ANALYSIS OF CHANGE IN ARCH FORM WITH PREMOLAR EXPANSION

The arch forms of 38 cases (53 nonextraction and 23 extraction arches) in which expansion, while maintaining arch form, was the objective of the practitioner, were analyzed before treatment, after treatment, an d an average of 6 to 8 years after retention, The cubic spline was use d to fit a curve representing arch form, By superimposing the spline c urves, changes in arch form were analyzed with the variables rebound c hange (RC), rebound index (RI), rebound number (RN), and stability num ber (SN), Traditional linear intraarch dimensions were also analyzed, Analysis of variance was used to determine differences between the max illary and mandibular arches and between the extraction and nonextract ion cases. Pearson correlation coefficients between spline variables a nd arch width variables were also computed. There was significantly mo re expansion in the maxiliary arch than the mandibular arch during tre atment, irrespective of extraction or nonextraction strategies. In the nonextraction cases, a greater amount of net expansion was achieved f or all dimensions for the maxillary arch as compared with the mandibul ar arch. Overall, a relatively high stability in arch form was found, The findings suggest that stability may not be related to the amount o f change produced during treatment. Significant expansion can be gaine d throughout the premolar regions and may be expected to be stable. Th e order of greatest net arch width gained was for the second premolars followed by first premolars, molars, and then the canines, The interc anine widths for both arches decreased toward pretreatment values, but were more stable in the maxillary arch in nonextraction cases, The cu bic spline permits measurement of change in arch form both during trea tment and retention periods.