Determining the Efficacy of Corneal Cross-linking (CXL) Protocols for Different Corneal Thicknesses: Theoretical Model and Experimental Validation

Abstract

Purpose : A theoretical model was developed to predict the stiffening effect of CXL for different corneal thicknesses and UV irradiation protocols. Ex vivo experiments were performed to verify the accuracy.

Methods : The theoretical model considered the reaction kinetics of riboflavin diffusion, photo-degradation, UV intensity, oxygen availability, production of singlet oxygen and extracellular matrix oxidation. For the experiments, freshly-enucleated porcine (n=66), murine (n=55) and rabbit (n=2) corneas were de-epithelialized and riboflavin was instilled for 30 minutes, prior to UV-irradiation. Different UV irradiation protocols (continuous, accelerated, pulsed), UV fluences (0.09 to 5.4 J/cm²) and irradiation times (30s to 30min) were analyzed. Stress-relaxation tests at 0.6 MPa were performed to measure the increase in corneal stiffness.

Results : The theoretical concentration of newly induced cross-links matched well with the experimental stress resistance in all groups (Figure 1). Standard CXL was more effective in thinner than in thicker corneas. The remaining stress after 120s of relaxation was: 322kPa (riboflavin) / 456kPa (CXL) in murine, 168kPa (riboflavin) / 213kPa (CXL) in rabbit and 9.3kPa (riboflavin) / 10.4kPa (CXL) in porcine corneas. The corresponding theoretical increase in cross-link concentration was: 1.03mol/m³ in murine, 0.63mol/m³ in rabbit and 0.28mol/m³ in porcine corneas. Murine corneas could be effectively cross-linked with only 1/60 of the standard UV fluence used in humans. Pulsed CXL was significantly less effective than standard continuous CXL (p<=0.017).

Conclusions : The proposed theoretical model predicts correctly the stiffening effect after CXL under different conditions. Increasing CXL efficacy in a clinical setting would require prolonged UV irradiation at reduced irradiances, or a higher oxygen tension in the environment. Pulsed CXL does not allow to speed up CXL or to increase its efficacy compared to continuous CXL of the same irradiation time.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

Scatter plot comparing the theoretical concentration of induced cross-links during CXL with the experimental stiffness increase directly after treatment.