A 3-dimensional analysis of molar movement during headgear treatment

Superimposition of serial cephalograms provides a limited description of tooth movement that could be complemented by data obtained from serial dental casts. The aim of this study was to develop a mathematical method for superimposing 3-dimensional data obtained from selected landmarks on longitudinally collected dental casts to describe maxillary first molar movement during headgear treatment. The material consisted of dental casts taken bimonthly from 36 children whose Class II Division 1 malocclusion was treated with straight-pull headgear during a 24-month period. Control data were collected from initial and final models of 38 subjects with a similar malocclusion who were not treated during a 24-month observation period. Spatial data from each subject's initial model were oriented similarly in an anatomically derived coordinate system, and a best-fit superimposition of palatal rugae landmarks from subsequent models allowed the measurement of molar movement. On average, headgear treatment resulted in distal movement of the molars, and the fitted net difference between treated and control subjects was 3.00 mm (SE, 0.37 mm; P < .001). Also, the headgear caused significantly more molar extrusion (0.56 mm; SE, 0.20 mm; P < .006) and buccal expansion (0.58 mm; SE, 0.17 mm; P < .001) on average than in the control group. Poor reliability of the method for measuring molar rotations indicated that they could not be determined accurately. Longitudinal description of molar movement for each subject revealed great individual variability in the amount and pattern of tooth movement. Several reasons could account for the wide range of individual variation and warrant exploration.