So I have sprained my middle finger (yes, that one) and it is hard to type. The splint looks a little like this one, although mine is longer. Luckily, if I'm wearing the splint, the sprain doesn't hurt.
How did I do it? I have no idea. I am easily injured, much to my dismay, but the only activity that could have caused it was that I went to the gym for a session with a trainer on weights. No trauma, just silly wear and tear.
So, I'll work on getting the hang of touch typing with only nine fingers better tomorrow, but this is all you're getting at the moment.