5.3.- From BigSIA to Zeiss Calculator

For beginners it can be difficult to understand how the mean obtained from BigSIA is introduced in the calculators. The reason is because calculators ask for the flattening effect at the incision location and not for the SIA magnitude and orientation. Here we represent an example for the Barrett Toric Calculator:

Imagine that you are obtaining the following mean SIA for temporal incisions at 180º in against-the-rule cases (0.43)@85º. This can be interpreted as with your followed protocol (temporal incision) you are steepening the 85º meridian 0.43D or that you are flattening 0.43 D at the 175º meridian, therefore what do I have to introduce in the calculator?

In the image below you will see that in the Zeiss calculator you have to insert the magnitude of the SIA (0.43 D) and the meridian on which the flattening effect is produced (175º) instead of the meridian on which you have conducted the incision (180º)


SIA calculator


The incision that is required for IOL implantation during cataract surgery induces astigmatism —this is referred to as surgically induced astigmatism (SIA)— so the total amount of astigmatism to be corrected with the toric IOL must consider the flattening (FE) or steepening (SE) effect on the meridian of the preoperative corneal astigmatism on which the IOL is going to be aligned. The SIA will depend on several factors, including incision type, location and size, and its design or architecture. 

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