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P<P, published online ahead of print May 9, 2008
(Investigative Ophthalmology and Visual Science. )
© 2008 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.07-1407
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Article

Rheology of Tear Film Lipid Layer Spread in Normal and Aqueous Tear Deficient Dry Eyes

Norihiko Yokoi 1*, Hideaki Yamada 2, Yutaka Mizukusa 3, Anthony J. Bron 4, John M. Tiffany 4, Takahisa Kato 5, and Shigeru Kinoshita 2

1 Department of Ophthalmology, Kyoto Prefectural University of Medicine, 465 Kajiicho, Hirokoji-agaru, Kawaramachi-dori, Kamigyo-ku, Kyoto, 602-0841, Japan
2 Department of Ophthalmology, Kyoto Prefectural University of Medicine, Kyoto, Japan
3 Kowa Co., Ltd., Tokyo, Japan
4 University of Oxford, Nuffield Lab. of Ophthalmology, Oxford, United Kingdom
5 Department of Mechanical Engineering, University of Tokyo, Tokyo, Japan

* To whom correspondence should be addressed. E-mail: nyokoi{at}ophth.kpu-m.ac.jp.


  Abstract

Purpose: To analyze the relationship between tear volume and tear film lipid layer (TFLL) spread. Subjects and Methods: Twenty-nine eyes from 22 subjects, including normal eyes and eyes with aqueous tear-deficient dry eye, were enrolled in this study. In all eyes, the radius of curvature (R: mm) of the central lower tear meniscus was measured with a video-meniscometer and interference images from the TFLL were recorded with a video-interferometer. The interference images were captured as still images every 0.05 seconds, and the relationship between the acquisition time for each image after a blink and the averaged heights of the spreading TFLL in the upstroke of the blink, were calculated. Results: In all cases, the time-dependent changes in TFLL spread could be described by the expression H(t) - H(0) = {rho}[1-exp(-t/{lambda})] [where H(t) = averaged height in mm at time = t; H(0) = averaged height at t = 0; {rho} = a constant; t = time in seconds; {lambda}: characteristic time in seconds], and a statistically significant correlation was found between those changes and the initial upward velocity of the spreading TFLL [H'(0) = dH(0)/dt] and R (r = 0.573, p = 0.003). Conclusions: This study demonstrated that the time-dependent changes of TFLL spread are compatible with the Voigt model of viscoelasticity, and that the initial velocity of TFLL spread after a blink decreased in proportion to the decrease of tear volume. There is potential interest in using this parameter to diagnose and evaluate the severity of aqueous tear deficiency.

Key Words: dry eyes, tear film, lipid layer spread, rheological model






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