Although laser has been widely used in various fields of medicine, it is the most widely used in the field of ophthalmology. This is because the eyeball itself is an optical system, and the light can reach the layers of the eyeball through the refractive interstitial. Due to the uniformity of the wavelength of the laser and the good directivity, lasers of different wavelengths can be applied, and the target is accurately targeted. Different tissues of the eyeball play a role, so they are first applied to ophthalmology in the medical field, and the widest range has formed a branch of laser medicine - laser ophthalmology.
First, laser treatment of eye diseases
1. The effect of different wavelength lasers on ocular tissue
The ocular tissues in different parts have obvious differences in the absorption of lasers of different wavelengths due to the different pigments. When laser treatment is selected, the laser should first be considered to have a high absorption rate in its target tissue. The refractive interstitial and other tissues on the path absorb as little as possible. In general, the shorter the absorption rate of melanin for the shorter wavelength, the difference is not great; the oxygen-containing hemoglobin has a high absorption rate for blue, green, and yellow light, but does not absorb substantially for red and infrared light. Lutein has a higher absorption rate for blue light. Therefore, blue, green and yellow light are commonly used in iris, anterior chamber tissue, retinal pigment epithelial layer and neovascular membrane. Among them, blue light can be absorbed by lutein, so it can not be used in macular area to avoid damage to retinal neuroepithelial layer. Although red light and infrared light can only rely on the absorption of melanin, they can penetrate thin diarrhea to reach the choroidal lining and retinal pigment epithelial layer, and are not absorbed and scattered by lutein, so they are often used in refraction. The quality is not clear, the retina has thin hemorrhage, the macular area, etc., but the effect on the pigment-free or de-pigmented area is poor, and it is easy to damage the deep tissue of the fundus due to the strong penetrability. Ultraviolet light with a wavelength shorter than 295 nm is mostly absorbed by the corneal tissue and cannot reach the intraocular tissues, so it is currently only used for corneal surgery.
2, the principle of laser treatment of eye diseases
After the laser acts on the eyeball and is absorbed by the tissue, a series of changes occur in the eye tissue, which is the basis of laser treatment.
1. Photoinduced fever
It refers to the process in which biological tissue absorbs laser energy and converts its light energy into heat energy. It is the most common method for laser treatment of eye diseases. Due to the difference in the level of local tissue reaction caused by heat, there are a series of reactions such as heat-induced warming, solidification, vaporization, perforation and cutting, which affect the level of eye tissue reaction, in addition to the laser power density, and the irradiated tissue. It is related to the absorption rate of the laser energy of the corresponding wavelength, the duration of the laser irradiation, and the like. Photoinduced heating can also lead to secondary physical and chemical reactions such as pressure and chemistry.
2. Photochemical action
It refers to the chemical reaction caused by biological tissue absorbing laser energy and converting light energy into chemical energy. There are four main types: photodecomposition, photo-oxidation, photopolymerization, and photo-sensitization. Photodegradation and photosensitization are common in ophthalmic treatment. The former uses an ArF excimer laser with a wavelength of 193 nm as a "cold knife" to decompose biochemical bonds and "cut" the cornea. A typical example of the latter is the treatment of retinoblastoma with photodynamic therapy.
3. Electromagnetic field action
Light is a changing electromagnetic wave, and a series of biological effects caused by electromagnetic interactions in biological tissues and optical bands are called electromagnetic electromagnetic fields. Among them, it is mainly a strong electric field. For ordinary light, since the optical power density is very low, the biological action of the electric field is not noticed. However, the laser makes the light energy highly concentrated in space. For example, the technique of Q-switching, mode-locking, etc., and its high concentration in time can generate a considerable electric field strength, thereby causing obvious biological effects.
4, photoinduced pressure
Lasers with a certain power density can also produce photo-induced pressure. This pressure can be generated for a variety of reasons, such as laser radiation, thermal vaporization recoil, thermal expansion, expansion-induced ultrasound, field-induced scattering, and field-stretching. Etc. This photoinduced pressure acts on the eye to produce a biological effect.
5. Principle of vaporization, cutting and punching
The high-power density continuous wave laser acts on the biological tissue and is absorbed by the biological tissue to cause heat. When the temperature reaches 100 ° C, the liquid in the tissue with water content of 60% to 80% begins to boil, and steam pressure appears, but Because the surface is closed, like a pressure cooker, when the laser energy is continuously absorbed, the temperature and air pressure in the tissue rapidly increase until the elastic limit of the sealed structure exceeds, and the steam breaks through the surface and the tissue debris is also trapped by the air flow.
Generally speaking, "vaporization" refers to the burning of lesions and neoplasms, that is, surface vaporization. If it is linear vaporization, it is called cutting. If it is point vaporization, it is called perforation. For a particular tissue that absorbs the corresponding energy, the depth at which vaporization occurs is in contrast to the time and power density of the laser illumination.
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