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Contact lenses.

Contact lenses are becoming increasingly popular because they now represent a viable alternative to glasses, especially for those who play sport, they are often much more practical. On the market if they are several, each with features and / or special instructions, and often make a right choice is particularly difficult. We try to do some 'clarity to support a choice "prudent". Contact lenses should be used carefully to avoid problems. There are different types. A first general distinction can be made with lenses: soft, semi-rigid or gas permeable, rigid.

TYPES OF CONTACT LENSES

Soft contact lenses

Soft contact lenses are also called "hydrophilic" because they consist largely of water and are those that adapt better to the natural characteristics of the eye. They can, in fact, to ensure proper oxygenation and hydration. They are the most comfortable to the eye, but may be due to intolerance because of their surface that absorbs (like a sponge) the foreign substances that are deposited more easily than other types of lenses. For this reason, proper maintenance is essential and their frequent replacement. For this, in fact, you can find soft contact lenses "disposable"-type daily, weekly, biweekly and monthly. Limited time with these lenses, thanks to constant change, the risks of infection are greatly reduced. The most popular are those that completely eliminate the need for daily maintenance and problems related to possible infection (for deposits of foreign substances on the lens itself). They are suitable for everyone and the radius of curvature of this type of lens is suitable for most of the corneas, ensures good oxygenation of the eye and superior comfort compared to other types.

Semi-rigid contact lenses

Semi-rigid contact lenses or "gas permeable" are slightly more flexible than rigid ones, represent the evolution and practice have changed. Their structure is very similar to that of rigid, but allows a better diffusion of oxygen in the cornea. They are, however, less well tolerated than soft, and should be used initially because the eye will gradually get used to it. From the hygienic point of view, however, are better than soft, because they do not absorb foreign substances that are deposited on top of them. They are recommended if you suffer from lack of watering of the eye and where the curvature of the cornea is not regular (as in the high degree of astigmatism and keratoconus).

Rigid contact lenses

They are made of a material not flexible, rigid, and for this structure, the oxygen does not reach the cornea and the only exchange with the outside is only possible by means of continuous movements that makes the lens on the corneal surface (being rigid, does not fit perfectly to the cornea). Usually this type of lens has a very low initial tolerance and, therefore, the adjustment period is very long. It must therefore, as with semi-rigid, the eye gradually preparing to use them, making periodic check-ups by a specialist to assess immediately any incidents of intolerance. Their structure allows for safer cleaning (you deposit a lot less foreign substances).
Now this type of lenses are virtually obsolete, their place being preferred semi-rigid or gas permeable, which constitute the evolution . The choice of hard lenses, therefore, occurs only in exceptional cases.

Cosmetic contact lenses

This type of lens was placed on the market recently. It is not corrective prostheses only, but mostly a cosmetic aid requirements associated with the image of the person. In some cases, may help the treatment of certain diseases such as amblyopia or ocular opacities. Carriers can choose from colored lenses and cosmetic lenses hydrophilic real. The difference is in the final aesthetic result: the first partial induce variations of color in the iris, the second, made ​​with sophisticated technology, completely altering eye color. The cosmetic lenses contain a high percentage of dye: it decreases the oxygen permeability of the cornea and this is a limit on the duty condition.

TIPS FOR THE USE OF CONTACT LENSES

Maintenance of the contact lens is designed to keep intact over time, the chemical and physical properties of the material. It is carried out using solutions that allow for the proper preservation and disinfection of the lenses and protect the deposits of foreign substances and the formation of mycobacteria, often cause unpleasant infections. The maintenance of these lenses is divided into several processes:

  • Cleaning

  • Disinfection

  • Rinse

  • Lubrication

Cleaning

The cleaning action is essential for the removal of mucus from the surface of the lens and cosmetics, and precedes the action of the same disinfectant. The presence of these compounds decreases the effect of disinfection carried out posteriorly, decreases the wettability of the material and the ease of use.
The cleaning agents may be of a nature anionic, nonionic or amphoteric. Seconds are the most commonly used surfactants, because of their characteristic to emulsify fat, dissolving the deposits and removing contaminants present in the tear.

Disinfection

Disinfecting action has as its primary objective the prevention of a disease state from an initial causative agent on the surface of the contact lens. The solutions used for disinfection are composed of one or more preservatives such as benzalkonium, chlorhexidine and thimerosal, and a chelator such as EDTA.

Rinse

The action of rinsing have the function of removing waste from the action of disinfection, maintain the wettability of the surface and play the role of buffer, or to maintain the pH values ​​of neutral or slightly basic (pH range = 7,0 / 7,4). The solutions are mainly salt. Should be used only suitable liquids: Contact lenses should never rinse under running water, but that can cause serious eye infections (caused, for example, a disease such as keratitis).

Lubrication

Lubrication action is necessary to maintain the hydrophilicity of the material, essentially hydrophobic. The action of the tear film coating also protects the lens surface during application, preventing the transmission from the fingers of sebum deposits.
The components most commonly used as wetting agents are polyvinyl alcohol, polyethylene oxide the hydroxyethyl cellulose and methyl cellulose.

Property of the materials for contact lenses

The main properties of the materials used to keep in mind for the construction of the contact lenses are: to oxygen permeability, wettability, hardness, resistance to deposits, thermal conductivity, specific weight and thickness, biocompatibility.

In particular:

Oxygen permeability

Is a key feature for a good tolerance of contact lenses, since the presence of oxygen is an indispensable factor for corneal metabolism. The ability to transmit oxygen is referred to as "value Dk", "D" represents the coefficient of gas diffusion through the material, while "k" is a constant that indicates the amount of oxygen present in the material itself.

Wettability

It defines the ability of a liquid to cover a solid surface and has particular importance in relation to the tear film, the maintenance is a necessary condition for compatibility between eye and lens.

Biocompatibility

Is defined in this way the complete absence of adverse reactions by the body into a material. The scientific research applied to contact lenses has led to the creation of numerous materials that have this particular feature.