Comparing tooth development timing between ethnic groups, excluding nutritional and environmental influences

This study compared the timing of dental development in Finns and Somalis, by examining the age of attainment of Demirjian stages [3] in seven left mandibular teeth. Staging was performed by the same examiners on DPRs from individuals that were all born and living in Finland, thereby ruling out variability between examiners and minimizing sampling and environmental bias. The current results revealed that none of the seven teeth analysed showed in all the ages of transition between adjacent stages an earlier development in Somalis nor in Finns. Within each tooth the lowest (or highest) mean ages of attainment between the two ethnic groups varied without any pattern. The magnitude of all observed differences in mean ages of attainment between these ethnic groups was minimal with, as an obvious consequence, a low impact on clinical and forensic age assessment practice. Moreover, within each tooth none of the transitions to an adjacent stage was consequently quicker for one of the two ethnic groups. This allows us to conclude that both ethnic groups have a similar timing in the development of the lower left permanent teeth.

The current results are largely in agreement with a number of studies stating only minor differences in the timing of tooth development between different ethnic groups [8, 10, 12, 19, 27]. Yet, there are studies in disagreement with the present findings, suggesting that children of African ancestry are significantly more advanced in dental development than populations of European ancestry [9, 20, 32]. There are two concerns to be made in evaluating these results. First, a fair comparison between the current study results and the results of the studies mentioned is not possible because various methodologies were used to establish the comparison(s) between ethnic groups. In fact, Liversidge et al. [12] and Phillips & van Wyk Kotze [8] compared allocated developmental stages. In that method the degree of development within a stage was not taken into account (E.g., an individual in the beginning of an allocated stage has a different timing of dental development than an individual at the end of the same stage). Uys et al. [9] compared mean ages in a stage. By analogy with the previous two studies, in this method, the degree of development within a stage is not taken into account and the fact that each stage covers an unequal age period is not considered. Davidson & Rodd [20], Esan & Schepartz [32], Metsäniitty et al. [27], Willems et al. [10] compared differences in mean estimated age and chronological age. In fact, a derivative of tooth development timing is compared, namely a parameter for the performance of the used age estimation method. The considered parameter is based on allocated developmental stages. It does not take into account the aforementioned degree of development within a stage, and, in addition, parameter variability is caused by the used age estimation method and the methods applied to establish the parameter. Angelakopoulos et al. [19] validated the reproducibility of the Cameriere’s maturity index [33] based on the Cameriere age estimation method [34]. The results confirmed that Cameriere’s maturity index is reproducible in the compared groups, but new population-specific models provide superior age estimation accuracy. Second, in all the studies discussed, the timing of development was not directly considered, but derived or related parameters were evaluated. In the current study, the available developmental moments at which the average transition from one stage of development to the next stage took place, were compared. As a result, several specific moments in the timing of tooth development were compared for each tooth and separately for M and F.

As a prerequisite for a reliable comparison, data on the exact dates of birth and chronological age at DPR exposure of all studied individuals were available. In fact, in Finland, hospital provides the necessary details of all births to the Population Information System, where the birth of a child is registered. The parents must report the child’s name and native language to the Digital and Population Data Services Agency (DVV). In cases when the child is born at home or in a place other than a healthcare unit, the assisting health care professional must notify DVV. If a child is born without the assistance of a healthcare professional, the mother must notify a healthcare unit or healthcare professional of the birth, and these instances then notify the DVV. The birth of the second and further generation(s) of Somalis, as well as of the Finns included in this study, were recorded by this governmental agency. The data of DPR exposure were recorded in the dental patient files.

A key question when establishing and comparing normative tooth development charts for distinct ethnicities is to what extent eventual differences in development reflect true diversity in dental development or replicate environmental factors. Notably, (deficient) nutritional status was reported not to have an effect on the rate of tooth development [14, 16], but conversely, advanced dental development was reported in children with high-calorie intake [18] and in overweight or obese children [15, 17]. In the present study, a highly similar nutritional and environmental status was secured, as far as possible, by local sampling of Finns and Somalis, not only living but also born in Finland. This means that both ethnic groups had similar access to equal nutrition, health care and living conditions from birth until radiographic exposure. Nevertheless, it cannot be ruled out that in current times both groups have access to their original diet. Although climatological circumstances were similar, the Somali group may have suffered from deficiency of vitamin D due to the long, dark winter periods in Finland as dark-skinned individuals [35]. While this could impair the formation of dental enamel, it would presumably not affect the rate of dental development [36, 37].

In studies comparing tooth development between ethnic groups, considerably little attention has been paid to a uniform retrospective radiographic sampling, particularly the clinical indications for the sampled exposures. DPRs are not taken routinely in any age group, and in terms of radiation protection, certainly not in children [38]. Typical indications for taking DPRs in young individuals are associated with deviations from normal dental development, such as delayed eruption due to various reasons, planning of orthodontic treatment, planning of third-molar extraction, or issues with dental health. Differences in all these indications may exist between ethnic groups, thus reflected in retrospective DPR collections. Factors that may both affect timing of permanent tooth development and be associated with the likelihood of undergoing DPR exposure include deep caries lesions and extractions of primary teeth [39, 40], certain malocclusion types [41, 42], and hypodontia [43‐46]. Comparative studies are therefore best performed in locations that offer equal accessibility to preventive and restorative dental care and orthodontic treatment in all social and ethnic groups. In the present study, identical clinical indications have most likely justified the radiographic exposures in both ethnic groups within the community-based Finnish health care, free of charge for all children.

A disadvantage of the present study is the relatively small Somali sample size, especially at both ends of the sampled age range. As a consequence, certain developmental stages had to be grouped, implying that for particular teeth distinctive stages could not be evaluated and compared. One factor to always consider, is a possible secular trend affecting the dental development [23]. Most of the DPRs of both Finns and Somalis were from the time-period from 2002 to 2016. The oldest DPRs, selected to cover younger age groups of ethnic Finns, were largely left out from the final comparative analysis, minimizing the effect of possible secular trends.

In the future, a comparative study of the current two ethnic samples excluding environmental and nutritional influences on third molar development could enhance the knowledge on age estimation outcomes in particular for the forensically important age period around 18 years.