Use of accurate data for planning toric IOL surgery is critical to optimizing astigmatism outcomes.
This article was reviewed by Noel A. Alpins, MD
Outcomes of toric IOL surgery can be improved when the planning considers the flattening effect (FE) of the phacoemulsification incision rather than the surgically induced astigmatism vector (SIA) and uses total corneal topographic astigmatism (CorT Total) or measured total corneal power (TCP) instead of an estimated value, according to Noel A. Alpins, MD.
“The incisional flattening effect (FE) is more accurate than the surgically induced astigmatism vector (SIA) and CorT Total or measured TCP is more accurate than estimated total keratometry values,” said Dr. Alpins, medical director, NewVision Clinics, and clinical professor of ophthalmology, Melbourne University, Melbourne, Australia.
Dr. Alpins explained that the flattening effect of the phaco incision, which is calculated from the analysis of previous surgeries, should be used rather than the SIA because the flattening effect represents the astigmatic reduction effect at the site of the incision.
In contrast, the SIA is composed of a flattening/steepening component, which is the effect in the direction of the incision, as well as a torque component, which rotates the astigmatism but does not reduce the astigmatism.
“When there is any amount of SIA that is off-axis from the incision, the SIA is an overstatement of the amount of change occurring at the incision site,” Dr. Alpins said.
Research showing how SIA can vary considerably depending on various factors also highlights the potential pitfalls of using a manufacturer-recommended SIA of 0.10 D as a default value for toric IOL power calculations.
For example, the astigmatically active incision is most active when operating on the steep meridian and more astigmatically neutral when operating on the flat meridian.1 In addition, it has been shown that the magnitude of the SIA varies depending on the amount of astigmatism present, increasing by 0.37 D for every 1 D increase in astigmatism.2
Citing results from a study he conducted analyzing data from operating on 1,300 eyes of 650 patients, Dr. Alpins demonstrated that the flattening effect of his 2.2-mm incision differed significantly between right and left eyes and was three times greater than a manufacturer’s recommendation to use an SIA value of 0.10 D for incisions ≤2.50 mm.
In his study, Dr. Alpins determined that the SIA averaged 0.64 D and the mean flattening effect was around 0.3 D.1
2. Chang SW, Su TY, Chen YL. Influence of ocular features and incision width on surgically induced astigmatism after cataract surgery. J Refract Surg. 2015;31(2):82-88.
3. Alpins NAM, Ong Jm Randleman J, Nevyas-Wallace A, Stamatelatos G. Corneal topographic astigmatism based on total corneal power data (CorT Total): a benchmark for total corneal astigmatism. Cornea. 2019 Oct 9 ePub ahead of print.