EyeWorld Asia-Pacific September 2014 Issue

52 EWAP rEfrActivE September 2014 Views from Asia-Pacific WANG Zheng, MD Professor of Ophthalmology, Zhongshan Ophthalmic Center 54 Xianlie South Road, Guangzhou 510060, China Tel. no. +86-13903002594 gzstwang@gmail.com T opography-guided ablation has been used for a number of years in Asia. Its main application has been to correct problems after initial refractive surgery. Many reports show that it is effective for enlarging optical zone and correcting decentration and irregular astigmatism. However, there’s no standard method of doing it, and there is little scientific data to assess the results. So to some extent it is more art than science. There’s a learning curve. Surgeons need to learn how to pick the right map and how topredict refractive changes. Although the algorithm of the state-of-the-art systems has been improved in terms of predicting effect on refractive changes, the two-step method is often needed for complicated eyes. In many cases, there’s limited residual corneal stroma left for the treatment. When planning topo-guided treatment, it is necessary to leave some tissue for possible further refractive ablation. Topography-guided treatment requires very accurate and precise eye registration and tracking. For very distorted corneas, even slight offset may compromise the surgical outcome. Topography or tomography? Each has its advantages and disadvantages. Placido- based topography has better repeatability if tear film is normal. Schiempflug tomography gets a more comprehensive picture of the cornea, both anterior and posterior surfaces. But the repeatability of its curvature data is not as good as Placido-based systems. For virgin eyes, wavefront-guided ablation is theoretically a better approach because it takes the entire optical pathway into account, not only the cornea. But in reality, the current wavefront technology is so unreliable that sometimes the result becomes less predictable. Topography-guided ablation on virgin eyes is a relatively new application of this technology. Patients with preop asymmetrical cornea may benefit from it. But for those with symmetrical cornea there may not be advantages. More clinical studies are needed to compare topography-guided treatment with conventional and wavefront-guided treatments. Editors’ note: Prof. Wang has no financial interests related to his comments. The current - from page 51 Advantages and disadvantages “The advantage of topographies is that you can get very good repeatable data,” Dr. Cummings said, indicating that whole eye wavefront-derived data is simply not as repeatable. Outside the U.S., there are two systems that can generate topography- guided ablation, he said. One is based on Placido disk, the Topolyzer (Alcon, Fort Worth, Texas, U.S.); the other is based on Scheimpflug, the Oculyzer (Alcon). In the U.S. clinical trial, only the Topolyzer was used. “Even though they’re two different systems, they almost invariably come up with the same treatment plan,” Dr. Cummings said. With the Scheimpflug device, it does not matter if the eye is slightly dry, while the other is more dependent on a good tear film and has slightly less good data in the middle due to the scotoma caused by the centrally placed camera on the Topolyzer. Even though one system might be better than the other in a particular situation, when you look at the ablation profile that the maps have generated, they normally look identical, Dr. Cummings said. Where there is missing data, the systems usually fill in the gaps. The disadvantage with topography-guided ablation, Dr. Cummings said, is that especially in a complicated eye, the key treatment is regularization of the cornea, and it is not always clear the effect this will have on refraction. The topography-guided retreatment procedure is less predictable in what kind of refraction you are going to get, he said, recommending that surgeons using topography-guided ablation treat it as two stages, first using it with no refractive input to simply regularize the cornea. Then after 6 months, there will be better surface and endpoint refraction and the refractive component could be treated. Dr. Durrie said that results in the U.S. were extremely good. One thing that stuck out in the data was that patients seemed to have less night glare and halos than had been seen in other clinical trials. The results were not only as good as other clinical trials with this technology, but they had some distinct high-quality vision results. However, he stressed that currently in the U.S., this approval only applies to “normal virgin eyes” because all data from the clinical trial is from eyes that had