Contact lenses extended wear

Acanthamoeba ocular infection was first described in 1915. Acanthamoeba keratitis can occur in both healthy and immunocompromised individuals and is initiated by contact with contaminated water. Most Acanthamoeba keratitis cases described in the mid-1980s involved daily-wear soft contact lens wearers who were using saline made from distilled water and salt tablets. Cases have also been described in extended-wear soft contact lens wearers and rigid contact lens wearers. In a survey of corneal specialists, it was found that 85% of the reported cases were in contact lens patients using primarily daily-wear or extended-wear soft lenses. 

Epithelial microcysts are an abnormal corneal response at the cellular level to chronic hypoxia from contact lens wear. When present, they tend to be observed in soft contact lens wearers, particularly those wearing extended-wear lenses. A hypoxic state can result in the development of microcysts due to such causes as... 

Fleiszig SM, Efron N, Pier GB. Extended contact lens wear enhances Pseudomonas aeruginosa adherence to human corneal epithelium. Invest Ophthalmol Vis Set 1992 33(10) 2908-16. Marechal-Coutois CH, Delcourt JC. Klinische Resultate einer Untersuchung mit einer 70%ig hydratisierten Linse die Linse Tp 70 (Yumecon). Contactologie 1981 3D 89. Pickering CAC, Bainbridge B, Birtwistle IH. Occupational asthma due to methylmethacrylate in an orthopedic theater sister. BMJ (Clin Res Ed) 1986 292 1362. 

Fig. 4. (Photo)-Rigid oxygen permeable contact lenses and a new generation extended wear hydrogel lens incorporate a variety of organosilicon substituted monomers (courtesy L. Seidner, Permeable Technologies).

Many materials are employed in the production of soft contact lenses. The material with the widest application in the industry is HEM A [13]. HEM A became a popular soft contact lens because of its clarity, comfort, good vision, and excellent wetting characteristics. HEMA is a hydrogel, and hydrogels are cross-linked polymer networks that are swollen in water. Hydrogels are typically classified by whether they are neutral or ionic. Anyone who has had a piece of dirt in the eye understands the need for good wettability when it comes to a contact lens. The wettability of a soft contact lens in the ocular environment is a function of the surface chemistry while it is hydrated, which determines the biocompatibility of the lens [6]. A lens surface that does not wet will cau.se discomfort and potentially increased deposits. These deposits can affect vision and the physical properties of the lens [3,4]. The major drawback of HEMA is the permeability of the material to oxygen [12]. Extended wear can result in increased vascularization of the cornea... 

Leahy, C. D., Mandell, R. B., and Lin, S. T., Do disposable lenses solve the problems of extended wear Contact Lens Spectrum, April (1989). 

As one can see from Eq. 6, the permeability of a soft contact lens is directly proportional to its water content (Figure 5.53). Whereas the moisture content of early PHEMA lenses was only ca. 35%, more recent silicone-based hydrogels contain up to 70% water. Though hydrophilic lenses are able to absorb water and permit extended wear without comeal irritation, an increase in water content also corresponds to lower mechanical strength that may result in tearing/scratching. 

Silicone rubber, however, shows the highest permeability rates for these gases, and in fact, silicone mbber is used in blood oxygenators required for open heart surgery. It is also used in extended wear contact lenses since the transport of O2 and CO2 through the lens allows the cornea to respire. Thus, cloudiness and rainbows are not generally experienced, even after continuous wear for a mmifh. 

WiUis S.L., Court J.L., Redman RP., Wang J.-H., Leppard S.W., O Bryne V.J., Small S.A., Lewis A.L., Jones SA. A novel phosphorylchoUne-coated contact lens for extended wear use. Biomaterials 2001 22 3261-3272... 

The search for Si-Hy materials for CL was driven by the belief that a high oxygen supply to the cornea will prevent most of the adverse events associated with contact lens wear [7, 8], Ideally, such materials will also allow a safe extended or continuous CL wear (uninterrupted use of lenses day and night for 7 to 30 days) [9]. 

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