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160 rev port estomatol med dent cir maxilofac. 2019;60(4):155-162

yl methacrylate (MMA) as the monomer. This similarity allows
a smooth diffusion of the reline monomers into the denture
base resin and forms a complex and strong network in the
interface. On the other hand, the lowest mean values of mi-
crotensile bond strength were obtained in both the control
and the 2.5%-CHX loaded Kooliner. This result may be due to
the composition of its monomer isobutyl methacrylate, a
high-molecular-weight monomer that makes the dissolution
of the PMMA in the denture base resin surface difficult and
leads to a less effective penetration of the reline resin into the
24
denture base. Similar results were found in previous studies
when the three resins were submitted to thermal aging con-
ditions. 27-30
Loading the three reline acrylic resins with CHX did not
change their total surface energy after chemical aging. So, the
second hypothesis is not rejected. Similar results were found
Figure 4. Boxplot of microtensile bond strength (MPa) in a previous study where the same percentage of CHX incor-
distribution among experimental groups after chemical poration showed a null effect on total surface energy after
aging [Kooliner – 0% vs. 2.5% (p=0.818); Ufi Gel Hard – 0% thermal aging. However, it seems that thermal aging in water
34
vs. 5% (p=0.310) and Probase Cold – 0% vs. 5% (p=0.004)]
leads to lower total surface free energy than chemical aging in
saliva (the method used in the present study), probably be-
cause of the different aging environments. 34,35
difference (p=0.144) was found between Kooliner and Ufi Gel No differences were found in the microtensile bond
Hard groups. strength values between control and experimental groups of
Neither Kooliner (p=0.818) nor Ufi Gel Hard (p=0.310) bond Kooliner and Ufi Gel Hard. Probase Cold was the only resin
strength to the denture base was affected by CHX incorpora- whose microtensile bond strength decreased, since the 5%-
tion. However, loading Probase Cold with 5% of CHX resulted in CHX group had lower values than the control. This indirect
lower (p=0.004) µTBS values than the control group (Figure 4). acrylic resin is composed of pre-polymerized poly(methyl
The failure mode was predominantly adhesive (79.4%) and methacrylate) particles, and it is polymerized with an indirect
was not affected by CHX incorporation (Kooliner, p=1.000; Ufi method, under high temperature and pressure. Those results
Gel Hard, p=0.125; Probase Cold, p=0.549). may be explained by the incorporation of CHX within the poly-
mer matrix of the material, which probably led to higher dis-
tances between the molecules of the polymer net and less
Discussion homogeneity in their structural arrangement, therefore weak-
16
ening the bond strength. Nevertheless, Probase Cold speci-
Exposing reline acrylic resins to oral pH fluctuations may mens loaded with 5% CHX presented higher values of μTBS
affect their physical and biomechanical properties. 40,45,46 than any of the other materials studied. Thus, the third null
Also, the maximum cumulative release of CHX reaches hypothesis is rejected.
higher levels at pH=3 than at pH=7. 31,47 However, although Regarding the failure modes, the most observed failure in
all specimens in the present study were immersed in artifi- Kooliner was adhesive in both groups, justified by the distinct
cial saliva with a cyclic procedure of 6 hours at pH=3 inter- chemical composition of this reline resin and the denture base
changing with 18 hours at pH=7 for 28 days, the incorpora- polymer, and proved by the lower μTBS values observed in the
tion of CHX in the reline acrylic resins studied only affected present study. Concerning Ufi Gel Hard, the most frequent fail-
the microtensile bond strength of the Probase Cold to the ure mode was cohesive, probably caused by an increase of the
denture base. bond strength promoted by the specific adhesive used before
Since differences were found between the surface free the relining procedure. Also, the cohesive failures on this res-
energy and microtensile bond strength of the three reline in were higher in the experimental group than in the control
acrylic resins, the first null hypothesis is rejected. These dif- group. Thus, the presence of CHX may weaken the internal
ferences may result from the distinct chemical composition structure of the Ufi Gel Hard and promote failures in the poly-
and structural arrangement of the three reline acrylic res- mer instead of in the bond interface. Both control and exper-
ins. 23,24 Kooliner had lower total surface free energy values imental groups of Probase Cold showed a predominance of
than Ufi Gel Hard and Probase Cold, probably because of its adhesive failures. The robust and complex network formed by
surface’s porous structure. Air voids are entrapped when mix- PMMA molecules and their strong internal structure can ex-
ing the powder and liquid components as a consequence of a plain the absence of cohesive failures on Probase Cold. 50,51
rapid polymerization reaction. 48,49 Regarding microtensile Although microhardness and flexural strength after chem-
bond strength, Probase Cold presented higher values than the ical aging of chlorhexidine delivery systems have already been
52
other tested resins, probably because its chemical composi- studied, other properties should be evaluated, and more lab-
tion is similar to the denture base resin: they are both based oratory studies that mimic the oral environment and clinical
on polymethylmethacrylate (PMMA) polymer and have meth- trials are needed.