Date of Award

Spring 2001

Document Type

Thesis - Restricted

Degree Name

Master of Science (MS)



First Advisor

Oyen, Ordean

Second Advisor

Stier, Erwin

Third Advisor

Bradley, Thomas


The purpose of this study was to investigate the spontaneous mandibular arch response to rapid palatal expansion (Haas-type) as the sole orthodontic intervention. Particular attention was paid to the occurrence of clinically significant effects and long-term "formula" post-treatment outcomes stability. This study differs from other published reports on the effects of RPE on the mandible in that many of them were of shorter duration, and/or the associated treatment protocols also involved the use of orthopedic and/or orthodontic fixed appliances. Data was obtained from orthodontic records including dental casts taken at each of the four different assessment stages for 30 patients (18 females and 12 males). All patients were non-extraction cases presenting Class I malocclusion and transverse maxillary/mandibular skeletal discrepancies treated during the early/mid-mixed dentition. Posterior and anterior mandibular arch width values, plus arch length and arch perimeter were measured directly on study casts obtained at pre-expansion "formula", short-term follow up "formula", progress "formula" and long-term follow up "formula". Descriptive and inferential analyses of the six variables at the four assessment stages for all 30 individuals were performed. Differences between pre-expansion and short-term follow up resulted in statistically significant (p < 0.001) increases for mandibular intermolar widths (lingual and occlusal values). During the transition to the permanent dentition an expected significant (p < 0.001) decrease occurred in arch length and arch perimeter. Also, in this period there was a decrease of 25% from the initial net gain for the intermolar width (occlusal value), while the lingual values remained unchanged. Further, differences between the short-term and the long-tenn follow ups, resulted in a mandibular intermolar width (occlusal value) net gain of 50%, and the expected significant (p < 0.001) decrease for the mandibular arch length and arch perimeter. Finally, there was a remarkable stability in the mandibular intermolar width (lingual value) and intercanine width (occlusal value) indicating that the increase in the width dimension in the mandibular arch was in response to the orthopedic effects of RPE in the early/mid-mixed dentition and that the stability was maintained until adulthood. Based on the findings of this study, several statistically significant changes were noted in the mandible that could be attributed to RPE. However, no clinically adverse effects were observed. As a matter of fact, the long-term effects associated with this form of maxillary intervention were positive especially for the mandibular arch width dimensions.



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