EyeWorld India March 2018 Issue

by Stefanie Petrou Binder, MD EyeWorld Contributing Writer Limiting prediction error to improve IOL power accuracy IOL power calculations using state-of-the-art methods for normal and complex eyes have reached high levels of reliability; however, certain limitations to accuracy are inherent O ne of the challenges eye surgeons encounter when calculating IOL power is that no single diagnostic modality or formula can be applied to all eyes. Oph- thalmic surgeons need to take different approaches to determine IOL strengths for phakic and pseu- dophakic eyes, eyes with astig- matism, long and short eyes, as well as eyes that have undergone previous refractive surgery. The success and safety of IOL implantation reflects decades of evolution in surgical technique and measurement methods, as well as in understanding of the sources of potential error. Reaching prediction limits “Precise measurements and IOL calculation formulas are essen- tial to achieve the high accuracy refractive outcomes demanded of premium IOLs,” said Jack Hol- laday, MD, MSEE , clinical pro- fessor of ophthalmology, Baylor College of Medicine, Houston, who spoke on the topic at the 2017 ASCRS•ASOA Symposium & Congress. “As we stand today, the prediction error ‘floor’ is associ- ated with a standard deviation of 0.31 D and a mean absolute error of 0.24 D. These values are close to the best that we can achieve. With these values, 90% of our patients would achieve within a half di- opter of their predicted refraction for ‘all comers,’ meaning all cases coming to our practice.” Dr. Holladay explained that prediction error, the difference between the actual and predicted refraction, was always Gauss- ian (normal distribution). “The average outcomes of standard deviation for doctors today is 0.50 D (not 0.31 D above),” he said. “Gaussian statistics tell us that if something has a standard devia- tion of 0.50 D, 67% of cases are within that (± 0.50 D). The mean absolute error is always 80% of the standard deviation, so if the average standard deviation is 0.50 D, the mean absolute error, on average, is 0.40 D. Good surgical outcomes today show a standard deviation of 0.40 D and a mean absolute error of 0.32 D, which corresponds to 78% of patients achieving within 0.50 D of the predicted refraction. The best I have ever seen is a standard devia- tion of 0.31 D and a mean absolute error of 0.24 D, which is equiva- lent to 90% of ‘all comer’ patients achieving within 0.50 D.” In a recent study, 1 Dr. Holladay and co-researchers investigated the standard deviation associated with IOL power prediction in current surgical practice, incorporating data from 13,301 patients and more than 140 surgeons. They found that the standard deviation in this large collective was 0.4439 D and mean absolute error was 0.34 D, corresponding to 77% of cases that achieved within 0.50 D predicted refraction. Sources of error Although prediction outcomes nearing 80% on average are very satisfactory for most surgeons, there is some room for improve- ment to reach 90%, which might be achieved through a better understanding of potential sources of error. A study that investigated the sources of error in IOL power calculations by Sverker Norrby identified 16 variables that con- tributed to prediction error, the first five of which explain almost 99% of error. 2 The highest source of error is from the prediction of the effective lens position (ELP)— the prediction of where the IOL is ultimately placed, for which precise anatomic eye measure- ments can greatly reduce error. It accounts for 35% of the prediction error, with a standard deviation of 0.31 D and a mean absolute error of 0.24 D. ELP was first predicted by Binkhorst in the early 1980s using only the measurement of the axial length. Today, seven variables have been identified as influencing the prediction of ELP, among them: axial length, K, anterior chamber depth, lens thickness, horizontal white-to-white, age, and pre-cata- ract refraction. The second most relevant source of error according to the study’s outcomes is refraction, ac- counting for 27% of the prediction error. Axial length measurement accounts for 17%, the corneal power accounts for 12%, and the pupil for 8% for a standard devia- tion of 0.391 D and mean absolute error of 0.31 D of IOL prediction error, the last of which is not usu- ally measured or used in power calculations, according to Dr. Holladay. Using variables to predict IOL power Surgeons have four IOL power selection methods at their dis- posal, which include different IOL vergence formulas for thin and thick lenses, ray tracing, and the neural network. Each method uses a combination of variables to predict IOL power as accurately as possible. Thin IOL vergence formulas include the Binkhorst formula, Holladay, and SRK. They require five measurements, including anterior K, refraction, IOL power, ELP, and axial length. Thick IOL vergence formulas include the Holladay 2, Olsen 2, and Barrett 2. Nine variables are required here, which include those used for IOL 28 March 2018 EWAP CATARACT/IOL