The author never explains to readers why the loss of her arm is significant. That is left up to individual readers, and an argument could certainly be made that the arm being lost at the end of book is significant for no other reason than explaining how she managed to lose the arm on her "last trip home" as the beginning of the novel states. At that point in the story, readers do not know anything about the upcoming time travels, so we are completely dumbfounded by the description that her arm is somehow crushed or existing within a wall.
"That was when I realized your arm wasn't just stuck, but that, somehow, it had been crushed right into the wall."
"Not exactly crushed."
You could also argue that the lost arm is symbolically important, and you could argue that Dana has left an actual piece of herself in the past. She time traveled back and forth between two time periods. With the loss of her arm, it could be said that she now exists in both time periods at the same time. This could be...
(The entire section contains 3 answers and 704 words.)