Hi Gwin and welcome,
I haven't heard of Blue Cross denying coverage for a fusion, ADR- yes but a fusion, this is a first.
I have two suggestions - there's a book on how to appeal your insurance co - (Mark - help me out here). You can borrow it from Mark's library.
Second, you have the right to ask for a peer to peer appeal, which means your doctor can speak via phone to the same type of doctor, eg neuro-neuro, ortho-ortho, all being back specialists. It's then up to your doctor to convince his couter-part of the necessity of this surgery.
Other than that, insurance companies are in the business of making money. They are not your friend despite the friendliness of whoever you speak to. They pretend they're on your side, Often we must prove our disabilities and even then
In case you can't tell, I hate insurance companies.
Dale