EyeWorld Asia-Pacific June 2012 Issue

June 2012 11 EWAP FEATURE continued on page 13 “By definition topography- guided treats on the visual axis, not on the geometric center,” he explained. “In my mind, hyperopic LASIK should only be topography- guided LASIK because it treats on the visual axis versus the center of the cornea. If you treat a hyperopic eye as the opposite of myopia, you’re invariably misplacing the ablation temporally.” Surgeons in Greece also have incorporated corneal collagen crosslinking into this hyperopic treatment, which may change the thinking that hyperopia “is something that regresses,” Dr. Kanellopolous said. Just comparing imaging alone in irregular corneas, Dr. Kanellopolous said topography systems are superior because “it’s extremely rare that you are not able to obtain reproducible maps with topography, whereas wavefront imaging of these corneas is very challenging.” Limitations Still, each system has its limits. “Topography-guided is limited as being able to have a good picture of what the spherical equivalent of that eye would be after the procedure,” Dr. Kanellopolous said. As with any surgical procedure, there are risks. “When you increase the index of irregularity and incorporate it into treatment, Johan A. HUTAURUK, MD Director, Jakarta Eye Center Jl. Cik Ditiro 46, Menteng, Jakarta 10430 Tel. no. +62-21-3193-5600 Fax no. +62-21-390-4601 johan.hutauruk@jakarta-eye-center.com E arly studies of custom LASIK using the ray tracing technology have shown superior results compared with wavefront-guided LASIK. This is actually another way to say that wavefront-guided LASIK is not yet a perfect technology, not to mention that the corneal flap creation during LASIK will also induced wavefront errors that are not detected during pre-LASIK examination. Wavefront-guided LASIK works fine for a patient with normal cornea, symmetric astigmatism profile and young patients. But for the patient with high astigmatism and corneal irregularities, in my experience, topography-guided LASIK still gives better visual outcomes compared with wavefront-guided LASIK. LASIK surgeons in the Asia-Pacific region are fortunate to have the opportunity of “playing” the topographic features in doing LASIK compared with our colleagues in the U.S. This is very effective for LASIK retreatment. I have seen unhappy patients after LASIK, mostly because of corneal irregularities, decentered ablation, or optical zones that do not cover the large mesopic pupil. Those problems are challenging, and these cases where topography-guided LASIK will fit in to help our patients. Corneal flaps also contribute problems in some patients due to formation of microstriae that reduce the visual acuity or at least the contrast sensitivity in a perfectly good LASIK ablation profile. Surface ablation is certainly helpful to eliminate this kind of flap problem, but there is something that needs to be done to make surface ablation more acceptable for surgeons and patients, mostly regarding the painful recovery and corneal haze. I believe that custom LASIK should be the future of surface ablation with ray tracing technology. Diagnostic tools for wavefront errors also play a major role in LASIK outcomes. Hopefully in the future, there will be a wavefront diagnostic machine that can measure our patient while lying flat on the bed just before the LASIK procedure, so that cyclotorsion between diagnostics while sitting and treatment while lying on the bed will no longer be a problem. Editors’ note: Dr. Hutauruk has no financial interests related to his comments. Views from Asia-Pacific John S. M. CHANG, MD Director, Guy Hugh Chan Refractive Surgery Centre Hong Kong Sanatorium and Hospital 8/F Li Shu Pui Block, Phase II 2Village Roa d, Happy Valley, Hong Kong Tel. no. +852-2835-8885 Fax no. +852-2835-8887 johnchang@hksh.com W avefront-guided LASIK is now quite mature and produces excellent results. The ideal system should also include the aspheric treatment to remove the induced spherical aberration. Topograhy-guided LASIK is useful for enlarging the optical zone, decentered ablation, irregular cornea. In order for treatment to be most effective we need better and faster eye trackers, iris registration, pupil centroid shift, and active and passive eyetracking. When you do not have the above features, treatment can make things worse, that is, you can make the valleys deeper and the mountains steeper. Studies have shown that 2° or more of torsional error will likely induce HOAs already. Cyclotorsion can be an issue. We reported that active cyclotorsion during surgery can be as large as 13.3°, which would under-correct astigmatism by 40% and induce significant HOAs. Topography-guided laser should be based on the corneal vertex not on the visual axis since there are still arguments as to where exactly the visual axis is. The cornea vertex can be quite reliably reproduced for each patient. Ray tracing takes all the refracting surfaces into account e.g. anterior and posterior cornea surface, the anterior posterior lens surface as well as the overall wavefront and structure of the eye. However, each measurement creates a small amount of error which can add up to a large error in treatment, there is still some work that needs to be done. I look forward to using this when it is available. Editors’ note: Dr. Chang has no financial interests related to his comments. you’re becoming extremely demanding of the centration and reproducibility of that treatment,” Dr. Kanellopolous said. “You increase the gains, but you quadruple the risks. Say I envision an irregularity that is a map of Greece, for instance, and I ablate that irregularity of Italy, then I exacerbate that irregularity by three times.” The difficulty is exacerbated because most images of patients’ eyes are taken while a patient is sitting, but the patient is ultimately treated while lying on the operating table. “Therefore we have issues of cyclorotation, different pupillary dilation status, and a potential shift of the visual axis,” Dr. Kanellopolous said. “The [issue] in my mind going into anything customized is that you’re increasing the risks of creating a bigger problem than the problem you are trying to treat.” The next step Investigators are now trying to marry the advantages of both the wavefront- and topography-guided systems. “It’s called ray tracing, where they include topographic data, wavefront data, and axial length data, and anticipated tissue response to the treatment into an extremely calculated formula,” Dr. Kanellopolous said. “This is not yet