In the early eighties,most of the material development was focused on rigid gas permeable (RGP) lenses.The addition of siloxane and later fluorine components to RGP materials enabled the manufacture of lenses with superior Dk/t,Refinements to RGP materials,in which the Dk/t was balanced with other desirable properties,led to lenses which could be worn overnight with an encouraging degree of success.The first reports of overnight use with RGP lenses appeared in 1984 for aphakia［28］,and 1985 for cosmetic wear.RGP lenses seemed a pronising alternative,offering ease of handling,ease of care,durability,high Dk/t,compatible surface characteristics,good tear exchange,deposit resistance,decreased limbal vessel response,and superior visual performance in many cases［2］.

　　Although the critical oxygen requirement of even extended wear can be reached with RGP lens materials,these lenses can cause other problems which are predominantly related to the mechanical properties of the lens.These complications include:discomfort which can lead to discontinuation of lens wear［2,29］,3&9 o'clock staining,lens adherence and corneal distortion.

　　Infection

　　Poggio et al［30］found the annualized incidence of ulcerative keratitis was 20.9 per 10,000 persons using extended-wear soft contact lenses for cosmetic purposes and 4.1 per 10,000 persons using daily-wear soft contact lenses for cosmetic purposes.According to Schein et al［31］,extended wear of contact lenses presents a 10 to 15 times greater risk of developing microbial keratitis than wearing lenses on a daily wear basis.

　　It is now a well established fact that all soft contact lenses cause hypoxia during overnight wear［1,2,7,30,31］and extended wear of soft lenses is more likely to cause microbial keratitis and contact lens associated peripheral ulcers.It appears that hypoxia is associated with Pseudomonas Aeruginosa (PA) binding to human epithelial cells (in vitro) as a result of wearing contact lenses［32］.lmayasu and co-workers［33］reported the same results in rabbits.The conclusion from both studies is that hypoxia from wearing low Dk/t lenses produces changes to the epithelial cells which facilitates PA binding.It has also been shown that PA is the most common pathogen associated with microbial keratitis from soft lens extended wear［34］.

　　Under closed eye conditions,tear secretion and lens movement decreases resulting in very little or no tear exchange.It has been shown that extended contact lens wear can substantially increase corneal epithelial permeability to fluorescein［35］.This is presumably due to corneal hypoxia and reduced tear exchange which suggests that these factors are responsible for altering the barrier function of corneal epithelium and may make the cornea more susceptible to infections［36］.

　　6　The future

　　Convenience and safety are major motivating factors in the choice of a mode of vision correction for refractive errors.Extended wear of contact lenses is associated with greater convenience than daily wear,but questions remain concerning the safety and risks associated with this modality.Studies collectively suggest that corneal hypoxia during extended wear is a potential causative mechanism in infectious keratitis which implies that a highly oxygen permeable soft lens may minimise or solve this problem.To improve oxygen supply,overall lens thickness may be reduced,lens movement and water content can be increased.However,these efforts have not provided sufficient oxygen to meet corneal requirements for safe extended wear.

　　Recently,new high-Dk soft lens materials,which contain both hydrophilic and silicone components have become available.The hydrogel component provides many of the desirable aspects of traditional soft lenses,such as comfort,conformity and surface compatibility.The silicone component provides a transport route for oxygen.Two silicone hydrogel contact lenses have recently become available with permeability values ranging from (100～175)×10-11and water contents ranging from 26%～34%.Fonn and his colleagues found 2.2% corneal swelling immediately on eye opening after overnight wear of theses lenses,compared to 1.8% in the non lens wearing control eyes.

　　However,it should be remembered that there are other important considerations for successful contact lens wear than oxygen transmissibility,such as lens comfort,deposit resistance,vision,fit and convenience.

　　from Centre for Conact Lens Reseearch,School of Optometry,University of Waterloo,Canada,N2L 3G1

　　作者单位：加拿大滑铁泸大学视光学院角膜接触镜研究中心(加拿大，N2L 3G1)

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(收稿：1999-08-04)

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