Chapter 7: The Astigmatism Indices

target induced astigmatism vector (TIA) surgically induced astigmatism vector (SIA) correction index (CI) Alpins Method The Alpins’ Method has become the standard in Ophthalmology, for treating and analysing astigmatism Let’s discuss the basic terminology. The three fundamental vectors are the target induced astigmatism vector or the treatment - TIA, the surgically induced astigmatism vector SIA- the difference between the postop and preop astigmatism in both magnitude and orientation and the difference vector or DV which quantifies how much you missed the target by. When you look at the relationship between these vectors you can work out some indices that gives you a good idea of how well the surgery is performed. First one is really the ratio between the SIA and TIA known as the correction index. If the SIA is greater than the TIA you have over corrected the astigmatism, or if the SIA smaller than the TIA it means you under corrected the astigmatism. The angle of error is the angular difference between the SIA and the TIA, and the magnitude of the error is the magnitude difference between the two. An additional parameter you have mentioned in your book is the flattening index – what does this tell us about the success of the procedure? The flattening index tells us the amount of astigmatism reduced at the intended meridian. If you divide this by the amount of astigmatism you were intending to correct, that is, the TIA, then it should be 1.0 because you have effectively corrected all the astigmatism at that targeted orientation.