You cannot "keep" someone's dog because you deem them to be abusive. You need to contact the police and then turn the dog over to animal control. If the owner does not claim the dog you might be able to adopt after the required amount of time which varies by city and state. I'm wondering why you say the dog has a broken kneecap? I've never heard of this before. Are you sure it's not luxating patella?
__________________ “Never argue with a fool, onlookers may not be able to tell the difference.”  Mark Twain |