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C. A. Woods, J. Varikooty, N. Keir, D. Fonn; Investigation of a Novel in vivo Method for Measuring the Refractive Index of Soft Contact Lenses Using Refractometry. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5655.
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To determine whether the refractive index of a soft contact lens can be evaluated using refractometry while the lens remains on the eye and to compare this with more traditional ex vivo refractometry assessments.
In order to establish this novel in vivo assessment of contact lens refractive index, a slit lamp apparatus was modified to incorporate a customized AtagoTM hand refractometer. Nine adapted symptomatic soft contact lens wearers wore two contact lenses (lotrafilcon B and etafilcon A), one on each eye in randomised order. A double-masked study design was utilized. In vivo refractive index was determined from the relative brix scale measurements immediately following lens insertion and after one and ten hours of lens wear. Ex vivo refractometry was measured after ten hours of lens wear for comparison. Mean ± standard error of the mean is reported.
In vivo refractive index values at baseline were 1.422±0.001 (lotrafilcon B) and 1.405±0.004 (etafilcon A); following 1 hour of lens wear, values were 1.424±0.002 and 1.405±0.004, respectively; and following ten hours of lens wear, values were 1.425±0.001 and 1.412±0.003, respectively. At the end of the ten hour wearing period, ex vivo refractive index values were 1.424±0.001 (lotrafilcon B) and 1.412±0.001 (etafilcon A).
This novel adaptation of refractometry was able to measure the refractive index of soft contact lenses and it’s variation during periods of wear. End of day measurements using in vivo and ex vivo refractometry were comparable. Traditional methods involve lens removal; therefore monitoring refractive index during lens wear requires an alternative technique. This novel in vivo method was able to assess refractive index and hence lens hydration without lens removal, which may further our understanding of the relationship between hydration of soft contact lenses and symptomology.
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