Topography-guided ablation is an effective technique for managing highly distorted corneas, such as those with keratoconus, corneal ectasia, corneal trauma, and irregularities caused by previous surgery.
Conventional refractive surgery can correct regular modalities such as myopia, hyperopia, and astigmatism, but managing distorted corneas (such as irregular astigmatism) is much more complex with this technique, he said; it is more difficult to measure the irregularities in these eyes as well as to transfer to the excimer laser platform a treatment that is specific for the irregularities.
"Wavefront-guided is a helpful entity. It can improve quality of vision in some eyes that are irregular. It does have limitations in delivery when the irregularity is very significant, though," he said. "The second disadvantage of wavefront in some eyes is that it requires a lot of tissue to smooth out irregularities. So in eyes with less tissue (such as previously treated eyes and/or ectasia cases) where you don't have much 'room' to spare as far as corneal thickness, that becomes an issue as well."
Topography-guided ablation has several features that make it a good choice for more severe irregularities. The topography unit captures the actual shape of the cornea and produces a precise contour map that is transferred directly into the excimer laser. The laser then performs the correction according to those specifications.
"It can be quite specific at 'taking down' all the peaks and 'bringing up' the valleys of irregular corneas," said Dr. Kanellopoulos, medical director of LaserVision.gr Institute in Athens and associate clinical professor, Department of Ophthalmology, New York University School of Medicine.
"Topography-guided [ablation] can be very specific and can image almost 99% of the corneas, so it can make available a customized treatment in almost all eyes," he said. "Wavefront can be captured in about 70% of the cases, so we're increasing the number of eyes that we potentially can help."
He has used corneal topography (Allegretto Wave Topolyzer, WaveLight) for the past 5 years and has experience treating the most extreme corneal irregularities, such as keratoconus and corneal ectasia. "We have been able, using corneal topography, to treat some of these eyes effectively in conjunction with a way to stabilize progressive ectasia," Dr. Kanellopoulos said.
"The second advantage is that topography-guided [ablation] requires less than half of the tissue that wavefront-guided procedures require to give us the same smoothing effect. It's very conservative as far as how much tissue we're removing, so where tissue preservation is of the essence, we can be much more efficacious with topography-guided [ablation]," he continued.
Topography-guided procedures also may be an effective alternative to penetrating keratoplasty if ectasia has developed, the cornea has begun to thin, and corneal irregularity have developed after refractive surgery.