![]() ![]() With respect to local strains within the arch, the predictions of beam theory are straightforward. In other words, shapes that structurally stiffen the arch may increase the fraction of applied anterior temporalis and medial pterygoid loads transmitted thru the arch relative to more structurally compliant shapes, thereby increasing stress (and, consequently, strain) in adjacent non-arch regions. Second, arch shape could affect global strain patterns in the cranium beyond the arch by altering load paths. It is reasonable to hypothesize that the precise manner in which morphological variation influences local strains could be predicted by beam theory, insofar as the zygomatic arch is, from a geometrical perspective, arguably the most beam-like structure in the entire cranium. First, arch shape could affect local strain patterns within the arch itself. ![]() Variation in zygomatic arch shape could affect cranial strains in two key ways. Curtis et al., (2011) suggests that these studies may be compromised by a failure to consider the functional role of the temporalis fascia, but the solution proposed by them (modeling the temporalis fascia as a series of applied forces acting along the superior margin of the arch) is not obviously realistic. Many studies using FEA to study feeding biomechanics have noted exceptionally high strains in the zygomatic arch ( Strait et al., 2005, 2007, 2009, 2010 Kupczik et al., 2007 Smith et al., 2015a, b Cox et al., 2013 Dumont et al., 2005 Santana et al., 2010 Tseng, 2009 Wang et al., 2010). ![]() Hylander and Johnson (1997) have shown that during feeding the zygomatic arch in macaques bends in parasagittal and transverse planes and twists around its axis. Steep strain gradients have been shown to span the length of the zygomatic arch in macaques, pigs and cats ( Hylander and Johnson, 1997 Herring et al., 1996 Herring, 2001 Buckland-Wright, 1978 Rafferty et al., 2000 Strait et al., 2005, 2007) with parasagittal bending and shearing forces concentrated in the anterior arch. Skulls are scaled for viewing and do not reflect actual size of species. a: Modern human, b: Giant Panda, c: Baboon, d: Coyote, e: Wood Rat, f: Glyptodon. Variation in zygomatic arch shape across mammalian taxa. Although the importance of the zygomatic arch for masticatory function is not disputed, the precise mechanical consequences of these apparent shape differences remain unclear. The shape of the zygomatic arch in mammals is remarkably variable, ranging from almost cylindrical (rounded in cross section) in macaques, some rodents and humans to elliptical in baboons to deep and blade-like in pandas ( Figueirido et al., 2012, 2013, 2014), gorillas ( Wroe et al., 2010), felids ( Christiansen, 2008), canids ( Milenkovic et al., 2010 La Croix et al., 2011), thylacinidae ( Attard et al., 2014), pigs ( Rafferty et al., 2000 Herring and Mucci, 1991 Herring et al., 1996 Teng et al., 1997 Freeman et al., 1997) and chimpanzees, to inferiorly flanged as in the Pleistocene armadillo subfamily Glyptodontinae ( Figure 1). Formed by the union of the zygomatic process of the temporal bone and the temporal process of the zygomatic bone, it is from this beam-like structure that the masseter muscle, a major jaw adductor, originates. The zygomatic arch plays a critical role in the mammalian masticatory system. Even though the arch has simple beam-like geometry, we fail to find a simple mechanical explanation for the diversity of arch shape. Furthermore, although modeling the arch as solid cortical bone did not have the effect of elevating strains in other parts of the face, as had been expected, it does have a small effect on stress associated with masseter contraction. One exception is that possessing a blade-like arch leads to elevated strains at the postorbital zygomatic junction and just below the orbits. We find that the shape of the zygomatic arch has local effects on stain that do not conform to beam theory. Here, we examine the mechanics of the zygomatic arch using a series of finite element modeling experiments in which the cross section of the arch of Pan troglodytes has been modified to conform to idealized shapes (cylindrical, elliptical, blade-like). A stiffer arch may lead to an increase in the relative proportion of applied muscle load being transmitted through the arch to other cranial regions, resulting in elevated cranial stress (and thus, strain). We expect zygomatic arches with different cross sectional shapes to vary in the degree to which they resist local bending and torsion due to the contraction of the masseter muscle. Based on geometry, the arch can be hypothesized to be a sub-structural beam whose ability to resist deformation is related to cross sectional shape. Mammalian zygomatic arch shape is remarkably variable, ranging from nearly cylindrical to blade-like in cross section. ![]()
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