title: Rheology of tear film lipid layer spread in normal and aqueous tear-deficient dry eyes.
abstract: OBJECTIVE
  To analyze the relationship between tear volume and tear film lipid layer (TFLL) spread.
  
  
  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. Interference images were captured as still images every 0.05 second, 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) is the averaged height in millimeters at time t, H(0) is the averaged height at t = 0, rho is a constant, t is time in seconds, and lambda is the characteristic time in seconds. 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.
authors.affiliation: Department of Ophthalmology, Kyoto Prefectural University of Medicine, Kyoto, Japan. nyokoi@koto.kpu-m.ac.jp
authors.forename: Norihiko
authors.initials: N
authors.lastname: Yokoi
authors.forename: Hideaki
authors.initials: H
authors.lastname: Yamada
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Mizukusa
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Bron
authors.forename: John M
authors.initials: JM
authors.lastname: Tiffany
authors.forename: Takahisa
authors.initials: T
authors.lastname: Kato
authors.forename: Shigeru
authors.initials: S
authors.lastname: Kinoshita
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015352:Dry Eye Syndromes / Q000378:metabolism*; D005260:Female; D006801:Humans; D007368:Interferometry; D008027:Light; D050356:Lipid Metabolism / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D012212:Rheology; D013666:Tears / Q000378:metabolism*; D013997:Time Factors