Power Calculation Methods for the Post–Keratorefractive Procedure Eye
The double-K method uses the pre-LASIK corneal power (or, if unknown, 43.50 D) to calculate the ELP, and the post-LASIK (much flatter) corneal power to calculate the IOL power. These calculations can be performed automatically with computer programs.
The double-K method is only one of more than 20 methods proposed over the years to either calculate the true corneal power or adjust the calculated IOL power to account for the errors discussed in the preceding sections. Some methods require knowledge of pre–refractive surgery values such as refractive error and K reading. Many of these methods have come in and out of favor based on studies investigating their accuracy. More recent theoretical or ray tracing formulas (Olsen, Barrett) may offer a more accurate alternative in abnormally sized eyes in which previous refractive procedures have been performed, especially corneal refractive surgery. It is up to the surgeon to keep abreast of the most accurate available methods.
It is not possible to describe all these methods in this chapter. Several excellent calculators are available online, including tools from Warren Hill and Graham Barrett at ASCRS.org, APACRS.org, and www.doctor-hill.com, as well as the Hoffer/Savini LASIK IOL Power Tool, which can be downloaded for free (see the Hoffer reference below).
Perhaps in the future there will be a more satisfactory method of measuring true corneal power by use of topography and advanced measuring techniques. At present, the ideal method for use with post–refractive surgery patients has yet to be determined.
Abulafia A, Hill WE, Koch DD, Wang L, Barrett GD. Accuracy of the Barrett True-K formula for intraocular lens power prediction after laser in situ keratomileusis or photorefractive keratectomy for myopia. J Cataract Refract Surg. 2016;42(3):363–369.
Alió JL, Abdelghany AA, Abdou AA, Maldonado MJ. Cataract surgery on the previous corneal refractive surgery patient. Surv Ophthalmol. 2016;61(6):769–777.
Fernández-Buenaga R, Alió JL, Pérez Ardoy AL, Quesada AL, Pinilla-Cortés L, Barraquer RI. Resolving refractive error after cataract surgery: IOL exchange, piggyback lens, or LASIK. J Refract Surg. 2013;29(10):676–683.
Hill W, Wang L, Koch D. IOL Power Calculation in Eyes That Have Undergone LASIK/PRK/RK. Available at iolcalc.ascrs.org.
Hoffer KJ. The Hoffer/Savini LASIK IOL Power Tool. Available at www.iolpowerclub.org/post-surgical-iol-calc. Accessed June 3, 2013.
Koch DD, Liu JF, Hyde LL, Rock RL, Emery JM. Refractive complications of cataract surgery after radial keratotomy. Am J Ophthalmol. 1989;108(6):676–682.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.