# DO I HAVE TO USE DOUBLE-K METHOD WHEN CALCULATING IOL POWER AFTER LASER REFRACTIVE SURGERY?.

Posted by on August 7, 2015 in Blog | Comments Off on DO I HAVE TO USE DOUBLE-K METHOD WHEN CALCULATING IOL POWER AFTER LASER REFRACTIVE SURGERY?.

DO I HAVE TO USE DOUBLE-K METHOD WHEN CALCULATING IOL POWER AFTER LASER REFRACTIVE SURGERY?.

Double K method was describe by Dr Jaime Aramberri in 2003 (Intraocular lens power calculation after corneal refractive surgery: Double-K method. J Cataract Refract Surgery 2003, 29(11):2063-2068).

The rationale for using double-K method is because most calculating formulae use keratometric power twice in their algorithms:

First to calculate the power of the lens to be implanted in the vergence formula, that is the power the cornea has at the moment of the lens surgery. If the cornea underwent refractive surgery, its power has logically changed, and here we should introduce the value we obtain in keratometers/topographers, etc once the proper corrections after refractive surgery have been made.

Second, the formulae use the keratometry power to calculate the position the intraocular lens will have after lens surgery (ELP), and this theorical position has to be calculated with the original power the cornea had before refractive surgery. Here we have to introduce the pre refractive surgery corneal power.

Out of the third and forth generation formulae, Holladay 1, SRK/T, Holladay 2 and Hoffer Q, follow this pattern of calculation. Haigis formula does not need double-K as it does not use K readings to calculate postoperative ELP.

How important is this?, how much impact has on the final result?.

Lets see a simulation:

Lets asume an eye with an axial length of 24,5 mm and we want to implant an IOL with an A constant of 118,9. In the left column of the table we see different preoperative mean K readings (40, 43,5 and 45) and in the top row different postoperative mean K readings after corneal refractive surgery (39, 36 and 32)

39                          36                  32

40                                23,3                       26,6               30,9

43,5                             23,3                       26,6               30,9

45                                23,3                       26,6               30,9

In this first table we do not use double-K and calculate using Holladay 1 formula. We can see in the rest of the cells the power of the lens to be implanted for emmetropia. The result is independent of the preoperative K readings as we have not used the double-K method. If you have a corrected K of 39 diopters, you get a suggested power for emmetropia of 23,3 diopters IOL regardless of your preoperative K being 40, 43,5 or 45.

In the next table we see the results for the same situations when we use the double-K method.

39                          36                  32

40                                23,5                       27,6               32,9

43,5                             24,5                       28,8               34,2

45                                25,0                       29,37             34,9

As we can see the predicted lens for emmetropia changes significantly, and using double-K for IOL calculation after corneal refractive surgery is mandatory if you chose formulae using K readings to calculate the ELP.

You should note that double-K calculation is completely independent of the adjustments you have to do to the K measurements done by the current keratometers/topographers that, as you know, overestimate the power of the cornea after refractive surgery.

SO, YES, IF YOU USE THIRD AND FORTH GENERATION CALCULATION FORMULAE, USE DR. ARAMBERRI´S DOUBLE-K CALCULATION. IF YOU USE HAIGIS FORMULA, THIS IS NOT NECESSARY.

Dr José R. Villada MD MRCOphth

Medish Directeur. Vision Ooglaseren. The Netherlands

Director Médico. Clinica Baviera Albacete. España