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Population variation in tooth jaw and root size A radiographic study of two populations in a high-attrition environment.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 79:197-206 (1989)
Population Variation in Tooth, Jaw, and Root Size:
A Radiographic Study of Two Populations in a
High-Att rition Environment
PATRICIA SMITH, YOCHANAN WAX, AND FANNY ADLER
Department of Anatomy and Embryology (P.S.), and Department of
Statistics (Y,W., F.A.), Hebrew University, Jerusalem, POB 11 72 Israel
KEY WORDS:
Teeth, Jaws, Roots, Mandible, Australia, New
Zealand alveolar bone height
ABSTRACT
Radiographs were taken of the jaws of skeletal remains of two
populations of different-phenotype Prehistoric Australians from Roonka and
Early New Zealanders (Maoris). On these radiographs crown, root, and corpus
size were measured. Corpus height was subdivided into alveolar bone height,
defined a s the bone superior to the mandibular canal, and basal bone height,
defined as that inferior to the mandibular canal. Both between and within the
two populations there was a significant and negative correlation between
crown size and corpus height. The differences between the two populations in
corpus height were associated with differences in alveolar bone height rather
than basal bone height and support hypotheses associating continued eruption
of adult teeth with growth of the alveolar bone. The findings also support
previous studies that have shown only a low correlation between crown size,
root size, and corpus height.
In studies of tooth-jaw relations in human
populations, tooth size is usually defined in
terms of crown size, with no reference to root
size or form, although the two show only a
low correlation (Garn et al., 1978; Rosing,
1983; Smith et al., 1986). Moreover, neither
root size nor crown size appears to be highly
correlated with jaw size in recent populations
(Anderson et al., 1977; Moore et al., 1968;
Lavelle, 1962). In investigations of the functional significance of evolutionary trends in
the hominid dentofacial complex, more attention then needs to be paid to the relation
between the size and proportions of the
crown, roots, and jaws. It must also be recognized that the relationship between these
three variables is dynamic and changes
throughout life.
With function, the crowns of the teeth
reduce in size, through occlusal and interproximal attrition, and the amount of tooth
material so lost varies with the diet and other
activities that cause dental attrition (Molnar,
1971; Smith, 1984). In contrast the roots normally show little change in size with age or
function, although in rare cases root resorp-
0 1989 ALAN R. LISS, INC.
tion or excessive deposition of cementum
may occur (Gustafson, 1950). The latter has
been described a s relatively common in Eskimos, where it has been attributed to excessive
forces exerted on the affected teeth (Pedersen,
1949).Corpus height may increase or decrease
throughout adult life, depending on the extent
of increase in alveolar bone deposition with
continued eruption of the teeth (Murphy,
1959; Ainamo and Talari, 1975), or alveolar
resorption in the presence of periodontal disease (Loe et al., 1979).
Data on Near Eastern populations suggest
that crown size, root size, and corpus height
show only a low correlation (Smith et al.,
1986). However, it may be argued that the
majority of the populations examined in this
study were utilizing relatively soft and nonabrasive foods, so that the findings are not
typical of populations in whom functional
demands on the dentition are high. The present study was designed to examine these
assumptions, by investigating the associaReceived August 7, 1987; revision accepted August 3, 1988.
198
P. SMITH ET AL.
tion between crown, root, and corpus height
dimensions in prehistoric Australians (Aboriginals from Roonka) and New Zealanders
(Maoris). Both suffered rapid and severe
attrition but differed in dietary base and
food preparation techniques. They provide a
good case study of the extent and pattern of
phenotypic variation in the teeth, roots, and
jaws and the effects of severe attrition on
corpus height.
MATERIALS AND METHODS
This study was carried out on the mandibles of skeletal samples of prehistoric Australians and New Zealand Maori. The Australian sample was excavated from Roonka,
a site on the Murray river in southeastern
Australia and dates from 7000 BP to 1800 AD
(Pretty, 1977). It is housed in the South Australian Museum, Adelaide, Australia. The
Australians were hunters and gatherers, exploiting riverine resources that included fish,
shellfish, and birds as well as marsupials
and plants (Pretty, 1977). The individuals
from Roonka were of moderate stature (males
167 cm) with dolichocranic heads and relatively short faces and small jaws (Prokapec,
1979). Their teeth were exceptionally large,
with severe attrition and little caries or periodontal disease (Smith et al., 1988).Although
the sample covers a period of some 7000
years, it shows no unidirectional diachronic
trends in skeletal morphometrics or pathology, suggesting little change in the gene pool
or environmental stress over the entire period.
The Maori sample, housed at the University of Otago, Duneidin, New Zealand, covers
a shorter time span (circa 1100-1900 AD) but
is more heterogeneous since it is largely composed of individual specimens from different
sites within New Zealand. The population
sampled was tall with exceptionally large
faces, large “rocker” mandibles, characteristic of Polynesians, and teeth of moderate size.
They were horticulturalists, but their diet
included seals, fish, and shellfish, as well as
wild birds and plants, and especially fern
root, which caused rapid and severe dental
attrition (Taylor, 1963; Shawcross, 1967;
Houghton, 1978a, 1980).
Criteria for inclusion of a specimen in the
study was the presence of a t least three teeth
(premolars or permanent molars) with fully
formed roots. Age and sex were determined
by examination of the skeleton (Krogman,
1962) and compared with that recorded by
Prokapec (1979) for the Australians and by
Houghton (unpublished catalogue) for the
New Zealanders. Only a few female specimens fitted the criteria for inclusion in this
study, so analysis was restricted to male
specimens. Corpus height was measured directly on the mandibles, between M1 and M2,
and compared with measurements taken on
the radiographs to assess distortion and
magnification. Radiographs and measurements were taken as described in Smith and
coworkers (1986), with the addition of two
new measurements of alveolar and basal
bone, as described below.
Radiographs were taken of the premolar
and molar teeth, using dental occlusal films
taped parallel to the dental arch in the
premolar-molar region (Fig. 1). Where both
sides were intact, radiographs were taken of
both sides. The mandible and film were positioned such that the film was a t right angles
to the X-ray beam and at a distance of 1.5 m
from the energy source. Along each mandible
a 4-cm aluminium wedge was placed to
standardize density and provide a scale. On
the radiographs, digitized tracings were recorded of crown outline, root outline, and corpus height.
The crown and root were demarcated by a
line joining the cervical margin of the enamel
on the mesial and distal aspect of each tooth.
Mesiodistal crown length (CW) was traced
parallel to this line, and the maximum value
was recorded. For the premolar roots the
width midway along the root length was
traced parallel to this line (RW1 and RW2),
and root height (RH) was traced parallel to
the long axis of the tooth to the root tip. For
the molars, lines were constructed joining the
root apices and used to measure the distance
between them (RW3, RW4, and RW5). Root
height was measured from these lines to the
line demarcating the crown and root.
Corpus height was traced interproximally
from the interdental crest to the base of the
mandible, mesial to each of the molars and
the first and second premolar. From the tip of
the premolar root and mesial roots of the
molars a line was drawn parallel with the
line defining corpus height crossing the inferior dental canal. The distance from the root
apex to the lower border of the inferior dental
canal was then traced and used as a measure
of “alveolar bone” (IDC), while the distance
from the inferior dental canal to the lower
border of the mandible was used as a measure of “basal bone” (LB). From these computerized tracings, we obtained crown height,
TOOTH AND JAW VARIATION
199
Fig. 1. Radiograph of molar region of the mandible.
Arrow shows upper limit of basal bone.
crown length, cross-sectional root area, root
height, root width, corpus height, alveolar
bone height, and basal bone height for each
tooth.
Measurements of corpus height taken on
the mandibles were compared with those
taken from the radiographs, to assess possible distortion and/or magnification on the
radiographs. No significant differences were
found. A further 15 radiographs were redigitized, then compared with the original measurements to determine experimental error
related to location of landmarks. This was
calculated after Dahlberg (1940) and found to
average 2-396 for parameters examined. It
was greatest for measurement of basal bone
and alveolar bone, because of difficulties in
locating the lower border of the inferior dental canal.
Statistical analysis
On the specimens from Roonka, radiographs were taken of both left and right
sides, and the paired Wilcoxon and student’s
t-tests were used to check for right-left
asymmetry. The correlation between the two
sides was also examined to evaluate the
effect of averaging the measurements of both
sides. Because of the relatively small number
of observations, the differences in crown root
and jaw proportions between the two populations was assessed through several statistical procedures. Scattergrams were plotted to
show the marginal distribution of the parameters for each population as given by the corresponding distribution points on each axis.
The projections of the points on the axes
represent the measurements of the variable
associated with the axis.
Univariate comparisons on each parameter and tooth were performed using the t-test
and Mann Whitney test. The KolmgorovSmirmov test was applied when the distribution seemed to have a different shape in the
two populations. Spearman’s rank correlation coefficients were calculated between variables in each population. Multivariate characterization of the differences between
Roonka and Maoris was obtained using the
linear discriminant technique and analysis
of covariance.
Discriminant analysis defined the linear
function of the variables measured in each
tooth that best discriminates between the
populations. The efficiency of the discrimination is estimated by the misclassification
error rates (Lachenbruch, 1975). I n order to
determine if the differentiation between the
two populations is expressed similarly in
each tooth, the correlation between the discriminant analysis scores for adjacent teeth
was examined. The contribution of each
parameter to the difference between the two
populations was also derived from the analysis of covariance technique. Through this
method, the independent contribution of any
200
P. SMITH ET AL
specific parameter to the difference between
the two populations is assessed after adjusting for the effect of the other variables. The
relationship between covariance adjustment
and discriminant analysis in choosing discriminating variables is discussed in Rao
(1966), and the use of covariance analysis is
discussed in Grizzle and Sen (1983).
RESULTS
A relatively high correlation was present
between the right and left sides of the same
individual, while no consistent statistical differences were found between them. Accordingly, for those individuals with both sides
present, averaged measurements were used
in all further analyses. The difference between
the two populations in any one parameter a s
well as the difference in the relationship of
any two parameters in the second molar can
be deduced graphically from Figures 2-5. The
other teeth studied showed a similar relationship, with maximum crown height and width
and root measurements larger in Roonka and
corpus height greater in Maoris. The marginal distribution of the parameters for each
population is shown by the corresponding
distribution of the points on each axis. The
projections of the points on the axes represent the measurements of the variable associated with the axis.
The range of values recorded for crown
height and mesiodistal diameter clearly differs in the two populations (Fig. 3). The maximum values are those of unknown teeth, and
these are consistently larger in Roonka. The
univariate analysis clearly shows that the
two populations differed in all parameters
(Table 1). Mean values for crown height,
width, and cross-sectional area were consistently higher in Roonka, a s was root size for
all teeth except the third molar. Corpus
height was greater in the Maoris, and this
was mainly due to differences in alveolar
bone height (Table 1). Statistically significant differences between populations were
most marked in the second molar and least
marked in the first premolar. In both populations correlations between the various crown
parameters and those of the bone tended to
be negative, and this applied especially to the
correlation between crown height and corpus
height (Table 2). Correlations between root
height and area and corpus height were,
however, positive. Population correlation
coefficients and the bivariate scattergrams
further indicate that correlations among
crown and root parameters tended to be positive, while correlations between alveolar bone
height and basal bone height tended to be
negative.
The multivariate analyses (discriminant
and covariance analyses) were based on
crown height, crown width, root area, and
corpus height. The contribution of each specific variable to the difference between the
populations is summarized in Table 3. The
results indicate that not all the variables
included in the analysis contributed significantly to the relatively good separation between the two groups. Crown width showed
the highest discriminatory power in every
tooth. Corpus height contributed to the differentiation in the second premolar and third
molar, and crown height contributed to the
differentiation in the second premolar. Root
area gave poor discrimination in all teeth.
The high correlation between the discriminant analysis scores indicate that the overall
difference between the two populations is
preserved in adjacent teeth (Figs. 6, 7). Discriminant analysis carried out with basal
bone and alveolar bone entered as separate
variables showed that alveolar bone made
the main contribution to differences in bone
height.
Covariance analysis supports the above
results and those implied by the signs obtained from the standardized coefficients
(Table 4): namely, significantly higher adjusted mean mesiodistal diameters in Roonka
and significantly higher adjusted means for
bone height in the Maoris in the second premolar and third molar. Controlling for the
relatively higher mesiodistal diameter and
lower bone height in Roonka yielded a lower
adjusted crown height in this group a s compared to the Maoris. The higher values
obtained for unadjusted crown height in the
Roonka teeth seem to be due to the observed
differences in mesiodistal diameter and bone
height.
DISCUSSION
The discriminant analysis demonstrates
the marked differences present between the
two populations (Figs. 8-11). Crown height
and mesiodistal diameter gave good discrimination between them. However, while corpus
height was greater in the Maoris, it seems
that this was mainly due to differences in
alveolar bone height. The contribution of
basal bone height was nil for the second
premolar and first two molars, while root
201
TOOTH AND JAW VARIATION
cw4
c 144
cn4
I
I*
.
120
108
RW4
0
LB4
96
cn4
0
21
.
0
O
..
35
.
. *
O
0
49
D
.
63
77
CH4
Figs. 2-7. Scattergrams showing distribution of parameters studied for the second molar. Solid circles represent
values recorded for the Australians, and open starred circles represent values recorded for the New Zealanders.
Fig. 2. Values plotted for alveolar bone height (IDC4)
against crown height (CH4).
Fig. 3. Values plotted for crown width (CW4) against
crown height (CH4).
Fig. 4. Values plotted for root width (RW4) against
crown height (CH4).
Fig. 5. Values plotted for basal bone height (LB4)
against crown height (CH4).
dimensions were similar in the two groups.
The discriminant analysis provides further
evidence of the independent contribution of
tooth size and corpus height to the differences between the two populations. The analysis of covariance provides complementary
information with respect to the differences
between the two populations and assesses
the contribution of each parameter.
The Maori face and mandible is large, and
the ascending ramus is high (Houghton,
1978b; Pietrusewsky, 1984). The mandible,
like that of other Polynesians, is characterized by a large “rocker” form that is due to a
convex inferior border. Houghton (1978a,b)
attributed this to buckling of the basal bone
during growth. Darling and Levers (1975)
have described the important role of alveolar
202
P. SMITH ET AL
1
I - ... . .
..
1471
1
7
0
I
RA4
..
.0..
.
0
9
133
GO..
0
"
cw4
. *
0
.
:
b b
119
0
D
.
1100
1 3 0 1
I
0
0
ti*
...
00
v
*
Q
*
O
0
105
0
0
0
0
5
0
9oot
ci
u
260
I
o
300
340
380
1
220
0
I
260
300
340
~ n 4
B H ~
Fig. 2-7. (Continued)
Fig. 7. Values plotted for crown width (CW4) against
Fig. 6. Values plotted for root area (RA4) against corcorpus height (BH4).
pus height (BH4).
TABLE 1 . Two-tailed P-values for differences between
Roonka and Maoris in tooth parameters, using t-test or
the Kolmagorou-Smirnov test when the distribution in
the two s a m d e s differed1
Variable/tooth
Pml
Pm2
M1
M2
M3
CH
CW
CA
RH
1DC
LB
RW
RA
BH
.32
.06
.13
.97
.15
,000
,005
.0001
.10
,000
.03
.35
.002
,000
.18
.352
,043
.033
.79
.10
.I8
.607
.I1
.I2
.I0
.05
,033
.02
,0053
,563
.002
.003
.013
.78
.09
.80
.7Ei2
,005
.03
.21"
,573
.0033
,223
.573
,003"
1 Crown size is larger in Roonka. 2 Root size is larger in Roonka
for all teeth except M3. 3 Total bone height and alveolar bone
height values are lower m Roonka
2Mean values are similar.
3Mean values in Roonka are smaller than those of Maoris.
For all other values, mean values in Roonka are greater than those
of the Maoris.
bone growth in maintaining the teeth in
occlusion after eruption. The exceptionally
large height of the corpus in the molar region
of the Maoris a s compared to Roonka may
then be due to compensatory growth of the
alveolar bone as the ramus increases in
height and the basal bone bends during
growth, since it is not related to tooth or root
size.
With similar root area but larger teeth, the
potential force exerted per unit area to the
bone in Roonka is greater than in the Maoris.
This study is two-dimensional only and does
not discuss buccolingual diameters of the
teeth or roots. Corpus width of the two groups
examined here is, however, similar. Mean
values are 15.7 mm in Roonka and 15.5 mm
in the Maoris in the first molar region.
The Roonka sample represents one of the
most robust, largest-toothed Holocene populations known from Australia (Brown, 1982;
Smith, 1982).The severe attrition of the teeth
and prominent areas of muscle insertion on
the bone attest to powerful and severe masticatory stress experienced by this population
and borne out by ethnographic accounts of
eating habits. The condition of the teeth and
jaws of earlier Aboriginal populations from
Cobool Creek and Kow Swamp (Brown, 1982;
Thorne, 1976) shows very clearly that heavy
dental function was the norm throughout
Australian prehistory. We find, however, in
the Roonka population, as indeed in other
Australian aboriginals, a combination of
large teeth and small corpus height (Solow et
al., 1982). Indeed molar corpus height in the
Roonka sample is similar to that of a recent
Near Eastern population, although crown
and root size in them is some 30% smaller
than that found in the Roonka population
(Smith et al., 1986).
Prehistoric Polynesian populations from
Hawai and Tonga have comparatively little
203
TOOTH AND JAW VARIATION
TAB1,E 2. Within-populationassociation of tooth parameters: Spearman correlation coefficients
for the second molar in Roonka and Maori samdes
C€[4
0.5551
1 CH4
2 cw4
3 RA4
4 CA4
5RH4
6 IDC4
7 LB4
8 RW4
9 BH4
cw4
0.5540
-0.C683
0.E 099
-0.2486
-0.5093
-0.5816
-0.C444
-0.4885
0.3726
0.7471
-0.1121
-0.3446
0.0099
0.4236
-0.3132
RA4
CA4
-0.2634
-0.1154
0.9396
0.6546
-0.2334
0.0711
0.5948
-0.0124
0.0124
0.6360
0.2539
-0.1697
-0.5286
-0.2556
0.1222
-0.4420
Maori
RH4
-0.6579
-0.3997
0.6346
-0.6539
0.1082
-0.0861
0.1497
0.4729
IDC4
LB4
RW4
BH4
-0.062
-0.1940
-0.4060
-0.0423
-0.4006
-0.4060
-0.1883
0.2779
-0.3469
0.3892
-0.2510
0.2057
0.1667
0.5127
0.2944
-0.1631
-0.1948
0.2930
-0.3732
-0.0651
-0.3089
0.0999
0.4904
0.3118
-0.3010
0.1974
0.5357
0.0319
0.0572
-0.3091
Roonka
TABLE 3. Discriminant analysis results: standardized coefficients
and classification dates for each tooth and variable
VariabWtooth
CH
cw
RA
BH
Constant
Correct
classification
rates (W)
Pml
Pm2
M1
M2
M3
,515
-1.705
-.01068
22199
2.68589
1.19
-3.00
.00051
.382
3.67
-.0196
-1.170
,017
,053
10.79
.28
-1.17
-.a26
.09
10.83
.59
-34
.O1
.28
2.8039
59.4
85.4
70.0
78.6
TABLE 4. Aiialvsis of covariance-summary table
Adjusted means
Dependent
variable
Col ariates Tooth Maori Roonka P-value
1
CH
CW, BH, RA
BH
CH, CW, RA
2
3
4
5
1
2
3
4
CW
RA
CH. RA.BH
BH, CH,CW
5
1
2
3
4
5
1
2
3
4
5
45.93
51.57
45.86
52.05
51.79
342.39
347.66
311.08
291.76
305.47
72.65
71.16
114.55
40.76
40.16
46.02
47.54
47.01
330.35
316.38
307.73
282.80
281.99
80.54
80.14
119.56
118.51 129.24
112.48 178.03
716.27 723.54
743.12 742.28
1,066.42 1,104.58
1,128.75 1,209.18
1,105.51 1,063.85
*
,0017
,9621
,3296
.1753
,2900
*
.7275*
,4785
,0356
,0128
,0000
,0395
-0011
.0784
,858
(*)
,492
(*)
,470
*The difference be ween the populations depends on the covanate
levels. P-values are for equality of adjusted means.
dental attrition (Smith unpublished data).
The Maoris are Polynesians and in contrast
to the Austr.dian aboriginals were horticulturalists w i t h similar traditions to those of
other Pacific: Polynesians; presumably like
69.2
them they were accustomed to a low-abrasive
diet. Houghton (1978a, 1980) has emphasised
the seventy of the New Zealand climate,
especially in the South Island. He suggested
that the climate deteriorated over time, reducing the yields of horticulture and resulting in
increasing dependence on abrasive fern roots
as a dietary staple.
The Maori and Roonka samples were then
descended from populations that had experienced different selective pressures on the
teeth and jaws in the past, and this presumably accounts for the small teeth of the
Maoris despite their recent exposure to a
high-attrition diet (Smith, 1982). However,
both populations showed a similar response
to attrition, in that corpus height increased
a s crown height and width decreased. As the
correlation coefficients show (Table 2), the
increase in corpus height was mainly due to
a n increase in the height of the alveolar bone
and may be attributed to alveolar growth
associated with continued eruption of the
teeth as described by Ainamo and Talari
(1975).
The correlation coefficient between loss of
crown height and increased corpus height
was, however, low, suggesting that the two
events were not highly correlated. This assumption receives some support from the
204
P. SMITH ET AL.
2
0
3.2
V
0
1
4
off
0
V
'
0
V
1.8
0
V
0
.
.
s1
1
-2
. .
. .
. . .... . .
0
52
ao
0
.
-
0
O
0
V
*
0 .
0.
.
- 1.6
.v
-3
- *5
--5o
-50
15
53
SZ
0
V
32
1.
0
0
00
9
0
0
V Q
0
s3
0
.
..
a.
.. ...
..
s4
*
Q(1
O
- .9
.1.f
-2.5
.
.
-.
....
..
0
0
0
0
0
m
-
0
0
1.6
.
0
0.1
0
0
0
0
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-
- 0
-16
v
.
0
-.50
0 .
1.5
a5
54
-12
0.0
1.2
2-4
$55
Figs. 8-11. Plot of the discriminant analysis scores for
adjacent teeth. S1 = scores for the first premolar; S2 =
scores for the second premolar; 53 = scores for the first
molar; S4 = scores for the second molar; and S5 = scores
for the third molar. Symbols as in Figures 2-7.
findings of Murphy (1959) and Tallgren
(1957). They found that in populations with
severe attrition, lower facial height (which is
largely governed by the combined height of
alveolar bone and crown height) decreases,
while it increases in populations with little
attrition. Murphy (1959) also reported that
the observed variation in facial height is
associated with growth of alveolar bone as
well as active eruption and attrition of the
teeth. This means that while continued eruption in adult life is a n ongoing process that
compensates for loss of crown height through
attrition, it does not seem to be a very finely
tuned process. This is achieved by continued
remodeling of the mandibular condyle in
older individuals to compensate for changes
in the level of the occlusal plane, as described
by Mongini (1975) and Richards (1984).
The pros and cons of a physiologic as
opposed to a pathologic basis for alveolar
resorption have been discussed a t great length
over the years, with some workers claiming
that under normal conditions, there is physiological migration of the gingival attachment
to maintain constant clinical crown height
following attrition. However, such claims are
invariably derived from populations with a
205
TOOTH AND JAW VARIATION
high-cereal diet, which is conducive to plaque
accumulation and so to periodontal disease
and alveolar bone loss. This probably accounts for the marked differences in alveolar
bone loss reported by Lavelle and Moore
(1969) between 8th and 17th century British
populations. Moreover, close examination of
the frequently quoted study by Philippas
(1952) on Greek skeletal remains shows that
in only 50% of the attrited teeth was clinical
crown height equivalent to that of unworn
teeth. In the other 50% clinical height was
considerably greater, and this Philippas acknowledged as due to the pathological resorption of alveolar bone from periodontal disease. We would argue that in the other 50%
periodontal disease was also present, but in a
less advanced form that approximated the
height of unworn teeth. Since both attrition
and periodontal disease are age related, the
rae of both must occasionally coincide, a s
found by Newman and Levers (1979) for
Anglo-Saxons and by Whittaker et a]. (1982;
1985) for a Romano-British sample. That the
association is accidental we infer from the
fact that there is no correlation found between the two in the majority of populations
studied. This is especially true when the
populations are eating a n abrasive but lowcarbohydrate diet as i n the Australians
studied by Murphy (1959), Amerindians
studied by Goldberg et al. (1964), and the
Australians and New Zealanders studied
here.
Clinical studies have shown that active
eruption, even under extreme conditions, is
usually associated with growth of alveolar
bone (Ainamo and Talari, 1975;Ingber, 1974).
Mechanically this is important, since reduction of crown height, without accompanying
reduction of the area of root attached to the
bone, is mechanically advantageous. If on
the contrary, crown height is maintained at a
constant level, through apical migration, the
clinical crown-root ratio gradually increases.
This produces a n unfavorable crown-root
ratio that cannot withstand lateral forces
and contributes to the rapid progress of periodontal disease.
In conclusion, this study provides further
evidence of the low correlation present between crown size, root size, and corpus height
both within and between populations. The
results also show that for all variables studied, the differences are maximal in the
second molar region. These results conform
to those previously obtained for Near Eastern
populations of different phenotype and den-
tal function. They suggest that factors other
than crown and root size must be considered
in the analysis of evolutionary trends in the
hominid mandible.
ACKNOWLEDGMENTS
We are indebted to the Departments of
Dental Radiography of the Faculties of Dentistry at the Universities of Adelaide and
Otago for use of their equipment and assistance in taking the radiographs on which
this study was based. We would also like to
thank G. Pretty, South Australian Museum,
and P. Houghton, University of Otago, for
permission to examine the specimens described here and T. Brown for his hospitality
and assistance with all phases of the study.
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