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Hello, this is Dr. Jess. I am sorry to hear about your dog. Is this wound right over the spine? Is there any way to take a photo to post here?
Is this something your dog has had its whole life, or only recently?
And the hole eventually showed up where the bump was? Any drainage from the area?
So one of the things that comes to mind for me with what you are describing is what is called a dermoid cyst or sinus. This is like a birth defect where the separation of the skin and the nervous system did not occur completely during embryonic development. Here is an article on the condition: https://www.acvs.org/small-animal/dermoid-sinus
Because it sounds like she had a lump for years and years which then opened up,I wonder if she had a dermoid cyst which became infected/irritated and eventually opened.
Without being able to put my hands on her it is very hard for me to really get a good idea of how this area feels/looks, so this is just one thing that came to my mind, but you could ask your vet if they think this is a possibility.
Another possibility is a sebaceous cyst that was incompletely removed the first time around. Here is an article on those: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=2603.
Fistula are usually the result of some foreign body or infected area deep below the skin trying to work its way out, or we can see them around the anus as a perianal fistula, but don't see them on the back very often.
HEllo, just checking to see if you got my response ?