The basic indication for a transplant is loss of vision due to corneal opacification—in other words, the inability to see through an injured or diseased cornea. Among the causes of corneal opacification severe enough to limit eyesight are injury, infection, inherent corneal disease such as keratoconus or corneal dystrophy, and corneal damage from previous eye surgery.
Other indications for a transplant are for relief of pain or discomfort, immediate repair of infectious or degenerative perforations in the cornea, and possibly cosmetic reasons.
When the cornea is damaged either by injury, disease, or hereditary conditions, it may become swollen or scarred. These scars cause the cornea to scatter or distort light resulting in reduced vision, sometimes to the point of blindness.
For more information see About Corneal Transplantation, American Academy of Ophthalmology.