EyeWorld Asia-Pacific March 2015 Issue

IOL Calculations March 2015 22 EWAP SECONDARY FEATURE Views from Asia-Paci c FAM Han Bor, MD Senior Consultant & Head, Cataract & Implant Service The Eye Institute @ Tan Tock Seng Hospital 11 Jalan Tan Tock Seng, Singapore 308411 Tel. no. +65-6357-7726 Fax no. +65-6357-7718 famhb@singnet.com.sg T he importance of IOL power calculations for cataract surgery is similar to that of preoperative refraction for refractive surgery. Good outcome in refractive surgery hinges on good refraction. Similarly, accurate biometry and IOL power calculations have a great impact on good postoperative refractive outcome in cataract surgery. Most IOL formulae today work well but none of them are perfect. There is still room for improvement. The improvement in IOL power calculations is in tandem with technological improvement in biometry and enhanced understanding in visual optics. Precision in biometry was signi cantly enhanced with the introduction of partial coherent interferometry or optical biometry. The latest in optical biometry utilizing swept-source OCT will improve the precision of biometry even further. Repeated measurements from swept-source OCT is even more consistent than partial coherent interferometry. Most formulae work well in normal eyes. It is at the two extremes that the performances of the formulae diverge. Haigis formula has three constants. I nd triple-optimized Haigis works well across a wide spectrum of axial lengths. When I need to implant IOLs for which I do not have my own personalized Haigis constants, I can rely on Haigis triple-optimized constants on the ULIB. I nd the Haigis constants listed on ULIB functionally accurate. Haigis is useful this way. The only problem with the Haigis formula is that the ULIB constants are generated for IOLMaster and cannot be simply ported over to biometry done with ultrasound. I also nd the Barrett Universal formula works very well and reliably across most axial lengths. It’s a real pity it’s not available on any biometry system and needs to be computed online risking typographical errors. For shorter eyeballs, I use Haigis (triple optimized) and Barrett Universal. For longer eyeball, I prefer to use SRK-T and Barrett Universal. For diverging (negative power) IOL, I prefer to use SRK-T but with an A-constant speci cally optimized for negative power range. For post-LASIK, Haigis-L and Shammas-PL are fairly reliable. Barrett True-K formula is another wonderful formula for post-refractive surgery. Intraoperative aberrometry is another exciting piece of technology that can enhance outcomes. It will not replace IOL power calculation but it will be a good adjunctive to enhancing postoperative refractive outcome. Ray-tracing is also an exciting approach to IOL power calculation. Ray-tracing should improve accuracy in all eyes particularly in atypical eyes and in post-LASIK eyes. Editors’ note: Dr. Fam has no nancial interests related to his comments. IOL - from page 21 Dr. Savini agreed, noting that differing axial lengths require different formulas. “For very short eyes I use the Hoffer Q formula, which has been shown to be the best in these cases by many studies,” Dr. Savini said. “For long eyes, I rely on the SRK/T for the same reason. In long eyes, I also follow the approach suggested a few years ago by Prof. Haigis, who dramatically changes the constants of the IOLs.” He also does not use the Holladay 2 formula as “it does not offer any advantage and it requires many more data [and time] points compared to the others,” he said. “I am also awaiting the results of the Hoffer H5 formula, which considers gender and race, but this will still take some time.” Non-virgin eyes Patients who present post-laser vision surgery, either LASIK or PRK, pose a different challenge, Dr. Savini said. Data he presented at the 2014 ASCRS•ASOA Symposium & Congress indicated the Savini method combined with double-K SRK/T, the Seitz/Speicher method combined with the same double-K SRK/T, the Masket method with SRK/T, and the Shammas no history method were the most accurate, he said. 1 “These methods allow me to have the same accuracy as with unoperated eyes, i.e., more than 70% of cases within 0.5 D of intended refraction,” Dr. Savini said. Dr. Savini does not change his formula of choice when other morbidities such as corneal disorders, diabetes, or glaucoma are present. Ray tracing The advantage of ray tracing is that it only requires Snell’s law and needs no other assumptions than the physical shape and refractive index of the material, Dr. Olsen said. This is “far more accurate than the first-order approximations used by the thin lens formulas, and it has the advantage over thick lens formulas that higher order aberrations (HOA) can be taken into account.” “[Ray tracing offers] full independence from perioperative data; surgeons just have to enter the axial length and software will predict the position of the lens, its power, and the final refraction, taking into consideration the corneal power and asphericity,” Dr. Savini said. Dr. Olsen added that ray tracing “is recognized as the most effective tool in optical engineering—it makes sense to use this technique on the eye.” “Most formulas only use a single constant, the A-constant or the anterior chamber depth [ACD] constant, which normally is derived from the actual postoperative refraction, back- calculating what the constant should have been to ensure an average prediction error of zero,” Dr. Olsen said. “When this ‘optimization’ has been accomplished, the performance of the formula is compared using metrics of the scatter like the standard deviation, the mean absolute error, or the median error.” But the C-constant defines the physical IOL position from the preoperative ACD and lens thickness, Dr. Olsen said, meaning it is not dependent upon the K-reading or the axial length. In a recent study, 2 Dr. Olsen and Peter Hoffmann, MD , evaluated 2,043 cases to determine the accuracy of the C-constant for ray tracing and found it a “promising concept.” In continued on page 25