Changes in the molar relationship between the deciduous and permanent dentitions: a longitudinal study.

The purpose of this study was to describe the changes in the molar relationship from the deciduous dentition to the permanent dentition in 121 subjects from the Iowa Longitudinal Growth Study. In addition, an attempt was made to determine the association between the various dentofacial variables and the changes in the molar relationship in 55 persons (33 male and 22 female subjects) with normal occlusion. All subjects were evaluated at three stages of dental development: stage I, completion of the deciduous dentition (means age = 4.94 years); stage II, when permanent first molars initially erupt into occlusion (means age = 6.91 years); and stage III, at the completion of eruption of the permanent dentition excluding third molars (means age = 13.01 years). The following sets of variables were evaluated: molar relationship, mesiodistal crown diameters of single and groups of deciduous and permanent teeth, dental arch widths, arch lengths, and various cephalometric dentofacial variables. Correlation coefficients and regression analyses were used to assess the relationships between these measurements and the changes in the molar relationship from the deciduous to the permanent dentition. The findings indicate that of the 242 sides evaluated in the deciduous dentition, 61.6% developed into a Class I molar relationship, 34.3% into Class II, and 4.1% into Class III. Those sides that started with a distal step in the deciduous dentition proceeded to develop into a Class II molar relationship in the permanent dentition. Of the sides with a flush terminal plane relationship in the deciduous dentition, 56% progressed to a Class I molar relationship and 44% to Class II in the permanent dentition. The presence of a mesial step in the deciduous dentition indicates a greater probability for a Class I molar relationship and a lesser probability for a Class II molar relationship. In the 55 subjects who achieved normal occlusion, the magnitude of change in the molar relationship was 1.91 mm in male subjects and 1.64 mm in female subjects. On the average, these cases had a mesial step in the deciduous dentition of 0.8 mm in male subjects and 1.0 mm in female subjects. There was a favorable difference between the maxillary and mandibular leeway spaces of 1.3 mm in male subjects and 1.1 mm in female subjects. There was also a favorable decrease in the Wits appraisal of 1.2 mm in male subjects and 0.6 mm in female subjects.(ABSTRACT TRUNCATED AT 400 WORDS)