Changes in Corneal Biomechanical Properties With Different Corneal Cross-linking Irradiances

To evaluate whether different corneal cross-linking (CXL) irradiances, all with the same delivered total energy, achieve similar increases in corneal material stiffness.One hundred twenty-six healthy white Japanese rabbits were randomly divided into seven groups (n = 18 each). After removing the epithelium of the left corneas, six groups were exposed to riboflavin (0.22% concentration by volume) and ultraviolet-A (370 nm) at different CXL irradiations, all with the same total dose (5.4 J/cm2), ranging from 3 mW/cm2 for 30 minutes to 90 mW/cm2 for 1 minute. The left corneas of the seventh group were exposed to riboflavin without irradiation. Twelve corneas of each group were prepared for inflation testing, where they were subjected to internal hydrostatic pressure simulating intraocular pressure, whereas the other six specimens were processed for electron microscopy measurements of fibril diameter and interfibrillar spacing. The inverse modeling process was used to estimate the tangent modulus of the tissue, which is considered an accurate measure of the material stiffness.The stiffening effect of CXL decreased when using high irradiation/short duration settings. Compared with the group with no irradiation (NUVA group), the tangent modulus increases reduced from 212.5% in the 3mW/30min group to 196.8% in the 90mW/1min group. These increases were significant (P < .05) in the 3mW/30min and 9mW/10min groups, but became insignificant in other CXL groups. The interfibrillar spacing in the anterior 50 μm of the corneal stroma also reduced with high irradiation/short duration settings, changing from 20.05 ± 1.89 nm in the NUVA group down to 13.06 ± 2.07 and 14.37 ± 1.90 nm in the 3mW/30min and 9mW/10min groups, respectively. These changes were significant (P < .05) between non-adjacent groups, but became non-significant otherwise. The corresponding changes in fibril diameter were small and nonsignificant in all cases (P > .05).Because the effect of CXL in stiffening the tissue and reducing the interfibrillar spacing consistently decreased with reducing the irradiance duration, the Bunsen-Roscoe law may not be readily applicable in the CXL of corneal tissue. [J Refract Surg. 2018;34(1):51-58.].