People alwqays view this as paradoxal. Because, in appearence, when you arrive in the past, the machine or mean by which you travelled there wasn't yet invented and may not be in the time it was when you originally traveled. If you go back in time, You go back to a present in which nothing that came after ever happened. Since the first time it happened, you weren't there. Your presence there already changes the state of that point in time (think butterfly effect).
So your time machine, or whatever the mean used to travel in time is an anachronism. But that would be seen as such only to those from that past age. For you, you're still stuck in the reality, with its sets of laws. You wouldn't start aging only when the years finally reach the year you left from, would you? Now, If you influence past events, leading to an impossibility for you to even exist in the state you arrived in the past, it won't really change a thing at all, since biologically speaking, you are still what you are because of what you experienced in your life. Your memory may end up being the only thing that proves what you went through. But I don't believe the universe will try to balance things out, or collapse on itself because your presence in the past can be viewed as a paradox. That the sensible world around you is aware of it or not, you still went through the experiences that lead you to time travel in the first place.
Here's a quote: "Think of an experience from your childhood. Something you remember clearly, something you can see, feel, maybe even smell as if you were really there. Afterall, you were there at the time, weren't you? How else would you remember it? But here is the bombshell: you weren't there. Not a single atom that is in your body today was there when that event took place... Matter flows from place to place and momentarily comes together to be you. Whatever you are, therefore, you are not the stuff of which you are made." -Steve Grand"
But your apparent paradox is inconsequential.
If time travel ever exist, a paradox could never be, because the point in time where you would arrive would start a series of events that'd lead to you actually travel back to that point. Like, if I take your example, the very first time you were to lose your leg, you wouldn't have, because your future self would have already been there to prevent you from losing it.
But now, why would you have gone back to time in the first place, then?
You wouldn't.
If you look at it by drawing a straight time line, putting events in sequences, you notice that the point at which you arrive in the past isn't part of a progression, but a simultaneity. The very first time that this given event happens, you and your time-traveling self are already both there. The event from the past, and the presence of your future self in that same moment are both part of what shapes the moments onward. You were there twice during those events. So the event that you describe, traveling in time to prevent something from happening is impossible. It would already have happened as if you prevented it in the first place. In fact, "prevent" is not the right word. The events would actually have happened this way, since you were already there from the future. You can't change what happened, since you were already part of that time line's conditions.
