Corneal refractive index-hydration relationship by objective refractometry.

Abstract

PURPOSE: To compare an objective (VCH-1) with a manual subjective Abb&#xe9; 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&#xb0;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 (&#x394;RI) was 0.00071 (standard deviation &#xb1; 0.0029, 95% confidence interval &#xb1; 0.0058). Root mean square difference between measurements obtained by the refractometers was 0.0024. There was no relationship between &#x394;RI 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.