2020–2021 BCSC Basic and Clinical Science Course™
11 Lens and Cataract
Chapter 7: Preoperative Considerations for Cataract Surgery
IOL Power Determination
Classic Regression Formula
Although it is no longer regularly used for IOL calculation, the SRK formula developed by Sanders, Retzlaff, and Kraff in the 1980s helps illustrate the relative impact of AL and keratometric power in attaining the proper implant power. A small error in AL can have a much larger impact than a small error in keratometry.
P = A − (2.5L) − 0.9K
P = lens implant power for emmetropia (diopters)
L = axial length (mm)
K = average keratometric reading (diopters)
A = constant specific to implant to be used
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.