When courts must decide whether a young person without valid documents is a minor or an adult, forensic experts turn to the skeleton for answers. The growth plates, or epiphyses, of long bones close in a predictable sequence during adolescence, and the state of that closure offers a window into chronological age. Now a team of German researchers has taken a closer look at one specific stage of knee development, asking whether a coarse category long used in magnetic resonance imaging could be split into finer steps to yield more precise legal answers. Their pilot study, published in the International Journal of Legal Medicine, shows that the finer classification is technically feasible, but it also reveals sobering limits on what this particular bone region can tell investigators about the most important legal age thresholds.
The work centers on the proximal tibial epiphysis, the growth plate at the upper end of the shinbone just below the knee joint. In 2018, Vieth and colleagues introduced an MRI-based staging system for this region that has since been validated by several independent research groups. The system’s most celebrated feature is its stage 6, which in male individuals was observed only after the eighteenth birthday, making it a potential marker of legal adulthood. Because magnetic resonance imaging involves no ionizing radiation, unlike computed tomography of the clavicles that is currently recommended when hand skeletal development is complete, MRI of the knee has been promoted as a more ethically acceptable tool for age estimation in living subjects.
Yet the earlier validation work by the same German group hinted at something else: stage 4, a mid-range category in which the growth plate is partially fused, might contain hidden structure. If the stage could be subdivided, perhaps the substages would align more closely with legally relevant ages such as fourteen, sixteen, or eighteen years. The new pilot study, led by Natia Chitavishvili of Jena University Hospital together with Daniel Wittschieber, Maria L. Hahnemann, and colleagues, set out to test that idea using a cohort of fifty-seven T1-weighted MRI examinations, twenty-four from males and thirty-three from females, all previously assigned to Vieth stage 4.
The technical logic behind the subclassification rests on a thin line of intermediate signal intensity that appears within the growth plate on T1-weighted turbo spin-echo images. This line represents the advancing front of bony fusion across the metaphysis. The researchers divided its extent relative to the total metaphyseal width into three categories: substage 4a when the line spans less than one-third of the width, substage 4b when it spans between one-third and two-thirds, and substage 4c when it exceeds two-thirds. Three experienced readers, a clinical radiologist, a pediatric radiologist, and a forensic medicine specialist, assessed only coronal images and evaluated every slice of the epiphysis, blinded to each subject’s age and sex throughout.
The results confirmed that every one of the fifty-seven cases could be placed into one of the three substages, demonstrating that the morphological criteria are workable in practice. Reliability statistics supported the scheme as well. Intra-observer agreement, measured when the first reader repeated all assessments two months after the initial consensus reading, yielded a Cohen’s kappa of 0.751, and inter-observer agreement between the first two readers reached 0.663. Under the conventional interpretation scale, both values fall within the range of good agreement, suggesting that different trained readers can apply the substages consistently.
The age data, however, tell a more complicated story. Among male subjects, the minimum ages at which substages 4a, 4b, and 4c were first observed were 15.45, 15.68, and 15.23 years respectively, meaning all three substages appeared only after the fourteenth birthday. Among females, the corresponding minimum ages were 12.64, 12.57, and 13.18 years. Statistically significant sex differences emerged for substages 4b and 4c, consistent with the well-documented pattern in which girls reach skeletal milestones roughly two years earlier than boys. In both sexes, substage 4c showed a particularly wide spread of chronological ages, weakening its value as a narrow age indicator.
What does this mean for the courtroom? The authors conclude that the substages may assist in verifying completion of the fourteenth year of life, at least in males, since no male in the cohort displayed any substage before age 15. But the crucial thresholds of sixteen and eighteen years, which drive decisions about criminal responsibility, asylum, and majority in many jurisdictions, could not be reliably verified with this subclassification. The broad age distributions, especially within substage 4c, mean that an individual at that substage could plausibly be anywhere within a multi-year window that straddles the legal boundaries that matter most.
The findings fit into a broader research landscape. Krämer and colleagues demonstrated as early as 2014 that substage assessment of the proximal tibial epiphysis by MRI was feasible, using an adapted system derived from the clavicle staging schemes of Schmeling and Kellinghaus, and reported minimum ages in comparable ranges. Ottow and colleagues expanded on that work in 2017 with 658 knee MRI scans from individuals aged twelve to twenty-four. More recently, Ekizoglu and colleagues introduced novel substages based on the Dedouit classification using MERGE sequences in 559 individuals, reporting minimum ages that likewise fell within similar age bands. Notably, the Dedouit approach has been criticized because it relies on absolute measurements of growth plate layer thickness, which may vary with body size and thereby introduce bias, whereas the Vieth system and its new substages rest on proportional, morphological criteria.
The authors are candid about the limitations of their pilot. With only fifty-seven cases, the numbers assigned to each substage were small, and substage 4a appeared in just five individuals, which limits the statistical power of the analysis. The retrospective design meant that scans came from different scanners at 1.5 and 3.0 Tesla with non-uniform clinical protocols, a heterogeneity that mirrors real-world practice but complicates precise morphological differentiation. The cohort, drawn from hospital records in Thuringia, was also predominantly Western Caucasian with a largely high socioeconomic background, raising questions about generalizability to the diverse populations that forensic age estimation actually serves. The researchers emphasize that the subclassification cannot currently be recommended for routine forensic practice without validation in larger, independent study populations.
Even so, the study adds a meaningful piece to the puzzle of skeletal maturation. It demonstrates that the intermediate signal line within the fusing tibial growth plate progresses in a graded, readable fashion, and it maps that progression onto chronological age with sex-specific reference values. Future prospective studies with standardized MRI protocols and larger multicenter cohorts may clarify whether the substages gain diagnostic value, particularly around the fourteenth-year threshold in males. For now, the message for forensic science is one of tempered expectation: the knee can speak about age, but its vocabulary at stage 4 remains too coarse to settle the sixteen- and eighteen-year questions that matter most in law.
Subject of Research: MRI-based subclassification of proximal tibial epiphysis ossification for forensic age estimation
Article Title: Subclassification of Vieth stage 4 in MRI of the proximal tibial epiphysis: a pilot study for forensic age estimation
Article References: Chitavishvili, N., Wittschieber, D., Papageorgiou, I., Malich, A., Mentzel, H.-J., & Hahnemann, M. L. (2026). Subclassification of Vieth stage 4 in MRI of the proximal tibial epiphysis: a pilot study for forensic age estimation. International Journal of Legal Medicine. https://doi.org/10.1007/s00414-026-03991-y
Image Credits: AI Generated
DOI: 10.1007/s00414-026-03991-y
Keywords: forensic age estimation, MRI, proximal tibial epiphysis, Vieth staging, growth plate, ossification, knee joint, forensic radiology, radiation-free imaging, skeletal maturation, legal age thresholds, pilot study
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Ophelia Keating. (September 25, 2026). MRI Study Splits Bone Growth Stage Into Three Substages to Sharpen Forensic Age Tests. Scienmag. https://scienmag.com/mri-study-splits-bone-growth-stage-into-three-substages-to-sharpen-forensic-age-tests/
Ophelia Keating. “MRI Study Splits Bone Growth Stage Into Three Substages to Sharpen Forensic Age Tests.” Scienmag, 25 September 2026, https://scienmag.com/mri-study-splits-bone-growth-stage-into-three-substages-to-sharpen-forensic-age-tests/. Accessed 25 September 2026.
Ophelia Keating. “MRI Study Splits Bone Growth Stage Into Three Substages to Sharpen Forensic Age Tests.” Scienmag. September 25, 2026. https://scienmag.com/mri-study-splits-bone-growth-stage-into-three-substages-to-sharpen-forensic-age-tests/
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Tags: adolescent skeletal maturity assessmentchallenges in legal age verificationepiphyseal closure in long bonesfine-grained staging of bone growthforensic age estimationforensic radiologyforensic radiology techniquesgrowth plateknee jointknee joint development in forensic sciencelegal age determination methodslegal age thresholdslimitations of MRI in age testingMRIMRI classification of growth stagesMRI-based bone growth stagingossificationpilot studyproximal tibial epiphysisproximal tibial epiphysis developmentradiation-free imagingskeletal maturationVieth staging
