In vivo dehydration of silicone hydrogel contact lenses

PURPOSE: To benchmark the performance of new-generation silicone hydrogel contact lenses in terms of their in vivo hydration characteristics and to highlight the possible clinical ramifications of any changes observed. METHODS: Thirteen subjects (four men and nine women with a mean age of 24.8 +/- 5.0 years) wore a silicone hydrogel lens (PureVision, Bausch & Lomb, Rochester, NY) in one eye and a conventional hydrogel lens (ACUVUE 2, Johnson & Johnson Vision Care, Jacksonville, FL) in the other eye for 4 weeks on an extended-wear basis. A gravimetric method was used to determine lens water content and dehydration during the intended lifespan of the lenses. RESULTS: For the PureVision lens, the water content was 38.3% +/- 0.9%, 35.2% +/- 1.1%, and 35.3% +/- 1.7% at baseline and after 2 and 4 weeks of wear, respectively (F=28.4, P<0.0001). For the ACUVUE 2 lens, the water content was 58.1% +/- 0.6% and 52.1% +/- 1.3% at baseline and after 2 weeks of wear, respectively. Thus, after 2 weeks of wear, absolute dehydration was 2.8% +/- 1.8% and 6.0% +/- 1.3% for the PureVision and ACUVUE 2 lenses, respectively (t=6.8, P<0.0001). The mass of deposition was calculated to be 568 +/- 457 microg for the PureVision lens and 1,660 +/- 499 microg for the ACUVUE 2 lens (t=5.1, P=0.0003). CONCLUSIONS: The ACUVUE 2 lens underwent a greater degree of lens dehydration, causing a reduction in oxygen permeability (3.6 barrer), and deposition after 2 weeks of extended wear. The loss of water from the PureVision lens was paradoxically associated with a 6.0-barrer increase in oxygen permeability after 4 weeks of extended wear.