EyeWorld Asia-Pacific March 2015 Issue

March 2015 IOL Calculations 21 EWAP SECONDARY FEATURE continued on page 22 IOL power calculations and biometry by Michelle Dalton EyeWorld Contributing Writer Physicians offer their thoughts on potential formulas S ince the introduction of the IOL, researchers and clinicians alike have been trying to determine the best formula to use to implant the best possible lens for a particular patient, based on numerous characteristics that include refractive error, axial length, ocular comorbidities, and previous surgery. Improved conditions for accurate IOL power calculation include the advent of small incision surgery, corneal scanning devices, IOL manufacturing improvements, laser biometry, and enhanced capsulorhexis techniques. Determining which of the many potential formulas is best depends on all the above, experts say. “All formulas are still far from perfect, as the rate of eyes within 0.5 D of the refractive target is only about 75%,” said Giacomo Savini, MD , G.B. Bietti Eye Foundation, Rome, Italy. With one-quarter of patients AT A GLANCE • The Fyodorov, Colenbrander, and Binkhorst formulas were among the first, created in the late 60s and early-to-mid 70s. • Differing axial lengths require different formulas. • Fourth-generation formulas used today include the Holladay II, Preussner, and Olsen. left with a suboptimal result, “I am investigating the role of corneal asphericity and lens thickness even in thin lens third-generation formulas,” he said, adding that he presented some data on the topic at the 2014 ASCRS•ASOA Symposium & Congress. 1 IOL power calculation formulas started as theoretical, with the Fyodorov, Colenbrander, and Binkhorst formulas in the late 60s and early-to-mid 70s, said Thomas Olsen, MD , University Eye Clinic, Aarhus, Denmark. The fourth- generation formulas used today include the Holladay II, Preussner, and Olsen, he said. Dr. Olsen is the author of the Olsen formula and the originator of the PhacoOptics PC software (IOL Innovations, Aarhus, Denmark) using the newly described C-constant approach for the prediction of the IOL position. Normal eyes Healthy eyes (those without any comorbidities or previous surgery) can have fairly accurate readings from the third-generation formulas, including Hoffer Q, Holladay 1 and SRK/T, Dr. Savini said. “I also rely on the Haigis formula, although I find it difficult to optimize its constants by myself. I am interested in investigating whether the C-concept by Dr. Olsen will carry any advantage, but until evidence of any advantage from different studies is available, I will go on with the standard formulas,” he said. A modern IOL power calculation formula should fulfill a number of requirements in order to be safe and effective, Dr. Olsen said, including that it should be accurate in “normal eyes.” “In fact, almost any formula would work within the ‘normal’ range by establishing average values for the outcome based on the clinical environment,” he said. For example, the SRK I formula used “little optical modeling” and was effective in the normal range, but “not so in the extreme range.” Noting that modern diagnostic instruments can provide significantly more data to help determine which lens would be best for an individual patient also means the term “normal” may need to be explored further, Dr. Olsen said. “Do we limit it to normal axial length? Normal K-reading? Normal anterior segment? What about the posterior curvature of the cornea?” he said. “The challenge is to take advantage of the more detailed information in our clinical practice.” Ray tracing software allows accurate no-history IOL power calculation even in cases like decentered radial keratotomy with residual corneal astigmatism. Source: Giacomo Savini, MD