Eyeworld

EW REFRACTIVE 138 March 2018 Presentation spotlight by Stefanie Petrou Binder, MD, EyeWorld Contributing Writer Nevertheless, the error associat- ed with lens position is significant, necessitating a reliable prediction of the ALP. Dr. Olsen proposed a meth- od to directly measure the dimen- sions of the crystalline lens in an- other study of 2,043 cases in which he evaluated the accuracy of the C constant for ray tracing assisted IOL power calculations using the Olsen formula, compared with the Haigis, Hoffer Q, Holladay 1, and the SRK/T formulas. He found that the IOL tended to locate within the empty capsule bag at a certain location that could be described with a constant. 3 "Modern OCT techniques confirm the concept of the C con- stant," Dr. Olsen explained. "We demonstrated IOL position using the C constant in a fairly consistent pattern in 1,730 eyes and compared the predicted to the observed value of ACD with a correlation coeffi- cient of r=0.9. Almost 90% of the cases were within ±0.25 mm. This is something we can use in IOL power calculations. We got rid of the K reading and the axial length, and we replaced the prediction of the IOL position by the anterior chamber depth and the lens thickness. The C constant defines the IOL position in terms of the crystalline lens anato- my, not dependent on the K reading or the axial length. This is especially important in post-LASIK cases. The C constant defines the position of the IOL as a fraction of capsular bag thickness. Our work shows good outcomes for different axial lengths in short but especially long eyes." Recent studies have shown good results using Dr. Olsen's method. In one investigation that compared the prediction accuracy in nine IOL power calculation formulas using two optical biometers, the Olsen formula was the most accurate with optical low coherence reflectometry (OLCR) measurements and signifi- cantly better than the best formula with partial coherence interferom- etry (PCI) measurements, regard- less of AL. 4 In another study on prediction accuracy of pre-installed formulas on two optical biometers, Wetzlar, Germany] offers data on curvature in two principle meridians and can map the whole elevation matrix," he said. "Using Pentacam data and importing it in optical en- gineering 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 calculations." Dr. Olsen explained that tracing 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 metrics, which correlated better with ray tracing readings. All formulas that are based on standard K readings will have a hyperopic er- ror 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 implant, and know something about the config- uration of the IOL. Back calculating using ray tracing can give the IOL curvatures and power. In a study of 767 pseudophakic eyes that de- scribed 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 pseudophakic eye. 2 "We can measure this by LENSTAR [Haag-Streit, Koniz, Switzerland] biometry, which is very accurate," Dr. Olsen said. "The implanted 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 can be accurately described using exact ray tracing and modern biometric techniques." 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 translat- ed into a refractive 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, keratome- try is key. It can be a source of error. We need to take readings from at least two instruments and use topog- raphy 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 calculations. "The Pentacam [Oculus, The battle goes on between optical and statistical IOL power calculation formulas A s biometry measurements become more accurate, physical optics can be applied with greater suc- cess than ever. Specialists think that IOL power calculations break down rather simply into predicting the actual physical lens position (ALP) and calculating the refractive status of the pseudophakic eye. In a presentation at the 2017 ASCRS•ASOA Symposium & Con- gress, Thomas Olsen, MD, Aarhus University, Denmark, brought attendees up to speed on where we stand now regarding IOL power calculations using ray tracing. "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 ef- fective lens position (ELP), however, is a virtual distance, and the cornea and IOL are assumed single planes. The first IOL power calculation for- mula by Fyodorov is still in use by Ray tracing 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

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