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.).