Cross-linking techniques

Respecting certain technical parameters in the application of corneal cross-linking (CXL) will help physicians achieve greater consistency in outcomes when treating patients with keratoconus, Professor Theo Seiler MD, PhD, told delegates attending a joint ESCRS/EuCornea symposium on cross-linking at the 36th Congress of the ESCRS in Vienna. Since the introduction of CXL into clinics, many changes to the original Dresden protocol have been proposed, such as new light sources with shorter treatment times due to higher irradiances and new solutions with shorter imbibition times. “I think now is a good opportunity to answer some of the questions that my colleagues frequently ask about the application of CXL in their clinics – how much energy to use, as well as various questions around oxygen and riboflavin use,” he said. Key parameters of the Dresden protocol included removing the central 8-10mm of the epithelium and applying a 0.1% riboflavin solution in 16% dextran to the corneal surface followed by 30-minute exposure to 365nm ultraviolet light with an irradiance of 3mW/cm2, said Dr Seiler. “We now know that effective CXL entails three key ingredients: ultraviolet light, riboflavin and oxygen," he said. In terms of ultraviolet parameters, Dr Seiler noted that lamps on the market now offer UV-irradiances ranging from 3mW/cm2 to 45mW/cm2. While the treatment time could be reduced with higher power densities, the key question is whether shorter irradiance duration diminishes the efficacy of the CXL treatment, said Dr Seiler. Two different groups who initially evaluated the topic came up with completely opposite conclusions, said Dr Seiler. His own study of the issue, however, concluded that using higher UV irradiances combined with shorter treatment times reduces the efficacy of the treatment. “Shorter operation times may be achieved by increasing the irradiance from 3mW/cm2 up to 15mW/cm2 without loss of efficacy if the application time is adjusted,” he said. For riboflavin application, using hydroxylpropyl methylcellulose (HPMC) solution offers some clear advantages over Dextrane, said Dr Seiler. “With HPMC the cornea swells by 8%, whereas with dextran 20% it shrinks significantly, which occasionally led to epithelial damage. So using HPMC shortens imbibition time from 30 minutes to 10 minutes and also makes it safer,” he said. For effective cross-linking to occur, a certain amount of oxygen is required to be present in the cornea, said Dr Seiler. Whereas the standard Dresden protocol occurs slowly enough for the oxygen to replenish, some accelerated CXL protocols consume too much oxygen too quickly. Theo Seiler: c/o claudia.kindler@iroc.ch

Tags: cornea, corneal cross-linking, keratoconus