of the axial length and of the corneal curvature. ELP is the distance between the anterior corneal surface and the implant’s optical center. This value cannot be measured before surgery and thus has to be estimated. The difference between the different formulas lies in the estimation of ELP.