The cornea is the clear, transparent, anterior covering of the eye. The cornea provides almost 70% of the eye's refractive power, and is composed almost entirely of a special type of collagen. The cornea is composed of five layers: the epithelium, Bowman's membrane, the stroma, Descment's membrane, and endothelium. Together, these layers are approximately 5ml (500 microns) thick. The cornea normally contains no blood vessels, but because it contains nerve endings, damage to the cornea can be painful.

Corneal transplant is used when vision is lost in an eye because the cornea has been damaged by disease or traumatic injury. Some of the disease conditions that might require corneal transplant include the bulging outward of the cornea (keratoconus), a malfunction of the inner layer of the cornea (Fuchs' dystrophy), and painful swelling of the cornea (pseudophakic bullous keratopathy). Some of these conditions cause cloudiness...