In Herman Melville's classic Moby Dick, Captain Ahab wears a prosthetic leg made out of whalebone ivory because the whale Moby Dick destroyed Ahab's leg in an earlier incident. Then, in Chapter 106, Ahab injures his ivory leg when being lowered into a boat. Melville writes of an earlier episode before the sailing of the Pequod: "his ivory limb having been so violently displaced, that it had stake-wise smitten, and all but pierced his groin." In this violent incident, Ahab's ivory leg nearly causes his own destruction, as it almost pierces his groin. After the incident in which he injures himself being lowered into the boat, he instructs the carpenter on the Pequod to make another leg made out of a whale's jaw. Ahab's whalebone leg is a constant reminder of his vendetta against Moby Dick.
Ahab's former injury and his memory of what Moby Dick did to him forever cause him to suffer. As Melville writes in Chapter 106, "Nor, at the time, had it failed to enter his monomaniac mind, that all the anguish of that then present suffering was but the direct issue of former woe." In other words, the monomaniacal Ahab, who always has Moby Dick on his mind, has a constant reminder of how much Moby Dick has caused him to suffer. Ahab's leg is a continual source of distress, and his pain makes him obsessed about killing Moby Dick.