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The relationship of facial asymmetry and bite force to handedness in Iraqi adult
sample
Dr.Hayder A. Kadhim, B.D.S., M.Sc.
Dr.Hayder F. Saloom, B.D.S., M.Sc.
Dr.Suhad M. Ali, B.D.S., M.Sc.
Abstract
Facial asymmetry either functional and/or structural is a normal finding in clinically
symmetrical faces. The aim of this study was to investigate the relationship of the amount and
direction of facial asymmetry in clinically symmetrical faces with class I normal occlusion to
handedness, and to discover if there is any relation for occlusal bite force with handedness and
facial asymmetry in Iraqi Arab adult sample.
The sample included 60 untreated Iraqi adult persons, 30 right handers, 30 left handers and
each group consisted of 15 males and 15 females, with age range 18-25 years. For each subject a
posteroanterior radiograph was taken, and then a maximum molar bite force was measured digitally
in the first molar area on each side.
All subjects showed skeletal asymmetry although they have normal occlusion, and it was
significantly greater in right handers than in left handers (P<0.05). In right-handers, the mean facial
area on the left side was significantly greater than that on the right side (P=0.000). In left handers,
the mean facial area on the right side was found to be significantly greater than that on the left side
only in females (P=0.004). The bite force, in both groups, was larger in males than in females, with
significant difference in right handers (P=0.03) and highly significant difference in left handers
(P=0.001).
Facial dimensions tend to be larger in males than in females. Skeletal asymmetry was highly
correlated to handedness. Bite force was independent of handedness with significantly greater molar
bite force in males than females.
Keywords: facial asymmetry ,bite force, handedness, normal occlusion.
Introduction
Each individual shares many
characteristics with the rest of population
but still unique in his own sense. This
uniqueness is exhibited due to variations
in size, shape and relationship of
skeletal, dental and soft tissue facial
structures. These variations may also exist
within the same individual on the opposite
sides of median sagittal plane of face 1.
Lateral asymmetry (functional and
structural) is a common finding in human
and animal. The various aspects of the
lateral asymmetry, particularly in human,
long have been the subject of the study and
research, especially in relation to
handedness, asymmetries of the two
hemispheres of the brain, facial asymmetry
and the relation between them 2. Some
studies of normal asymmetry have
documented no significant difference
between right and left hemiface size 3, 4,
while other studies have found that the
right hemiface is usually wider than the left
1, 4, 5.
Investigations into the location and
extent of the asymmetry have used the
Orthopantomographs 7-9, three-dimensional
laser scanning 10. Postero-anterior (PA)
Cephalometric studies have typically used
to study the presence of a certain degree of
asymmetry in the bony face 11.
Brain functional asymmetry is well
established in human for language and
motor regions and it correlates with
handedness. Right cerebral cortex regulates
movement of the left side of the body,
while the left cerebral cortex regulates
movement of the right side, and more than
90% of the human population is naturally
more skilled with the right hander than
with the left 12.
Bite force is one of the components of
the chewing system, which may be
influenced by dental occlusion,
craniofacial morphology, and masticatory
muscle thickness 13.
Facial asymmetry was found to be
related to handedness in both
genders14,other study 15 suggested that
craniofacial asymmetries might be a
consequence of crossbite malocclusion, but
no previous study had been made to
correlate the handedness and facial
asymmetries with the masticatory bite
force, so this study aimed to discover if
there is any relation of occlusal bite force
with handedness and facial asymmetry in
Iraqi adults.
Material and Methods
Out of (195) clinically examined Iraqi
subjects, only (60) subjects (30 right-
handers and 30 left-handers each group
consists of 15 males and 15 females) were
selected as they fulfill the criteria of the
sample specification, with an age range of
18-25 years. They were mostly collected
from the undergraduate students of the
College of Dentistry, University of
Baghdad and Al-Mustansiriyah University.
A small part was selected from patients
who were attending College of Dentistry,
University of Baghdad.
None had apparent clinical facial
asymmetry; enamel crack, heavy filling or
periodontal disease in the first molar;
history of orthodontic treatment and/or
orthognathic surgery; deviation of the
mandible during opening and closing; Uni-
or bilateral posterior cross bites and open
bite; temporomandibular joint dysfunction;
documented systemic disease. All the
subjects showed Full permanent dentition
excluding third molars; Symmetrical molar
occlusion on right and left sides which is
CL.I angle classification, no or minimal
crowding and/or spacing not exceed 2mm..
Hand preference was assessed by
asking each participant about his/her
preferred hand for working and writing.
Extraoral examination was done through
inspection to check for any obvious facial
asymmetry, any deviation of the mandible
on closing and opening. The intraoral
examination was done to check fulfillment
of subjects for other required sample
criteria.
To measure the surface areas of the
facial regions, posteroanterior radiographic
cephalograms for each participant was
taken. The angle of the subject’s head
adjusted until the Frankfort plane became
horizontal and parallel to the floor. The
subject was asked to occlude the teeth in
centric occlusion.
Maximum molar bite force
measurement was performed while each
participant was sitting relaxed on a chair in
upright position and looking forward; with
both back and hand rest and the feet easily
reached the ground. After covering the
biting element of the device with a new
plastic tube, put the device in the first
molar area and ask the participant to bite
firmly for a few seconds as much as he/she
can. This measurement was repeated twice
for each side in reversed order after 2-3
minutes interval. The bite force was
displayed digitally (in Newton); the highest
value was taken for each side.
To assess the relative asymmetry in
the component areas of the facial complex,
the right and left halves of the face were
divided into several triangles using skeletal
landmarks as a reference points on the
radiographs, so that they were joined by
drawing lines 14. Then the area of each
triangle was calculated and compared with
that on the opposite side. The following
triangles were drawn on the cephalograms,
figure (1): Triangle A (cranial base)
between the extremes superior extent of
the head of the condyle and mesial extent
of the head of the condyle and Sella.
Triangle B (lateral maxillary region)
between Sella, Mastoidale and the root of
zygoma. Triangle C (upper maxillary
region) joining Sella, Anterior Nasal Spine
and the root of the zygoma. Triangle D
(middle maxillary region) has drawn
between the roots of zygoma, upper molar
points and the Anterior Nasal Spine.
Triangle E (lower maxillary region)
joining Anterior Nasal Spine, upper molar
points, and the point of intersection of a
line drawn between the bilateral upper
molar points and the arbitrary anatomical
axis., Triangle F (dental region) drawn
between upper molar points, upper incisal
point, and the point of intersection of a line
joining the upper molar points and the
anatomical axis. Triangle G (mandibular
region) drawn between the Condylar
points, Gonion, and Menton.
Data analyses were conducted by the
application of the SPSS (version15), the
statistical analysis included:
1. Descriptive Statistics: Including means,
standard deviation, standard error,
statistical tables and figures.
2. Inferential Statistics:
i. Independent samples t-test: for
comparison between groups and
genders.
ii. Paired sample t-test: to assess the
asymmetry of right and left
measurements for both genders and
both groups.
Results
Table (1) shows the mean and standard
deviation of facial dimensions in both
genders for both groups. It can be noted
that almost all the facial areas appeared
larger in males than in females in both
right and left handers.
Figure (2) shows that the greatest
degree of asymmetry could be seen in
mandibular region, while the least amount
of asymmetry could be noted at dental
region.
In table (2), it is found that a very
highly significant difference (P ≤ 0.000)
between the right handed and left handed
groups in almost all measured areas, except
the upper maxillary area the difference is
highly significant (P=0.004) and lateral
maxillary area the difference is significant
(P=0.02), in female group.
The gender difference was significant
(P=0.04) at the mandibular area in the right
handers and highly significant (P=0.007) in
the left handers at the lateral maxillary
area, table (3).
The facial areas in the right handed
subjects appear larger on left side than that
on right side in both genders, so that the
side difference is highly significant (P ≤
0.01) in all that areas, table (4).
In left handed subjects, the facial areas
tend to be larger on the right side with a
highly significant difference (P ≤ 0.01) in
males at lateral and lower maxillary areas,
and in females at lower maxillary,
mandibular and total facial areas.
Table (5), illustrates the mean values
and standard deviations of maximum molar
bite force in both right handed and left
handed subjects. As shown in table (6), the
mean maximum bite force was greater in
males than in females with significant
difference (P=0.03) in right handed
subjects, and highly significant difference
(P=0.001) in left handed subjects.
Discussion
The sample selected for this study
comprised of subjects in age range from 18
to 25 years, since most of the growth of
craniofacial bone could be completed after
16 years of age 16. In descriptive statistical
analysis, sometime a high standard
deviation can be seen; this could be related
to the size of the measured areas that is
proportional to the total size of skull,
which cannot be standardized to the whole
sample of the study.
Gender was always considered an
important factor that influencing all of the
facial areas. Regardless the significant
difference, the present study demonstrated
that almost all facial areas were larger in
males than in females, in both right handed
and left handed groups, this could be due
to sex hormones effect on the formation of
facial contour attribute to these differences
between males and females, which become
very evident by adolescence. The male
bony structure is bolder, more prominent,
with dominance of the forehead, nose,
chin, and stronger contour of the mandible,
this comes in agreement with many studies
8, 9, 14, 17-20.
There is a general tendency of the
inferior landmarks to deviate more
frequently and at greater distances than the
more superiorly located landmarks,
because the mandible continue grows for
longer time so it is more likely to exhibit
more deviation. Because the growth of the
mandible largely seen at the condylar
regions 21, the mandible is likely to show
gradual deviation during growth period, as
if it swings with a condylar head on the
affected side as its center of rotation, this
comes in agreement with many studies 8, 9,
11, 22-25.
As the facial skeleton could be
considered as a unit built of a number of
semi-independent regions, the dento-
alveolar region, and lower parts of the
nasal cavities show a greater response to
functional adaptation. These suggestions
could be supported by the present study as
the least amount of asymmetry was noted
in the dental region, which could be related
to the functional adaptation due to
masticatory forces. This finding suggested
that subjects having normal occlusion with
upper and lower midlines coinciding could
normally showed asymmetry of the two
sides of face; this comes in agreement
some other studies 8, 9.
In both genders, the right handers
showed significantly greater degree of
asymmetry than left handers. The facial
asymmetry could be related to asymmetry
in brain tissues, so the general trend for left
handers to show a reduced asymmetry
could be related to less difference between
the two left and right hemispheres of brain.
In right handed subjects, it was obviously
seen that the left side of the face was
significantly larger than right side and this
was true for all the facial areas. The left-
shifted facial asymmetry in right handers
may be accounted for by the asymmetrical
development of the brain, and a left
dominance in various cerebral regions is
obvious in right-handers. Consequently,
the development of various parts of brain
especially the temporal lobes might be
responsible for flattening out of the cranial
base and anterior displacement of the
nasomaxillary segment; and the greater
development of the temporal lobes on the
left would cause a greater displacement of
the nasomaxillary segment on the left side,
resulting in wider left side14.
In general, the right facial areas tended
to be larger than the left facial areas in
these subjects. However, the significant
differences in only some areas, while in
most areas the difference was insignificant.
It can say that the facial asymmetries in
left handers were not quite opposite to
right handers. The asymmetry in lateral
maxillary area of females was even
resemble to that seen in right handers, i.e.
the mean on the left was greater than that
on right but with insignificant difference.
In left handed people, the right planum
temporale and other parts of brain are not
consistently larger than left ones 26, 27.
Therefore, this inconsistency in facial
lateralization of left-handers is, in fact,
could be a general feature of left handers
with that cerebral lateralization. The right
shift theory suggested that an inconsistent
cerebral lateralization may be observed in
some of the left handers, if there is no
right-shift gene to produce language on the
left brain. The inconsistency of cerebral
laterality in left-handers was also reported
for dichotic listening and visual field
asymmetries 28. These relations between
facial and other asymmetries in left
handers support that the cerebral
lateralization may be essential for facial
asymmetry, this comes in accordance with
Keles et al. 14.
The findings of this study showed that
males have significantly greater molar bite
force than females in both right and left
handed groups. This indicates that gender
is an influencing factor in magnitude of
maximum molar bite force; this comes in
accordance with AL-Sam 29, Ferrario et al.
30, Regalo et al. 31, Bonakdarchian et al. 32,
Duygu et al. 33, Al-Saadi 34.
The gender difference in bite force
could be explained by the anatomical
difference between males and females that
in turn lead to greater muscular potential
for males. The masseter muscles of males
have type 2 fibers with larger diameters
and greater sectional areas than those of
females. One of factors that might
contribute to the composition of the muscle
fibers is the gender- related hormonal
difference 35 ,so the muscles of males can
exert more force during body activity (as in
biting) 31.
The hormonal effect on muscles would
appear in post-pubertal period. At that
time, ketosteroids (Androgens) which are
hormones that exert musculinizing effect,
is formed by two ways, from cortisol and
cortisone by side chain cleavage in the
liver and from Testosterone conversion,
excreted more in males 36, therefore
muscles mass increases in males at a
significantly greater rate than in females.
Thus, continued muscle development may
account for gender-related bite force
difference between males and females.
In general, all variables showed a
difference between the right handed and
left handed groups, but statistically
insignificant, this mean that the molar bite
force was not related to handedness.
However, the mean bite force of right
handers appeared larger than the left
handers. This could be attributed to the
larger facial dimensions of skull for right
handers, this comes in agreement with
Raadsheer et al.37, 38 which reported that
there was a positive relationship between
bite force and the facial dimensions.
The clinical outcome of this study is
that neither dental occlusion nor bite force
was related to facial asymmetry which in
turn highly correlated with handedness.
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Table (1) Descriptive statistics of facial dimensions for both genders and in both
groups.
Table 2:
Difference between left and right handers in the degree of asymmetry for both
genders.
Left handed group
Right handed group
GROUP
Female
Male
Female
Male
SEX
SE
SD
M
SE
SD
M
SE
SD
M
SE
SD
M
Areas (mm2)
20.74
80.33
354.21
20.5
79.41
391.87
18.33
71
354.13
29.64
114.79
471.47
Cranial
base(A)
63.57
246.21
1081.39
64.32
249.11
1128.5
85.45
330.93
1262.27
92.36
357.69
1182.91
Lateral
maxillary(B)
63.46
245.8
1202.93
91.98
356.23
1225.8
59.37
229.93
1430.25
75.06
290.71
1480.85
Upper
maxillary(C)
38.24
148.1
545.74
31.72
122.86
578.76
28.05
108.64
546.85
51.73
200.35
629.33
Middle
maxillary(D)
33.61
130.18
437.48
28.54
110.54
444.87
18.11
70.14
437.71
34.7
134.39
469.9
Lower
maxillary(E)
15.57
60.3
135.86
23.8
92.2
134.25
26.78
103.73
237
23.19
89.8
173.36
Dental
region(F)
112.49
435.66
1997.61
118.99
460.84
2388.88
97.15
376.27
2237.73
111.73
432.74
2494.96
Mandibular
(G)
Group difference for females
Group difference for males
Variable
P value
T-test
P value
T-test
0.001
***
-3.54
0.001
***
-3.6
Cranial base(A)
0.02
*
-2.45
0.000
***
-4.93
Lateral maxillary(B)
0.004
**
-3.13
0.001
***
-3.77
Upper maxillary (C)
0.000
***
-4.26
0.001
***
-3.58
Middle maxillary(D)
0.000
***
-4
0.000
***
-6.33
Lower maxillary(E)
0.001
***
-3.58
0.001
***
-3.92
Dental region(F)
0.000
***
-5.28
0.000
***
-6.04
Mandibular (G)
* = Significant difference.
** = Highly significant difference.
*** = Very highly significant difference.
Table 3: Gender difference in the degree of asymmetry for both groups.
Table 4: Comparison between the right and left measurements for both groups
and both genders by using paired t-test.
Gender difference in left handed group
Gender difference in right handed group
Variable
P value
T-test
P value
T-test
0.18
(NS)
1.38
0.1
(NS)
1.68
Cranial base(A)
0.007
**
2.92
0.89
(NS)
0.14
Lateral maxillary(B)
0.91
(NS)
0.11
0.54 (NS)
-0.63
Upper maxillary (C)
0.58
(NS)
-0.56
0.82
(NS)
0.23
Middle maxillary(D)
0.96
(NS)
-0.04
0.23
(NS)
-1.23
Lower maxillary(E)
0.3
(NS)
1.06
0.5
(NS)
0.68
Dental region(F)
0.85
(NS)
0.2
0.04
*
-2.22
Mandibular (G)
Left handed
Right handed
GROUP
Females
Males
Females
Males
SEX
P value
T-test
P value
T-test
P value
T-test
P value
T-test
Variable
0.985
(NS)
0.019
0.085
(NS)
1.85
0.000
***
-5.31
0.002
**
-3.76
Cranial base(A)
0.622
(NS)
-0.505
0.002
**
3.86
0.01
**
-4.13
0.006
**
-3.25
Lateral maxillary(B)
0.174
(NS)
1.433
0.35
(NS)
-0.96
0.005
**
-3.37
0.007
**
-3.12
Upper maxillary(C)
0.077
(NS)
1.906
0.25
(NS)
1.18
0.000
***
-4.59
0.001
**
-4.37
Middle maxillary(D)
0.037
*
2.299
0.000
***
5.48
0.000
***
-6.41
0.001
**
-4.08
Lower maxillary(E)
0.511
(NS)
0.674
0.06
1.98
0.001
***
-4.07
0.003
**
-3.64
Dental region(F)
0.004
**
3.505
0.29
(NS)
1.09
0.000
***
-4.66
0.000
***
-4.59
Mandibular (G)
0.004
**
3.436
0.12
(NS)
1.62
0.000
***
-9.8
0.000
***
-10.29
Total facial area
NS = Non-Significant.
* = Significant difference.
** = Highly Significant difference.
Table 5: Descriptive statistics of right and left measurements of bite force for
both genders and in both groups.
Table 6:
Gender differences for bite force in both groups.
Figure 1: Drawing of triangles on a Posteroanterior radiograph.
Group
Gender
Side
Variable
M
SD
SE
Right hander
Males
Right
540.47
186.3
48.1
Left
556.27
181.74
46.92
Females
Right
411.73
160.84
41.53
Left
401.67
155.23
40.08
Left hander
Males
Right
573.67
206.66
53.36
Left
601.27
173.7
44.85
Females
Right
376.93
89.37
23.07
Left
404.87
133.94
34.58
Gender difference in
Left handed group
Gender difference in
Right handed group
Variable
P value
T-test
P value
T-test
0.001
**
3.54
0.03
*
2.34
Bite force
* = Significant difference.
** = Highly Significant difference.
Figure 2: Mean values of the degree of asymmetry for genders in both groups.
0
20
40
60
80
100
120
area (mm²)
Right handed
males
Right handed
females
Left handed
males
Left handed
females