Transepithelial Crosslinking Using Femtosecond Laser Machined Epithelial Microchannels for Riboflavin Delivery

Abstract

Purpose : Ultraviolet corneal collagen crosslinking (UVA CXL) via photoactivation of riboflavin (Rf) is an effective treatment for Keratoconus, and in development as a refractive procedure. A drawback of UVA-CXL is the need for epithelial debridement to achieve adequate stromal Rf concentrations, leading to discomfort, delayed recovery, and risk of infection. Transepithelial (TE) CXL using chemical disruption of the corneal epithelial barrier has been shown to have limited success in addressing these concerns. Also an intact epithelium could act as a barrier to irradiation, preventing effective TE UVA-CXL. This study describes a femtosecond laser (FS) approach to machine corneal epithelial microchannels (MC) for enhanced stromal Rf penetration combined with nonlinear optical (NLO) CXL to avoid both epithelial damage and shielding of irradiation.

Methods : Using a 1030nm FS laser with 5μJ pulses, 2μm diameter by 25μm long MC were machined into the epithelium of ex vivo rabbit corneas at a density of 100 MC/mm². Rf penetration through the MC was then determined by applying 1% Rf in PBS with osmolarity ranging from 200-450 mOsm for 30 minutes followed by removal of the cornea and extraction from the central stromal button. Stromal Rf concentrations were compared between osmolarity groups. Eyes treated with MC UVA-CXL were compared to eyes treated with BAK UVA-CXL using collagen autofluorescence (CAF) to determine the effect of CXL. CXL was performed both with and without continuing drops of Rf solution to determine the shielding effect of the solution on the surface of the eye.

Results : Rf levels were highest in eyes treated with 250 mOsm solution. No eyes treated with TE UVA CXL with continued dripping produced measurable CAF in either group. Eyes without further dripping after the start of CXL did produce CAF. Additionally organ culture showed a normal corneal epithelium following both MC alone and MC NLO CXL, while BAK alone, BAK UVA CXL, and MC UVA CXL treated corneas all showed extensive epithelial and stromal damage at 24 hours post treatment.

Conclusions : FS epithelial MC enhance stromal penetration of Rf for corneal CXL, but both the epithelium and a layer of Rf solution shield the cornea from UVA irradiation, inhibiting CXL. Data suggests that combining MC with NLO CXL minimizes damage, avoids shielding, could lessen patient discomfort, speed recovery, and decrease risk of infection.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.