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PUBLICATION EXCLUSIVE: How to adjust IOL power in unusual cataract cases

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With newly developed methods of IOL power calculation, we are approaching 90% or higher accuracy of being within 0.5 D of the intended refractive target. The Barrett Universal formula uses improved algorithms, the Hill-RBF method relies on artificial intelligence with large data sets, and the Ladas Super Formula incorporates both novel methods as well as crowd-sourced big data to continuously hone its results. But even with these techniques of calculation, there is still a need to adjust the IOL power in unusual cataract cases.

Changing from one IOL design to another

The A-constant of the IOL reflects many variables such as effective lens position, lens geometry, refractive index and more. If we are planning to implant a particular IOL in the capsular bag but then decide to switch to a different IOL, we may need to adjust the power in order to keep the same refractive outcome. This may be the case if a particular power of a specific IOL is not available or if we decide to switch from one brand to another to better match the patient’s tissue.

The difference in A-constant between the two IOL types is the same as the adjustment needed to the IOL power. For example, if we are planning to insert an IOL of power +20 D and A-constant of 119.2 in the capsular bag, but then we decide to switch to a different IOL design with an A-constant of 118.7 while still implanting it in the capsular bag, we need to drop the IOL power by 0.5 D to +19.5 D to have the same refractive outcome as originally planned.

This also applies for choosing an appropriate IOL power for an anterior chamber IOL. If the original IOL for in-the-bag placement calls for a power of +20 D with an A-constant of 119.2, but then we decide to implant an anterior chamber IOL with an A-constant of 115.7, we need to drop the anterior chamber IOL power to +16.5 to have the same refractive result. This drop from +20 to +16.5 is 3.5 D, which is the same as the difference in A-constants (119.2 minus 115.7).

• Click here to read the full publication exclusive, Back to Basics, published in Ocular Surgery News U.S. Edition, November 25, 2016.