Dental appliances and methods for expanding the lower dental arch. The appliances may be removable mandibular expander devices for efficient arch expansion while being comfortable to wear and easy to use. The appliances may include a lingual portion that is shaped and sized to apply an expansion force to lingual surfaces of one or more teeth. The shape and size of the lingual portion may be configured to distribute the forces among posterior and/or anterior teeth. A series of expander devices may be designed to provide the expansion forces in a stepwise fashion toward a target lower arch configuration according to a treatment plan