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Contact lenses cover the cornea, blocking normal oxygen, tears, and secretions. This is also the root cause of people's discomfort when wearing lenses. Eyes, like breath, repeat the process of entering oxygen and releasing carbon dioxide. However, the oxygen supply is lowered when wearing contact lenses. At this time, if there is not enough oxygen, waste accumulates, and the concentration becomes high (forming high osmotic pressure), and water comes in from other structures to solve this problem. The problem is that this process increases the parenchymal volume in the eye structure, which leads to the appearance of corneal edema.
Oxygen is the energy source for the eye. This is the biggest problem with contact lenses for the eyes. The structure of the contact lens is simply the front and rear curves, diameters, edges, and lacrimal lenses.
The curve can be thought of as curvature. Since the eyeball has the shape of a sphere, the lens has curvature according to the sphere. Since the degree of curvature varies from person to person, the curves on the lens also vary from person to person. The curve affects the fit, with tighter fits that are greater than the curvature of the eye, and flat fits that are smaller. In the latter case, the wearability of the lens and the ability to exchange oxygen and tear in the eye are degraded.
The diameter is necessary because each person has a different pupil size. For example, on average, female pupils are larger than males. In addition, the diameter is an important factor in fitting condition correction.
The edge is affected by the gap. Edge crevices serve as tear exchange, lens removal, tear meniscus formation, and lens fixation. Therefore, even if it is too small, a problem occurs even if it is too large, so more caution is needed.
A teardrop lens refers to a tear layer formed between the lens and the cornea.
Contact lenses include hard oxygen transmission lenses (RGPs), soft contact lenses, silicone hydrogel lenses, biocompatible lenses and biomimetic lenses, silicon lenses, and PMMA lenses. Among them, the most popular hard oxygen transmission lens and soft lens silicone hydrogel lens will be described.
Many people don't recognize the difference between hard and soft lenses. Hard lenses are literally hard lenses, and soft lenses are soft lenses. Rigid lenses, of course, don't feel good to wear, but they're hard enough to last longer than soft lenses. This basic principle makes it easier to understand. From now on, I will explain the difference between a hard lens and soft lens based on the basic principle.
Why are hard lenses hard, and why are soft lenses soft? The reason lies in the water content of the material. Hard lenses have little water content. So its strength is hard, the initial fit is bad. As a result, adaptation periods and testing times are long and must be carefully determined. What are the advantages of hard lenses over soft lenses? It is oxygen permeable. Hard lenses have a small diameter and high oxygen permeability because they are made of silicon, which is a material through which oxygen passes well. In addition, correction of astigmatism is possible. Since soft lenses do not correct astigmatism, a toric soft lens should be used. However, hard lenses may correct astigmatism to some extent. As a result, hard lenses are suitable for those who need oxygen permeation and astigmatism correction.
Recently, the most popular lens is a silicone hydrogel lens. In other words, silicone hydrogel lens is a lens that satisfies oxygen permeability and soft lens for the hard lens. Of course, the effect is less than a hard lens or a soft lens, but there is a big advantage in that it satisfies both. Silicone hydrogel lenses are also slower to dehydrate than soft lenses. When wearing the lens, when the tears that have accumulated on the surface of the eye evaporate, we feel dry. This is called dehydration of the lens. Silicone hydrogels slow down the process and reduce dryness. Incidentally, the dehydration of the lens occurs in the order of hard silicon hydrogel soft, which is the molecular structure. Soft lenses have a large molecular structure, so there is a lot of space for tears to enter. This will absorb more tears. In addition, the silicone hydrogel lens has a small bi-molecular structure, so dehydration is slow, and the hard lens is smaller, resulting in slower lens dehydration.
Hard lenses include PMMA (methyl polymethacrylate), cab, silicone, silicone acrylate, and Fsa fluoro silicone acrylate. PMMA is the material used when contact lenses were first introduced. The characteristic of PMMA is its high strength, which is much higher than the current hard lens. However, due to poor wettability, poor fit, and poor oxygen permeability, they are rarely used at present. The cab is not currently well used for the same reason. As silicon appeared in hard lenses, oxygen permeability increased greatly. However, silicone is hydrophobic and therefore not comfortable to wear. To protect this, silicone acrylate was added by adding a hydrophilic component. The most widely used material is fsa fluoro silicone acrylate, which is one step further. Compared to other materials, the eye is less congested and can be worn relatively well by dry eye patients.
Soft lenses include HEMA, MMA, and MA. The most representative material is HEMA. HEMA has good hydrophilicity, flexibility, and wettability, so it has a good fit but low oxygen permeability. MMA is used in soft lenses to reduce water content or increase hardness.