Abstract
Purpose :
Stromal oxygen availability affects the balance between Type I and Type II reactions during corneal crosslinking (CXL). As oxygen depletes, anaerobic reactions dominate and crosslinking efficiency decreases. This effect is more pronounced with high irradiances (high oxygen consumption) and in epi-on procedures (low oxygen diffusion). This study provides ex vivo measurements comparing stromal oxygen dynamics and biomechanics of high-irradiance, epi-on CXL under normoxic and hyperoxic conditions.
Methods :
Ex vivo porcine eyes were partially de-epithelialized to approximate the thickness of human epithelium. Eyes were held in a chamber for control of temperature, humidity, intraocular pressure, and oxygen concentration (21% or >90%). Test eyes were dosed with a two-part trans-epi riboflavin formulation (ParaCel Part One / Part Two). Positive control eyes were dosed with a 0.1% riboflavin solution in HPMC.
Stromal oxygen was measured with a fiberoptic probe inserted into a laser-cut flap at a depth of 230 mm. Oxygen was continuously monitored before, during, and after UV illumination. An irradiance of 30 mW/cm2, dose of 10 J/cm2, and 1 sec: 1 sec pulsing were used, with and without application of supplemental oxygen. Negative (no CXL) and positive “Dresden protocol” (epi-off, 3 mW/cm2, 5.4 J/cm2, no pulsing, room air) controls were also performed. Stiffness was measured using a biaxial extensiometer after crosslinking.
Results :
When supplemental oxygen is provided at the anterior surface, mid-stromal supply and demand are balanced during UV illumination in high-irradiance epi-on CXL. Conversely, mid-stromal oxygen depletes to zero under normoxic conditions for both high-irradiance epi-on and epi-off Dresden protocols. Biaxial extensiometry showed an increase in biomechanical strength when CXL is conducted in a hyperoxic environment.
Conclusions :
Supplemental anterior oxygen increases stromal oxygen availability despite increased consumption and decreased diffusion in high-irradiance epi-on protocols, giving improved corneal stiffening in ex vivo porcine eyes.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.