Forensic age estimation based on development of third molars: a staging technique for magnetic resonance imaging

Abstract

Background: The development of third molars can be evaluated with medical imaging to estimate age in subadults. The appearance of third molars on magnetic resonance imaging (MRI) differs greatly from that on radiographs. Therefore a specific staging technique is necessary to classify third molar development on MRI and to apply it for age estimation.

Aim: To develop a specific staging technique to register third molar development on MRI and to evaluate its performance for age estimation in subadults.

Materials and methods: Using 3T MRI in three planes, all third molars were evaluated in 309 healthy Caucasian participants from 14 to 26 years old. According to the appearance of the developing third molars on MRI, descriptive criteria and schematic representations were established to define a specific staging technique. Two observers, with different levels of experience, staged all third molars independently with the developed technique. Intra- and inter-observer agreement were calculated. The data were imported in a Bayesian model for age estimation as described by Fieuws et al. (2016). This approach adequately handles correlation between age indicators and missing age indicators. It was used to calculate a point estimate and a prediction interval of the estimated age. Observed age minus predicted age was calculated, reflecting the error of the estimate.

Results: One-hundred and sixty-six third molars were agenetic. Five percent (51/1096) of upper third molars and 7% (70/1044) of lower third molars were not assessable. Kappa for inter-observer agreement ranged from 0.76 to 0.80. For intra-observer agreement kappa ranged from 0.80 to 0.89. However, two stage differences between observers or between staging sessions occurred in up to 2.2% (20/899) of assessments, probably due to a learning effect. Using the Bayesian model for age estimation, a mean absolute error of 2.0 years in females and 1.7 years in males was obtained. Root mean squared error equalled 2.38 years and 2.06 years respectively. The performance to discern minors from adults was better for males than for females, with specificities of 96% and 73% respectively.

Conclusion: Age estimations based on the proposed staging method for third molars on MRI showed comparable reproducibility and performance as the established methods based on radiographs.

Figure 1

Yellow boxes depict the stack of scanned MRI slices. (A) Coronal MRI showing all four third molars enclosed within the boxes of sagittal slices. (B) Coronal MRI showing all four third molars enclosed within the box of axial slices. (C) Sagittal MRI showing the right third molars enclosed within the box of coronal slices.

Figure 2

Definition of MR crown height and MR root length on MRI. Lines are perpendicular to the tooth axis. Distances (arrows) are evaluated along the tooth axis. Line D is at the distal cusp tip, while M is at the mesial cusp tip. Line C represents the midpoint between the distal and mesial cusp tips. Line H is at the pulp horns, which are both at the same level. Line A is at the most apical point of the roots, which are all at the same line. The distance between lines C and H is the MR crown height. The distance between lines H and A is the MR root length. In this case MR root length is more than one and a half MR crown height, so the tooth is in stage 5.

Figure 3

(A, B) Lower right mandibular third molar depicted on two consecutive MRI slices. Slice (A) is situated more buccally than slice (B). The pulp chamber has a trapezoidal shape, corresponding to stage 3. To exclude stage 4, MR crown height and MR root length have to be evaluated as illustrated in images (C, D). (C) Copy of image (A), with marked landmarks and distances to consider in order to allocate a stage. Lines are perpendicular to the tooth axis. Distances (arrows) are evaluated along the tooth axis. Line Dc is at the distal cusp tip, while Mc is at the mesial cusp tip. Line C represents the midpoint between the distal and mesial cusp tips. Line Dh is at the distal pulp horn while line Mh is at the mesial pulp horn. Line H represents the midpoint between the distal and mesial pulp horns. Line Da is at the most apical point of the distal root. Line Ma is at the most apical point of the mesial root. The distance between lines C and H is the MR crown height based on this slice. The distance between lines Dh and Da is the distal MR root length, while the distance between lines Mh and Ma is the mesial MR root length. (D) Copy of image (B). Both cusp tips are at the same level on this slice, represented by line C. MR crown height is larger than on the previous image, whereas the distal MR root length is smaller. To allocate a stage, MR crown height on image (D) is the most appropriate, while the distal MR root length on image (C) is the most appropriate. Because the third molar is tilted bucco-lingually, the observer has to scroll through consecutive slices to decide on the most appropriate measures to consider. In slice (C) the crown is transsected more buccally, so part of the crown is not depicted. By contrast, the distal root apex is situated more buccally than the crown, so it is best depicted in slice (D). Because the distal root is shorter than MR crown height, this third molar is in stage 3.

Figure 4

Lower right mandibular third molar depicted on consecutive MRI slices from buccally to lingually. MR crown height is most appropriately measured on slice D. When MR root length would only be based on slice C, stage 3 would be allocated. In fact the tooth is in stage 4, since MR root length on slice B is slightly longer than MR crown height and the root is depicted over several slices (keeping in mind that slice thickness is 2 mm).

Figure 5

Schematic representation of developmental stages of third molars on MRI. Mineralized tissues appear black on MRI. By contrast, the dental follicle, pulpal tissues, the periodontal space and saliva appear white. The upper panels show stages for multiradicular molars, while in the lower panels stages for monoradicular molars are shown (this also corresponds with the appearance of the palatal root in upper molars).

Figure 6

Representative examples of third molars in developmental stages 0 to 3, in the upper (A-E) and lower jaw (F-J). For some stages different appearances are illustrated. (A) Stage 0. The crypt of the third molar shows no calcification. It is seen as a clearly delineated white area. (F) Stage 1. Cusp tips are seen as separate black areas within the crypt. (B, G) Early stage 2. Cusps are fused. The roof of the pulp chamber is quite flat. (C, H) Late stage 2. The roof of the pulp chamber is more curved than in (B, G). Notice that in (H), the mesial side of the pulp chamber is more mature than the distal side. Thus, for staging the distal side should be considered. The distinction between early and late stage 2 was considered too subjective to consider them as separate stages. (D, I) Stage 3. Notice the pointy appearance of the pulp horns. No furcation was present. (E, J) Stage 3. Notice the furcation. In (J) the distal pulp horn appears curved on this sagittal slice. However, scrolling through the slices and including the coronal slices in the assessment, it was clear that both pulp horns were pointy, like an umbrella top.

Figure 7

Representative examples of third molars in developmental stages 4 and 5, in the upper (A-C) and lower jaw (D-F). In (A-C) palatal roots are depicted. For stage 5 different appearances are illustrated. (A, D) Stage 4. Notice that the distal root in (D) is less developed than the mesial root. (B, E) Early stage 5. Root walls are clearly funnel shaped at the root apex. (C, F) Late stage 5. The funnel shape of the root walls at the apex is more subtle than in (B, E). The distinction between early and late stage 5 was considered too subjective to consider them as separate stages. Moreover, variability in root length would hinder a subclassification of stage 5.

Figure 8

Representative examples of third molars in developmental stages 6 to 8, in the upper (A-D) and lower jaw (E-H). In (A-D) palatal roots are depicted. For stage 8 different appearances are illustrated. (A, E) Stage 6. The width of the root canal differs depending on tooth anatomy. Still, parallel root walls are clear. Notice that the thin dentin at the apex in (A) might give the impression of a small funnel shape. However, it is stated in the criteria for stage 5 that the funnel shape should be more extensive than is seen in this example. Therefore, stage 6 is appropriate. (B, F) Stage 7. The apices have clearly started closing. In (F) remnants of the dental follicle can be seen as white areas surrounding root apices. (C, G) Stage 8. The apical dentin is relatively thin, but clearly continuous. (D, H) Stage 8. Not only is the apical dentin continuous, but in these examples it is also relatively thick.

Figure 9

Influence of agenetic third molars on the posterior density. The posterior density becomes smaller as a function of increasing information (plot obtained from males).

Figure 10

Visualisation of the ad-hoc procedure to construct an approximate confidence interval without the need to model the multivariate correlation structure between the indicators. The black continuous curve refers to the posterior density assuming conditional independence. The blue dashed curve presents the density obtained after application of the ad-hoc procedure (Boldsen et al. 2002). At the top right are the stages for each third molar with their respective point prediction of age based on the maximum likelihood estimate, the 95% prediction interval and the posterior probability to be a minor. At the top left are the combined results, first without correction and second after correction applying the ad-hoc procedure. Note that without correction the prediction interval is too narrow.

Figure 11

Graphs comparing chronological age (dots) with the point (triangle) and interval (line) prediction in females (A) and males (B). The point prediction is the ML estimate.

Figure 12

Posterior density for all possible homogenous stage patterns (same stage for all third molars) in males. When all third molars are in stages equal to or lower than three (3333), the distribution of age is right-skewed. This smoothly evolves to a left-skewed age distribution when all third molars are fully mature (8888). Around the age of 18 years, most individuals have third molars in stage 5. Per situation the probability to be adult is represented by the area under the posterior density curve to the right of the 18 years threshold (blue vertical line).