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Jerry R. Paugh, Fiona Stapleton, Lisa Keay, Arthur Ho; Tear Exchange under Hydrogel Contact Lenses: Methodological Considerations. Invest. Ophthalmol. Vis. Sci. 2001;42(12):2813-2820. doi: https://doi.org/.
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purpose. To characterize hydrogel lens tear exchange and to apply an optimized
method to compare tear exchange of a marketed hydrogel lens to that
measured with a prototype silicone hydrogel lens.
methods. Fluorophotometry and a nonpenetrating tracer (70-kDa FITC-dextran) were
used with a single extended-wear soft contact lens (EWSCL) material on
11 subjects to characterize tear-exchange kinetics. Twenty to 30
measurements were obtained over a 30-minute period to allow accurate
modeling and estimation of the several tear-exchange parameters.
Calculated values included tear-replenishment rate
(TRR), elimination rate (ER), and the
time for 95% of the signal to be eliminated
(T 95). Major experiments were (1) comparison
of ER under controlled and physiological conditions, (2)
comparison of right and left eyes, (3) repeatability of ER and T 95 on five occasions,
and (4) comparison of a marketed lens (oxygen permeability [Dk]
28 × 10−9 [cm/sec][ml O2/ml
mm Hg]) to a prototype silicone hydrogel lens (Dk 140 ×
10−9 [cm/sec][ml O2/ml mm Hg]).
results. Tracer elimination behind a hydrogel contact lens (CL), up to 30
minutes after insertion, was optimally described by double-exponential
kinetics. Physiological ER (5–30 minutes after CL
insertion) was optimally described by single-exponential kinetics.
Overall, physiological ER was 8.8% ± 3.8% per minute,
and T 95 was 31.0 ± 16.1 minutes
(n = 76 and 72 determinations, respectively).
Differences between right and left eyes in ER and T 95 were not significant at the 0.05 level.
No difference in ER or T 95 was found between habitual and controlled blinking. Mean TRR was 0.67% ± 0.26% per blink (n = 11 determinations). No differences were shown between ER or T 95 measurements over time. A prototype
highly oxygen-permeable silicone hydrogel lens showed higher ER than did a marketed hydrogel lens
(P < 0.01).
conclusions. Estimates of postlens tear exchange using a slit lamp fluorophotometer
are similar to previously reported rates using similar
fluorophotometric techniques. Fluorescent decay behind a hydrogel lens
is most precisely described using a double-exponential curve equation
and tear exchange may be described using ER, TRR and T 95, although the T 95 may be the least reliable of these
measures. The technique appears capable of discriminating between lens
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