A plastic surgeon stated that he can inject facial fat using cannulas inserted into the corners of my mouth. Pros and cons?

Facial Fat Grafting is an excellent way to restore lost facial volume.  The procedure can be performed under local anesthesia in the office, and takes about an hour.  Tiny incisions are needed to inject the harvested fat, easily hidden in other regions of the face.  The corners of the mouth is one approach, but other approaches may be better, with depending on exact location of areas of fat needed.  Be sure to consult with a board certified plastic surgeon. Good luck!

Mercer401,
     Fat grafting is a common technique, and can be a great tool in restoring lost volume to the face.  It is permanent and often more natural result than fillers can give as it is your own native tissue.  There are a number of techniques, but it is common to make a few select puncture sites for the cannula and place the fat as needed for each patient.  Common insertion sites include corners of the mouth, mid cheek, temples, and pre jowl areas.  

Incisions can be used from almost anywhere. I would avoid injections through the  mouth itself to minimize the risk of contamination.