New knowledge about keratomileusis and treatment of undercorrection or regression of refractive error after LASIK. Undercorrection and regression are two different concepts: undercorrection is caused by deviation from the target design during the surgery or during the first week after the surgery, e.g. the target is to correct 1000 degrees of myopia (-10D myopia), but the patient still has 100 or 200 degrees of myopia left when examined the next day or the first week after the surgery. This is called undercorrection; regression is when the patient progresses to the same refractive state for a longer period of time, such as six months or more than a year after surgery (or even longer than 5 or 10 years), for example, myopia in a previously myopic patient and hyperopia in a previously farsighted patient. Undercorrection is mainly a matter of surgical design, treatment environment (including space, equipment, etc.) and accuracy of optometry, and there is also intentional undercorrection. Since the corneal thickness of some patients may not meet the standard (e.g., the patient’s corneal thickness is only 500um or so and the degree is a little high), some degrees can be deliberately reserved for the patient after obtaining the patient’s consent so as not to cause the corneal tissue to be too thin and affect the safety of the surgery. Other patients are in need of a certain amount of myopia for occupational reasons, so that they can see or read close up. The main reason for regression after myopia surgery is due to changes in corneal biomechanics. The corneal tissue is not as strong as plastic or glass, and the environment it is in is affected by both atmospheric pressure and intraocular pressure, especially after the cornea thins its ability to resist intraocular pressure decreases. In some highly myopic patients, the intraocular pressure rises at a certain age, and with this rise in pressure, the thinned cornea will bulge forward, meaning that the refractive power of the cornea changes and the corneal surface becomes steeper again. This change from the shape of the cornea that was designed to flatten at the time of surgery or the shape of the cornea in the early postoperative period is the main cause of regression. When it comes to treatment, the main cause of regression is a change in corneal biomechanics, related to the pressure on the cornea. There is now some consensus that the early stage of regression, especially after LASIK surgery to correct high myopia, requires a reduction in intraocular pressure, so some anti-glaucoma drugs that have a lowering effect on intraocular pressure should be used, which is a good treatment for regression. In the past, glucocorticoids or other medications may be chosen to try to prevent regression, which may be useful in early stages, especially in patients with subepithelial haze after superficial excimer laser keratomileusis, but the principle of this procedure is not the same as LASIK, where the preferred treatment for visual regression several years after LASIK should be to reduce intraocular pressure. After the regression of refractive error in patients with hyperopic myopia, if the regression is due to corneal dilation or steepening of corneal curvature, it is recommended to first reduce intraocular pressure, and then to observe the patient’s refractive status and corneal morphology to ensure stability, and if the patient’s corneal thickness is within the safe range, the patient can be operated again, or it is called enhanced treatment. The mechanism of myopia and the progress of research The mechanism of myopia is complex, but in general there are two major causes: first, it is genetically related, which is a hereditary factor; second, it is environmentally related, which is an environmental factor. Research on myopia has not progressed to a particularly satisfactory level so far because of the complexity of both genetic and environmental factors, as evidenced by the increasing incidence of myopia in China. Despite a variety of measures (e.g., making students do eye exercises, doing more outdoor activities, and reducing the stress of schoolwork), the results are not very obvious at this point. Another hot research topic that has received a lot of attention, and one that ophthalmology professionals agree on, is the use of drugs such as low concentration atropine eye drops, which act to block the M receptors and block the signal that stimulates the eye to grow longer. Many ophthalmologists in the literature, especially in Taiwan and Singapore, are conducting long-term treatment and intervention with low-concentration atropine for myopia development, which is a new research direction; the second is the increasingly familiar corneal shaping technique, i.e., wearing corneal shaping lenses, or OK lenses for short. The principle of this shaping lens is to temporarily flatten the center of the cornea, which is achieved through epithelial thinning or epithelial redistribution, and it is only a temporary shape change. Now there is a principle of myopia occurrence that is related to peripheral vision or peripheral overcorrection. While ordinary glasses have just the right refractive state for central correction, it is possible for the periphery to be in a state of overcorrection. In this case it is possible to stimulate a gradual lengthening of the anterior and posterior diameters of the eye. This is one of the more important reasons why myopia cannot be controlled and is always progressing. The flattening of the cornea with a keratoplasty lens can prevent the periphery from becoming overcorrected. In terms of this principle, OK lenses can play a role in preventing the development of myopia, but the real mechanism remains to be further explored. Advantages of corneal topographers capable of measuring posterior corneal surface morphology The use of ultrasound to measure central corneal thickness is more reliable in terms of the currently accepted gold standard for ophthalmic measurement of corneal thickness, but the ultrasound method has many inconveniences, such as the need for patients to undergo contact measurements under surface anesthesia, which may result in corneal epithelial damage and cross-contamination problems. In addition, ultrasound measurement is performed at a single point rather than the entire cornea, so the thickness is only known at the point of contact, not the entire cornea. This optical band scanning corneal topographer, represented by Pentacam, Sirius, Orbscan, etc., can scan the entire corneal image in a few seconds and reflect the overall corneal thickness. The thinnest area can be identified from the center to the periphery, so it is also possible to determine whether a patient has a tendency to cone the cornea based on the pattern of corneal thickness distribution, which cannot be done by ultrasound alone. Thus, corneal scanning topography reflects the overall thickness of the cornea, but is also very fast and does not touch the cornea. In addition, it can also reflect the shape of the anterior surface of the cornea, the shape of the posterior surface and even the depth of the anterior chamber, providing a reference for many clinical procedures. Trends in excimer laser keratomileusis The development of excimer laser keratomileusis began in the 1980s and became popular in the 1990s. Excimer laser keratomileusis has evolved over the past two to three decades and is becoming increasingly personalized and optimized for design and treatment. The further refinement of eye tracking and positioning techniques has led to better visual quality and visual outcomes after surgery. Excimer laser will become more and more precise and refined, and it can design the optimal and most suitable plan for each patient according to the different corneal morphology or aberration of each patient. In addition to the correction of traditional refractive errors, it can also be designed to compensate for old eyesight for older patients, so that while improving distance vision, it also retains a fairly good near vision.