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Implantation of a toric IOL in eyes with low pre-existing cylinder has benefit for significantly reducing postoperative astigmatism, but surgeons should be mindful of the importance of minimizing surgically induced astigmatism when using a low-cylinder power toric IOL.
"It is very difficult to correct low corneal astigmatism if the surgical incision creates astigmatism that is 50% or more of what you are trying to treat," said Dr. Ernest, founder, TLC Vision of Michigan, Jackson. "Implantation of the toric IOL offers a more precise approach to correction of low-power astigmatism."
Outcomes of toric IOL implantation to correct low pre-existing cylinder were investigated in a retrospective chart review of eyes that underwent uncomplicated cataract surgery in 2009 and had preoperative corneal astigmatism of 0.75 to 1.37 D with no prior history of corneal surgery. Preoperative keratometry was measured in all eyes with repeat testing using manual keratometry, automated keratometry, topography, and with the optical coherence interferometer (IOLMaster, Carl Zeiss Meditec).
For the entire population, mean preoperative corneal astigmatism was ~1.06 D, and it did not differ significantly comparing the toric IOL and standard IOL groups. There was also no difference between IOL groups in proportions of eyes having with-the-rule (WTR), oblique, or against-the-rule (ATR) astigmatism.
The low-power toric IOL model implanted corrects 1.03 D of astigmatism at the corneal plane. None of the eyes with spherical IOLs had intraoperative limbal relaxing incisions. In all eyes, phacoemulsification and IOL implantation was performed through a 2.2-mm square, sutureless posterior limbal incision.
"Knowing there is minimal SIA with this incision technique, the Toric IOL calculator from Alcon was not required or used," Dr. Ernest said.
Follow-up ranged from 2 weeks to 6 months. SIA was calculated for a subset of eyes with postoperative keratometry results, and the calculation was also performed with eyes divided into subgroups based on the axis of the measured preoperative corneal astigmatism. Mean SIA was 0.25 ± 0.14 D in both lens groups and also did not differ between subgroups categorized by axis of astigmatism, Dr. Ernest noted.
Mean residual refractive astigmatism was 0.26 ± 0.25 D among eyes with the toric IOL implanted and 0.97 ± 0.37 D for eyes receiving the spherical IOL (p < 0.0001).
"In the toric IOL group, residual refractive astigmatism was 0.5 D or less in 95% of eyes, 0.25 D or less in [more than] 60%, and no eye had more than 0.75 D of residual refractive cylinder," Dr. Ernest said. "The results were quite different in the control group, where postoperative refractive astigmatism was 0.50 D or less in only about 12% of eyes and exceeded 0.75 D in almost 60%."