A palatal expander is a device in the field of orthodontics which is used to widen the upper jaw (maxilla) so that the bottom and upper teeth will fit together better. This is a common orthodontic procedure. Although the use of an expander is most common in children and adolescents 8–18 years of age, it can also be used in adults, although expansion is slightly more uncomfortable and takes longer. A patient who would rather not wait several months for the end result by a palatal expander may be able to opt for a surgical separation of the maxilla. Use of a palatal expander is most often followed by braces to then straighten the teeth. It is believed that expansion therapy should be started in patients either before or during their peak growth spurt. To obtain maximal skeletal changes, the therapy is typically initiated at a very early age. Expansion therapy performed after the peak growth spurt will lead to more dental changes than skeletal which leads to tipping of buccal teeth. Indications for a palatal expander Maxillary expansion is indicated in cases with a difference in the width of the upper jaw to the lower jaw equal to or greater than 4 mm. Typically this is measured from the width of the outside of the first molars in the upper jaw compared to the lower jaw taking into account that the molars will often tip outward to compensate for the difference. Rapid palatal expansion is also used in cleft palate repair, and to gain room for teeth in patients with moderate crowding of the teeth in the upper jaw. Types of expansion Rapid palatal expansion Rapid palatal expansion (RPE) or Rapid Maxillary Expansion (RME) is an expansion technique where expansion of 0.5 mm to 1 mm is achieved each day until the posterior crossbite is relieved. The expander works by turning a key inside the center of the expander. The turn of this key will push the arms of the expander. For stability purposes, the RPE usually remain in the patient's mouth anywhere between 3–6 months, but this time may vary between patients. This is often known as "six month retention period" during which the bone fills the gap in the maxilla that was created by the expansion process. To prevent any type of relapse, a retainer is given to the patient to keep the teeth in proper alignment.