The reason that an eye transplant is so difficult is because of the complex nerve and tissue attachments necessary for the procedure. The surgeons, who spent 21 hours transplanting chin tissue, part of a face, a nose, and all of the necessary muscles, as well as an eye, practiced the operation on 15 cadavers before attempting it on James, their first live patient.
The eye transplant procedure was brand new, so although surgeons built on existing methods, they had to invent some new techniques for this groundbreaking operation.
