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Dental arch shape and tooth wear variability.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 82~385-395 (1990)
Dental Arch Shape and Tooth Wear Variability
STEPHEN MOLNAR AND N'A M. MOLNAR
Department of Anthropology, Washington University, St. Louis, Missouri
63130
KEY WORDS
Australia, Dental Anthropology, Occlusion
ABSTRACT
The rapid rate of tooth wear frequently reported among
certain contemporary aboriginal populations has often been attributed to
dietary form and abrasives. Several investigators have reported a close
correlation between food bulk and the wear planes formed over the dental
arches, i.e., steep oblique wear vs. flat horizontal planes. In this investigation
we demonstrate that arch shape is an additional and a significant factor
influencing the distribution of wear facets and exposed dentin over occlusal
surfaces. We examined 64 dental stone casts of Aboriginals from Yuendumu,
Central Australia, born between 1900 and 1940. These casts offer a record of
the variety of tooth wear and arch forms and their interrelationships. This
group of individuals, some subsisting on abrasive and some on soft diets, have
dentition which exhibit various wear rates and wear patterns probably due to
the diversity of arch shape, size, and occlusal relationships. Hypsiloid or
U-shaped maxillas had a more buccally directed wear in contrast to the
parabolic or hyperbolic forms, which exhibit a heavier lingual loading. Varying
occlusal conditions also contribute to differing wear patterns over the arches.
Individuals with alternate intercuspation, for example, have a more horizontally directed wear. These and other conditions of shape, size, and occlusion
emphasize the importance of morphological factors in the production of tooth
wear rates and patterns in addition to dietary abrasives.
One of the most notable features of the
dentition of prehistoric and some recent human populations is the heavy wear over
occlusal and interproximal surfaces. Since
the degree and form of these worn surfaces
varies among individuals and populations,
the goal of many studies has been to understand the causes so the dental remains could
be read as a record of ast oral environments, a record that wou d offer evidence of
the bulk and abrasive nature of the diet and
food preparation methods. With the rise of
interest in dental anthropology, many reports have been directed to the description
and comparison of these conditions of tooth
wear among populations with diverse diets.
People dependent on a hunting and gathering subsistence wear their anterior teeth
more rapidly than agriculturists, who, most
frequently, show rapid and extensive destruction of molar crown surfaces (Hinton,
1981). Furthermore, the shapes or angles of
the worn occlusal surfaces may differ between the two population groupings. The
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0
1990 WILEY-LISS, INC
hunter-gatherer dentition tends to have flattened horizontal wear planes, whereas agriculturism produces oblique, concave surfaces during the course of wearing away
enamel and dentin (Smith, 1984). These observations suggest diets requiring different
chewing modes and varying lengths of time
to reduce the food bolus prior to swallowing.
Distinctions have also been reported between males and females within a population; females generally wear their teeth
more rapidly than do males from early adolescence on. The evidence for this sexual
dimorphism comes from studies of Eskimos
(Pedersen, 1938), Australians, (Campbell,
1938; Molnar et al., 1983a,b), and American
Indians (Molnar, 1971).However, no consistent differences were observed amon certain other populations, such as me ieval
skeletons from Denmark (Lunt, 1978). The
weight of the evidence, though, seems to be
f
Received October 26,1988; accepted February 21,1989.
386
S. MOLNAR AND I.M. MOLNAR
in favor of greater wear among females because of the rougher portions of food consumed (Heithersay, 1959)and because of the
heavy craft use of the teeth applied to the
chewing of items such as animal skins and
plant fibers (Molnar, 19721.
In addition to differences of wear over the
anterior and posterior teeth, considerable
variation in the degree of wear between the
arches has also been reported. The maxillary
teeth wear to a greater degree than do the
mandibular teeth in many populations (Molnar, 1971). The reverse is true in certain
populations; for example, prehistoric as well
as contemporary Australian Aboriginals
tend to wear mandibular teeth earlier and at
a greater rate than their opposite members
in the maxilla (Molnar et al., 1983a,b). A
complete explanation for these contrasts is
still lacking, but it may be, as McKee (1985)
has suggested, that face form and premolar
eruption precedence influences the differences between upper and lower teeth.
Once cusps have been flattened and
broader dentin areas are formed, the occlusal wear plane over the molar tooth rows
may undergo a rotation or reversal of its
oblique slope from the first to the third molars. This occlusal plane reversal, described
by man as a helicoidal plane, is probably
due to t e different relative widths of maxillary and mandibular arches at each of the
molars (Campbell, 1925) or to the axial tilt of
the molars (Smith, 1986).This form has been
described more recently as a pattern to be
expected in dentitions subjected to a lifetime
of heavy attrition (Richards and Brown,
1986).Besides the extent of wear, the incongruency of upper and lower arch widths is
significant.
The way in which the arches occlude is
another important factor in the study of diet
and force loadings, with its resultant wear
pattern. The “classes” of occlusion and what
is a normal or a “natural”form have received
extensive examination (see Brace, 1977,for a
review).This examination of occlusal classes
relates principally to populations whose
softer diets allow them to retain intact cusps
throughout their lifetimes. The study of people living under harsh dental conditions (i.e.,
hi hly abrasive diets) requires a somewhat
di erent erspective and is limited because
of an ear y loss of cusp eminences during
childhood and adolescence. Under such conditions, occlusion in the adult may be more
appropriately described as “functional,”
E
k
P
which raises the still unresolved question of
the function of cus s in maintaining arch
relationships when t ey are lost so soon after
tooth eru tion. Additionally, considerable
note has een taken of the “edge-to-edge
bite,” i.e., the meeting of the u per and lower
anterior teeth at their incisa surfaces. The
question of whether this occlusal form is
genetic or functional1 caused has been considered in studies o tooth surface loss in
dental1 harsh environments. The weight of
the evi ence suggests that the “edge-to-edge
bite” is a result of function since Australian
Aboriginal or Eskimo children on soft
“western-type” diets develop overbites even
though their parents and grand arents (who
subsisted on harsh abrasive oods) had a
“primitive” edge-to-edge condition (D’Amico,
1961). However, at least one study has documented that 50% of a contemporary aboriginal group living in the New Guinea Eastern
Highlands possess an “edge-to-edge bite”
even though their teeth are largely unworn
because of a soft diet (Boyd, 1972).
A articular form of occlusion, however,
whic relates to patterns of tooth wear is
“alternate intercus ation,” a condition in
which a much broa er maxilla prevents bilateral interdigitation of cusps when the
jaws are brou ht together. The ran e of arch
dimension differences and their fgrequency
were recently described b Brown and coworkers (1987), who note that 25% of the
cases show alternate intercuspation among
the grou of Central Australians we discuss
below. TYlis condition of occlusal incongruency contributes to a particular pattern of
wear, as we shall describe.
In sum, tooth wear patterns and rates of
wear vary wide1 for a number of reasons as
outlined in Tabres 1A and 1B. In addition,
another major contributing factor to variability is that the cheek teeth do not always
function as a single unit. Teeth erupt sequentially and thus begin to function as part
of the masticatory unit at different times;
premolars and canines reach position in the
occlusal plane before the second molars in a
majority of humans, for example. Hence,
tooth wear also may be sequential, which
causes wear facets to appear in a diversity of
ositions and these facets enlarge at diferent rates as enamel and dentin loss
progresses. The effect of tooth position, arch
size and shape, and eruption timing are all
factors to be considered in addition to dietary
consistency. In this report, we consider the
K
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B
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fl
B
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387
ARCH SHAPE AND TOOTH WEAR
TABLE I A . Principal causes
Dietarv
of tooth wear
Nondietarv
Natural abrasives (e.g.,
wind blown sand)
Introduced abrasives
(e.g., ashes)
Food bolus consistency
Tool use of teeth
Chewing of gums or
leaves
Chewing of animal
skins, fibers, or sinew
Tooth cleaning
Attrition from tooth to
tooth contact
TABLE 1B. Factors contributing to gradients o f
tooth wcar
Tooth eruption sequences
Enamel thickness
Cusp morphology
Root angulation
Occlusion
Arch shape
Craniofacial shape
Diet form and abrasiveness
Paramasticatory functions
born between 1900 and 1940 and ranged in
age from 13.9 to 50 years at the time the
dental casts were made. They had reached
adolescence or adulthood before the establishment of Yuendumu, and many of them
had lived from one to four decades under
aboriginal conditions. Their dental casts
record a range of tooth wear patterns and
arch shapes. Some casts had to be rejected
because of tooth exfoliation due to extreme
wear or advanced age, but there were 64
individuals whose casts were suitable for
study. These subjects were selected on the
basis of completeness of their dentition, the
presence of clearly distinguished wear facets
or patches of ex osed dentin, and a lack of
dental disease. bhis selected group represents a broad range of tooth wear patterns,
and their arch shapes encompass most of
those shapes reported for any population.
METHODS
contribution of arch shape differences to the
variation in wear patterns over the occlusal
surfaces.
MATERIALS
Dental stone casts of 64 individuals were
selected from the collection of 1,717 casts
representing 446 individuals of Walbiri and
Pintubi groups living a t the Yuendumu settlement in Central Australia, 285 km from
Alice Springs. These casts had been collected
by Barrett, Brown, and coworkers between
1951 and 1972 during the course of a
semilongitudinal growth study (see Brown
and Barrett, 1973).The bulk of the collection
consists of serial casts taken from children
between the ages of 6 and 18 years. In our
earlier study, the rate and degree of tooth
wear of 68 of these children were measured
by the reduction of cusp height and the increase of the wear facet area on each of four
serial dental casts made from each individual during childhood and adolescence
(Molnar et al., 1983a,b). These children and
adolescents had spent their lives at Yuendumu, where their diet was main1 a modern, low-abrasive type. Neverthe ess, the
rate of tooth wear was substantially greater
than Australians of European descent because of food preparation contaminants and
the occasional addition of some aboriginal
foods to supplement their usual diets.
The balance of the cast collection consisted
of single cast sets of 89 individuals who were
i
The degree of tooth wear is recorded on an
ordinal scale (la),
a method previously used
and standardized on other samples (Molnar,
1971; Molnar et al., 1983a; Scott, 1979;
Smith, 1984).This scoring method identifies
the location and relative size of the wear
facet by tooth and tooth uadrant. Distribution of wear over the arc es is evaluated by
comparisons of wear scores for each tooth;
e.g., the scores of 3 4 for the first molars in
an arch whose anterior teeth have scores of
6-7 would indicate an anteriorly directed
wear, the result of a more continuous and
heavy use of the anterior teeth. Such evaluation of relative wear of arch regions is aided,
of course, by com arisons of wear scores on
the other teeth. l!he numerical codes of the
tooth wear ranges, the numbers of subjects,
and their arch forms are listed in Table 2.
The hi hest wear scores in one or more dental arc quadrants determine the lacement
of an individual into light, me&um, and
heavy wear groups.
Orientation of wear of the maxilla is also
classified by reference to the relative size of
wear facets or areas of exposed dentin on the
buccal vs. lingual cusps, which suggests a
buccal-lingual orientation of occlusal load.
Likewise, the relative wear of mesial-distal
cus s and of adjacent teeth indicate mesia -distal loading. Subjects are grouped accordingly into four wear pattern groups, illustrated in Figure l, a method that follows
those applied previously to the Yuendumu
1
t
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388
S. MOLNAR AND I.M. MOLNAR
TABLE 2. Yuendumu dental casts’
Arch shapes
A2
Wear degree
Females
Light (1-2)
Medium (3-4)
Heavy (5-7)
Totals
Males
Light (1-2)
Medium (3-4)
Heavy (5-7)
Totals
B.’
c
D
1%
20+
-
1
-
1
4
1
-
7
8
6
5
1
-
4
4
2
3
1
8
5
3
2
10
-
1
5
4
1
-
2
3
1
5
3
-
-
13
11
2
7
4
3
-
4
3
1
-
14
11
5
1
45+
5
3
8
7
40+
-
4
2
2
8
2
Ages (years)
30t
35+
3
5
4
5
25+
-
-
-
6
-
1
-
-
2
-
-
1
1
-
4
1
-
1
1
‘Wear de rees (see texl); arch shapes (after Boyd, see text); ages (see Table 2 for individual ages).
?Two m a t s with arch shape A/R included.
”Three males and four females with arch shape B/C included
serial casts (see Mckee, 1985; McKee and
Molnar, l988,1988a,b).
Both Grou s 1and 2, for example, have the
same bucca -lingual horizontal wear pattern. Group 1, however, shows more tooth
surface loss over the posterior arch region, a
distal gradient of wear as the arrow in
Figure 1indicates. In contrast, Group 2 has a
mesial gradient, with the premolars, canines, and mesial cusps of the first molar
showing the most wear. Groups 3 and 4
exhibit a steep buccal-lingual slope, the lingual cusps wearing more rapidly. A distally
directed wear of oup 3 contrasts to the
mesial wear and t e anterior arch loadin s
of grou 4.Each wear pattern group is su dividecfinto wear degree A, light wear without exposed dentin, and B, heavier wear with
one or more patches of exposed dentin. This
classification method uses only the maxillary occlusal surfaces to define wear direction and occlusal loading because a mandibular arch tends to follow the same pattern as
the opposing maxilla (oblique to oblique and
horizontal to horizontal), except in those
rare and unusual occlusal conditions as described below.
Arch shapes are classified by a subjective
method, an evaluation of the apparent curvature formed by the relative positions of the
postcanine tooth rows and the anterior teeth.
This follows the methods of Heithersay
(1959), Boyd (1972), and others. The photographs in Figure 2A-D illustrate four shape
categories: hyperbolic (shape A), diverging
tooth rows; parabolic (shape B), the tooth
rows are less divergent; hypsiloid (shape C),
the post canine tooth rows are nearly parallel and form a “ U shaped palate; and elli isoid (shape D), convergent tooth rows in t e
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%
i:
posterior molar region. In addition to arch
shape, the photographs show the range of
wear patterns and their groupings.
Arch diameters are also used as a method
to compare variations of arch shapes. These
diameters (in mm) are taken as the distance
between the central points of interproximal
contact facets, a method previously used by
Bjork (1953) and McKee (1985) in their description of dental arch shapes. Initially, the
diameters between all left and right teeth
were measured at both mesial and distal
contact facets. Comparisons of these diameters showed that the distal interproximal
points on the first molars and the mesial
interproximal points on the canines proved
t o be most useful for shape com arisons. The
ratios of the canine to first mo ar diameters
of each arch are listed as maxilla C/M and
mandible C N in Table 3A,B and are arranged in ascending order, together with
Boyd’s shape classification of the maxillas.
Occlusion or “fit”betweenupper and lower
arches frequently is difficult to define, and
the various methods have generated much
discussion and many publications as noted
above. For comparisons of wear and arch
sha e differences, we are more concerned
wit the relative size of the upper and lower
arches and intercuspal positions of the
isomeres than with a class or group. Therefore, we record three types of observations on
arch occlusion: the traditional Angle classifications for reference, the ratio of mandibular
to maxillary diameter at the canine (Mand
CMax C), and at the first molar points
(Mand M/Max M). These arch diameter ratios easily identify individuals with narrower mandibles at either the canine or the
molar, or at both regions, a condition that
P
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389
ARCH SHAPE AND TOOTH WEAR
buccal
A
Group 4
Buccal to lingual oblique
wear, mesial gradient
0
M2
M1 P 4 P 3 C
A
Group 3
Buccal to lingual oblique
wear, distal gradient
3
B
.....
I
A
Group 2
Horizontal wear w i t h a
mesial gradient
0
M2
i
A
Group 1
Horizontal wear w i t h a
distal gradient
M1 P 4 P 3 C
B
lingual
Fig. 1. Wear distribution. Occlusal wear facets are
indicated by shaded areas and exposed dentin by the
darker shading. The orientation of wear is indicated by
arrows next to the diagrams of a portion of a dental arch
quadrant from canine to second molar. Each of the four
P
I representative
,
of an orientation of wear, is subed into A (light wear) and B (medium to heavy
iv1
wear).
390
S. MOLNAR
AND I.M. MOLNAR
Fig. 2. Examples of arch shapes. A hyperbolic (A), 21-year-old male. B: parabolic (B),
30-year-old male. C: hypsiloid (C), 38-year-old female. D: ellipsoid (D),
30-year-old female.
varies from only a slight difference of arch
diameters to an extreme condition called
“cross-bite” or “alternate intercuspation.”
Such individuals either have problems in
achieving a symmetric occlusion or cannot
interdigit upper and lower molar cusps bilaterally in one or more positions along the ost
canine tooth rows. This condition an its
variability among the Aboriginals at Yuendumu has been described in detail by Brown
and coworkers (see Brown et al., 1987).
old female (ID 138)who is included because
her dental casts record the effects of a harsh
diet. Her teeth show, even at this young age,
enamel loss far greater than some persons
several decades older. Associations exist between arch form and orientation of occlusal
wear for most of the sample and some of
these associations are statistically significant.
Maxillary arch shapes vary between the
opened diverging tooth rows of the hyperbolic sha e (A) and the parallel or near parRESULTS
allel toot rows of the hypsiloid shape (C). A
The dental casts of these 30 males and 34 majority of arches (31%) are classified by
females exhibit a variety of arch shapes, visual a praisal as (A) hyperbolic. Parabolic
sizes, and tooth wear patterns. Their ages (B) is t e next most frequent (27%); the
range from 15 to 55 years, plus a 13.9-year- hypsiloid shape (C) is found on 25% of the
a
K
K
391
ARCH SHAPE AND TOOTH WEAR
TABLE 3A. Arch shape and wear group-Females‘
ID/age (years)
338129.5
81128.5
70140.1
347125.5
343121.5
73/14.9
80117.1
426/35.9
404130.5
67/32.9
265124.5
91130.5
474/38.5
71132.5
615124.1
346117.5
69/24.5
66/24.5
78136.1
401130.5
396125.5
399123.5
342119.5
264124.5
402128.1
79136.1
331115.9
82129.1
339129.5
60129.9
2551 18.1
345138.5
72131.1
138113.9
Class
occl.
X-bite
I
I
I
X-bite
I
I
X-bite
X-bite
I
I
I
I11
X-bite
X-bite
I
X-bite
X-bite
I
I1
II
I
I1
I
X-bite
I
***
X-bite
X-bite
I
I
X-bite
I
I
Mandlmax
Max
C/C
M/M
Shape
C/M
,531
.77
.78
.86
.77
.77
.75
.78
.77
.78
.75
.90
.92
.96
.92
.90
.90
.92
.94
.88
.94
.92
.97
.98
.87
.94
.88
.93
.RO
.92
.91
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
D- e 11i p
B-parab
B-parab
B-parab
B-parab
B-parab
B-parab
D-ellip
B-parab
B-parab
.62
.65
.67
.67
.67
.67
.68
.68
.69
.69
.69
.70
.70
.70
.71
.71
.71
.71
.71
.72
.72
.72
.73
.73
.74
.74
.75
.75
.75
.76
.78
.78
.79
.80
.81
.78
.76
.92
.72
.79
.86
.74
.70
.78
.77
.76
.78
.78
.70
.72
.73
.72
.76
.76
.75
.71
.78
.88
.92
.96
.92
.92
.90
.94
.93
.94
.96
.90
.96
.94
.90
B/CB1CBIGB-parab
C-hypsi
C-hypsi
C-hypsi
C-hypsi
B/CC-hypsi
C-hypsi
B/CC-hypsi
C-hypsi
C-hypsi
C-hvusi
Mand
C/M
Wear
type
.55
54
54
.62
.57
.58
.55
.57
.60
.57
.66
.58
.56
.61
.69
.58
.60
.88
.58
.55
.64
.61
58
1A
4A
.62
.62
.58
.58
.59
.58
.60
.66
.62
.60
.69
2B
4A
2A
4A
3A
2B
1A
4A
4A
1B
2A
2B
1A
3A
1A
2A
4A
3A
4A
4A
1A
1A
1A
2B
4A
2B
2B
2B
2A
1R
2R
1B
]ID/age, individual identification number and age. Class occl. angle clsssifications or occlusion note: X-bite, alternate intercuspatiun
(Brown et al. 1987);Mn-Bccl,(niandible,buccal version;seeID 26, male); ***, difficultto occlude, noclassification possible. Mand/max, ratiu
ofdiametersofinandibletomaxillaat thecanines(C/C)and thefirstmolars(M/M).Archshape,classesofarchshapeafterRoyd(seetext).
Max C/M, ratio of maxillary canine and first molar diameters. Mand C/M. ratio of mandibular canine and first molar diameters. Wear
group, patterns of wear; degree and direction (see Fig. 1). Tables are arranged by maxillary canindmolar ratios.
casts. The balance, or nine maxillary arches
(14%),are difficult to classify by this subjective method. Only two subjects can be called
ellipsoid (D), and both are females. Otherwise there is no difference between the sexes
in the shape of their dental arches.
This subjective classification system of
Boyd and Heithersay is a useful method for
an initial sorting of dental arch variabilit
and corresponds well to maxillary arc
shapes defined by the ratio of the canine to
molar diameters; therefore, Table 3A,B list
both methods for comparison. Shape A
ranges from 5 9 to .68 caninelmolar diameter
ratio, which contrasts with the .73-.80 ran e
of the hypsiloid shape (C). The paraboic
shape (B) was intermediate (.69-.72), and
the maxillas a t either end of this range are
more difficult to distinguish from shapes A
or C. Because of the difficulty in establishing
K
boundaries of a continuously varying trait
like form or shape, the range of diameter
ratios is arbitrarily divided between the
lower or higher end of the ranges of the forms
A and C, and the intermediates are listed as
form A/l3 or form B/C.
The mandibles are graded also by diameter ratios, as indicated in Table 3A,B,but not
by subjective classification since all mandibular arches are parabolic in form, as classified by Boyd (1972). There is more of a
narrowing at the canine region in several
individuals, and this can be evaluated by
comparing ratios of mandible to maxilla diameters taken at the canines and at the
molars. These comparisons indicate a “goodness” of fit or degree of congruency between
the upper and lower teeth upon occlusion.
The mandible/maxilla ratios vary from .68 to
.94 (at the canines) and .8&1.09 (at the
392
S. MOLNAR AND I.M. MOLNAR
TABLE 3B. Arch shapr and wear group-Maks’
ID/age (years)
Class
occl.
I11
X-bite
Ill
4W24.6
487/24.5
543L10.5
447/22.5
533/25.5
33/21.1
534/25.6
466/30.5
283/15.9
410/19.1
42 1/26.5
26/55.5
475/22.5
54 1/25.5
545/30.5
I
X-bite
I
X-bite
X-bite
I
111
I
Mn-bcl
I
I
X-bite
I
270i25.5
12/22.1
333/ 19.9
30/26.1
336/39.5
476/25.5
31/23.1
282/ 17.9
430/25.1
269/16.5
17/18.1
453/24.1
570/25.1
10/30.9
X-bite
X-bite
2231’25.5
X-bite
I
I
I11
I
I1
111
I
I
I1
I1
I
Mand/max
C/C
M/M
.94
.79
.76
.77
.72
.86
.74
.77
.78
.78
.79
.78
.74
.74
.73
.77
.74
.74
.79
.78
.76
.73
.74
.76
.79
.81
.71
.81
.68
.75
Shape
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A-hyper
A/BA/BB-parab
B-parab
B-parab
B-parab
B-parab
B-parab
B-parab
B-parab
H/CH/CH/CC-hypsi
C-hypsj
C-hypsl
C-hypsi
C-hvosi
.93
.a7
.93
.86
.86
.94
.91
37
.91
.94
.91
1.09
.91
.92
1.00
.86
.88
.89
.98
.94
.94
.91
.93
.96
.96
.92
.90
.92
.a9
.92
Max
C/M
Mand
.59
59
.62
.63
.64
.64
.56
C/M
52
57
.54
.65
.G5
59
.65
.66
.67
.68
.68
.68
.69
.69
.69
.70
.70
.71
.71
.72
.72
.72
.73
.73
.75
.76
.57
.56
.56
59
.xj
.77
.78
.53
.49
56
.55
.50
.62
.59
58
57
.59
.58
57
57
.57
.60
.66
.60
61
58
.64
Wear
type
4A
2B
4A
3A
2A
1B
2A
2A
3A
2A
3A
2B
3A
1B
2R
4B
2A
1B
4A
4B
4A
3R
3A
4B
3A
2A
2A
1B
ZB
1A
‘lI)/age, individual identification number a n d age. Class occl, angle rlassifications or occlusion note: X-bite, alternate intercuspation
(Brown et al. 19R7); Mn-Rccl,(mandible, buccal version; see ID 26, malc); ***, difficult to orrlude, no classification possible. Mand/max, ratio
uf‘diametersofmandibletomaxillaatthecanines(C/C) andthefirstmolarsIM/M).Archshape,classesofarchsha~eaftcrBoydkeetext)
Max C/M, ratio uf maxillary canine and first molar diameters. Mand C/M. ratio of mandibular canine and first molar diameters. Wear
group, patterns of wear; degree and direction (see Fig. 11. ‘rahles are arranged by maxillary canindmolar ratios.
molars), which suggests one explanation for
the diversity of occlusal load distribution
evidenced by wear patterns recorded in
Table 3A,B and diagramed in Figure 1.
Comparisons of wear patterns with arch
shape show some interesting and distinctive
associations. Considering the slope of the
worn occlusal surfaces relative to the tooth
axis, horizontal wear (see lA,B and 2A,B in
Fig. 1)occurs most frequently on the teeth of
both arches of the five males and ten females
who have h psiloid-shaped palates. x2 tests
show that t is distribution is significant for
females (P = .02) and males (P = .05). Horizontal wear occurs on palates of other shapes
as well; nine females (palate sha es A to B/C)
exhibit this type of wear, as o 11 males.
These individuals have the type of occlusion
called “alternate intercuspation” (X-bite),
which prevents bilateral intercuspation and
contributes to a broader lateral movement of
the mandible. The association of horizontal
K
cf
wear with X-bite is significant by x2 test
(P = .01, females, P = .02, males).
Steep buccal to lingual slope of the wear
lanes on the maxilla molars with the
in a1 to buccal slope o the o posing mandib ey teeth (the oblique wear o groups 3 and
4,Fig. 1) tend to occur most frequently on the
broader, more symmetrically shaped maxillas (form B and some form A). The mandibles
are only slightly narrower in the postcanine
region, and there was an even interdigitation of the opposing cusps of teeth on both
sides of the arches. There is wide individual
variation in wear patterns in these individuals, however, and this wear pattern-arch
shape relationship is not statistically si
icant. Five females and four males, wit out
X-bite, have flat horizontal wear even
though their arches are parabolic or hyperbolic shaped.
Occlusion varies somewhat, but few subjects have other than class 1,though there
P
y ?
Y
f-
ARCH SHAPE AND TOOTH WEAR
are m a y with pronounced overbite and
overjet. Edge-to-edge” bite is rare amon
this group; there are only six females an
nine males with incisors meeting at the incisal edges. All these individuals have considerable wear over the tooth rows, which
a ain supports the observation that this type
o incisor-canine relationship of the opposing arches is the result of function. No helicoidal wear is seen even though there are a
number of subjects with moderate to heavy
wear and exposed dentin over the molars,
who might be expected to have such wear.
lated to orofacial morphology (Molnar et al.,
1983a; McKee and Molnar, 1988a). Other
important contributions to occlusal wear
patterns are the shapes and sizes of the
upper and lower arches; to investigate these,
we examined the casts taken from adults
living at Yuendumu at the time the growth
study was in progress.
Our results of the comparisons of arch
diameter and shape show some interesting
relationships to the pattern of occlusal wear.
The relative arch widths and arch symmetry
are important influences, and this is demonstrated by the statistically significant association of horizontal wear with hypsiloid
shaped maxillas. The opposing mandibles of
these individuals were narrower and parabolic-shaped and would require broad lateral
chewing motions to use the teeth efficiently
to reduce a tough, abrasive food bolus. Such
vigorous mastication would bring more of
the buccal-lingual surfaces into contact and
result in flattening of cusps on both the
buccal and lingual sides of the upper and
lower molars. This type of broad lateral motion is apparent even in those adults whose
lightly worn dentition gives evidence of a less
abrasive diet.
The majority of the subjects have parabolic- or hyperbolic-shaped maxillas and
more often tend to have obliquely directed
wear (types 3A, B or 4A, B in Fig. 1)than the
horizontal types (lA, B and 2A, B in Fig. 1).
The parabolic or hyperbolic shapes suggests
a symmetr of occlusion with the opposing
parabolic-s aped mandibles, and oblique
molar wear patterns are expected, as documented in the several studies of prehistoric
populations cited above. However, the wear
pattern distributions within these two
groups are not statistically significant.
There are 11 of 22 females and 11 of 25
males with shapes A, B, or B/C with horizontal wear types. These exceptions to the expected oblique patterns can be explained by
reference to their occlusion; seven of the 11
males and six of the 11 females have the
X-bite form. This X-bite or alternate intercuspation requires a broader lateral chewing
motion (see Brown et al., 1987)and results in
a more even horizontal wear, as is the case
with the hypsiloid individuals. The association of this occlusal condition with horizontal
wear is also significant.
The remaining nine individuals, four
males and five females, who would be exected to have an oblique wear pattern and
gave the horizontal form instead and none
!i
H
DISCUSSION
The distribution of wear over the occlusal
surfaces may be expected to conform to a
certain attern, as outlined in several previous stuges of rehistoric dentition. The lingual cusps oft e maxillary postcanine teeth
wear more rapidly than do the buccal cusps.
This produces an oblique slope from the buccal to lingual side and the reverse is true for
the mandibular teeth (see Leigh, 1925;
Mehta, 1969; Molnar, 1971). The degree of
these oblique angles change over the more
distal arts of the arches as the width of the
mandi le, relative to the maxilla, increases;
such change describes a helicoidal plane
(Campbell, 1925; Richards and Brown,
1986). However, there are numerous departures from this expected pattern due to a
variety of causes; some are related to dietary
consistency and some are influenced by morphological differences (see Lunt, 1978; Tobias, 1980; Smith, 1984). One major departure is the more horizontally directed molar
wear seen among nomadic hunters described
by Smith (1984); other differences have been
described as being due to paramasticatory
functions of the dentition (Barrett, 1960)and
occlusal variations (Brown, et al., 1987;
D’Amico, 1961).
These studies of prehistoric dentitions still
leave many questions about the development of occlusal wear unanswered, so in our
previous study we took measurements of the
wear facets seen on the serial casts ofAboriginal children at the Yuendumu settlement
(Molnar et al., 1983a,b).Because these casts
were taken from those 6-18 years of age, a
record of rate of wear and its pattern of
develo ment could be obtained. This confirme the results of the studies of prehistoric dentition. There were individual variations, some of which could be explained by
sexual dimorphism and some variations re-
R
t:
cp
393
K
394
S. MOLNAR AND I.M. MOLNAR
have the X-bite occlusion. Three of the four
males may not be easily explained, though
two have narrower anterior mandibular
arches, which might be a contributing factor.
The wear on the dental arches of the fourth
male (ID 26) can be easily understood because of the eater intermolar diameters of
the mandib e (mandible, buccal version).
One of the five females has an irregularly
sha ed mandible making it difficult to occlu e (ID 3311, two of the others have narrow
mandibles in the anterior region but there is
no ready explanation for the two other individuals. In summary, most of the wear patterns conform to expectations, and those individuals who do not may be explained by
reference to their unusual occlusal conditions.
The wear patterns of the sample show
some mesially and some distally directed
types with both horizontal and oblique forms
regardless of arch shapes, which may relate
to face shape. No helicoidal wear is in evidence, though some of the subjects have
advanced wear comparable to that observed
on prehistoric dentition and would be exected to have formed the helicoidal planes.
!'here is no explanation that we can offer
expect to lead a small sample size and to
note that, ased on our observations of hundreds of precontact Australian skulls, helicoidal wear is not a re lar feature. Prehistoric peoples as we 1 as contemporary
populations subsisting on dentally harsh diets generate a great variety of dental conditions, including a diversity of wear patterns.
Considering the entire Yuendumu dental
cast collections, including the serial casts of
the children, there is less than expected
tooth wear. The relative lack of wear of the
children of the growth study can be understood because of their softer diet provided by
the station food rations from the late 1940s.
However, it is more difficult to understand
the lack of wear among some of those persons
who came to maturity before the settlement
was established. Only 22 persons of the 64
had the amount of wear, expected for their
age. Some of the 18-20 year olds had flattened cusps with exposed dentin, and several
of the 30-40-year-old adults did not. These
adults on1 had distinctive wear facets on
their near y intact cusps, but the wear still
represented a greater amount of enamel loss
for their age than comparable European subjects. Several of the older adults, however,
had worn their teeth to the 7-8 degree,
equivalent to what has been observed among
P-
1
g
K"
P
Sam les of prehistoric Australian skulls
(Mopnar et al., 1989). The subjects with
lesser wear for their ages had, presumably,
been living on softer European diets for some
time before they arrived at the settlement,
but where, when, and for how lon needs to
be established. This diversity in t e rate of
loss of the enamel crown and of the underlying softer dentin could be taken as a record of
dietary differences and of life-style.
a
CONCLUSIONS
Our interpretation of these results is that
arch shape, relative size, and occlusion are
important factors in wear pattern development. The rate of wear is another matter,
and, as we and others have demonstrated,
the rate is due to abrasives introduced into
the mouth by food or b nondietary chewing.
There is no single toot wear pattern typical
of either the grou of children enrolled in the
growth study or t e adult residents at Yuendumu we discuss in this report. This is despite any similarity or dissimilarity of diets,
living habits, or paramasticatory jaw functions. Both groups developed some wear at
early ages, but it progressed more rapidly for
those ersons who had spent at least part of
their ife on an aboriginal diet. To establish
the former life-styles of these individuals, an
investigation of earlier records and family
histories could be carried out to identify the
original residence of those persons with the
lesser amounts of tooth wear. For instance,
did they live in towns or on cattle stations
before the establishment of Yuendumu -in
1946, or did they arrive straight from their
foraging lifestyle with minimal prior contact
with Europeans? Relating the dental evidence in this way is another useful application of the dental record to ethnographic
inquiries in addition to its valuable use as an
estimate of age.
Finally, and most significantly, because
we may establish the types of diet, life-style,
and age for a majority of this population, a
more accurate description can be made of the
degree of influence that these variables have
on tooth wear. Such a description can then
contribute to a clearer interpretation of rehistoric Aboriginal dental remains, w ich
also show a considerable variation in patterns and rates of dental wear.
i
R
P
g
ACKNOWLEDGMENTS
This study was supported in part by NSF
grant BNS79-06859 and by the Eustace memorial research fund, University of Ade-
ARCH SHAPE AND TOOTH WEAR
laide, Australia. We express our appreciation to Dr. Tasman Brown for his many
kindnesses and his permission to study the
Yuendumu collections.
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