Symphyseal surface strain during in vitro human mandibular wishboning

Area of Science:

• Human evolutionary biology
• Biomechanics
• Functional morphology

Background:

• The human chin's evolutionary purpose is debated, with some hypotheses suggesting it is an adaptation to withstand masticatory stresses.
• Understanding the biomechanical role of the chin, particularly during bending (wishboning) of the mandible, is crucial for testing these hypotheses.

Purpose of the Study:

• To theoretically model and empirically measure symphyseal surface strain in human mandibles during wishboning.
• To investigate if the human chin is associated with reduced strain effects predicted by curved beam theory.
• To determine strain gradients on the symphyseal surfaces under simulated wishboning loads.

Main Methods:

• Theoretical strain calculations were performed on five human mandibles based on morphometric criteria.
• Strain gauges were applied to the same mandibles, which were then subjected to simulated wishboning loads.
• Empirical strains were recorded and compared with theoretical predictions to assess curved beam behavior and strain disparities.

Main Results:

• Human mandibles function as curved beams during wishboning, as predicted by theoretical models.
• Empirical strain measurements exhibited greater variation on both labial and lingual symphyseal surfaces than theoretical models predicted.
• These findings suggest the chin is associated with larger-than-expected strain gradients.

Conclusions:

• The human chin's morphology is unlikely to be an adaptation for countering wishboning loads.
• The presence of a chin correlates with increased strain gradients, contradicting optimality criteria for adaptation.
• This suggests wishboning may not be a significant functional load in human mastication or poses different structural risks compared to other primates.