REINFORCEMENT OF POLYMETHYLMETHACRYLATE (PMMA) RESIN WITH PERFORATED METAL PLATES
Figure 1. Dentaurum Strengthening Grid
causes of fracture appearing to be poor fit and
lack of balanced occlusion.8 In another survey on
the prevalence of types of fractures, published
in 1994, it was reported that 29% were repairs
to mid-line fractures most commonly seen in the
upper denture.9
It was also reported that 63% of dentures had
broken within 3 years of their insertion.10 Complete
dentures often fracture during normal masticatory
function, despite the fact, that an edentulous
patient can only exert occlusal forces of 15 to 25%
that of dentate patients.11
Maxillary dentures are subject to bending
deformation, with tensile stresses occurring at the
labial and lingual aspect of the incisors.4 A midline
fracture of single maxillary complete denture base,
especially in patients who have retained their
natural mandibular teeth, is at times an inevitable
problem.9
Therefore, over the years, various approaches to
strengthening acrylic resin have been suggested.
Basically these efforts can be divided into two ways.
The first one is to modify the resin composition in
order to become tougher (high impact resins). The
other way is to incorporate strengthening scaffold
made out of metals (mesh, wires, cast plates ore
frameworks) or fibers (glass-, carbon-, polyamide,
or aramid fibers).12
Such incorporated scaffolds were not really
increasing the strength of the dentures. A study by
Smith13 reported that the addition of glass fibers
did not provide substantial improvement to the
tensile strength.
Untreated fibers act as inclusion bodies in the
acrylic resin mixture and instead of strengthening,
actually weaken the resin.4 Furthermore, fibers are
difficult to place, create polishing problems, if they
surface and may be an aesthetic problem due to
their color. Metal frameworks, meshes, wires or
cast plates have the same effect as untreated fibers.
Figure 2. Wax/grit strengthener plate in injection
molding flask with sprues.
Arrows show reposition areas without wax
covering the grid
Studies investigating reinforcement with mesh and
a braided wire plate did not report a significant
improvement in the transverse strength of acrylic.
However, incorporating silanated glass fibers into
acrylic resin improved the fracture strength. Vallittu
et al 199414 found a linear relationship for the
increase. The more fibers were incorporated the
greater the increase in strength.
Metal reinforcement could fail at the resin/
strengthener interface since areas of stress
concentration occur around embedded materials.
Various approaches have been used to improve
the adhesion between the metal surface and
acrylic resin such as sandblasting, silanization
and metal adhesive resins. The effect of the metal
strengthener’s surface roughness on the fracture
resistance of the acrylic denture base material was
investigated by Vallittu (1992).15 The investigation
showed that the surface roughening of the metal
wires used to reinforce the acrylic resin denture
base material increased the fracture resistance of
the test specimens. The best results were achieved
by sandblasting.15 In Europe, preformed perforated
gold plated steel plates (Grid strengthener,
Dentaurum GmbH &Co, KG, Ispringen, Germany)
(Fig.1) are sold as strengthener for full dentures,
which simplify the incorporation into dentures.
However the manufacturer does not give any
instructions how to treat the surface before being
incorporated into the PMMA resin.
Based on literature data, one would expect a
strengthening effect, if such plates were bonded to
the resin. Therefore the purpose of the study was
to test the strengthening effect of the perforated
metal plates as a function of the bonding strategy.
The null hypotheses were as follows:
1. The metal grid strengthener does not strengthen
acrylic resin.
2. The different surface treatments do not affect
strengthening.
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