Spontaneous lower arch changes with and without second molar extractions.

This study describes the spontaneous changes that occurred in the lower arches of 41 children in whom orthodontic treatment was restricted to the upper arch only. In one group, lower second molars were extracted (n = 18), whereas in the other, these teeth remained in situ (n = 23). A two-phase treatment involved distal movement of the buccal segments with an "en masse" appliance and extraoral traction, followed by edgewise mechanics to retract or align the anterior teeth. Only study casts were examined. The occlusal aspects of the lower casts were recorded with the reflex microscope. Models were examined at the start of treatment, at the completion of buccal segment retraction, and when active appliance therapy was complete. The following arch measurements were recorded: intercanine and intermolar width, arch length and perimeter, and the degree of crowding in both the labial segment and the arch as a whole. In the extraction sample, upper buccal segment retraction was associated with increases in all lower arch dimensions and a reduction of crowding. In the nonextraction group, the response was inconsistent. Although arch length and perimeter reduced, intercanine and intermolar dimensions enlarged with minimal alteration in the labial segment crowding. During the second stage of therapy, dimensions generally decreased: arch length, perimeter, and the intercanine width returned to their pretreatment values in the extraction group and were further reduced in the nonextraction sample. However, both arch length and perimeter remained significantly longer in the extraction group. Intermolar width remained stable in both groups, whereas crowding increased. Over the 2-year observation period, arch length, arch perimeter, and crowding exhibited statistically significant differences in behavior between the two groups.