The medial canthus is an arch-shaped fold from the medial canthus to the nasal side, usually extending downward from the upper lid, and in a few cases upward from the lower lid, partially or completely obscuring the medial canthus and lacrimal folds, showing a wide medial canthus. Because the sclera on the nasal side of the lid is overly obscured, it gives the appearance of an internal strabismus, which affects the aesthetics of the eyes and appearance. In severe cases, it can partially obscure the nasal visual field and impede visual function. In the East, mild entropion is more common and normal. More than 50% of Chinese and 40-90% of Japanese and Koreans have this fold. However, it is rare in Westerners (less than 3%). It is most common in Mongolians and is a racial characteristic, hence the name Mongolian crease. Nowadays, because many beauty seekers want to have a pair of larger and nicely shaped eyes, canthal correction surgery has become more popular in recent years. The causes of canthus: most Asians have too much skin on their upper eyelids and Caucasians have not enough skin in the vertical direction. There are also abnormal attachments of the levator aponeurosis in the medial canthus, attachment of the orbicularis oculi muscle, muscle contracture below the medial canthus, and less orbital prominence. Some patients with congenital medial canthus may have small lid fissures, ptosis, microphthalmos, and other congenital developmental abnormalities of the eye. Some patients with congenital entropion may have small lid fissures, ptosis, microphthalmia, and other congenital abnormalities of the eye. Acquired canthus is mainly formed by trauma, burns, and scar contraction after canthal surgery and is often irregular and unilateral. It is often associated with adjacent tissue injuries and deformities, such as injury to the medial canthal ligament, lid fissure adhesion displacement, lacrimal duct injury, and nasal orbital fracture.