Long-term Effects of Riboflavin Ultraviolet-A–Induced CXL With Different Irradiances on the Biomechanics of In Vivo Rabbit Corneas

PURPOSE: To evaluate the long-term effects of ultraviolet-A corneal cross-linking (CXL) with different irrandiances on the biomechanical properties of rabbit corneas and the corresponding changes in stromal microstructure. METHODS: The study involved the left eyes of 85 healthy white Japanese rabbits, randomly divided into five groups (n = 16 to 18 each). After removing the epithelium, the first four groups were exposed to riboflavin (0.22% concentration by volume) and ultraviolet-A (370 nm) at different CXL irradiations but with the same total dose (5.4 J/cm 2 ). The four groups were defined as standard CXL (SCXL; 3 mW/cm 2 for 30 minutes, n = 17), accelerated CXL1 (ACXL1; 9 mW/cm 2 for 10 minutes, n = 16), accelerated CXL2 (ACXL2; 18 mW/cm 2 for 5 minutes, n = 17), and accelerated CXL3 (ACXL3; 30 mW/cm 2 for 3 minutes, n = 17). The control group (n = 18) was treated with riboflavin without ultraviolet-A exposure. Nine months after CXL, 10 corneas from each group were tested ex vivo under inflation, and the tangent modulus (Et) was estimated using an inverse analysis process. The remaining six to eight specimens in each group were examined by electron microscopy to determine the mean fibril diameter and interfibrillar spacing. RESULTS: The SCXL and ACXL1 groups showed statistically significant differences in Et at all stresses (0.005, 0.010, and 0.015 MPa) analyzed compared to the control group (all P < .01), but the differences were non-significant in the ACXL3 group ( P = 1.000, .785, and .679, respectively). For the ACXL2 group, there was no statistical difference in Et under the low stress of 0.005 MPa ( P = .155), but the differences became significant at 0.010 and 0.015 MPa when compared with the control group (all P < .05). CONCLUSIONS: CXL had a significant effect on corneal biomechanics in both standard and accelerated procedures. However, standard CXL was the most effective, and this effectiveness decreased gradually with increasing ultraviolet-A power intensity. [ J Refract Surg . 2022;38(6):389–397.]