[Cross-sectional study of the evolution of the primary dentition: shape of dental arches, overjet and overbite].

The aim of this study was to evaluate the characteristics (shape of dental arches, overjet and overbite) of the primary dentition of 6- to 39-month-old children from four nurseries of Rio de Janeiro, Brazil, and to associate them to gender and dentition phase. It was observed that 68.6% of the children presented round upper arch, while 31.4% had triangular upper arch; 92% of the children presented U-shaped lower arch and 8% had square-shaped lower arch. Moderate overjet was observed in 38.3% of the children, and slight overjet, in 30.3%. Severe (26.6%) and negative (25.5%) overbites were the most prevalent modalities of that condition. There was no association between gender and the studied characteristics. There was statistically significant association between the dentition phase and the shape of the arch, overjet and overbite (p < 0.001, chi-square test). It was possible to observe, in the present study, that early signs of malocclusions appeared when first primary molars erupted, as the posterior vertical dimension of occlusion increased. It was, thus, verified that early signs of malocclusions appear as the primary dentition develops. It is very important that the first dental visit occur during the first year of age, since it allows the pediatric dentist to prevent or carry out an early diagnosis of malocclusions in the primary dentition.