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Back calculation helps identify causes of residual astigmatism
The data collected can change the way clinicians plan cataract surgery, a study suggests.
An online back-calculator can help surgeons better plan future cataract surgery, according a study.
The retrospective data review analyzed 12,812 records of patients who had undergone toric IOL implantation, with a mean postoperative refractive astigmatism of 1.89 D. The data were taken from the back-calculator developed by two of the study’s authors, Berdahl and Hardten, and included material collected on astigmatismfix.com from mid-2012 through 2015.
The back-calculator is designed to find the toric IOL orientation that will most effectively reduce refractive astigmatism. It does this by including current orientation, cylinder power and manifest refraction in its calculations.
The study found that although 30% of IOLs were at their preoperative calculated orientations, 90% were not at ideal orientation.
“This suggests there are deficiencies in the preoperative calculation or changes to the cornea at the time of surgery that might affect the calculated orientation,” the study authors wrote. “Preoperative effects would include things such as variability in surgically induced astigmatism, effects of posterior corneal astigmatism that are not accounted for and variability in keratometric measurement.”
The study said that 37% of participating eyes with postoperative astigmatism could experience a reduction in astigmatism to 0.5 D or less if the IOL was reoriented in the eye.
The study also found that increased cylinder power resulted in significantly higher refractive astigmatism (P < .01), but refractive astigmatism did not differ according to IOL manufacturer.
The authors acknowledged significant postoperative astigmatism does not occur at a high rate. They noted a 2016 meta-analysis of 13 clinical trials showing 6% to 16% of eyes that had undergone toric IOL implantation had more than 1 D of such residual astigmatism, depending on IOL model and cylinder power.
Nonetheless, they said, “a better understanding of the factors associated with suboptimum outcomes could be helpful.”
One of the study authors, Richard Potvin, MASc, OD, pointed out several keys to making the proper adjustments.
“A surgeon should ensure that the biometry they have collected is repeatable,” he said. “As Epitropoulos et al have shown, dry eye, among other things, can affect keratometry, which will affect toric IOL implantation.
“Surgeons should also consider using a formula that compensates in some fashion for posterior corneal astigmatism,” Potvin continued. “These considerations are likely to minimize the likelihood of an astigmatic surprise after surgery.
“If [surgeons] do end up needing the back-calculator because of significant refractive astigmatism after surgery, the only two critical pieces of the puzzle are knowing what lens is implanted and at what orientation, and having a good postoperative refraction,” Potvin said. “This is sufficient because the relative change in astigmatism as the lens is rotated will apply directly to the postoperative refractive astigmatism.”
Potvin also stressed that surgeons should set proper expectations for patients.
“In general, explanations preoperatively are interpreted by the patient as counseling,” he said. “Explanations after the fact are interpreted as excuses. It is important that the surgeon communicate to patients before their initial surgery that a toric IOL is designed to reduce or eliminate their astigmatism, but that, human biology being what it is, it is not always 100% effective in all patients on the first go.”
Potvin pointed to studies suggesting that 70% to 80% of eyes show a residual refractive astigmatism of 0.5 D or less after the initial toric IOL implantation. – by Joe Green
- References:
- Epitropoulos AT, et al. J Cataract Refract Surg. 2015;doi:10.1016/j.jcrs.2015.01.016.
- Kramer B, et al. J Cataract Refract Surg. 2016;doi:10.1016/j.jcrs.2016.09.017.
- For more information:
- Richard Potvin, MASc, OD, can be reached at Science in Vision, 6197 Dye Road, Akron, NY 14001; email: rick@scienceinvision.com.
Disclosure: Potvin reports he is a consultant for Alcon Laboratories, Haag-Streit and Oculus.