Contact Lenses explained
Contact lenses
 Corneal contact lenses are real prosthesis, born with the aim to improving wearer's visual acuity, are sometimes the only possible correction, on the other hand glasses can not always give a full binocular vision. The complexity of construction is truly remarkable, and the simplicity with which too often are fitted, can frustrate the efforts taken to produce them. In fact, the construction the geometry varies from lens to lens and in many cases the right eye need of a different geometry from the left one. The power of the contact lens is realised in the central optic zone which represents the largest portion the lens. At the edge of this area will usually be an intermediate curvature and at the end of this one a third curvature  that ends with the lens edge joining with the anterior curvature. The difference in curvature between the front and rear surfaces determines the power in air of the lens. The peripheral curvatures are used to fit the lens in an appropriate manner on the peripheral cornea and to create an appropriate movement of the lens itself (after each blink), which creates an exchange of tears under the lens during the whole time of use of the corneal lenses. Very important is also the central thickness of the lens and the peripheral one as well as the width of the intermediate and final curve. All these parameters should be considered by the Optomnetrist that has to fit the ideal lens for each eye. Geometry of the design may vary, as we said, and it goes from two curves (Central and peripheral) to five curves (central and 4 more), to elliptical lenses to aspheric lenses (no curves but a progressive opening from the central curvature toward the edge of the lens), conic lenses, etc. Materials are many, both with regard to soft hydrophilic lenses and for Gas-Permeable lenses. Hydrophilic soft lenses, almost all bi-curves, have the characteristic of containing water and the choice of the appropriate lens must be made not only on the geometry, but also on the hydrophilic content that can vary from 36% up to 75%. The higher the water content of the lens, the greater the passage of oxygen through the lens and then the supply of oxygen to the cornea, but only if tears are enough to maintain hydrated the lens to the same percentage. New material with silicone, require less water in order to transmit the same amount of oxigen to the cornea, so are more indicated for eyes with less amount of tears or for prolonged wearers. Another very important point to consider is the wearing time. Gas-Permeable lenses ensure the contribution of oxygen to the cornea through their porosity and not through the water they don't contain. The more the material is porous (with the addition of fluorine and silicone) and the greater will be the contribution of oxygen to the cornea, but it will be easier to scratch and break. Also will dirty more easily, forcing a more thorough maintenance. The production techniques are also many. With regard to the gas-permeable lenses they are all built for turning, with computerized lathes or manual semi-automatic one with a considerable commitment of personnel and with a considerable waste of material. The hydrophilic lenses are made rarely nowday for turning (such as gas permeable) and then hydrated, but mainly by molding and/or cast-molding. These last two types do not waste material and can be performed on the assembly line, but the final quality is lower, while the reproducibility is higher than the lathe tachnique. Disposable lenses are made with the last procedure explained. To conclude, the choice of contact lens have to be done essentially by the optometrist, specialist in contact lenses fitting who can evaluate whether it is really the case to fit contact lenses (in order to safeguard patient's eyes) and the best type to use to best fit the future wearer. After the first fitting must be made at least two other after care visits to make sure that all is fine, especially regarding for the eye health. 
 
The power of a contact lens immersed in a liquid, such as, for example the tears, is different from that seen in air because of the different index of refraction (density difference). The air has n = 1.00, tears have n = 1.3375.
Hypoxia, the major obstacle to continued use

Hypoxia is the major obstacle to continued use 
 
The pathogenetic mechanism implicated in the occurrence of major side effects of the anterior segment is hypoxia. The cornea, to carry out its vital activities, needs oxygen, like any other tissue, but not having a blood supply meets its own needs oxygen attingendolo from various sources. 
 
In the condition of open eye the cornea meets its requirements for oxygen from the atmosphere (pO 2 = 155 mmHg) by means of the tear film and dall'umor vapor, which provides such gas to the endothelium and to the third baseline of the stroma, ie to the rear surface of the cornea. 
 
In the condition of the vessels of the conjunctiva closed eye lid make the most amount of oxygen (pO2 = 55 mmHg) while a small amount comes from vessels in the limbus and dall'umor vapor. Therefore, the performance of oxygen equivalent (EOP) to reach your eyes closed, 75 mm Hg, equivalent to an atmospheric concentration of 8.1%, is lower than that obtained in the condition of eyes open 155 mm Hg, corresponding to 20.9%. 
 
The reduced supply of oxygen leads to an alteration in the metabolism of keratocytes: the rate of aerobic metabolism is reduced (Krebs cycle), by the very energy performance (from 1 mole of glucose are obtained 36 molecules ATP), while increasing the anaerobic low energy yield. 
 
As a result to occur is an excessive amount of lactic acid that accumulates in the stroma, induces a vigorous booster osmotic of water which results in corneal edema. The average thickness of the center point of the cornea is about 500-570 µm. 
 
The accumulation of lactate, also, determines corneal acidosis that is linked to hypoxia and hypercapnia. The high concentration of carbon dioxide (CO2) has a role of particular interest. As soon as the CO2 in the atmosphere tries to return from the cornea, accumulates behind the lac soft. This leads to a further lowering of the pH of the stroma and epithelium. 
 
Bonanno and Polse have shown that the pH of the corneal stroma to the normal state is approximately 7.55 when the eyes are open and 7.37 when these are closed. By decreasing the transmissibility to gases with slow gas-impermeable, also decreases the pH stromal at a level of 7.1. This change in pH produces an endothelium bullous (blebs) and acidosis may be responsible for the prolonged corneal endothelial polimegatismo occurring in carriers of lac over time, or one eye closed, for example during a prolonged ptosis. Even the integrity of the epithelium, in such conditions, is less because it reduces the function of the Na + / K + ATPase employee, which guarantees the deturgescenza ie the relative dehydration with consequent corneal stromal edema and epithelial the clinical importance is manifested by striae and microcysts. The edema is superficial epithelial, stromal and what is deep and can be up to the severe form of bullous keratopathy. Edema is an insult to tissue, in addition to reduced reparative epithelium, hesitates in continuous solutions that offer a possible way to pathogenic invasion. 
 
The biomicroscopic examination can highlight different events as a function of the affected area. On the surface epithelial folds are observed whose origin is mixed: mechanical and edematous; they can achieve a mosaic appearance. Epithelium we have: microcysts and vacuoles, neovascularisations, colors, pseudodentrities. In the stroma we have striae and folds, dellen (localized thinning of the stroma linked to poor distribution and tear on the appearance of dry area in correspondence of a zone of lack of congruence between the eyelid and ocular surface), opacity, corneal infiltrates, sterile, neovascularisations. The endothelium is a tissue monostratificato thickness of about 5 µm. The combination of the phenomena related to hypoxia and acidosis, following the use of CL, is the basis of changes in the endothelial mosaic and are: endothelial blebs or bullae, endothelial pleomorphism, abnormal density 
 
Physical and physiological measurements of oxygen permeability 
 
The two chemical-physical parameters in such a way that incontrovertible qualify the passage of oxygen through a CL are: 
 
The permeability: DK is the product between D is the diffusion coefficient of a gas and K which is that the solubility of oxygen through a biomaterial as a function of temperature. There is a logarithmic relationship between the water content and oxygen permeability due to the fact that oxygen diffuses across the water contained in the hydrogel polymers. Thus the higher the water content the higher is the oxygen permeability. But the lac with high water content are more brittle and thicker. The greater thickness determines a lower transmissibility as well as a less comfort. 
 
The transmissibility: Dk / t is the product of permeability by the thickness (t) of lac. This factor takes into account the thickness that varies from center to periphery of lac. The central value of the thickness is usually specified, while the real thickness of the entire surface of a lac it is not because it is variable in the different points. 
 
The permeability and transmissibility are the two parameters that predict the biocompatibility of a lac, in fact a CL is biocompatible if it is oxygen permeable. 
 
Silicone hydrogel contact lenses 
 
From the above it is clear that the major obstacle to continued use of oxygen is reduced and this has been made numerous efforts to reach the production of new CL which exceed the general limits of the old generation with lac hydrogel copolymers that Hema not allow an adequate flow of oxygen. 
 
In particular, the polyHEMA 40% of hydration CL allow a flow of 0,00264 ml/Cm, those of 70% of hydration 0,00441l ml/Cm in the condition of eyes open, eyes closed, respectively 0,000736 ml/Cm and 0,00122 ml/Cm. These values are lower than both the normal corneal oxygen demand that has been set at 0,005 ml/Cm from Hill and Fatt in 1964 and then at 0,007 ml/Cm from Larke and other in 1981. 
 
Form a copolymer that combines the properties idrophilic of Hydrogel with that hydrophobic of silicone was been possible with the indtroduction of a hydroxyl group. This produces a copolymer with an excellent oxygen-permeability although it has a low water content and that allows the realization of lac that can be used for a long time, without causing the foreign body sensation or pain. 
 
Lies in the two-phase structure the big innovation of these lenses: the oxygen-permeable hydrophobic phase is composed of monomers and macro-monomers that contain siloxane, fluorosilossano or triple carbon bonds, while the phase ion-permeable hydrogel contains monomers, together with the polyethylene. Normally two materials that are in different phases have different refractive indices, but the key discovery of these new materials is the presence of two continuous phases that guarantee a clear vision. 
 
In conclusion, knowing the limit of the amount of oxygen necessary to the cornea to avoid anoxia and then the edema determined by Harwitt and Bonanno in 125 units (DK), some manufacturers have developed highly biocompatible materials can provide a large oxygen transmission both during phase than during the eyes open to eyes closed. Today, these lenses are available on the market and an Optometrist preparation will give the best recommendations on the use of these lenses and their management over time.