Did you have any dental work prior that could have caused the fistula? It might be worth getting a list of the materials that were used during your dental treatment to be sure that none of them irritated the gum. Typically they use standard bleach to clean out the canals. And gutta percha to fill them....not any formaldehyde. Ask for your records and not ask them directly. If you ask what did you use, and they used a combination of formaldehyde and gutta percha, they will just say gutta percha. Since you went to an endo, they probably did not use formaldehyde, but not 100%. Just tell them you want a copy of your records (both dentist and endo) because you are going to an oral surgeon. They will tell you that they will send them to the OS, but insist that you want to take them yourself. I had a very bad experience with a root canal and I proceed with extreme caution. I look at all of my xrays and make sure that I know what materials that are using...down to the novacaine injection. I will never let them use septocain or articane. They are more toxic than lidocaine and mericaine (spellings may not be right but they are close).