This is just my opinion, because I've never actually GIVEN femur/marrow bones, but I would think they would be fine for Yorkies. I think the problem with them is more for big dogs, who can put the whole bone in their mouth and *try* to eat them, therefore causing tooth damage/breakage. As for Yorkies, I'm not sure that they could even begin to *try* to eat the bone so in theory, it would just be recreational chewing for them (like scraping the sides of the bone, etc.). I don't know.
I have heard of many other YT users giving these, though.