Tear exchange and oxygen reservoir effects in silicone hydrogel systems.
OBJECTIVE
The effects of oxygen reservoir and tear exchange are known for conventional hydrogel contact lenses. This study attempted to (1) confirm their presence in lenses of a silicone hydrogel (SH) material and (2) evaluate their individual and combined contributions to hypoxic relief of the cornea.


METHODS
Corneal oxygen uptake rates were measured polarographically for 10 OD corneas immediately after 300-sec periods (equivalent to 60+ blink cycles) of: (1) nonblink wear of a 0 Dk/L polymethylmethacrylate (PMMA) cap lens; (2) nonblink wear of that cap lens with a SH lens inserted between it and the cornea; (3) regular blinked wear of that same lens combination; (4) nonblinked wear of the SH lens alone; and (5) normal open-eye, non-lens wear. The rates for each eye had a ratio with its own baseline (no lens) rate, and the total sample mean was calculated for each condition from those individual eye means.


RESULTS
(1) Under nonblink conditions, a 28% reduction in corneal oxygen demand was observed when a SH lens was inserted under the PMMA cap lens versus without. (2) When regular blinking was added to that lens combination, corneal oxygen demand decreased another 8%, for a total of 36%. The combination of SH insertion and blinking did achieve a statistically significant difference (P<0.05 by the Dunnett test) from the nonblink, 0 Dk/L, maximum deprivation condition.


CONCLUSIONS
The statistically significant hypoxia reduction observed with the SH lens insertion and blinking indicates the additive presence of two factors: (1) a lens reservoir effect caused by the SH lens and (2) a bulk-flow tear exchange effect caused by blinking. Their respective contributions to the reduction of corneal oxygen demand over the period studied were found to be in the ratio of 3.4:1.
Florkey
Lindsay
L
College of Optometry, The Ohio State University, Columbus, OH 43218-2342, USA.
Fink
Barbara A
BA
Mitchell
G Lynn
GL
Hill
Richard M
RM
D000328:Adult; D001672:Biocompatible Materials; D001767:Blinking / Q000502:physiology; D003261:Contact Lenses; D003263:Contact Lenses, Hydrophilic; D003315:Cornea / Q000378:metabolism*; D005260:Female; D006801:Humans; D000860:Hypoxia / Q000378:metabolism; D008297:Male; D010100:Oxygen / Q000378:metabolism*; D010539:Permeability; D011048:Polarography; D019904:Polymethyl Methacrylate; D012826:Silicone Elastomers; D013666:Tears / Q000378:metabolism*
D001672:Biocompatible Materials; D012826:Silicone Elastomers; D019904:Polymethyl Methacrylate; D010100:Oxygen