EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
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EW REFRACTIVE SURGERY 38 November 2014 by Lauren Lipuma EyeWorld Staff Writer Indications The idea is to tailor use of topo- graphic ablations to cases where it is a distinct advantage—those cases being keratoconus, pellucid mar- ginal degeneration, previous radial keratotomy (RK), and previous laser ablation (LASIK or PRK) with small optical zones or irregular astigma- tism, Dr. Stein said. "In terms of previous surgeries, it's a big indication to use topogra- phy-guided [ablation] because we're assuming that the change that's occurred is from the previous refractive corneal surgery," Dr. Lin said. "Therefore, any induced changes are best treated by treating the cause, which is the cornea, rather than trying to treat the lens." Thanks to the success of crosslinking in increasing corneal strength, patients with keratoconus and ectasia can now undergo topog- raphy-guided PRK to correct their refractive error. Refractive surgeons in line with the anatomical visual axis, a benefit in patients who have large angle kappas, Dr. Lin said. Whereas wavefront measure- ments are limited to the size of the dilated pupil, topography can map the entire cornea. By expanding the optical zone, topography-guided treatments have been effective in improving the symptoms of glare, halos, and poor night vision associated with refractive surgery. "When putting in the final refraction, if the topography astig- matism lines up with the manifest astigmatism, there are no conflicts," Dr. Lin said. "However, if the corneal topography astigmatism axis is not in agreement with the manifest astigmatism axis (e.g., high lentic- ular astigmatism or early cataracts), the surgeon then has to decide which to bias more." As topography-guided custom ablation systems gain approval in the U.S., experienced users offer insights and pearls for getting started O utside of the U.S., topography-guided procedures—both LASIK and PRK—have gained traction as the second incarnation of customizable refrac- tive surgery. After the FDA approval of the first topography-guided laser systems in 2013, U.S. surgeons can now include them in their surgi- cal armamentarium, but should be aware of several key differences between topography-guided and wavefront-guided procedures. Surgeons David Lin, MD, FRCSC, clinical director, Pacific Laser Eye Centre, and clinical associate professor, University of British Columbia, Vancouver; and Raymond Stein, MD, FRCSC, associate professor of ophthalmol- ogy, University of Toronto, and medical director, Bochner Eye Institute, Toronto, shared the most vital information about topography- guided treatments and their pearls for getting started. Topography versus wavefront Topography measures more points of curvature on the cornea than wavefront imaging, providing a much more detailed image of the refractive surface and delivering better treatment for those with highly irregular corneas. "In an ideal cornea, which is completely symmetrical, you don't need topography-guided treatment because any treatment will work," Dr. Lin said. Asymmetric corneas, however, require a different ablation strategy because the topography is not necessarily uniform. Using a combination of myopic and hyper- opic ablations based on the topogra- phy yields better visual results and removes less tissue than is typical of wavefront-guided or traditional surgery. All topographies are based on the corneal apex instead of the pupil center, so these treatments are more What you need to know about topography-guided laser treatments Corneal topography and ablation profile for a patient undergoing topography-guided PRK, showing the areas of myopic and hyperopic ablations Source: Raymond Stein, MD, FRCSC S ometimes I find it difficult to fathom that the excimer laser was approved almost 20 years ago here in the U.S. The first laser treatments were oblate with minimal ablation zones that resulted in very good Snellen visual acuity, but there was much more glare and halo than we have today. Ten years ago we progressed to wavefront-optimized and wavefront treatments that dramatically improved quality of vision and patient satisfaction. The next great breakthrough appears to be topographic-guided treatments, which should become readily available in the U.S. in 2015. With topographic ablations we will have the opportunity to treat irregular corneas that were not treatable with the current generation of lasers. This takes LASIK and PRK into the realm of a therapeu- tic treatment and will help those patients most in need of a refractive solution. This will include patients with previous corneal surgery such as radial keratotomy, penetrating keratoplasty, and suboptimal results from previous LASIK or PRK. The second most common reason we perform corneal transplantation in the U.S. is for keratoconus. If we can reduce the number of patients requiring a PK or take patients who are forced to wear gas permeable contact lenses for visual rehabilitation and move them into glasses or soft contacts, we will be providing a significant service. In this article we are afforded a glimpse into the future of LASIK and PRK with topographic ablations by two leading Canadian refractive surgeons who have vast experience with this technology. Many patients will not benefit significantly as compared to our present treatments, but there will be subgroups of patients where topographic ablations will be a distinct advantage. Identifying the patients who will best respond to topographic ablations will be the next challenge for refractive surgeons. I am eager to embrace this new technology that should offer improved out- comes for many of our patients interested in refractive surgery and improved quality of life for patients with irregular corneas who have been searching for a better solution than a penetrating keratoplasty or gas permeable contact lens. Eric Donnenfeld, MD, refractive editor Refractive editor's corner of the world "In terms of previous surgeries, it's a big indication to use topography-guided [ablation] because we're assuming that the change that's occurred is from the previous refractive corneal surgery. Therefore, any induced changes are best treated by treating the cause, which is the cornea, rather than trying to treat the lens." –David Lin, MD, FRCSC