Eyeworld

147 EW INTERNATIONAL March 2015 rior corneal elevation and residual bed thickness after LASIK or PRK in this study, suggesting that residual bed thickness alone may not be an accurate predictor of post-refractive keratectasia." Dr. Jhanji suggested that flap ablation and a deep ablation zone could cause the cornea to weaken. That said, the precision of femtosecond lasers could account for the significant drop in the in- crease in posterior corneal elevation compared to previous studies, which mainly analyzed microkeratome- based LASIK, he noted. In this study, posterior corneal elevation change amounted to no clinical significance. "It would be especially interest- ing to see if there are any differences between LASIK and surface ablation over a long-term period," Dr. Jhanji said. EW Editors' note: Dr. Jhanji has no finan- cial interests related to his comments. Contact information Jhanji: vishaljhanji@cuhk.edu.hk by Matt Young and Gloria D. Gamat EyeWorld Contributing Writers a longer wavelength allows better light penetration and less scatter through the LASIK flap interface or with corneal haze after PRK." Results Swept-source OCT (Casia SS-1000, Tomey, Nagoya, Japan) found that the mean change in posterior corne- al elevation was 4.88 +/–0.47 µm in the femtosecond LASIK group and 3.67 +/–0.48 µm in the PRK group at 1 month. At 1 year, the change dimin- ished to 2.92 +/–0.46 µm in the LASIK group and 2.72 +/–0.46 µm in the PRK group. "The findings of our study suggested that there was a mild but significant forward protrusion of the posterior cornea after femtosecond laser-assisted LASIK and PRK," Dr. Jhanji wrote. The study also has implications for the stability of PRK versus LASIK, as well as the importance of residual bed thickness. "In our study, the posterior cor- nea fluctuated during the first post- operative year after LASIK, whereas it stabilized as early as 3 months after PRK," he said. "Moreover, there was no correlation between poste- study investigator Vishal Jhanji, MD, associate professor of ophthal- mology, cornea and external eye diseases, Chinese University of Hong Kong. "It works at a wavelength of 1,310 nm and has a speed of 30,000 axial scans per second. The axial resolution is less than 10 µm and transverse resolution is 30 µm. The device measures tissue with a maxi- mum width of 16.0 x 16.0 mm and a maximum depth of 6.0 mm." The topo-pachy map that SS-OCT generates, according to Dr. Jhanji, is composed of 16 radial B-scans (cross-sections) with evenly spaced angular intervals. All that amounts to a total scan time of 0.3 seconds. "Swept-source optical coher- ence tomography can measure and compute corneal topography from the posterior corneal surface direct- ly," Dr. Jhanji wrote in the study. "It is believed that swept-source optical coherence tomography is able to im- age the posterior cornea better than Scheimpflug photography because of its shorter scanning time [0.3 versus 2 seconds] and longer wavelength of light source [1,310 versus 475 nm]. A shorter scanning time greatly reduces motion artifacts, whereas Previous technology found up to 50 µm of protrusion post-refractive surgery—what about now? C orneal ectasia after LASIK ranks in seriousness to endophthalmitis after cataract surgery. While the severity of each problem differs, these continue to be a sig- nificant issue when they occur after their relative surgeries. Ectasia continues to mystify, to some extent. Forme fruste keratoco- nus, numerous LASIK procedures, and even high myopia are just some of the risk factors for the disease, but nothing predicts with 100% accura- cy who will get ectasia. Refractive surgery itself has been known to potentially cause forward protrusion of the posterior cornea, suggesting that minor bulging of the eye happens in some cases. Given the wide variability of re- ports—sometimes no protrusion and sometimes up to 50 µm of it—the accuracy of the technology used to measure the cornea often has been called into question in this regard. One of the latest reports out of Hong Kong, using the latest OCT technology, provides insights into whether refractive surgery causes corneal bulging, whether different surgical methods cause different outcomes, and whether this is significant. Corneal protrusion in Hong Kong In Hong Kong, 48 patients (98 eyes) underwent analysis at the Chi- nese University of Hong Kong Eye Centre. Results were published in December 2014 in Ophthalmology. Researchers investigated poste- rior corneal elevation change 1 year after both femtosecond LASIK and PRK in these patients using some of the latest technology—swept-source OCT (SS-OCT). "SS-OCT uses a balanced pho- todetector for spectrally resolved interferometric detection," said Detecting corneal bulging with swept-source OCT Optical coherence tomography compared to histology Source: Mohamed Abou Shousha, MD, and J. Wang

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