Effects of rapid-slow maxillary expansion on the dentofacial structures

Background: To date, no study has determined if rapid followed by slow maxillary expansion (also termed 'semi-rapid' expansion) has the same effects on the dentofacial skeleton as rapid maxillary expansion.Objective: To determine the vertical and sagittal changes in the facial skeleton during and following rapid then slow maxillary expansion (R-SME).Methods: Bonded maxillary expansion appliances were used to separate the maxillae over six days by activating the midline screws twice a day. The screws were then activated three times a week until sufficient expansion was obtained (Mean: 3.4 months) and used as retainers for six months. Cephalometric measurements at the start of expansion (T1), end of expansion (T2) and end of retention (T3) were compared with paired t-tests. Pearson correlation coefficients were used to determine the associations between the expansion (dental and skeletal) and the cephalometric changes.Results: The maxillae moved forward a small, but statistically significant, extent during expansion. The upper molars were extruded and the mandible 'rotated' downward and backward. Although the vertical height of the facial skeleton (SN/GoMe, S-Go, N-Me, ANS-Me) increased significantly during expansion, the changes were small and highly variable. Some dimensions (SN/GoMe) relapsed during retention, while others (S-Go, N-Me) increased.Conclusions: Rapid then slow maxillary expansion caused a small, but statistically significant, forward movement of the upper facial skeleton, a small downward and backward rotation of the mandible and a small increase in face height. The changes were similar to those found during rapid maxillary expansion. (Aust Orthod J 2010; 178-183