(Weiner & Wagner 1998). However, similar investigations on dental enamel,
another important family of hard connective tissues, have not been well developed,
although there are many structural, mechanical, and functional similarities between
these biomaterials. As the hardest connective tissue in the human body, dental
enamel consists of 96% mineral, 4% organic material, and water. Enamel has
unique properties, including extraordinarily high hardness, outstanding resistibility
to wear, and stability over a lifetime of use within the physically demanding
environment of the oral cavity (Ge et al. 2006). The enamel structure and its
biomineralization have therefore been of interest in order to provide theoretical
basis for both the treatment of enamel disease and the biomimic synthesis of novel
biomaterials.
Recently, the hierarchical assembly of enamel structure was investigated and
depicted via various microscopic explorations from nanoscale to microscale, which
supposedly is related to its functions and the physical requirements placed upon it in
the oral cavity. On the other hand, the mechanical diversity within enamel was
reviewed and proposed to have close corresponding relationships with the microstructure of enamel in terms of hierarchy. Human enamel taken from mature third
molars was explored (Cui & Ge 2007). Integrating the microscopic observations
revealed the high complexity of the well-organized enamel structure in terms of
hierarchical assembly. Based on these observations, seven hierarchical levels of the
microstructure were proposed and described, using a scheme representing a complete
spectrum of the organization in detail, covering a range from microscale to nanoscale:
hydroxyapatite crystals (Level 1) at first form mineral nanofibrils (Level 2); the
nanofibrils always align lengthways, aggregating into fibrils (Level 3) and further
thicker fibers (Level 4); prism/interprism continua (Level 5) are then composed of
them. At the microscale, prisms assemble into prism bands (Level 6), which present
different arrangements across the thickness of the enamel layer (Level 7). Analysis
of the enamel and bone hierarchical structure suggests similarities of scale distribution at each level. The study also aimed to understand further the structural–mechanical relations at each hierarchical level.
The prevailing concept of enamel structure is that the basic elements of enamel
are nanosized fibril-like hexagonal hydroxyapatite crystals, which are further
attached into groups. The most readily apparent structural blocks of enamel, termed
prisms and interprisms, are imposed on this arrangement. It has been proved that in
both sites the composition is identical; the only substantial difference is the
orientation of the crystals. The crystals in the prisms, particularly along the center,
tend to align lengthways and lie parallel to the prism axis, but deviate more and
more as their distance from the center increases.
In the latter location, the crystals tend to orientate perpendicular to the incremental lines. Where the crystals within these two locations meet, the structure is
discontinuous, leaving a gap, the prism sheath. The prisms and interprisms further
construct a distinct structural pattern in the enamel. Generally, human molar enamel
contains a superficial layer of enamel with parallel prisms, so-called radial enamel,
within which the prisms are orientated radially and intercept the occlusal surface
166
Q. Feng
another important family of hard connective tissues, have not been well developed,
although there are many structural, mechanical, and functional similarities between
these biomaterials. As the hardest connective tissue in the human body, dental
enamel consists of 96% mineral, 4% organic material, and water. Enamel has
unique properties, including extraordinarily high hardness, outstanding resistibility
to wear, and stability over a lifetime of use within the physically demanding
environment of the oral cavity (Ge et al. 2006). The enamel structure and its
biomineralization have therefore been of interest in order to provide theoretical
basis for both the treatment of enamel disease and the biomimic synthesis of novel
biomaterials.
Recently, the hierarchical assembly of enamel structure was investigated and
depicted via various microscopic explorations from nanoscale to microscale, which
supposedly is related to its functions and the physical requirements placed upon it in
the oral cavity. On the other hand, the mechanical diversity within enamel was
reviewed and proposed to have close corresponding relationships with the microstructure of enamel in terms of hierarchy. Human enamel taken from mature third
molars was explored (Cui & Ge 2007). Integrating the microscopic observations
revealed the high complexity of the well-organized enamel structure in terms of
hierarchical assembly. Based on these observations, seven hierarchical levels of the
microstructure were proposed and described, using a scheme representing a complete
spectrum of the organization in detail, covering a range from microscale to nanoscale:
hydroxyapatite crystals (Level 1) at first form mineral nanofibrils (Level 2); the
nanofibrils always align lengthways, aggregating into fibrils (Level 3) and further
thicker fibers (Level 4); prism/interprism continua (Level 5) are then composed of
them. At the microscale, prisms assemble into prism bands (Level 6), which present
different arrangements across the thickness of the enamel layer (Level 7). Analysis
of the enamel and bone hierarchical structure suggests similarities of scale distribution at each level. The study also aimed to understand further the structural–mechanical relations at each hierarchical level.
The prevailing concept of enamel structure is that the basic elements of enamel
are nanosized fibril-like hexagonal hydroxyapatite crystals, which are further
attached into groups. The most readily apparent structural blocks of enamel, termed
prisms and interprisms, are imposed on this arrangement. It has been proved that in
both sites the composition is identical; the only substantial difference is the
orientation of the crystals. The crystals in the prisms, particularly along the center,
tend to align lengthways and lie parallel to the prism axis, but deviate more and
more as their distance from the center increases.
In the latter location, the crystals tend to orientate perpendicular to the incremental lines. Where the crystals within these two locations meet, the structure is
discontinuous, leaving a gap, the prism sheath. The prisms and interprisms further
construct a distinct structural pattern in the enamel. Generally, human molar enamel
contains a superficial layer of enamel with parallel prisms, so-called radial enamel,
within which the prisms are orientated radially and intercept the occlusal surface
166
Q. Feng