The present study confirmed that the novel classification system by Vieth et al. [18] can also be applied to MRI data from routine diagnostics using both 1.5 T and 3.0 T scanners as well as other T2 sequences than the T2-TSE SPIR sequence used in the original study. However, it has to be emphasized that T2 sequences with fat saturation are necessary in order to determine Vieth stages. Our observations are in line with two other recent studies that also retrospectively applied the classification system by Vieth et al. [18] to MRI of the knee joint in two Turkish study populations [20, 21]. As also stated by Saint-Martin et al. (2015) [22] as well as by Gurses and Altinsoy (2020) [20], we likewise obtained similar results with MR image material from 1.5 T and 3.0 T scanners, and we were able to identify all relevant structures required for stage determinations despite the fact that T2-TSE SPIR sequences were not available.
Table 4 shows a comparison of the so far existing studies investigating MRI of the PTE using the classification system by Vieth et al. [18]. The data of the present study corroborated that, at least in male individuals, MRI of the PTE is suitable to determine majority when using the classification system by Vieth et al. [18]. Although we observed minimum ages for stage 6 above the age of 18 years in both sexes (20.27 years in males and 18.55 years in females), we are aware that, especially with regard to forensic age estimation practice, both the original study by Vieth et al. [18] and the two above-mentioned Turkish studies [20, 21] have already found minimum ages for stage 6 in females below the age of 18 years. Thus, our results are not in contradiction with the previous data. However, our data can only be considered a strong and valuable support of the more comprehensive and prospectively collected age data presented by Vieth et al. [18].
Table 4 Comparison of the studies investigating MRI of the PTE using the classification system by Vieth et al. [18] This result reveals the most relevant limitation of our study: the retrospective study design. Despite the acquisition of retrospective knee MRI data from two large tertiary care hospitals of a 10-year period, we were able to include 413 cases only, which is considerably lower than the 694 cases of the original study by Vieth et al. [18]. The lower case number may therefore explain not only the lack of female individuals below the age of 18 with a PTE showing stage 6, but also the comparably higher minimum age in males concerning stage 6.
Besides the limitations, the present study has also some noteworthy strengths. Unlike the other two subsequent studies from Turkey [20, 21], which both stated a lack of information on the socio-economic status of their study patients, a high socio-economic status of the present Central German study cohort can be assumed because the vast majority of the subjects can be considered part of a Western Caucasian population. This is of great relevance because, as also stated by Vieth et al. [18], it is well known from earlier studies [24, 25] that the Western Caucasian ethnicity with high socio-economic status displays the fastest progression of skeletal maturation. Hence, the application of the minimum ages of a reference study investigating a population with high socio-economic status, which are the most relevant when using the so-called “minimum age concept” (e.g., in criminal proceedings) [2], will rather lead to an underestimation of age when applied to other ethnicities, particularly to those with lower socio-economic status.
Another strength is the relatively even distribution of our study subjects across age groups and sexes, which was not the case in the two Turkish studies [20, 21]. However, both an even age distribution and data on the reference population (regarding genetic/geographic origin) and socio-economic status, respectively, belong to the AGFAD criteria required for reference studies in forensic age estimation [3].
MRI of the PTE has already been investigated for the purpose of forensic age diagnostics by several other authors with some other staging systems. As early as in 2010, Jopp et al. [26] studied a small pilot sample of 41 young males between 15 and 19 years using 1.5 and 3.0 T MRI and a 3-category system based on the epiphyseal-diaphyseal osseous fusion of the PTE. In 2012, Dedouit et al. [10] used 290 MRI scans of patients aged between 10 and 30 years and introduced a 5-stage system for evaluating MRI of both knee epiphyses based on the absolute measureable thickness of growth plate layers. However, although the presence of a stage 5 is possibly able to determine age of majority [10, 27], the general approach of the staging system has been criticized as the physical dimensions of anatomical structures are associated with the individual’s body height and might therefore be unsuited for absolute measurements [18]. Later studies applied the classical staging system composed of 5 main stages according to Schmeling et al. [28] and 6 sub-stages according to Kellinghaus et al. [29] to T1-weighted (closest-to-bone) MRI sequences of both knee epiphyses [16, 17]. However, this approach finally did not facilitate the determination of majority in either sex. Concerning this matter, Vieth et al. [18] pointed out that the Schmeling/Kellinghaus system does not take into account the watery components and soft tissues of the osseous structures.
Hence, the staging system by Vieth et al. [18] aimed at considering more aspects of the morphological appearance of the epiphyseal-diaphyseal fusion. Although, as also stated by the authors themselves [18], the relevant landmarks of each stage are mainly drawn from the T1-weighted sequence, stage 5 and 6, which are crucial for the question of majority, have exactly the same appearance in T1 and absolutely require information from the T2-weighted sequence. Recognizing the faint hyperintense signal, defining the difference between stage 5 and 6, also represented a very small, but the most striking difficulty in our study. Possibly, this may be due to its relative novelty. Moreover, it still remains an interesting question what is the origin of this hyperintense signal. It is known, that T2 changes within the bone are much more sensitive to remodeling processes than T1 signal. For example, edema can be identified much longer in the event of trauma or inflammation. This could also apply to modeling processes in the epiphyseal areas.
In the present study, cases with different ossification stages in T1 and T2 only occurred very rarely. In order to merge the information of T1 and T2 into a single ossification stage, the rules previously developed for the combination of multiple CT slices of the medial clavicular epiphysis [6, 30] were modified as follows: stage 2 (in one sequence) + stage 3 (in the other sequence)→stage 3, stage 3 + stage 4→stage 4, stage 4 + stage 5→ stage 4, stages 5 and 6 can only be distinguished by T2 [18].
Although the staging system by Vieth et al. [18] was originally developed for both knee epiphyses, it may have potential for other growth plates as well. Recently, it has already been applied to the proximal humeral epiphysis [31]. The authors investigated a study population of 315 individuals and found minimum ages in either sex above the age of 18 years for both stage 5 (19.32 in males, 19.20 in females) and stage 6 (22.21 in males, 22.19 in males). Future studies will show whether the approach is able to prevail.