Long-term clinical outcome of rapid maxillary expansion as the only treatment performed in Class I malocclusion

The purpose of this study was to investigate the long-term clinical responses of rapid maxillary expansion as the only treatment performed in Class I malocclusion using the Haas-type appliance. The longitudinal sample consisted of 90 sets of study models from 30 consecutive patients (12 males and 18 females) selected on the basis of the following inclusion criteria: all patients (1) had a Class I malocclusion with transverse maxillary/mandibular skeletal discrepancies, (2) were treated nonextraction in the early/ mid mixed dentition, (3) presented with mandibular dental arches with mild or no crowding, and (4) had no subsequent comprehensive orthodontic treatment implemented in either the maxilla or the mandible. The mean age was 8.2 years when treatment was initiated. Treatment outcomes were evaluated at pretreatment A1, short-term follow-up (one year after A1) A2, and long-term follow-up (four years after A2) A3. The changes in maxillary arch width and arch length were quantified and compared among assessment stages A1, A2, and A3 using the Student's t-test. The results demonstrated a highly significant increase in maxillary arch width in both the short- and long-term follow-ups. The arch width increased significantly during treatment and decreased slightly during the long-term follow-up. The long-term clinical response demonstrated the efficacy and stability of this type of treatment in achieving maxillary arch width. The follow-up examination during the early/mid/permanent dentition confirmed the validity of overtreatment.