Removable appliances or plates are used for relatively minor tooth movements, particularly in children who still have deciduous teeth. They are worn all the time except for when cleaning (yes, they're even worn when eating) or for sports requiring a mouth guard or swimming.
The most common orthodontic problem treated by upper removable appliances (URA) is crossbites of the front teeth. Crossbites of the back teeth can also be corrected and thumb-sucking habits can be helped. In some cases, this early treatment may be the only treatment that the patient ever requires but most patients will still need more complex treatment with braces later.
Functional appliances – twin blocks
These are removable appliances that force the lower jaw into a more forward position when biting. This repositioning of the lower jaw creates a potential space for the growth of the lower jaw at the back, particularly the jaw joint, lengthening the lower jaw itself. It is designed to take optimum advantage of any inherent and natural growth of the patient’s lower jaw. There is no evidence that it grows the lower jaw more than the patient’s potential.
Functional appliances are only used in actively growing patients for quite specific types of bites and severities of bites, and are only the treatment of choice for a relatively small number of cases. They're used as part of early treatment and fixed braces are almost always necessary afterwards.