Inspiration from the Wear of Human Teeth - Dental Biotribology - page 162

8.1.3 Compact and Orderly Microstructure

As mentioned above, the enamel possesses a much better wear resistance than the
dentin. Microexaminations reveal that the excellent antiwear properties of enamel
are mainly attributed to its compact and orderly microstructure.
Human enamel, the most highly calcifi ed tissue in the body, is one of those
unique natural substances that still cannot be effectively replaced by artifi cial restor-
ative materials. The enamel is composed of hierarchical structures. Based on its
primary structure, the enamel can be regarded as a fi ber-reinforced composite. As
described in Chap. 1 , keyhole-like rods (6-8

m in diameter) are embedded in the
interrod enamel, the matrix. The rods are over 95 % mineralized, while the interrod
enamel is rich in protein and mostly a result of the incoherence of combining crys-
tals of different orientations [ 9 ]. Hence, the Young's modulus and the hardness are
lower in the interrod enamel than those in the area of the rods. The rod sheath, a
natural coupling agent, locates where the enamel rods meet the interrod enamel ; the
rod sheath consists of more protein than both the interrod enamel and the rods.
Obviously, the primary structure of enamel is a compact alternate arrangement of
mineral and organic phases.
Considering the physiological function of enamel, the enamel rod is its
functional unit. The rods align in parallel and run approximately perpendicular
from the DEJ toward the tooth surface. Each rod consists of tightly packed hexago-
nal carbonated hydroxyapatite particles, which are covered by a nanometer-thin
layer of enamelin and orient along the rod axis [ 9 , 10 ]. The hydroxyapatite particles
are the fundamental hierarchical structure level, which then assemble into nanofi -
brils and fi bers level by level. The fi bers have a mean width of 68.3 nm and a mean
thickness of 26.3 nm [ 9 ]. Finally, the fi bers assemble into an enamel rod through a
unique arrangement,
An important mechanical function of the composite nature of enamel is related
to its antiwear performance. As described above, the interrod enamel is a weaker
phase that can be easily worn out. However, the protein-rich interrod enamel could
act as a stress buffer for the brittle enamel rods. When occlusal stress is applied on
the surface of enamel during chewing, it is carried mostly by the high-stiffness rods,
producing little plastic deformation and then low wear loss, while the soft interrod
enamel may produce a large deformation and then partially dissipate the stress,
preventing the rods from cracking. An additional contribution of enamel rods to the
tribological behavior of enamel is attributable to their orientation. Mass [ 11 ] pointed
out that the variation in the crystallite orientation of prismatic enamels may contrib-
ute to optimal dental function through the property of differential wear in function-
ally distinct regions of teeth. As described in the Chap. 3 , the orientation of the
enamel rods plays an important role in the wear resistance of enamel, which is
related to the alignment of fi ber-like apatite crystals and the composite nature of
enamel rods.

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