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J. J. Doutch, C. Tucker, A. J. Quantock, P. A. R. Ade, K. M. Meek; The Correlation Between Corneal Transmission in the Near/ Mid Infrared and Corneal Hydration. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3508.
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The infrared transmittance of the corneal stroma has been approximated to that of the equivalent path length of liquid water. It is known that there is a linear relationship between corneal hydration and corneal thickness, thus as tissue hydration is increased, it is reasonable to suppose that the equivalent path length of liquid water should also increase. Here, we aim to determine the relationship between transmitted near infrared intensity and corneal hydration to assess the potential for the use of infrared light for non-invasive corneal hydration measurements.
Fresh bovine corneas were equilibrated to a physiological hydration level, and near and mid infrared spectra obtained using a Fourier transform spectrometer concentrating on the 3.3µm - 1.5 µm region. Corneas were then swollen by immersion in a saline solution, and spectra taken at 15 minute intervals for 1 hour. Wet weights were taken at all stages. Samples were subsequently placed in an oven for at least 48 hours to obtain a dry weight, and hydration levels extracted from the wet weight/dry weight ratios.
A linear relationship was found between transmitted intensity and stromal hydration. With 2.22µm selected as a convenient point of analysis, it was discovered that the dry, mainly collageneous mass of cornea affected infrared transmission at low hydration levels. Using the linear relation between corneal wet weight and transmitted intensity a normalisation protocol was employed to factor out the effect of small variations in dry weight between individual specimens. The thickness/hydration relationship was used to calculate the corresponding stromal thickness in each case. Using these values the absorption coefficient was calculated for each hydration. Unexpectedly, this was found to change with hydration, reaching a minimum at hydration value slightly above physiological, before gradually increasing back to its physiological value.
Corneal transmittance in the infrared assumes a linear relationship with hydration. This property could be used as a non-destructive tool for assessing corneal hydration for research and tissue banking purposes, and also has potential for introduction to the clinical realm. We suspect that the unexpected change in the absorption coefficient is caused by changes in the arrangement of water molecules within the collagen matrix with increasing hydration.
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