Relating Nanostructural Properties of the Cornea, its Macroscopic Transparency and Compressibility

Abstract

Purpose : To establish a few-parameter model of corneal stromal structure on a nanometric and micrometric level and predict macroscopic parameters like transparency and compressibility.

Methods : Transmission electron microscopy (TEM) images of the stroma from n=16 healthy and edematous corneas were analyzed using a custom-developed algorithm permitting to extract the pair-correlation function of collagen fibrils, thus quantifying the structural order on the nanometric level. The predictions of a few-parameter model were adjusted to fit the data. This physical model reduces the complex electrostatic interaction between fibrils to a single typical interaction distance and predicts optical properties while taking into account only single scattering processes. The photon mean-free path (l_s, indicating the penetration depth of a coherent plane wave) was calculated using literature values for collagen and interstitial liquid refractive indices. Corneal compressibility under osmotic pressure was similarly estimated and compared to data obtained from a study relating corneal thickness to the concentration of a deturgescence solution.

Results : The results are summarized in Table 1. Strong agreement was found between the extracted and predicted pair-correlation functions, as well as between measured and estimated compressibility. The observed l_s values for healthy corneas (around 7 mm) and edematous corneas (17 mm) were more than 10x or 20x superior to typical stromal thicknesses, respectively.

Conclusions : Agreement between model predictions and experimental results suggests that the complex electrostatic interactions among collagen fibrils may be adequately described by a reduced model using a single typical interfibrillar interaction distance.
Our study shows that under physiological conditions the perturbation of the collagen fibril structure cannot explain transparency reduction with developing edema. The fact that l_s increases indicates that the dilution of the fibril density has a stronger effect than the increase in scattering cross section with edema.
Light scattering effects, reducing visual acuity, contrast sensitivity and increasing glare, may be attributed to micrometric artefacts developing with edema, such as swelling keratocytes and formation of collagen-free liquid pockets often called “lakes.”

This is a 2020 ARVO Annual Meeting abstract.

 

Data derived from TEM images of the stroma of 11 healthy and 5 edematous human corneas.