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# Centroid SIA calculation - ASCRS website

This article is in continuation to my write up in this blog ‘The Why & What of Centroid SIA involving astigmatism correction’.

I would strongly recommend you to read the article before you go ahead with this update. Recently the Double Angled Plot for Centroid SIA calculation has been launched on the ASCRS website.

Astigmatism is a vector of both magnitude and axis. Hence, a correct Surgical Induced Astigmatism (SIA) would involve a vector analysis that would involve the principles of Trigonometry and Maths. In Trigonometry, zero degree and 180 degree are not the same meridian, but in cataract surgery for astigmatic correction, they are very much the same.

Nothing wrong, but to apply the principles of science to cataract, we need to make a level playing ground, so that our assumptions are not unscientific. Hence the concept of Double Angle Plot, or doubling the angle or axis so as to match 360 deg Cartesian coordinates.

Various proponents of this method have found that their Centroid SIA has been minimal, with Graham Barrett presenting his data to as low as .1 diopter. Graham Barrett of Barrett Universal II calculator fame advocates a Centroid SIA of .1.

With this low value of SIA, you do not run the risk of assuming abnormal SIA values, which may otherwise screw up your low astigmatism TORIC IOL outcomes. Furthermore, in his presentation at APSCRS at Chang Mai in 2018, Barrett presented data of how the mean method of scalar SIA calculation is high on standard deviation, which proves the old method of calculating SIA cannot be trusted anymore.

Good News !! For those who are scientifically oriented, the double angle plot of calculating Centroid SIA is now available in the ASCRS website : http://ascrs.org/astigmatism-double-angle-plot-tool

Click on the above link, to help you download the tools for calculating Centroid SIA through Double Angled Plots.

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