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Carabelli's trait and tooth size of human maxillary first molars.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 132:238–246 (2007)
Carabelli’s Trait and Tooth Size of Human Maxillary
First Molars
Edward F. Harris
Department of Orthodontics, College of Dentistry, University of Tennessee, Memphis, TN 38163
KEY WORDS
tooth morphology; tooth development; cusp size; tooth size
ABSTRACT
Carabelli’s trait is a morphological feature that can occur on the protocone of human maxillary
molars. This study tests the hypothesis that Carabelli’s
trait is correlated statistically with the dimensions of the
crown’s four principal cusps or whether, as a cingular
feature, the trait truly accretes onto an otherwise unaffected crown. Computer-assisted image analysis was
used to measure the 6 intercusp distances and 12 angular relationships among cusp tips on the permanent first
molar of 300 young adult American whites. Carabelli’s
complex was scored using an 8-grade ordinal scheme.
Crown size was quantified in three ways, namely as 1)
maximum mesiodistal and buccolingual diameters, 2) the
6 intercusp distances, and 3) the 12 angular cusp
arrangements. There was no sex difference in the morphological expression of Carabelli’s trait in this sample.
Overall crown size and intercusp distances were significantly and progressively larger in molars with larger
Carabelli’s trait expressions. There are graded size
responses between crown size (mesiodistal and buccolingual diameters), sizes of the four principal cusps, and
morphological stage of Carabelli’s complex, though the
statistical relationships are appreciably stronger in
males than females. Carabelli’s trait occurs preferentially in larger molars. In contrast, angular (shape) relationships among cusp tips are not discernibly affected by
trait size in either sex. There is the situation, then, that
Carabelli’s trait is developmentally correlated with
crown size, but with no apparent alteration of cusp
arrangements, suggesting that the increases are isometric across the occlusal table. Why the association is much
weaker in females remains speculative, but these data
provide yet another line of evidence that, within a population, tooth size is associated in a positive fashion with
crown complexity. Am J Phys Anthropol 132:238–246,
2007. V 2006 Wiley-Liss, Inc.
Carabelli’s trait is a morphological feature that occasionally occurs on the mesiolingual aspect of maxillary
molars in humans (i.e., on the lingual surface of the protocone), especially in peoples of European extraction
(Meredith and Hixon, 1954; Turner and Hawkey, 1998).
The trait develops from the cingulum, and it ranges in
morphology from a faint groove or furrow to a cusp outline without lingual prominence up to a cusp of size
equivalent to the molar’s principal cusps (Dietz, 1944;
Kraus, 1959; Scott, 1980; Turner et al., 1991).
The adaptive significance of Carabelli’s trait, if any,
remains speculative. A homologous trait occurs in the
great apes, and the trait is of considerable antiquity in
humans (Korenhof, 1960). It is most common in people of
European extraction even though these groups tend to
have small mesiodistal (MD) and buccolingual (BL) crown
dimensions (Harris and Rathbun, 1991; Hanihara, 1998).
The larger cusp forms of the trait may provide additional
surface area that helps resist occlusal attrition (e.g., Dahlberg, 1963), thus extending a molar’s functional life in an
abrasive environment. This argument was strongly promoted by Begg (Begg, 1954; Begg and Kesling, 1971). The
contention is that, as the molars’ main cusps are worn
down by abrasion, Carabelli’s cusp will occlude between
the metaconid and entoconid of the lower molar, thereby
extending the dentition’s function. Several researchers
question this scenario since 1) the frequency of cases with
a Carabelli’s cusp large enough to provide this advantage
is small in any population and 2) the frequency of Carabelli’s complex is highest in Caucasians who are characterized by small tooth sizes brought about by comparatively
rapid tooth size reduction over the past several millennia
(Brace and Mahler, 1971; Frayer, 1978). Alternatively,
Dahlberg (1963) suggests that Carabelli’s cusp—most
common on the first molar—helps compensate for size
reductions of the posterior molars. Schwarz (1927) noted
that a large Carabelli’s cusp’s interdigitation between the
metaconid and entoconid might somehow enhance occlusion and, possibly, trituration, at least until the crowns
are abraded flat.
Garn has suggested that there are positive associations
between crown size and crown complexity (Garn et al.,
1966a,b; Garn, 1977). Garn does not discuss Carabelli’s
trait explicitly; instead, his principle is that, within a
population, larger teeth tend to be morphologically more
complex. By extension, Carabelli’s cusp should be more
common in larger teeth within a sample. Garn’s principle is supported by the positive intertrait associations
documented by Keene (1968), Turner (1969), Lombardi
(1975), Scott (1977a,b, 1979), Kieser and Becker (1989),
and others. Importantly, trait associations do not occur
just within a developmental field—where the dental elements can be viewed as meristic series (Bateson, 1894)—
but commonly among different tooth types, suggesting a
broader, more fundamental level of morphogenetic integration.
C 2006
V
WILEY-LISS, INC.
C
Correspondence to: Edward F. Harris, University of Tennessee,
Department of Orthodontics, College of Dentistry, Room S301, 875
Union Ave., Memphis, TN 38163. E-mail: eharris@utmem.edu
Received 21 November 2005; accepted 8 August 2006
DOI 10.1002/ajpa.20503
Published online 31 October 2006 in Wiley InterScience
(www.interscience.wiley.com).
CARABELLI’S TRAIT AND MOLAR SIZE
239
Fig. 1. Three examples of the cusp form of Carabelli’s trait in contemporary American whites. Top row: Occlusal views of cusps
(asterisk) that occur on the lingual of the protocone. The left and right examples also have occlusal metallic restorations. Bottom
row: Corresponding sections oriented through Carabelli’s cusp to show the dentin component. Enamel is thickest over the apex of
the dentin projection, so while enamel accentuates the cusp’s prominence, this cingular feature has the same tissue structure as the
tooth’s main cusps, both dentin and enamel. Since the dentinoenamel junction was the interface between the inner and outer
enamel epithelium during tooth development (e.g., Ooë, 1981), the obvious topography in these mature teeth suggests a separate
enamel knot for, at least, the more prominent expressions of Carabelli’s complex. Kraus and Jordan (1965) likewise illustrate several developing molar specimens where the inner enamel epithelium bulges out from the protocone in the presumptive area of the
Carabelli trait, thus carrying a formative dentin component that augments the enamel component of this feature.
Pertinent information stems from embryological studies of enamel knots (e.g., Jernvall et al., 1994; Thesleff
and Jernvall, 1997; Thesleff et al., 2001; Luukko et al.,
2003). Enamel knots are transient sites of nondividing
cells that form on the inner enamel epithelium (IEE)
during the cap and bell stages of tooth formation (Butler,
1956). Substances in the knots promote rapid cell proliferation of adjacent structures, thus creating sites of cusp
formation. Several factors known to regulate crown size
and cusp pattern are known to be active in enamel
knots, notably fibroblast growth factors, epidermal
growth factors, and bone morphogenetic proteins (e.g.,
Kettunen and Thesleff, 1998; Thesleff, 2003; Kassai
et al., 2005; Plikus et al., 2005).
Larger forms of Carabelli’s trait—those with a free
apex—can approximate the occlusal area and height of a
principal cusp. There is, as yet, no specific evidence that
Carabelli’s cusp is initiated by an enamel knot (most
studies of epithelial signaling centers are conducted in
mice, which lack a cingulum; see Cohn, 1957), but, as
Kondo and Townsend (2006) suppose, it is reasonable to
surmise that the same developmental events that initiate formation of other cusps are involved. Figure 1
shows three examples of molars with the Carabelli trait,
and the corresponding sections through the feature dis-
close an obvious dentin component in addition to thicker
enamel atop the feature. The question arises whether
Carabelli’s trait extracts its size from the developing
IEE, or whether, as a cingular element, it simply supplements the tooth size.
The present study is a statistical analysis of the dependency in the statistical sense between measures of
molar crown size and gradients of expression of Carabelli’s complex. The purpose of the present study is to test
the two competing hypotheses listed in the prior paragraph—so far as can be inferred from study of the completed phenotypes. In addition to overall crown size,
intercusp distances were measured among the four principal cusps on permanent maxillary first molar and the
size differences were tested among subsamples based on
form and size of Carabelli’s trait.
MATERIALS AND METHODS
The sample was drawn from North American whites
(n ¼ 300). These young adults (127 males, 173 females)
were free of any condition known to affect growth. Fullmouth dental casts were taken with rigid trays and
poured immediately in dental stone to prevent distortion.
Dental charting had been conducted by direct intraoral
American Journal of Physical Anthropology—DOI 10.1002/ajpa
240
E.F. HARRIS
TABLE 1. Frequency distribution of Carabelli’s trait, by sex1
Males
Females
Total
Grade
n
%
n
%
n
%
0+1
2
3
4
5
6+7
Total
47
23
10
15
23
9
127
37.0
18.1
7.9
11.8
18.1
7.1
63
33
16
25
17
19
173
36.4
19.1
9.2
14.5
9.8
11.0
110
56
25
40
40
28
300
36.7
18.7
8.7
13.3
13.3
9.3
1
Scoring used the ordinal grading system of Dahlberg (1963);
grades are collapsed here into 6 categories (shown in left column) based on sample sizes.
Fig. 2. Top: Example of a right human maxillary first molar
with the names of the four principal cusps. Arrow points to the
lingual prominence of Carabelli’s trait on side of the protocone.
Bottom: Illustration of the six linear intercusp distances,
defined by the cusp tips, and the numeric codes of the cusps
suggested by Gregory (1916; also see Scott and Turner,
1997:18). Similarly, the 12 three-point angles among the cusps
are defined by the cusp apices, such as \134, the acute angle
defined by the paracone, metacone, and hypocone.
examination to identify all sites of caries and restorations. Odontometrics of this sample have been described
previously (Harris and Burris, 2003). Maximum MD and
BL crown dimensions were measured with sliding calipers following the guidelines of Moorrees (1957). There
were few occlusal surfaces visibly affected by attrition in
these Americans living on an essentially grit-free diet,
and those subjects were omitted. Cusp tips were marked
with minute pencil dots, and a standardized digital photograph of the occlusal table of each molar was taken
(Hlusko et al., 2002), including millimetric scales positioned at the molar’s occlusal table. A computer-assisted
image analysis program (SigmaScan Pro 5.0; SPSS, Chicago, IL) was used to compute the 6 intercusp distances
and the 12 three-cusp angles on each tooth (see Fig. 2).
Other details of the method, including technical error
estimates, are given in Harris and Dinh (in press). Not
all data were collected on every tooth, primarily because
of some dental restorations obscuring some details. The
tooth (left or right) with better definition was used from
each individual; if there was no difference, the left tooth
was measured. All data were collected for M1 and M2,
but the frequency of Carabelli’s complex is too low on
the second molar to warrant analysis.
Carabelli’s trait complex was scored visually as absent
or present, and, when present, the size and morphology
were scored against a 7-grade scheme of Dahlberg (1963)
and Scott (1980). These seven grades (plus the category
of trait absence) were then combined into 6 groupings on
the basis of sample sizes; specifically grade 1 was pooled
with 0 and grades 6 and 7 were combined for analysis.
Size and morphology of Carabelli’s complex were scored
from visual inspection of the dental casts because minor
forms are not visible in an occlusal view. All scores were
made twice, and repeatability accuracy using these 6
groups was 99.3%. Grade 0 is absence of the trait.
Grades 1–4 are pits and furrows of various sizes and
shapes, but without any obvious prominence buccolingually. Grade 5 denotes a small tubercle without a free
apex. Grade 6 is a broad, moderate size tubercle, and
grade 7 is a large, prominent cusp nearly equal in height
with the principal molar cusps.
Statistical analysis relied primarily on factorial analysis of variance (ANOVA) models with the classes of Carabelli’s complex as the independent variable. Statistics
were calculated using JMP 5.0.2 (SAS Institute, Cary,
NC).
RESULTS
The overall prevalence of Carabelli’s trait in this
North American sample is 63% (Table 1), which agrees
with this feature being characteristic of Caucasian peoples (Kraus, 1959; Scott, 1980; Turner and Hawkey,
1998). By chi-square analysis (v25 ¼ 5.4; P ¼ 0.37), there
is no sexual dimorphism in trait frequency (see Fig. 3).
This lack of difference in frequencies needs to be viewed
specific to the present sample because assessments of
some other groups have disclosed the trait complex to be
more common in one sex or the other, but significance
occurred because of different aspects of the trait distributions in different groups. Goose and Lee (1971) found
a sex difference in British whites, with the major source
of significance being a higher frequency of the trait (all
forms) in males. Kaul and Prakash (1981) studied an
Asian Indian group; here the major source of the significant sex difference was an excess of cuspal forms in
females. Townsend and Brown (1981) also found a difference in Australian Aborigines and it too was primarily
due to an excess of the trait in females. Kieser and Preston (1981) studied the Lengua of South America and
found a sex difference due in about equal parts (in terms
of cell chi-square values) to an excess of males without
American Journal of Physical Anthropology—DOI 10.1002/ajpa
241
CARABELLI’S TRAIT AND MOLAR SIZE
size and crown diameter in the subsample of males but,
while the trend is similar in females, it does not reach
statistical significance (P ¼ 0.28 for the MD dimension;
P ¼ 0.08 for BL).
Table 2 lists the results of whether trait expression
depends statistically on sizes of intercusp distances.
Does the expression of Carabelli’s trait depend on how
big the crown is—assessed at the occlusal table. Dahlberg (1951:169, 170) claimed—on the basis of visual
impressions—that a large Carabelli’s cusp develops at
the expense of protocone size. Reid et al. (1991) and
Kondo and Townsend (2006), on the other hand, found
that occlusal base areas were positively associated with
Carabelli’s trait size. In the present dataset, all tests
exhibited a significant trait-by-sex interaction when
assessed with two-way ANOVA. Looking at the results
within each sex separately, there are two features consistent across all 6 intercusp distances: 1) the association
is positive and highly significant (P < 0.01) in males
regardless of whether the intercusp distances are oriented primarily mesiodistally or buccolingually and 2)
the explained variance is appreciably lower in females,
where just one distance achieves significance statistically. The model R2 (i.e., the variation in intercusp distances explained by the trait’s size) is 24 and 14% for
the MD and BL diameters in males, respectively, but the
corresponding values are only 4 and 6% in females.
the trait and an excess of females with prominent
grades. On the other hand, Scott (1980) studied five contemporary groups, with no significant sex difference in
any of them. Numerous other researchers have, likewise,
reported no discernible sexual dimorphism (e.g., Garn et
al., 1966c; Alvesalo et al., 1975; Rusmah, 1992; Falomo,
2002). In sum, results seem to be specific to the group
being studied, without any predictable pattern.
The driving question in the present study is whether
Carabelli’s trait is tied developmentally to tooth size,
where size consists of three sets of variables: 1) maximum MD and BL crown diameters, 2) the 6 intercusp
distances measured across the molar’s four main cusps,
and 3) the 12 three-point angles taking all combinations
of the main cusps. Of note, intercusp distances and
angles are measured at the occlusal table whereas maximum MD and BL crown diameters occur at the heights
of contour along the collum of the crown—well gingival
to the occlusal table. Indeed, maximum BL breadth generally occurs near or at the molar’s cementoenamel junction (Zeisz and Nuckolls, 1949).
Differences in mean crown size partitioned by expression of Carabelli’s trait was assessed by factorial analysis
of variance. Initial tests between crown size and Carabelli’s trait disclosed significant trait-by-sex interactions,
so Table 2 provides the results of one-way ANOVA
within each sex. The source of the interaction is the
same for both crown diameters (see Fig. 4), namely that
there is a significant, positive association between trait
DISCUSSION
Carabelli’s trait arises from the cingulum (Osborn,
1907; Gregory, 1922) even though the protocone’s cingulum is not discernible on the completed human molar
(Korenhof, 1960; Kraus and Jordan, 1965). But, since
the tooth crown forms developmentally from the presumptive IEE apically, there must be biochemical signals
that control marginal morphology well in advance of
cingulum formation per se. This is apparent when a
large Carabelli’s cusp is present of size comparable with
that of the protocone. Little is known yet of what modulates size and shape of the molar’s cervical loop, but it is
clear from tooth formation (e.g., Ooë, 1981) that the IEE
needs to fold soon enough for the accessory cusps, styles,
and crests to develop their occlusal heights well ahead of
the basal structures from which they arise. Data in
Kraus and Jordan (1965) suggest that the most occlusal
cusps begin to mineralize (with dentinogenesis advancing ahead of amelogenesis) on the order of 6 months
before the basal cingular features take on their definitive
morphology.
It is, perhaps, to be anticipated that size of Carabelli’s
complex is tied to crown size. It is a cingular feature,
but it has intimate morphological associations with the
protocone. Indeed, the minor expressions of the complex,
such as pits, grooves, and furrows, are expressed only on
Fig. 3. Frequency distributions, by sex, for Carabelli’s complex. Based on these categories, sexual dimorphism is nonsignificant in this sample. Overall trait frequency in the sample is
63%.
TABLE 2. Descriptive statistics, by sex and grade of Carabelli’s expression, and tests for among-grade differences in crown size1
Grades 0 + 1
Variable Sex n
x
SD
Maximum crown diameters
MD size M 42 9.7 0.50
F 55 9.8 0.49
BL size M 46 11.3 0.58
F 61 11.2 0.63
1
Grade 2
n
x
SD
Grade 3
n
x
21 10.0 0.45 9 10.2
28 9.7 0.55 14 9.7
22 11.6 0.71 10 11.8
33 11.0 0.55 15 11.2
Grade 4
x
SD
Grade 5
SD
n
n
x
0.54
0.47
0.71
0.37
14 10.3 0.35 19 10.5
23 9.8 0.44 14 9.9
15 11.9 0.62 22 11.8
24 11.0 0.43 17 11.4
SD
Grade 6 + 7
n
x
SD
0.72 8 10.2 0.36
0.69 16 10.1 0.52
0.62 9 12.0 0.48
1.08 18 11.4 0.48
Analysis of variance
R2
df
0.244
0.042
0.141
0.060
5, 107
5, 144
5, 118
5, 162
F
P
6.89 <0.0001
1.26 0.2821
3.89 0.0027
1.99 0.0822
Counts are of individuals, not teeth.
American Journal of Physical Anthropology—DOI 10.1002/ajpa
242
E.F. HARRIS
the lingual border of the protocone without extension to
the cingulum. Korenhof (1960), with access to the dentinoenamel junction of the molars, describes a crista that
he termed the cingulum–protocone crest. It courses from
the apex of the protocone (at the dentinoenamel junction) to the apex of Carabelli’s cusp. He notes that size of
this crista is proportionate to size of the cusp, and this
crest probably is the most occlusal (so first to form) feature shared in common by this trait and the crown
proper. This is evidence for the associated development
of these two features since the presumptive morphology
of both is established at the IEE. So, while paleontolo-
Fig. 4. Mean crown size partitioned by expression of Carabelli’s trait. For both dimensions (mesiodistal and buccolingual),
crown size increases with trait expression in males, but the
trend is weak and nonsignificant in females.
gists emphasize the cingular origin of Carabelli’s complex, its ontogeny is shown by this crista (as well as
other features) to stem from at least as early as the
establishment of the morphology of the IEE during the
bell stage well prior to crown mineralization.
More compelling in this light are the observations
from several researchers that the positive forms of Carabelli’s complex have a dentinal component extending
from the protocone (e.g., de Terra, 1905; Korenhof, 1960;
Kraus and Jordan, 1965), and it has long been recognized that prominent Carabelli’s cusps also possess an
extension of the pulp horn (Fabian, 1928).
The present study shows that crown size per se is
indeed correlated with expression of Carabelli’s trait.
Small M1 crowns are more likely to exhibit no trait
expression, and increasing crown size increases the likelihood of cuspal expression of Carabelli’s complex. Reid
et al. (1991, 1992) and Kondo and Townsend (2006) likewise found a positive, \dose dependent" relationship
between size of Carabelli’s trait and crown size that they
measured as basal crown areas. As here, Reid and coworkers found that size of the whole crown (i.e., all four
principal cusps) was affected, not just the protocone.
Reid’s group studied a sample of South African Bushmen
(mostly males); the present study shows that analogous
relationships occur in a group of Western European
extraction with appreciably different trait frequencies
and genetic backgrounds.
Results in Tables 2 and 3 show that the associations
between trait size and tooth size are stronger in males
than females. Statistically, the larger R2 for males stems
from the greater crown size increments among Carabelli’s trait grades than in females. Noss et al. (1983) also
found a stronger relationship in males than females
(their Table 2), but the underpinning genetic causes of
the sex differences remain unclear. The association in
males is much stronger than can be explained by proportionate tooth size differences between the sexes.
Data in Kondo and Townsend’s study (2006) likewise
disclose the higher level of statistical associations
between measures of crown size and Carabelli’s trait in
males than females (their Table 3), and their working
hypothesis is that the sex difference is due to an
extended interval of mitotic activity in the IEE in males
prior to stoppage by dentinal bridging. Bigger teeth in
individuals in a population are so because of faster rates
or extended intervals of growth or both. This explanation
TABLE 3. Descriptive statistics, by sex and grade of Carabelli’s expression, and tests
for among-grade differences in intercusp distances1
Grades 0 + 1
Grade 2
Grade 3
Grade 4
Grade 6+7
Analysis of variance
Variable
Sex
n
x
SD
n
x
SD
n
x
SD
n
x
SD
n
x
SD
R2
Distance 1–2
M
F
M
F
M
F
M
F
M
F
M
F
22
32
22
32
21
32
22
32
21
32
21
32
6.2
6.5
6.5
6.9
4.7
4.9
5.0
5.1
8.7
9.0
5.8
6.1
0.64
0.47
0.58
0.63
0.58
0.51
0.46
0.51
0.76
0.76
0.53
0.75
12
18
12
18
12
18
12
18
12
18
12
18
6.7
6.5
7.2
6.8
5.2
4.9
5.4
5.0
9.5
9.1
6.5
6.2
0.68
0.42
0.66
0.58
0.53
0.56
0.45
0.47
0.55
0.68
0.44
0.74
5
11
5
11
5
11
5
11
5
11
5
11
6.9
6.4
7.2
6.8
5.0
4.8
4.6
5.2
9.2
9.1
6.5
6.2
0.66
0.64
0.65
0.49
0.54
0.46
0.68
0.64
0.50
0.81
0.42
0.46
12
16
12
16
12
16
12
16
12
16
12
16
6.8
6.4
7.0
6.9
5.1
5.0
5.5
5.5
9.6
9.4
6.3
6.3
0.66
0.56
0.36
0.58
0.52
0.40
0.74
0.57
0.84
0.63
0.48
0.54
7
10
7
10
7
10
7
10
7
10
7
10
7.5
6.7
8.2
7.1
6.0
5.4
5.4
5.4
9.9
9.7
6.8
6.5
1.07
1.20
1.45
1.01
0.77
0.79
0.59
0.36
0.41
0.51
0.50
0.54
0.253
0.016
0.352
0.018
0.311
0.108
0.203
0.128
0.308
0.101
0.356
0.030
Distance 1–3
Distance 1–4
Distance 2–3
Distance 2–4
Distance 3–4
df
5,
5,
5,
5,
5,
5,
5,
5,
5,
5,
5,
5,
61
86
61
86
60
86
61
86
60
86
60
86
F
P
4.14
0.28
6.63
0.32
5.42
2.07
3.11
2.53
5.35
1.94
6.63
0.54
0.0027
0.9218
<0.0001
0.8993
0.0004
0.0763
0.0146
0.0345
0.0004
0.0967
<0.0001
0.7487
1
Prior analysis disclosed significant grade-by-sex interactions, so the tests here are within each sex. R2 is fraction of total variance
explained by the ANOVA model.
American Journal of Physical Anthropology—DOI 10.1002/ajpa
M
F
M
F
M
F
M
F
M
F
M
F
M
F
M
F
M
F
M
F
M
F
M
F
\214
\142
\132
\324
\134
\213
\314
\124
\213
\143
\234
\123
Sex
Variable
21
32
22
32
21
32
21
32
22
32
21
32
21
32
21
32
21
32
21
32
22
32
21
32
106.1
105.4
69.0
71.9
107.9
106.9
75.6
75.8
47.2
44.8
31.0
31.2
44.6
43.6
32.7
32.4
59.6
60.7
39.2
40.4
63.8
63.5
42.9
43.2
9.81
9.20
9.20
5.73
8.93
7.28
10.57
7.49
6.96
4.67
4.66
4.18
6.46
4.53
4.74
3.94
7.02
7.21
5.84
5.12
5.34
4.76
7.67
6.09
Grades 0 + 1
n
x
SD
12
18
12
18
12
18
12
18
12
18
12
18
12
18
12
18
12
18
12
18
12
18
12
18
n
105.5
106.5
72.6
71.4
106.6
108.2
75.2
74.0
44.8
44.2
31.7
31.1
44.2
44.1
32.4
31.2
60.8
62.1
40.8
40.4
62.6
63.9
42.6
42.4
6.34
9.93
5.01
6.95
6.95
5.26
6.38
6.49
2.96
4.85
3.15
4.75
4.33
3.65
3.30
2.57
5.96
7.60
4.85
4.84
6.06
5.27
5.47
6.29
Grade 2
x
SD
5
11
5
11
5
11
5
11
5
11
5
11
5
11
5
11
5
11
5
11
5
11
5
11
n
100.6
107.2
74.1
71.0
109.7
106.1
75.7
75.5
38.5
46.0
32.1
30.2
42.5
43.0
28.3
33.0
61.9
61.3
42.2
40.7
67.1
63.1
47.3
42.1
9.50
5.82
6.43
3.78
4.58
3.94
3.87
2.31
7.61
5.54
4.09
2.50
4.59
2.70
3.53
3.37
5.84
2.53
5.76
2.58
2.93
4.35
6.17
4.57
Grade 3
x
SD
12
16
12
16
12
16
12
16
12
16
12
16
12
16
12
16
12
16
12
16
12
16
12
16
n
108.1
110.1
69.3
70.4
108.5
105.8
73.9
73.8
47.1
48.2
30.4
30.2
44.8
44.8
32.5
33.8
61.1
61.5
38.5
40.0
63.6
61.2
41.4
39.6
5.18
6.03
7.06
5.26
6.42
6.21
3.33
5.12
5.96
4.41
4.20
2.87
5.22
3.49
3.76
3.73
4.57
5.14
4.89
4.38
6.58
5.34
3.56
4.45
Grade 4
x
SD
9
5
9
5
9
5
9
5
9
5
9
5
9
5
9
5
9
5
9
5
9
5
9
5
n
108.3
102.0
70.0
73.6
108.7
110.3
72.7
74.0
45.2
42.2
29.9
34.1
44.1
46.5
31.1
29.7
63.4
60.0
40.0
39.3
64.9
64.3
41.3
43.8
7.89
4.47
6.03
4.09
5.18
10.82
6.15
9.94
5.91
3.67
3.35
3.66
4.38
6.46
3.24
6.71
5.92
4.35
4.54
4.22
5.56
4.97
6.03
3.43
Grade 5
x
SD
7
10
7
10
7
10
7
10
7
10
7
10
7
10
7
10
7
10
7
10
7
10
7
10
96.8
107.2
76.2
71.0
107.9
109.0
78.8
72.7
41.2
46.6
36.1
31.7
45.5
46.6
31.7
31.6
55.4
60.7
40.1
39.3
62.5
62.4
47.2
40.9
14.83
16.75
6.99
11.25
6.97
4.87
14.42
14.70
5.95
7.87
5.93
8.44
5.31
8.51
5.51
2.01
11.57
9.53
4.13
4.17
5.44
9.71
10.30
14.90
Grade 6+7
n
x
SD
0.063
0.041
0.027
0.048
0.083
0.038
0.086
0.135
0.029
0.031
0.054
0.090
R2
0.93
0.63
0.51
0.78
1.50
0.81
1.48
2.51
0.35
0.30
0.83
0.4621
0.6737
0.7716
0.5669
0.1918
0.5472
0.2002
0.0325
0.8815
0.9127
0.5282
0.2376
Grade
P
1.37
F
1.92
0.86
0.02
0.29
0.50
0.29
0.27
2.27
0.51
0.16
0.21
1.57
F
P
0.1684
0.3541
0.8946
0.5916
0.4813
0.5916
0.6058
0.1338
0.4773
0.6930
0.6476
0.2125
Sex
1.00
0.59
0.39
0.64
1.39
0.30
1.50
2.62
0.44
0.55
1.10
1.59
0.4176
0.7111
0.8526
0.6715
0.2300
0.9114
0.1940
0.0266
0.8213
0.7384
0.3626
0.1656
Interaction
F
P
Two-way analysis of variance
TABLE 4. Descriptive statistics, by sex and grade of Carabelli’s expression, and tests for among-grade differences in intercusp angles
CARABELLI’S TRAIT AND MOLAR SIZE
243
American Journal of Physical Anthropology—DOI 10.1002/ajpa
244
E.F. HARRIS
is wholly consistent with known size relationships
(delineated in their study), but it may not account for
the sex difference in the intensity of morphological integration between traits, as judged by the statistical correlations (i.e., variance accounted for) that are higher in
males than females.
Of course, studies that find sex differences in tooth
size typically invoke genes on the sex chromosomes since
there is no other parsimonious explanation (Tanner et al.,
1959; Ogata and Matsuo, 1992; Ogata et al., 1995). The
numerous studies by Alvesalo on tooth sizes in people
with chromosomal aberrations (e.g., Alvesalo, 1997)
document in the aggregate that genes on both the X and
Y chromosome affect tooth size. The permanent first
molar mineralizes its crown perinatally (Kraus and Jordan, 1965), and we may need look no further than the
fetal increases in testosterone and estrogen levels (e.g.,
Tapanainen et al., 1981; Migeon and Wisniewski, 1998)
to account for size differences, though other downstream
factors may also be involved. The occurrence of sex differences in size during infancy is not surprising, since it
also accounts for the significant sexual dimorphism in
deciduous tooth crown sizes (Harris and Lease, 2005)
that form wholly or predominantly in utero (Lunt and
Law, 1974).
Moss and Moss-Salentijn (Moss and Moss-Salentijn,
1977; Moss, 1978) put forth the insightful argument that
tooth size differences between the sexes might be due to
enhanced enamel thickness in males, but quantitative
analyses have shown this supposition to be wrong
(Moore, 1998; Gantt et al., 2001; Harris et al., 2001;
Schwartz and Dean, 2005). Instead, the developing tooth
achieves its dimorphic size prior to mineralization, when
the size and shape of the IEE is established, which
becomes the interface between the enamel and dentin
(e.g., Arey, 1965). Males have larger tooth crowns
because the dentin and pulpal components are larger;
enamel thickness, in turn, is not dimorphic (Harris and
Hicks, 1998; Moore, 1998; Zilberman and Smith, 2001).
One assumes that factors on the Y chromosome act early
in development to promote mitotic rates or otherwise
increase size of the IEE, so that definitive tooth size is,
on average, somewhat larger in males (Alvesalo and Portin, 1980; Lahdesmaki and Alvesalo, 2005). There is no
information as yet what causes the enamel knots to be
positioned farther apart in males so that the intercusp
distances (and basal occlusal areas) are larger on the average than in females.
One supposition is that distances between the formative enamel knots is the same in both sexes and that
sexual dimorphism occurs by enhanced growth in males
between the cusps up until size at the dentinoenamel
junction is set by bridging of mineralized tissues (Moss
and Applebaum, 1957; Butler, 1967a,b). Observations on
the sequential bridging across the various cusps (Ooë,
1981; Kraus and Jordan, 1965) argue against this,
though, because there is no statistical difference between
the sexes in cusp arrangements (Harris and Dinh, in
press).
Kondo and Townsend (2006) emphasized the valuable
point that there probably is no cause-to-effect directionality to the coincidence of larger crowns and larger trait
expressions. Bigger teeth do not cause trait expression;
instead, formative events that modulate presumptive
cusp positions (i.e., intercusp distances) and overall
crown size are likely also to modulate likelihood of presence and expression of Carabelli’s trait.
One might suppose that size of Carabelli’s trait would
alter cusp arrangements (Table 3). Angles defined by the
cusp apices characterize the arrangement of the cusps
(see Fig. 2). Since Carabelli’s trait derives from the protocone, the mesial cusps might be affected more than others,
notably the variable hypocone composing the talon. In
fact, this is not the case because all 12 intercusp angles
are statistically independent of Carabelli’s trait (Table 4).
This discloses an interesting contrast. On one hand, MD
and BL dimensions are significantly associated with trait
size as are the intercusp distances. In contrast, the angular relationships are not affected, implying that crown
size varies without associated changes in cusp arrangements. Certainly there is considerable patterned variation
in cusp arrangements of this molar (detailed in Harris
and Dinh, in press), but equally certainly, cusp arrangements are refractory to size of Carabelli’s trait, including
this feature’s cuspal forms.
The statistical and, presumably, biological independence
between the angular intercusp arrangements and Carabelli’s complex may be accounted for by the temporal differences in tooth formation. Primary and secondary enamel
knots develop during the cap and bell stages, respectively,
when the future occlusal surface is being delimited, as least
as defined at the IEE. Uneven amelogenesis and intercuspal increases prior to when growth at the IEE is stopped by
dentinogenesis both alter this formative arrangement of
cusps, but these events precede growth of the cervical loop
(Keene, 1982). Peretz et al. (1997, 1998a, b) and Townsend
et al. (2003) also have commented on the modest correlations between dimensions of the occlusal surface and traditional MD and BL dimensions, implying that different control mechanisms are operative. The statistical independence between intercusp angles and all forms of Carabelli’s
trait would seem to epitomize these temporal signaling differences.
The quantitative results of the present study disagree
with the anecdotal impression of Dahlberg (1951) and
others who supposed that Carabelli’s cusp encroaches on
size of the protocone—or that this accessory cusp deflects
position of the protocone or other main cusps. The present
study is not without precedence, of course, since de Terra
(1905), Korenhof (1960) and others have shown that crown
size (maximum MD and BL diameters) tend to be larger in
teeth exhibiting a cusp form of Carabelli’s complex.
Scott (1979) found a positive association between the
size of Carabelli’s trait and that of the hypocone on the
first molar. He used ordinal scales for these two features,
but the results are concordant with the present findings,
namely that a large hypocone predisposes for the expression of Carabelli’s trait. Conversely, to paraphrase Scott’s
conclusion, hypocone reduction precludes rather than
enhances the expression of Carabelli’s trait. Keene
(1965, 1968) had found similar associations, also using
visually-graded morphological scales, namely that Carabelli’s trait is less common on three-cusped first molars
(i.e., those without a hypocone), and when the second
molar has three rather than four principal cusps, and
when third molars are congenitally absent. These various associations collectively reaffirm Garn’s (1977) principle that larger teeth, within a population sample, have
a tendency to be more complex morphologically.
CONCLUSIONS
Results from this study suggest the following developmental scenario: size of Carabelli’s trait (scored on a
American Journal of Physical Anthropology—DOI 10.1002/ajpa
CARABELLI’S TRAIT AND MOLAR SIZE
morphological scale of increasing prominence) is associated with factors affecting overall crown size, including
the spacing among cusp tips that depends on cusp sizes.
Factors determining crown size at the IEE appear to develop early on since separation and elevation of the IEE
responsible for a cuspal form of Carabelli’s trait needs to
form close in time relative to those of the crown proper
since this aspect of the IEE can be virtually the same
coronal height as the occlusal table. On the other hand,
the angular arrangements of the cusps are not discernibly affected; there is, then, the situation where factors
modulate crown size without an accompanying change in
shape, at least at the crown’s occlusal table.
ACKNOWLEDGMENT
I thank my colleague Barry Owens for preparing the
specimens shown in Figure 1.
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American Journal of Physical Anthropology—DOI 10.1002/ajpa
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