Mechanical Impact of Incisor Loading on the Primate Midfacial Skeleton and its Relevance to Human Evolution

Abstract

The midfacial skeleton in the human lineage demonstrates a wide spectrum of variation that may be the consequence of different environmental and mechanical selective pressures. However, different facial configurations may develop under comparable selective regimes. For example, the Neanderthal high and projected face and the Inuit broad and flat face are hypothesized to be the consequence of (1) life in a cold climate, and (2) excessive paramasticatory stresses focused on the anterior dentition. In this study, the second of these two hypotheses is tested using finite element analyses of a monkey skull. Results indicate that incisor loading induces heavy stress in the anterior midface of macaques. Additional analyses using incremental increases in the anteroinferior tilt of the skull to simulate different magnitudes of facial projection revealed that comparable muscular force generates less stress in a less-projected face. However, the findings of our final analyses, which attempted to combine biting with the incisors and pulling with the hands, differed from the analyses that mimicked only incisor loading (without any sort of anterior pulling component). These findings suggest that shortening the face may be the most effective way to compensate for anterior dental loading but not necessarily offset the forces incurred when using the anterior dentition as a vice for various paramasticatory behaviors. Although Neanderthals may have frequently loaded their anterior dentition, countervailing selection pressures, such as the inclusion of tough foods in the diet that demanded molar grinding, may have selected for a longer face with a lower load-to lever-arm ratio. Anat Rec,

The Mechanical Significance of the Temporal Fasciae in Macaca fascicularis: An Investigation Using Finite Element Analysis

“…The meshed geometry was imported into commercially available FEA software (ANSYS version 12, ANSYS), and all muscle and bone structures were defined with six or eight noded (plane strain) second-order elements. Bone was specified with a Young's modulus of 17 GPa and a Poisson's ratio of 0.3 (consistent with direct measurements and within the ranges applied by others; Strait et al, 2005;Witzel and Preuschoft, 2005;Dumont et al, 2009;Wang et al, 2010), and the temporalis muscle with a Young's modulus of 10 MPa and a Poisson's ratio of 0.3. Notional thermal expansion properties were additionally assigned to the muscle elements so that their expansion (bulging) could be simulated, and a solution reference temperature of zero and an expansion coefficient of 0.07 (/ C) were assigned.…”

“…2008; Dumont et al, 2010;Wang et al, 2010), and generally report large inferior bending of the zygomatic arch and high strain gradients across the skull. When we include a complete deep temporal fascia into our FE model of a macaque skull we note a considerable reduction in peak strains, resulting in a more uniform distribution of strains across the entire zygomatic arch (Figs.…”

“…Computational techniques such as finite element analysis are popular for exploring the mechanical function of skulls (e.g., Ross et al, 2005;Kupczik et al, 2007Kupczik et al, , 2009Strait et al, 2007Strait et al, , 2009Wroe et al, 2007Wroe et al, , 2010Curtis et al, 2008;Dumont et al, 2010;Wang et al, 2010), but it is rare that the temporal fasciae are taken into account (Witzel et al, 2011). As a result most analyses, particularly in skulls with relatively weak zygomatic arches, predict large inferior bending of the arch producing elevated levels of strain and high strain gradients across the skull.…”

The Global Impact of Sutures Assessed in a Finite Element Model of a Macaque Cranium

“…Anterior tooth loading is associated with higher facial strain magnitudes than posterior tooth loading (Strait et al, 2008(Strait et al, , 2009Wang et al, 2010b), yet here the presence of sutures had a higher proportional impact during molar loadings. This might be related to increased muscle force asymmetry, which is pronounced during molar bites.…”

“…In premolar loadings, muscle forces of the balancing side were in the middle of the balancing muscles forces for molar and incisor. (Strait et al, 2008;Kupczik et al, 2009;Wang et al, 2010b). Thus facial sutures might have different functional effects during different loading scenarios.…”