Three-dimensional analysis of third molar development to estimate age of majority
Introduction
An important emerging challenge in recent years for forensic practitioners around the world is age diagnosis in living persons. The increasing numbers of undocumented individuals moving across international borders has led to growing pressure from legal systems to provide more accurate age estimates. In many countries the legal age limit is between 14 and 21 years, with age 18 years being the most frequent legal demarcation between child and adult status [1], [2], [3], [4].
Several methods have been recommended for forensic age diagnosis in criminal, civil and asylum proceedings to determine whether an individual has reached the age of majority when the date at birth is uncertain [1], [5]. These methods include physical examination, determination of anthropometric measures and evidence of sexual maturation stage, radiographic examination of the left hand, and dental examination including orthopantomography (OPG). If skeletal development of the hand is complete, additional X-ray examination of the clavicles should be used [6], [7], [8], [9]. All recommended procedures should be used in combination to increase diagnostic accuracy and improve the likelihood of identifying any age-relevant developmental disorders [5], [6], [10], [11].
In dental examinations, dental mineralization and eruption are the two main parameters used in forensic age estimation from birth to adulthood. Dental mineralization comprises a more uniform and continuous set of changes than eruption, and may be less influenced by external factors compared to other measurable criteria of maturity [5], [12]. In addition, dental mineralization is slower and delayed compared to skeletal bone [13], and correlates with different morphological stages of mineralization that can be determined by evaluating dental X-rays [14]. For these reasons, several methods have been proposed to define distinct dental mineralization stages [15], [16], [17]. However, age estimation becomes more complicated in individuals aged 14 years or older because most teeth are fully developed, and the only teeth still growing are the third molars [18]. Despite their highly variable patterns of formation (agenesis is frequent and the age of complete mineralization varies widely), third molars are one of the few sources of useful data to estimate biological age in young persons near the legal age of majority, in the absence of better dental indicators. This makes the study of third molars, in conjunction with skeletal maturity, an appropriate method for estimating the age of undocumented juveniles or asylum seekers [19]. Among the numerous methods developed to evaluate third molar maturation [20], [21], [22], [23], the scoring system of Demirjian et al. appears to be the most reliable in terms of both observer agreement and the correlation between stages [17], [18], [19], [24], [25], [26], [27], [28], [29], [30].
Nevertheless, the methods proposed thus far to assess third molar mineralization are usually based on the evaluation of an OPG, a two-dimensional panoramic radiograph of both maxillary arches [17], [18], [19], [24], [25], [26], [27], [28], [29], [30]. Therefore, when bone structures or teeth overlap, roots cannot be clearly visualized, and it is enormously difficult to discern differences between two consecutive maturational stages. Because of these difficulties, alternative imaging modalities such as computed tomography (CT) and magnetic resonance imaging (MRI) are making inroads into medicolegal age estimation [31], [32], [33], [34], [35], [36], [37], [38]. In connection with dental age estimation, it has been shown that CT scans cause no magnification errors due to geometric distortion, which is a common problem in conventional radiography. In addition, CT scans are able to provide accurate, measurable three-dimensional (3D) images of the third molar that can be rotated in space [39], [40], [41], [42], which facilitates assignment of the correct mineralization stage.
To achieve more accurate visualization and assessment of third molar mineralization from multi-slice helical CT (MSCT) images, we developed a new computer forensic analysis tool called DentaVol©. This tool makes it possible to isolate a 3D image of a third molar from adjacent and superimposed mineralized structures, in order to study qualitative and quantitative parameters of dental development such as maturational stage and third molar volume. Therefore, the aims of this study were (1) to determine third molar maturational stages according to Demirjian's schemes from MSCT images displayed by DentaVol© in a population sample of individuals of known chronological age, (2) to measure third molar volumes using DentaVol© in the same sample, and (3) to evaluate the accuracy of each procedure to assess third molar development in order to identify individuals 18 years of age or older.
Section snippets
Sample
The sample consisted of MSCT images of 230 maxillary third molars and 230 mandibular third molars from 135 individuals aged between 14 and 23 years (62 females, age range: 14.00–22.92, mean: 18.23; 73 males, age range: 14.00–22.83, mean 17.98), collected retrospectively from the database of the Diagnostic and Image Center of University Hospital Dr. Eleuterio González, Autonomous University of Nuevo Leon (AUNL) in Monterrey, Mexico. An additional sample of MSCT images from 30 individuals aged
Assessment of third molar mineralization stages
Demirjian's stages of tooth maturation were determined for all third molars. The kappa value for intra-observer agreement was 0.87 (95% CI: 0.78–0.97). For inter-observer agreement, the kappa value was high for all third molars, ranging from 0.84 (95% CI: 0.76–0.92) for tooth 38 to 0.90 for teeth 28 and 48 (95% CI: 0.84–0.96).
Age distribution was determined for different mineralization scores according to Demirjian's stages. Table 1 displays the percentage frequency distributions for age group
Discussion
Among methods for estimating the chronological age of individuals around the age of 18 years, analysis of third molar mineralization stage has been proposed as one of the most relevant procedures in the absence of other reliable biological markers during late adolescence [5], [9]. In most studies tooth mineralization is evaluated in an OPG, and dental age is estimated by comparing dental mineralization status in a person of known or unknown chronological age with dental developmental surveys and
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgments
Appreciation is expressed to the Diagnostic and Image Center of University Hospital Dr. Eleuterio González in Monterrey, Mexico, and Prof. Dr. Eloy Girela-López from the University of Cordoba, Spain, for help with obtaining MSCT images. We thank Dr. Francisco J. Rodríguez-Prados, Prof. Dr. Juan Carlos Torres-Cantero and Dr. Francisco Soler-Martínez for their technical expertise and scientific advice in the design and update of the DentaVol© software. We thank K. Shashok for improving the use of
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