Surgical debridement of an infected cornea can also be used to reduce organism load and excise devitalized tissue from the cornea. It may also improve the efficacy of medical therapy by promoting penetration of medication into deeper layers of the cornea.[3][11] In cases of corneal ulceration or perforation, or if corneal scarring is severe, keratoplasty may be required.[25][26] This typically involves full thickness transplantation of the cornea from a healthy donor eye. The size of the graft should be kept as small as possible, as larger grafts carry a great risk of host rejection, and due to the possibility of graft revision surgery.