High-resolution corneal strain maps after patterned corneal cross-linking (CXL)

Abstract

Purpose: To develop a spatially highly resolved, non-invasive and contact-free technique to measure corneal strain distribution in rodent eyes, and subsequently to investigate the effect of patterned CXL. Methods: Freshly enucleated rat eyes were obtained and subjected to repeated biomechanical characterization based on OCT elastography. Measurements were performed under different treatment conditions: (i) virgin, (ii) after epithelial removal and hypoosmolar 0.5%riboflavin instillation for 30 min, (ii) after patterned accelerated CXL treatment at 9 mW/cm2 for 10 min with either bow-tie, central or peripheral UV irradiation patterns. For each measurement, the eye was fixed in a silicon mold mounted within a pressure chamber and aligned with the OCT system. An ambient pressure variation of approx. -2 mmHg was induced, while before and after a volume OCT-scan was taken. Corneal deformation was determined by analyzing magnitude and phase changes of the raw interference OCT signal. Results: In the cross-sectional image, virgin corneas (i) showed a rather homogenous compression across the entire stroma. Riboflavin instillation (ii) reduced corneal compression, while after CXL treatment (iii), the sign of the strain changed and a bi-phasic distribution became apparent. In the enface image, central corneal tissue presented a well-defined distinct strain in regions that were subjected to UV than those without. No local differences were observed in the posterior stroma. In the anterior stroma, an irregular strain distribution was present, likely arising from dehydration effects. Conclusions: Non-contact OCT elastography based on ambient pressure modulation is a promising technique for the analysis of spatially resolved corneal strain. It holds promise to facilitate an earlier diagnosis of corneal ectasia, but also to improve the identification of material properties for numerical simulations of refractive surgeries. Patterned CXL treatment is effective in locally modifying corneal strain distribution.