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154 rev port estomatol med dent cir maxilofac. 2021;62(3):150-156

mented with zinc phosphate.The mean values of fracture re-
sistance reported were higher than those in the present study.
The differences in composition between the composite resin
™
studied by Attia et al. and Cerasmart270 affect the modulus
22
of elasticity and flexural strength of these materials, which are

properties that influence their fracture resistance. It should
also be noted that Attia et al. used dentin as a substrate as
opposed to a cobalt -chrome alloy.
16
Wheyrauch et al. analyzed the influence of the type of
cement on the fracture resistance of full -contour crowns
milled from CAD/CAM blocks of various ceramics and a nano-
-hybrid indirect composite resin. Several types of cement were
used, and they found no correlation between the two variables.
Differences between the materials’ compositions may be re-
Figure 5. Bar chart for fracture resistance mean values sponsible for the incompatibility of their results with those of
23
and standard deviation, in Newtons. the present study. Nakamura et al. also studied the influence
of various types of cement on the fracture resistance of full-
-contour zirconia crowns and found no significant differences

tically significant differences were observed between the two between groups.This result may be due to the high strength
groups (p<0.001), as group 1 registered a mean value higher and hardness inherent to that material. 24
than group 2, with a difference of 866.4 N, which translates There are limitations to this in vitro study. Instead of pre-
into a 207.3% increase in the fracture resistance. pared natural teeth, a single cobalt -chrome alloy die was used
All crowns failed and were not repairable. All fractures for the cementation of all 20 crowns tested. The sandblasting
were confined to the crowns, and none progressed to the die. can be speculated to have caused dimensional alterations in
Group 1 crowns fractured at the crown -die interface, with the die, increasing the cement gap for the next cementations
crown fragments completely separating from the die. Group 2 and, therefore, influencing the fracture resistance and reduc-
crowns failed by chipping without separation of the crown ing standardization. Considering a potential actual increase of
material from the die. cement gap, recent studies using CAD/CAM milled crowns
from both conventional ceramics and resin nanoceramics
showed that different cement thicknesses had no significant
Discussion effect on fracture resistance. 25,26 Furthermore, studies with
similar methodology also had standard deviations similar to
A higher mean value and greater standard deviation were ob- the ones of the present study. 15,16,20,23 Thus, there is no clear
tained in group 1. Cementation with self -adhesive resin ce- evidence that the standardization was reduced.
ment showed significantly higher results of fracture resist- Nevertheless, the sandblasting caused micro -mechanical
ance than cementation with RMGI. Thus, the null hypothesis retentions on the die’s surface. In addition, the moduli of elas-
was rejected. The mode of failure also differed between the ticity of cobalt -chrome and dentin are different, which may
two groups. have led to results different than expected. Thus, it would be
The mean values of fracture resistance were 1284.3±340.19 pertinent to use dentin as a substrate and one die per cement-
N for group 1 and 417.9±106.35 N for group 2. Ferrario et al. ed crown.
noted that the average maximum bite force in premolars was Another possible limitation is the use of putty to stabilize
21
206.01 N in women and 291.36 N in men. The present study the crown and die set. Being a flexible material, it may have
obtained a minimum fracture resistance value of 285N, corre- allowed some movement even though the crown and die set
sponding to a crown cemented with RMGI. Therefore, almost was firmly fixed to the test machine. However, the paper chart
every crown withstood higher loads than the average bite forc- from the software of the testing machine, in all specimens,
es in the premolar area. demonstrated a linear curve until fracture occurred. Therefore,
Notwithstanding the lack of studies regarding the influence there is no indication that the putty elastomer impacted the
of the type of cement on the fracture resistance of crowns applied force or influenced the results obtained, but less flex-
made from the resin nanoceramic featured in this study, it is ible materials, such as gypsum or epoxy resin, could have been
possible to verify concordant results in similar papers. 14,15 Lim a more suitable alternative.
and Lee’s study demonstrated that a self -adhesive cement The crowns cemented with self -adhesive resin cement
14
increased the fracture resistance of an indirect composite res- were tested immediately after light -curing. Thus, there was no
in by 29%, compared to zinc phosphate. However, they regis- post -curing time, which is described as increasing resin ce-

tered mean values significantly lower than those of the present ment’s properties, namely surface hardness. 27-29
study, presumably due to using rectangular blocks as speci- Another limitation of this study is the lack of cyclic and/or
mens instead of crowns with adequate marginal adaptation. thermomechanical load since restorations in the clinical en-
15
Attia et al. demonstrated that full -contour crowns milled vironment fracture mainly due to fatigue. 30-33 Given the afore-
from indirect composite resin CAD/CAM blocks and cemented mentioned limitations, the direct transfer of results to in vivo
with RMGI showed superior fracture resistance than those ce- conditions is limited.