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S. Dennis, S. Khan, K.M. Meek; Changes in the Refractive Index of the Cornea when the Stroma Swells . Invest. Ophthalmol. Vis. Sci. 2003;44(13):886.
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Purpose: To examine the change in refractive index of bovine corneal stroma as solvent enters. Methods: Measurements of refractive index were taken from the anterior (epithelium) and posterior (endothelium) sides of swollen de-membranated bovine corneas using a benchtop Abbe refractometer. Results: As solvent enters the tissue, the average refractive index of the stroma (ns) reduces. The shape of the ns versus hydration graph can be explained using Gladstone and Dale’s law of mixtures. We also confirmed in bovine cornea earlier observations in pigs and humans (Patel et al, J. Refract. Surg. 11 100-105, 1995) that at physiological hydration the refractive index of the anterior stroma is greater than that of the posterior stroma (p<0.01). Furthermore, we have shown that in swollen corneas the refractive index of the posterior stroma is less than that of the anterior (p<0.01), likely reflecting the preferential distribution of water to the posterior stroma during swelling. Conclusions: We have previously shown that collagen fibrils swell very little when solvent enters the stroma (Meek et al., Biophys J. 60 467-474,1991) so their refractive index (nf) does not change. The solvent entering the interfibrillar space causes a reduction in its refractive index (ne), and hence a reduction in the overall refractive index of the stroma. This increased mismatch between nf and ne should lead to an increase in light scattering, particularly in the posterior stroma, although the magnitude of this effect has yet to be determined.
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