Spontaneous mandibular arch response after rapid palatal expansion: a long-term study on Class I malocclusion.

The purpose of this study was to investigate the spontaneous mandibular arch response to rapid palatal expansion as the sole orthodontic intervention. Particular attention was paid to clinically significant effects and long-term (mean age, 11.3 years) posttreatment stability in 30 Class I malocclusion patients treated during the early and mid mixed dentition. Measurements were made directly on 120 dental casts obtained at 4 assessment stages: pre-expansion (A1), short-term follow-up (A2), progress (A3), and long-term follow up (A4). Comparisons between A1 and A2 showed statistically significant (P < .001) increases for intermolar widths (lingual and occlusal values). During the transition to the permanent dentition, a significant (P < .001) decrease occurred in arch length and arch perimeter. There was a 25% intermolar width (occlusal value) decrease from the initial net gain, whereas the lingual values remained unchanged. Comparisons between A2 and A4 showed an intermolar width (occlusal value) net gain of 50% and a significant (P < .001) decrease for the arch length and arch perimeter. There was remarkable stability in intermolar width (lingual value) and intercanine width (occlusal value), indicating that the increase in the mandibular arch width dimension was in response to the orthopedic effects of rapid palatal expansion in the early and mid-mixed dentition and that the stability was maintained until adulthood.