Effects of rapid-slow maxillary expansion on the dentofacial structures

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.

To determine the vertical and sagittal changes in the facial skeleton during and following rapid then slow maxillary expansion (R-SME).

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.

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.

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.