This decade has seen many new concepts cropping up with regard to the concept of astigmatic correction with TORIC IOLs. First, Douglas Koch's path breaking work on Posterior Corneal Astigmatism, followed by the concept of Centroid SIA by Abulafia and others. The concept of Centroid SIA and its importance has been explained in this blog in two separate articles.
A new work and no less promising than what we have been already introduced to is the concept of Total SIA by Dr Jack Holladay.
Straight to business , what is the concept of Total SIA ? Total SIA is the vector difference between the postoperative refractive astigmatism at the corneal plane and the pre operative Keratometry reading. The Total SIA, that is postoperative refractive astigmatism minus preoperative keratometric astigmatism , according to Holladay, will include post operative posterior corneal astigmatism changes, astigmatism resulting from IOL tilt and decentration, etc. However, Holladay cautions that the postoperative refractive astigmatism should be vertexed to the corneal plane to determine the Total SIA.
"For example, if you took someone who had a spherical cornea and you made a temporal incision, what we're finding is that person ends up with a little induced against-the-rule on his refraction even though the K readings show that he got a little with-the-rule. What that tells us is there's more going on than just the change in the front surface of the cornea," Dr. Holladay was quoted to say in an article in Eye World published in February, 2014 by Erin L. Boyle EyeWorld Senior Staff Writer.
Using Total SIA therefore may be an improvement over traditionally calculated SIA, even if such SIA has been calculated on the centroid method. By traditional SIA, I mean the post operative Keratometry values minus the pre operative Keratometry values. As Holladay advocates, the later does not consider the PCA and any astigmatism that may arise from IOL tilt and decentration. We all know, that our eyes do have tilt and decentration. The natural tilt of the IOL in the vertical axis , temporal decentration , may result in an average ATR astigmatism. Holladay says, this may be to the tune of .20D.Holladay also advocates for a stable post operative refraction at six months for Total SIA calculation. This ofcourse is taking into fact the healing effect of corneal incisions.
With Toric IOLs, the parameters needed for analyzing astigmatic changes are far more complex than in lasik or laser based astigmatic keratotomy. In cataract surgery with TORIC IOLs, the parameters required are - a) refraction , b) corneal anterior astigmatism, c) Posterior corneal astigmatism, d) IOL alignment , e) IOL tilt which induces astigmatism with Toric as well as non Toric IOLs, f) effective IOL toricity at corneal plane which can be calculated but not measured. In the editorial in JCRS volume 44 Issue 10, October 2018, Abulafia and other authors proposed the term ACTUAL POSTOPERATIVE REFRACTIVE ASTIGMATISM (at the corneal plane) to understand astigmatism change.
The concept of Total SIA is no doubt a novel addition in the world of astigmatism correction with TORIC IOLs. What remains to be seen, its acceptance, and the ease of calculation through SIA calculators. Keep in mind, a vast majority of surgeons and optometrists are still on the old SIA magnitude of an average of .5 diopters. How the Total SIA will shape up, will depend on a large extent to the shift of this mindset.