In vivo tear-film thickness determination and implications for tear-film stability

Purpose. Previous measurements of tear-film thickness in vivo are limited and cannot be easily applied in a clinical setting. A novel technique to measure tear-film thickness indirectly is introduced here, requiring only a slit lamp, video camera, and computer. A recent fluid mechanical theory relates tear-film thickness h, to the tear meniscus radius R, tear surface tension sigma, tear viscosity mu, and upper lid velocity U. This theory yields the result that h/R = 2.12 (mu U/sigma)(2/3). All parameters except h/R are taken as known physical constants, and R was measured for each subject, allowing the above equation to establish h. Tear-film breakup was also evaluated and correlated with tear-film thickness. Methods. A clinical study was performed in which aqueous tear-film thickness was determined for 45 subjects, including 24 non-lens subjects, 15 hydrogel contact lens wearers, and 6 RGP lens wearers. R was measured by instilling fluorescein dye in the form of an eyedrop and videotaping the tear meniscus in profile. Tear-film breakup was videotaped through the ocular port of the slit lamp and evaluated based on a severity scale. Results. Aqueous tear-film measurements are in the same range as literature values, with most measured values falling between 6 and 12 mu m. Average tear-film thicknesses for non-lens, hydrogel, and RGP subjects are 10.4, 6.5, and 5.8 mu m, respectively. Tear-film breakup is most severe in subjects with thin tear films, especially in contact-lens wearers. Conclusions. Tear-him thickness is an important parameter that varies among individuals. These variations correlate with differences in tear-film stability.