Eyeworld

67 EW REFRACTIVE November 2017 by Stefanie Petrou Binder, MD, EyeWorld Contributing Writer better measurements and advanced machines based on swept source OCT. The final ingredient is how best to incorporate this new level of accuracy into existing formulas. To this end, I have modified my own formula to incorporate total ker- atometry as an option in the online Barrett Universal Toric Calculator v2.0." He elucidated that while swept source biometry could enhance both post-refractive and toric cylinder calculations, it also has the poten- tial to influence spherical power prediction. This would involve the use of the same formula, leaving the effective lens position component for prediction intact, and plugging in the actual measured corneal values in the Gaussian solution for the vergence prediction. The Barrett Universal II seems to be amenable to this small modification that allows it to include the extra information for spherical power predictions. According to recent data from a paper presented at the 2017 ASCRS•ASOA Symposium & Con- gress that compared five different power calculation formulas in 34 patients, refractive outcomes with the Barrett Universal II were better than the rest, 85% within 0.5 D and 94% within 0.75 D, in predicting the spherical equivalent. Using the same data set for spherical equivalent using true keratometry, the percent- age of cases within 0.5 D improved to 91%, and 97% were within 0.75 D. "This is likely reaching the ceiling for prediction accuracy of formulae," Dr. Barrett said. "All the formulas do benefit from total keratometry. The ben- efit, however, is more noticeable with the Barrett Universal II. The potential for post-refractive, toric cylinder, and even spherical power prediction, with this new method of measuring the cornea, is something that is going to have a significant impact on how we measure and how we predict outcomes in the future," Dr. Barrett said. EW Editors' note: Dr. Barrett has no finan- cial interests related to his comments. Contact information Barrett: graham.barrett@uwa.edu.au second. "You can take an optical slice and rotate it 360 degrees and reconstruct the 3-D architecture of the anterior segment," he explained. "With traditional keratometers, we measure the anterior surface, and the power of the cornea is provided by the so-called keratometer index, but this is only an assumption," Dr. Barrett said. "A more sophisticated approach is to utilize a Gaussian solution for the power of the cornea, using assumed values for the posteri- or radius and the posterior power of the cornea. Now with swept source OCT, you can take that equation and put in the actual values for the radius of the posterior cornea, and corneal thickness. This is called true or total keratometry (TK) in the IOLMaster 700. Standard Ks and TK values are highly reproducible and highly repeatable, for both pow- er and axis. What is important is that although there are individual differences between the TK and the standard K, collectively, these differ- ences are within 0.25 D for 95% of cases. So you can use your standard constants when utilizing this new value. As things stand today, all the ingredients are there. We have moves back and forth, changing the interference pattern. This allows the surgeon to detect the eye structures and measure important parame- ters, such as the axial length. Swept source is very similar, but instead of having a moving reference mirror, it uses a full-spectrum super lumi- nescent diode light source that is swept back and forth, changing the interference pattern. "Surgeons can use this technology to measure axial length with high precision; you can also now measure in the same system, simultaneously, the central corneal thickness, anterior cham- ber depth, and lens thickness," Dr. Barrett said. The IOLMaster 700 (Carl Zeiss Meditec, Jena, Germany) uses a com- bination of swept source biometry and the latest Barrett formulas that take the posterior corneal surface into account. It has a scan depth of 44 mm, a scan width of 6 mm, a fixation scan width of –1 mm (on retina), with a resolution in tissue of 22 µm. According to Dr. Barrett, it is not only the extra precision of the technology that is important, but the speed of acquisition, allowing approximately 2,000 A scans per Swept source OCT makes its way into practice S teady advances in oph- thalmologic devices and technologies have been ac- companied by a flourish of heightened expectations, shared by eye surgeons and patients alike. The best way to meet these expectations is through painstaking precision. Accurate measurements, particularly for axial length and corneal power, are essential for the precise and reliable calculation of IOL power needed to achieve desired postoperative refractions. The tech- nology behind swept source OCT allows for extreme measurement accuracy and is likely to have an impact on refractive predictions. Swept source technology is currently incorporated into many of the new generation biometers, with implications for how physicians can now measure the cornea. This new technology provides eye surgeons with "true keratometry," as op- posed to standard keratometry that assumes the power of the cornea as provided by the keratometer index. Graham Barrett, MD, Lions Eye Institute and Sir Charles Gairdner Hospital, Nedlands, Western Austra- lia, spoke on the benefits of swept source OCT in cataract surgery at the 2017 ASCRS•ASOA Symposium & Congress as part of a session titled, "Optimizing Outcomes: What Am I Doing Differently?" "Swept source OCT is the aspect of my practice that I am and will be doing differently," Dr. Barrett said. "It has the potential to effectually change our surgical outcomes. There are new, expensive technologies that have been disappointing. With the new technology behind swept source OCT, however, this is not the case. I think it will be a major advance in predicting refractive out- comes after cataract surgery." Swept source OCT is not very far off from partial coherence interfer- ometry (PCI). The development of PCI in 2000 was one of the most important landmarks in the histo- ry of keratometry, leading to the technology that we have today. The basis for PCI optical biometry is a modified Michelson interferometer, which has a reference mirror that Impacting refractive outcome predictions Presentation spotlight The Barrett Toric Calculator soon to be released with a measured PCA option Source: Graham Barrett, MD

• Blank Page
• Blank Page (1)