Infraocclusion: Dental development and associated dental variations in singletons and twins.

Objective: The aim of this study was to investigate the prevalence of selected dental variations in association with infraocclusion, as well as determining the effects of infraocclusion on dental development and tooth size, in singletons and twins.
Design: Two samples were analysed. The first sample comprised 1454 panoramic radiographs of singleton boys and girls aged 8-11 years. The second sample comprised dental models of 202 pairs of monozygotic and dizygotic twins aged 8-11 years. Adobe Photoshop CS5 was used to construct reference lines and measure the extent of infraocclusion (in mm) of primary molars on the panoramic radiographs and on 2D images obtained from the dental models. The panoramic radiographs were examined for the presence of selected dental variations and to assess dental development following the Demirjian and Willems systems. The twins' dental models were measured to assess mesiodistal crown widths.
Results: In the singleton sample there was a significant association of canines in an altered position during eruption and the lateral incisor complex (agenesis and/or small tooth size) with infraocclusion (P<0.001), but there was no significant association between infraocclusion and agenesis of premolars. Dental age assessment revealed that dental development was delayed in individuals with infraocclusion compared to controls. The primary mandibular canines were significantly smaller in size in the infraoccluded group (P<0.05).
Conclusion: The presence of other dental variations in association with infraocclusion, as well as delayed dental development and reduced tooth size, suggests the presence of a pleiotropic effect. The underlying aetiological factors may be genetic and/or epigenetic.
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