EyeWorld Asia-Pacific December 2020 Issue

EWAP DECEMBER 2020 9 FEATURE New on the Barrett Suite 2020 T he Barrett Suite of IOL Calculators provides surgeons around the world with some of the most accurate IOL calculation formulas available today. The Suite’s creator, Graham Barrett, MD, constantly upgrades the system to address different challenges faced by surgeons. Post-refractive PCA The first new feature addresses the challenge of predicting refractive outcome after cataract surgery in patients who have undergone previous LASIK, PRK, or RK. Tissue ablation following these procedures results in an altered, more aspheric contour. However, according to Dr. Barrett, the major reason for the difficulty in prediction is the altered relationship between the anterior and posterior radii of the cornea. Dr. Barrett developed the True K formula to improve outcome prediction in this context and the formula has proved to be more accurate than other methods for patients that have undergone laser correction for myopia, 1 hyperopia, 2 and compared with other methods. 3 Last year, Dr. Barrett modified the True K formula to incorporate the measured posterior cornea—a modification he called the True K TK. To access this method online via the APACRS website (www.apacrs.org ), first enter the data and select calculate. Select the measured posterior cornea option which then becomes available. On the new page that appears, select the device and enter the measured posterior cornea—the power or the radii as listed in the IOLMaster 700 as PK1 and PK2 or corresponding values from other devices such as Scheimpflug devices. Select calculate, and when select the universal formula from the menu above, IOL power and predicted refraction will now be based on the True-K formula but with the standard True K (63.3% within 0.5 D) compared to other no- history solutions (Haigis L 31.7%, Haigis TK 50.0%, Shammas 46.7% within 0.5 D) (Figure 1). A larger series of 72 eyes that had undergone previous myopic and hyperopic refractive surgery 4 confirmed that True K with inclusion of posterior cornea provided the most accurate predictive option in patients who had prior myopic and hyperopic measured posterior cornea— that is, the True K TK. Looking at data from 60 eyes with a history of previous myopic LASIK, combining his cases with those of Michael Lawless in Australia and Tun Kuan Yeo in Singapore, Dr. Barrett compared the error in predicted refraction of the True K TK to the standard True K formula, Haigis L, Haigis TK, and Shammas formula. He found that the spherical power was more predictable with the True K TK (70.0% within 0.5 D of predicted refraction) and the laser correction and undertook cataract surgery without prior refractive surgery information. Version 2 of the True K Toric calculator now also uses either predicted or theoretical posterior corneal astigmatism (PCA) or measured PCA, as well as an integrated K calculator that allows entry of anterior cornea measurements from three different devices to calculate a new integrated K or median vector for toric IOL power calculation. “This is particularly helpful when the K of different Figure 1. Spherical power prediction comparing different IOL power formulas. Source: Graham Barrett, MD From the lecture by Graham Barrett, MD, Australia Professor Lions Eye Institute, Perth