I'm surprised the five answers here so far all advocate, rather assuredly, that you should explain your injury first thing.
I've worked with a bunch of people that wouldn't give a rat's ass why you have an injury. They're there to figure out whether you have the skills to do the job and whether you will work well in their organization. Spending that time discussing your injury wouldn't even cross their minds.
I would also say that as an interviewee, don't forget that you are also screening them. Do you really want to work with people who are so concerned about why you were injured? Is that really the most important thing for them to be thinking about?
Of course, if you really need the job and opportunities aren't looking great, yes, you can cover your bases by explaining your injury. But keep this in mind: not only would an interviewer who is suspicious enough to think you got into a fight be suspicious enough to think you are making up an excuse, but focusing on your injury first thing shows a lack of confidence.