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Jeremy Wernli, Silvia Schumacher, Eberhard Spoerl, Michael Mrochen; The Efficacy of Corneal Cross-Linking Shows a Sudden Decrease with Very High Intensity UV Light and Short Treatment Time. Invest. Ophthalmol. Vis. Sci. 2013;54(2):1176-1180. doi: 10.1167/iovs.12-11409.
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Standard treatment in cases of progressive keratectasia is UV-triggered corneal cross-linking. For irradiances larger than 10 mW/cm2 and treatment times below 10 minutes, the scientific proof of a biomechanical strengthening effect is insufficient. The authors investigated the biomechanical strengthening of ex vivo corneal tissue treated with irradiances between 3 mW/cm2 and 90 mW/cm2 and illumination times from 30 minutes to 1 minute, respectively.
A total of 100 porcine eyes received riboflavin + UV treatment (constant irradiation dose of 5.4 J/cm2) with different intensities and illumination times and were randomly assigned into 10 groups. A control group (80 eyes) was not irradiated but underwent the same treatment otherwise. Young's modulus at 10% strain was determined for each strip after uniaxial stress-strain measurement. A Kruskal-Wallis test was used for statistical analysis.
A statistically significant difference (α = 0.01) was found between the median value of Young's modulus of the treatment groups up to 45 mW/cm2 (illumination times from 30 minutes to 2 minutes) compared with the control group. There was no statistically significant difference between the treatment groups from 50 mW/cm2 up to 90 mW/cm2 (illumination times of less than 2 minutes) and the control group.
The ex vivo results of corneal cross-linking performed in porcine corneas show that the Bunsen-Roscoe reciprocity law is only valid for illumination intensities up to 40 to 50 mW/cm2 and illumination times of more than 2 minutes. Further experiments are necessary to validate these results for in vivo human corneal tissue. Additionally, safety aspects at high intensities must be investigated.
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