Table 1 Inclusion and exclusion criteria

Inclusion criteria

 Eight incisors have been completely replaced, and primary canines and primary molars have not started to be replaced

 Angle class I or Class II

 Incompetent lips

One or more of the following features: ① Crowding: ≤ 4 mm; ② Anterior teeth overjet I°-II°; and ③ Anterior teeth deep overbite

Exclusion criteria

 Angle class III

 Severe skeletal class II

 Moderate to severe crowding

 High angle, open bite tendency and skeletal deviation

 Poor compliance and coordination

 Adenoid, tonsil hypertrophy with indications for resection but no surgery for an obstructed nasal airway

 Allergy to oral materials