Maxillary expansion: a meta analysis.

The utility of maxillary expansion has been equivocal for more than 100 years. The advent of meta-analysis and evidence-based learning has provided an opportunity to look objectively at this treatment modality. Medline was searched from 1978 to 1999 for all studies examining the stability of transverse expansion of the human maxilla using initial search terms of maxillary expansion and palatal expansion which were limited to those English language and human subjects. The more than 5000 articles were reduced to 12 based on the defined inclusion and exclusion criteria. The two investigators participating in the study were blinded, the studies' authors and origins blacked out and the evaluation coded and scored. A cumulative 'Meta evaluation score' was computed for each study. Six studies remained for the final analysis. The mean expansion after adjustment according to the principles of meta analysis was 6.00 mm with a standard deviation of 1.29 mm. Of the 6-mm average, 4.89 mm was retained while wearing retainers. Five of the papers provided retention data and post-retention data, but only three studies provided both retention and post-retention data. The average age of patients in these reports was 10.8 years. The 6-mm average expansion with retention in the short-term (<1 year) yielded a 4.71-mm residual expansion. Subsequently, this expansion during the short-term post-retention period was reduced to 3.88 mm. Finally, in the long-term post-retention study period only 2.4 mm of the residual expansion was reported to have remained. This 2.4 mm of expansion remaining after more than a year or more of post-retention period was no greater than what has been documented as normal growth. There is insufficient data to conclude that any useful expansion beyond that can be expected through normal growth was retained.