EyeWorld Asia-Pacific June 2018 issue

June 2018 EWAP REFRACTIVE 57 Ray tracing by Stefanie Petrou Binder, MD EyeWorld Contributing Writer The battle continues be- tween optical and statisti- cal IOL power calculation formulas A s biometry measure- ments become more accurate, physical optics can be applied with greater success than ever. Special- ists think that IOL power calcula- tions break down rather simply into predicting the actual physical lens position (ALP) and calculating the refractive status of the pseu- dophakic eye. In a presentation at the 2017 ASCRS•ASOA Symposium & Congress, Thomas Olsen, MD , Aarhus University, Denmark, brought attendees up to speed on where we stand now regarding IOL power calculations using ray trac- ing. “Ray tracing is more precise than first order approximations used in thin lens formulas that rely on K readings and axial length, which are used by most cataract and refractive surgeons,” Dr. Olsen said. “The effective lens position (ELP), however, is a virtual dis- tance, and the cornea and IOL are assumed single planes. The first IOL power calculation formula by Fyodorov is still in use by the SRK/T and Holladay formulas, giving us a method to estimate the lens plane. But accuracy has greatly increased over the 50 years since he developed his formula.” Sources of error “Biometry is getting so accurate that it is no longer a significant source of error,” Dr. Olsen said. “Ultrasound error was perhaps ±0.20 mm and laser error ±0.02 mm. This translated into a refrac- tive error of 0.05 D with the laser, which is insignificant. IOL power calculation error comes from other sources, including axial length, K reading, and ELP/ALP. Next to the IOL formula, keratometry is key. It can be a source of error. We need to take readings from at least two instruments and use topography to check the axis.” Paraxial ray tracing has been used to calculate the total corneal power by adding the anterior and posterior corneal surfaces and fac- toring in a correction. This left the assumed keratometer index as the only variable. Most surgeons still use 1.3375, but according to Dr. Olsen, 1.3315 is a better estimate, making a large impact on calcula- tions. “The Pentacam [Oculus, Wet- zlar, Germany] offers data on cur- vature in two principle meridians and can map the whole elevation matrix,” he said. “Using Pentacam data and importing it in optical engineering software allows ray tracing on any surface with given physical dimensions. Ray tracing is used to track light based on Snell’s law. We constructed a front and back surface of the cornea using this data, which allowed us to model the cornea. As you know, power is the inverse of the focal length, so we can get the effective corneal power out of these calcula- tions.” Dr. Olsen explained that trac- ing corneal power by ray tracing has revealed the K reading of 1.3375 as 1 D higher than the true power of the cornea, as well as higher than other Pentacam met- rics, which correlated better with ray tracing readings. All formulas that are based on standard K read- ings will have a hyperopic error in eyes with low IOL power (i.e., long eyes). 1 Calculating IOL power inside the pseudophakic eye According to Dr. Olsen’s research, IOL power measurements in the pseudophakic eye are possible if we know the refraction, K reading, axial length, position of the im- plant, and know something about the configuration of the IOL. Back calculating using ray tracing can give the IOL curvatures and power. In a study of 767 pseudophakic eyes that described a method for back solving the power of an IOL in situ based on laser biometry and ray tracing analysis, he accurately described the optics of the pseu- dophakic eye. 2 “We can measure this by LEN- STAR [Haag-Streit, Koniz, Swit- zerland] biometry, which is very accurate,” Dr. Olsen said. “The im- planted IOL power correlated with the calculated IOL power. There was no bias with axial length or IOL power. There is evidence that the optics of the pseudophakic eye Raw 3-D data from Pentacam tomography scan of front and back surface of the cornea imported by Zemax ray tracing analysis software and examined for best focus (sharpest point spread function) with an IOL inserted behind the pupil Source: Thomas Olsen, MD continued on page 58