Human mandibular shape is associated with masticatory muscle force

Abstract

Understanding how and to what extent forces applied to the mandible by the masticatory muscles influence its form, is of considerable importance from clinical, anthropological and evolutionary perspectives. This study investigates these questions. Head CT scans of 382 adults were utilized to measure masseter and temporalis muscle cross-sectional areas (CSA) as a surrogate for muscle force, and 17 mandibular anthropometric measurements. Sixty-two mandibles of young individuals (20-40 years) whose scans were without artefacts (e.g., due to tooth filling) were segmented and landmarked for geometric morphometric analysis. The association between shape and muscle CSA (controlled for size) was assessed using two-block partial least squares analysis. Correlations were computed between mandibular variables and muscle CSAs (all controlled for size). A significant association was found between mandibular shape and muscle CSAs, i.e. larger CSAs are associated with a wider more trapezoidal ramus, more massive coronoid, more rectangular body and a more curved basal arch. Linear measurements yielded low correlations with muscle CSAs. In conclusion, this study demonstrates an association between mandibular muscle force and mandibular shape, which is not as readily identified from linear measurements. Retrodiction of masticatory muscle force and so of mandibular loading is therefore best based on overall mandibular shape.

Full text

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Human mandibular shape is
associated with masticatory
muscle force
Tanya Sella-Tunis1,2, Ariel Pokhojaev1,2,3, Rachel Sarig2,3, Paul O’Higgins
4 & Hila May1,2
Understanding how and to what extent forces applied to the mandible by the masticatory muscles
inuence its form, is of considerable importance from clinical, anthropological and evolutionary
perspectives. This study investigates these questions. Head CT scans of 382 adults were utilized to
measure masseter and temporalis muscle cross-sectional areas (CSA) as a surrogate for muscle force,
and 17 mandibular anthropometric measurements. Sixty-two mandibles of young individuals (20–40
years) whose scans were without artefacts (e.g., due to tooth lling) were segmented and landmarked
for geometric morphometric analysis. The association between shape and muscle CSA (controlled
for size) was assessed using two-block partial least squares analysis. Correlations were computed
between mandibular variables and muscle CSAs (all controlled for size). A signicant association was
found between mandibular shape and muscle CSAs, i.e. larger CSAs are associated with a wider more
trapezoidal ramus, more massive coronoid, more rectangular body and a more curved basal arch. Linear
measurements yielded low correlations with muscle CSAs. In conclusion, this study demonstrates an
association between mandibular muscle force and mandibular shape, which is not as readily identied
from linear measurements. Retrodiction of masticatory muscle force and so of mandibular loading is
therefore best based on overall mandibular shape.
e inuence of masticatory muscle action on the development of craniofacial morphology has received con-
siderable attention in the dental literature (see review article by Pepicelli et al.1). Since bone adapts to loads by
remodeling to reach the optimal form to withstand them (Wollf’s law)2, it has been hypothesized that craniofacial
skeletal form is largely determined by mechanical loading (e.g.3–6). is has been supported by many clinical and
experimental studies. us, an association exists between muscle cross-sectional areas, which are approximately
proportional (excluding pinnate muscles) to force generation, and craniofacial morphology, as found by studies
using a range of methodological approaches (e.g., nite elements, CT models, strain gauges)7–12. Accordingly, it
was established that facial types are associated with bite force, i.e. brachycephalic pattern with strong bite force
and dolichocephalic with weak bite force7,13,14. Experimental studies show that the decreased functional demands
on mandibles of animals fed a so diet results in structural changes in the masticatory muscles15, as well as mor-
phological alterations of the mandible, such as reduced size of the alveolar bone16–18.
Mandibular form and development have been extensively studied (e.g.19,20). Yet, how common measurements
of human mandibular morphology and size covary with masticatory muscle forces has not been investigated in
detail. is is a signicant shortcoming for clinicians and anthropologists alike, since knowledge of how mas-
ticatory muscle force and mandibular form covary could enable the latter to be used to reconstruct diet and
food preparation techniques in ancient populations. Although several studies have shown associations between
craniofacial and mandibular shape and dierent feeding strategies21–24, eorts to reveal dietary habits and food
preparation techniques from the oral apparatus have focused mainly on the study of oral pathologies such as
caries, periodontal diseases, ante-mortem tooth loss, and attrition25,26.
1Department of Anatomy and Anthropology, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv, Tel Aviv,
69978, Israel. 2Shmunis Family Anthropology Institute, Dan David Center for Human Evolution and Biohistory
Research, The Steinhardt Museum of Natural History, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv, Tel
Aviv, 69978, Israel. 3The Maurice and Gabriela Goldschleger School of Dental Medicine, Sackler Faculty of Medicine,
Tel Aviv University, Ramat Aviv, Tel Aviv, 69978, Israel. 4Centre for Anatomical & Human Sciences, Department of
Archaeology and Hull York Medical School, University of York, Heslington, York, YO10 5DD, UK. Correspondence and
requests for materials should be addressed to H.M. (email: mayhila@tauex.tau.ac.il)
Received: 18 October 2017
Accepted: 27 March 2018
Published: xx xx xxxx
OPEN
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e current study was therefore carried out to gain greater insight into the associations between muscle forces
and mandibular morphology. Such a study requires living individuals and is best established using computerized
tomography (CT) scans in which bone and so tissue shadows are visible. More so, muscle cross-sectional areas
(CSA) from CT, magnetic resonance imaging and ultrasound scans can be used as a surrogate for the peak forces
that can be generated by the masticatory muscles7,9,11,12,27–33.
e aims of this study were to identify associations between masticatory muscle force (as estimated by CSAs)
and mandibular shape and to relate variations in specic muscle CSAs (masseter and temporalis) to specic
aspects of mandibular shape and size variation. Two hypotheses were tested: H01 - no association exists between
the CSAs of the masseter and temporalis muscles and mandibular shape; H02 - no associations exist between
temporalis and masseter CSAs and anthropometric (linear and angular) measurements of the mandible. e rst
hypothesis was examined using shape variables derived from landmark data. e second hypothesis was tested
using Pearson correlations to assess relationships between muscle CSAs and mandibular variables. e second
analysis was carried out for practical reasons since archeological mandibles are sometimes too fragmented to
readily allow shape analysis.
Material and Methods
e study included 382 individuals (193 males and 189 females) aged 18–80 years who had undergone a head
and neck CT scan at Carmel Medical Center, Haifa (Brilliance 64, Philips Medical System, Cleveland, Ohio: slice
thickness 0.9–3.0 mm, pixel spacing 0.3–0.5 mm, 120 kV, 250–500 mAs, number of slices 150–950 and Matrix
512*512), between the years 2000 and 2012. All CT scans were carried out for diagnostic purposes, where a
CT scan was medically necessary. Inclusion criteria were as follows: age between 20 and 80 years, intact lower
incisors, and at least two teeth of the posterior unit (premolars and/or molars) on each side. Exclusion criteria
included the absence of the lower incisors; dental implants and metal restorations that interfere with imaging
and so, measurement; prominent facial and mandibular asymmetry; craniofacial, temporomandibular joint, or
muscular disorders; trauma; previous surgery on the head and neck region (based on medical les or signs on the
skull); and technically aberrant CT scans. is study was approved by the ethical board of the Carmel Medical
Center, Israel (number: 0066-11-CMC) and followed their guidelines.
Evaluating muscle areas (Force). CSAs of the masseter and temporalis muscles (which reect peak force)
were measured using the planar mode for sectioning CT stacks, and the ‘region of interest’ tool for tracing out-
lines and measuring areas available on the Brilliance Workspace Portal (Philips v. 2.6.1.5). Masticatory muscle
CSAs were measured following the method of Weijs and Hillen28 (Fig.1). e muscle CSA was controlled for
mandibular size using either mandibular centroid size (in GM analyses) or the geometric mean of the mandibular
linear measurements (MGM - for analyses of anthropometric data; see statistical analysis section).
Evaluating mandibular shape using the geometric morphometrics. 62 mandibles (30 males and
32 females) were segmented and reconstructed from the CT stacks using Amira (v6.1). Semi-automated seg-
mentation of CT sections was carried out based on grey level thresholds. Manual renement of segmentation
was carried out where needed. e inclusion criteria for this group were: age 20–40 years to control for age eect
Figure 1. Muscle cross-sectional area measurement following Weijs and Hillen28. Masseter (1) area was
estimated by tracing it on the CT scan sectioned 3 cm ventro-cranially to the jaw angle, 30° relative to the
Frankfurt horizontal plane. Temporalis (2) area was measured one cm cranially to the zygomatic arch, parallel
to the Frankfurt horizontal plane.
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on muscle CSAs and CT scans with no artefacts that may interfere with the segmentation (e.g., tooth lling and
dental crown). e 3D form of the mandible was characterized using 35 landmarks and 60 curve semi-landmarks
(representing 13 curves; Tables1 and 2; Fig.2). e landmarks, curves and curve semi-landmarks were placed on
the mandibular surface mesh using the EVAN Toolbox soware (v.1.71) and semi-landmark sliding was carried
out to minimise bending energy34.
Evaluating mandibular shape and orientation using linear and angular measurements. 17 lin-
ear, CSA and angular measurements of the mandible were obtained. ese include traditional measurements and
non-standard ones that are feasible due to the use of CT scans35 (Table3; Fig.3). All measurements were taken
directly from CT scans using the Brilliance Workspace Portal (Philips v. 2.6.1.5). All linear measurements were
controlled for mandibular size using the MGM (square roots of the CSA measurements of the mandible were
divided by the MGM) following the principles presented in Jungers et al.36. is accounts for the eects of general
size when assessing how the resulting indices covary with muscle CSAs, also scaled for MGM.
Statistical analysis. Statistical analyses of landmarks, indices and angular measurements were carried out
using PAST (v. 3.15) or SPSS (v.22.0). e threshold of signicance was taken as p = 0.05 in this study.
Landmark based analyses. Reliability: Intra- and inter-observer variation in the shape of mandibular
landmark congurations was assessed using ve randomly selected mandibles. To assess intraobserver variation,
one researcher (AP) placed the landmarks and curve semilandmarks twice on each mandible with a week-long
interval between landmarking sessions. To assess inter-observer variation, the set of landmarks was placed by an
additional independent researcher (GA). To examine variations in shape, Principal components analysis (PCA)
was carried out following a General Procrustes Analysis (GPA) of the landmark data, which eliminates dierences
in orientation, location, and size37. e signicances of Procrustes distances within and between repeated meas-
urements of specimens and by researchers were assessed via permutation tests (1000 random permutations)38.
Mandibular shape and muscle CSAs. For the 3D shape analysis, Cartesian coordinates were converted into shape
variables through GPA. PCA was carried out to examine shape variation in the general population. Since mandib-
ular size aects shape variation19–21 we controlled for allometry. Shape variables were regressed and standardized
on centroid size (allometrically adjusted). A linear regression of log square roots of muscle CSAs on log centroid
size (i.e., the independent variable) was used to allometrically adjust muscle CSAs.
Two-block Partial least squares (2B-PLS) analysis was carried out, separately for males and females, on allo-
metrically adjusted muscle CSAs as one block and the adjusted shape variables as the second block, to examine
the association between shape and muscle CSAs when allometry is accounted for. Visualization of shape changes
along the PLS vector was carried out by warping the mean surface mesh using a triplet of thin plate splines (TPS)
in the EVAN Toolbox (v. 1.71)39.
Landmark Denition
1 Gnathion e inferiormost point of the mandibular body in the midsagittal plane
2Infradentale anterior e anteriormost point of the mandibular alveolar border in the midsagittal plane
3 Linguale e genial tubercle
4Infradentale posterior e postero-superior point of the mandibular alveolar border in the midsagittal plane
5 Pogonion e anteriormost point in the midsagittal plane
6+7 C-P3 e anteriormost point between canine and 1st premolar (le and right, respectively)
8+9 P4-M1 e anteriormost point between 2nd premolar and 1st molar (le and right, respectively)
10+11 M1-M2 e anteriormost point between 1st and 2nd molars (le and right, respectively)
12+13 Mental foramen e anteriormost point of mental foramen (le and right, respectively)
14+15 Root of ramus e anteriormost point of the ramus rim at the level of the alveolar ridge (le and right,
respectively)
16+17 Gonion e point on the projection of the bisection of the mandibular angle (le and right, respectively)
18+19 Lateral condyle From a superior view, the lateralmost point of the condyle (le and right, respectively)
20+21 Center of condyle From a superior view, the central point of the condyle (le and right, respectively)
22 Medial condyle From a superior view, the medialmost point of the condyle (le and right, respectively)
24+25 Sigmoid notch e inferiormost point of the mandibular notch, when the mandible is positioned in the
mandibular plane (le and right, respectively)
26+27 Coronion e superiormost point of the coronoid process (le and right, respectively)
28+29 Mandibular foramen e inferiormost point of the mandibular foramen (le and right, respectively)
30+31 Alveolar process - lingual aspect From a superior view, the intersection between a line tangent to the lingual alveolar process of
the molar teeth and a line, perpendicular to it, passing through the ramus root (le and right,
respectively)
32+33 Anterior condyle e anterosuperior point of the mandibular notch (le and right, respectively)
34+35 Posterior condyle e posteriormost point of the condyle at its center (le and right, respectively)
Table 1. Denition of landmarks placed on the mandibular surface.
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Analysis of linear measurements. Reliability. Anthropometric measurement reliability was assessed
using 15 randomly selected mandibles. To assess intraobserver variation in the linear and angular dimensions,
a single researcher (TST) carried out the measurements twice with a two-week interval between each attempt.
To assess interobserver error, measurements were taken by an additional independent researcher (HM or VS).
Intraclass correlation coecient (ICC) analysis was carried out to examine the reproducibility of the measure-
ments and was interpreted according to the categorization method of Cicchetti40.
Mandibular linear measurements and muscle CSAs. A Kolmogorov-Smirnov test was carried out to test for the
normality of distributions of the variables. Logarithmic transformation was carried out for variables that did not
distribute normally. e association between muscle CSAs and mandibular measurements, both controlled for
mandibular size (MGM), were assessed by calculating Pearson correlation coecients. Data were controlled for
sex (analyses were carried out separately for males and females) and age (using the partial correlation test).
Data availability. e datasets analyzed during the current study are available from the corresponding
author on request.
Results
Reliability analysis. Permutation tests of Procrustes distances indicated that dierences in shape among
repeated measurements of specimens were signicantly greater than those among specimens, when landmarks
were placed by the same researcher (p < 0.01). No signicant dierences in shape distances between researchers
were found (p > 0.05). ICC results for the reproducibility of the linear, CSA and angular measurements showed
good to excellent agreement (0.84≤ICC≤0.995 for intraobserver variation and 0.71≤ICC≤0.996 for interob-
server variation)40.
Mandibular shape variation. 37% of shape variation in the sample is explained by the rst and second
principal components of shape (PCs) (Fig.4). Most female mandibles are located in the lower quadrants, whereas
males are scattered mainly in the upper quadrants. e main aspect of shape variation represented by the rst PC
comprises changes in the shape of the mandibular body, which, warping along PC1, varies from being more tri-
angular (right) to more rectangular (le). e main aspect of shape variation represented by the second PC relates
Curve Denition # of
sLMs
1+2Mandibular body (le and right) Passing from the Ramus root (LMs 14/15) along an oblique line to the
midheight of the mandibular symphysis 8
3+4Anterior rim of ramus (le and right) Passing from coronion (LM 26/27) to ramus root (LM 14/15) 10
5+6Inferior margin of mandibular body (le
and right) Passing from Gonion (LM 16/17) to Gnathion (LM 1) 10
7+8Posterior rim of ramus (le and right) Passing from posterior condyle (LM 34/35) to gonion (LM 16/17) 10
9+10 Mandibular notch Passing from anterior condyle (LM 32/33) to coronion (LM 26/27) on the
superior border of the mandibular notch 10
11 Anterior symphysis Passing from infradentale (LM 2) to pogonion (LM 5) in the midsagittal
plane 3
12 Inferior symphysis Passing from pogonion (LM 5) to linguale (LM 3) in the midsagittal plane 6
13 Posterior symphysis Passing from linguale (LM 3) to orale (LM 4) in the midsagittal plane 3
Table 2. Denitions of curves placed on the mandibular surface and number of curve semi-landmarks (sLM).
Figure 2. Landmarks (blue), curves (red) and curve semi-landmarks (yellow) placed on a 3D surface mesh of a
mandible, see Tables1 and 2 for denitions.
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to the ascending ramus which varies in shape between an elongated narrow parallelogram (lower) to a wide low
trapezoid (upper), with the coronoid process varying in shape between an elevated-narrow-pointed structure to
a low-wide-rounded one (Fig.4).
The association between mandibular shape and muscle CSA. 2B-PLS analyses between the rst
singular warps (SW1) of mandibular shape and the CSAs of the masseter and temporalis muscles, for both males
Measurement Denition
Bi-gonial breadth Distance between right and le gonion
Mandibular angle e angle formed by the inferior border of the mandibular body and the posterior border of the ramus
Mandibular angle width e distance between the gonion and deepest point on the concavity connecting the anterior border of
the ramus with the mandibular body
Mandibular angle width CSA e cross-sectional area of the mandibular body along the mandibular angle width line
Ramus length e distance from the highest point on the condyle to the gonion
Ramus width e distance between the anterior and posterior indentations of the mandible ramus
Ramus width CSA e cross-sectional area of the mandibular ramus along the ramus width line
Coronoid width e distance between the deepest point on the mandibular notch and the anterior border of the coronoid
process
Coronoid width CSA e cross-sectional area of the mandibular ramus along the coronoid width line
Coronoid height e vertical distance between the most superior point of the coronoid process and the coronoid process
width line, perpendicular to it
Mandibular body length e distance from the most anterior point of the chin to a line placed along the posterior border of the
ramus
Mandibular body height
(P1-P2 and M2-M3) e vertical distance from the alveolar crest between the 1st and 2nd premolars, or distal to the 2nd
mollar, to the inferior border of the mandibular body
Mandibular body CSA (P1-P2
and M2-M3) e cross-sectional area of the mandibular body along the body height line
Symphysis thickness In the midsagital plane, the distance between the Pogonion and the most posterior point of the symphysis
Chin height e distance between the menton and the deepest point of the concavity between the posterior
infradentale and pogonion
Table 3. Linear, angular and cross-sectional area (CSA) measurements of the mandible.
Figure 3. Linear, angular and cross-sectional area measurements of the mandible, see Table3 for denitions.
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and females, yielded high and signicant correlations (r = 0.734, p < 0.001 and r = 0.697, p < 0.001, respectively)
(Fig.5). e visualization of the PLS for both males (Fig.6) and females (Fig.7) demonstrates that mandibles with
large muscle CSAs manifest a wider more trapezoidal-shaped ramus, more massive coronoid, rectangular body
and a curved basal arch. Mandibles with small CSA are characterized by a tall and narrow ramus (more like a
parallelogram) with a pointed coronoid, triangular body and a more triangular basal arch.
The association between mandibular metric characteristics and muscle areas. Associations
between linear measurements and muscle CSAs controlled for MGM appear in Table4. Most mandibular
measurements manifested either signicant, small correlations or no signicant correlations with muscle CSAs
(Table4). is analysis has yielded three types of parameters: 1. Parameters not associated with muscle CSAs:
mandibular angle, mandibular angle width, coronoid width, coronoid width CSA, body length, body height at
premolars and its CSA. 2. Parameters associated with muscle CSAs in either males or females. For females: bigo-
nial breadth (with masseter CSA). For males: mandibular angle CSA (with both muscle CSAs), ramus width and
its CSA (with temporalis CSA), body height at molar (with temporalis CSA), body height CSA at molar (with
both muscle CSAs) and symphysis thickness (with temporalis CSA) and chin height (with masseter CSA). 3.
Parameters associated with muscle CSAs for both males and females: ramus length (with masseter CSA) and
coronoid height (with both muscle CSAs).
Discussion
e current study shows that mandibular shape varies to a certain extent as a function of the forces applied to
it by the temporalis and masseter muscles (Fig.5). is is anticipated based on prior studies; “…the size and
shape …of the jaws should reect muscle size and activity”13 (p. 136). e major aspects of mandibular shape
that covary with muscle CSAs, independent of sex, are, with larger CSAs, a wider trapezoidal ramus, a massive
coronoid, a more rectangular body and curved basal arch. In contrast, mandibles with a tall and narrow ramus
Figure 4. Principal component analysis of shape variation in the studied sample: Shape variables following
general Procrustes analysis. e rst two Principal Components (PCs) explain 37% of total variance.
Figure 5. Plot of SW1 (mandibular shape) against SW1 (muscle CSA) from a two block partial least squares
analysis in males (a) and females (b). Scores on these axes are signicantly correlated (r = 0.734, p < 0.001 and
r = 0.697, p < 0.001, respectively).
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Figure 6. Warpings along SW1 of mandible shape in males. Large muscle CSAs are associated with a wider,
more trapezoidal ramus, more massive coronoid, rectangular body and a more curved basal arch. Mandibles
with smaller muscle CSAs are characterized by a tall and narrow ramus (more like a parallelogram) with a
pointed coronoid, triangular body and a more triangular basal arch.
Figure 7. Warpings along SW1 of mandible shape in females. Large muscle CSAs are associated with a wider
more trapezoidal ramus, more massive coronoid, rectangular body and a curved basal arch. Mandibles with
smaller muscle CSAs are characterized by a tall and narrow ramus (more like a parallelogram) with a pointed
coronoid, triangular body and a more triangular basal arch.
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(parallelogram-like), a more pointed coronoid, a more triangular body and a more triangular basal arch were
associated with smaller muscle CSA (Figs6 and 7). In the absence of studies that directly measure the associa-
tion between mandibular shape and masticatory muscle CSAs, our discussion is largely based on circumstantial
evidence, namely, the associations between mandibular morphology and dental attrition (i.e., indicating exten-
sive function of the masticatory muscles) and mandibular morphology and subsistence economy (i.e., soer
diet requires less mastication force). For example, several anthropological studies have reported an association
between excessive attrition and broad mandibles41–44. It has been shown that agriculturalists (soer diet) had
relatively short and broad mandibles with a tall, angled ramus and coronoid process, whereas hunter-gatherer
populations (harder diet) have relatively long and narrow mandibles with a short, upright ramus and coronoid
process24. ese results are in agreement with our observations.
Modern population studies oer similar insights. For example, individuals suering from bruxism manifest
broad mandibles45–47; subjects with strong bite forces tend to have a low mandibular plane angle and wide man-
dible, whereas those with weak bite force tend to have a high mandibular plane angle and narrow mandible1,48,49.
Direct evidence for an association between mandibular morphology and masticatory muscle force comes
from clinical studies. For example, in individuals suering from myotonic dystrophy of the masticatory muscles
a greater mandibular angle and excessive vertical growth of the mandible was reported (e.g.6,50); and enlargement
of the coronoid process was observed in individuals with temporalis muscle hyperactivity51.
Several animal experimental studies provide further support for this association. For example, pigs raised on a
so rather than a normal diet, manifested changes in jaw morphology and dental arch dimensions52; and reduced
function of the masticatory system in rats caused changes in the width, height and thickness of the alveolar pro-
cess and smaller cross-sectional area of the bone16,17,53,54.
Of the 17 linear parameters used in our study, 10 manifested signicant low associations with muscle CSAs.
Yet, these associations varied with sex and muscle (temporalis and/or masseter). Only two linear measurements
(coronoid height and ramus length) showed signicant, but weak, associations with muscle CSAs in both males
and females when controlled for size. ese results coincide with our shape analysis and highlight some of the
biomechanical factors involved in mandibular design. For example, the anterior ramal border, from coronoid
process downward, is under considerable tension during mastication55, potentially explaining the involvement
of the temporalis and masseter muscles in shaping the ramus and coronoid. e increase in mandibular CSAs
at the ramus, mandibular angle and body at the molar region, with muscle CSAs is in accordance with previous
studies suggesting that the thickening and increase in height of the posterior part of the mandibular body with
increased muscle strain is to enable the mandible to resist the parasagittal and transverse bending stresses, which
are concentrated in these regions56–59. e idea of bone apposition over areas with increased demand to withstand
bending force has been demonstrated in several human and animal studies (e.g.16,17,60).
Finally, all linear measurements in our study show, aer correction for size, low correlations with muscle
CSAs. is raises the question of why our ndings do not support those of previous studies (e.g.14,28,49) that found
high correlations. is might be because studies suggesting much higher correlations between mandibular linear
measures and mastication force (e.g.28,49) did not correct their data for mandibular size. It is noteworthy that very
few allometrically adjusted anthropometric variables show signicant correlations with muscle CSAs. ose that
do, largely reect the ndings of the PLS analyses of Figs5–7 in that they measure ramus and coronoid form.
However, given the strength of these associations they are likely useful only to predict the strength of masticatory
Measurement
Masseter CSA Temporalis CSA#
Males Females Males Females
Bigonial breadth#0.093 0.407** 0.050 0.099
Mandibular angle −0.088 −0.028 0.053 −0.079
Mandibular angle width 0.126 0.003 0.168 0.068
Mandibular angle CSA 0.194*0.033 0.307** 0.031
Ramus length 0.290** 0.280** 0.152 0.047
Ramus width 0.121 −0.078 0.214*−0.035
Ramus width CSA 0.099 0.078 0.258** 0.039
Coronoid width 0.021 −0.131 0.082 0.044
Coronoid height −0.350** −0.272** −0.282** −0.130
Coronoid width CSA 0.055 0.097 0.173 0.148
Body length 0.048 0.093 −0.091 0.072
Body height at premolar −0.065 0.018 0.003 −0.081
Body height at molar#0.151 0.046 0.185*−0.062
Body height at premolar CSA 0.127 0.134 0.127 0.010
Body height at molar CSA 0.211*0.114 0.336** 0.032
Symphysis thickness 0.124 0.176 0.198*−0.013
Chin height 0.189*−0.048 0.062 −0.032
Table 4. Partial correlations1 between masticatory muscle CSAs and mandibular measurements$. 1Control for
age. $Muscle CSAs and mandibular measurements, except for mandibular angle, were controlled for mandibular
size (MGM). #Following logarithmic transformation. *p < 0.05; **p < 0.01.
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muscle action among sample means rather than individuals. Indeed, the weak correlations shown by all variables
stand in contrast to the PLS analyses of landmark data which nd signicant overall associations. is nding
emphasizes the need to take a multivariate or landmark based approach to dietary retrodiction in archaeological
populations. Even with such an approach, population loading history is most reliably inferred, rather than the diet
or masticatory muscle force of any one individual.
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Acknowledgements
e authors wish to thank the Dan David Foundation and the Israeli Science Foundation (grant no.1116/16) for
their nancial support.
Author Contributions
S.T.T. carried out the metric measurements. M.H. created the G.M. protocol and P.A. applied it on the mandible
sample. M.H. and S.R. carried out the statistical analysis. M.H. and O.P. wrote the main manuscript text. All
authors read and approved the nal manuscripts.
Additional Information
Competing Interests: e authors declare no competing interests.
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