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William J. Benjamin, Huagang Chen, Owen Gan; Apparent Tonicity of Contact Lenses: Part 2. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6486.
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To determine if apparent hypertonicity of a conventional and a silicone-hydrogel contact lens worn up to 8 hrs by wearers with end-of-day dry eye symptoms was influenced by conditioning in different contact lens care (CLC) solutions. Differences between CLC solutions were evaluated. The tonicity progression and comparison of lenses were reported in Part 1 (Optometry & Vision Science 87:Abstract 105869, 2010).
Apparent Tonicity (AT) of soft lenses was assessed with a vapor pressure osmometer upon removal from the eyes of 4 wearers with end-of-day symptoms of dry eye. Lenses were treated overnight in 3 different CLC solutions before wear: ReNu MultiPlus (Bausch & Lomb, Inc., Rochester NY), Opti-Free RepleniSH (Alcon Laboratories, Inc., Ft. Worth TX), and Clear Care (CIBA Vision Corporation, Duluth GA). Lenses were measured at time 0 and then after 2, 4, & 8 hrs of wear. Identities of care regimens were doubly masked and new lenses were supplied for each period of wear because testing was destructive. Subjects participated for 18 days in which the CLC solution was staggered. A total of 192 osmotic readings were calibrated according to linear regressions derived from measurements after equilibration in reference solutions of 100, 300, & 500 mOsm/kg.
An analysis of variance revealed a significant effect of CLC solution [p = 0.0003] but not interactions between CLC solution and wearing duration [p = 0.2200] or lens type [p = 0.4474]. The mean tonicities found at time 0 for ReNu (278 mOsm/kg) and Opti-Free (285) were significantly lower than that of Clear Care (319). The mean tonicities found from 2 to 8 hrs of wear for ReNu (356 mOsm/kg) and Opti-Free (343) were significantly lower than that of Clear Care (410).
Surprisingly, the apparent tonicity of soft contact lenses was influenced by the CLC solution in which the lenses were equilibrated even up to 8 hrs of wear. The apparent tonicity associated with equilibration in one CLC solution approached the average threshold for awareness of hypertonicity upon instillation of eye drops (430 mOsm/kg). More data on other types of soft contact lenses and CLC solutions are needed in order to see if these findings are representative over the spectrum of soft lenses and lens care products.
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