You can use the LOR for your school, then. But as far as other programs, the LOR will not carry as much weight as a clinical letter. If the prof will write a GREAT LOR, you can use it as a 4th LOR for applications since most ortho programs will take 4 LORs. For the programs that take only 3 or even just 2 LORs, I would use clinical letters instead unless your clinical letters are very bad. Another way to use the prof's LOR for more programs is to do a research rotation with him in which you get some signifcant work done. I think that would make his LOR more legit than to say that you are just the TA for his class.
Well, that's my take. The final decision as always is up to you.
