EyeWorld Asia-Pacific September 2013 Issue

65 EW PHARMACEUTICALS Septemebr 2013 the cystotome which many of us in the region would be familiar in the adult scenario,” he said. Using forceps, he said, “grasp and regrasp multiple times”, lifting the capsule up toward the microscope. “Lifting up, you can control the size and you can fashion the rhexis depending on your IOL fixation—in-the-bag or capture,” he added. “Frequent grasping will avoid disruption of the anterior vitreous face and will allow you to achieve the correct size, depending upon your strategy.” In very small eyes, the standard technique may be difficult, and Dr. Vasavada suggests going through the pars plana and performing PCCC with the forceps curve reversed. This technique, he said, allows surgeons to stabilize the environment, enough even for in- the-bag implantation that might otherwise be difficult in very small eyes. Dr. Vasavada also recommends using triamcinolone at various stages in pediatric cases, and after PCCC in adult cases to visualize the anterior vitreous face, particularly strands that may have entered the anterior chamber. While common in pediatric cases, PCCC has various uses in adults, from clearing the visual axis, particularly in cases with densely fibrotic posterior capsules (for which Dr. Vasavada recommends using microincision scissors), to converting a posterior capsule rupture into a stable bag structure for IOL implantation. Intact or left open with a rhexis, the posterior capsule influences the outcome of IOL implantation. Dr. Barrett wrapped up the combined symposium by discussing the role of the posterior capsule in effective lens position and predicting IOL power. “We all have to try and be as accurate as possible because that is what helps predict our refractive outcome,” said Dr. Barrett. “That’s what our patients judge us on—not just having a complication-free surgery, but actually what is the outcome of this surgery.” Modern instruments, he said, allow surgeons to measure parameters such as axial length and keratometry “quite accurately”— that often leaves one variable: the position of the posterior capsule, which determines effective lens position. Dr. Barrett considered three ways to improve prediction of effective lens position: through surgery, through more accurate measurement of the position of the posterior capsule, and through calculation. In terms of surgery, there is no doubt that the femtosecond laser produces a more circular capsulorhexis than manual surgery, though he noted that at 1 month the standard deviation is “not as tight…as time goes on after surgery, that round rhexis may not necessarily stay round.” But what’s important to understand is how the rhexis influences the position of the posterior capsule and the lens implant. Dr. Barrett cited the results of one study that showed the difference in lens position between a 4-mm rhexis (on the optic) and a 6-mm rhexis (completely off the optic): “The difference with this quite extreme situation is only 0.1 mm or 0.13 D.” The second option, more accurate measurement of the position of the posterior capsule, is made possible by modern instruments, perhaps most famously the ORA intraoperative wavefront aberrometer (WaveTec Vision, Aliso Viejo, Calif., USA). However, Dr. Barrett noted that the correlation between intraoperative refraction and 1-week postop autorefraction is only “moderate, fairly weak”. “This may be good for an expected large error such as postrefractive surgery, but not at the 0.1-, 0.25-D accuracy that we’re after,” he said. Meanwhile, he said, Oliver Findl, MD , Vienna, Austria, is currently at work investigating the potential of intraoperative OCT— work Dr. Barrett said has thus far produced “quite encouraging” results. The final option is to improve prediction through optimized mathematical formulas. Dr. Barrett developed his own Universal and True-K formulas. Dr. Barrett developed the Universal formula to be applicable throughout a wide range of different axial lengths. It is, he said, basically a thick lens formula in which both the effective lens position as determined by the lens power and the shape of the lens are variables taken into account. “This formula is able to account for negative IOLs in calculating extreme high myopes, which is not always the case with other formulae,” he said. The True-K formula, on the other hand, is a variant of the Universal formula for post-LASIK patients. “Using prediction today, we are able to manage axial lengths— short, long, medium and we are good at predicting patients post refractive surgery,” said Dr. Barrett. “The final challenge or the last frontier is toric calculations, and this is more complicated because you have magnitude as well as direction of your error.” Dr. Barrett believes that while toric IOLs are often characterized as premium lenses, they should in fact be considered a standard of care. He thus developed his own toric calculator—again based on the Universal formula—that calculates toric power on the corneal plane, and contains an algorithm to account for the posterior cornea. Using this formula, he said, 72% of patients end up with less than 0.5 D of error. Dr. Barrett’s Universal, True-K and toric formulas are available on the recently revamped APACRS website (www.apacrs.org ). EWAP Editors’ note: The doctors have no financial interests related to the content of this article.