In this classical novel, the captain wears a prothesis in his leg made out of whalebone ivory because a white whale that gave the name to the book, destroyed his leg in an earlier incident.
The suffering that Captain Ahab has it's not only physical, it is emotional, not having his leg, means that he has lost part of his manhood. It's a symbol for energy and determination, that he finally one day he will go back and hunt the whale and get even because of that incident.