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Enamel structure and microwear An experimental study of the response of enamel to shearing force.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 85:31-49 (1991)
Enamel Structure and Microwear: An Experimental Study of the
Response of Enamel to Shearing Force
MARY c. MAAS
Department of Biological Anthropology and Anatomy, Duke Uniuersity
Medical Center, Durham, North Carolina 27710
KEY WORDS
Dental microwear, Enamel microstructure,
Abrasion
ABSTRACT
The anisotropic fracturing and differential wear properties of
enamel microstructure represent factors that can obscure the predictive
relationship between dental microwear and diet. To assess the impact of
enamel structure on microwear, this in vitro experimental study examines the
relative contributions to wear of three factors: 1) species differences in
microstructure, 2) direction of shearing force relative to enamel prisms and
crystallites, and 3) size of abrasive particles. Teeth oflemur, Ouis, Homo, and
Crocodylus, representing, respectively, the structural categories of prismatic
patterns 1,2,and 3 and nonprismatic enamel, were abraded by shearing forces
(forces having a component directed parallel to abraded surfaces) and examined by scanning electron microscopy. Striation width increased with particle
size for nonprismatic, but not for prismatic, specimens. Direction of shear
relative to prism and crystallite orientation had a significant influence on
striation width in only some prismatic enamels. The different responses of
prismatic and nonprismatic enamels to abrasion reflect the influence of
structure, but at the level of organization of crystallites rather than prisms per
se. Such interactions explain in part the inability of striation width to
discriminate among animals with different dietary habits. Heteroscedasticity
and deviations from normality also may confound parametric analyses of
microwear variables. Variation in crystallite orientation in prismatic enamels
may contribute to optimal dental function through the property of differential
wear in functionally distinct regions of teeth.
Dental wear patterns offer abundant information about dental function and diet of
living and fossil mammals. On a general
level, for example, the well established relationship between dental wear facets and
chewin patterns offers a reliable picture of
the evo ution and diversity of mammalian
mastication (see, e. ., Butler, 1952, 1973;
Crompton, 1971; Mil s, 1955,1973; Kay and
Hiiemae, 1974; Ka , 1977). To infer more
detailed dietary in ormation, other studies
have focused on microscopic features of dental wear (microwear) (e.g., Walker et al.,
1978; Fine and Craig, 1981; Gordon, 1982;
Teaford and Walker, 1984; Teaford, 1985,
1988; Grine, 1986, 1987; Kay, 1987; Grine
and Kay, 1988; Harmon and Rose, 1988;
Walker and Teaford, 1989). The notion that
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0 1991 WILEY-LISS, INC.
microwear atterns can be used to reconstruct specir)ic dietary habits is based on two
assumptions. The first is that microwear
variation is controlled primarily b two factors, both of which pertain direct y to diet:
the physical properties of foods and ma
tude and direction of chewing force.
second assumption is that microwear variation within functionally homologous wear
surfaces is less than that between functionally distinct surfaces or between individuals
with different diets.
An important series of studies of living
rimates with different feeding habits (i.e.,
Rard-object feeders and soft-object feeders)
P
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_ _ _ _ ~
Received J u n e 25,1990; accepted October 5 , 1990.
32
M.C. MAAS
documented statistically significant associations between microwear and diet (Teaford
and Walker, 1984; Teaford, 1985; Teaford
and Oyen, 1989).With an eye to more precise
documentation of dietary variation, other
studies have uantified microwear patterns
of species wit less marked dietary differences and of o ulations of the same species
that occu y J f g r e n t habitats (e.g., Teaford,
1986; So ounias et al., 1988; Teaford and
Robinson, 1989). The relationship between
diet and microwear is, not surprisingly, less
clear-cut in these cases; some, though not all,
metrical parameters of microwear failed to
discriminate among dietary groups. These
more ambiguous results may indicate a limitation to the predictive resolution of microwear analysis; they also may reflect the
failure of the predictive models to account for
some critical determinant of microwear morphology. One potentially critical factor is the
microstructure of tooth enamel (Rensberger,
1978, 1983; Walker et al., 1978; Puech and
Albertini, 1983; Walker, 1984; Boyde and
Martin, 1982, 198413; Fortelius, 1985; Puech
et al., 1986; Walker and Teaford, 1989). If
the structure and physical properties of
enamel are variable, rather than constant as
has been assumed, then structural variation
in enamel may contribute to variation in
microwear.
1
P
ENAMEL MICROSTRUCTURE
tion (Gwinnett, 1967). In nonprismatic
enamels, enamel crystallites are arran ed
with their long axes parallel to one anot er
and roughly perpendicular to the tooth surface. Its structural organization is uniform
and relatively homogeneous and therefore is
expected to have little differential effect on
wear.
In contrast to nonprismatic enamels, prismatic enamels are structurally heterogeneous, reflecting both their prismatic organization and the arrangement of individual
crystallites that make up the prisms.
Enamel prisms are bundles of similarly oriented mineral cr stallites separated from
areas of different y oriented crystallites by
pronounced discontinuities in orientation
(prism boundaries). Differences in crosssectional arrangements of prism boundaries
represent one cause of variation in patterns
of fracture and wear, since prism boundaries
represent potential planes of weakness in
enamel and therefore potential routes of
referential fracture (Powers et al., 1973a;
oyde, 1976a; Rasmussen et al., 1976; Hassan et al., 1981). Prism cross-sectional shape
and packin differ among species but also
within singe teeth. For example, a single
primate tooth may contain not only nonprismatic enamel but three different prism packing patterns (Boyde and Martin, 1987). A
second, and well established, source of differential wear concerns the orientation of prism
long axes relative to the wear surface (see,
e.g., Boyde, 1984a,b).Prism orientation also
varies within teeth as well as among taxa. A
third potential source of differential wear
ertains to the orientation of enamel crystalites, which differ between prisms and interprismatic material, and also among enamel
t pes (Fig. 1).The influence of all three of
t ese structural factors will differ according
to the plane of a wear facet or other functional surface.
These microstructural (prism) and ultrastructural (crystallite) features represent an
as ect of the enamel wear process that is
in ependent of diet. An understanding of its
influence may elucidate unanswered questions concerning the nature of abrasive wear
and therefore ambiguities in the relationship between microwear and diet.
a
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7
Enamel, a composite tissue that is some
95% mineral by weight, is the hardest of the
three dental tissues (enamel, dentine, and
cementum) and is tough and resistant to
brittle fracture (Boyde, 1976a). Enamel is
structurally heterogeneous among taxa (e.g.,
Boyde, 1964) and within individual teeth
(e.g., Boyde, 1976a; Boyde and Martin,
1984a, 1987;Martinet al., 1988; Skobeet al.,
1985; Stern and Skobe, 1985; Stern et al.,
1989). Importantly, the structural heterogeneity of enamel, both intra- and interspecifically, confers anisotropic properties of fracture and wear (Powers et al., 197313; Boyde,
1976b, 1984a,b; Rasmussen et al., 1976;
Hassan et al., 1981).
The two major structural categories of
mammalian enamel are nonprismatic and
prismatic (Fig. 1). Nonprismatic enamel is
characteristic of the outer enamel of many
EXPERIMENTAL DESIGN
mammalian teeth as well as of the enamel of
most reptiles. It is reported to be harder than
The model of tooth abrasion presented
prismatic enamel (Hodge and McKay, 19331, here differs from previous microwear analyperhaps because of its greater mineraliza- ses, which routinely handle force and abra-
z
B
33
ENAMEL STRUCTURE AND MICROWEAR
A
B
Fig. 1. Three-dimensional block diagrams depicting
cross-sectionalpacking arrangement of prisms and relative orientations of crystallites in prisms, interprismatic
enamel, and nonprismatic enamel (not to scale). Apical
and cervical (relative to the apicocervical axis of a tooth)
are towards the top and bottom of each block, respectively. Prism boundaries are shown as solid lines. A.
Nonprismatic enamel; crystallites are oriented parallel
to one another. B: Pattern 1 enamel; prism boundaries
appear as complete circles and prisms are packed in a
hexagonal arrangement and separated by interprismatic material with differently oriented crystallites. C:
Pattern 2 enamel; prism boundaries are incomplete
cervically and are acked in longitudinally oriented
rows, separated by sgeets of inter rismatic material. D:
Pattern 3 enamel; prism bounfaries are incom lete
cervically, but risms are packed in a hexagonay arrangement, anlinterprismatic material is restricted to
the cervically located ‘tail” regions of prisms.
sive particle properties as variables but have
implicitly treated the physical properties of
enamel as constants. This model includes
enamel microstructure as a variable and
treats wear as a mechanical process consisting of interaction among three factors: 1)
physical properties of food, 2) vectors of
chewing force, and 3) structural properties of
teeth. To predict diet from microwear patterns, the influence of physical properties of
food and vectors of chewing force must predominate and their interactlon must produce a consistent result (i.e., a characteristic
wear pattern).
The ex eriments described here detail the
resultsnio! vitro abrasion by shearing force;
experiments in which force was directed normal to the abrasion surface are reported
elsewhere (Maas, 1988, 1989). The shearing
experiments tested the influence of variation in particle size and enamel structure;
article shape and hardness, magnitude of
Force, and direction of force relative to the
abrasion surface were “extrinsic” experimental constants (those variables not de endent on the structure of the enamel itse 0.
The effect of particle size was tested by
alternating abrasive grit size among experiments. The influence of enamel structure
was tested at two levels, by varying two
different factors. First, the effect of variation
in crystallite arrangement was tested by
comparing abrasion patterns among species
representing the three different prism packing patterns and non rismatic enamel. Second, the effect of di erences in crystallite
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34
M.C. MAAS
orientation was tested by abrading enamel
specimens in different directions relative to
an apicocervical axis. Tests of the effects of
different directions of shear also addressed
the influence of differences in prism pattern,
since asymmetry in prism boundaries might
produce different responses to abrasive
force, depending on the direction of force.
The experimental series consisted of 36 experiments, each defined by unique combinations of the three factors: differences in
prism packing pattern (represented in these
experiments by different species), differences in crystallite orientation relative to
direction of shear, and particle size (Table 1).
The three variables are designated species,
direction, and grit, respectively.
Striation width was selected to represent
microwear variation. Striation width is of
special interest, because, although it is routinely recorded in microwear studies, it typically fails to discriminate among different
dietary groups (see, e.g., Teaford, 1986;
Solounias et al., 1988; Ryan and Johanson,
1989; Teaford and Robinson, 1989). Even in
cases in which mean striation widths differ
between groups, within-group variation can
obscure the statistical si ificance of between group differences. T is suggests that
striation width re resents an attribute of
microwear morpho ogy influenced by factors
not generally considered in microwear analyses.
The null hypothesis
The hypothetical framework consisted of
the null hypothesis and two alternative hypotheses. The null hypothesis predicts that
the effects of the experimental variables,
structural factors (species differences in
enamel and orientation of prisms and crystallites relative to the direction of shear), and
particle size should be random. According to
the null hypothesis, there should be no statistically significant difference in striation
width between experiments defined by different attributes of enamel structure or by
different sizes of abrasive grit.
The influence of particle size
The first alternative hypothesis predicts
that abrasive grit size is the primary determinant of striation width. According to this
hypothesis, striation width should vary directly with the size of abrasive particles,
given constancy of force vectors, although
some variation in striation width would be
P
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TABLE 1 . Experimental design
Grit size
SDecies
Direction of shear
(urn)
Crocodylus
(nonprismatic)
Lemur fuluus
(pattern 1)
Apicocervical
240 (73)
Mesiodistal
400 (23)
Ouis aries
(pattern 2)
45' to apicocervical
600 (14)
Homo sapiens
(pattern 3)
expected because of the irregularity of abrasive particle shape (and therefore variation
in contact area between the abrasive and the
enamel surface). In addition, because particle shape regularity increases with decrease
in size (Wright, 1968), sample variance
should decrease with decline in grit size, if
the size of contact area is the prime determinant of striation width.
The influence of enamel structure
The second alternative hypothesis predicts that structural characteristics of
enamel would have the predominant influence on striation width. Because of the structural complexity of enamel, there are four
separate predictions associated with this hypothesis.
1.If enamel structure influences striation
width, then only nonprismatic enamel
(which is comparatively homo eneous in
structure) would show a random tistribution
of striation widths (the null hypothesis) or a
clear correlation between abrasive particle
size and striation width (the first alternative
hypothesis).
2. If arrangement of prism boundaries is
the most im ortant structural influence on
striation wi th, enamel would be removed
referentially in units of whole
herefore, striation width distri utions
would vary accordin to prism dimensions.
This should occur on y when abrasive particle dimensions are greater than the dimensions of prism boundaries and therefore are
likely to induce fracture alon prism boundaries. When particles are sma ler than prism
diameters, striation widths would show either a random distribution or vary with particle size.
3. If arrangement of prism boundaries is
the most important structural influence, the
effect of direction of shear (relative to the
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grisms.
B
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ENAMEL STRUCTURE AND MICROWEAR
35
apicocervical axis of the tooth) should var
among enamel types for enamels in whic
potential planes of weakness (prism boundaries) are asymmetric (patterns 2 and 3) and
when particle diameters are greater than
rism dimensions. This is because rism
oundaries in pattern 2 and 3 ename s are
arranged so that the most continuous and
linear lanes of weakness occur in an apicocervica direction. The continuity and/or linearity of prism boundaries is interrupted in
other directions (Fig. l),and such interruptions increase the amount of energy required
to propagate a fracture and therefore reduce
the ease of abrasion in those directions
(Boyde, 1976a; Hassan et al., 1981;
Pfretzschner, 1986).
4. If crystallite orientation influences
enamel abrasion, then, for prismatic enamels, striation width would differ according to
prism packing pattern and according to direction of shear because crystallite orientation varies between prism types and between
risms and interprismatic material (Fig. 1).
!his variation would be expressed for abrasion by particles smaller than prism diameter.
E
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MATERIALS AND METHODS
Materials
Abrasive particles consisted of three sizes
of commerciallyproduced silicon carbide grit
(Buehler, Ltd., Waukegan, IL). Silicon carbide is hard enough t o abrade enamel
(Wright, 1968) and has previously been used
in experimental studies of enamel abrasion
(Wright, 1968; Ryan, 1979a,b).The grit sizes
were selected to maximize size-related differences: 240 grit (maximum particle
diameter = 73 Fm), 400 grit (maximum particle diameter = 23 km), and 600 grit (maximum particle diameter = 14 m) (Fig. 2).
The abrasives are irregular in s ape, but it is
important to emphasize that the degree of
shape variability in these abrasive grits is
comparable to that found in naturally occurring abrasives that have been implicated in
the microwear process (Kay and Covert,
1983).
Size of particles relative to prism diameters was an important aspect of the test of
structural influence. Particles greater than
rism diameter test the effects of prism
goundary arrangements on striation width
and particles smaller than prism diameter
test the effects of crystallite orientation. On
t
Fig. 2. SEM micrographs of silicon carbide abrasive
grits. A 240 grit; B: 400 grit; C: 600 grit. Bar = 100 pm.
36
M.C. MAAS
average, particles abrade enamel with a contact area approximately one-tenth their
maximum dimension (Wright, 1968). Consequently, although the maximum diameter of
all three grits is greater than that of the
average dimension of enamel prisms for the
experimental species (3-8 pm), it was expected that the abrasive contact area of 600
(14 pm) grit would most frequently be
smaller than average prism diameter and
that such small contact areas would be less
frequent for 400 (23 pm) grit and infrequent
for 240 (73 pm) grit.
The experimental specimens represented
four species, each characterized by one of the
four structural categories of enamel found in
extant and extinct mammals. In addition t o
the differences in prism morphology, the
enamel types represent differences in crystallite orientation (Fig. 1).Cheek teeth of
Lemur fulvus (brown lemur), Ovis aries
(sheep),and Homo sapiens correspond to the
prismatic enamel patterns 1, 2, and 3, res ectively. Teeth of Crocodylus rhombifer
uban crocodile) represent nonprismatic
enamel. Although all four structural types
ma be found in single teeth of some mamma1ys, a single pattern redominates in each
of the four species se ected here, ensuring
control of maximum structural differences
between experiments.
Specimen preparation
Molars of Lemur, Ouis, and Homo extracted and stored in 70% ethanol and drystored shed teeth of Crocodylus were cleaned
ultrasonically with ethanol, rinsed, rehydrated in distilled water for 24 hr, and allowed to air dry. Preliminar experiments
demonstrated that the dif? erent storage
methods had no effect on response to abrasion (Maas, 19881,a conclusion that supports
the results of other hardness and abrasion
tests that utilized rehydrated air-dried and
alcohol-stored specimens (Miihlemann,
1964; Boyde, 1984a,b). After rehydration,
each specimen was mounted with epoxy cement on an aluminum scanning electron
microscope (SEMI stub.
Flat facets were prepared on horizontally
oriented tooth surfaces using a series of increasingly fine grades of abrasives (600,800,
1,200grade polishing aperand 1.0-0.05 pm
alumina oxide polis ing powder). In all
cases, the abrasive facet was prepared on an
undamaged, nonocclusal surface. The polished facet was maintained in a level orientation during preparation and abrasion by
(8
f
1
means of a specially designed specimen
holder (Fig. 3). The horizontal orientation of
the facet ensured that magnitude and direction of force were consistent amon all ex eriments, with no variation due to t e ang e of
the specimen surface. The horizontal facet
orientation also eliminated the potential
problem of distortion of microwear dimensions during SEM viewing that results when
wear facets are not oriented perpendicular to
the electron beam (see, e.g., Grine, 1986;
Gordon, 1988).
An abrasive sliding device (Fig. 3) ensured
consistenc of direction and magnitude of
shearini&-ce for all of the experimental
trials. T is device consisted of a glass plate
resting on flexible wheels above the specimen holder. A fixed amount of abrasive rit
was applied to a glass slide and attache to
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2
specimen holder with a sin le stroke. Abrasion facets (the areas of t e specimens in
contact with the abrasive late) were
roughly equivalent in size for a specimens;
therefore, magnitude of force (kglmm') was
considered constant among experiments.
To test the factor of direction of shear, the
apicocervical axis of the tooth relative to the
orientation of the plate was adjusted for each
experiment. Each of the four enamel types
was tested at three orientations relative to
the apicocervical axis of the tooth: parallel to
the apicocervical axis, 45" to the apicocervical axis, and 90" to the apicocervical axis
(mesiodistal). These directions are respectively designated A-C, 45", and M-D in subse uent discussions.
!'ourteen of the 36 experiments were replicated. Following each experimental trial,
the abraded s ecimen was removed from the
specimen hol er and cleaned, and the abrasion facet was marked with a diamond stylus
adjacent to the densest occurrence of microwear features. Vinyl polysiloxane molds
Regular Set; 3-M Go.,
casts (Araldite
hardener; CibaGeigy Corp., Hawthorne, NY)were made of
each specimen. The epoxy casts were
mounted on SEM stubs and, by comparison
with camera lucida drawings of the originals, positioned identically to the enamel
specimens.
Following the abrasion experiments, the
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ENAMEL STRUCTURE AND MICROWEAR
37
Fig. 3. Abrasion device for shearing experiments. A Tooth in specimen holder; B: plate containing
abrasive grit; C: weights. See text for explanation of the abrasion procedure.
enamel specimens were etched with 0.5%
H3P04for 10-15 sec to highlight microstructural features. Epoxy replicas and etched
enamel s ecimens were examined in a JEOL
35C SE , in secondary electron mode at 10
kV accelerating volta e. SEM examination
of specimens include photo a hic documentation of abrasion facets or 0th epoxy
replicas and original enamel specimens.
Four areas of microwear on each replica were
photographed at x 400 for measurement of
microwear features. Identical areas were
documented on the original specimens for
evaluation of the structural features underlying the microwear. The stylus mark facilitated consistent orientation of enamel s ecimens and epoxy resin replicas in the EM
chamber.
Data collection
For each specimen, the orientation of
prisms relative to the enamel surface was
determined by examination of micrographs
and specimens classified in one of three nominal prism orientation categories: 1) transverse, 2) oblique, and 3) mixed (fields including prisms with transverse and with oblique
orientations). Classification of prism orien-
d
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!i
tation provided a means of evaluating the
relationship of striation width to prism long
axis orientation, by comparing replicate experiments that differed only in prism orientation.
The objective of collection of metrical data
from experimentally produced microwear
features was to evaluate the variability of
feature dimensions within and between experiments. Width of each striation (based on
a single site) was recorded with a Graf-Pen
sonic digitizer (Science Accessories) and
computer from tracings of enlarged projections of micrograph negatives. Striation
width was defined as a line perpendicular to
the lon axis of a striation. In addition, micrograp s were evaluated visually to assess
size and distribution of wear features and
structural features, including prism boundaries and orientation of prism and crystallite
long axes.
StatisticaE analysis
Most microwear studies have ado ted
nonparametric (e.g., Teaford and WaR( er,
1984) or parametric analyses of individual
sample means (e.g., Grine, 1986; Teaford,
1988; Solounias et al., 1988; Teaford and
a
38
M.C. MAAS
Robinson, 19891, because microwear data
tend not to meet the assum tions of parametric statistical tests (e.g., omogeneity of
variance, normality, and independence).
However, in this stud , an important goal is
evaluation of the inf uence of sample variability on among-group differences, and
parametric tests of equality of variance are
the most appro riate.
Striation wi th distributions for each experiment were tested for deviations from
normality by G tests (Sokal and Rohlf, 19811,
and the entire experimental test was examined for homoscedasticity (homogeneity of
variance) (Bartlett’s test and Fmax test;
Sokal and Rohlf, 1981). Because no single
transformation succeeded in normalizing
95% of the experiments or reducing heteroscedasticity to an acceptable limit, two alternative methods of parametric analysis were
adopted. The first alternative, the Games
and Howell method for multiple comparisons, is appropriate for inherently heteroscedastic data (Games and Howell, 1976; Sokal
and Rohlf, 1981). The second alternative is
appropriate for both heteroscedastic and
nonnormally distributed data. It entailed
transformation of individual variates to
ranks, and the application of the analysis of
variance (ANOVA)procedure to the ranked
variates (Iman, 1974; Conover and Iman,
1976,1981).
R
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B
Factorial ANOVA of the rank-transformed
data was used to evaluate the contribution of
each factor ( it size, species differences, and
direction o shear) and the interaction
among factors to the overall variance of the
experiments. The design was a model I
ANOVA testing main effects (the contribution of each factor to the overall variance)
and interaction (the effect of the dependence
of one factor on the level of another factor);
model I ANOVA is appropriate because all
factors re resented fixed treatments rather
than ran om effects (Sokal and Rohlf, 1981).
A series of planned comparisons tested
which ex erimental means or groups of
means di fered from others. The planned
comparisons include tests for significant differences among subgroups defined by each
experimental factor (species, grit size, direction of shear) and tests for significant differences among groups defined by unique combinations of factors (e.g., among grit sizes for
each direction of shear and each species,
among species for each grit size and each
direction of shear). Table 2 lists the questions addressed by each planned comparison.
ff
cp
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RESULTS
Observations
The experiments produced discrete and
nonoverlapping elongate microwear fea-
TABLE 2. Planned comparisons among experimental subsets
Hypotheses
First-order interactions
1. Does microwear width differ according to grit size, for each species
(all directions combined)?
2. Does microwear width differ according
for each grit size
- to soecies
(all directions combined)?
Does microwear width differ according to direction of shear for each species
(all grit sizes combined)?
Interactions
Species X grit
Species X direction
Does microwear width differ according to direction of shear for each grit
size (all species combined)?
Does microwear width differ according to grit size, for each direction of
shear (all species combined)?
Direction X grit
Does microwear width differ according to grit size for each direction of
shear (all species combined)?
Second-order interactions
7. Does microwear width differ according to grit size for each direction of
shear, for each species?
8. Does microwear width differ according to species for each direction, within
each grit size?
Species X grit X direction
9. Does microwear width differ according to direction of shear for each grit
size, within each species?
39
ENAMEL STRUCTURE AND MICROWEAR
tures (striations). Striations coursed freely
across prism cores (Fig. 4). For pattern 1
there was no discernible pattern of difference in striation dimensions between those
portions crossing prism cores and those
crossing interprismatic regions (Fig. 4A).
For pattern 2, enamel striations generally
were better defined where they passed
across prism heads than where crossing in-
terprismatic regions, but they did not deviate along prism boundaries (Fig. 4B). Prism
boundaries apparently exercised no influence on striation width for pattern 3 enamel
specimens (Fig. 4C); striations were no more
variable than those viewed on etched nonprismatic enamel specimens (Fi . 4D).The
relationship of striation boun aries and
prism boundaries was the same for both
s
Fig. 4. Micrographs of abraded enamel (etched for 10 sec with 0.5% H,PO,). A Lemur (pattern 1
enamel); B: Ouis (pattern 2 enamel); C: Homo (pattern 3 enamel); D: Crocodylus (nonprismatic enamel),
Bar = 10 km.
40
M.C. MAAS
large (greater than prism diameter) and
small (less than rism diameter) striations;
there was no evi ence that larger striations
were produced by fracture along prism
boundaries and removal of whole prism fragments. Variation in striation width is therefore not a direct reflecton of differences in
prism size.
As expected from previous descriptions of
prism orientation in surface enamel ofHomo
and Ouis (e.g., Boyde, 1964, 1969, 1976b),
almost all the human specimens exhibited
transverse prisms (relative to the facet
plane), whereas most Ovis specimens were
characterized by combinations of oblique
and transverse prisms (mixed), with considerable variation in the obliquity of the prism
long axes. For Lemur specimens, prism orientations were either transverse or oblique.
Consequently, comparisons between prism
orientation categories for a single species,
direction of shear, and grit size were limited.
Five experiments (Lemur 400 grit/M-D and
600 grit/M-D, Ouis 240 g r i m - D and 400
grit/M-D, Homo 600 gritI45") included replicates re resenting different prism orientations. ean striation width was greatest
within each replicate pair for the specimen
with more obliquely oriented prisms, with
the exception of the Ouis 240 grit/M-D experiment.
Analysis of variance
The complex relationship between striation width and the factors considered in this
experimental stud is illustrated by the plot
of mean -+ 1 stan ard deviation of natural
log-transformed striation widths for each
experiment (Fig. 5). Any obvious correlation
between striation widths and any single factor (grit size, species differences, or direction
of shear) is obscured by the degree of overlap
among ex eriments.
The in uence of interactions among factors on striation width is clear from the
results of the overall ANOVA (Table 3). Not
only was the effect of each factor significant,
but the second-order interaction among all
three factors (species X grit x direction) was
highly significant. In other words, the effect
of each factor varied according to the expression of the other factors. Consequently, any
statement about the effect of a single factor
must be qualified by its dependence on the
other factors (Kempthorne, 1975; Sokal and
Rohlf, 1981). For these experiments then,
the conclusion that each main effect (species,
B
600
M-D
- c
- H
-
0
- L
- c
600
600
H
c
A-C
4
0
I L
I
- c,
H
- 0
- L
450
t o
I
A-C
U
240
4
i
450
I
-c
I
0
L
U
I
s
B
R
Ln STRIATION WIDTH Iuml
*
Fig. 5. Mean 1 standard deviation natural logtransformed striation width, by ex eriment. L, Lemur
attern 1);0, ouis (patFern 2!; H,Romo ( attern 3); C,
8rocodylus (nonprismatic). Grit sizes (240flargestl,400
[intermediate], 600 [smallest]) and direction of shear
relative to the apicocervical tooth axis (45,', A-C [apicocervical], M-D [mesiodistal]) are indicated in the column
on the left.
grit, and direction) contributed significantly
to differences between experiments is not
very meaningful, since the effect of each was
dependent on the other factors. Nevertheless, the presence of significant interaction
among factors is an im ortant characteristic
of the wear process. nteraction, tested by
planned comparisons, includes first-order
interactions (interactions between two factors, summed over all levels of a third factor)
and second-order interactions (interactions
among all three factors).
First-order interactions: Species and grit,
species and direction, direction and grit
First-order interactions usually are not
tested when second-order interactions are
P
41
ENAMEL STRUCTURE AND MICROWEAR
TABLE 3. Summary ANOVA for shearing experiments
for rank-transformed data
Source of variation
Main effects
Species
Grit
Direction
First-order interactions
Species X grit
Species X direction
Grit X direction
Second-order interactions
Species X grit X direction
Error
D.F.
F
3
2
2
65.14***
21.03***
35.51***
6
6
6
5.78***
12.40***
8.55***
12
3.013
6.75***
*** P < 0.001.
significant. In this case, however, due to the
fact that the critical value for the secondorder interaction was so small (because of
the large number of error degrees of freedom
in this analysis), further analysis was expected to be of biological, if not statistical,
significance.
The first-order interactions are illustrated
graphically in Figure 6, where each point
represents the mean value of a subset of
experiments, defined by unique combinations of two factors summed for all levels of
the third factor. Each species in the plot of
mean striation widths for the species x grit
interaction (for each species and each grit
size, averaged for all directions of shear)
shows different trajectories (Fig. 6A).Nonparallel trajectories denote significant interaction among factors (Sokal and Rohlf,
1981).Therefore, the plot illustrates how the
effect of grit size on striation width differed
according to enamel structure at the species
level. The trajectories differ most markedly
between prismatic enamels and nonprismatic enamel.
Surprisingly, for the prismatic enamels
mean striation widths for the 240 grit experiments (all shear directions combined) were
smaller than mean striation widths for the
smaller grit experiments (400 and 600grits).
Only the nonprismatic Crocodylus enamel
showed the relationship predicted by the
first alternative hypothesis (particle size as
the main determinant of striation width) of
decrease in striation width with decrease in
grit size. In contrast, for the three prismatic
enamels the relationship between mean striation width and grit size was op osite that
predicted. Although the plannei compari-
sons were not significant for an single species (unlike the significance o the overall
anova), the contrast between prismatic and
nonprismatic enamels suggests that the effect of particle size differs for the two enamel
ty es.
!?he significance of the apparent dichotom in abrasion response between rismatic
an nonprismatic enamels is clari ied when
the effects of direction of shear are considered. The influence of shear direction was
tested in two of the first-order interactions,
s ecies x direction and direction x grit.
&en all grit sizes were combined, the species X direction interaction (Fig. 6B) resulted in greater mean striation width for
A-Cshear for all four of the species, although
the differences were significant only for Lemur. Plots of mean striation width for each
direction and each grit size, summed over all
species (direction x grit) (Fig. 6C),show that
for 45"shear and M-D shear mean striation
width decreased with decrease in grit diameter. However, the plot for mean striation
width by it size for A-C shear differed
dramatical y from the other two: Striation
width increased with decrease in grit size.
The differences in striation width were significant only for the 600 grit oup, summed
over all species, but the very ifferent trajectories of the plots suggest stron ly that striation width was influenced by actors other
than grit size when shear was directed a icocervically. In sum, analysis of first-or er
interactions indicates that the effects of grit
size on striation width are mitigated by species differences (expressed in the dichotomy
between prismatic and nonprismatic enamels) and by direction of shear (for prismatic
enamels).
Second-order interactions: Species, grit,
and direction
The interaction among all three factors
(second-order interactions) is de icted from
a single perspective, grouped y species,
each point representing the mean value for a
single experiment (Fig. 7). If there were no
significant interactions among factors, then
the tra'ectories of the plots of mean striation
width
direction of shear for each species
would e arallel. Instead, each species
shows a dif erent pattern for mean striation
widths according to grit size and direction of
shear. Only the nonprismatic enamel (Crocodylus)specimens approach the pattern expected if grit size were the prime determi-
P
B
P
Y
f.
B
cp
E
ii
P
42
M.C. MAAS
SPECIES X DIRECTION
SPECIES X GRIT
fi
I
I-
Lemur
0
3
z
2000-
2
4
a
I-
cn
0 1600W
z
2
cn
z
d 1200-
z
'\i
z
w
4
c
o
d
y
l
u
\
s
a
---
800
240
A
400
8
GRIT SIZE
0
45
600
A-C
M-D
DIRECTION OF SHEAR
DIRECTION X GRIT
I
I-
P
3
2000
0
z
U
I-
cn
eool
240
C
400
600
GRIT SIZE
Fig. 6. Plots oftwo-way (first-order)interactions. A: Species and grit (all directions of shear);B: species
and direction (all grit sizes); C: direction and grit (all species).
43
ENAMEL STRUCTURE AND MICROWEAR
0 VIS
LEMUR
/450
,."*
1800
M-D
1300
M-D
A-C
Y
z
z
2
2
800
240
BOO
400
000
B
GRIT SIZE
240
400
800
GRIT SIZE
CROCODYLUS
HOMO
A-C
0
W
z
B
v)
z
450
M-D
1300
U
a
Lz
2
k
z
A
e
c
o
o
400
240
l
800
GRIT SIZE
D
4 00
240
800
GRIT SIZE
Fig. 7. Plot of three-way (second-order) interactions among factors (species, grit, and direction),
grouped by species. A Lemur; B: Ouis; C: Homo; D: Crocodylus.
nant of striation width (decrease in striation
width with decrease in grit size). For the
three prismatic enamels, the apicocervical
shear experiments show an increase in mean
striation width with decrease in mean grit
size. Furthermore, for the three prismatic
enamels mean striation width is greatest for
apicocervical shear of all the shearing directions, but only for the two smaller grit sizes
(400 grit and 600 grit).
Multiple comparisons
Multiple comparison tests (total experimentwise error rate of 0.05 for 630 possible
comparisons) general1 sup ort the interpretations based on d O V A . q h e most striking aspects of the multiple comparisons are
the absence of significant differences amon
the nonprismatic enamel experiments an
the frequent occurrence of significant differ-
%
44
M.C. MAAS
ences between nonprismatic and prismatic
enamel experiments. Amonf prismatic
enamels, there are few signi icant differences between pattern 2 and pattern 3 experiments, although both differ significantly
from pattern 1experiments.
DISCUSSION
The Null Hypothesis
Based on the significant results of the
analysis of variance and multiple comparisons, the null hypothesis that striation
widths are random according to the experimental factors of particle size, species differences in structure, and direction of shearing
force can be rejected. However, the significant interaction among all three factors
demonstrates that the effect of any one experimental factor was dependent on the expression of the other factors. Therefore, the
alternative h potheses of microstructural
influence an particle size influence outlined above must be reevaluated.
The influence of particle size
The clear lack of association between average particle size and average striation width
for all the prismatic enamel experiments
demonstrates no support for the conjecture
that width of microwear features reflects the
size of abrasive particles (see, e.g., Ryan,
1979a; Wells et al., 1982; Solounias et al.,
1988; Teaford and Robinson, 1989). However, nonprismatic enamel experiments, in
contrast to prismatic enamel experiments,
did show a strong association between average width and average particle size.
The silicon carbide grit particles used in
these experiments are, like naturally occurring abrasives, irregular in shape. Kay and
Covert (1983) sug ested that particle shape
and hardness, ratfler than size per se, may
be the important determinant of striation
morphology, and Walker et al. (1978) explicitly noted that the area of contact between
grit and enamel will vary for irregularly
shaped particles. As predicted, variance in
striation width declined with decrease in
average particle size, presumably because of
the greater regularity in sha e of smaller
abrasives (400 and 600 grit a rasive particles are more uniform in sha e than 240 grit
silicon carbide grit particles! Therefore, although particle shape variability may contribute to the lack of correlation between
avera e striation width and average particle
size, t is factor should be least important for
B
E
a
very small particles. In these experiments,
however, it was the width of striations produced b the smallest sized particles that
deviate the most from the predicted pattern
of the particle size hypothesis. In any event,
whether particle shape, enamel microstructure, or some other variable was the mitigating factor, there is no evidence from these
experiments that striation width is directly
correlated with particle size when prismatic
enamel is abraded by shearing force. A variety of substances potentially produce microwear. Among the most frequently mentioned are exogenous mineral particles and
plant phytoliths; both are characterized by
properties of shape and hardness similar to
the silicon carbide used in these experiments. Given the results of this study, similarity in striation width cannot be used uncritically as evidence of similarity in size of
abrasive particles.
The influence of enamel microstructure
Differences in striation width distributions did not correspond directly with prism
packing patterns, but did differ according to
prismatic or nonprismatic structure. The
distinction between the two enamel types
appears to lie in their responses to the different directions of shear. Direction of shear
had a differential effect on striation width for
prismatic enamels and not for nonprismatic
enamel.
The structural attributes of prismatic
enamel that potentially contribute to anisotropic wear properties are 1) the arrangement of groups of similarly oriented crystaland inter rismatic
lites into
material, 2) t e orientation of the ong axes
of risms, and 3) the orientation of individua enamel crystallites or subparallel crystallite groups (Ronnholm, 1962; Boyde,
1964; Boyde and Fortelius, 1986; Maas,
1988) (Fig. 1).The first two characteristics,
prism packing atterns and prism orientation, appl to t e enamel at a microstructural leve , whereas the third (enamel crystallites) is an expression of structure on the
smaller, ultrastructural scale. All three
structural attributes form a regular rather
than random structural attern, because
enamel structure reflects t e mor holo
enamel-secreting cells during toot deveY opO f
ment.
Although the correspondence was not perfect, the hypothetical model did explain some
of the differences in wear between prismatic
B
irisms
P
P
P K
R
45
ENAMEL STRUCTURE AND MICROWEAR
and nonprismatic enamels observed in these
experiments. First, the model predicted that
the effects of nonstructural factors would be
expressed most clearly for nonprismatic
enamels and that, for prismatic enamels,
structure would mitigate the effect of particle size. The experimental results, which
showed different patterns of striation width
for prismatic and nonprismatic enamels,
were consistent with this prediction. Second,
the model identified two potential structural
influences on striation width, the arrangement and orientation of prism boundaries
and the orientation of cr stallites. The difference in scale between t e microstructural
features (prisms) and ultrastructural features (crystallites) provided a means of testing the relative contribution of each aspect of
enamel structure to striation width.
The lack of a consistent attern of sigf
for
nificant between-species c ifferences
prismatic enamels, demonstrated by the
multiple comparison tests, sup orts the conclusion that rism size and pac ing arrangement per se $o not influence striation width.
This statistical evidence is consistent with
observations that striations coursed across
prism cores and through interprismatic material and were not constrained by arrangements of prism boundaries. It is possible that
differences in the orientations of prism long
axes among specimens of the same species
obscured between-species differences and
precluded a fair test of the prismatic structural influence hypothesis. However, the
pairwise multiple com arison results indicate that this was not t e case.
The observation that the three prismatic
enamels, and, in particular, pattern 2 and
pattern 3 enamels, showed larger than expected striation widths for apicocervical
shear for the experiments with small abrasives (400 and 600 grits) but not the experiments with large abrasives (240 grit) supports the hypothesis that orientation of
crystallites, rather than attributes of prism
boundaries, is the important structural constraint on microwear. These abrasion experiment results also are consistent with experimental studies of fracture that have
documented the propensity for enamel to
fracture parallel to rather than perpendicular to crystallite long axes (Rasmussen et al.,
1976) and are consistent with suggestions
that enamel is preferentially removed in
units smaller than enamel prisms (Boyde
and Fortelius, 1986). The mechanical expla-
t
!i
K
nation lies in the orientation of enamel crystallites within risms and inter rismatic
material: Crysta lite long axes wit in prism
bodies are parallel to the prism long axis but
have a more apicocervical orientation in
prism tails or interprismatic regions, particularly for attern 2 and attern 3 prisms (see
Fig. 1). his preferre orientation allows
greater ease of abrasion, and therefore
greater striation width, when shear is directed apicocervically. When the long axis of
a prism is oriented obliquely, the net orientation of crystallite long axes also is more
oblique, for all three prism patterns. In this
study, experiments in which the net orientation of prisms was oblique to the surface
were characterized by greater mean striation widths, for any given grit size, than for
otherwise identical experiments in which
the net orientation of prisms (and therefore
crystallite long axes) was ierpendicular to
the direction of force. Bot these observations, the larger striation width for shear in
an apicocervical direction and the larger striation widths when prism long axes were
oriented more oblique to the wear surface,
are consistent with the hypothesis that crystallite orientation influences abrasive wear.
P
8
g
a
CONCLUSIONS
This analysis specifically evaluated the
im act of within-individual variability on
differences between microwear samples and
found that in most cases within-sample differences obscured those between groups.
Analysis of individual means, the approach
commonly adopted in microwear studies
whose goal is discrimination among groups,
avoids this problem by ignoring withinsample variance. Although arguably a propriate in such cases, ignoring varia ility
within individuals also ignores an inherent
property of microwear distributions, a property that reflects the multiplicity of factors
that influence microwear morphology.
In this study, variability in enamel microstructure was in part exemplified by differences among four species. In fact, the same
degree ofvariation (three prism packing patterns and nonprismatic enamel) can be
found within single teeth (e.g., Boyde and
Martin, 1987). Likewise, differences in
prism orientation, demonstrated here between different specimens of the same species, also may characterize individual wear
surfaces. Therefore, the most careful control
of phylogenetic and functional consider-
E
46
M.C. lMAAs
ations may not be enou h t o eliminate the
potentially confounding actor of microstructural variation from comparative wear studies.
The causal complexity of the tooth wear
rocess demonstrated by this study is the
[kely explanation for the inability of parametric analyses of striation width to discriminate among some dietary variables.
Likewise, variation in other microwear
parameters also may be explainable in part
in terms of a complex causal relationship
that includes both dietary factors and factors
independent of diet. If that is the case, there
may indeed be a limit to the explanatory
ower of microwear pattern analysis. That
Emit can be defined on1 by more rigorous
analyses of the process o tooth wear, as well
as its pattern (see, e.g., Teaford, 1988;
Teaford and Oyen, 1989).
Other factors whose influence on microwear morphology has et to be assessed
fully include magnitude o force, variation in
particle shape and hardness, and cumulative effect of wear on morphometrics of individual features. One study (Peters, 1982)has
suggested that particle hardness may influence resence or absence of microwear produce by compression but indicated no relationship between size of wear features and
particle hardness. The effects of variation in
magnitude of force has been tested under
conditions of com ression (Maas, 1988,
1989)but only to a imited extent for shearing force (Ryan, 1979a). Other potential influences on microwear that have yet to be
tested empirically for enamel include the
effects of multiple abrasion events (see, e.g.,
Powers et al., 1973b; Teaford, 1988). In this
study it has been shown that experimentally
controlled tests can refute previous assumptions of causation; further experimental
studies should provide the necessary information for evaluating the relative contributions of all these factors in a rigorous and
meaningful manner.
It has recently been claimed that the
causes of tooth wear, although a legitimate
research roblem, are not of primary relevance to ietary reconstruction (Walker and
Teaford, 1989). Walker and Teaford argued
that demonstration of correspondence between wear pattern and diet is enough to
establish a predictive relationship, regardless of their causal relationship. Certain1
the increasing number of comparative stu ies documenting significant differences in
P
F
F
B
P
cp
B
wear patterns between animals with major
differences in diet attests to the utility of
microwear for dietary reconstruction. Nevertheless, misinterpretations of the causes of
diagnostic wear patterns, arguably irrelevant when comparing very different patterns of microwear, may be of considerable
importance when attempting to assess more
subtle differences in microwear and therefore in diet. An understanding of the determinants of tooth wear facilitates assessment
of the potential of different parameters to
discriminate dietary differences; if a parameter varies with nondietary factors, as is the
case with striation width, there is little point
in evaluating its morphology or distribution,
On the other hand, analyses of attributes of
microwear that are unequivocally controlled
by factors related to diet should ield useful
results. Although this study in&cates that
striation width is not such an attribute, variables such as microwear feature shape, density, and erhaps orientation ap ear to be
more close associated with diet. ven if the
mechanica determinants of a wear pattern
are not the primary concerns of dietary reconstruction studies, knowledge of their
causes is necessary for appropriate and informed analysis of the data.
In addition to the cautionary note regarding causal interpretation of microwear, this
study raises questions about the relationshi of tooth wear and enamel structure. It is
we known that tooth form changes with
wear and that for many mammals changes
due to wear serve to optimize the biological
role of the dentition (see, e.g., Berry and
Poole, 1974; von Koenigswald, 1982;
Fortelius, 1985; Stern et al., 1989). Among
some primates, tooth wear serves to maintain occlusal function by, for example, maintaining sharp shearing crests or enlarging
crushin basins (Kinzey and Rosenberger,
1976). T is ontogenetic component of dental
function, which is clearly related to differential wear, has yet to be investigated. As was
pointed out previously, differential rates of
wear are close1 related to enamel structure
(see, e.g., Rens erger and von Koenigswald,
1980; von Koenigswald, 1982; Boyde,
1984a,b; Walker, 1984). For the most part,
structurally constrained wear properties
have been considered in relation to relatively
substantial wear and in relation to prominent structural features such as decussation
zones (groups of similarly oriented prisms).
This study demonstrates that ultrastruc-
E
pr
K
a
il
ENAMEL STRUCTURE AND MICROWEAR
tural variation, the arrangement and orientation of crystallites in prisms and interprismatic material, contributes to variation in
size of wear features. Such differences may
ultimately control the amount of wear in a
particular region of a tooth.
Prismatic enamel is characterized by regular patterns of microstructural and ultrastructural variation, including the distribution of prismatic and nonprismatic enamel
within teeth, differences in proportions of
prismatic and interprismatic enamel, and
differences in prism orientation that ma
contribute to regional differences in toot
function. It has been suggested that the evolution of prismatic enamel is linked to the
maintenance of functionally critical occlusal
relations between diphyodont teeth (e.g.,
Stern et al., 1989). The relationship between
differential wear and ultrastructural variation may prove to be an important as ect
both of age-related wear and of dental aiaptation and evolution in mammals.
K
ACKNOWLEDGMENTS
The material presented here represents
an as ect of the author’s dissertation, completelat SUNY Stony Brook, and I thank the
members of my committee, Drs. Fred Grine,
John Gwinnett, Bill Jungers, Lawrence Martin, and es ecially Dr. David Krause for their
advice an assistance during the course of
this research. Drs. R.F. Kay, D.W. Krause,
M.J. Ravosa, and Ms. S.G. Strait provided
helpful comments on an earlier draft of the
manuscript. Josef Muenni drafted Fi
1 and 5, and I also than him for p E :
graphic and engineering assistance. The
American Fellowship Program of the American Association of University Women generously provided financial su port during the
completion of this researck. This research
was supported by the Doctoral Program in
Anthropologcal Sciences, SUNY at Stony
Brook, and NSF grant BNS-8611198.
B
a
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