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Changing occlusal variation in Pima Amerinds.

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Changing Occlusal Variation in Pima Amerinds
Department of Anthropology, Southern Illinois Uniuersity Carbondale,
Illinois 62901 (R.
S. C.), Indiana University School of Dentistry,
Indianapolis, Indiana 46202 (R.H. Yl?), and Uniuersity of Chicago,
Chicago Illinois 60637 (A.A.D.)
Dental, Occlusion, Pima, Diet consistency,
Malocclusion, Deciduous teeth, Tooth replacement, Arch size
Occlusal variables and arch measurements have been recorded on 341 Pima Amerinds pertaining to two samples, one of older individuals raised on traditional diets, the other of younger individuals raised on
refined commercial foods that are typical of modern urbanized people. Permanent dental occlusion is significantly more variable from defined ideals in the
younger sample, despite their fewer deformities resulting from progressive
aging. The youths had relatively narrower palates, correlating with a general
trend among industrialized populations and also among experimental animals
fed soft diets. Relative rate of deciduous tooth loss and replacement did not
correlate with any occlusal variable.
Modern industrialized Western society is
characterized by highly variable dental occlusion, often times exceeding limits set by
orthodontic consensus for acceptable or good
occlusion. Approximately 50% of all American youths show occlusion for which orthodontic treatment is rated mandatory or
desirable (Kelley and Harvey, 1977). As with
other clinical specialists, orthodontists tend
to show more interest in correction than in
finding causes. Major textbooks and review
articles (cf. Corruccini and Whitley, 1981)
concur in considering malocclusion “not abnormal” and predominantly genetic in origin, hence nonpreventable.
Various anthropologists or anthropologically oriented dentists have periodically
shown that nonindustrialized, nonurbanized
populations manifest much lower malocclusion prevalence (see Niswander, 1966; Lombardi and Bailit, 1972; Kaul and Corruccini,
1983 for literature review). The transition
from the ancestral state of anatomically ideal
occlusion to prevalent bad occlusion has been
shown to happen in several groups within
one generation upon the acquisition of “modern” (i.e., processed) diet, to annul both the
normal-condition and genetic-cause concepts.
Similar to heart disease and diabetes which
are “diseases of civilization” or “Western dis-
0 1983 ALAN R. LISS. INC
eases” (Trowel1 and Burkitt, 1981)that have
attained high prevalence in urban society
because of environmental factors rather than
“genetic deterioration,” a n epidemiological
transition (Omran, 1971) in occlusal health
accompanies urbanization.
Western society has completely crossed this
transition and now exists in a state of industrially buffered environmental homogeneity.
The relatively constant environment both
raises genetic variance estimates (since
environmental variance is lessened) and
renders epidemiological surveys largely
meaningless because etiological factors are
relatively uniform. Nevertheless most occlusal epidemiological and heritability surveys
are conducted in this population rather than
in developing countries currently traversing
the epidemiological transition. In this regard, anthropologists have a vital contribution to make in integrating the cross-cultural
approach with occlusal epidemiology. Appropriate study populations are disappearing
with development. One study sample is the
Pima, a reservation Amerindian group from
which one of us (A.A.D.) collected about ten
thousand dental casts from 1948-1970.
Received January 10, 1983; revision received June 30, 1983;
accepted July 6, 1983.
These casts have been studied for evidence
of odontometric heritability (Potter et al.,
1968, 1983)and differences with fluoride variation in well water. An initial study (Irie
and Dahlberg, 1967)presented some occlusal
data and concluded that permanent dental
occlusion did not change between ages 1216. Extending this comparison to older Pima
offers the advantage of assessing occlusal
change occurring with dietary change. The
Pima traditionally relied on a heavy bean
dietary staple grown in their own gardens.
Tortillas were also a favorite, the indigenous
corn long-cooked and stone-ground. More recently, approximately since 1950, the Pima
do not grow their beans, as the all-important
country store became the source of commercial beans and processed corn meal. In postwar years the children started receiving good
government sponsored dental care, while this
has not reached the adults.
The dietary transition, considered to be a
factor in the dental occlusal epidemiological
transition, was fairly subtle in this population, entailing a change from homegrown to
commercially purchased versions of the same
foods with the latter more refined and more
cariogenic. The effect of this and other
changes on the Pima occlusion have not yet
been considered and are the subject of the
present study.
A stratified cross-sectional sample from the
cast collection was used, in order to represent
the various collection times. Records provided exact age, sex, date of casting, and
family. If yearly serial casts of the same individual were available, the most recent one
(when the individual was oldest) was used.
Only those casts were scored that had no
missing teeth (congenitally or otherwise), and
a t least all but one permanent tooth fully
erupted (excluding consideration of M3) such
that the full permanent occlusion alone could
be assessed. For older individuals (over age
40) it was necessary to allow up to two missing teeth to improve sample size. There were
as many as six yearly casts over a nine-year
period for younger Pima. Most of the casts of
older Pima come from the earlier (1949-1957)
series of collections, such that the effects of
aging on dentition could be cancelled in the
comparison with younger Pima, many of
whom were cast between 1963-1969. Average age was 14.7 for young and 27.7 for old
Pima. We established lack of correlation of
occlusion and age (between 11-30 years) in
the young sample, similar to Irie and Dahlberg (1967) and also true of the much larger
samples of Kelley and Harvey (1977).
Taking 1950 as roughly the cut-off time for
traditional Pima diet and considering that
dietary effects on permanent occlusion probably act up to 12 years of age, when deciduous premolars are replaced and M2s are
erupted, we would expect individuals 30 or
younger (by 1969, the last year of casting and
the baseline date for this study) to show occlusal effects from dietary change and those
over 30 (by 1969) to have developed occlusally during the traditional diet period. The
former group was defined as the younger
sample and the latter group as the older sample. Every case had two chronological data,
the number of years ago born from 1969
(YAB) and actual age at casting. Thus the
younger sample consisted of 274 Pima with
YAB of 30 or less, and there were 67 older
Pima with YAB of 31-50. The minimum age
for individuals in the young sample was 11.
The variables studied include arch measurements, and measurements and observations of dental occlusal variation (not reflective of “skeletal malocclusion”) taken and
modified from the criteria making up the
Treatment Priority Index (TPI) described in
Kelley and Harvey (1977):
Arch Size. Lower arch length (left) was
measured from the most anterior point between the cervical margins of the medial incisors to the distobuccal margin of the crown
of MI. Upper arch length was from the interincisal point to the distobuccal margin of M1.
Arch breadth was measured between the
buccal grooves of the MI and M1.
Overjet. This is the anterior distance from
the most mesial part of the labial surface of
the upper central incisor to the labial surface
of the opposing lower incisor, measured perpendicular to the coronal plane.
Overbite (anterior). With the casts in centric (convenience) occlusion, this is the
amount of vertical overlap of the upper central incisor over the lower central incisor
taken as a ratio of the total crown height
(cervix to incisal edge) of the lower incisor.
Cross-bite (posterior). This measures buccal-lingual deviation in occlusion of postcanine teeth (Kelley and Harvey, 197750). Its
extent was measured by the distance the mesiolingual cusp of M’ deviated from centric
occlusion with the central occlusal fossa of
M1 taken as a ratio (in tenths) of the bucco-
lingual size of the M’ mesiolingual cusp. The
measurement was positive if buccal crossbite c M 1 positioned too far to buccal) occurred, and negative when lingual cross-bite
was observed, defining crossbite type. The
absolute value of this is used to define extent
or amount of crossbite. The measure was incremented for bilateral crossbite.
Buccal segment relation (BSR). This assesses mesial-distal relations of first molars
in centric occlusion (Kelley and Harvey,
1977:49).It was measured as the amount the
mesiobuccal cusp of M1 deviated from ideal
alignment over the buccal groove of MI,
again taken as a proportion of the cusp’s size
(that is, a score of 0.5 indicates that the mesiobuccal M1 cusp directly overlies the mesiobuccal cusp of MI). The BSR type
measurement was positive for distoclusion
(mandibular orthognathism) and negative for
mesioclusion (mandibular prognathism), and
again absolute value gives BSR extent. Bilateral BSR discrepancy incremented the
Rotation and displacement (R/D).The rotatioddisplacement score is the sum of the
number of teeth rotated or displaced noticeably from ideal alignment, teeth rotated more
than 45 degrees or displaced more than 2 mm
being counted twice. Anterior and postcanine
teeth were considered separately, then added
to give the total R/D score.
Treatment priority index (TPI). This combined index, designed originally to assess
need for orthodontic treatment, is computed
according to the method of Kelley and
Harvey (1977). However, our only purpose in
using it is to derive a n overall measure of
occlusal variation that compounds the individual measures. The TPI is only a n epidemiologic variable in the present study and
has only internal comparative (not clinical)
With the exception of R/D, all the TPI components are measured along a continuous
scale from positive to negative values. Thus,
cases of mandibular overjet and of open-bite
were entered as negative overjet and negative overbite, respectively. All measurements were made by the same investigator
(R.S.C.) with Mitutoyo dial calipers calibrated to 0.02 mm.
Like the arch measurements, the occlusal
variables constitute ratio-interval data so
that parametric statistical techniques may
be applied. There was considerable positive
skewing in some variables. The one-tailed F
test was employed to directionally test the
hypothesis that younger and older Pima can
be considered a s having been sampled from
the same statistical universe regarding variance (frequency and amount of departure
from the norm), and the t test was used on
the mean values. The F test is more appropriate for measurements of ‘<type’’of occlusal
discrepancy and the t test for “extent.”
Premature Deciduous Tooth Loss
The effect of eruption sequence and premature deciduous tooth loss (PDTL) has
caused speculation in discussions of malocclusion etiology. Early shedding of a deciduous tooth, premature eruption of its successor, and a n excessive time lag between
those two events are interrelated (Miller et
al., 1965; Singh, 1980). Earlier epidemiological results have been quite inconclusive regarding the PDTL factor (Lundstrom, 1955;
Miyamoto et al., 1976; Baume, 1973; Corruccini et al., 1983; Anderson et al., 1980). The
longitudinal series of Pima casts allows a
unique test of PDTL influence in a n occlusally variable population. Teeth lost prematurely due to advanced caries could be
identified and considered as a separate category from prematurely lost teeth with no
detectable carious involvement.
A PDTL score was devised to record noticeable degrees of deciduous tooth exfoliation
occurring prematurely from the normal
schedule. Due to maturational variation and
secular trends (the young sample erupting
second molars a year earlier on average than
the older sample), it is important to compare
tooth succession by a n absolute time schedule and by a relative scale that compares a
given tooth to others which ordinarily should
be shed earlier or later. Among the younger
Pima sample normative ages fell into four
categories: permanent incisors replacing deciduous incisors (lower incisors appearing
first; central incisors occluding first) a t age 8
or 9, permanent anterior premolar replacing
its deciduous predecessor a t age 10, canines
and posterior premolars appearing a t age 11
(lower canines appearing first), then by age
11-12 the anterior permanent premolar
achieves occlusion, followed by canines and
posterior premolars and eruption of the second molars. A deciduous tooth class lost prematurely by one year counted as one point,
including maximally 1/2 point counted for a
tooth erupting one year earlier than either a
normally synchronous antagonist or a n anti-
BSR type and extent are both significantly
more variable in younger Pima, signifying
greater average deviation around stipulated
norms, and mean BSR extent is significantly
larger in the younger Pima. The abnormality
is predominantly with mandibular molars
being too far anterior, which is usually the
case for non-European groups. Only Western
European-derived whites tend to show predominant “Class 11” malocclusion with the
upper arch displaced anteriorly. When the
sexes are separated, differences are still
highly significant (x2 = 8.6; d.f. = 1) for
males; only eight of 48 older male Pima have
cusp-to-cusp or worse BSR anomaly, compared to 96 out of 239 younger males. There
is no difference in the much smaller female
Postcanine tooth rotation and displaceRESULTS
ment is nearly twice as prevalent in the
The sexes were very unbalanced with re- young sample with a highly significant difspect to sample size, a total of 287 males and ference between the groups. Displacement of
54 females having been sampled in younger anterior teeth is also less in the older Pima
and older groups. The bias resulted from too raised on traditional diet, but more variable
few younger females having been cast at a n (A few older Pima had very severely crowded
old enough stage of dental maturation (11-12
incisors due to mesial drift.). Although 16.4%
years). Every variable was tested for sex dif- of older Pima showed perfect anterior tooth
ference, and none was found with the excep- alignment versus 10.6% of the younger, the
tion of the metrical arch variables, to agree difference is not significant based on the
with Irie and Dahlberg (1967) for Pima, and sample. Total rotation and displacement is
Kelley and Harvey (1977)for large American significant between the groups, reflecting the
samples. The exception was BSR type, where high incidence of this disorder in the postthe females showed significantly less devia- erior teeth of the younger group.
tion from the norm so that this variable was
The TPI is smaller, though not significant,
compared between old and young samples and less variable in older Pima. Again age
with the sexes separated.
affects results as some older cases have tooth
Table 1 gives basic statistical results. In loss, attrition and mesial migration of teeth
terms of incisor overjet, there is a more an- that increases TPI scores, while many others
terior maxilla in the younger sample to show maintain remarkably good occlusion. Thus
a general trend of modernization (an edge-to- there is a distributional difference, 13 older
edge incisor bite actually being the norm in (19.4%)as opposed to 19 younger (6.9%) indiaboriginals). The difference is not significant viduals having perfect TPI scores, i.e., no
statistically; the mean of about 3 mm is a t appreciable deviation from ideal anatomical
the normative value for American white and occlusion in any variable. The difference is
black youths as stipulated by the criteria of significant a t P = .01 by x2 and Kolmogorovthe TPI (Kelley and Harvey, 1977).However, Smirnov tests.
older Pima are less variable.
The TPI is a valuable method of combining
Overbite is quite similar among old and different occlusal variables, but carries unyoung, both samples showing average incisor fortunate connotations of clinical assessoverlap of three-tenths of the lower incisor’s ment. Another approach is to combine
crown height; again this is within the ortho- occlusal variables by canonical variates analdontic norm.
ysis to find whether a statistical combination
Crossbite deviations of transverse occlusal that most effectively discriminates older from
relations occur mostly with maxilla to the younger samples is significant. This analylingual side, as in all human populations. sis, where the input included all the nonThere is slightly more crossbite in younger metrical variables, resulted in Mahalanobis’
Pima, but the difference is not significant.
D = 0.69(F = 2.78, P < .01) and the means
mere. Since four categories are recognized
and one must be held constant within the
sequence against which to gauge the others,
this PDTL score ranges from zero to three by
The role played by PDTL in creating occlusal variation could be studied only in the
younger subsample where longitudinal casts
from about ages 6-12 were available; there
were 190 such serial cases. Seven classes of
PDTL were erected (from zero to three by 1/2point increments) but the zero and the
half-point classes were merged to create a
minimum sample of 18 in any class. The numbers were normally distributed over these
classes. One-way analysis of variance for six
groups was conducted for each occlusal variable according to PDTL.
32 1
TABLE 1. Descriptive statistics and t and F tests for arch measurements (mm)and occlusal variables
Younger Pima (n
BSR (type)
BSR (extent)
Posterior R/D
Anterior R/D
Total R/D
arch length
arch breadth
arch length
arch breadth
- 0.12
Older Pima (n = 67)
*Significant a t the .05 level
** .06 > p > .05.
being 6.5 standard errors apart on the discriminant vector. Some 68.3% of the old are
discriminated from 64.5% of the young individuals, considerably removed from determinative identification but significantly greater
(P < .001) than the random expectation of
50%. The coefficients of discrimination emphasized the variables distal R/D and BSR
extent in effecting this separation.
From the table, arch lengths are significantly greater in younger subjects, while the
difference in arch breadths, though larger in
the older, is less significant. This is the maxillary shape trend found in all surveyed industrializing groups with rising malocclusion
incidences, that is, relative palatal narrowing
(and deepening), relating to the “maxillary
collapse syndrome” frequent in American
youths (Kelley and Harvey, 1977) and to general facial narrowing (Davies, 1972).The relative effect on length and breadth also relates
to aging effects: mesial tooth drift, accentuated by some tooth loss and interstitial
attrition, reduces arch length with age. Correlation between maxillary arch length and
YAB, with effects of chronological age removed by partial correlation, shows zero
rather than significantly negative correlation. Among arch measurements, only maxillary breadth correlated significantly
(negatively) with TPI.
For occlusal variation in relation to PDTL
in the younger sample only, analysis of vari-
ance showed no significant differences. The
highest F-ratio for any measurement or occlusal variation was 1.85 for distal tooth displacement (.lo > P > .05). PDTL scores in
the upper half of the range had somewhat
higher R/D counts. The only other variable
with a relatively high partitioned variance
was overbite (F = 1.691, with the biggest
increase in occlusal variation again occurring with PDTL scores of 2 or above. The
smallest F ratios were associated with arch
Correlations between PDTL and occlusal
variables were examined. All correlations
were very nearly zero except those for overbite (r = 0.15) and R/D (r = 0.141, both borderline significant correlations indicating
slight increase in vertical incisor overlap and
in rotated teeth with more PDTL teeth. One
major reason for considering PDTL important to increased occlusal variation with urbanization is the possible relation to caries
which, if serious, could cause early tooth loss.
For cases with macroscopic carious destruction in deciduous teeth (n = 301, a notation
was made and these were later removed from
the sample (non-carious n = 160).Among the
non-carious cases, correlations were consistently larger (though the difference is slight)
between PDTL score and occlusal variables
than in the carious cases, where PDTL correlations were all near zero. Whatever the
relation between tooth succession and occlu-
sion may be, it seems to be unconnected to
The older Pima, raised less recently on less
refined diets than the younger Pima, are significantly less variable from occlusal ideals
in spite of their greater deleterious age
changes. The most obvious suspects as causal
agents for this difference are dietary change
and genetic change.
Genetic admixture could only have been
extremely slight in the intergenerational
time. Orthodontists have often invoked the
notion that “race mixture” causes malocclusion (refuted by Chung et al., 1971; Horowitz
and Osborne, 1971), in that one may inherit
jaw size from one parent which may be disharmonious with tooth size inherited from
the other, or malfitting upper and lower jaw
profiles from two different ethnic predecessors. Other than this notion, it could also be
theorized that the white U S . population has
a higher frequency of deleterious alleles for
occlusion through accumulating mutations,
and that admixture with whites raised the
Pima malocclusion prevalence. However,
these mechanisms are both very unlikely,
due to the short time involved and the lack
of evidence of increase in genetic flow. What
little admixture that has been documented
among Pima seems more toward Mexican
In considering environmental correlates of
occlusal variation, no single study on humans can offer adequate controls. A crosscultural approach, where many societies urbanizing under different conditions may be
examined, holds the most hope for achieving
a n understanding of the causes. Other studies documenting a sudden deterioration of
occlusal norms have remarked upon the diet,
particularly its physical consistency, and how
that changes with Western contact. Such
study populations have included Eskimos
(Waugh, 1937a, 1937b; Williams, 1943; Price,
1936; Wood, 1971), Australian aborigines
(Clinch, 1951), Chinese (Liu, 1977; Goose,
1972), Appalachian Americans (Corruccini
and Whitley, 1981),Europeans (Lavelle, 1968;
Davies, 19721, Asian Indians (Kaul and Corruccini, 1983; Corruccini et al., 1983), Melanesians (Lombardi and Bailit, 1972;
Corruccini and Pacciani, 19831, Amerinds
(Niswander, 1966), and reviews of several
populations (Klatsky, 1948; Price, 1935;
Hunt, 1961).
In the Pima occlusal transition, dietary
consistency and its relation to chewing stress
likewise seems to be a possible factor. There
is considerable theoretical and experimental
support for this concept. It has been shown
that vigorous chewing during growth affects
midfacial, palatal, and chin growth favorably
(Hunt, 1959, 1961) by transmission of forces
through the alveolus, and that lessening the
biomechanical demands of diet causes a reduction of temporomandibular joint size
(Hinton and Carlson, 1979; Corruccini and
Handler, 1981). The maxillary narrowing
that invariably accompanies occlusal deterioration on a population level has been induced in laboratory rats (Watt and Williams,
1951; Beecher and Corruccini, 1981a), macaques (Beecher and Corruccini, 1981b), and
squirrel monkeys (Corruccini and Beecher,
1982; Beecher et al., 1983) fed artificial soft
diets. Malaligned teeth and abnormal occlusal contacts were also significantly greater
for the soft diet group than hard diet controls
in the squirrel monkey study.
Other environmental correlates of the
maxillary collapse syndrome and related
malocclusion which co-occur with urbanization have been suggested. Premature deciduous tooth loss seems a t most to be a minor
contributing factor. Chronic allergy, respiratory infection and mouth-breathing (Marks,
1965; Harvold et al., 1973) have a few effects
similar to soft diet but respiration and posture lack a logical functional relation to tooth
displacement. Interproximal attrition once
was thought to increase relative arch space
and maintain isomeric dental relationships
in the aborigines with gritty diets (Begg,
19541, but the universal applicability of this
mechanism is doubtful (Brace, 1977; Lombardi, 1982; Kaul and Corruccini, 1983) especially a s it does not explain crowding in
the 12-20 age range prior to significant attrition. In this age range, malocclusion has become the most expensive medical condition
in the average American family.
It could be that biased sampling operated
in the older group, since only individuals
with nearly intact dentitions were scored and
these may have had better occlusions than
excluded cases when younger. Only a n extended longitudinal design could answer this;
we hope the facts that most cases age 40-t
were excluded, and that the different sample
average ages of 15 and 28 span a period of
little occlusal change, mitigate this problem.
From our data, we conclude that the change
in oral function resulting from dietary
change is a likely factor in deterioration of
occlusion in younger Pima. Physical anthropologists and geneticists are suited to test
this theory in various rapidly urbanizing societies where more drastic occlusal changes
(as well as other increases in chronic noninfectious disease conditions) are shown, especially since dental clinicians are often less
than maximally appreciative of cross-cultural data and their implications for preventive therapy.
This study was supported in part by NIH
NIDR DE05771 (to R.H.Y.P.), NSF BNS
8119875 (to R.S.C.), and an award from
Southern Illinois University Office of Research Development and Administration.
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