Experimental observation, theoretical models, and biomechanical inference in the study of mandibular form

Abstract

Experimental studies and mathematical models are disparate approaches for inferring the stress and strain environment in mammalian jaws. Experimental designs offer accurate, although limited, characterization of biomechanical behavior, while mathematical approaches (finite element modeling in particular) offer unparalleled precision in depiction of strain magnitudes, directions, and gradients throughout the mandible. Because the empirical (experimental) and theoretical (mathematical) perspectives differ in their initial assumptions and their proximate goals, the two methods can yield divergent conclusions about how masticatory stresses are distributed in the dentary. These different sources of inference may, therefore, tangibly influence subsequent biological interpretation. In vitro observation of bone strain in primate mandibles under controlled loading conditions offers a test of finite element model predictions. Two issues which have been addressed by both finite element models and experimental approaches are: (1) the distribution of torsional shear strains in anthropoid jaws and (2) the dissipation of bite forces in the human alveolar process. Not surprisingly, the experimental data and mathematical models agree on some issues, but on others exhibit discordance. Achieving congruence between these methods is critical if the nature of the relationship of masticatory stress to mandibular form is to be intelligently assessed. A case study of functional/mechanical significance of gnathic morphology in the hominid genus Paranthropus offers insight into the potential benefit of combining theoretical and experimental approaches. Certain finite element analyses claim to have identified a biomechanical problem unrecognized in previous comparative work, which, in essence, is that the enlarged transverse dimensions of the postcanine corpus may have a less important role in resisting torsional stresses than previously thought. Experimental data have identified subperiosteal cortical thinning as a culprit in diminishing the role of cross-sectional geometry in conditioning the strain environment. These observations raise questions concerning the biomechanical significance of mandibular form in early hominids, fueling persistent arguments over whether gnathic morphology can be related to dietary specialization in the "robust" australopithecines. Nonmechanical explanations (e.g., tooth size or body size) for Paranthropus mandibular dimensions, however, are not compelling as competing hypotheses. Both theoretical and experimental models are in need of refinement before it is possible to conclude that the jaws of the "robust" australopithecines are not functionally linked to elevated masticatory loads.

Remodeling dynamics in the alveolar process in skeletally mature dogs

“…Another reason for examining remodeling is the differences in the mechanical environment between the maxilla and mandible. Strain gauge and experimental data suggest that the mandible of monkeys and humans is exposed to torsional forces (Hylander and Crompton, 1986;Daegling and Hylander, 2000;Ravosa et al, 2000). However, the thin maxillary bone is suggested to resist compressive force of mastication.…”

Predicting Skull Loading: Applying Multibody Dynamics Analysis to a Macaque Skull

“…These technologies, and in particular the former, have been used for decades in the automotive and aerospace industries to reliably predict structural performance of mechanical systems. Applying FEA and MDA to biological systems seems a logical progression, and indeed mechanical modeling in relation to the masticatory apparatus of humans and other nonhuman primates has previously been conducted (Koolstra and van Eijden, 1995;Daegling and Hylander, 2000;Koolstra, 2003;Sellers and Crompton, 2004;Ross et al, 2005;Strait et al, 2005;Ichim et al, 2006;Kupczik et al, 2007). A common way of estimating loading conditions is to compute physiological crosssectional areas (PCSA) to approximate the peak forces that can be generated by muscles and, where available to further refine loadings using data from experimental analyses of muscle activation (e.g., Ross et al, 2005).…”

Mammalian Oral Rhythms and Motor Control

“…One of the greatest challenges is to achieve reasonable agreement between experiments and models (Daegling & Hylander, 2000). Biomechanical data and issues with respect to the oromandibular complex are discussed in detail elsewhere (Daegling & Hylander, 2000;Douglas, 1996, Gallo, 2005van Eijden, 2000). Examples of some of the challenges and issues will be presented, below.…”