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/cm2). The four groups were defined as standard CXL (SCXL; 3 mW/cm2 for 30 minutes, n = 17), accelerated CXL1 (ACXL1; 9 mW/cm2 for 10 minutes, n = 16), accelerated CXL2 (ACXL2; 18 mW/cm2 for 5 minutes, n = 17), and accelerated CXL3 (ACXL3; 30 mW/cm2 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.].

Effect of Mydriasis-Caused Intraocular Pressure Changes on Corneal Biomechanical Metrics.

Purpose: To evaluate the dependence of biomechanical metrics on intraocular pressure (IOP). Methods: 233 refractive surgery patients were included in this study-all were examined 3 times with the Corvis ST before and after dilation, and the differences (∆) in the main device parameters were assessed. The data collected included the biomechanically corrected IOP (bIOP), the central corneal thickness (CCT), and six dynamic corneal response (DCR) parameters, namely DA, DARatio2mm, IIR, SP-A1, CBI, and SSI. Participants were divided into three groups according to the changes in patients' bIOP after mydriasis. Results: Intra-operator repeatability was generally high in most of the DCR parameters obtained before and after dilation. The mean changes in bIOP and CCT after dilation were -0.12 ± 1.36 mmHg and 1.95 ± 5.23 µm, respectively. Only ∆DARatio2mm, ∆IIR, and ∆CBI exhibited a statistically significant correlation with ∆CCT (p < 0.05). The changes in all DCR parameters, especially ∆DA and ∆SP-A1 were also correlated with ∆bIOP (p < 0.01)-a 1-mmHg change in bIOP was associated, on average, with 5.612 and -0.037 units of change in SP-A1 and DA, respectively. In contrast, the weakest correlation with ∆bIOP was exhibited by ∆SSI. Conclusion: Most corneal DCR parameters, provided by the Corvis ST, were correlated with IOP, and more weakly with CCT. Changes experienced in CCT and IOP should therefore be considered in studies on corneal biomechanics and how it is affected by disease progression and surgical or medical procedures.