 Corticosteroid has been used for the treatment after phototherapeutic keratectomy PRK. Dexamethasone, a potent steroid can retard the corneal epithelial healing rate and has numerous serious complications, especially glaucoma. Loteprednol, a new generation of corticosteroid has less potency than dexamethasone and was proposed to has less side effect regarding intraocular pressure IOP elevation. Therefore, Investigators would like to compare the effect of dexamethasone and loteprednol on corneal epithelial healing rate after PRK and risk of glaucoma. 