Biomedical Engineering Reference
In-Depth Information
called “molars.” The sequence and age of dental eruption
are discussed with growth and development (see also
“Dental and skeletal maturation” above).
Deciduous teeth are generally smaller than their
successor teeth. Anterior deciduous teeth have many of the
same characteristics as their permanent counterparts.
Deciduous cheek teeth, on the other hand, more closely
resemble permanent molars rather than the premolars
which succeed them. This has resulted in the frequent
misidentification of these premolars as molars. The total
number of deciduous teeth is 20 in Old World monkeys and
apes and 24 in NewWorld monkeys. As in humans, the first
permanent molars (which have no predecessors) usually
erupt before the deciduous teeth are replaced by their
permanent successors.
The permanent dentition of higher primates is usually
complete soon after skeletal maturation is achieved. In
some species, particularly the smaller NewWorld primates,
dental maturity may be accelerated and may precede
skeletal maturity. The dental formula (
James, 1960
) for Old
World monkeys and apes is written as 2.1.2.3./2.1.2.3. for
a total of 32 teeth (
Figure 4.6
). This formula indicates two
incisors, one canine, two premolars, and three molars in an
upper quadrant and an identical lower quadrant pattern. The
dental formula for New World monkeys other than Calli-
trichidae is written as 2.1.3.3./2.1.3.3. for a total of 36 teeth
(
Figure 4.7
). The dental formula for Callitrichidae, in
which only two molars are present in each quadrant, is
usually 2.1.3.2./2.1.3.2. for a total of 32 teeth.
The two most anterior teeth in each quadrant are I1 and
I2. The crown of each incisor is generally broad and spat-
ulate. The single root is usually conical. The roots of upper
incisors are usually rounder and longer than those of lower
incisors. The upper incisors are the only teeth rooted in the
premaxillary bone. The incisors of primates play an
important role in food acquisition.
Posterior to the incisors and the premaxillary suture is
the canine. Both its crown and root are conical. The size
and shape of this tooth vary greatly among primate species,
between the sexes, and between the maxillary and
mandibular dental arches. The canine usually projects
beyond the occlusal plane of the regular tooth row and
a diastema is frequently evident between the upper lateral
incisor and the canine. In this arrangement the lower canine
usually fits into the diastema during occlusion and thus lies
anterior to the upper canine (
Figure 4.2
). As the jaw closes
the posterior border of the upper canine slides against the
adjacent lower premolar. This facilitates honing of the
posterior aspect of the upper canine to a razor sharp edge in
species where the canine projects well beyond the tooth
row. In terrestrial species with marked sexual dimorphism
the upper canine of the adult male may project as much as
seven times the height of the other teeth in the dental row.
In contrast, arboreal species with little sexual dimorphism
FIGURE 4.7
NewWorld monkey permanent dentition (Cebus apella,
female). Note the number of premolars in each quadrant, the relative size
and shape of the premolars versus molars, and the shape of the incisors.
(Drawings by Nancy Hong.)
generally exhibit very slight canine projection, with the
notable exception of gibbons. The very large upper canine
seen in some primates is generally considered to have
a social rather than feeding function.
The cheek teeth of primates are used for mastication
and are divided into two groups based on evolutionary
development as well as dental morphology. The teeth
immediately posterior to canines are the premolars. In the
bicuspid premolars of higher primates, the lateral (outer or
buccal) cusp is normally larger. The only exception to the
usual bicuspid premolars in higher primates is the P
3
(lower
P3) in Old World monkeys and great apes. In this exception
P
3
is unicuspid, and its anterior surface forms a sloping
shoulder which the upper C abrades against during jaw
closure thus sharpening the distal canine edge. Upper
premolars generally have three roots whereas lower
premolars have two (see
Remane, 1960
, for a detailed count
of roots of teeth in various species). The range of species
variability in premolars is greater than in other teeth.
As already indicated, the number of premolars in each
quadrant is not constant for all higher primates but divides
into two distinct patterns along evolutionary lines. New
World monkeys have retained three, i.e. P2, P3, and P4, of
the original four primitive mammalian number. Old World
monkeys and apes (as well as humans) have retained only
