Why did my Bridge Last Less Than One Year, How Long Does a Bridge Usually Last? I Floss my Teeth 3 Times a Day? (photo)

My dentist made a 3 unit fixed bridge for me(tooth #3, 4,5) She cut the old bridge open and found deep decay. After cleaning, more than half of the tooth was gone. She spent almost a year, made 3 bridges on same tooth: 1st did not fit; 2nd one lasted 5 month; 3rd one lasted less than 1 year, then the gum around the bridge swollen, #3 had to be pulled. could you tell me why? Is it ok to use tooth #5, a small weak tooth, #3, a 10yrs old root canal, deep decay, 2/3 gone tooth, as abutment teeth?