Masticatory stress and the mechanics of “wishboning” in colobine jaws

Understanding the mechanical features of cortical bone and their changes with growth and adaptation to function plays an important role in our ability to interpret the morphology and evolution of craniofacial skeletons. We assessed the elastic properties of cortical bone of juvenile and adult baboon mandibles using ultrasonic techniques. Results showed that, overall, cortical bone from baboon mandibles could be modeled as an orthotropic elastic solid. There were significant differences in the directions of maximum stiffness, thickness, density, and elastic stiffness among different functional areas, indicating regional adaptations. After maturity, the cortical bone becomes thicker, denser, and stiffer, but less anisotropic. There were differences in elastic properties of the corpus and ramus between male and female mandibles which are not observed in human mandibles. There were correlations between cortical thicknesses and densities, between bone elastic properties and microstructural configuration, and between the directions of maximum stiffness and bone anatomical axes in some areas. The relationships between bone extrinsic and intrinsic properties bring us insights into the integration of form and function in craniofacial skeletons and suggest that we need to consider both macroscopic form, microstructural variation, and the material properties of bone matrix when studying the functional properties and adaptive nature of the craniofacial skeleton in primates. The differences between baboon and human mandibles is at variance to the pattern of differences in crania, suggesting differences in bone adaption to varying skeletal geometries and loading regimes at both phylogenetic and ontogenetic levels.Keywords craniofacial skeleton; biomechanics; function; adaptation; Micro-CT; primate Elastic properties and their variations in individuals are important for understanding bone adaptations and quality at the tissue level (Dechow et al., 1992(Dechow et al., , 1993Dechow and Hylander, 2000; Schwartz-Dabney and Dechow, 2003;Wang and Dechow, 2006;Wang et al., 2006; Rapoff et al., 2008). They are especially indispensable for exploring the biomechanics and adaptation of primate skulls using Finite Element Analysis (FEA) (Strait et al., 2005(Strait et al., , 2009Wang and Dechow, 2006;Wang et al., 2006; Rayfield, 2007). We have * Please send page proofs and direct correspondence to: Dr Qian Wang, Division of Basic Medical Sciences, Mercer University School of Medicine, 1550 College Street, Macon, GA 31207. Phone:478-301,4043; Fax: 478-301,5489; wang_q2@mercer.edu NIH Public Access Author ManuscriptAm J Phys Anthropol. Author manuscript; available in PMC 2011 April 1. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript systematically collected elastic properties from the craniofacial skeletons of three species including modern humans (crania and mandibles), macaques (crania), and baboons (crania) (Peterson, 2002; Peterson and Dechow, 2003; Dechow, 2002a, 2003;Dechow, 2006, Wang et al, 2006)...

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Regional, ontogenetic, and sex‐related variations in elastic properties of cortical bone in baboon mandibles

Wang¹,

Ashley²,

Dechow³

2009

“…In attempting to explain its functional significance, a number of studies have addressed the possible relationship between symphyseal shape and the mechanical stresses generated within the symphyseal region during mastication (e.g., Beecher, 1977Beecher, , 1979Hylander, 1984Hylander, , 1985Hylander, , 1988Ravosa, 1991;Daegling, 1992Daegling, , 2001Ravosa and Hylander, 1994;Fukase, 2007;Daegling et al, 2008;Fukase and Suwa, 2008;Daegling and McGraw, 2009;Koyabu and Endo, 2009;Anton et al, 2011;Groning et al, 2011;Panagiotopoulou and Cobb, 2011). Specifically, a well-developed superior transverse torus (STT) and/or an obliquely inclined symphysis seen typically in cercopithecine mandibles have been interpreted as mechanical adaptations to withstand lateral bending in the transverse plane (wishboning) during mastication, because an effective way to counter this type of bending is to increase the labiolingual thickness of the symphysis (e.g., Hylander, 1984;Ravosa, 1991;Daegling, 1992, Hylander and Johnson, 1994Panagiotopoulou and Cobb, 2011).…”

Section: Introductionmentioning

confidence: 99%

Interspecies difference in placement of developing teeth and its relationship with cross‐sectional geometry of the mandibular symphysis in four primate species including modern humans

Fukase

2011

The form of the anthropoid mandibular symphysis has recently been addressed in association with spatial requirements for the forming anterior teeth. To evaluate potential relationships between the symphyseal shape and teeth further, the growth patterns of the symphyseal region and the positioning of the tooth crypts were examined using CT data, comparing four primate species (modern humans, chimpanzees, Japanese monkeys, and hamadryas baboons) with varied symphyseal curvature and tooth size. First, results showed that interspecies differences in overall mandibular shape including symphyseal inclination and bicanine width are consistently expressed throughout postnatal ontogeny, although local symphyseal configurations related to the superior transverse torus (STT) tended to change considerably during growth in chimpanzees. Second, the four species were found to exhibit differentiated formation positions of the incisor and canine crypts. In particular, I2 developed between I1 and C in humans with a broad bicanine space and small teeth, whereas it was positioned posterior to I1 and above C in the cercopithecines with an extremely narrow bicanine space. In chimpanzees, despite the large bicanine width, I1 and I2 grew with a large antero-posterior overlap owing to their large size. These results indicate that the dental positioning is determined in concert with the size balance of the available mandibular space and forming teeth. Finally, the positions/contours of I2 crypt were shown to correspond strongly with the STT across the taxa. This suggests that interspecies differences in symphyseal shape should be interpreted partially by the species-specific positional relationships of the developing anterior teeth.

“…A combination of factors, not necessarily mutually exclusive, have been proposed as determinants of variation in anterior mandibular morphology, including sexual dimorphism, facial orientation, dentition, and phylogenetic constraints (e.g., Shea, 1985Shea, , 1987 Brown, 1997;Plavcan and Daegling, 2006; Cobb and Baverstock, 2009a,b;Cobb and Panagiotopoulou, 2011). However, the greatest attention has been paid to adaptation of the mandible to resist loads during intraoral food processing (e.g., Hylander, 1984Hylander, , 1985Hylander and Johnson, 1994;Daegling and Hylander, 2000;Daegling, 2001).The current state of knowledge regarding the mechanical significance of the symphysis is based on a combination of allometric studies, in vivo strain gauge studies, and geometric abstraction when modeling the mandible as a curved beam (e.g., Daegling, 1993Daegling, , 2001Hylander, 1984Hylander, , 1985Hylander and Johnson, 1994;Ravosa and Hylander, 1994;Ravosa, 1996Ravosa, , 1999Ravosa, , 2000Hylander et al, 1998Hylander et al, , 2000Daegling and Hylander, 2000;Ravosa et al, 2000;Daegling and McGraw, 2009). …”

mentioning

confidence: 98%

The mechanical significance of morphological variation in the macaque mandibular symphysis during mastication

Panagiotopoulou

Cobb

2011

Catarrhine symphyseal morphology displays considerable variation. Although this has been related to dentition, phylogeny, sexual dimorphism, and facial orientation, most emphasis has been given to the functional significance of the symphysis to mechanical loading during mastication. The current state of knowledge regarding the mechanical significance of the symphysis is based on a combination of in vivo experimental and comparative studies on Macaca fascicularis. These approaches have provided considerable insight into the stereotypical patterns of loading in the symphyseal region during chewing and hypotheses related to the associated symphyseal morphologies. Finite element analysis (FEA) was used to assess how in silico manipulation translates into the mechanical loading hypotheses previously proposed experimentally. In particular, this study tests the form-function relationship of the symphysis of an adult M. fascicularis mandible during lateral transverse bending and dorsoventral shear of the mandibular symphysis, and a series of modified hypothetical morphologies including absence/presence of tori and variation in the inclination and depth of the symphysis. FEA results of this study support previous findings that stresses associated with lateral transverse bending and dorsoventral shear of the mandibular symphysis can be minimized via an increased labio-lingual thickness in the superior transverse torus, an oblique symphyseal inclination, and/or an increased symphyseal depth. The finding that reduction of strains related to lateral transverse bending and dorsoventral shear can be achieved through a number of different morphologies contributes to our understanding of the influence of morphological and/or developmental constraints, such as dental development, on symphyseal form.