The Normal Cornea
The cornea is the transparent window at the front of the eye. The majority of the thickness of the cornea is made up of fibres known as the corneal stroma.
On the outer surface of the cornea is a thin layer of cells - the epithelium - which forms a protective barrier to prevent infections getting into the eye. If the epithelium is damaged it usually heals rapidly because the cells can multiply very quickly to cover any exposed area of stroma.
On the inner surface of the cornea lies the endothelium - a single layer of fluid-pumping cells. The endothelium is necessary to keep the cornea clear, and if the endothelium fails the cornea becomes water-logged and cloudy. These cells do not recover well if they are damaged as they cannot divide to make up for lost numbers.
Corneal disease is the second most common cause of visual impairment globally, with about five million people blind, and many more with varying degrees of visual impairement. Infections such as trachoma and herpes simplex my cause corneal scarring, and vitamin A deficiency can lead to corneal melting and perforation.
When the full thickness of the cornea is damaged, as for instance in perforating injury, the full thickness of the cornea can be replaced by a corneal graft (penetrating keratoplasty). When the corneal damage or disease is limited to the corneal stroma, as for instance in keratoconus, then a partial thickness or deep anterior lamellar graft may be appropriate. When only the endothelial layer has failed, the cornea may be restored to normal hydration and clarity by an posterior or endothelial lamellar graft.
Recently, techniques have been devised to transplant just the outer corneal epithelium, or the inner endothelium, when these cell layers need replacement. In severe corneal disease, a conventional graft may have a poor chance of success, and in such cases an artificial cornea such as the AlphaCor, or a keratoprosthesis such as the Legeais keratoprosthesis may be required.