Daily contact lenses » Permeability of contact lenses for oxygen

Opticians believe that permeability of contact lenses for oxygen is one of the main conditions for their successful application. Scientific studies agree that the number of adverse effects caused by chronical hypoxia (oxygen deficiency) of the cornea decreases with increasing oxygen permeability of contact lenses materials.

Needs for corneal metabolism

It is also shown that demands of cornea for oxygen feed are individual for each of wearers. There are people who arbitrarily prolong the period during which they should wear contact lenses. They sleep with them on although contact lenses are not designed for this purpose. Even though no adverse effects occur or at least in not such an extent that would be expected. However, there are wearers, who follow principles relating to the lenght of wearing of contact lenses, even though they have some problems. This findings show that needs for corneal metabolism are highly individual. Currently, there are no means, which it would be possible to find out the need for oxygen feed of the cornea in each of wearers by. The only way to avoid complications caused by its deficiency is to apply contact lenses with the highest oxygen permeability to each wearer from the beginning.

Needs of corneal metabolism can change depending on external conditions and situations, which a client inhere in. These include for example, altitude, aerobic endurance sports, many-hour daily wearing, stay in smoky, air-conditioned or otherwise polluted environments. Because we never know to what a situation we can get it is better to consider the possiblity, that a time of increased needs of corneal metabolism may come.
The cornea perceives the lack of oxygen when eyelid is closed, even without contact lenses. This deficit naturally increases with contact lens. This implies that the longer the lens is in the eye, the higher the risk of a complication in consequence of corneal hypoxia. This also occured in the 80’s, when there was a development of lens application for prolonged wearing. However, these lenses were not made of sufficiently permeable material. Complications were so frequent that the number of wearers of such lenses began to decrease again. Another turning point came at the end of 90’s, when silicon-hydrogel lenses were developed.

The cornea and the need for oxygen

Chronical hypoxia is a major health complication and can have a serious consequences. These include swelling of the cornea (the Sattler veil), opacity of the corneal centre, creation of striae, vacuoles and microcysts. In the long-term, hypoxia can cause a total exhaustion of the cornea, which can lead to ceasing of use of contact lenses.

It was also found that contact lenses with a lack of permeability may cause damage of corneal metabolism, damage of corneal integrity, decrease epithelial thickness, weaken the stroma or increase endothelial polymegatism and a lumbar injection. The formation of corneal vascularization represents a serious problem. Hypoxic cornea becomes more susceptible to penetration of infections.

It is proven that all types of contact lenses influence a division and restoration of corneal epithelium to some extent. Lenses with the smallest impact are silicon-hydrogel and of one day use. The reason for etiolation of epithelium is probably an imbalance between production of new cells of basal epithelium and removal of old epithelial cells from the surface of the cornea. The consequence of this phenomenon is that less cells reach the surface of the cornea and a central epithelium weakens. Studies show that prolonged and improper wearing of contact lenses with low permeability for oxygen disrupted epithelial metabolism, oxygen feed was reduced and total weakening of epithelium occured.

The lack of oxygen in the periphery of the cornea can lead to a critical situation not only in the cornea but the entire eye. The periphery of the cornea is the only source of epithelial stem cells. These are necessary for correction of corneal erosions. Loss or damage of stem cells dividing may have unpleasant aftermath that may be expressed by repeated erosions, chronical keratitis and vascularizations.

When comparing the corneal state of people wearing silicon-hydrogel and one-day lenses with high permeability for oxygen with the cornea of people, who do not use contact lenses at all, there are almost no differences between the state of their corneas.

Oxygen permeability is important particularly for those, who use lenses with higher vergency or lenses stronger on the edge or those working in dry and dusty environments. Working with the computer can also have an affect to oxygen feed to some extent. Around a third of the wearers of contact lenses suffer from high myopia, astigmatism or hyperopia, so the thickness of their lenses, in the center or periphery, reaches 0.35 mm. People with presbyopia, whose number is not small, also need contact lenses with a greater thickness.

It seems that a large number of wearers do not take instructions of ophthalmologist seriously and occasionally sleep with contact lenses on, even though they are not designed for this purpose and have a low amount of permeability. Some percentage of wearers keep contact lenses even during the night. The solution are lenses with higher permeability, which allow also an occasional nap without a big risk. However, the wearer should not unnecessarily misuse it and arbitrarily violate a mode of wearing. The high amount of permeability is convenient, regardless whether you intend to wear contact lenses daily or in a prolonged mode of wearing.

If we want to maintain a sufficient oxygen feed to the cornea, especially silicon-hydrogel and one day contact lenses should be used. Their oxygen permeability is so high that the cornea is affected by wearing of contact lenses only in a very small extend and they have hardly any impact to the state of the cornea.