Measurement of mineral density from the cemento-enamel junction to occlusal/incisal edge
MD readings were averaged for each third for the discoloured and the non-discoloured teeth. Non-discoloured teeth displayed a gradient of increasing MD from the cervical third to the occlusal/incisal third. The discoloured teeth also showed a similar gradient (Fig. 1A(1)). The differences between the discoloured and non-discoloured teeth were statistically significant for the cervical third (t(19) = 2.422, p = 0.0128 (one-sided)) and the middle third (t(19) = 2.8172, p = 0.0055), but not for the occlusal/incisal third (t(19) = 1.0677, p = 0.1495).
Measurement of mineral density from dentino-enamel junction to external surface
The MDs were calculated from a representative cross-sectional slice for each tooth. The difference in MD between discoloured and non-discoloured enamel was statistically significant for only the middle (t(18) = 2.5293, p = 0.0105) and outer thirds (t(18) = 4.5253, p ≤ 0.0001) (Fig. 1A(2)). For both the discoloured and non-discoloured groups, the least mineralised part was the outer third. The MD values sampled from unaffected areas within the discoloured teeth were similar to the MDs recorded from the non-discoloured teeth, with no statistically significant differences. This suggests that in the discoloured teeth, the reduction in MD is restricted to discoloured areas. There was a statistically significant difference in MD between the affected/discoloured and unaffected regions within the discoloured teeth for the middle and outer thirds of enamel. The average MD for unaffected areas at the inner third was 2.27 g/cm3 (SD = 0.20), 2.38 g/cm3 (SD = 0.14) for the middle third and 2.21 g/cm3 (SD = 0.16) for the outer third.
Research using XMT to assess the MD of enamel report the range of densities for sound enamel at between 2.3 and 3.1 g/cm3. The MD of non-discoloured enamel in the present study is towards the lower end of the normal reported range. The MD of discoloured enamel in this study had a range of 1.96–2.22 g/cm3, thus reflecting a true hypomineralisation.
Our findings show that the MD was not constant throughout the enamel. When traced horizontally, the non-discoloured teeth showed a positive gradient of MD from the CEJ to the cuspal/incisal edge, a finding consistent with previous studies. The higher MD at the cuspal/incisal edge may reflect the gnarled nature of enamel in that area, where the enamel rods are twisted over each other. The lower MD of the cervical third may be an artefact of the partial volume effect. Partial volume effect occurs when a region of interest adjacent to the outer surface incorporates part of the dark background, making the MD reading appear lower. This is a possibility particularly for cervical area readings because the enamel at this point is very thin, making it difficult to identify a region of interest that excludes the dark background. The MD readings of the discoloured teeth showed the same gradient from the CEJ to the cuspal/incisal region, although the average MDs were lower. The most severely affected area is the middle third, with an average of 7.4% reduction in MD compared with non-discoloured enamel.
In sagittal sections of discoloured and non-discoloured teeth, the MD decreased from the middle third to the outer third. For both discoloured and non-discoloured samples, the lowest MD was located within the outer third, although the discoloured teeth had significantly lower MD in the outer enamel. There is some disagreement in the literature regarding MD trends when traced from the DEJ to the external enamel surface. Some studies reported an increasing gradient from the DEJ to the external surface, whereas others documented similar findings to our study. The lower MD of the outer third of both discoloured and non-discoloured samples found in the current investigation may be, in part, attributed to the partial volume effect. Alternatively, low MD at the outer surface may indicate that the teeth were unerupted or newly erupted. Finally, ion exchange may have occurred between the burial environment and the outer enamel of the teeth in this archaeological sample, causing the low MD reading at the outer surface.
To further confirm the hypomineralised nature of the affected areas in the discoloured teeth, the MD was measured for non-discoloured areas in the discoloured teeth. Those non-discoloured areas were found to have normal MD, which confirms that only the discoloured areas were truly hypomineralised. The significant difference in the MDs between the discoloured and the non-discoloured teeth from the ‘Atele burial sites is in contrast to previous findings for teeth collected from the prehistoric site of Ban Non Wat, Thailand. This strengthens the argument that identifying teeth as hypomineralised based solely on their macroscopic appearance is not possible.
This paper is the first step in providing a fuller understanding of the life histories of these infants and children in Chiefdom Period in Tonga, a period of increasing hierarchy and interactions through trade networks. In establishing that these discoloured teeth are evidence for physiological disruption, our future work will quantify macroscopically all deciduous dental enamel defects in the sample. The analyses of the prevalence, timing of these defects during dental development, and the relationship between these defects and dental caries will be informative to the epidemiology of stress and oral pathology of the infants and children, and childhood diets. In establishing the prevalence of these defects, this may be interpreted in the wider context of disease from previous palaeopathological research and dietary information that is being published. Infectious diseases such as treponematosis and hookworm were likely significant contributors to ill health in prehistoric Pacific populations, and there is palaeopathological evidence for infectious and metabolic disease in a high proportion of infants and children from ‘Atele. A hypothesis can be proposed that infection and periodic undernutrition were causative agents in the development of the observed hypomineralisation.
Further research of alternative methods to identify hypomineralisation in bioarchaeology studies could include DIAGNOdent (DIAGNOdent, KaVo, Biberach, Germany), a small hand-held laser fluorescence device used in dentistry for diagnosing dental caries.