I’m not a dentist so these definitions are not strict.
(1) After you have your braces removed, a retainer is something you put on to your teeth every night while you sleep. The objective is to keep your straight teeth in place. There are plastic and metal retainers.
(2) As I’m sure you know there is a treatment called Invisalign. Transparent braces are put into your teeth and their objective is to align your teeth, to straighten your teeth. These objects look identical to (plastic) retainers but are called aligners.
How do you say “retainer” and “aligner” in french and what are their genders? If there are no french equivalents for these nouns then that’s fine, please let me know. Thank you for your help. 😊✌🏽