Many are needed because of keratoconus, a disorder of the shape of the cornea, or corneal disease, often caused by cataract surgery.

BBC: Eye transplants 'hit by Alder Hey'

Keratoconus causes the collagen fibers in the eye to weaken.

FORBES: A New Hope for Those With Down Syndrome...And a Rare Eye Disease

People with advanced keratoconus see life through beveled glass.

FORBES: A New Hope for Those With Down Syndrome...And a Rare Eye Disease

In order to restore the eye to its original shape, those with advanced keratoconus generally undergo a cornea transplant, a procedure with a long recovery time that demands the patient refrain from physically stressing the eyes for months.

FORBES: A New Hope for Those With Down Syndrome...And a Rare Eye Disease