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Jeffery Schafer, Robert Steffen; Evaluation of Surface Water Characteristics of Novel Daily Disposable Contact Lenses Using Refractive Index Shifts after Wear. Invest. Ophthalmol. Vis. Sci. 2013;54(15):487.
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© ARVO (1962-2015); The Authors (2016-present)
Several novel daily disposable contact lenses have been introduced with unique water characteristics. Nesofilcon A lenses are described as having 78% water, the same water content as the cornea throughout the lens matrix. Delefilcon A lenses are described as having a surface water content of 80% and a bulk water content of 33%. The delefilcon A high water content at the surface is reported to be the result of a surface modification using a copolymer of polyamidoamine and poly(acrylamide-acrylic acid) which is highly anionic (negative charge). During wear, these high water materials are exposed to air and tear components that may change the properties of the lenses. The objective of this study was to investigate surface water characteristics using refractive index shifts after wear with delfilcon A lenses compared to nesofilcon A and etafilcon A lenses.
Twenty subjects wore each of the three lens types in a randomly determined order for 15 minutes. The worn lenses for each subject were measured for surface refractive index on the Metricon M-2010 Prism Coupler. To establish baseline refractive index values, unworn lenses of each type were also measured for refractive index directly from the package.
The mean changes in refractive index (unworn - worn) were 0.006 for nesofilcon A lenses, 0.012 for etafilcon A lenses and 0.093 for delefilcon A lenses. With the highly accurate measurement capability of the Metricon instrument, (routine refractive index accuracy of ± 0.00053 and standard deviation from 0.0008 to 0.0046), the difference between unworn and worn average values were statistically significant for each lens, p<0.0001.
Lenses with higher surface water content have a surface refractive index closer to the refractive index of water (1.33), while lenses with lower surface water content will have a higher refractive index. The refractive index is typically 1.46-1.48 for a 20% water lens and 1.37-1.38 for a 75% water lens. The results of this study show a change in mean surface refractive index for the delefilcon A lenses from 1.34, typical of >80% water, to 1.43, typical of a 33% water, following just 15 minutes of wear due to a change in the water content at the surface. There was no change in refractive index at the surface following lens wear for either the 78% water nesofilcon A lenses or the 58% water etafilcon A lenses.
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