As of the end of 2003, the U.S. Food and Drug Administration (FDA) approved LASIK to correct refractive errors in the following ranges: myopic spherical lenses 0.00 to -15.00D (less than 1500 degrees); hyperopic spherical lenses +0.50 to +6.00D (less than 600 degrees); and astigmatic column lenses 0.00 to 6.00D (less than 600 degrees). For LASIK, the main factors that limit the correction of myopia and astigmatism include: corneal thickness, corneal morphology and postoperative visual quality. ① Corneal thickness factor: According to the Munnerlyn formula, the laser cutting depth = d2 ´ D/3, the unit of optical zone diameter (d) is millimeter (mm), while the unit of laser cutting depth is micron (mm); D indicates the absolute value of corrected refraction. For example, to do 6 mm optical zone diameter correction myopia-5.00D (500 degrees of myopia), the laser cutting depth is 62 ´ 5 / 3 = 60 microns (mm). That is, with a 6mm optical zone diameter, for every 100 degrees of myopia correction, the thickness of the cornea to be cut is 12 microns. Studies have shown that the corneal flap of LASIK is weak against intraocular pressure, and therefore a sufficient thickness of the subflap stromal bed must be preserved to reduce the possibility of postoperative corneal dilatation and secondary cone corneas. The remaining corneal stromal bed thickness can be roughly calculated using the following formula: remaining stromal bed thickness = preoperative central corneal thickness – flap thickness – laser cutting depth. For example, if the preoperative central corneal thickness is 530 mm, the flap thickness is 130 mm, and the laser cutting depth is 60 mm, the remaining corneal stromal bed thickness is 530-130-60 = 340 mm. Currently, most surgeons believe that the remaining corneal bed thickness after LASIK should be at least 250 mm and/or one-half of the preoperative central corneal thickness. However, this number is derived from clinical statistics only and is not based on any tangible laboratory findings. Even if the stromal bed is preserved at 250 mm after LASIK, there is no guarantee that corneal dilatation will not occur after surgery. Therefore, the higher the degree of myopia corrected and the thicker the flap, the thinner the postoperative stromal bed will be, and the greater the likelihood of secondary cone corneas. From this point of view, the superficial cutting technique, LASEK or Epi-LASIK, has certain advantages over LASIK because the superficial cutting technique only creates a corneal epithelial flap with a thickness of about 50-60 microns, which is thinner than the corneal flap of ordinary LASIK and preserves the biomechanical structure of the cornea better after surgery. ② Corneal morphological factors: the cornea flattens after LASIK correction of myopia; it steepens after correction of hyperopia. For each myopic diopter correction, the cornea flattens by 0.8D (80 degrees), while for each farsighted diopter correction, the cornea steepens by 1.0D (100 degrees). Therefore, the surgeon is able to anticipate the postoperative corneal curvature (refractive power) value before surgery. For example, if the preoperative corneal refractive power is 42 D, after correction of -10 D (1000 degrees) myopia, the corneal refractive power will be 42-10´0.8 = 34 D. Studies have shown that excessive flattening or steepening of the cornea will affect postoperative visual quality and increase the aberration of the eye. Therefore, postoperative corneal refractive power below 34D or above 50D should be avoided, and a steep or flat cornea can affect tear film adhesion, which is one of the causes of dry eye after LASIK. ③ Post-operative visual quality factors: From the Munnerlyn formula, it can be derived that to correct the same refraction, the laser cutting depth is proportional to the square of the optical zone diameter. Therefore, clinically, the optical zone can be reduced or zoned cutting methods can be used to conserve corneal tissue for the purpose of retaining sufficient postoperative residual corneal stromal bed thickness. These methods, however, can significantly increase the spherical aberration, especially in dark illumination situations when the patient’s pupil is dilated, producing symptoms such as glare, halos and even ghosting, which can significantly decrease the quality of vision. Therefore, LASIK has certain limits for refractive error correction, and should not be forced before surgery, taking into account corneal thickness, corneal morphology, pupil size and other factors. Some patients who are not suitable for LASIK, especially those with high refractive error, can be considered for lens refractive surgery, such as lens eye IOL implantation, refractive lens removal combined with IOL implantation, etc.