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Mandibular Biomechanics and Development of the Human ChinDepartment of Oral Sciences, Faculty of Dentistry, University of Otago, Dunedin, New Zealand Correspondence: * corresponding author, jules.kieser{at}stonebow.otago.ac.nz
The development of the chin, a feature unique to humans, suggests a close functional linkage between jaw biomechanics and symphyseal architecture. The present study tests the hypothesis that the presence of a chin changes strain patterns in the loaded mandible. Using an anatomically correct 3-D model of a dentate mandible derived from a CT scan image, we analyzed strain patterns during incisal and molar biting. We then constructed a second mandible, without a chin, by ‘defeaturing’ the first model. Strain patterns of the second model were then compared and contrasted to the first. Our main finding was that chinned and non-chinned mandibles follow closely concordant patterns of strain distribution. The results suggest that the development of the human chin is unrelated to the demands placed on the mandible during function.
Key Words: mandible • chin • hominid evolution • FEA
The chin, or mentum osseum, defined as an inverted T-shaped elevation in the midline of the mandibular symphyseal region, flanked by 2 variable scallop-shaped depressions on either side of a central keel, is thought to be unique to humans (Aiello and Dean, 1990; Schwartz and Tattersall, 2000). While some early studies have pointed to non-mechanical explanations for the evolution of the human chin (Weidenreich, 1941; Riesenfeld, 1969), several authors have proposed that the emergence of the chin in early anatomically modern humans may in fact be explained in biomechanical terms (Robinson, 1913; DuBrul and Sicher, 1954; Daegling, 1993; Dobson and Trinkaus, 2002). Evidence for this view comes from 3 sources. First, the in vivo studies of mandibular loading in anthropoids (Hylander, 1984; Hylander et al., 2000) show that the anthropoid mandible experiences predictable patterns of twisting and shearing throughout the power stroke of mastication. These include wishboning (lateral transverse bending), dorsoventral shear, and vertical bending in a coronal plane. Second, comparative studies of the mandibular symphysis in hominoid primates (Daegling, 1993, 2001) suggest that symphyseal morphology is functionally linked to the biomechanics of wishboning of the mandibular corpus. Third, Dobson and Trinkaus (2002) showed that there are important biomechanical consequences to structural changes in the mandibles of late-Pleistocene humans, particularly in the resistance to vertical bending. However, they also showed that the development of a chin appears to be independent of resistance to wishboning. This, together with the fact that the chin emerged at a time of decreased dental use and mandibular shortening (Ackermann and Cheverud, 2004), suggests a more complex causation than previously proposed. The objective of the present study was to test the hypothesis that the presence of a chin changes the strain pattern in the loaded mandible.
Mandibular Modeling and Meshing We developed a 3-D model of an adult dentate mandible from a CT scan image (Siemens Somaton Plus Imager) set to 1-mm slice thickness with 0.5 interpolations, yielding a stack of 117 slices. The skeletal specimen was obtained under the standard protocol of the New Zealand Anatomy Act. Using in-house software and based on the grayscale analysis of the slices, we generated initial meshes for the cortical bone and teeth. We achieved the solid conversion by patching the meshes with rational surfaces (NURBS) using a generic 3D CAD package (Rhinoceros 3D modelling for Windows, v 3.0, Robert McNeel & Assoc., Seattle, WA, USA). To create socket spaces, we subtracted the root shapes of the teeth from the volume of the cortical and medullary bone. The periodontal ligament was not represented. As in previous such studies, parameters of our model were determined by measurements on a single skull (Barbenel, 1972; Baragar and Osborn, 1984; Korioth et al., 1992).
We then constructed a second, chinless, mandible, by ‘defeaturing’ our first model so that all characteristics of the chin were removed. However, the cortical bone thickness was kept the same in both models (Fig. 1
We meshed both models using 1.5-mm parabolic tetrahedral elements, yielding 147,015 elements for the chinless and 141,506 elements for the chinned model. The small size and high order of elements ensured proper strain calculation (Fig. 1  ).
Loads, Constraints, and Materials Constituent materials (bone, dentin, and articular cartilage) were taken as homogenous and linearly isotropic. We assumed that, during normal physiological function, bone was loaded within its elastic range, and also that its behavior could be replicated numerically with linear elastic equations (Vollmer et al., 2000; Cowin, 2001). Although bone is an anisotropic material, other workers (Daegling and Hylander, 1998) and our previous work (Ichim et al., 2006) showed that isotropic models of the mandible were capable of discerning meaningful biomechanical differences. The material properties of hard-tissue elements were extracted from the literature; the elastic modulus for cortical bone and dentin was taken as 14700 MPa, and that of medullary bone as 490 MPa, with a Poisson’s ratio of 0.3. The elastic material placed over the articular condylar surface was taken to have an elastic modulus of 6.1 MPa and a Poisson’s ratio of 0.49 (O’Brien, 2002).
Analysis and Post-processing For the molar bite, the scaling factors of the muscle force were different between the working and balancing sides (Korioth et al., 1992), while for the incision, we used bilateral symmetry of adductor forces. The bite forces generated were 257.6 N during incision and 757.9 N during molar biting. The FE analysis was performed with the use of Cosmos DesignStar v.4 (Structural Research & Analysis Corp., Los Angeles, CA, USA). Numerical values of strains were taken from 4 strain-reading sites on both models: one along the midline, the others in symmetrical pairs immediately distal to the canine, the second premolar, and the second molar. Each site had 5 reading points, one on the lower border, and 2 pairs (buccal and lingual) placed near the alveolar margin and at half the corpus height.
Emphasis was placed on the magnitude of the equivalent strain distributions, which are associated with the von Mises’ stress distribution within the bone. The equivalent strain gives a measure of the amount of elastic distortion in the body, and it is calculated from the component principal strains (
This strain-based approach ensured the compatibility of our results with Frost’s mechanostat model (Frost, 2003).
The distribution of strains along the lower border and mid-corpus of both mandibular models is closely concordant with the distribution of lower border strain (Fig. 2  ). While this pattern is mirrored on the balancing side of the mandible, the working side strains are maximized in the first molar and tail off toward the end of the tooth row.
A similar concordant pattern emerges for strains along the alveolar aspect of the chinned and non-chinned mandibular models loaded for molar biting (Fig. 2 ).
For molar biting, midline strain in the chinned mandible was 884 µ
The total maximum displacements of chinned and non-chinned mandibular models under similar loading conditions were almost identical (1.1 mm for incision and 1.5 mm for molar loading).
In this study, we tested the hypothesis that the presence of a chin changes the strain of the loaded mandible. It is known that morphological variation in bone is due predominantly to its self-optimizing remodeling. Changes in the shape of a particular bone can result from adaptation of the bone’s density and internal architecture to functional stress (Fyhrie and Carter, 1986; Carter et al., 1987), or to changes in the strain energy density, SED (Huiskes et al., 1987). In principle, the relation between bone function and its architecture can be what Frost (2003) refers to as overadequate, but never inadequate. According to his mechanostat hypothesis, bone strains in the range of 1500–3000 µ  cause bone deposition, while strains below the 100–300 µ range result in bone resorption (Frost, 2003). In our study, calculated strain values for both the chinned and flat mandibles were within the normal bone maintenance levels of the mechanostat during molar biting (Fig. 4). Furthermore, the difference between the strains on the lingual and the buccal aspects of the symphyseal region, with the latter being smaller, confirms the findings of Korioth et al.(1992) and is close to those determined on the loaded human mandible (Throckmorton et al., 1992). As a caveat, we acknowledge that, because the bite forces generated by our model are larger toward the upper limit of those recorded in humans (Koolstra, 2002), this may influence interpretations of the mechanostat.
If there is equilibrium between force resistance and symphyseal design, then we have to conclude that the development of a chin does not in fact confer rigidity to the mandible during function. This would seem to conflict with the findings of those who have suggested that the chin provides either buttressing to a mandible being squeezed together by the pterygoid muscles (DuBrul and Sicher, 1954), or wishboning (Hylander, 1984). In a previous paper, we used a simple, non-anatomical model of symphyseal shapes (chin, flat and lingual buttresses) subjected to wishboning and torsional forces, to show that none of these shapes was more biomechanically advantageous than the others (Ichim et al., in press). This study confirmed these results: Strains in chinned and non-chinned mandibles with normalized area and mass show minimal differences during molar loading (Figs. 2 , 3); hence, a chinned symphysis cannot be said to act in lowering the bone strain during mastication. Daegling (2001) has pointed to a possible solution to the problem of the development of a chin by arguing that the inclination, rather than changes in cross-sectional shape of the symphysis, represents a morphological solution for minimizing the effects of wishboning during mastication. We suggest an alternative position—that the development of the human chin is in fact unrelated to the functional demands placed upon it by mastication. Nonetheless, considering the differences in both inclination and contour between these 2 symphyses, as well as that the shapes differ in more than just the presence or absence of a chin, as witnessed by the straight vs. internally convex lingual contours of the two, we believe that Daegling’s explanation is not necessarily contradicted. A final point to make concerning the chin is that it is of recent origin. Neither archaic humans nor H. neanderthalensis has it. Our study addresses the chin from a masticatory perspective and shows that, in mechanical terms, having a chin is no better than having a non-chinned mandible. Hence, our results support the conclusions of O’Connor et al.(2005), that masticatory biomechanical adaptation does not underlie variation in the facial skeleton of later Pleistocene Homo. In a larger biological context, if the retraction of the human mandible over time presented certain special constraints in the oral cavity (Smith, 1984), then the chin might represent a mechanically effective design that offers an adequate adaptive solution for competing functional demands. While we are unable to point to the generative force behind the development of the uniquely human chin, we feel that the mandibular model and biomechanical analysis described here will provide tools to resolve this issue in the future.
This paper has greatly benefited from the comments of two anonymous referees and was funded by a University of Otago Research Grant as well as a Deputy Vice-Chancellor’s award to the Craniofacial Biomechanics Group. Received for publication March 21, 2005. Revision received February 15, 2006. Accepted for publication March 23, 2006.
Journal of Dental Research, Vol. 85, No. 7,
638-642 (2006)
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