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Human mandibular shape is associated with masticatory muscle force


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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.
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ScienTiFic REPORTS | (2018) 8:6042 | DOI:10.1038/s41598-018-24293-3
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.36). 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)712. 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 bone1618.
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 strategies2124, 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:
Received: 18 October 2017
Accepted: 27 March 2018
Published: xx xx xxxx
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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 muscles7,9,11,12,2733.
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. 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. 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 variation1921 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,
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,
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.
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 (0.84ICC0.995 for intraobserver variation and 0.71ICC0.996 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
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
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
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).
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 mandibles4144. 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 mandibles4547; 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 regions5659. 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57 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
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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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
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... En effet, la morphologie crânio-faciale se dessine par le remodelage successif que subissent les structures osseuses au cours de leur croissance (Enlow, 1966;Loth & Henneberg, 2001). Le développement de forces musculaires, en particulier au cours de la puberté, modèle la forme du crâne et de la mandibule et conduit à la formation de crêtes et reliefs osseux au niveau des zones d'insertion musculaire (Bejdová et al., 2018;Sella-Tunis et al., 2018). Du fait d'une musculature plus développée et d'une force masticatoire accrue, les individus masculins ont tendance à présenter un système squelettique plus robuste. ...
... Les individus édentés partiellement ou totalement génèrent des forces de mastication plus faibles que les dentés complets du fait de la perte des contacts occlusaux (Hatch et al., 2001). Cette réduction de l'activité musculaire peut mener à des changements de conformation de la mandibule, en particulier au niveau des zones d'insertion des muscles masticateurs, à savoir la région goniaque, condylienne et le processus coronoïde (Sella- Tunis et al., 2018). De plus, la perte dentaire induit une résorption au niveau des procès alvéolaires, puisque leur principale fonction est d'apporter un soutien structurel à la dentition (Cawood & Howell, 1988;Enlow et al., 1976). ...
La mandibule est un os du massif facial aux caractères dimorphiques très marqués, pouvant être employé dans l'estimation du sexe d'un individu. Étant le seul os mobile de l'extrémité céphalique, elle peut être séparée du crâne dans des contextes archéologiques ou médico-légaux. Par ailleurs, il s'agit d'un os soumis à des phénomènes de remodelage osseux de par la perte dentaire et les contraintes mécaniques qu'il subit au cours de la fonction masticatoire. L'objectif de ce travail était, d'une part, d'améliorer la performance des techniques d'estimation sexuelle à partir de la mandibule et, d'autre part, d'avoir une meilleure compréhension des changements morphologiques que subit cette structure au cours du vieillissement. Dans un premier temps, nous avons étudié le rôle de la mandibule dans le dimorphisme sexuel de l'extrémité céphalique. Des méthodes métriques et morpho géométriques, basées sur le positionnement informatique de landmarks, ont été employées sur 120 examens tomodensitométriques d'individus âgés de 23 à 84 ans. Nos résultats ont montré que la morphométrie géométrique offre une précision de diagnose sexuelle supérieure à la méthode métrique traditionnelle. Par ailleurs, le crâne présente une plus grande précision d'estimation sexuelle que la mandibule, quelle que soit la méthode d'analyse employée. Enfin, le taux de prédiction correcte du modèle mandibulaire s'améliore à partir de 40 ans. Dans un deuxième temps, nous avons analysé l'effet du vieillissement et de la perte dentaire sur la conformation mandibulaire, ainsi que l'impact de ces changements morphologiques sur l'identification du sexe d'un individu. 14 landmarks mandibulaires ont été placés sur 160 examens tomodensitométriques de sujets âgés de 40 à 79 ans. Nos analyses par morphométrie géométrique montrent que le dimorphisme sexuel mandibulaire demeure significatif avec le vieillissement et que les changements de conformation mandibulaire débutent à 50 ans. En revanche, la sénescence affecte différemment les individus masculins et féminins : le processus semble être plus précoce, plus rapide et plus accentué chez la femme, et les changements de conformation touchent des zones différentes selon le sexe. Par ailleurs, l'édentement, en particulier la perte de calage dentaire, entraîne des modifications de conformation différentes entre hommes et femmes. Il tend ainsi à estomper le dimorphisme sexuel de taille et à accentuer le dimorphisme sexuel de conformation.
... Masticatory muscle function is regarded as one of the determinants of mandibular shape [38]. A previous finding indicated that mandibles with smaller muscle force were characterized by a tall and narrow ramus (more like a parallelogram) [39]. In a clinical setting, when there are several impacted permanent teeth and/or supernumerary teeth on panoramic radiography with diagnostic uncertainty, the contour of the mandible and zygomatic arch should be emphasized. ...
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Background: Cleidocranial dysplasia (CCD) is a rare and underdiagnosed congenital disorder in dentistry. The purpose of this study was to illustrate and quantify the maxillofacial bone abnormalities detected on panoramic radiographs from a relatively large retrospective case series and to provide a series of diagnostic references for dentists to indicate the presence of disease and help in making an early and accurate diagnosis. Methods: The dental panoramic radiographs of thirty CCD patients aged 11 to 45 years (18 males and 12 females) were examined retrospectively. The dentition states, including supernumerary teeth and impacted teeth, were recorded. Twelve quantified measurements were adopted to determine the abnormalities of maxillofacial bones, including the degree of the zygomatic arch downward bend, bicondylar breadth, ramal height, mandibular height, mandibular aspect ratio, mandibular body height, condylar height, coronoid height, distance between the coronoid process and the condyle, bigonial width, gonial angle and best-fit gonial circle diameter. The Wilcoxon rank-sum test was used to compare the findings of the CCD patients with those of their matched controls (n = 300). Results: Supernumerary teeth were detected in 27 patients (90.0%), and all 30 patients presented impacted teeth. Compared to the matched controls, the CCD patients had a significantly larger degree of zygomatic arch downward bend (ZAD), a larger diameter of the best-fit gonial circle (BGC), and a shorter distance between the coronoid process and the condyle (DCC) in panoramic radiographs (P < 0.001). According to the reference cutoff values established from the 5th or 95th percentile of the measurements in the control group, ZAD higher than 6.90 mm, DDC less than 22.37 mm and BGC higher than 52.41 mm were significantly associated with the CCD features identified. Other panoramic measurements were not significantly different between the two groups. Conclusions: Panoramic radiographs had great value in the diagnosis of CCD. In this study, we identified some dental and maxillofacial features on panoramic radiographs from a relatively large retrospective case series of CCD. A series of reliable quantitative indicators were provided for dentists that can indicate the presence of disease and improve the diagnostic specificity.
... Increased neuromuscular activity and training such as masticatory forces, can result in muscle hypertrophy, metabolic and cross-sectional area changes, and may change the fibertype composition of the muscle towards a larger percentage of slow-type fibers [33]. Low angle group with increased muscle activity [34] have larger muscle cross-sectional area [35] as well as wider trapezoidal-shaped ramus, bigger coronoid, rectangular body, curved basal arch and wider MS. ...
This study aimed to investigate the general mandibular symphysis (MS) shape variation among Class III skeletal base, using geometric morphometric analysis. Pre-treatment lateral cephalometric radiographs of 254 patients aged 11-40 years old, with Class III skeletal base (ANB <1°) and lower incisor angle (<99°) were included. Nine-landmarks with x and y coordinates were identified on MS using TPSDig2 software, then exported into Morpho J for shape and statistical analysis. Principal component analysis showed that three main shape dimensions with a total variance of 74.6% represented the majority variation of samples. Procrustes Anova showed the shape of MS in Class III skeletal base to be mainly influenced by gonial angle, incisor inclination and sex (P<0.0001). Canonical variate analysis showed that high gonial angle groups had significantly narrower and elongated MS whereas low gonial angle groups had wider, bulbous and rounded MS (P<0.0001). The ratio of alveolar part to basal part was 1:5 in low gonial angle and 2:3 in high gonial angle. Males had significantly taller MS with narrower B point area compared to females (P<0.0001). Retroclined incisors exhibited taller and retroclined alveolar parts (P<0.0001). The shape of MS in Class III skeletal base varied at the alveolar part, basal part or both and it is influenced by gonial angle, incisor inclination and sex. Hence, understanding the shape variation of MS is important to aid orthodontic treatment planning.
... Furthermore, bones and muscles are in close crosstalk; a better understanding of the relationship between craniofacial deformities and muscles helps understand the relationship between malocclusion and mandible deformities [24,25]. Rodrigues et al. revealed that experimental removal of the masseter muscle during the growing period in rats induced atrophic changes and shortening of the whole mandible [26]. ...
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(1) Objective: This study aimed to evaluate the association between unilateral premolar scissors bite and mandibular symmetry of adults via the 3D distance mapping method. (2) Methods: A total of 53 cone-beam computed tomography (CBCT) images of adults with unilateral premolar scissors bite were set as study samples. A total of 53 age- and sex-matched samples without scissors bite were in the control group. Three-dimensional mandibular models and seven mandibular functional units, including condylar process (Co), coronoid process (Cr), mandibular ramus (Ra), mandibular angle (Ma), alveolar process (Ap), mandibular body (Mb), and chin process (Ch) were constructed and mirrored. After superimposition of the original and the mirrored models, 3D distance maps and deviation analysis were performed to evaluate the mandibular symmetry and morphology. (3) Results: In the study group, the matching percentages of the entire mandible (50.79 ± 10.38%), Ap (67.00 ± 12.68%), Mb (66.62 ± 9.44%), Ra (62.52 ± 11.00%), Ch (80.75 ± 9.86%), and Co (62.78 ± 13.56) were lower than that of the entire mandible (58.60 ± 5.52) (p < 0.01), Ap (73.83 ± 8.88%) (p < 0.01), Mb (72.37 ± 8.69%) (p < 0.01), Ra (68.60 ± 7.56%) (p < 0.01), Ch (85.23 ± 6.80%) (p < 0.01), and Co (67.58 ± 10.32%) (p < 0.05) in the control group. However, Cr and Ma showed no significant difference (p > 0.05). (4) Conclusions: The 3D distance mapping method provided a qualitative and quantitative mandibular symmetry and morphology assessment. Mandibular asymmetry was found in adults with unilateral premolar scissors bites. Mandibular functional units, including the alveolar process, mandibular body, mandibular ramus, chin process, and condylar process, showed significant differences, while no significant difference was observed in the coronoid process and mandibular angle.
... 50 An experimental investigation foundthat consumption of soft diet offersreduced masticatory activity on the mandible resulting in morphological remodelling of the mandible,such as reduce alveolar bone size. 51 In consonance with Yamada and Kimmel 52 , craniofacial growth has a direct relationship with dietary habit and masticatory function, specially affecting the mandible, which mayinfluence third molar impaction.The growth pattern of ramus of the mandible is related to resorption of its anterior surface and deposition of its posterior surface. Regardless of whether this process is in disequilibrium or not, the mandibular third molars will typically do not have enough space to erupt. ...
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Background: Third molars positions and eruption patterns tend to be unpredictable in most cases. Substantial diversity exists globally among modern human races in the prevalence of third molar impaction. Aims and Method: This study aimed to investigate the prevalence and pattern of third molar impaction amongBangladeshi adults. Digital panoramic radiographs of 5923 patientswith the mean age of 35.90 ± 10.76 years olds were retrieved from database and evaluatedusingPlanmecaRomexis software.Demographics, gender and sidedifferences wereanalysed using SPSSversion 26.0. Result:Approximately, 46.2% of the adult population had third molar impaction where significant impaction occurred in the mandible compared to maxillary arch. However, no significant differences were observed among gender and side distribution. The most common type of third molar angulation in the maxilla and mandible was distoangular (55.9%) and mesioangular (36.6%), respectively.Comprehension of demographic and morphological variations in third molar impaction will lead to an understanding of third molar impaction assessment, which will aid in understanding the evolutionary origins of an important condition adversely affecting modern peoples. Bangladesh Journal of Medical Science Vol. 21 No. 03 July’22 Page: 717-729
... Moreover, Sella-Tunis et al. showed that the shape of a wide mandible is related to the strength of the large masticatory muscles. They showed that larger muscles controlled for sizes correlated with a wider, more massive coronoid, more trapezoidal ramus, more curved basal arch, and a more rectangular body [16]. Finally, Gionhaku et al. and Kiliaridis described the relationship between brachycephalic appearance and a strong masticatory force. ...
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In this study, we evaluated changes in the masseter and lateral pterygoid muscles in the prognathic mandible group after a mandibular setback by comparing the volume-to-length ratios. Preoperative and postoperative 1-year computed tomography was used to calculate the volume-to-length ratio of the lateral pterygoid and masseter muscle in 60 Korean individuals. Three-dimensional images were reconstructed, the results of which showed no significant differences in the volume-to-length ratios of the masseter and lateral pterygoid muscles after a mandibular setback (p > 0.05). This result was found for both vertical ramus osteotomy and sagittal split ramus osteotomy, and for both males and females. No significant differences in the volume-to-length ratio of the masseter and lateral pterygoid muscles were found up to 1 year after a mandibular setback. Therefore, this study can contribute to the prediction of soft-tissue profiles after mandibular setback.
Introduction: The morphology of the human face varies broadly, with genetic and environmental factors determining these variations. Examining variations in the 3-dimensional (3D) craniomandibular morphology and identifying related factors (eg, sex differences) are important in orthodontic clinics. This study observed shape variations in the 3D facial morphology of Japanese adults showing skeletal Class 1 malocclusion and examined the association of sexual dimorphism with shape variations. Methods: Sixty cone-beam computed tomography images of Japanese adults (30 males and 30 females) with skeletal Class I malocclusion were employed. In each cone-beam computed tomography image, wire mesh fitting was conducted as previously described. A principal component (PC) analysis after Procrustes registration and the PC clustering method was conducted to observe the shape variations. A PC regression analysis was conducted to determine the sexual morphologic characteristics. Results: Nine PCs depicting 62% of the morphology were determined. Four typical phenotypes were found, mainly related to mandibular protrusion (PC1) and the vertical divergence of the face (PC2). PCs related to sex determination were PC3 (robustness of the mandibular angle in males), PC5 (greater size and shape of the coronoid and mastoid processes in males), and PC7 (greater maxillary width in males), accounting for 16% of total variations. Conclusions: The major shape variations in skeletal Class 1 subjects were related to nonsexual dimorphic characteristics (ie, mandibular protrusion and facial divergence). Sexual dimorphic characteristics were evaluated in detail and accounted for 16% of total morphologic variations.
If the mandible is missing, a large section of the lower face must be interpreted and estimated for shape and form. This is the case of Dante Alighieri whose original mandible was never found. In this study a new virtual reconstruction of his mandible was designed based on a mathematical method. In this work a new virtual reconstruction of the mandible was designed based on linear measurements of the skull. A three-dimensional standard mandible was designed and modelled on the size of Dante's skull, previously virtually reproduced by scanning the plaster model made by the anthropologist Fabio Frassetto in 1921 with a 3D scanner and imported into a 3D graphic modelling software. After the preliminary work to reconstruct the mandible, the skull was completed and a new 3D virtual facial reconstruction of Dante was developed according to the methods commonly used in forensic contexts.
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Genome-wide association studies (GWAS) identified thousands of genetic variants linked to phenotypic traits and disease risk. However, mechanistic understanding of how GWAS variants influence complex morphological traits and can, in certain cases, simultaneously confer normal-range phenotypic variation and disease predisposition, is still largely lacking. Here, we focus on rs6740960 , a single nucleotide polymorphism (SNP) at the 2p21 locus, which in GWAS studies has been associated both with normal-range variation in jaw shape and with an increased risk of non-syndromic orofacial clefting. Using in vitro derived embryonic cell types relevant for human facial morphogenesis, we show that this SNP resides in an enhancer that regulates chondrocytic expression of PKDCC - a gene encoding a tyrosine kinase involved in chondrogenesis and skeletal development. In agreement, rs6740960 SNP is sufficient to confer a large difference in acetylation of its cognate enhancer preferentially in chondrocytes. By deploying dense landmark morphometric analysis of skull elements in mice, we show that changes in Pkdcc dosage are associated with quantitative changes in maxilla, mandible, and palatine bone shape that are concordant with the facial phenotypes and disease predisposition seen in humans. We further demonstrate that the frequency of the rs6740960 variant strongly deviated among different human populations, and that the activity of its cognate enhancer diverged in hominids. Our study provides a mechanistic explanation of how a common SNP can mediate normal-range and disease-associated morphological variation, with implications for the evolution of human facial features.
Diffusion Tensor Imaging (DTI) is a technique developed from Magnetic Resonance Imaging (MRI), which uses a mathematical form diffusion tensor to measure the movement of water molecules in biological tissues in vivo. By performing fibre tracking using diffusion tensor data, we can study the micro-structure of biological tissues in a non-invasive way. Skeletal muscle plays a significant role in force and power generation that contribute to maintaining body postures and to controlling its movements. DTI fibre tracking may re-construct the skeletal muscle in a fascicle level. Procrustes analysis is a landmark-based method for studying the shapes of objects. In this paper, we explore using Generalised Procrustes Analysis to study the fascicle shapes that we have collected in medial gastrocnemius muscles from 6 healthy adults by using DTI technology. This is an innovated attempt of using Procrustes analysis to find the trend of the changes of fascicle shape when foot is in plantarflexion and dorsiflexion, by clustering method.KeywordsProcrustesShapeAnalysisDTISkeletal muscleFascicle
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Significance Agriculture changed not only human culture and lifeways, but human biology as well. Previous studies indicate that softer agricultural diets may have resulted in a less robust craniofacial morphology in early farmers. However, obtaining reliable estimates of worldwide subsistence effects has proved challenging. Here, we quantify changes in human skull shape and form across the agricultural transition at a global scale. Although modest, the effects are often reliably directional and most pronounced in craniofacial features that are directly involved in mastication.
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Sex estimation of skeletal parts is of great value even in the DNA era. When computed tomography (CT) facilities were introduced to forensic institutes, new possibilities for sex estimation emerged. The aim of this study was to develop a CT-based method for sex estimation using the mandible. Twenty-five CT-based measurements of the mandible were developed and carried out on 3D reconstructions (volume rendering) and cross sections of the lower jaw of 438 adult individuals (214 males and 224 females). Intraobserver and interobserver variances of the measurements were examined using intraclass correlation coefficient (ICC) analysis. Five discriminant functions were developed using different states of completeness of the mandible. The success rates of these equations were cross validated twice. The measurements were found to be highly reliable (for intraobserver 0.838 < ICC < 0.995 and for interobserver 0.71 < ICC < 0.996). For a complete mandible, the correct classification rate was 90.8%. For incomplete mandibles, the correct classification rates varied from 72.9 to 85.6%. Cross-validation tests yielded similar success rates, for the complete mandible 89% and for the incomplete mandible 67.5 to 89%. We concluded that CT techniques are appropriate for estimating sex based on the mandible size and shape characteristics. Suggested discriminant functions for sex estimation are given with data on the correct classification rates.
Objectives: To examine the femoral midshaft morphological characteristics in hunter-gathering Natufian and farming Pre-pottery Neolithic (PPN) populations in the southern Levant and relate these to changes in mobility, physical stress, and diet. Materials and methods: 32 Natufian, 41 PPNB, and 26 PPNC femora, dating from 14,900 to 8,250 cal BP, were studied. Femoral diaphyseal cross-sectional images were obtained from CT scans. Dedicated software was used to measure cross-sectional breadths, areas, cortical bone thickness, rigidity, and strength. Results: Two general temporal trends in femoral bone architecture were observed: (1) a continuous decline in the relative amount of bone tissue (cortical area/total area) due to expansion of the medullary cavity and (2) an increase in circularity (decrease in anteroposterior/mediolateral ratios) together with an overall decline in bone rigidity and strength, mainly apparent in the later PPNC. The first trend suggests a gradual decline in nutritional quality and health continuing from the Natufian through the late Neolithic. The second trend is interpreted as a result of increased sedentism with the full establishment of agriculture. Discussion: The transition to food production in the southern Levant was accompanied by reduced physical stress and mobility, with the most marked effects occurring toward the end of the PPN with increasing sedentism. Deterioration of nutrition and health also occurred, but more continuously from the beginning of the PPN. Thus, environmental changes associated with the agricultural transition in this region of the world were gradual and prolonged, with direct dietary effects more apparent earlier than reductions in mobility. Am J Phys Anthropol, 2016. © 2016 Wiley Periodicals, Inc.
The aim of this investigation was to measure the functional effects of different occlusal strains on the growth of the dentoalveolar process of the rat mandible. Forty-two growing male albino rats were divided into three groups: one group received a hard diet, one a soft diet, and the remainder were killed at the beginning of the experiment. The experimental period was 28 days. After the animals were killed, the mandible halves were prepared, and undecalcified transversal sections of 120 μ were cut. Microradiograms were taken of the sections from the dentoalveolar molar region, and morphometric analysis was performed by means of a computer-assisted image analysis system. Significant differences were found in the alveolar process of both the molars and the incisor. The cross sectional area and the width of the molar alveolar process, and the width of the molar periodontal space were found to be smaller in the soft diet group. The alveolar process of the incisor was characterized by a smaller transversal area and poorer bone quantity, poorer bone 'quality' and a shorter vertical dimension of the process in the group with the reduced masticatory load. It seemed that high occlusal strain induced bone adaptation in the molar alveolar process of the mandible. This was to support the loading on the occlusal surface of the molars during mastication in the hard diet group. The increased strain also induced changes in the alveolar process of the incisor, which improved the mechanical resistance of this bone region to better withstand the bending forces applied to the tooth during biting.
The human skull is gracile when compared to many Middle Pleistocene hominins. It has been argued that it is less able to generate and withstand high masticatory forces, and that the morphology of the lower portion of the modern human face correlates most strongly with dietary characteristics. This study uses geometric morphometrics and finite element analysis (FEA) to assess the relationship between skull morphology, muscle force and cranial deformations arising from biting, which is relevant in understanding how skull morphology relates to mastication. The three-dimensional skull anatomies of 20 individuals were reconstructed from medical computed tomograms. Maximal contractile muscle forces were estimated from muscular anatomical cross-sectional areas (CSAs). Fifty-nine landmarks were used to represent skull morphology. A partial least squares analysis was performed to assess the association between skull shape and muscle force, and FEA was used to compare the deformation (strains) generated during incisor and molar bites in two individuals representing extremes of morphological variation in the sample. The results showed that only the proportion of total muscle CSA accounted for by the temporalis appears associated with skull morphology, albeit weekly. However, individuals with a large temporalis tend to possess a relatively wider face, a narrower, more vertically oriented maxilla and a lower positioning of the coronoid process. The FEAs showed that, despite differences in morphology, biting results in similar modes of deformation for both crania, but with localised lower magnitudes of strains arising in the individual with the narrowest, most vertically oriented maxilla. Our results suggest that the morphology of the maxilla modulates the transmission of forces generated during mastication to the rest of the cranium by deforming less in individuals with the ability to generate proportionately larger temporalis muscle forces.