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P. Ramamoorthy, L. T. Sinnott, J. J. Nichols; Contact Lens Material and Tear- Related Factors Associated With Hydrogel Lens Dehydration. Invest. Ophthalmol. Vis. Sci. 2009;50(13):6340.
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The purpose of this study was to determine the association between hydrogel contact lens material characteristics, tear osmolality and contact lens dehydration.
The data for this analysis were derived from automated refractometry measurements of currently worn hydrogel contact lenses and tear osmometry measures of 380 participants of the Contact Lens and Dry Eye study (CLADES) (Nichols et al, IOVS, 2005, 2006). CLADES was a large cross-sectional study on contact lens wearers with a nested case control arm. An automated refractometer (CLR 12-70, Index Instruments, Cambridge, UK) was used to measure water content and refractive index of currently worn lenses. Dehydration was determined in two ways, as the difference between nominal and measured: (1) water content or (2) refractive index. After refractometry, 200-nL tear samples were obtained and analyzed using a commercial tear osmometer (Advanced Instruments, Needham Heights, MA). High vs. Low water content, ionicity, and tear osmolality measures were analyzed as candidate predictors of dehydration (using both water content and refractive index) as the outcome variables in multiple regression models.
The overall measured and nominal water content values were 51.59 ± 7.3 (%) and 50.98 ± 13.89 (%) respectively, while the measured and nominal refractive indices were 1.43 ± 0.02 and 1.41 ± 0.02. Linear regression modeling for dehydration as difference in water content revealed that high water content and ionic hydrogel lens materials were associated with greater dehydration (p < 0.0001 for both) than low water content and nonionic materials. Tear osmolality was not significant; however it was retained as a control variable. When dehydration was assessed as the difference in refractive index, only high water content was significantly associated with dehydration (p<0.0001) after controlling for tear osmolality.
High water content and ionic nature of hydrogel lens materials are significantly associated with hydrogel lens dehydration, with the former being more strongly associated. Such dehydration changes could in turn lead tomay result in important clinical ramifications such as reduced oxygen transmissibility, greater lens adherence and reduced tear exchange.
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