With so many flowers in sight, how should you choose your glasses?

When you can't see clearly, wearing glasses is the choice of many people. But do you really know how to choose glasses? How do they help people see clearly? What are scleral lenses and RGP lenses? Find out in this article.

Written by reporter Wang Xueying Edited by Liu Zhao

New Media Editor/Lv Bingxin

Interview experts:

Song Hongxin (Deputy Director and Chief Physician, Ophthalmology and Optometry Center, Beijing Tongren Hospital)

Zhang Jingshang (Chief Physician of Ophthalmology, Beijing Tongren Hospital)

"Round and bright, the same on both sides, feet on the ears, waist across the bridge of the nose", what is the object? The answer is actually very simple: glasses.

As the secrets of optics were uncovered by scientists, glasses came into being. Over the past few hundred years, glasses have changed from being a "special" accessory for the nobility to a common item for ordinary people. But did you know that there are actually many kinds of glasses? Let's take stock of the wide variety of glasses!

Myopia, hyperopia...why can't we see clearly?

To understand glasses, we must first talk about why humans cannot see clearly. The working mechanism of the eye is similar to that of a camera. The cornea, aqueous humor, lens and vitreous body are the refractive system of the eye, just like the "lens" of the "camera" of the eye, and the retina is the film of the "camera".

When light passes through the "lens" of the eyeball and focuses to form an image, for an eyeball with a normal refractive system, the image will fall impartially on the "film" of the eyeball. At this time, we can see a clear image. If there is a refractive error, such as myopia, hyperopia or astigmatism, what the human eye sees will be blurry.

The most common type of myopia is axial myopia, which is caused by the length of the eye's axial distance - the distance between the cornea and the retina of the eyeball's front and back diameter - becoming longer. In this case, the light is focused and formed in advance, and the image cannot be accurately placed on the "negative film". On the contrary, for people with axial hyperopia, because the length of the eye axis is insufficient, the image formed by the convergence of light will fall behind the "negative film". In this case, people will see things far away more clearly than those close by.

Few people know that the length of the human eye axis is not fixed, but will grow longer as people age: the eye axis of a newborn baby is very short, about 16 mm, which is a typical "hyperopia"; after 3 years old, the eye axis of a child will grow rapidly until it stops in adulthood. During this process, a person's vision will gradually change from "hyperopia" in childhood to "emmetropia" in adulthood. For an adult with normal vision, the average length of the eye axis is generally about 23.5 mm to 24 mm.

Although it will continue to grow during development, the eye axis of a person cannot grow in the opposite direction. In other words, the eye axis either remains the same and stops growing, or continues to grow and grows in length. In this case, for people with refractive errors, the simplest way to correct their vision is to use "external forces" to "adjust" the light entering the human eye, such as placing a pair of optical lenses in front of the eyes.

Myopia glasses and hyperopia glasses: using lenses to help the eye axis

Myopia glasses and hyperopia glasses are the most common types of glasses for correcting vision. As mentioned above, axial myopia is the most common type of myopia. The reason why modern people are myopic, apart from genetic factors, is unhealthy living habits, such as staring at computers for a long time, using mobile phones, and not paying attention to eye hygiene, which can easily cause the eyes to be in a state of long-term adjustment tension, causing eye congestion, increased intraocular pressure, and reduced elasticity of the eyeball wall, which in turn leads to the elongation of the eye axis.

▲The correction principle of myopia and hyperopia (Drawing by Wang Xueying)

Myopia glasses are made by using the principle that concave lenses can disperse light to lengthen the image distance. The lenses are made of concave lenses, which are thin in the middle and thick at the edges. Specifically, a pair of concave lenses is placed in front of the myopic eyes to allow the light to be dispersed before entering the eyeball, so that the light can accurately form an image on the retina after passing through the lens of the eyeball.

Compared with myopia, hyperopia is slightly different. As mentioned above, under normal circumstances, the eye axis of infants with physiological hyperopia will continue to grow as the eyeball develops, but some people are affected by genetic, environmental and other factors, and their eye axis cannot grow to a normal length, eventually leading to axial hyperopia. Although axial hyperopia makes it clearer to see far away, it is more difficult to see near, especially for those with high hyperopia, which is very likely to cause strabismus and amblyopia. Therefore, hyperopia should also be corrected in time.

The principle of hyperopia glasses is exactly the opposite of that of myopia glasses. Hyperopia glasses mainly use the principle that convex lenses can gather light, allowing the light to "gather" before entering the eyeball. The lens uses a convex lens, which is thick in the middle and thin at the edge.

It is worth noting that although presbyopia and hyperopia appear similar in that both cause difficulty in seeing close objects but clarity in distant objects, and both glasses use convex lenses, there are essential differences between the two: presbyopia is a physiological phenomenon, the result of natural aging and the gradual weakening of the eye's accommodation function, while hyperopia is a pathological phenomenon.

Astigmatism lens: "Cylindrical lens" debut

In addition to myopia and hyperopia, there is another refractive error called astigmatism, which is mainly related to the cornea of ​​the eye.

When discussing myopia and hyperopia, people are based on the premise that the eye's refractive system is regular, that is, the refractive power in all directions is the same, but the human eye is obviously not a perfect ball, but more like a horizontally placed egg.

▲The differences in objects seen by myopia, astigmatism and myopia and astigmatism (Photo source: Shanghai Children's Hospital)

In this case, the cornea has a strong refractive power in the vertical direction, causing the vertical light rays to converge first, while the cornea has a weak refractive power in the horizontal direction, causing the horizontal light rays to converge later. Even if there are no problems with the lens, axial distance, and position of the retina, the image seen by people will be "double", as if the light rays were scattered, and this is what we call astigmatism.

Although few people mention it when fitting glasses, there are actually many types of astigmatism: according to the difference in the vertical and horizontal diopter, astigmatism is also divided into three categories: along-the-rule astigmatism, against-the-rule astigmatism and oblique axis astigmatism. Among them, along-the-rule astigmatism is the most common type of astigmatism.

▲Principle of cylindrical mirror (Drawing by Wang Xueying)

Since astigmatism is the abnormal refractive power of the cornea in a certain direction, it is impossible to correct vision using spherical lenses such as concave or convex lenses. For this reason, scientists have come up with another type of lens - cylindrical lens, which is the part obtained by cutting a cylinder along its axis.

In the vertical direction, the cylindrical lens is a straight line without a curved surface, so it does not refract light; in the horizontal direction, the cylindrical lens has a curved surface and can refract light. Compared with a spherical lens that can focus light into a point, a cylindrical lens focuses light into a line, and astigmatism is caused by the inconsistent focusing effect of the spherical surface in a certain direction. Using a cylindrical lens can correct this.

It is worth noting that in real life, astigmatism often occurs together with myopia or hyperopia. Therefore, when making lenses, people will combine spherical lenses and cylindrical lenses into one - convex cylindrical lenses are used for hyperopic astigmatism, and concave cylindrical lenses are used for myopic astigmatism. However, it is difficult to distinguish them easily with the naked eye.

Orthokeratology lenses and RGP lenses: the new generation of contact lenses?

In addition to the common myopia glasses, hyperopia glasses and astigmatism glasses, orthokeratology lenses and rigid gas permeable contact lenses (RGP) are also "Internet celebrity" products in the current eyewear industry.

Orthokeratoloy Lens, also known as OK lens, is a special purpose RGP. Its inner surface is composed of multiple arc segments, which can make the curvature of the central area of ​​the cornea flat within a certain range after wearing. Through the physical effects of negative pressure suction and positive pressure compression, the corneal epithelial cells are rearranged and the curvature of the front surface of the central area of ​​the cornea becomes flat, while the peripheral cornea becomes steeper.

In this way, the light passing through the center of the cornea can be imaged exactly on the retina, thereby temporarily reducing the degree of myopia and improving naked eye vision, while effectively controlling the growth of the axial distance and slowing down the progression of myopia.

▲There is a clear difference in the size of soft contact lenses, hard contact lenses and scleral lenses (Chart by Wang Xueying)

Different from orthokeratology lenses, which are used to control the development of myopia, RGP lenses are mainly used to improve the visual quality of people with high refractive errors and irregular astigmatism. For example, the frame glasses worn by people with high myopia are essentially a magnifying lens. Since there is a certain distance between the eyeball and the lens, the objects seen by the human eye will be reduced to a certain extent. However, RGP lenses will not have similar problems because they can fit closely to the eyeball.

In addition, due to the special properties of RGP material, which significantly increases the amount of oxygen passing through, it performs better in oxygen permeability, firmness, wettability and precipitation resistance than traditional contact lenses (i.e. soft corneal contact lenses), causes less damage to the cornea, and is more suitable for long-term wear. In view of this, the market generally regards RGP as a new generation of contact lenses.

When in use, orthokeratology lenses are usually used at night while sleeping and are worn for about 8-10 hours. After being taken off during the day, the naked eye vision can almost reach the best corrected vision. The effect is most significant for people aged 8-18. RGP is just the opposite. The wearer needs to wear it during the day and take it off at night when sleeping. This is the same as ordinary contact lenses, and there is no greater restriction on the age of adaptation.

Since entering the Chinese market in the late 1990s, orthokeratology has been receiving mixed reviews: at the beginning, the quality of orthokeratology was uneven, and the lack of attention to wearing and care led to many cases of eye diseases in children after wearing them. However, today, orthokeratology technology has matured, and many cities' tertiary hospitals and specialist hospitals have even opened orthokeratology service programs.

"Now the technology (of orthokeratology lenses) has been very mature, and there will be almost no problems if you wear them correctly according to the doctor's instructions," Zhang Jingshang, chief ophthalmologist at Beijing Tongren Hospital, told reporters.

In this regard, Song Hongxin, deputy director and chief physician of the Ophthalmology and Optometry Center of Beijing Tongren Hospital, also said that there will be tear protection between the orthokeratology lenses and the cornea. As long as the parameters are appropriate, the safety of fitting the glasses is "very high". "Moreover, the corneal epithelium of the human eye has the function of self-repair. It will generally be replaced naturally once every 24 hours, and there will be a major renewal every 7 days. Therefore, if corneal epithelial damage occurs, as long as it is treated promptly and correctly, the cornea will be repaired as soon as possible."

Scleral lenses: "crossing" the cornea

In recent years, scleral contact lenses have become more and more popular.

As a large-diameter hard, highly oxygen-permeable contact lens, the diameter of a scleral lens can reach between 15 and 25 mm. People may be curious: ordinary contact lenses are so small that they are difficult to wear, and scleral lenses are larger than them. Will they be comfortable to wear?

It turns out that the design of scleral lenses is different from that of ordinary contact lenses. Instead, they have a special dome formed in the middle. This special shape allows them to directly "cross" the cornea, allowing the lens to extend to the sclera without contacting the cornea, allowing the sclera to bear the entire weight.

▲Schematic diagram of scleral lens (drawing/Wang Xueying)

"In fact, there is nothing special about the scleral lens technology. First, its diameter is larger than that of hard contact lenses, and second, it does not contact the cornea, but the sclera," Song Hongxin told reporters. The diameter of a general corneal contact lens is only 11 to 12 mm, while the diameter of a scleral lens "exceeds the black pupil of the human eye, but directly contacts the white of the eye." In theory, scleral lenses can correct vision, provide better visual quality, and improve the vision of patients with irregular astigmatism. They are especially suitable for various corneal ectasia diseases represented by keratoconus and transparent marginal degeneration.

At the same time, before wearing the scleral lens, the lens will be filled with special fluids such as saline solution. The tear layer formed by the lens and the cornea can keep the cornea moist, thereby locking the moisture of the cornea and significantly improving symptoms such as dry eyes and eye aches.

Although there are more and more discussions about scleral lenses online, Song Hongxin also told reporters that at present, the use of scleral lenses is still mainly concentrated abroad, "because the State Food and Drug Administration has not yet approved it, it is not common in China." ■

Produced by: Science Central Kitchen

Produced by: Beijing Science and Technology News | Beijing Science and Technology Media

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