There is an idea that contact lenses are specifically indicated for short periods of wearing (in other words, replacements): so users will buy new ones more often. But the fact is that the time indicated by the manufacturer really cannot be ignored. Imagine that on your eye, in direct contact with it, there is a foreign body, from a material unknown to you, with properties unknown to you. What will happen to the eye during prolonged contact with this body, you do not know. But the manufacturer knows. Therefore, it limits the period of wearing the lens, depending on its properties. Therefore, it is necessary to strictly follow the rules of storage and cleaning: how, in what solution to store lenses, how to wash the container, how often to change the solution, etc.
The replacement period is the allowed life of the lenses, in fact, wearing them. The parameters of the materials from which they are made (and they are all different) and the safety risks to eye health during this time are calculated. When they are no longer safe, it's time to change them.
What are these risks? While the lenses are on the eye, they are out of the sterile environment of the storage solution and absorb everything they come in contact with. Various deposits form on them - for example, protein and lipid from your tear fluid, as well as all kinds of pollution from the environment. In other conditions, these substances are washed out of the eye naturally, but remain with the lenses. Accumulating, they can lead to very unpleasant consequences, from irritation to infectious inflammation.
The volume and criticality of these deposits, as well as the consequences for vision, depend on how much you wear the lenses.
Contact lenses are one-day (replacement period – 1 day), two weeks (2 weeks), monthly (1 month), quarterly < /a>(3 months). Manufacturers indicate these indicators in accordance with the resistance of the material used to various deposits. Then the resistance is significantly reduced and there is a danger to the eyes. The most hygienic, though also the most expensive, option is disposable contact lenses, which are thrown away at the end of the day. It is very convenient and safe. The most economical option is three-month lenses. They need to be stored, cleaned, the risk of deposits, although minimized, is still there. But it's cheaper.
Another necessary parameter is the wearing mode, i.e. how much time per day we spend in lenses, and how much without them. It happens:
The mode is related to the properties of the lens material, including its oxygen permeability and moisture content. For the cornea of the eye, the supply of oxygen in the right quantities is very important, otherwise hypoxia begins, which leads to a decrease in vision and swelling. And the material from which the lenses are made has different oxygen permeability. The most popular are hydrogel and silicone hydrogel.
A material such as hydrogel is not breathable by itself, so the water it contains is responsible for supplying oxygen to the cornea. Hence, moisture saturation is important. The more water in hydrogel lenses, the longer oxygen will flow to the eyes. During wearing, the water gradually evaporates, less and less oxygen enters the eyes. And so that they keep their shape and are easy to handle, the water content should not exceed 85%. Hence the mode of wearing no more than 6 – 8 hours a day, you can not sleep in them.
Silicone hydrogel lenses are made from a more advanced and modern material. Due to the addition of silicones to hydrogels, they "breathe" and do not depend on the water content, but are somewhat stiffer than hydrogels. They allow enough oxygen to pass to the cornea for its normal metabolism for a long time. It was after the invention of silicone hydrogel that it became possible to produce contact lenses for continuous wear (up to 30 days without removing). Therefore, if you do not have the opportunity to remove lenses until late at night or, due to the work schedule, are forced to spend many hours or even days in them, then choose this particular material.
However, the parameters for different brands of silicone hydrogel lenses will also be different, so you should not wear lenses that are designed exclusively for daily wear without removing them.
To protect the health of your eyes and avoid a decrease in visual acuity, follow the rules for their use: