Tear mixing under a soft contact lens: Effects of lens diameter

PURPOSE: Tear exchange under a soft contact lens is modest, and higher exch ange rates may be necessary to reduce extended-wear complications; what is not known is the optimal soft lens design to increase tear mixing. We explo red the effect of lens diameter on tear mixing. METHODS: Twenty-three subjects wore four different soft contact lenses with diameters of 12.0, 12.5, 13.0, and 13.5 mm. Tear mixing was quantified by placing fluorescein isothiocyanate-dextran on the posterior lens surface, i nserting the lens, and monitoring the changes in fluorescence intensity in the postlens tear film. Tear mixing, expressed as the percentage decrease i n fluorescence intensity per blink, was estimated using an exponential mode l, Lens movement was videotaped and lens comfort was graded on a 50-point s cale (50 = excellent comfort). Subjects reporting a comfort level of less t han 35 were excluded. RESULTS: The mean +/- SE tear mixing rates were 1.82% +/- 0.17%, 1.61% +/- 0.16%, 1.34% +/- 0.17%, and 1.24% +/- 0.17% per blink for the 12.0-, 12.5-, 13.0-, and 13.5-mm diameter lenses, respectively. By regression analysis w e found that, on average, mixing under the 12.0-mm lens was 0.59% per blink greater than with the 13.5-mm lens (P = .0024). Lens diameter was a signif icant predictor of lens comfort, and adjusting for the effects of comfort w eakened the relationship between diameter and tear replenishment rate, alth ough the mean rate under the 12.0-mm lens was still 0.43% per blink greater than with the 13.5-mm lens (P = .0468). CONCLUSIONS: These data suggest that smaller-diameter soft lenses provide s ubstantially better tear mixing than larger lenses; however, even small len ses provide modest tear mixing compared with rigid contact lenses. (Am J Op hthalmol 1999;127:659-665. (C) 1999 by Elsevier Science Inc. All rights res erved.).