Contact lens shape dynamics during off-eye dehydration

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Over the course of lens wear, contact lenses dehydrate. This loss in water content may lead to not only discomfort, but also surface instability resulting in degradation of vision. To study the rate of dehydration and understand the associated lens shape changes, four commercially available daily disposable contact lenses were examined in a controlled laboratory environment. As a means to study the properties of the lenses alone, without any subjectivity associated with individual patients, this study was conducted off-eye.

The lenses under investigation included: BiotrueONEday, with a water content of 78 per cent, 1-Day Acuvue Moist with a water content of 43 per cent, 1-Day Acuvue TruEye (narafilcon B) with a water content of 48 per cent and Dailies AquaComfort Plus with a water content of 69 per cent. Four -3.00DS power lenses of each product type were allowed to dehydrate in an ambient environment for 20 minutes to observe the physical shape changes over time. All 16 lenses were allowed to dehydrate at the same time. At the time of the experiment, the average indoor temperature was 72°F (22.2°C) with a relative humidity of 30 per cent.

Each lens was taken from the package and blotted to remove excess packaging solution. A photograph was taken every two minutes over the 20-minute period. Although changes were occurring throughout the 20-minute period, three of the commercially available contact lens materials showed significant edge 'fluting' as a result of lens dehydration at the end of the 20-minute test period, while the Biotrue ONEday contact lens material did not (Figure 1).

Four pairs of photos for each lens type, at time zero and time 20 minutes, were presented to 12 subjects who rated whether the lens shapes shown in the two images were similar or different, using a 10-point rating scale where 1 corresponded to 'extremely similar' and 10 corresponded to 'extremely different'. A one-way ANOVA showed a statistically significant difference (p<0.0001) between the Biotrue ONEday lens (A) compared to Dailies AquaComfort Plus (D), 1-Day Acuvue TruEye (narafilcon B) (C) and 1-Day Acuvue Moist (B), where the mean rating scores were 1.9, 8.8, 8.4 and 6.8, respectively, indicating that the Biotrue ONEday lens, after 20 minutes looked 'extremely similar' to the time zero image (Figure 2).

Traditional clinical literature and belief suggest that higher water content lenses dehydrate more than lower water content lenses. In this study, the highest water content lens material demonstrated a more consistent lens shape over the 20-minute dehydration period compared to 1-Day Acuvue Moist, 43 per cent; 1-Day Acuvue TruEye (narafilcon B), 48 per cent and Dailies AquaComfort Plus, 69 per cent lenses. These findings suggest that the Biotrue ONEday material with 78 per cent water content may have unique properties relative to water content retention.

? Rosa Lee is senior product design engineer and Dr Alexis Vogt optical physicist at Bausch+Lomb, Rochester, NY