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T.R. Karkkainen, M.R. Utech, H.J. Chavda, M.L. Roberts; Dehydration Rates of Ionic and Non-ionic Hydrogel Contact Lenses . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3715.
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Purpose: Dehydration of hydrogel contact lenses often leads to patient complaints of dryness and may reduce wear time. Factors affecting lens dehydration include lens thickness, power, water content, tear parameters and ambient environment. One aspect that has received little attention in the literature is the affect of lens charge on dehydration. The purpose of this study is to compare the in-vitro dehydration rates of ionic and non-ionic contact lenses. Methods: Twenty lenses from each FDA group II (hilafilcon material) and group IV (methafilcon material) classifications were used. In addition another group II material (alfafilcon) often used in marginal dry eye lens wearers was included for comparison. The lenses used were controlled for power, lens thickness and water content. Room temperature and humidity were controlled and continuously monitored. Evaporation rate was determined by a gravimetric method similar to techniques described by other investigators. Each lens was removed from its packaging and excess solution discarded from the lens surface. Lens mass was determined using a Mettler AE50 scale (Columbus, OH) that had been previously calibrated. Measurements were determined at the following time intervals: 0 minutes(baseline), 15 min, 1 hour and 4 hours. Each lens was then dehydrated overnight in a chamber containing anhydrous calcium sulfate. The mean loss of mass over time was calculated and an ANOVA was applied to determine significant differences between the means. Results: The mean and standard deviation for the change in lens mass over time was 34.10±0.017mg, 29.15±.014mg and 23.42±0.008 for the hilafilcon, alphafilcon and methafilcon materials respectively. The ANOVA revealed a significant difference (p<0.001) between the overall means during the measurement period. The methafilcon material was noted to have the least amount of mass lost over the first 15 minutes and also dehydrated less between the 1 and 4 hour storage periods. Conclusions: When other dehydration variables are controlled for group IV methafilcon lenses dehydrate significantly less than group II lenses. We hypothesis the effect is secondary to the overall negative charge of the material causing water to be retained within the lens matrix. This information is important when deciding what material to use on patients with marginal dry eye and also should be considered when determining the best lens for use in solution/drug delivery to the ocular surface. Future studies should examine the in-vivo dehydration response and evaluation of additional materials in these groupings.
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