Dental age estimation: periodontal ligament visibility (PLV)—pattern recognition of a conclusive mandibular maturity marker related to the lower left third molar at the 18-year threshold

Recently, the DARLInG research group demonstrated that root pulp visibility (RPV) provides a strong indication that a subject at or near the 18-year threshold can be assigned to above the 18-year threshold with confidence (Lucas et al. 2016 submitted). This is an important threshold as increasing numbers of adult looking subjects seeking asylum in Europe and North America are claiming to be under 18 years. This has led to a number of publications aimed at using the development of the lower third molar as an indication of mature status. The most common age threshold is 18 years as this, in legal terms, is the defining age between child and adult status [1].

At the 18-year threshold, the third molar is the only tooth that has not completed development. The effective use of the lower third molar to determine whether or not a subject is over 18 years was first described by the American Board of Forensic Odontology (ABFO) [2]. This was also the first time that the eight-stage system of tooth development stages (A–H) devised by the Anglo-Canadian research team at the Institute of Child Health in London [3] was used for forensic age estimation. An important aspect of the ABFO study was the reporting of summary statistics including the mean (\( \overline{x} \)) and standard deviation (sd) which were then converted to probability values. Thus, it was possible, for the first time, to easily tabulate the probability that the client was at least 18 years old using, in particular, stage H which is the final stage of development. The application of the principle of probability estimates has since been applied at the age of criminal responsibility in the UK [4, 5]. This proved to be a highly reliable method of assigning a child to the correct position above or below the 10-year threshold. The usual approach when assigning a subject to below or above the 18-year threshold is to estimate the cumulative probability with the cutoff at 18 years of age. This is usually based on calculations using the normal distribution [2, 6]. However, it has been shown recently, using the gold standard of chronological age as a comparator, that in the age range 17.5 to 19.5 years, there is a high risk of an incorrect assignment [7].

This raises concerns that a large proportion of children will be treated as adults, and alternatively, a large proportion of adults will be treated as children. This is inappropriate and lends support to the view that when assigning a subject to below or above the 18 threshold, it is important to be sure that the assignment has a very high level of probability. It was concluded that “Completed root growth in wisdom teeth is observable at ages less than 18 years.” [8].

An important statement as in the UK the assignment for a subject to over (or under) 18 years is based on the balance of probability that is to say greater (or less than) 50 %. In the USA, this is known as the preponderance of the evidence [9].

Other human biological growth markers (HBGM) assist in this process. One of these is the relative visibility of the periodontal ligament as seen on dental panoramic tomographs (DPT) [10]. A four-stage system was devised [10] whereby the decrease in periodontal ligament visibility (PLV) with increasing age demonstrated the potential to show that a subject is below or above the 18-year threshold. The importance of this is that using the data derived in that paper [10], it might be possible to determine whether or not an individual subject is over 18 years old from a single DPT. It is important to note that this applies only when the lower third molar has completed growth i.e., stage H of the eight-stage system [3]. Thus, the use of PLV may assist in determining whether or not a subject without birth records is above or below the 18-year threshold.

This report is aimed at using the mandibular maturity marker of PLV to determine whether this specific HBGM is a reliable indicator that a subject is over 18 years old in a UK-Caucasian group of females and males.

The use of periodontal ligament visibility is a compelling indication of a subject’s status as a minor (under 18 years) or above the age of majority (over 18 years). This HBGM demonstrates that when making age assessments at the 18-year threshold, it is helpful to be sure as to the “below/above” status of the individual for whom age estimation is being conducted [8].

There are a number of issues relating to the methodology of the periodontal ligament visibility categories. The first publication implies a clear pattern of gradual disappearance of periodontal ligament visibility as shown in the schematic drawings in the first publication on this subject [10]. These drawings are reproduced here (Fig. 1). The clear pattern of PLV shown in the schematic diagrams was therefore used as a guide for the pattern recognition of the approximate percentage of the absence of periodontal ligament on the radiographs.

A shortcoming of the original paper [10] is that there was no information on reproducibility of the assessments for the PLV categories. This is a concern as reliable assessment is the key to the establishment of accurate data arrays for each of the categories. Notwithstanding this, the high Kappa scores for within observer agreement and between observer agreement in this study provide strong support for the use of the four categories described in the original paper [10].

An issue of further importance is whether or not the mandibular maturity markers can be applied to ancestral groups other than UK Caucasians, the work from Germany [10] and Portugal [15] support this. The data in this paper are similar to that from Germany and Portugal and lend support to the view that PLV assessments may be applied to other ancestral groups.

The value of the PLV-A, PLV-B, PLV-C, and PLV-D is shown in Figs. 2 and 3 where the data array for each category is overlaid on the probability distribution histogram. The critical feature is the point estimate for the minimum value for data array C and also D. In both females and males, the minimum value is higher than the 18-year threshold. This is strong evidence that a subject with complete root development of the lower third molar, stage H in the eight-stage system [3], combined with PLV-C or PLV-D, is above the 18-year threshold. This work confirms that for a definitive assignment of a subject to above the 18-year threshold, it is necessary to identify a mandibular maturity marker such as PLV-C or PLV-D. Only then can a dental surgeon be confident that the subject under investigation has passed and is beyond the 18-year threshold. This paper gives further support to the seminal work on periodontal ligament visibility published in 2010 [10].