February 2015

 

COVER FEATURE

 

IOL calculations

Barrett Toric Calculator aims for accurate outcomes


by Vanessa Caceres EyeWorld Contributing Writer

 
   
Barrett Toric Calculator

Barrett Toric Calculator Source: Graham Barrett, MD

New formula considers lens position, posterior corneal curvature

In the quest for better surgical outcomes for toric IOLs, Graham Barrett, MD, has created the Barrett Toric Calculator. Dr. Barrett is a consultant ophthalmic surgeon at Sir Charles Gairdner Hospital, Nedlands, Western Australia; president of the Australasian Society of Cataract & Refractive Surgeons; and president of the Asia-Pacific Association of Cataract & Refractive Surgeons (APACRS). His work on the calculator began as part of a reflection on improving cataract surgery outcomes.

Of all the things that have happened over the last 10 to 15 years in cataract surgery, many of them make you wonder about the huge amount of money spent when the improvement for patients is modest or debatable, he said. With a toric lens, its quite different. You really change what you can do for the patient. However, using a toric lens requires more legwork for the surgeon than a conventional IOL. Its more demanding than a non-toric lens. If you want to get the best outcomes, you have to think about 4 things: 1) the device used to measure the cornea, 2) interpreting those measurements, 3) a measurement of prediction thats accurate, and 4) when you have the right method, you need something to let you put the lens on the right axis, he said. Those 4 concepts put the Barrett Toric Calculator in context.

About the calculator

Surgeons may feel bewildered by the sheer number of toric IOL calculators available, Dr. Barrett said. He said that his is unique because it takes into account the posterior cornea and considers lens position for each individual patient versus using what is known about the average eye. My formula uses the Universal II, which is a method of predicting IOL power to work out where the lens is and uses that to calculate what is the effect of the cylinder power at the cornea, he said. Dr. Barrett also devised the Universal II formula.

Dr. Barretts formula considers the thickness and shape of the lens as well, which provides a more sophisticated way of predicting and translating the cylinder power, he said. The formula is able to predict posterior corneal curvature without actually measuring it. The reason it can do so is I looked at all the observations that people have made, especially Doug Koch [MD, Houston], about the posterior cornea, he said. It has long been known that the posterior cornea tends to have a half diopter of against-the-rule effect on the astigmatism of the eye, he said. Although that has been known since the late 1800s, it has not been accounted for in other toric lens calculations, Dr. Barrett said. The reality is, if you have against-the-rule astigmatism, its aligned in the vertical meridian. I wondered why that is. Its odd and not obeying the rules you would expect from an optical surface. My basic theory is that the normal cornea tends to be elliptical. The diameter is wider in the horizontal meridian. Therefore, that means the curvature of the posterior cornea has to be steeper than the vertical. That means you have against-the-rule astigmatism because the posterior cornea is convex. That also explains why almost all corneas exhibit against-the-rule behavior, he said. Using that background, Dr. Barrett was able to calculate a measurement for the diameter of the cornea for each patient and calculate the posterior corneal curvature.

Looking at the evidence Barrett Toric article summary

Because the Barrett Toric Calculator is relatively newit has only been online since the summer of 2013clinical evidence to support it is just now reaching peer-reviewed journals. In a submission that is in press, Dr. Barrett said his calculator was found to be within half a diopter of residual astigmatism 75% of the time compared with only 33% of the time for the Alcon AcrySof calculator (Fort Worth, Texas) or the Holladay calculator. By adding the Baylor nomogram, the accuracy of the Alcon and Holladay calculators increased to 50%, he said. The [Barrett Toric] theoretical method is doing significantly better than actually measuring the posterior cornea, he said.

The Barrett Toric Calculator is available on the websites of ASCRS (www.ascrs.org/barrett-toric- calculator) and APACRS (apacrs.org). The APACRS website also features Dr. Barretts True-K formula and Universal II formula.

The calculator was also recently built into the LENSTAR LS900 (Haag-Streit, Koniz, Switzerland).

Dr. Barrett encourages surgeons to think carefully about other variables that affect their toric IOL use, such as selecting one primary tool to measure the cornea and then always using secondary tools, a concept he learned from Warren Hill, MD, Mesa, Ariz. You may have the LENSTAR or other tool for your primary measurement, but you also have to have a secondary tool to confirm your primary is giving you the correct reading, he said. Warren uses the analogy of a pilot who doesnt have just one instrument. The pilot has other instruments to make sure the primary instrument is correct.

Editors note: Dr. Barrett has no financial interests related to this article.

Contact information

Barrett: graham.barrett@uwa.edu.au

Barrett Toric IOL Calculator aims for accurate outcomes Barrett Toric IOL Calculator aims for accurate outcomes
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