Enzymatic Resistance of Corneas Crosslinked Using Riboflavin in Conjunction With Low Energy, High Energy, and Pulsed UVA Irradiation Modes

PURPOSE: To investigate the effect of various riboflavin/ultraviolet light (UVA) crosslinking (CXL) protocols on corneal enzymatic resistance. METHODS: A total of 66 enucleated porcine eyes, with the corneal epithelium removed, were divided into 6 groups. Group 1 remained untreated. Groups 2 to 6 received riboflavin/dextran for 30 minutes. Group 3 underwent standard CXL (SCXL) with 3 mW/cm(2) UVA for 30 minutes (total energy dose 5.4 J/cm(2)). Groups 4 and 5 underwent high intensity CXL (HCXL) using 30 mW/cm(2) UVA for 3 minutes (5.4 J/cm(2)) and 30 mW/cm(2) for 4 minutes (7.2 J/cm(2)), respectively. Group 6 was exposed to 8 minutes of 30 mW/cm(2) UVA in a 10-second on/10-second off pulsed-radiation mode (p-HCXL; 7.2 J/cm(2)). A central 8-mm disk from each cornea was submerged in pepsin digest solution at 23°C and measured daily. After 13 days, the dry weight was recorded from 5 samples in each group. RESULTS: The CXL-treated corneas took longer to digest than nonirradiated corneas (P < 0.0001). Differences in digestion time also were observed between CXL groups, such that, HCXL (5.4 J/cm(2)) < SCXL (5.4 J/cm(2)) < HCXL (7.2 J/cm(2)) < p-HCXL (7.2 J/cm(2); P < 0.0001). The dry weight of the SCXL (5.4 J/cm(2)) group was higher than the HCXL (5.4 and 7.2 J/cm(2); P < 0.001) and p-HCXL 7.2 J/cm(2) (P <0.05) groups. No difference was detected between the HCXL and p-HCXL 7.2 J/cm(2) groups. CONCLUSIONS: The intensity and distribution of the crosslinks formed within the cornea vary with different UVA protocols. The precise location and amount of crosslinking needed to prevent disease progression is unknown.