Purpose. To compare an objective (VCH-1) with a manual subjective Abbé refractometer (MSAR) and evaluate the refractive index (RI)–hydration (H) relationship for the corneal stroma.
Methods. Epithelial and endothelial layers were removed from a fresh postmortem ovine corneal buttons. RI was measured at both surfaces using (i) MSAR then (ii) VCH-1. The sample was weighed, slowly dehydrated under controlled conditions (2 h), and RI measures were repeated. Sample was oven dried (90°C) for 3 d to obtain dry weight and hydration at each episode of RI measurement.
Results. Average difference between individual pairs of measurements obtained using the two refractometers (ΔRI) was 0.00071 (standard deviation ± 0.0029, 95% confidence interval ± 0.0058). Root mean square difference between measurements obtained by the refractometers was 0.0024. There was no relationship between ΔRI and the mean of each measurement pair (r = 0.201, n = 40, p = 0.214). Linear regression revealed a significant relationship between RI and reciprocal of H at both surfaces as follows: anterior (i) RI = 1.355 + 0.111/H (r = −0.852, n = 20, p = <0.001), (ii) RI = 1.357 + 0.105/H (r = −0.849, n = 20, p = <0.001) and posterior (i) RI = 1.353 + 0.085/H (r = −0.882, n = 20, p = <0.001), (ii) RI = 1.350 + 0.088/H (r = −0.813, n = 20, p = <0.001).
Conclusions. VCH-1 measurements are in good agreement with MSAR. RI at the anterior stroma was consistently higher suggesting hydration is lower by 1.10 units (6%) compared with the posterior stroma. Dehydration increased RI at both surfaces by similar rates. Current hypothetical models are useful for predicting RI from H for the posterior, but not the anterior, stroma.
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Research, Development and Innovation Department, VISSUM, Instituto Oftalmologico de Alicante, Alicante, Spain
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