Arch expanding appliance

A removable polymer shell appliance corresponding to a stage of a treatment plan, the appliance comprising:

• a first tooth engagement region comprising a first set of teeth receiving cavities shaped to receive one or more of a patient's teeth on a left side of the patient;
• a second tooth engagement region comprising a second set of teeth receiving cavities shaped to receive one or more of the patient's teeth on a right side of the patient;
• and
• an arch region extending between the first and second tooth engagement regions, the arch region shaped to expand a dental arch of the patient according to the stage of the treatment plan, wherein the arch region comprises a first transverse section and a second transverse section to reinforce the arch region in a transverse direction, wherein the second transverse section has a raised height relative to the first transverse section,
• wherein the arch region, the first tooth engagement region, and the second tooth engagement region are formed of the same material, and
• wherein the arch region is 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.