Abstract
Purpose :
Brillouin frequency shift has proved beneficial in studying the mechanical changes of the cornea including characterizing crosslinking efficiency and monitoring the progression of keratoconus. Recently, the effects of water content on frequency shift have been investigated and Brillouin microscopy has been used to detect the changes in corneal hydration that accompany Fuchs Dystrophy. However, the specific contributions on frequency shift of hydration and mechanics independent of hydration due to the intrinsic solid component of the network have not been isolated. For this reason, Brillouin measurements are constrained to making relative modulus observations. Here, we successfully describe the relationship between Brillouin frequency shift, hydration, and solid mechanics within the cornea.
Methods :
Corneal buttons were separated into virgin and crosslinked groups. Following, each cornea was subjected to hydration/dehydration protocols in which the button was immersed in distilled water or left out in air. Then, the frequency shift of each sample was measured using Brillouin microscopy at varying probing angles in respect to the preferred collagen direction within the cornea.
Results :
In order to probe the intrinsic properties of the solid network, we quantified mechanical anisotropy by measuring Brilouin frequency shift at different angles relative to the collagen fiber direction. Brillouin microscopy did not lose sensitivity to mechanical anisotropy even at very high-water content (~96%). More so, Brillouin microscopy detected a significant difference in frequency shift between virgin and crosslinked corneas, notably independent of hydration. We built a mathematical model to relate hydration, solid mechanics, and Brillouin frequency shift and verified experimentally the validity of such model.
Conclusions :
Brillouin microscopy is sensitive to mechanical properties of the cornea and their changes both triggered by hydration and the intrinsic solid network. Our model successfully describes the relationship between Brillouin frequency shift, hydration, and solid mechanics; thus, decoupling the corneal hydration and solid mechanical contributions of Brillouin frequency shift. More so, this model paves the way towards obtaining a quantitative relationship between Brillouin measurements and gold-standard mechanical tests, offering a path for noninvasive and comprehensive measurements of corneal mechanics in vivo.
This is a 2020 ARVO Annual Meeting abstract.