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    Learn how curiosity propels astigmatism innovations

     

    Javal’s rule

    Louis Émile Javal, MD, a 19th-century ophthalmologist, first noted that he could not account for total ocular astigmatism by simply measuring the power of the anterior cornea. Known as Javal’s rule, he postulated the posterior cornea contributed about 0.5 D of against-the-rule astigmatism.

    “When we refract a patients, the ocular astigmatism does not always reflect the corneal astigmatism,” he said.

    Related: Cataract app helps surgeons customize IOL selection

    It was not until Douglas D. Koch, MD, reminded people a few years ago of the importance of the posterior cornea in toric IOL calculations during his own Innovator’s Lecture that Javal’s rule began to be understood.

    “If you ignore the posterior cornea, you’ll end up with about a 0.3 D overcorrection for patients with against-the-rule astigmatism and about 0.5 D overcorrection for those with with-the-rule astigmatism,” Prof. Barrett said. “You need to measure where the lens is sitting, and not where you think it’s sitting.”

    In a study of his own patients (n = 160) using the Holladay formula, Prof. Barrett found 28.8% were within 0.5 D of residual astigmatism. Using the Barrett formula, which included effective lens position and considers lens position for each individual patient versus what is known about the average eye, provided a better outcome, but not a “great” one, he said.

    Related: Customized IOL calculator helps cataract surgeons navigate lens choices

    “Nomograms are improving, but none are as accurate as an online calculator,” he said, adding his own True-K toric IOL calculator is now available online.

    Clinicians know from several large cohort studies that about 80% of patients have more than 0.5 D of astigmatism, and that 0.75 D of astigmatism has more of an impact on vision than 0.75 D of spherical error.

    Related: How new ‘evolving’ IOL formula maximizes lens accuracy

    “Some studies suggest that less than 0.5 D of astigmatism is really minor,” he said, who disagrees.

    He recommends using toric IOLs for even low levels of astigmatism, and has been able to get 86% of his patients with with-the-rule astigmatism and 83% of his patients with against-the-rule astigmatism to within 0.5 D.

    “Target a minimum residual cylinder in all your patients,” he said.

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