Changes in the refractive index of the stroma and its extrafibrillar matrix when the cornea swells

Abstract

The transparency of the corneal stroma is critically dependent on the hydration of the tissue; if the cornea swells, light scattering increases. Although this scattering has been ascribed to the disruption caused to the arrangement of the collagen fibrils, theory predicts that light scattering could increase if there is an increased mismatch in the refractive indices of the collagen fibrils and the material between them. The purpose of this article is to use Gladstone and Dale's law of mixtures to calculate volume fractions for a number of different constituents in the stroma, and use these to show how the refractive indices of the stroma and its constituent extrafibrillar material would be expected to change as more solvent enters the tissue. Our calculations predict that solvent entering the extrafibrillar space causes a reduction in its refractive index, and hence a reduction in the overall refractive index of the bovine stroma according to the equation n'(s) = 1.335 + 0.04/(0.22 + 0.24 H'), where n'(s) is the refractive index and H' is the hydration of the swollen stroma. This expression is in reasonable agreement with our experimental measurements of refractive index versus hydration in bovine corneas. When the hydration of the stroma increases from H = 3.2 to H = 8.0, we predict that the ratio of the refractive index of the collagen fibrils to that of the material between them increases from 1.041 to 1.052. This change would be expected to make only a small contribution to the large increase in light scattering observed when the cornea swells to H = 8.

FIGURE 1

Refractive index of stroma as a function of tissue hydration calculated using Gladstone and Dale's law. Dotted lines indicate confidence limits of the solid curve.

FIGURE 2

Refractive index of extrafibrillar matrix as a function of tissue hydration calculated using Gladstone and Dale's law. Dotted lines indicate confidence limits due to uncertainties in experimental data.

FIGURE 3

Refractive index ratio, m, as a function of tissue hydration, calculated using Gladstone and Dale's law. Dotted lines indicate confidence limits due to uncertainties in experimental data. The value m = 1 corresponds to the situation where the refractive indices of the collagen fibrils and the extrafibrillar matrix are equal. This was found not to be the case for either the physiological or the swollen stroma.

FIGURE 4

(a) Experimental values of the refractive index of bovine stroma (each point represents the average of anterior and posterior measurements from a given cornea) compared with the theoretical variation predicted by Eq. 21 (continuous line). (b) Linear regression of experimental and theoretical data in Fig. 4 a indicates a correlation coefficient of 0.78.

FIGURE 5

Refractive index measured at the posterior stroma (a) and anterior stroma (b). In each case, the continuous line is the theoretical prediction (Eq. 21).