I see. Some of your pictures have reminded me of my rabbits. Their backs are flexible also but not in the way as a hedgehog. Rabbits can somewhat flex both ways when they are in the air kicking and ricocheting off the walls and furnitureNo they aren’t. Hedgehogs are designed to “ball”. They’re really flexible and can get into the most extraordinary positions. A broken back suggests impact damage of some sort. Perhaps a car, but it’s odd there were no other wounds.