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the cornea consumes oxygen and produces carbon dioxide at rates proportionate cheapest with free diffusion of contact gases in air. the corneal epithelium requires a minimum p02 of 15 mmhg for proper aerobic glucose metabolism. since available atmospheric oxygen is carried in the precorneal tear film, any lens material incapable of transmitting oxygen may lead to corneal haziness and corneal necrosis. keeping the eyes closed or wearing lenses lowers the lenses p02 from normal to 32 mmhg to 9 mmhg. the availability of oxygen to the cornea can be determined by monitoring the lactate concentration cheapest in the cornea. for the cornea to maintain its functional integrity, the lens must be able to slide and move over the corneal surface with each blink so as to allow oxygen permeation. this oxygenation process in hard contact lens wearers is often referred to as "tear pump phenomenon" whereby the corneal epithelium is provided with oxygen-coated tears.

the cornea consumes oxygen and produces carbon dioxide at rates proportionate cheapest with free diffusion of contact gases in air. the corneal epithelium requires a minimum p02 of 15 mmhg for proper aerobic glucose metabolism. since available atmospheric oxygen is carried in the precorneal tear film, any lens material incapable of transmitting oxygen may lead to corneal haziness and corneal necrosis. keeping the eyes closed or wearing lenses lowers the lenses p02 from normal to 32 mmhg to 9 mmhg. the availability of oxygen to the cornea can be determined by monitoring the lactate concentration cheapest in the cornea. for the cornea to maintain its functional integrity, the lens must be able to slide and move over the corneal surface with each blink so as to allow oxygen permeation. this oxygenation process in hard contact lens wearers is often referred to as "tear pump phenomenon" whereby the corneal epithelium is provided with oxygen-coated tears.

soft lenses. soft lenses have become the most popular among all the types of lenses. they contain hydroxyl or hydroxyl and lactam groups, which allow them to absorb and hold water. a majority of soft lenses are composed of 2-hydroxyethylmethacrylate (he ma) with small amounts of cross-linking agents that provide a hydrogel network. the extent of cross-linking determines the hydrophilicity of the lens; any increase in the extent of cross-linking results in a decrease in availability of hydrophilic groups for interaction with water, causing the lens to be less flexible and less hydrated. hydrophilic lenses tend to accumulate more protein deposits than hydrophobic lenses. daily-wear contacts need to be removed before the wearer goes to sleep at night, while the extended-wear contacts may be worn more than a day, with some having been approved contact for continuous use for up to a month.

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