A radiographic and histological study of modern human lower first permanent molar root growth during the supraosseous eruptive phase

There is increasing focus on the relationship between root growth and the eruptive process in studies of primate dental development, and the first permanent molar (M1) is regarded as a key tooth in many of these comparative studies. In this study of modern human M1s, histological and radiographic data were compared. Rates of root extension were determined histologically in 20 M1s from individuals of known sex using data for daily incremental markings and the orientation of accentuated lines in root dentine. Mean values at the mesiobuccal enamel cervix were 4.3–5.4 μm per day and then rose to a maximum of 6.7–8.4 μm per day during the first 5 mm of root growth before gradually declining again to 2.8–3.6 μm per day towards apical closure. A sample of 101 orthopantomograms of children, where M1s were between the stages of alveolar eruption and complete eruption, were then used to determine total mesial tooth height and mesial and distal root lengths at four successive stages of eruption. At complete eruption, mean values for mesial and distal root lengths were 8–10 mm, respectively. Expressed as a percentage total of mesial tooth height these averaged 45.6–56.2%. Maximum rates of M1 eruption occur just prior to gingival emergence but did not coincide with maximum rates of root extension in this study. These results emphasise that rates of eruption and rates of root growth do not follow the same pattern of change during the supraosseous eruptive phase. They highlight the need for greater consideration of the role of the eruptive process in explaining differences in gingival emergence times in comparative studies of modern humans and fossil hominins.