Biomedical Engineering Reference
In-Depth Information
commercialized as hybrid, microhybrid, and condensable composites
[25]
. There was an improve-
ment in mechanical strength; however, the polishability was still a limitation. A maximum filler
load from 70 to 77 wt% was recorded then. Microfilled composites (10
100 nm) were not
suitable for high stress bearing areas (e.g., Class I, II, and IV restorations) of the dentition. The par-
ticle sizes of these hybrid composites were not similar to the size of the hydroxyapatite crystal,
dentinal tubule, and enamel rod. There was also a potential for compromise in adhesion between
the macroscopic restorative material and the nanoscopic (1
10 nm in size) tooth structure. So
nanofilled composite materials were introduced.
There are two distinct types of dental nanocomposites currently available: nanofills and nanohy-
brids
[26,27]
. Nanofills contain nanometer-sized particles (1
100 nm) throughout the resin matrix,
with no other large primary particles included. Nanohybrids consist of larger particles (400
5000 nm)
with added nanometer-sized particles. The use of nanoparticles addresses the aforementioned diffi-
culty by combining high mechanical strength with long-term polish retention in one material.
Another adhesive material used in orthodontics is GIC. GIC is also known as polyalkenoate
cement and contains components of silicate glass and polyacrylic acid
[28]
. GIC is translucent and
adhesive to tooth structure and has unique properties such as biocompatibility, anticariogenic action
(due to fluoride release), and adhesion to moist tooth structure. In addition, the coefficient of ther-
mal expansion for GIC is low and close to the values of tooth structure. Besides its advantages,
GIC has some disadvantages such as brittleness and inferior mechanical strength. The use of GIC
in orthodontics became popular during the late 1980s due to its fluoride-releasing potential which
made it highly attractive for orthodontic band cementation. It was shown that modifying conven-
tional GIC with nanohydroxy and fluoroapatite (
Figure 11.5
) and N-vinylpyrrolidone containing
polyacids improved its mechanical properties. When tested, it proved to have greater CS and higher
diametral tensile strength (DTS) and biaxial flexural strength (BFS) than the control group consist-
ing of conventional GIC
[16]
.
Recently, “nanoionomer,” which is resin-modified GIC (Ketac
N100 Light Curing Nano-
Ionomer), has been introduced to operative dentistry
[29,30]
. This light curing nanoionomer is com-
posed of fluoroaluminosilicate glass, nanofillers, and nanofiller “clusters” combined to improve
mechanical properties and high fluoride release. A recent study tested the commercially available
nanocomposite material (Filtek Supreme Plus Universal) and the nanoionomer restorative material
(Ketac
t
N100 Light Curing Nano-Ionomer) for orthodontic bracket bonding
[17]
. The study evalu-
ated their shear bond strength and failure site locations in comparison with a conventional light-cure
orthodontic bonding adhesive (Transbond XT). Orthodontic brackets should withstand high shearing
force in the oral cavity generated during the orthodontic teeth movement. The results of the study
demonstrated that conventional orthodontic composite had higher shear bond strength than that of
nanocomposite and the nanoionomer groups. The study concluded that nanocomposites and nano-
ionomers may be suitable for bonding but are inferior to conventional orthodontic composites.
In another study, an experimental composite adhesive material (ECA) containing silica nanofil-
lers and silver nanoparticles was compared with two conventional adhesives (composite and resin-
modified glass ionomer) to evaluate the surface characteristics, physical properties, and antibacte-
rial activity against cariogenic Streptococci
[18]
. Bacterial adhesion was measured by incubating
the adhesive discs for 6 h in saliva sample. The discs were washed with sterile phosphate buffered
saline and the number of adherent cells was determined using a Beckman LS-5000TA liquid scintil-
lation counter. The results of this study showed that ECA had rougher surfaces than conventional
t
