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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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

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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,”

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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

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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

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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

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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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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

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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

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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

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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

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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

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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

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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

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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-

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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

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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.

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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.

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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. LITERATURE CITED Barrett MJ (1960) Parafunctions and tooth attrition. J . West. SOC.Period. 8(4):142-143. Bjork J (1953) Variability and age changes in overbite and overjet. Am. J. Orthod. 39:779. Boyd RC (1972)AppendixN :a n odontometric and observational assessment of the dentition. In RA Littlewood (ed): Physical Anthropology of the Eastern Highlands of New Guinea. Seattle: University of Washington Press, pp 175-212. Brace CL (1977) Occlusion to the anthropological eye. In JA McNamara (ed): The Biology of Occlusal Development. Craniofacial Growth Series Monograph no. 7. Ann Arbor, Michigan: Center for Human Growth Development, pp 179-209. Brown T, Abbott AH, and Burgess VB (1987) Longitudinal study on dental arch relationships in Australian Aboriginals with reference to alternate intercuspation. Am. J. Phys. Anthropol. 72:49-57. Brown T, and Barrett MJ (1973)Dental and craniofacial growth studies of Australian Aborigines. In RL Kirk (ed): The Human Biology of the Aborigines in Cape York. Australian Aboriginal Studies, no. 44, Canberra: Australian Inst. of Aboriginal Studies, pp 69-80. Campbell TD (1925) Dentition and Palate of the Australian Aboriginal. Adelaide: The Hassell Press. Campbell TD ( 1938)Observations on the teeth of Australian Aborigines, Mt. Liebig, Central Australia. Aust. J. Dent. 4285-89. D’Amico A (1961) Functional occlusion of the natural teeth of man. J . Pros. Dent. 11:899-915. Heithersay G (1959)A dental survey of the Aborigines at Haast’s Bluff, Central Australia. Med. J . Australia 1:721-729. Hinton R J (1981) Form and patterning of anterior tooth wear among aboriginal human groups. Am. J. Phys. Anthropol. 54: 555-564, Leigh RW (1925) Dental pathology of Indian tribes of varied environmental and food conditions. Am. J . Phys. Anthropol. 8:179-199. Lunt DA (1978) Molar attrition in medieval Danes. In

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