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Asymmetry of the mandibular condyle in Haida Indians.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 70:119-123(1986)
Asymmetry of the Mandibular Condyle in Haida Indians
RAYMOND L.COSTA, JR.
Department of Oral Anatomy, College ofDentistry, University of Illinois at
Chicago, Chicago, Illinois 60680
KEY WORDS
Dental attrition, Fluctuating asymmetry
ABSTRACT The condyles of 72 aged and sexed Haida Indians were measured for anteroposterior and mediolateral diameter and their approximate
areas calculated. Dental wear was assessed for the same individuals. Asymmetry of condyle size did not appear to change with age. In a pair-wise analysis,
no relationship was found between the largest of a pair of condyles and the
most worn side of the dentition. The difference in size between each pair of
condyles (normalized for individual size) was plotted as a histogram and found
to have a normal distribution with a mean of 0 and no skewness. Condyle
asymmetry does not appear to be related to differential chewing forces but
more closely fits the model of fluctuating asymmetry.
Striking differences between the sizes of
the mandibular condyles can often be observed in individuals who display no obvious
temporomandibular joint (TIvIJ) pathology.
At least two hypotheses can be generated to
account for this asymmetry. The first of these
is a mechanical explanation. Several workers have demonstrated that the TMJ transfers chewing forces from the mandible to the
cranium during mastication (Mongini et al.,
1981; Hohl and Tucek, 1982; Standlee et al.,
1981; Brehnan et al., 1981; Hylander and
Bays, 1979; Hylander, 1979). This suggests
that the magnitude of joint loads over time
might be related to mandibular condyle size.
Humans tend to chew on one side of the
mouth at a time and most skulls, including
juvenile specimens, exhibit differential wear
on the right or left side of the dentition, suggesting that over a period of years more force
will be transmitted to one or the other condyle. Increased force transmission through
one condyle may be related to increased bone
growth in that condyle, at least until the
individual is fully grown. A prediction can
be made that, if a correlation exists between
condylar asymmetry and the more habitually used side of the dentition, then a distinct
positive or negative relationship should also
exist between the side of the mouth that has
the more worn teeth and the side that has
the largest condyle.
An alternative hypothesis is that the observed asymmetry of condylar size is simply
0 1986 ALAN R. LISS,INC
a case of fluctuating asymmetry. Many bilateral structures display little or no size asymmetry (e.g., limb bone length), since they are
under strict functional as well as genetic control during growth. Others structures, such
as teeth, are under genetic but not functional
control. Asymmetry in these structures is
oRen attributed to relaxation of genetic control because of increased environmental
stresses during development. This is sometimes called developmental noise, (see Siege1
and Doyle, 1975, for an example). In cases of
fluctuating asymmetry, either the right or
left side can be larger, hence the name. If
condyles fit the model of fluctuating asymmetry, a frequency histogram of the difference in size between the right and left sides
will be normally distributed and have a mean
of 0. In addition, there should be no skewness
or kurtosis (Van Valen, 1962).
While studying the paleopathology of the
TMJ in Haida Indians, we observed that most
skulls displayed considerable condylar asymmetry. In a few skulls, one condyle appeared
to be as much as 50%larger than the other.
This sample, which displays marked asymmetry of the mandibular condyles and far
greater attrition of the dental occlusal surfaces than is seen in modern clinic patients,
is used in this study to test the predictions
generated by the two hypotheses outlined
Received April 15, 1985; revision accepted October 14,1985
120
R.L. COSTA, JR
above. Although asymmetry of many bodily
structures has been studied, such a s cleft lip
and palette (Siege1 and Smookler, 1973) and
vertebrae (Taylor, 19831, asymmetry of mandibular condyles has not.
MATERIALS
A collection of Haida Indian skeletons housed at the Field Museum of Natural History,
Chicago, was used for this study. This sample
was collected by G.A. Dorsey and C.F. Newcombe between 1893 and 1905 from burials.
The bones are in excellent condition. Only
individuals having two undamaged, nonarthritic condyles (insofar as could be determined visually) were used for this study.
Additionally, only individuals with enough
teeth present to give a good indication of
dental attrition for each dental quadrant
were included. The study sample consisted of
72 individuals, 41 males and 31 females.
[This skeletal collection formed part of the
sample used by Cybulski for a study of cranial morphology in Northwest Coast American Indians. The reader interested in details
of this collection is referred to Cybulski
(19751.1
The Haida have inhabited the Queen Charlotte Islands, located off the coast of British
Columbia, Canada, since prehistoric times.
The traditional diet consisted primarily of
fish, shellfish, sea mammals, and some land
mammals. Several kinds of fish, notably
salmon, were taken in seasonal “runs,”
which formed the backbone of the Haida
economy. Fish not immediately consumed
were filleted and dried for future use. Berries
and some vegetables were gathered but were
not a major portion of the diet (Drucker,
1963). Although food was generally plentiful,
there were periodic shortages (Suttles, 1968).
The observed marked dental attrition was
probably due to the fibrous nature of the food
and preparation and storage techniques.
(Fish dried in the open air often become embued with airborne sand and silt.) By the
third decade of life, molar teeth were worn
well into the dentin, and individuals aged 50
years or older displayed many molars with
either secondary dentin or pulp chambers exposed by dental attrition.
METHODS
Each condyle presents its own irregular
shape, and the two condyles of one mandible
can be of different shapes. Condyle size was
estimated by taking the greatest anteropos-
terior width of the condyle and multiplying
this by the greatest mediolateral length to
give a n area in mm2. This method of calculating condyle area has been used previously
by Hinton (1983). This value is a n overestimate, rather than a n actual area measurement, but was deemed adequate for this
investigation in that the sue asymmetry,
rather than the actual sizes of the condyles,
was being investigated. Smith et al. (1983)
suggested using the formula for calculating
the area of a n ellipse for measuring condylar
area, but there is little reason to believe that
this actually increases accuracy.
Dental wear was recorded using a n 11-stage
scale derived from a five-stage scale previously used by Costa (1982). This method is
similar to those used by Murphy (1959) and
Smith (19841, both of whom used eight wear
stages. Each tooth was graded for amount of
occlusal wear based on the appearance of the
crown, where 0 represented a n unworn tooth
and 10 represented a tooth worn to the cementoenamel junction on a t least one side.
Wear values were averaged for the existing
first and second molars and premolars of each
dental quadrant. Quadrants were combined
to form left and right sides. Since third molars wear at a significantly different rate,
they were excluded from the study (Smith,
1984). Left- and right-side wear averages
were compared for each individual to determine the more worn side.
Each individual was assigned an age and
sex. Individuals for whom there were associated innominate bones were assigned a sex
based on dimorphism of Washburn’s index (P
x lOO)/I, where P is the length of the pubic
bone and I is the length of the ischium, and
width of the sciatic notch (Washburn, 1948).
Individual skulls were sexed using nonmetric traits, such as shape and angle of the
forehead and size of the mastoid processes.
Age at death was assigned in 5-year intervals based on age changes in the pubic symphysis, dental eruption timing, and suture
closure for those skeletons with intact pubic
bones. Using these specimens as a “knownage” sample, linear regression analyses of
dental wear were performed individually for
first, second, and third molars, all molars,
and all teeth. Correlation coefficients for first
and second molars ranged from r = .91 to r
= .94. Values for third molars, all molars,
and all teeth were somewhat lower. Because
the slopes for males and females were significantly different, sexes were kept separate.
121
HAIDA CONDYLE ASYMMETRY
Once regression lines were established, dental wear values for individual skulls were
interpolated into the regression formulae to
assign an age at death (Costa and German,
1986).
Condyle asymmetry was analyzed in two
ways. The largest condyle of each pair was
compared to the more worn side of the dentition for each individual in a pair-wise analysis. Binomial probability and (x2)tests were
used to check for significance. Fluctuating
asymmetry was investigated using the value
of (L - R)/(L + R),which is the difference
between condyle sizes divided by their sum.
This procedure normalizes individual size
differences and gives a signed value for
asymmetry to each individual. The results
are plotted as a histogram with right-condyle-larger mandibles represented as negative values, and left-condyle-largermandibles
as positive values. Another traditional measurement of asymmetry is the correlation
coefficient, r (Bailit et al., 1970), which can
be converted to Z scores for comparisons between groups. Angus (1982) notes that the
distribution of values within the sample can
significantly affect the r values of similar
samples. Since between-group comparisons
were not part of this study, this statistic has
not been calculated.
Males and females were examined separately (Table 2). Males have 54.8% and females 64.3% larger right than left condyles.
These values proved not to be significantly
different from a random outcome when
checked with a x2 test (significance level >
0.05; see Table 2). Males and females were
therefore combined. When larger condyles
are compared to the side with greater attrition, 31 individuals display an agreement
between sides, and 28 individuals do not. This
is nearly a random outcome, as is suggested
by the x2 test (see Table 2). There is only a
9.2%chance that this outcome is better than
random using a binomial test.
When (L - R)/(L + R) is plotted on a histogram (Fig. I), a normal distribution results,
with a mean of -0.012 and an S.D. of 0.062.
The median is 0.01. Since the median and
mean nearly coincide, the skewness is
close to 0.
DISCUSSION AND CONCLUSIONS
Condyle size is established during the first
three decades of life, whereas dental wear
increases with age until death. Some change
in chewing pattern could therefore arise late
in life (resulting, e.g., from loss of some teeth)
that might obscure a pattern that exited during the growth period. Although this may
have occured in some individuals, the results
RESULTS
shown in Table 1imply that size asymmetry
Condyle size asymmetry was compared t o of the mandibular condyles is consistent in
age for males and females (Table 1).When L this group from at least the second decade of
- R)/(L + R) for each sex and each 5-year life onwards. Given this consistency, the reage grouping were subjected to a x2 test, the sults of the pair-wise comparison shown in
values were consistent for all age groups and Table 2 suggest that there is probably no
both sexes: x2 < 0.98 for males, x 2 c .99 for significant relationship between the larger
females and both sexes combined. No age condyle of a pair and the side of the dentition
trends were noted. The age groups, accord- displaying the greatest dental attrition in
ingly, were combined.
this sample.
TABLE 1. Condyle size asymmetry {(L - R) / (L f R) x 10'1 for 72 Haida Indians
distributed by age and sex
Age bears)
Male (n)
Female (n)
Combined
15-20
21-25
26-30
31-35
36-40
4145
46 +
Total Individuals
Mean (r, - R) / (L
-0.08 (2)
0.17 (4)
-0.13 (9)
0.12 (4)
-0.26 (8)
0.10 (4)
0.06 (10)
41
-0.003
1.055,6DF
< 0.98
-0.14 (4)
-0.64 (3)
-0.47 (6)
0.29 (3)
-0.13 (2)
-0.18 (3)
-0.26 (10)
31
-0.219
0.048,6 DF
< 0.99
6
7
15
7
10
7
20
72
-0.111
0.038,13 DF
<0.99
X2
Significance (P)
+ R)
122
R.L.COSTA, JR.
TABLE 2. Haida condyle sue asymmetry us. most worn side of the dentition
Larger condyle
More worn side
Left
Right
Male
Right
Left
Sides agree
Sides disagree
Incidence of larger right side: 54.8%
x2: 0.095 (not significant at 0.05)
Female
Right
Left
Sides agree
Sides diagree
Incidence of larger right side: 64.3%
x2: 0.571 (not significant at 0.05)
Totals
Sides agree
Sides diagree
x2: 0.0381 (not significant at 0.05)
Binomial test: disagree: 0.5921
(9.2% chance that outcome is better
than random)
Same
10
10
17
19
9
7
6
3
9
2
14
9
7
5
3
2
31
28
h of
9
2
1
individuals
I
I
2
1.5
m.
1
R I G H T (+)
Fig. 1. Distribution of (L - R)/C
given are multiplied by lo2.
.5
0
.5
1
(-)
1.5
LEFT
+ R) for Haida males and females (combined). The values
The distribution of (J- RXCL + R) implies
that the asymmetry exhibited in this sample
can be adequately explained as a case of fluctuating asymmetry (Fig. 1).Although theoretically the population mean should be 0 in
cases of fluctuating asymmetry, Van Valen
(1962) notes that this is rarely the case and
is most often just to one side of 0, as is the
case in this sample. Further evidence of fluctuating asymmetry is given by the lack of
skewness.
This study suggests that increased chewing stress on one side of the mouth, as evidenced by increased dental attrition, does not
adequately explain condyle size asymmetry.
The pattern exhibited more clearly fits the
model of fluctuating asymmetry.
ACKNOWLEDGMENTS
I would like to thank Drs. H. Barghusen,
S. Herring, W. Greaves, and R. Scapino and
three unnamed reviewers for helping to prepare this paper for publication.
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