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  • Publication
    Diffusion of oxygen and riboflavin during corneal cross-Linking (CXL)
    (University College Dublin. School of Physics, 2017) ;
    Corneal Cross-Linking (CXL) is a one hour therapeutic treatment involving epithelial removal, application of riboflavin drops for 30 min. followed by UV light exposure to the cornea for the treatment of keratoconus, ectasia and infection. The photochemical reaction mechanism during CXL effectively halts the progression of corneal disease. The three main components of successful CXL are 1) the diffusion of riboflavin, 2) the presence of molecular oxygen (O2) and the formation of reactive oxygen species, and 3) UV interactions during CXL which results in the biomechanical strengthening in the corneal stroma, thus halting disease progression. Recent applications such as reduced treatment time, epithelial by-passing and the combination of advanced CXL protocols as an alternative to refractive procedures have been investigated. The purposes of this study are to 1) investigate the diffusion of riboflavin using intra-stromal channels in order to determine the effective diffusion coefficients as compared to traditional axial diffusion with epithelium on or off, and 2) investigate O2 behaviours during CXL in order to better understand Type I and II photochemical reactions using an established luminescence quenching technique.Measurements of riboflavin diffusion using intra-stromal channels were created by means of a mechanical stromal instrument in whole-mounted post-mortem porcine eyes. The use of fluorescent imaging along with numerical modelling allows determination of effective diffusion coefficients under different conditions.Time-Correlated Single Photon Counting Phosphorescence Lifetime Imaging (TCSPC-PLIM) is an innovative and complex method for fluorescence and phosphorescence lifetime measurements. Phosphorescence sensitive O2 based probes in soluble form have the ability to detect O2 concentrations repeatedly and non-invasively in living biological tissue. This study investigated the use of O2 sensitive phosphorescent probes using three models: (i) O2-sensitive nanoparticles in soluble solution of riboflavin without collagen, (ii) collagen type-I gel with O2-sensitive nanoparticles and riboflavin, and (iii) porcine eyes stained with O2-sensitive nanoparticles and riboflavin. Several O2-sensitive nanoparticle probes were evaluated. One probe was chosen (SII-A), displaying sufficient brightness, photostability and efficient in-depth staining. Results showed O2 behaviour after UV- induced CXL in all samples measured, concluding TCSPC-PLIM to be a novel, effective method in measuring CXL.
  • Publication
    Rate of riboflavin diffusion from intrastromal channels before corneal crosslinking
    (Lippincott Williams & Wilkins, 2016-03) ; ;
    Purpose To determine the diffusion of riboflavin from intrastromal channels through the effective diffusion coefficients compared with traditional axial diffusion with epithelium on or off. Setting Advanced Optical Imaging Laboratory, University College Dublin, and Wellington Eye Clinic, Sandyford, Dublin, Ireland. Design Experimental study. Methods The rate of diffusion in whole-mounted porcine eyes was monitored for a 30 minutes using an optical setup with a charge-coupled device camera and a bandpass filter (central wavelength 550 nm and 40 nm bandpass) to image the fluorescence under ultraviolet illumination (365 nm wavelength). For comparison, an isotropic corneal stroma with an annular channel was modeled numerically for different diffusion constants and boundary conditions. Results Numerical and experimental results were compared, allowing determination of the effective diffusion coefficient for each case. Experimental results for 6 different riboflavin solutions were in all cases found to be higher than for the common crosslinking (CXL) riboflavin protocol, where the diffusion constant is D0 = 6.5 × 10-5 mm2/sec. For the intrastromal channel, 2 isotonic solutions containing riboflavin 0.1% correlated with a diffusion constant of 5D0 = 32.5 × 10-5 mm2/sec. Hypotonic solutions and transepithelium had a higher diffusion coefficient approaching 10D0 = 65.0 × 10-5 mm2/sec, which is an order-of-magnitude increase compared with the typical diffusion coefficient found in standard CXL. Conclusions In this study, riboflavin had a faster stromal diffusion when injected into a corneal channel than when applied as drops to the anterior corneal surface. Further numerical modeling might allow optimization of the channel structure for any specific choice of riboflavin.
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