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To determine the impact of corneal cross-linking (CXL) performed over the flap (Standard CXL) or under the flap after flap lift on regional corneal stiffness using Brillouin microscopy.
After epithelium debridement, LASIK flaps were created on all intact fresh porcine eyes with a mechanical microkeratome. Then, CXL over the flap (S-CXL) or CXL under the flap (Flap-CXL) was performed. For S-CXL, riboflavin was dropped on the corneal surface followed by 3 mW/cm2 UV exposure with the flap in place for 30 minutes. For Flap-CXL, riboflavin was dropped on the stromal bed after reflecting the flap followed by the same UVA exposure with the flap replaced. Depth profile of stiffness variation and averaged elastic modulus of anterior, middle and posterior stroma were determined by analyzing Brillouin maps.
S-CXL had maximal stiffening impact at the corneal surface with a Brillouin shift of 8.40±0.04 GHz, while Flap-CXL had maximal stiffening impact 249±34 μm under the corneal surface (including 116±10 μm flap) with a Brillouin shift of 8.22±0.03 GHz (p<0.001). S-CXL increased longitudinal modulus by 6.69% in the anterior, 0.48% in the middle, and -0.91% in the posterior regions as compared to Flap-CXL which increased longitudinal modulus by 3.43% in the anterior (p<0.001), 1.23% in the middle (p<0.1), and -0.78% in the posterior regions (p=0.68).
The standard CXL technique for treatment after LASIK flap creation generates significantly greater stiffening effect in the anterior cornea than using a modified protocol with riboflavin administration under the flap. Minimal middle or posterior corneal stiffening occurs with either protocol.