Oxygen kinetics during CXL using symmetrically and asymmetrically pulsed UV-irradiation

Purpose To investigate oxygen kinetics during symmetrically pulsed and asymmetrically pulsed crosslinking (p-CXL) with and without supplementary oxygen at different irradiances and corneal depths. Design Experimental, laboratory study Methods In de-epithelialized porcine eyes, a femtosecond-laser generated tunnel was used to place a fibre-probe in corneal depths of 200 and 300 µm to measure the local oxygen concentration. After riboflavin imbibition, the corneas were irradiated at 9, 18 and 30 mW/cm 2 for 10 seconds On and 10 seconds Off; while the oxygen concentration was continuously measured until oxygen levels depleted below the oxygen sensor’s threshold (1%) or until stabilized. All experiments were performed under normoxic (21%) and hyperoxic (>95%) conditions and the obtained data were used to identify parameters of a numerical algorithm for oxygen consumption and diffusion. Following the algorithm’s development, the suggested asymmetrical pulsing values were experimentally tested. For 9, 18 and 30 mW/cm 2 the suggested tested pulsing schemes were 3 seconds On : 9 seconds Off, 2 seconds On : 9 seconds Off and 1 second On : 9 seconds Off respectively. Results The minimum, available stromal oxygen for p-CXL in normoxic environment was decreasing <1% for 9, 18 and 30 mW/cm 2 in 200 and 300 μm. Using optimized p-CXL, the minimum available oxygen increased to 3.8, 1.8 and 2.8 % at 200 μm, for irradiances of 9, 18 and 30 mW/cm 2 , respectively, where the periods exhibited an equilibrium state. At 300 μm, 1.1 % of oxygen was available for 30 mW/cm 2 . Using a hyperoxic environment, the oxygen concentration was 19.2% using 9 mW/cm 2 in 200 μm, dropping to 17.0% in 300 μm. At 18 mW/cm 2 , the concentrations were 3.9% and 1% in 200 and 300 μm, respectively. Using 30 mW/cm 2 , all oxygen was depleted below the threshold limit (1% O 2 ) for both depths. Using optimized pulsing in combination with hyperoxic environment, the oxygen concentration was 42.0% using 9 mW/cm 2 in 200 μm and 43.3% in 300 μm. At 18 mW/cm 2 , the concentrations were 24.7% and 16.1% in 200 and 300 μm, respectively. Using 30 mW/cm 2 , the minimum oxygen availability was 25.7% and 13.7% in 200 and 300 μm, respectively. Conclusion Supplementary oxygen during symmetrical and asymmetrical p-CXL increased the oxygen availability during corneal cross-linking. The pulsed irradiance and the hyperoxic environment potentially increased the efficacy of corneal cross-linking in deeper corneal layers and higher irradiances. The numerical algorithm for asymmetrical pulsing led to the quantification of “On” and “Off” times related to different scenarios such as irradiances.