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Corneal cross-linking using sunlight
First Author: E. Torres-Netto SWITZERLAND
Co Author(s): H. Abdshahzadeh S. Kling R. Abrishamchi F. Gilardoni N. Hafezi F. Hafezi
Keratoconus (KC) is globally underdiagnosed and corneal cross-linking (CXL) is its standard-of-care in modern ophthalmology. A dilemma, however, is that many people living in remote and underserved areas of the world do not have access to CXL technology. CXL prevents KC progression by increasing corneal stiffness via the combination of a photosensitizer with ultraviolet-A irradiation (UVA). If the latter could be substituted by daily sunlight exposure, then CXL could be performed using considerably less infrastructure. Unlike studies performed more than 20 years ago, our study evaluated corneal biomechanical properties induced by sunlight-riboflavin using clinical settings of riboflavin and UVA exposure.
The study was conducted at the Ocular Cell Biology Laboratory, Center for Applied Biotechnology and Molecular Medicine at the University of Zurich (Switzerland).
Forty-two porcine corneas were obtained from the slaughterhouse and divided into 3 groups. All groups had the epithelium removed and corneas from groups 2 and 3 were soaked with 0.1% and 0.5% riboflavin, respectively. Group 1 received no riboflavin and served as control. UVA-transmission was used to estimate stromal riboflavin concentration. The duration of sunlight exposure necessary to achieve a total fluence of 7.2J/cm2 was calculated according to live illuminance measurement (luminous-flux per unit-area). Eyes were exposed to sunlight using a UVA filter-free glass wet-chamber. The elastic-modulus of 5-mm wide corneal strips was calculated as an indicator of corneal stiffness.
The stromal riboflavin concentration in group 3 was higher by a factor 2.8 when compared to group 2. According to live illuminance measurements and stromal riboflavin concentration, the sunlight exposure duration varied between 16 and 45 minutes. Normal distribution was tested with both the Shapiro-Wilk and Kolmogorov-Smirnov tests and final analysis was performed using the Mann-Whitney test. Elastic-modulus at the range of 5-10% of strain showed significant differences between both groups and controls (p<0.010) but not between groups 2 and 3 (p=0.194). Groups 2 and 3 had a stiffening effect of 84% and 55%, respectively.
Sunlight exposure of ex vivo corneas soaked in both 0.1% and 0.5% riboflavin resulted in an increased corneal stiffness. Group 2 (0.1% riboflavin, longer UVA exposure) had a greater stiffening effect, likely due to the greater oxygen availability. This might open new alleys for the use of oral riboflavin and fractioned sunlight exposure as less invasive CXL techniques. In-vivo studies are currently underway to confirm this hypothesis.
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