A Retainer Case is a portable container for storing a retainer, clear aligners, or other orthodontic appliances. Retainer Cases are usually made of hard plastic. Some have a special place to hold the key for an expander. If you wear retainers or clear aligners, it’s a good idea to get into the habit of using a Retainer Case to store your retainer. Most retainers and aligner sets get thrown out because they were wrapped in a napkin at lunch or dinner. Your orthodontist usually gives you a retainer case when you get your retainers or aligners, but if you need extras, you can buy them online.