Introduction

Due to an increase in cross-border migration, the importance of forensic age estimation in living adolescents and young adults, on behalf of courts and government authorities, has increased significantly over the past two decades. In many European countries, the legally relevant age levels are between the 14th and 21st year of age [1]. In addition, these medical age estimation procedures are also used to ensure equal opportunities for up-and-coming adolescent athletes in world-class sports, where the competing athletes may make questionable claims regarding their age [2].

If a legitimation for X-ray examinations without a medical indication is available, the international and multidisciplinary Study Group on Forensic Age Diagnostics recommends using the following combination: a physical examination plus an X-ray examination of the hand plus a dental examination, including an orthopantomogram (OPG). If the skeletal development of the hand is already complete, an additional X-ray or CT examination of the clavicles should be carried out [3]. If it is possible to perform a CT examination, then this should be given preference over projectional radiography [4].

In order to increase the validity of age estimation, when there is no legitimation for X-ray examinations, researchers have focused their interest on X-ray-free imaging procedures over recent years. Now, numerous sonography and MRI studies are available for a variety of skeletal areas [2, 5–16].

Given that the embryological development of teeth takes place largely independently of the maturation of the skeleton [17], it is to be expected that the additional possibility of assessing the mineralization of third molars using an X-ray-free imaging procedure would reduce the range of scatter of the overall age estimate.

The aim of the present study was, for the first time, to generate statistical data for a large study population regarding the mineralization stages of third molars according to Demirjian et al. [18] using MRI.

Materials and methods

In total, 613 German male and female volunteers aged 12 to 24 years were prospectively examined in the time period May 2013 to March 2015 in the Department for Clinical Radiology of the University Hospital of Münster, Germany. It can be assumed that the study participants did not significantly deviate from the average German population, as regard their socioeconomic status. Table 1 shows the case numbers, separated by sex, per 1-year age cohort. The study received a positive vote by the relevant ethics committee (reference number: 2013-062-f-S). After being duly informed, all study participants gave their written consent to take part in the study. For minors, the written consent of the parents was also obtained.

Table 1 Age and sex distribution of the sample
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All examinations were performed using a Philips 3.0 T Achieva (gradient amplitude 80 mT/m, Philips Medical Systems, Netherlands). Subject of the MRI examination was the third molar of the left lower quadrant (tooth 38). For study participants whose medical case history indicated that their left lower third molar had been extracted, the right lower quadrant third molar (tooth 48) region was examined. By applying the high-resolution surface coil SENSE-NV 16, MRI scans are performed utilizing a T2 turbo spin echo (TSE) sequence (TSE factor = 13; TR = 2800 ms; TE = 80; flip angle = 90; sense = 1.5; NSA = 6; scan duration = 5:36 min; measured voxel size = 0.50/0.65/2.00 mm; reconstructed voxel size = 0.19/0.19/2.00 mm). A View Forum workstation (Philips Medical Systems, Netherlands) with a diagnostic monitor was used to evaluate the MRI images.

To assess the mineralization stages of the third molars, the classification system according to Demirjian et al. [18] was used. All stage assessments were carried out by a dentist experienced in third molar mineralization assessments (Y. G). To determine the intra-observer agreement, in 60 cases, the stage assessment was repeated after 4 weeks. To determine the inter-observer agreement, these same 60 cases were also assessed by another dentist experienced in third molar mineralization assessments (A. O.).

Statistical analyses were performed using IBM SPSS Statistics 22 (Version 22.0.0.0). Results were expressed as minimum, maximum, mean ± standard deviation, and median with lower and upper quartiles. Differences between the sexes were analyzed using the Mann-Whitney U test to determine their statistical relevance (p < 0.5, exact, two-sided). The kappa coefficients were calculated to determine intra- and inter-observer agreement.

Results

Of the 613 examined persons, 83 (13.5 %) had no lower third molar, on either side. In 13 cases (2.1 %), the mineralization stage could not be reliably assessed, due to insufficient image quality. As a result, the mineralization stage of the lower third molars could be assessed by means of MRI in 517 study participants (269 males, 248 females).

With the exception of stage A, all mineralization stages were present in our study population. Table 2 shows the measures of location and dispersion relating to the statistical description of the assessed mineralization stages. The measures demonstrate an increase in the mean values and medians of the chronological age of the test persons with an increase in the mineralization stages of the third molars.

Table 2 Statistical parameters for mineralization stages B-H
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Figures 1, 2, 3, 4, 5, 6, and 7 show examples of typical MRI results for the third molar mineralization stages B-H.

Fig. 1
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Stage B of tooth 38 (circle), 13.04-year-old male

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Fig. 2
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Stage C of tooth 38 (circle), 12.48-year-old female

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Fig. 3
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Stage D of tooth 38 (circle), 19.46-year-old female

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Fig. 4
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Stage E of tooth 38 (circle), 19.88-year-old male

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Fig. 5
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Stage F of tooth 38 (circle), 21.47-year-old females

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Fig. 6
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Stage G of tooth 38 (circle), 17.16-year-old male

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Fig. 7
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Stage H of tooth 38 (circle), 20.85-year-old female

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Mann-Whitney U test results demonstrated that mandibular third molars at stages C, E, F, and G showed significantly lower mean ages in males than in females.

The kappa coefficient was 0.89 for intra-observer agreement and 0.83 for inter-observer agreement.

Discussion

A range of different stage classification systems was proposed for assessing the mineralization of third molars [18–27]. In a comparative study, Olze et al. [28] demonstrated that the Demirjian stages provide the best results as regard estimate accuracy and reproducibility. For these reasons, the classification system of Demirjian et al. [18] was used in the present study.

While numerous OPG studies regarding the process of third molar mineralization over time have been published [29–33], to the authors’ knowledge, only two studies have been published to date regarding MRI-based assessments of third molar mineralization. In a feasibility study, Ottow et al. [34] demonstrated that the assessment of third molar mineralization in living persons is fundamentally possible. As part of a pilot study of 29 persons aged 13–26 years, for whom a medically indicated OPG had been carried out, Baumann et al. [5] performed subsequent MRI examinations of the molars, with only a brief time interval between the two examinations. For all molars, the mineralization stage according to Demirjian et al. [18] was assessed. A good correlation of the mineralization stages determined via orthopantomography and MRI was observed. However, the MRI method tended to yield slightly lower stages than the OPG method.

As no statistically significant differences have been observed, in studies carried out to date, between the left and right side of the mandible as regard the third molar mineralization process over time [35–39], it seemed justifiable to the authors to assess the mineralization stage of the third molar of the right lower jaw in cases where no third molar was present in the left lower jaw.

The present study examined 613 persons aged 12–24 years. In 83 of the examined persons (13.5 %), no lower third molar was present, on either side of the mandible. In 13 cases (2.1 %), the mineralization stage could not be reliably assessed, due to insufficient image quality. Therefore, the procedure used here seems suitable for routine use, for age estimation in practice.

Tables 3 and 4 show means and standard deviations for mineralization stages of tooth 38 in the populations investigated by Kahl and Schwarze [40] and Olze et al. [41] juxtaposed with figures from the present study for the lower third molars. It can be seen that the mean chronological age in the study by Kahl and Schwarze [40] is at almost all stages significantly lower than in the present study. By contrast, the mean chronological age in the study by Olze et al. [41] is for almost all stages higher than in the present study. One possible reason for these differences is the different age groups and the differing age distributions in the studies cited. Kahl and Schwarze [40] investigated people aged from 5 to 24, whilst Olze et al. [41] included people aged from 12 to 26 in their study. In addition, boys and girls at the younger end of the age range were overrepresented in the study by Kahl and Schwarze [40], whilst the study by Olze et al. [41] included significantly more subjects at the upper end of the age range than at the lower end. Gelbrich et al. [42] have shown that the age range and age distribution within a sample have a significant effect on mean chronological age values for mineralization stages.

Table 3 Means and standard deviations (in years) in males for mineralization stages of lower third molars (present study) and tooth 38 (Kahl and Schwarze [40] and Olze et al. [41])
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Table 4 Means and standard deviations (in years) in females for mineralization stages of lower third molars (present study) and tooth 38 (Kahl and Schwarze [40] and Olze et al. [41])
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It should also be noted that staging can also be affected by the imaging procedures used. For example, for clavicular ossification, it has been demonstrated repeatedly that by using different imaging methods for the same clavicle, different ossification stages were assessed [43–45]. Baumann et al. [5] also observed method-dependent differences as regard the mineralization stages in some of the examined molars.

In agreement with a number of other studies [39, 46, 47], the present study found a faster developmental rate for males in several mineralization stages, when a sex comparison of the results was carried out. By contrast, for developmental processes which take place at a younger age, boys show a slower developmental rate compared to girls [48–50].

Because people whose age is to be estimated are frequently from countries where no forensically useable reference studies are available, the question arises as to whether ethnicity exerts an effect on the chronological progression of third molar mineralization. In a comparative study on the chronology of third molar mineralization, Olze et al. [51] found that development in Black Africans was accelerated compared to Europeans, whilst development in Asians shows a relative retardation. The effect of ethnicity on the chronological progression of third molar mineralization must therefore be taken into account in age estimation practice.

A number of OPG studies have shown that third molar mineralization in Europeans of both sexes can be completed before reaching the age of 18 [52–54]. By contrast, the youngest female study participant with a mineralization stage of H in our study population was aged 19.57 years. This result should be verified in further studies.

In conclusion, it can be stated that magnetic resonance imaging is an X-ray-free alternative to orthopantomography in the assessment of third molar mineralization.