Hulk tells TMZ ... in October he went in for dental surgery to get implants. The doctor performed a sinus graft -- placing a donor bone in the mouth to support the implant. Hulk then went to a cosmetic surgeon to begin the implant process. When the doc removed the mold in Hulk's mouth, it ripped out around 50 stitches from the donor bone procedure.
Hulk says he's been dealing with the problem ever since ... his mouth became infected and the antibiotics just weren't doing the trick.
So recently Hulk had a CAT scan, which revealed -- get this -- that one of the doctors left a metal tack in his mouth.