Long-term release of monomers from modern dental ... - Dentsply

Monomer release from composite materials 73
Supreme XT, followed closely by Clearfil Core. Very
small amounts of monomers were eluted from Ceram X.
These findings support the results of Franz et al. (26),
who demonstrated that Filtek Supreme XT exhibits
higher cytotoxicity on L-929 fibroblasts than the other
composite materials tested.
According to the manufacturersÕ information, the filler
content of these materials is as follows: Ceram X, filler
content 76% by weight; Clearfil Core, filler content 78%
by weight; and Filtek Supreme XT, filler content 78.5%
by weight. These small differences in the filler loading
between the three materials could not be the reason for
our findings. It is more likely that the difference in the
chemistry between the three materials, the filler size, and
the distribution of the filler particles could have influenced
our results.
As far as Clearfil Core is concerned, no published data
exist concerning the elution of monomers. The decreased
degree of conversion of the chemically cured composites
compared with the light-cured materials (27) could
explain the high degree of elution of monomers from
Clearfil Core. Storage of the samples in ethanol immediately
after their manufacture could have influenced the
release of TEGDMA and BisGMA because of the fact
that the degree of conversion Clearfil Core at the end of
mixing is markedly lower than that of photoinitiated
systems. Although such a comparison of the composite
materials directly after mixing might be considered to be
quite unfair for the chemically cured composite, as
mentioned above, the use of this solution for sample
storage simulates the situation in the mouth. Chemicallycured
materials are generally used as core build-up
materials and therefore they should not come into direct
contact with saliva or oral soft tissues because they are
covered with a crown. However, these materials very
often remain in the oral cavity for a considerable period
of time before they are covered with the final restoration.
Additionally, it is possible that monomers can diffuse
through dentin into the pulp (28, 29).
In the present study, the ethanol samples were analysed
using LC–MS/MS. Using this method, substances
with the same mass and retention time can be identified.
The use of MS/MS has the benefits of separating different
molecular ions, generating fragment ions from a
selected ion, and then mass measuring the fragmented
ions. The fragmental ions are used for structural determination
of original molecular ions (23). Therefore, a
combination of LC and MS in the form of LC–MS/MS
can be very helpful to identify specific compounds with
similar structures, which is one of the greatest problems
encountered in analytical methods. In the present study,
the identification of specific molecules was studied.
However, it should be mentioned that beside these
molecules, other substances and degradation products
could be released from the composite materials. The use
of LC–MS/MS could be very helpful in their identification.
An important finding of the present study was the
small amount of monomers released from Ceram X.
Ormocer contains inorganic–organic copolymers in
addition to the inorganic silanized filler particles. It is
synthesized through solution and gelation processes (sol–
gel process) from multifunctional urethane and thioether(meth)acrylate
alkoxysilanes. By hydrolysis and
polycondensation reactions, the alkoxysil groups of the
silane promote the formation of an inorganic Si–O–Si
network, while the methacrylate groups are still available
for light-activated organic polymerization. Ormocers are
characterized by the novel inorganic–organic copolymers
in the formulation that allow the modification of
mechanical properties over a wide range (30). Hickel
et al. (31) suggested that the advantages of ormocer
include low shrinkage, high abrasion resistance, biocompatibility,
and caries protection. However, only
limited information is available on the elution of
monomers from ormocers and their toxicity. Brackett
et al. (32) found that materials with traditional compositions,
such as Filtek Supreme XT, were severely cytotoxic
throughout an 8-wk interval, but materials with
newer chemistries or filling strategies, such as Ceram X,
improved over time of aging in artificial saliva. This
appears to support the claims of better polymerization
and lower leaching in newer chemistries (33). The difference
in the chemical structure of the composite, and
the variation in the ratio of filler and monomer, have a
significant effect on the element release and cytotoxicity
level of the materials (34).
Regardless of the polymerization time, storage time,
and material, a greater amount of BisGMA was released
compared with the other monomers. As mentioned
above, 75% ethanol has a solubility parameter which
matches that of BisGMA and this could be the reason for
the higher amount of BisGMA released. Additionally,
BisGMA and TEGDMA differ markedly as far as their
chemical properties and reactive potentials are concerned.
(35). BisGMA is more reactive than TEGDMA,
but the fact that it presents a sterically hindered structure
precludes a higher degree of conversion (36). It has been
shown that the quantity of unreacted unsaturations in
the BisGMA/TEGDMA system can be qualitatively
correlated with the amount of BisGMA in the polymerizing
system (37, 38) and that neat TEGDMA
exhibits a degree of conversion that is about 10% higher
than that of neat BisGMA (37, 38).
In the present study, BPA was found to be eluted by
unpolymerized composite samples of Filtek Supreme XT
and Ceram X and by polymerized samples of Ceram X.
This is in contrast to previous studies where no BPA was
detected (15, 16, 25). However, detectable amounts of
BPA have been mentioned, also by other authors (34,
39), to be extracted from composite materials. Dental
composites typically contain monomers [such as
BisGMA and bisphenol A dimethacrylate (BisDMA)]
that are derived from BPA, but there is no known use for
BPA itself in dental composites. As these monomers may
leach from dental composites, their stability has been
studied under a variety of conditions to determine whether
they may hydrolyze to form BPA. BisGMA, the
base monomer for many resin composites, has been
found to be stable under various hydrolytic conditions
(25). However, two researchers have reported that Bis-
DMA is hydrolyzed to BPA, which probably accounts