LOR from research mentor?

Yes - it's perfectly acceptable, especially if it's a strong letter and the person has interacted w/ you enough to know you well. It's an even bigger plus if it's a well-known attending in the ortho community.

I got a great letter from a very well-respected PhD Ortho basic science researcher, and I know that it helped on my apps. Everyone has good clinical letters, and you will too. Yet, every time I go to an interview, the one that they want to talk about is the research letter. I used my letter, and milked it for all it was worth, and I'm completely convinced it got me interviews at some great programs that wouldn't be interested in me without it.