Abstract
Purpose: :
The tear film lipid layer is known to spread over the aqueous layer after a blink. In a previous study, time required until the lipid layer becomes stable after a blink (defined as spreading time) becomes longer in aqueous tear–deficient dry eyes compared to the normal aqueous sufficient eyes (Goto E, IOVS44, 2003). This study investigated the relationship between the speed of lipid layer spread and the aqueous tear volume using the rheological model.
Subjects and Methods: :
27 eyes from 20 subjects including normal volunteers and patients with aqueous tear–deficient dry eye [all female patients; aged 65.6 ± 11.9 (mean ± SD) years] were enrolled in this study. In all subjects, the radius (R: mm) of lower tear meniscus was measured by meniscometry (Yokoi N, BJO, 1999) and interference images from the tear film lipid layer were recorded digitally by DR–1TM (Kowa). Areas of the lipid layer during spreading were sequentially calculated every 0.05 sec from the recorded interference images. The time–dependent changes of areas of the lipid layer during spreading were analyzed rheologically using the Voigt model {S(t) = ρ[1–exp(–t/)]; S(t): area (mm2), ρ: constant number, t: second (sec), : relaxation time (sec)}, and the relationship between the initial speed of the lipid layer spread [S’(0) = dS(t)/dt | t=0] (mm2/sec) and R were analyzed.
Results: :
In all cases, time–dependent spreading of the tear film lipid layer was appropriately analyzed using the rheological Voigt model and a linear correlation was seen between S’(0) and R [S’(0) =656.8×R–87.7; r=0.793, p<0.001].
Conclusions: :
Rheological analysis demonstrated that the tear film lipid layer spreads with the behavior of a viscoelastic substance, and the initial spread speed of the lipid layer after a blink considerably increased dependent on the increase of aqueous tear volume.
Keywords: cornea: tears/tear film/dry eye