An orthodontic device, comprising: an anchorage structure, comprising: a left posterior teeth cap and a right posterior teeth cap configured to be removably worn on the left and right posterior teeth of a patient's maxillary dental arch, respectively; a palatal bar connecting the left posterior teeth cap and the right posterior teeth cap and configured to contact the patient's palate; and an expanded arch connected to the left and right posterior teeth caps and extending from the left posterior teeth cap to the right posterior teeth cap; a palatal expansion structure, comprising: a left anterior teeth part and a right anterior teeth part configured to be removably worn on the left and right anterior teeth of the patient's maxillary dental arch, respectively; and a plurality of expansion screws connecting the left anterior teeth part and the right anterior teeth part to each other and to the palatal bar, wherein the left and right posterior teeth caps, the palatal bar and the expanded arch transversely enclose the palatal expansion structure; and at least one protrusion (33, 33A, 33B) formed on an occlusal side of the expanded arch and configured to contact the patient's mandibular dental arch when the orthodontic device is worn. an anchorage structure, comprising: a left posterior teeth cap and a right posterior teeth cap configured to be removably worn on the left and right posterior teeth of a patient's maxillary dental arch, respectively; a palatal bar connecting the left posterior teeth cap and the right posterior teeth cap and configured to contact the patient's palate; and an expanded arch connected to the left and right posterior teeth caps and extending from the left posterior teeth cap to the right posterior teeth cap; a left posterior teeth cap and a right posterior teeth cap configured to be removably worn on the left and right posterior teeth of a patient's maxillary dental arch, respectively; a palatal bar connecting the left posterior teeth cap and the right posterior teeth cap and configured to contact the patient's palate; and an expanded arch connected to the left and right posterior teeth caps and extending from the left posterior teeth cap to the right posterior teeth cap; a palatal expansion structure, comprising: a left anterior teeth part and a right anterior teeth part configured to be removably worn on the left and right anterior teeth of the patient's maxillary dental arch, respectively; and a plurality of expansion screws connecting the left anterior teeth part and the right anterior teeth part to each other and to the palatal bar, wherein the left and right posterior teeth caps, the palatal bar and the expanded arch transversely enclose the palatal expansion structure; and a left anterior teeth part and a right anterior teeth part configured to be removably worn on the left and right anterior teeth of the patient's maxillary dental arch, respectively; and a plurality of expansion screws connecting the left anterior teeth part and the right anterior teeth part to each other and to the palatal bar, wherein the left and right posterior teeth caps, the palatal bar and the expanded arch transversely enclose the palatal expansion structure; and at least one protrusion (33, 33A, 33B) formed on an occlusal side of the expanded arch and configured to contact the patient's mandibular dental arch when the orthodontic device is worn.