Astigmatism is a type of refractive error, and the causes of astigmatism are usually divided into two types: the first type is physiological astigmatism, which is caused by congenital irregularities in the development of the cornea, and this type of astigmatism usually rarely exceeds 600 degrees. The second type of astigmatism is acquired astigmatism, which is caused by keratoconus due to acquired diseases, such as corneal inflammation, corneal ulcer, corneal trauma or cone cornea, etc. This type of astigmatism is usually large and extremely irregular, so it is difficult to improve the vision to normal with conventional frame glasses or corneal contact lenses. Therefore, if the condition is severe, corneal transplantation can be considered to improve the vision.