Assessing Corneal Elasticity as a Function of Hydration with Optical Coherence Elastography

Abstract

Purpose : Recent research has called the stiffening effects of CXL into question due to confounding hydration. Here, we quantify the Young’s modulus of in situ rabbit corneas as they were dehydrated with a 20% dextran solution, which mimics the “Dresden” CXL protocol. Optical coherence elastography (OCE) measurements were combined with a finite element method (FEM) to quantify the elasticity of the corneas.

Methods : A focused micro air-pulse induced low-amplitude (≤ 10 μm) elastic waves in rabbit corneas (N=10) in the whole eye-globe configuration. The waves were imaged by a phase-stabilized swept source OCE system. The corneas were hydrated with a 1X PBS solution for one hour. Then, a 20% dextran solution was topically added every 5 minutes for an additional hour. OCE measurements were made every 20 minutes.
The Young’s modulus was quantified by an error-minimization process. At each OCE measurement time point, the average central corneal thickness (CCT) was used to construct an FE model of the cornea. The Young’s modulus of the FE cornea was adjusted to minimize the error between the OCE-measured elastic wave speed and FEM-simulated speed.

Results : The CCT of the rabbit corneas increased slightly from 671±54 µm at 0 mins to 676±49 µm after the corneas were hydrated with PBS for 1 hr. After 60 mins of topical addition of the dextran solution, the corneal CCT thinned to 367±29 µm. The OCE-measured elastic wave speed was 3.18±0.24 m/s at 0 mins, which then slightly increased to 3.23±0.32 m/s after 1 hr of hydration with PBS. After 60 mins of topical addition of the dextran solution, the elastic wave speed decreased to 2.64±0.21 m/s. The FEM-reconstructed Young’s modulus at 0 mins was 500 kPa, which slightly decreased to 465 kPa after 1 hr of hydration with PBS. However, the corneal elasticity then dramatically increased to 800 kPa 1 hr after continual topical addition of the dextran solution.

Conclusions : The Young’s modulus of the corneas increased dramatically after the corneas dehydrated and thinned, demonstrating that corneal hydration and geometry must be accounted for when quantifying corneal elasticity.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.

 

Figure 1. (a) OCE-measured central corneal thickness (CCT) and OCE-measured elastic wave group velocity. (b) CCT and FEM-reconstructed Young’s modulus.