Biomedical Engineering Reference
In-Depth Information
After maturation, some protein fragments are retained primarily at
the incisal edges and proximal sides of rod boundaries defining a rod
sheath, 8 which help to define the rounded discontinuity seen around the
top of the 'fish scale' on rods. These discontinuities have two very
important mechanical functions. First, they define three-dimensional
cleavage planes that will deflect cracks. 9,10 This prevents cracks from
advancing straight through enamel to cause catastrophic macro-
mechanical failure, but instead spreads the damage laterally over a larger
volume. Second, the presence of minute quantities of protein remnants
allows limited differential movement between adjacent rods. 11 Limited
slippage also reduces stresses without crack growth. The minor
components of enamel, namely the protein remnants and water, have a
profound plasticizing effect. Enamel is known to be much more flexible
and softer than its major component, crystalline hydroxyapatite. 10,12
The result of such a highly precise regulation of the biomineralization
procedure is the prominent hierarchy of enamel structure ( Fig. 8-1 ) . 13
The smallest structural units are needle or plate-like hydroxyapatite
crystallites which are roughly of flattened hexagonal form in cross-
section 14 with mean width of 68 nm and mean thickness of 26 nm. 15 In
healthy human enamel, these hydroxyapatite crystallites are organized
and bundled together by organic molecules into larger-scale structures,
called 'keyhole-shaped' enamel rods. At this level, the directional
arrangement of the hydroxyapatite crystallites varies. Crystallite plates in
the central part of the rod are parallel to the rod axis while those near
the edge of the rod usually have an angle of near 45 degrees to the
longitudinal axis of the rods. 16 The arrangement of rods determines the
next level unit, namely enamel types. Human enamel consists of ~5 μm
diameter rods encapsulated by thin protein rich sheathes that are arranged
parallel in a direction perpendicular to the dentino-enamel junction (DEJ)
from dentin to the outer enamel surface. In some areas, rods may twist
together or change their direction slightly to reinforce the whole
structure. 17 This complex hierarchical structure has elicited a number of
attempts to model it, 18-21 however in all instances the emphasis has been
placed on the elastic rather than the non-linear deformation and
toughness of the structure.
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