LAUSR

We read the recent article by Knyazer et al1 on 1-year outcomes following cross-linking of thin keratoconic corneas. A concern arising from this study was that the same ultraviolet-A (UVA) light “on” time was used irrespective of the stromal thickness. A damage threshold of 0.35 mW/cm2 for the endothelium of animal eyes is known and the endothelium is exposed to only 0.18 mW/cm2 when the stroma is 400 μm thick.2,3 Therefore, the “on” time of UVA light should be titrated to minimize the risk of damage to the endothelium. The absorption coefficient of UVA light in riboflavin of different concentrations was presented in an earlier study.4 By using the Lambert-Beer equation, we present a simple calculator to decide the UVA light “on” time for an incident intensity of 3 and 9 mW/cm2 (Figure 1).4 The assumption behind the calculator was that the incident energy density on the stroma was 5.4 J/cm2 and this was attenuated at deeper depths of the stroma as determined by the Lambert-Beer equation. We used an absorption coefficient of 46.86 and 93.72 cm-1 for riboflavin concentrations of 0.1% and 0.2%, respectively.4 The following equations were used to determine the time: