DENTAL MATERIALS
likely cause drift of the grid during processing.
Previous studies have shown that fibers or mesh
without surface modification would not render
the denture base any strengthening effect. Some
studies even showed negative effect. Substantial
improvement of strengthening was observed
only when the surface of the strengthener fiber or
mesh was gritblasted, coating of bonding agent or
combination of the two.17
The results of this study showed basically that
the metal grid alone had a statistically significant
strengthening effect in flexural strength and
modulus.
However, the improvement of flexural strength
from the grids receiving gritblasting and coating
with bonding agent (Group 4) is not statistically
significantly from the control (Group 1).
Contrary to previous studies, the results show that
there is no statistical difference among the groups
comprising strengtheners. In other words, grit
blasting and coating of bonding agent provided
no additional benefit in enhancing flexural strength
and modulus.
Grit blasting is a well-recognized method of surface
roughening that leads to bonding enhancement.
In addition, the bonding agent used in the study
is an ethanol solution known to consist of three
distinct compounds; namely, silane methacrylate,
phosphoric acid methacrylate and disulphide
methacrylate.16
They are designed to bond to three different
surfaces: silane targeting silica (not present in the
test groups), a phosphate containing methacrylate
focusing on metal oxides (in this case chromium
oxides) and a disulfide methacrylate aiming for
gold.
The cause of inability of the bonding agent and grit
blasting to enhance flexure strength and modulus
should be clarified.
When glass fibers in the form of woven, loose and
continuous fiber bundles were first investigated for
reinforcing PMMA denture base resin, they were
either mixed discretely with the PMMA dough or
by lamination with glass cloth.18
The addition of fibers did not give a substantial
improvement to the tensile strength caused by the
lacking of adhesion between the polymer matrix
and the untreated fibers.
These fibers acted as inclusion bodies in the
PMMA resin mixture that actually weakened the
resin instead of strengthening.
Treating glass fiber with silane compounds before
incorporating in the PMMA dough presented a
potential of strengthening PMMA.19
If the fibers were made into cloth form instead
of loose form, untreated cloth form fibers had
104
demonstrated strengthening effect whereas
untreated loose glass fibers had a weakening
effect.17
Using metallic inserts in reinforcing PMMA
denture base resin yielded similar results, where
embedding metal fibers weakened the polymer
due to poor adhesion between the acrylic resin
matrix and the metal fiber inserts.17
Gritblasting17, silanization of the metallic insert20,
and metal bonding adhesive resin21 have been
reported to improve the adhesion between the
metal surface and acrylic resin.
It is of interest to observe that untreated metal grid
strengthened the acrylic resin but no additional
improvement was realized when the metal grid
had been gritblasted or coated with bonding
adhesive for metal surface.
This unexpected result could only be caused by
the design of the grid.
Metal grid used in this study is a perforated
stainless steel with staggered pattern of ø 2.5mm
holes and 1.25 mm between adjacent perforations.
When this grid is incorporated during the
processing described earlier, the resin fills the
perforated spaces (Fig. 2).
The result is an interpenetrating structure between
the metal grid and denture resin that interlocks the
resin and grid mechanically.
Therefore, as the resin component is being
stressed during testing, the metallic component
will be subjected to the same stress simultaneously.
In other words, metal grid strengthener exhibits
strengthening effect without the need of bonding
between the metal grid and the resin.
Although the study shows that resultant bonding
from gritblasting and adhesive coating would not
improve the flexure strength and modulus of the
denture base resin, microleakage can still occur
from lack of bonding or poor bonding between the
denture base resin and the cast metal framework
of a removable partial denture.
It can cause discoloration and staining of the
margins at the metal–resin interface over time.22
Therefore, the need of a durable bonding between
the metal grid and the denture resin remains
critical with respect to staining over time.
Should that type of bond be achieved by
gritblasting, adhesive resin or combination of the
two should be investigated further by additional
in vitro studies with specimens after a long-term
storage in appropriate solutions or through longterm clinical observation.
The presence of a metal grid within a denture
base resin qualifies the specimen design in this
study as a composite structure and the mechanical
property of the final structure will depend on those
of the metal grid and the resin.
STOMA.EDUJ (2015) 2 (2)