EyeWorld Asia-Pacific March 2018 Issue

power predictions in the thin ver- gence formulas plus information on posterior K and IOL radii and thickness. Ray tracing has huge potential, using the above variables with the addition of topographic information from the front and the back of the cornea, as well as the radii and asphericities of the front and back of the IOL (10+ vari- ables). Neural network works with the five variables (axial length, K, refraction, anterior chamber depth, and IOL power), computing 120 possible combinations between the variables to internally predict ELP precisely. According to Dr. Holladay, screening is a step the surgeon needs to take to improve measure- ment error. “Data screening identi- fies measurement error and allows the surgeon to repeat measure- ments, when necessary. Binocular screening is important especially with axial length differences of more than 0.3 mm, K differences of more than 1.0 D, and IOL power differences greater than 1.0 D. These outcomes should go up as red flags, and the measurements should be repeated. Monocular screening can identify axial length signal/noise ratios (S/N ratio) of less than 2.0 or if the K reading has a standard deviation in excess of 0.20 D. When these numbers pop up, those measurements should be repeated in patients because those are red flags,” he explained. “If we knew everything about the optical system exactly, then Snell’s law of refraction and ray tracing would be perfect in their predictions,” Dr. Holladay said. “But current accuracy limitations including axial length, K, and refraction measurement errors can only be improved by improving the technology. Corneal posterior surface measurements are evolv- ing. IOL radii, asphericity, and thicknesses are proprietary. You can’t get that from manufacturers for use in ray tracing, and that is something that we should change. Prediction of ELP is not exact, and it is continually evolving. It will never be exact, however, because it is a prediction.” According to Dr. Holladay, ray tracing will ultimately provide eye surgeons with the greatest accu- racy, once IOL properties are made available and the posterior cornea can be measured reliably. He high- lighted screening techniques to identify potential errors preopera- tively to help direct patient flow. He also thinks it is important that all surgeons personalize their lens constant to get the best results in their patients. EWAP References 1. Melles RB, et al. Accuracy of intraocular lens calculation formulas. Ophthalmol. 2017 Sep 23. Epub ahead of print. 2. Norrby S. Sources of error in intraocular lens power calculation. J Cataract Refract Surg. 2008;34:368–76. Editors’ note: Dr. Holladay has no financial interests related to his com- ments. Contact information Holladay: holladay@docholladay.com EWAP CATARACT/IOL March 2018 29 Cornea Ed Ad 1/2 vertical