Contact Lenses

 

The contact lenses are medical devices in plastic material, shaped like a small transparent cover, which are applied on the surface of the eye to correct refractive defects (myopia, hyperopia, astigmatism, etc.).

They have many advantages, but the eyes of some people do not tolerate them mainly because they limit the breathing of the surface of the eye (the cornea is subjected to hypoxia or lack of oxygen).

 

Historical Background

Adolf Fick, inventor of the modern contact lenses

The birth of the contact lens dates back to Leonardo da Vinci, which occurred in 1508 that by immersing the eye in a sphere containing water, there was a continuous optical between the inner surface of the glass ball, and the outside of the cornea.

 

Then Descartes, in 1636 he published The diopter, which perfects the idea of Leonardo, explaining that a tube filled with water and placed on the cornea, with a lens at the end that is perfectly comparable to the cornea itself, cancels or reduces the anomalies refractive eye.
Contact lenses are designed to be modern traced back to the discoveries of AE Fick, E. Kalt, a population equivalent Muller, respectively, in Switzerland, France and Germany. These lenses were glass material, for support on the sclera, of large diameter and poorly tolerated physiologically.
The first plastic lenses must be two American researchers, Dallos and Fleinbloom. The advantages compared to glass are immediate, significantly reducing weight. The first corneal contact lenses rigid born in 1950, with a diameter less than the cornea, designed by Bier.
In the early 60s two researchers Czechs, Lim and Wichterle planned the first hydrogel contact lenses, the soft. At the end of the 60s also began to be used the first rigid lenses hortocheratologic and in order to reduce myopia after their removal were designed and applied during the day with the purpose of modifying the corneal profile. The lenses for orthokeratology in 2002 have been approved by the FDA for the correction of myopia up to 6 diopters with astigmatism of up to 1.75. Today lenses for orthokeratology using super oxygen permeable materials and are used for corrective purposes only during sleep.

 

Coefficient of permeability and oxygen transmissibility

The coefficient of permeability is indicated by the term Dk, where D is equal to the diffusion coefficient of a gas through a material and K the coefficient of solubility of the same. This feature, namely the ability of a material to transmit it through the gas, is fixed for each polymer, but can vary with temperature. Factor strong influence in the passage of oxygen is also the thickness of the lens taken into consideration. A correct evaluation of the permeability can not therefore not apart from it. To then define the exact contribution of oxygen coming to the cornea with the application of contact lenses, one must speak of oxygen transmissibility, Dk place or the value in relation with the thickness l: Dk / l.

Types of contact lenses

Contact lenses are divided properly into two large families, according to the materials used for their construction:

Rigid lenses or glassy polymers
soft lenses or rubbery polymers
 

rigid lenses

The rigid contact lenses are built with material is not permeable to oxygen than with permeable materials.

Non-gas permeable hard lenses

PMMA

The PMMA (polymethylmethacrylate) is the only non-permeable material used. You get dall'esterificazioneacido methacrylic acid with methyl alcohol. This material is stable, hard, has high optical quality, it is not attacked by organic enzymes and is well tolerated by the tissues with which it comes in contact. For lack of polarity, does not absorb water and binds poorly with the substances contained in the tear film, tear or

Rigid gas-permeable lenses

Also known commercially with the erroneous name of semi-rigid.

CAB

The first material used for their construction was the CAB (cellulose acetate butyrate). Compared to PMMA is incrementing the flexibility of the polymer, but is also determined to dimensional instability which is restricted by increasing the thickness of the contact lens itself. Increasing the thickness decreases the oxygen transmissibility.

Siloxane copolymer

To improve the permeability and transmissibility of materials, the CAB has been added to a copolymer of siloxane. The siloxane is formed from a monomer of silicon, oxygen and a radical. This arrangement made ​​it possible to increase the oxygen transmissibility, while increasing its dimensional stability.

Fluoroacrilati

The lenses rigid gas-permeable latest generation see the choice of fluorinated materials, which have led to reduce the coefficient of friction between the eyelid and contact lens, increasing comfort, reduced the formation of protein deposits, reduced thickness while maintaining the dimensional stability having greater hardness ..

soft lenses

The soft contact lenses are made from polymers having the physical characteristic of softness. They are distinguished primarily in non-hydrophilic and hydrophilic materials.

Soft non-hydrophilic

Silicone lenses, have a high value of oxygen permeability, but they are essentially hydrophobic. To reduce this deleterious characteristic corneal application has arrived to add HEMA (Idrossimetilmetacrilato), the results are not fully satisfactory.

soft hydrophilic

Commercially the most widespread, are constituted by polymers of high hydrophilicity, linked with varying amounts of water. The ability to convey oxygen from this type of lenses is fundamentally dependent on its level of hydration. The transmissibility is still modest, since the thickness of these lenses adversely affects the supply of oxygen to the corneal epithelium. Soft lenses of the latest generation gel with medium-high hydration, overcomes these limitations by acting on its thickness. These lenses are disposable, or frequent changes, the most currently marketed. Disposable lenses allow to eliminate the risk of accumulation of deposits. Are changed every day (every day), or exist: fortnightly, or monthly (in need of routine maintenance, preservatives or soaps).

maintenance

The maintenance of the contact lens has the purpose to maintain the integrity in time of the chemical-physical characteristics of the material.

rigid lenses

Routine maintenance of these lenses can be divided into different processes:

cleaning
disinfection
rinse
lubrication
 

cleaning

The cleaning action is essential for removal from the lens surface of mucus and cosmetics, and precedes the disinfecting action of the same. The presence of these compounds decreases the effect of disinfection carried posteriorly, decreases the wettability of the material and the comfort of use.

The cleaning agents can be anionic, or amphoteric nonionica. The second are the tensioattiviemulsionare lipids, dissolving deposits and removing contaminants present in the tear. most used, for their characteristic of

disinfection

The action of disinfection has as its main purpose the initial prevent a pathological state coming from a causative agent present on the surface of the contact lens [1].

The solutions used for disinfection are composed of one or more antiseptics, such as benzalkonium, thimerosal and chlorhexidine, and a chelating agent, which the 'EDTA.

rinse

The rinsing action has the function to remove waste material coming from the action of disinfection, maintain the wettability of the surface and play the role of buffer, or to maintain the level of PH values ​​of neutral or slightly alkaline (PH interval = 7.0 / 7.4). The solutions used are predominantly saline.

lubrication

The action of lubrication is needed to keep the 'hydrophilicity of the material, essentially hydrophobic. The action of the tear film covering also protects the surface of the lens during application, preventing the transmission from the fingers of sebaceous deposits.

The components most commonly used as wetting agents are the 'polyvinyl alcohol, l' polyethylene oxide, l 'hydroxyethyl cellulose and methyl cellulose.


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