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J Dent Res 85(7):638-642, 2006
© 2006 International and American Associations for Dental Research

RESEARCH REPORT
Biomaterials & Bioengineering

Mandibular Biomechanics and Development of the Human Chin

I. Ichim, M. Swain, and J.A. Kieser*

Department of Oral Sciences, Faculty of Dentistry, University of Otago, Dunedin, New Zealand

* corresponding author, jules.kieser{at}stonebow.otago.ac.nz


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
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. 1Go). A uniform elastic material of 2-mm thickness was placed over the articular condylar surfaces of both models to provide both resilient restraint and freedom of movement for the condyles (Korioth et al., 1992).


Figure 1
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  		Figure 1. Models of chinned (a) and chinless (b) mandibles morphed from a CT scan showing cortical and medullary bone. The vectoral muscle components are applied to the muscular areas of origin (c). Meshed models as occlusally loaded for molar bite (d) and incision (e).

 
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. 1Go).

Loads, Constraints, and Materials
Our analysis was based on the methodology of Korioth et al.(1992), whose strain predictions were consistent with mandibular mechanics observed in human and non-human primates. We loaded both models with vector groups corresponding to the main masticatory muscles and their components: superficial and deep masseter, medial pterygoid and anterior, middle and posterior temporalis. The forces were prescribed on their anatomic insertion areas by their orthogonal components, where XY was the horizontal plane, ZY the coronal plane, and XZ the frontal plane. Also taken from Korioth et al.(1992) were the magnitudes of the muscle contractile forces (Mir), which were given by the product of the cross-sectional area of the muscle (XMi), a constant for skeletal muscles (K), and the scaled value of the muscle contraction relative to its maximum response for any task (EMGMI): Mir = [XMi·K]·EMGir.

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 each symphyseal shape, we performed two static linear FE analyses: one simulating incision, and the other molar biting using the computed values. To simulate incisor biting, we placed a set of fixed restrains on the incisal margin of the front teeth (Fig. 1eGo), and, for the molar bite, the occlusal surfaces of the first and second lower molars were fixed (Fig. 1dGo).

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 ({varepsilon}1, {varepsilon}2, {varepsilon}3) and Poisson’s ratio (v'), as defined by the following equation:


Formula

This strain-based approach ensured the compatibility of our results with Frost’s mechanostat model (Frost, 2003).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
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. 2Go). 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.


Figure 2
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  		Figure 2. Strain distribution along the lower border (top) and mid-corpus (bottom) of chinned and non-chinned mandibular models under molar biting. The left side is the working side.

 
A similar concordant pattern emerges for strains along the alveolar aspect of the chinned and non-chinned mandibular models loaded for molar biting (Fig. 2Go).

For molar biting, midline strain in the chinned mandible was 884 µ{varepsilon} lingually and 647 µ{varepsilon} buccally, and that in the non-chinned mandible was 841 µ{varepsilon} lingually and 744 µ{varepsilon} buccally (Fig. 3Go). During incision, midline strain on the lingual side of the chinned mandible was 1530 µ{varepsilon}, and that on the buccal side was 718 µ{varepsilon}. In contrast, midline strain in the non-chinned mandible during incisor biting was 1450 µ{varepsilon} on the lingual and 1411 µ{varepsilon} on the buccal aspect.


Figure 3
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  		Figure 3. Strain plots of chinned and flat-symphysis models during molar biting. The sections have the cancellous bone removed and are facing the working side of the corpus. Note the small differences in symphyseal low-strain for all 4 cases, on both the buccal and lingual aspects. Scale calibrated to a range of 100–2500 µ{varepsilon}.

 
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).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
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{varepsilon} cause bone deposition, while strains below the 100–300 µ{varepsilon} 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. 4Go). 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.


Figure 4
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  		Figure 4. Calculated strains as compared with the bone mechanostat (not to scale). Values for both chinned and non-chinned designs are within the maintenance interval (see text for details).

 
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. 2Go, 3Go); 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.


   ACKNOWLEDGMENTS
 
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 March 21, 2005; Last revision February 15, 2006; Accepted March 23, 2006


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Ackermann RR, Cheverud JM (2004). Detecting genetic drift versus selection in human evolution. Proc Natl Acad Sci USA 101:17946–17951.[Abstract/Free Full Text]

Aiello L, Dean C (1990). An introduction to human evolutionary anatomy. London: Academic Press.

Baragar FA, Osborn JW (1984). A model relating patterns of human jaw movement to biomechanical constraints. J Biomech 17:757–767.[Medline]

Barbenel JC (1972). The biomechanics of the temporomandibular joint: a theoretical study. J Biomech 5:251–256.[ISI][Medline]

Carter DR, Fyhrie DP, Whalen RT (1987). Trabecular bone density and loading history: regulation of connective tissue biology by mechanical energy. J Biomech 20:785–794.[Medline]

Cowin SC (2001). Bone mechanics handbook. 2nd ed. Boca Raton: CRC Press.

Daegling DJ (1993). Functional morphology of the human chin. Evol Anthropol 1:170–177.

Daegling DJ (2001). Biomechanical scaling of the hominoid mandibular symphysis. J Morphol 250:12–23.[Medline]

Daegling DJ, Hylander WL (1998). Biomechanics of torsion in the human mandible. Am J Phys Anthropol 105:73–87.[Medline]

Dobson SD, Trinkaus E (2002). Cross-sectional geometry and morphology of the mandibular symphysis in Middle and Late Pleistocene Homo. J Hum Evol 43:67–87.[ISI][Medline]

DuBrul EL, Sicher H (1954). The adaptive chin. Springfield, IL: Charles Thomas.

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Koolstra JH (2002). Dynamics of the human masticatory system. Crit Rev Oral Biol Med 13:366–376.[Abstract/Free Full Text]

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