Maxillary adaptation to expansion in the mixed dentition.

This study presents the findings of 162 patients who underwent rapid maxillary expansion during the early mixed dentition. Maxillary changes were evaluated through the analysis of serial dental casts. Arch dimensions were measured pre-expansion, immediately post-expansion, and at yearly intervals until the eruption of the first premolars. The expansion was effected with an acrylic rapid maxillary expansion appliance bonded to the posterior teeth for 5 to 6 months. A simple retention protocol was used post-expansion. The average increase in transpalatal width was 5 to 6 mm. During the post-expansion period, most of the arch width increases were maintained. For example, 90.5% of the original expansion at the first permanent molars remained after the first year, with slightly less overall expansion (80.4%) evident at the end of the observation period (2.4 years postexpansion). Maxillary dental arches that initially were narrow tended to retain a greater percentage of the achieved expansion than those with initially wider arch dimensions. In addition, maxillae with initially more lingually-inclined molars tended to retain more expansion than maxillae with initially more facially-inclined molars. Palatal vault height decreased very slightly during treatment, but returned to pretreatment values one year after expansion and increased slightly during subsequent time intervals. The results of this study indicate that the majority of increased arch dimensions in patients produced by early orthopedic expansion of the maxilla are maintained at the end of the transitional dentition.