Biomedical Engineering Reference
In-Depth Information
Table 1.2
Mechanical
properties of enamel and
dentin
Property
Dentin
Enamel
Young's modulus (GPa)
10.2-15.6
20.0-84.2
Shear modulus (GPa)
6.4-9.7
29
Bulk modulus (GPa)
3.11-4.38
45.65
Poisson's ratio
−0.11-0.07
0.23, 0.30
Compressive strength (GPa)
0.249-0.315
0.095-0.386
Tensile strength (GPa)
0.040-0.276
0.030-0.035
Shear strength (GPa)
0.012-0.138
0.06
Knoop hardness
57-71
250-500
Density (kg/m
3
)
2,900
2,500
Source
: From Braden [
14
]
Enamel is the hardest tissue in the human body, while dentin is usually considered
to be elastic and soft. Initially, the enamel, which has a thickness of 2-3 mm, is exposed
to the occlusal surface and chemical environment within the mouth. Enamel is
composed of 92-96 % inorganic substances, 1-2 % organic materials, and 3-4 %
water by weight [
15
]. Most of the inorganic substances are hydroxyapatite, which is
contained in the basic structural unit of enamel, the rod or prism. These prisms align
and run approximately perpendicular from the dentin-enamel junction toward the
tooth surface [
16
-
18
]. The high hardness of enamel is attributed to its high mineral
content, while its brittleness is due to its high elastic modulus and low tensile strength.
Investigations have shown that the mechanical properties of enamel vary with loca-
tions on a tooth, local chemistry, and prism orientation [
18
-
20
]. Finite element stress
analysis results showed that the enamel absorbs most of the applied chewing load
during mastication due to its greater stiffness as compared to dentin; therefore, mas-
ticatory forces tend to flow around the enamel cap to the root dentin [
21
].
Dentin is a hydrated biological composite composed of 70 % inorganic material,
18 % organic matrix, and 12 % water by weight, and its properties and structural
components vary with locations [
22
]. The structural composition of dentin includes
oriented tubules surrounded by a highly mineralized cuff of peritubular dentin and
an intertubular matrix consisting of type I collagen fibrils reinforced with apatite.
Between enamel and dentin, the dentin-enamel junction, a biological interface, may
dissipate stresses, inhibiting further crack propagation [
23
,
24
]. The dentin-enamel
junction has a high fracture toughness and, along with the more resilient underlying
dentin, supports the integrity of enamel by preventing its fracture during function.
Another possible mechanical function of the soft dentin beneath the dentin-enamel
junction may be related to the ability of a whole tooth to resist impact forces, which
often occur when the tooth is working [
25
].
The major functions of teeth, together with any restorations and implants, are
associated with speech, breathing, tasting, and chewing, and with supporting bone,
soft tissues, and the muscles of mastication. Initially, a human has 20 primary
(or baby) teeth [
26
]. These are eventually replaced, during childhood, with 32 per-
manent (or adult) teeth. To allow chewing, teeth are arranged into two opposing
arches in the mouth. Within each arch are different types of teeth (Fig.
1.2
), each of
