thank you for your response
Burning pain, as you describe, is typically related to nerves and generally nerve or nerve root compression.
You didn't mention when the fusion was done, and what type it was. Probably an anterior cervical discectomy and fusion? Also, was there any mention of a radiculopathy?
It could be, if this was very recent, this is a result of normal post operative nerve inflammation as part of the normal healing process.
However, yours sounds extreme. A revision surgery may be necessary, and if a radiculopathy wasn't done, it probably needs to be. (that's where they decompress the nerve roots by making the opening larger)