That's where an infection may set in. Personally I would get a vet or someone experienced in that field to take a look. Exposed bone is never a good idea, especially if there's nothing left to grow and cover it, it leaves an open door for bacteria and infection. But that's if I was in the same situation, I can fix many things on my chickens myself, but I can't fix everything. It may need a full amputation. But that's just my advice, perhaps someone more experienced in that situation ( I've only had one hen who had a crushed wing and it had to be amputated by the vet, she made a full recovery and still somewhat flew) would be able to shed some better light for you.