PRK (excimer laser refractive keratomileusis): The principle is that after scraping the corneal epithelium, excimer laser is applied to cut the anterior elastic layer and superficial stromal layer of the cornea, and the curvature of the cut cornea is reduced and the refractive power of the cornea is decreased, thus correcting myopia. The advantage is that the surgery is simple, but the disadvantage is that the recovery of vision is slow, the hormonal medication takes a long time, and there are more complications. LASEK (excimer laser subepithelial keratomileusis): The principle is to create a corneal epithelial flap and then apply the excimer laser to cut the anterior elastic and superficial stromal layers of the cornea, which reduces the curvature of the cornea and decreases the refractive power of the cornea, thus correcting myopia. LASEK is a procedure between PRK and LASIK for patients with low to moderate myopia with thin corneas, high curvature, and small lids. LASIK (excimer laser in situ keratomileusis): The principle is to use a microkeratome or femtosecond laser to form a corneal flap first, then use the laser to cut the corneal stroma, after cutting, the cut corneal flap is restored to its original position. After cutting, the curvature of the cornea is reduced, and the refractive power of the cornea is reduced, thus correcting myopia. The advantages are fast vision recovery and short hormonal medication time, but the skill requirement for surgeons is higher than PRK and LASEK. Epi-LASIK (subepithelial-excimer laser in situ keratomileusis): The principle is to use a microkeratome to first form an epithelial flap, and then use a laser to cut the corneal stromal layer under the epithelial flap, and after cutting, the cut flap is restored to its original position. After cutting, the curvature of the cornea is reduced, and the refractive power of the cornea is reduced, thus correcting myopia. The advantage is that it has some of the advantages of LASEK and LASIK, but it is actually still a superficial surgery with the main disadvantages of superficial surgery.