EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
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August 2013 Diagnostic technologies for February 2011 cataract surgery to a number of doctors. Currently the most common measurement for this is with a Scheimpflug device. Dr. Hamilton indicated that not many surgeons are using posterior corneal astigmatism at this time. "It's not something that is generally used right now," he said. However, he believes the technology will be useful in the future. There has been quite a bit of buzz recently about it. Dr. Hamilton said that not a lot of cataract surgeons have this more sophisticated topography technology; it's mainly refractive surgeons who have it. Why PCA is important Posterior corneal astigmatism is important when it comes to choosing an appropriate lens for a patient, especially when toric IOLs are involved. "Most devices only measure the front surface of the cornea and then adjust that measurement to take into account the contribution of the back surface of the cornea," Dr. Weikert said. Though this can be helpful for approximations, it can lead to assumptions. "Sometimes those assumptions are correct and sometimes those assumptions lead to errors." Dr. Hamilton said posterior corneal astigmatism measurements could be useful to "fine-tune our results for toric lenses for cataract surgery." Without imaging the back of the cornea as well as the front, inaccuracies in measurements can occur, which could lead to a decreased efficacy of toric intraocular lens implants. IOL calculators and PCA Dr. Hamilton said many of the toric IOL calculators do not take posterior corneal astigmatism into account. The toric calculator from Alcon (Fort Worth, Texas) does not have this ability. Although the Abbott Medical Optics IOL calculator (AMO, Santa Ana, Calif.) is more sophisticated, it does not account for posterior corneal astigmatism. The Alcon calculator gives one option for the astigmatic IOL power a surgeon should use, while the AMO calculator and the Holladay IOL Consultant (Holladay Consulting, Bellaire, Texas) give multiple options for the astigmatic IOL power, indicating what the residual will be in all of these cases. However, Dr. Hamilton said none of these IOL calculators are able to factor in the back of the cornea at this time. Age and other factors Age is one of the factors that needs to be considered when determining the measurement of the cornea. Dr. Weikert said a study on 700 corneas recently published in the Journal of Cataract & Refractive Surgery looked at this and measuring both the front and back surface of the cornea. "We found that as people age, the astigmatism on the front surface of the cornea tended to change from with-the-rule to against-the-rule, which is generally accepted," he said. However, he said that the back of the cornea adds effective againstthe-rule astigmatism that changes minimally with age. This is important to know because it can cause an error in measurement. Dr. Weikert said this could cause an overestimation of with-therule astigmatism in a younger person if the against-the-rule astigmatism from the back of the eye is not being measured. Similarly, the astigmatism could be underestimated in an older person if only measuring the front of the cornea. Although some of these differences may only be less than a diopter, Dr. Weikert said it's important to have accurate results, especially for toric lenses. "Basically, against-the-rule becomes more common with age," Dr. Hamilton said. In 80% of eyes, the back surface of the cornea has a with-the-rule shape, which subtracts from with-the-rule astigmatism on the anterior surface and does not change with age, he said. Leaving room for change or differences in measurements Despite wanting to have accurate and exact measurements for total astigmatism, surgeons sometimes have to adjust and leave room for change. Dr. Weikert said he will aim for a bit of with-the-rule astigmatism EW FEATURE 41 after cataract surgery to account for the against-the-rule shift that happens to many over time. Toric IOLs will generally be chosen to correct a specific amount of astigmatism, but this can change as patients age. One of the reasons that IOL calculators and measurement of posterior corneal astigmatism are so important is because they help a surgeon be even more accurate and know exactly the amount of astigmatism that needs to be factored in. Knowing surgically induced astigmatism and posterior corneal astigmatism can help obtain an even more accurate measurement. "I think industry is motivated to continually improve the accuracy of those measurements," Dr. Weikert said. Similarly, Dr. Hamilton said you could end up with very little expected residual astigmatism when using these toric IOL calculators, but in order to know for sure if the measurements are correct or if you will be overcorrecting or undercorrecting the astigmatism, the posterior corneal astigmatism needs to be considered. EW Editors' note: Dr. Hamilton has financial interests with Ziemer, Alcon, and AMO. Dr. Weikert has financial interests with Ziemer. Contact information Hamilton: Hamilton@jsei.ucla.edu Weikert: mweikert@bcm.tmc.edu Poll size: 302 EyeWorld Monthly Pulse EyeWorld Monthly Pulse is a reader survey on trends and patterns for the practicing ophthalmologist. Each month we send a 4-6 question online survey covering different topics so readers can see how they compare to our survey. If you would like to join the current 1,000+ physicians who take a minute a month to share their views, please send us an email and we will add your name. Email daniela@eyeworld.org and put EW Pulse in the subject line; that's all it takes. Copyright EyeWorld 2013