The ontogeny of the chin: an analysis of allometric and biomechanical scaling

Abstract

The presence of a prominent chin in modern humans has been viewed by some researchers as an architectural adaptation to buttress the anterior corpus from bending stresses during mastication. In contrast, ontogenetic studies of mandibular symphyseal form suggest that a prominent chin results from the complex spatial interaction between the symphysis and surrounding soft tissue and skeletal anatomy during development. While variation in chin prominence is clearly influenced by differential growth and spatial constraints, it is unclear to what degree these developmental dynamics influence the mechanical properties of the symphysis. That is, do ontogenetic changes in symphyseal shape result in increased symphyseal bending resistance? We examined ontogenetic changes in the mechanical properties and shape of the symphysis using subjects from a longitudinal cephalometric growth study with ages ranging from 3 to 20+ years. We first examined whether ontogenetic changes in symphyseal shape were correlated with symphyseal vertical bending and wishboning resistance using multivariate regression. Secondly, we examined ontogenetic scaling of bending resistance relative to bending moment arm lengths. An ontogenetic increase in chin prominence was associated with decreased vertical bending resistance, while wishboning resistance was uncorrelated with ontogenetic development of the chin. Relative to bending moment arm lengths, vertical bending resistance scaled with significant negative allometry whereas wishboning resistance scaled isometrically. These results suggest a complex interaction between symphyseal ontogeny and bending resistance, and indicate that ontogenetic increases in chin projection do not provide greater bending resistance to the mandibular symphysis.

The enduring puzzle of the human chin

“…As Hylander showed, coronal bending produces much lower magnitude strains than does wishboning, suggesting that coronal bending is less likely to produce the large stresses that the bone geometry is hypothetically arranged to counter. Also, as the human mandible grows and develops, its architectural rigidity to coronal bending is much reduced . If coronal bending were the load symphyseal form was primarily aimed at countering, then it is expected that as humans begin to chew a stiffer diet, the resistance to coronal bending should increase.…”

“…After all, if the basal mandible were to be reduced in size along with the alveolar process and dentition, humans would not possess a chin. Researchers supporting the hypofunction hypothesis all cite the different ontogenetic and functional modules of the mandible, as well as the superior-to-inferior growth cessation gradient, 10,19,20,23,[29][30][31][32][33]42,43 claiming that the two regions would experience differential speeds of evolutionary reduction. This is a plausible explanation, especially when considering norms of reaction and genetic assimilation in light of the potentially circuitous nature of a hypofunctional evolutionary mechanism.…”

“…Also, as the human mandible grows and develops, its architectural rigidity to coronal bending is much reduced. 43 If coronal bending were the load symphyseal form was primarily aimed at countering, then it is expected that as humans begin to chew a stiffer diet, the resistance to coronal bending should increase. Evidently, this is not the case.…”

Symphyseal surface strain during in vitro human mandibular wishboning

“…Due to this complication, relatively few analyses of primate mandibular masticatory strain have been conducted on the symphyses of living subjects. The preponderance of investigations have either relied on theoretical modeling or in vitro analyses (e.g., Korioth et al, ; Daegling ; Vinyard and Ravosa, ; Ichim et al, ; Ross et al, ; Daegling and McGraw, ; Bucinell et al, ; Gröning et al, ; Holton et al, ). Virtually all of our present understanding of primate in vivo masticatory strain has come from the Hylander‐led research group (e.g., Hylander, ; Hylander and Bays, ; Ross and Hylander, ; Hylander and Johnson, ; Hylander et al, ; Vinyard et al, ).…”

“…In comparing these two sets of analyses this research aims to address the following questions: First, assuming for the sake of argument that wishboning occurs during modern human mastication (see above), 1) does the modern human mandible function as a curved beam? This first question is worth investigating because some have suspected that the unusual shape of the modern human mandible eliminates curved beam effects (e.g., Daegling, ; Holton et al, ). Drawing on Eq.…”