Pierre-Hadrien Decaup

Objectives: The modern human mandibular symphysis displays a mental prominence, a unique trait in the human lineage. Beyond this shared pattern, considerable variability in shape and metrics exists. This heterogeneity is thought to result from multiple mechanisms, including biomechanical compensation, allometric and dietary effects, population genetics, or sexual dimorphism. While the influence of these processes on overall mandibular shape is thoroughly documented, limited information exists on their effects on the distribution of mandibular cortical bone. Thus, this study assessed the impact of local and global allometry, sexual dimorphism, and cortical area on the shape and size of the symphyseal cortical bone contours. Methods: We analyzed computed tomography images of 62 modern human mandibles using Elliptic Fourier Descriptors. The effects of the investigated variables on external and internal cortical bone shapes and sizes were tested using a nested multivariate approach and predicted shape reconstructions were used to visualize morphological variation. Results: In this sample, sex, and local allometry had significant effects on mandibular symphyseal shape and sex was the main predictor of local allometry. Cortical area significantly affected the shape and size of the external cortical bone contour and body weight influenced the size of the internal cortical bone contour. Conclusion: Sexual dimorphism, allometric effects, and biomechanical compensation impact the shape and size of the mandibular symphysis. External and internal cortical bone contours respond differently to these effects. Future studies could investigate the influence of other relevant factors on symphyseal shape and size.

En constituant l’élément mobile de l’appareil manducateur, la mandibule humaine joue un rôle clef dans les fonctions de ventilation, déglutition, mastication, préhension et phonation et supporte ainsi des régimes de charges complexes, s’exerçant à des intensités et fréquences variables. Bien que les interactions morphofonctionnelles liant cet environnement de charge et la forme externe de la mandibule humaine soient en partie documentées, l’approche morphofonctionnelle de l’anatomie osseuse mandibulaire interne demeure imprécise. Certains auteurs, tels que Demes et al. (1984) ont postulé que cette relation morphofonctionnelle entre l’environnement de charge et la distribution de l’os cortical mandibulaire pourraient être fondée sur l’existence d’une asymétrie faciolinguale. Dans ce contexte, le présent travail vise à interroger le paradigme « mécaniste » de Demes et al. (1984) à l’aune de nouvelles approches méthodologiques et ainsi répondre à la problématique suivante : « Est-ce que la distribution d’os cortical mandibulaire covarie avec l’environnement de charge chez les humains modernes ? ». À travers une revue systématique de la littérature (Decaup et al., 2023), une approche morphométrique comparative au sein de populations du passé (Decaup et al., 2024a) et une modélisation de cette problématique par la méthode des éléments finis (Decaup et al., 2024b, en révision) nous avons montré que la distribution de l’os cortical mandibulaire covarie, sous certaines conditions, avec la distribution des déformations dans le cadre d’un environnement de charge masticatoire unilatéral molaire. Cette covariation produit notamment des effets morphologiques plus marqués dans les groupes humains présentant un niveau de charge mandibulaire élevé. Cette covariation suit enfin un gradient antéropostérieur en faveur d’une relation moins marquée dans les régions antérieures symphysaires et plus marquée dans les régions postérieures du corpus mandibulaire. Le développement d’une approche couplant l’analyse morphométrique comparative et l’analyse par éléments finis en modélisation globale – locale permettrait de mieux interpréter l’impact biomécanique de la diversité biologique de distribution de l’os cortical mandibulaire au sein des populations humaines dans le cadre de recherches futures.

The human mandibular symphysis concentrates multiaxial loads during function and remodels throughout growth, but the precise mechanisms underlying cortical bone shape during growth remain relatively unexplored. Approaches based solely on thickness or external cortical contours provide only partial insights and do not capture the functional, genetic, and allometric interactions driving morphogenesis. In this study, we analyzed symphyseal cross‐sections from 63 subadult mandibles from two French archeological assemblages using elliptic Fourier analysis. Micro‐computed tomography volumes were reconstructed, and external and internal cortical outlines digitized and quantified with Fourier descriptors. Principal component analysis summarized shape variation, and linear models tested the effects of size, age, total cortical area, and geographic origin on each outline. A similar workflow was applied to the size variable expressed as the logarithm of centroid size to test the following predictors: age, total cortical area and archeological site of origin. Across cortical outlines, symphyseal shape was governed primarily by allometry, with a smaller additional contribution of total cortical area. Age significantly affected the inner cortical outline but not the outer one, and geographic origin had no detectable influence. Individuals examined before and after eruption of the first permanent molar occupied largely overlapping morphospaces, indicating a continuous developmental trajectory rather than a shift at eruption. Collectively, the results support bone‐tissue remodeling that progresses throughout growth and is dominated early by size‐related change, with the hypothesis that inter‐individual divergence emerges later. The approach provides a reproducible, resolved framework for quantifying bone remodeling across ontogeny and for testing form‐function relationships.

Objectives: To investigate the influence of non‐syndromic hypodontia on radiographic dental development and to estimate dental age in children with bilateral mandibular agenesis using a machine learning approach. Materials and Methods: This retrospective cross‐sectional study included 626 children aged 6–15 years (311 with dental agenesis, 315 matched controls). Dental age (DA) was assessed using the original Demirjian method. In cases with bilateral congenitally missing teeth in the mandible, DA was estimated using supervised machine learning models based on specific random forests, following a secondary‐diagnosis approach. Dental developmental delay was calculated as the difference between dental and chronological age (DA–CA) and compared across groups defined by agenesis status, severity and pattern. Multiple linear regression was applied to evaluate the effects of hypodontia, sex, chronological age and their interactions on DA–CA. Results: Eight random‐forest models were trained, achieving good age‐prediction accuracy (MAE = 0.08–0.28 years, R ² > 0.95). A 0.77‐year difference in dental development (95% CI 0.61–0.94) separated children with hypodontia from controls ( p < 0.001). The regression model confirmed that agenesis status, sex and CA were significant predictors of DA–CA, with an interaction between sex and agenesis. Unilateral or bilateral agenesis of mandibular second premolars was associated with a dental developmental delay compared with controls ( p _adj < 0.05). Conclusion: Our results were consistent with broader evidence linking hypodontia to altered developmental timing. Machine learning imputation offers a robust approach for missing teeth and can be implemented for age estimation in larger cohorts for orthodontic or forensic purposes.