Biomedical Engineering Reference
In-Depth Information
coupling agents, and the initiator
accelerator system. Despite the significant improvement of RBC,
restorative composites still suffer from several key shortcomings: deficiencies of mechanical
strength and high polymerization shrinkage, which are responsible for the shorter median survival
life span of RBCs (5
7 years) in comparison with amalgam (13 years)
[52]
, and secondary caries
and bulk fracture. Caries at the restoration margins is a frequent reason for replacement of existing
restorations, which accounts for 50
70% of all restorations.
During the past decade, more efforts have been focused on dental nanocomposite, with a
hope that contemporary nanocomposites with ceramic nanofillers should offer increased
esthetics, strength, and durability. However, research to date shows that most nanofillers provide
only incremental improvements in the mechanical properties with a few exceptions
[53]
. Variety
of calcium phosphates (CaPs), such as HAP, ACP, tetracalcium phosphate (TTCP), and dical-
cium phosphate anhydrous (DCPA) have been studied as fillers to make mineral releasing dental
composites. Skrtic et al.
[54]
conductedpioneeringresearchtoinvestigate the physicochemical
properties of dental composites containing unhybridized and hybridized ACP. Their research
demonstrated that hybridization of ACP fillers using agents, such as tetraethoxysilane (TEOS)
or ZrOCl
2
solution, improved the mechanical properties, e.g., biaxial flexural strength, of the
composites containing ACP fillers. However, the addition of both hybridized and unhybridized
ACP fillers generally degraded the biaxial flexural strength of the resin materials
[55]
.Itwas
hypothesized that the strength degradation compared to unfilled resin is attributed to poor dis-
persion and insufficient interaction between ACP and resin. Such hypothesis has been supported
by mechanical testing of dental composites containing particles with different sizes
[55]
.Both
nanosized and microsized HAP particles were also studied as dental fillers and the mechanical
tests indicated that microsized instead of nanosized HAP was favored in terms of mechanical
properties
[56]
.
From the point of view of composite mechanics, fibers are the preferred reinforced materials
compared to particles since fibers can provide larger load transfer and they can also facilitate some
well-known toughening mechanisms, such as fiber bridging and fiber pullout. Reinforcement with
high-strength inorganic fibers indeed demonstrates significant improvement on the mechanical proper-
ties of dental composite. Beyond the benefits of strengthening effects, it has been reported that fibers
can reduce the polymerization shrinkage as well
[57]
. The development of RBCs as an alternative to
dental amalgam has resulted in optimization of the particle size distributions and filler loading, result-
ing in an improvement in the mechanical properties
[58]
. In order to achieve superior esthetics,
submicron fillers were introduced to the development of RBC materials. However, filler loading of the
early “homogeneous microfill” RBC types was reduced due to a high surface-area-to-volume ratio,
thereby limiting mechanical properties. The introduction of “heterogeneous microfills” increased the
filler loading (
B
50 vol%), as prepolymers containing a high-volume fraction of silanated nanofillers
(
50 nm) were incorporated into a resin matrix containing discrete submicron particles. Although the
approach improved the flexural strength of “heterogeneous” RBCs (80
B
160 MPa) compared with
“homogeneous” microfills (60
80 MPa), the mechanical properties remained inferior to hybrid RBC
systems, which are loaded to approximately 55
65 vol% and possess flexure strengths in the region of
120
145 MPa
[59]
.
Microfilled composites comprise silicon dioxide filler particles with less than 100 nm in
diameter in conjunction with prepolymerized organic fillers, aggregated by crushing them into
larger filler particles. Nowadays, the most commonly used resin composites, i.e., microhybrids
