Objectives: To evaluate the stability of the dental changes that occurred during combined, maxillary and mandibular expansion over a period of six years, post-expansion. Methods: Study casts and lateral cephalograms were used to evaluate 29 patients with moderate to severe arch length discrepancy. Each subject was appraised at the end of four time periods: pre-orthodontic treatment phase 1 (T1), post expansion (T2), end of orthodontic treatment phase 2 (T3), and post retention (T4). Mean age at T1 was 9.3 years in females and 10.1 years in males. Inter-molar width, inter-canine width, arch circumference, arch length, and crowding were measured from each set of dental study casts. Lower incisor inclination was measured from the lateral cephalograms. Results: One-way ANOVA testing revealed a significant mean increase in mandibular inter-canine width T1-T2 (3.6mm, p<.001), and the expansion was maintained into post retention (p<.001). Lower arch circumferences increased 9.4mm (p<.001) from T1 to T2, was sustained to T3 (p=.002) but ultimately returned near T1 status at T4. Mandibular anterior crowding significantly decreased T1 to T2 (p<.001) and was sustained through study periods T3 and T4 (p<.001). No significant differences were found to exist among the four study time periods for cephalometric variable IMPA (p>.05). Conclusions: Concurrent expansion of maxillary and mandibular dental arches during the mixed dentition was effective for the sustained resolution of lower dental arch crowding.

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