Long-term stability of rapid palatal expander treatment and edgewise mechanotherapy

Previous studies on long-term stability of orthodontic treatment primarily have focused on the stability of the lower arch treated with edgewise appliances. The aim of this study was to evaluate the long-term stability of the upper and the lower dental arches of patients treated with a rapid palatal expander. The sample comprised of 165 dental casts randomly selected from patients who had been out of retention for 8 to 10 years at a mean age of 30 years. Measurements were made directly on dental casts obtained at the three time intervals: before treatment, after treatment, and after retention. Differences over time between the upper and the lower dental arches and between intervals were analyzed by a two-way multivariate analysis of variance (MANOVA) and post hoc Bonferroni t tests. Differences between after treatment and after retention were statistically significant (P < 0.006) for all except lower intermolar width. However, only for lower and upper arch lengths and perimeters were the differences greater than 2.0 mm. Treatment with the rapid palatal expander presented good stability for upper intercanine width, upper and lower intermolar widths and incisor irregularity. Lower intercanine, arch length, and perimeter presented poor stability.