Abstract
Removable polymer shell appliances. An appliance may include first and second tooth engagement regions including corresponding first and second sets of teeth receiving cavities shaped to receive a patient's teeth on respective left and right sides of the patient. The appliance may include an arch region shaped to expand a dental arch of the patient according to a stage of a treatment plan. The arch region may extend between the first and second tooth engagement regions. The arch region may include one or more raised portions extending transversely across the arch region to reinforce the arch region in a transverse direction. The arch region and the first and second tooth engagement regions may be formed of the same material. The arch region may be offset in a vertical direction to form a gap between the arch region and a patent's palate when the appliance is worn over the patient's teeth.