One thing to remember is that this dog can't bite at this stage. Pinch with the gums perhaps, but the owners should be able to open the mouth and dodge the lone canine.
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Since the dog has been to the vet and been checked out, it seems that Simon can enjoy a good quality of life--that is what is important. If he had advanced dental disease it is possible that he had lost teeth for a while unnoticed. (This isn't saying Simon's people neglect him, just he is an elderly fellow and this does happen with aging.) Chewing cage bars may have taken out the rest of the teeth. Dogs frequently swallow the teeth, so you may not see evidence. The dog's 4th premolar (the carnassial tooth) is a beast to extract from what I've been told. It typically needs stitches and the upper roots can extend into the sinus cavity. Nasal bleeding is common when this tooth is pulled. There would be evidence of surgical removal. And I do doubt surgery was performed over a weekend. Just hoping Simon does well and has many, many more tail wags for his family.