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Walter L. Nash, Mary Mowrey-McKee, Alan Landers; An Evaluation of Two Methods for the Ex Vivo Analysis of Lens Osmolality. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6497.
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To develop a reliable method to determine the contact lens osmolality of samples collected from clinical studies for ex vivo analysis taking care to reduce evaporation from lenses and assay materials.
In the first study, lotrafilcon A and comfilcon A lenses were collected from the eye by clinicians aseptically and placed directly into a pre-weighed eppendorf tube containing 200 µL of a 100 mOsm/kg elution solution using clean forceps. In the second study, the same lens materials were collected from the eye by clinicians aseptically and placed directly into a pre-weighed eppendorf tube, no longer containing elution solution, using clean forceps. For the samples collected in the second study, eppendorf tubes containing only the lens sample were weighed and a 200 µL of a 100 mOsm/kg elution solution was then added to each sample. In both studies, samples which contained elution solution and lens sample, were again weighed and then incubated for a minimum of 18 hours at 33±2 ºC to equilibrate. Samples and controls were removed from the incubator, reweighed, and agitated on a vortex for approximately 15 seconds. Next the osmolality of each elution control was measured once, while the osmolality of each sample elution was measured in duplicate. Samples were then dried at 55±2 ºC for a minimum of 20 hours with the eppendorf tube’s cap off and subsequently reweighed. For both methods, the lens osmolality was calculated by determining the weight of the elution solution added to each tube, wet weight of the lenses, dry weight of the lenses, water weight of the lenses, % water content and weight change due to 33±2 ºC incubation. These values were then used to calculate the final lens osmolality per sample.
Lens osmolality data collected on worn lenses in the first study exhibited a relative standard deviation for comfilcon A and lotrafilcon A of 26 % and 51 %, respectively, and 15 % and 28 % using the modified methodology. A significant drop in lens osmolality was observed for both lens materials using the modified methodology.
Methods for measuring lens osmolality are highly dependent upon maintaining the lens water content volume and the elution solution volume throughout the procedure.
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