610 the journal of prosthetic dentistry volume 81 number 5

TOLEDANO ET AL
THE JOURNAL OF PROSTHETIC DENTISTRY
MAY 1999
613

---

However, this bonding is not so strong and does not
produce an adequate marginal sealing.
20,21
The increase in leakage of Dyract resin composite
also could be attributed to thermal expansion mismatch
with tooth substance, which is reported to be signifi-
cantly higher than that of conventional cements and
also less than that of composites,
9,10
perhaps due to
their different chemical composition. Leakage of com-
posite resin restorations may be attributed to a con-
traction gap produced by polymerization shrinkage and
expansion and contraction with temperature changes,
because the coefficient of thermal expansion of com-
posites is different from that of the dental hard tissues.
Glass ionomer cements exhibit limited shrinkage dur-
ing setting and their coefficient of thermal expansion is
similar to that of dentin.
4
Mitra and Conway
9
reported
that Fuji II LC and Vitremer materials had coefficients
of thermal expansion of 31.5 and 11.5 ppm/°C,
respectively, and Silux Plus microfilled composite 56.6
ppm/°C 7 days after curing. Dyract has a composition
closely related to the microfilled composites and has a
coefficient of thermal expansion of 40.52 ppm/°C
(P Hammesfahr, verbal communication, 1998). This
may explain why Dyract resin composite is more sus-
ceptible to thermal stresses than the other materials.
Also, because the resin component of the material
adheres poorly to the cervical dentin than to enamel,
this justifies, in part, that the Dyract resin composite
revealed more leakage at the gingival margin than at
the enamel margin.
Although Vitremer glass ionomer cement displayed
microleakage values between those of Fuji II LC and
Dyract materials, there was no statistically significant
difference among the 3 materials. Some authors have
pointed out that significant dimensional changes and
surface hardening can occur after initial light curing of
the resin component of resin-modified glass ionomers,
and further contraction continues for the first 24 hours
as the material matures.
10,11
Because both Vitremer
and Fuji II LC glass ionomer cements contain approx-
imately the same percentage of resin, which is less than
that for Dyract composite, it could be thought that this
is another reason to explain the different microleakage
patterns.
10,11
Uno et al
16
considered that the differ-
ences observed between Vitremer and Fuji II LC glass
ionomer cements might be due to differences in matu-
ration of setting reactions.
Although the results obtained from this study may
not be directly extrapolated to the clinical situation,
they provide some information regarding the perfor-
mance of the restorative system evaluated. Independent
long-term clinical data are still required.