Effect of surface tension on tear meniscus curvature

Purpose: To determine by optical coherence tomography (OCT) the effect of surface tension on reliability of tear meniscus curvature measurements Methods: Dimensions of the lower tear meniscus (LTM) were obtained in normal humans eye (n=22). The radius of curvature was calculated in two way. In the classical approach (system n.1), operator inputs were provided to identify three touch points used to fit a circle (R). In the second approach (system 2), since tear molecules close to epithelia of cornea and conjunctiva could interact differently with them, two radii of curvature in the LTM were calculated: the inner radius (ri), where the tear touches the cornea, and the outer radius (re), where the tear touches the lid. Results: The average R was 496.59 ± 24.28 μm. The ri and re were 401.34 ± 32.12 μm, 563.10 ± 29.14 μm, respectively. Significant differences were found between ri and re (Wilcoxon Signed Ranks Test = -3.095, p = 0.002), as well as R and both ri and re (Wilcoxon Signed Ranks Test = -3.095, p = 0.002). Also considering the mean value (rM) between the two radii (ri and re) were revealed significant discrepancies (Wilcoxon Signed Ranks Test = - 2.585; p = 0.01). Conclusions: Preliminarily, our study has demonstrated the difficulty in obtaining a reliable approximation of the radius of curvature of meniscus by the classical system. The system 2, considering the effect of the surface tension on interaction between tears and epithelia, appears to offer a different measure with respect to system 1. Therefore, the second approach could be more representative of the real state of health of ocular surface and could be useful to achieve more reproducible and comparable measurements of tear meniscus curvature.