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 have a case of 2D of corneal astigmatism with steep meridian at 180º (ATR case). The Net Astigmatism is 2.35D at 0º-180º instead of 2D because we have selected the Predicted PCA (Posterior Corneal Astigmatism), therefore as the measurement was taken with IOL Master 500 (Ksim considering only anterior corneal surface) the nomogram corrects the possible undercorrection in ATR cases by means of overcorrecting the corneal cylinder derived from anterior corneal surface.

 

 

We have not still introduced the SIA, but here there is a very important thing to know. Although the incision was located at 180º by the surgeon, the flattening effect was at 175º. Therefore, we have to introduce in the calculator the incision location at 175º instead of 180º. In incision SIA we would introduce 0.43 D as it is shown below.

 

 

The new Net Astigmatism is 1.93@1º after correcting posterior corneal astigmatism by nomogram estimation and introducing the flattening effect at the calculator. The IOL is suggested to be located at 1º instead of 180º because of the flattening effect at 175º instead of at 180º.