A permanent retainer, called a lingual retainer, is one that is glued to the inside part of your teeth (the part of the teeth by the tongue). It is not visible. Usually, a permanent retainer lasts about 5-7 years. It can become unglued during a dental cleaning, normal wear and tear over time, and if you bite into something very crunchy.
A permanent bonded retainer is a great option after finishing orthodontic treatment with braces or Invisalign since it will keep the teeth from moving even the slightest bit. The downside of a permanent retainer is that it might collect some plaque or calculus around it if you are not brushing and flossing properly.
If a patient would like a permanent retainer, Dr. Markova usually recommends it on the lower front teeth only. These are the teeth that are most likely to shift and crowd up over time. A bonded retainer is sometimes done on the upper teeth, but since you use the upper teeth to bite into things, the retainer on the upper teeth has a higher risk of falling off.
This is an example of a patient that finished with Invisalign orthodontic treatment and has a bonded lingual permanent retainer on the inside of the lower teeth: