Biomedical Engineering Reference
In-Depth Information
of the remineralization, interference with pellicle and plaque formation, and the inhibition of micro-
bial growth and metabolism. For this purpose, fluoride-releasing filler systems are used, such as
strontium fluoride (SrF
2
) or yttrium trifluoride (YbF
3
) and leachable glass fillers
[18-20]
. The filler
systems release fluoride by means of an exchange reaction of water diffusion into the composite and
fluoride release from the particles. One of the major disadvantages of fluoride release is that it may
cause voids in the matrix when the fluoride leaches out of the material. Furthermore, most of the
fluoride is already released during the setting reaction, followed by a smaller amount of long-term
fluoride release.
3.2.1.2 Nonreleased Antibacterial Agents
Filler modification may also produce a nonreleased antibacterial agent. For example, Yoshida et al.
[9]
reported that an experimental restorative composite containing silver-supported fillers inhibits the
growth of
S. mutans
, upon direct contact with the silver composites, with no release of silver ions.
Contact inhibition of bacteria was also achieved by a restorative composite containing bactericide-
immobilized filler
[21]
. The antibacterial component utilized was 12-methacryloyloxydodecylpyrid-
inium bromide (MDPB). MDPB was found advantageous over agent-releasing materials in terms of
maintaining favorable mechanical properties after aging; and over silver-immobilized agents in terms
of color stability (an imperative feature in restorative composites). The MDPB activity is assumed to
be by contact inhibition. It is presumed that the agent cannot penetrate fully through the cell wall or
membrane, unlike free antimicrobials. Hence, the effect of the filler-modified MDPB-containing com-
posites is not as intensive as the materials which release antibacterial agents.
3.2.2
Matrix Phase Modification
Matrix phase modification is basically easier to accomplish than filler modification because it is rela-
tively easy to alter resins. Similarly to filler modification, matrix modification involves two approaches:
(1) addition of a released antibacterial agent and (2) addition of nonreleased antibacterial agent.
3.2.2.1 Released Antibacterial Agents
Released agents are soluble components capable of diffusing in aqueous environments. One exam-
ple for soluble antibacterial agents is organic fluoride components added to the matrix. Matrix-bound
fluorides include acrylic-amine-HF salts, methacryloyl acid-fluoride, and acrylic-amine-BF
3
[22]
.
Unfortunately, fluoride levels leached from composites are much lower than levels released from
conventional or resin-modified glass ionomers. Furthermore, it is not proven by prospective clini-
cal studies whether the incidence of secondary caries can be significantly reduced by the fluoride
release of restorative materials
[23]
. An additional soluble antimicrobial used as an added agent in
restorative composites is chlorhexidine
[24]
. It has been demonstrated that chlorhexidine inhibits bac-
terial growth around restorative composites. However, resins such as poly(ethyl methacrylate) and
tetrahydrofurfuryl methacrylate incorporating chlorhexidine or chlorhexidine-diacetate-based sys-
tems release up to 50% of the agent within 14 days
[25,26]
. An additional agent added to restorative
composites was benzalkonium chloride which is a quaternary ammonium compound. Benzalkonium
chloride was added to an orthodontic composite based on the concept that a small amount of this
compound could provide antibacterial properties without significantly affecting the physical proper-
ties of the material. This addition resulted in enhanced antimicrobial properties
[27]
.
