A transepithelial PRK (TransPRK) procedure (SmartSurface, Schwind eye-tech-solutions) provides the benefits of one-step, no-touch surface ablation plus rapid visual recovery, said Diego de Ortueta, MD.
The technique--performed with an excimer laser system (AMARIS) and pulse technology (SmartPulse Technology [SPT], both Schwind eye-tech-solutions)--is able to create a smoother corneal surface. With the novel pulse technology, the laser spots are delivered based on a three-dimensional fullerene structural model of the cornea that accurately represents the cornea’s curved surface and allows spots to be placed closer together, particularly in the periphery.
“The difference in smoothness can be seen on electron micrographs,” said Dr. de Ortueta, medical director, Aurelios Augenzentrum, Recklinghausen, Germany. “Laboratory bench testing shows that compared with the original software for [the excimer laser], SPT reduces surface roughness by about 60%, from 749 to 272 nm local standard deviation.”
Results of a retrospective study comparing two consecutive groups of 125 eyes each that underwent TransPRK with or without pulse technology showed the benefit of the procedure, Dr. de Ortueta noted. Predictability of the refractive outcome was equally good in both groups as was safety, with no eyes losing 2 or more lines of Snellen visual acuity. At the time of bandage contact lens removal on postoperative day 4, however, visual acuity was 20/25 or better in 80% of patients in the SmartSurface group compared with just 55% of those treated without the new pulse technology.
“In addition, the corneas of the eyes in the SmartSurface group appeared clearer postoperatively,” he said.
The idea of TransPRK has always been attractive to refractive surgeons because of the elegance and simplicity of removing the epithelium with the excimer laser instead of by mechanical or chemical debridement, Dr. de Ortueta noted.
Absence of a systematized “plug-and-play” option for performing the procedure hampered its adoption, but that obstacle was overcome in 2009 when the manufacturer implemented single-step TransPRK for the excimer laser.
“With this technique, the laser fires the aspheric PRK treatment profile first and then switches to a defined-depth radial PTK mode for aspheric epithelial ablation,” Dr. de Ortueta explained.
“The ORK-CAM software of the laser compensates for the slight differences in photoablative rates of the stroma and the epithelium, and the applied defined epithelial thickness profile is based on literature values and mean epithelial profiles of large-based populations so that it is thinner at the center and thicker at the periphery,” he said. “Treatment without this aspheric ablation would remove about 10 μm of stromal tissue and induce a myopic-like correction of approximately -0.75 D.”
The software also takes into account the greater loss of energy for pulses delivered at the periphery and the change of the corneal curvature during the treatment.