One of the more curious aspects of this incredible story is the way that Dana, as she returns to the present from the past for the last time, loses her arm as she returns and is literally disabled by her experiences and time travelling. You might find it interesting to read what Butler herself said about this ending and how it relates to the novel as a whole. Let us remember that the whole motif of time travelling is designed to force us as readers to experience slavery and its grim realities first hand, just as Dana is forced to experience them. We are not able to enjoy them from the luxury of our armchairs, and the way that the two radically different time periods are juxtaposed forces us to confront the horrors of slavery, just as Dana so often has to struggle to accept what is going on in spite of her 21st century values. Dana is profoundly changed by her experience of living out the reality of slavery in the past, and this is reflected by her amputation. Note what Butler says about this:
I couldn't really let her come all the way back. I couldn't let her return to what she was, I couldn't let her come back whole and that, I think, really symbolises her not coming back whole. Antebellum slavery didn't leave people quite whole.
Dana's loss of her arm therefore is a powerful reflection of the true impact of slavery and how those that survived it were not left "quite whole." Dana's exposure to slavery in all of its grim reality has to have some lasting impact on her, and she, just like her ancestors, has to bear that impact for the rest of her life. The loss of her arm is therefore a powerful symbol of the impact of slavery.