ARTÍCULOS ORIGINALES
Effect of surface treatments on the bond strength of different resin teeth to complete denture base material
Carolina B. Meloto1, Laís R. Silva-Concílio2, Renata C.M. Rodrigues-Garcia1, Giancarlo T. Canales1, Célia M. Rizzatti-Barbosa1
1 Department of Prosthesis and Periodontology, Piracicaba Dental School, University of Campinas, Piracicaba, Sao Paulo, Brazil.
2 Department of Prosthodontics, Dental School, University of Taubaté, Taubaté, São Paulo, Brazil.
CORRESPONDENCE Lais Regiane da Silva Concilio Rua: Expedicionario Ernesto Pereira, 110 12020 270, Taubate, SP, Brazil email: regiane1@yahoo.com
ABSTRACT
Although different commercial brands of artificial teeth are available in the market, debonding from the denture base is still an issue when rehabilitating edentulous patients with conventional or implant-supported complete dentures. The purpose of this study was to investigate the effect of surface treatments on the bond strength of four artificial teeth brands to a denture base material polymerized by microwave energy. Forty anterior artificial teeth of each brand (BioluxR, TriluxR, Biotone IPNR, and Vipi Dent PlusR) were bonded to denture base material (VipiWaveR). Before processing, groups of ten specimens of each brand received surface treatment: control, monomer application (MA), air abrasion (AA) or diatoric cavity (DC). After processing, a blinded examiner conducted the bond test by applying load to the specimens (0.5 mm/min, to 45°). Data were analyzed by one-way ANOVA followed by Tukey's test (α=0.05). BioluxR teeth have stronger bonding to denture base than TriluxR (p<0.05) in control group; higher bond values than Biotone IPNR (p<0.05) in MA group; and higher bond strength than Vipi Dent PlusR and TriluxR (p<0.01) in DC group; AA had no differential effect for any of the brands. With regard to the effect of the surface treatments on bond strength within groups of commercial brand, statistical analysis revealed no difference among them. According to results in general, BioluxR teeth had the strongest bonding to the denture base material polymerized by microwave energy. Results may assist dentists in selecting denture teeth from the standpoint of shear bond strength.
Key words: Artificial teeth; Acrylic resins; Surface properties.
Influência do tratamento de superfície em dentes artificiais na resistência de união em bases de próteses totais confeccionadas em resinas acrilicas termopolimerizáveis por energia de microondas
RESUMO
Diferentes marcas comerciais e tipos de dentes artificiais estao disponiveis no mercado, entretanto o descolamento do dente artificial a base da protese ainda e um problema em reabilitacoes com proteses totais convencionais e implanto-suportada. O objetivo deste estudo foi verificar o efeito de diferentes tratamentos de superficie em quatro marcas de dentes artificiais e sua influencia na resistencia de uniao a base de protese confeccionada em resina acrilica termopolimerizavel por energia de microondas. Foram utilizados quarenta dentes artificiais anteriores de quatro marcas comerciais (BioluxR, TriluxR, Biotone IPNR e Vipi Dent PlusR) distribuidos aleatoriamente em 4 grupos (n=10) de acordo com o tratamento de superficie utilizado: controle (sem tratamento), aplicacao de monomero (MA), abrasao a ar (AA) ou cavidade com broca (DC). Apos os tratamentos de superficie as amostras receberam procedimentos de inclusao e polimerizacao usualmente utilizados para resinas acrilicas ativadas por energia de microondas (VipiWaveR). Apos o processamento, foi realizado o ensaio de resistencia de uniao (microcisalhamento), em equipamentos para ensaios universais. Os resultados foram analisados estatisticamente (ANOVA e Tukey, α=0,05). Os resultados evidenciaram que os dentes Biolux R apresentam forte resistencia de uniao a base da protese de TriluxR (p <0,05) no grupo controle; no grupo MA o maiores valores foi da marca Biotone IPNR (p <0,05); no grupo DC a maior resistencia de uniao foram das marcas Vipi Dent PlusR e TriluxR (p <0,01); O grupo AA nao apresentou influencia para nenhuma marca de dente avaliada. Com base nos resultados obtidos, e possivel concluir que nao houve diferenca estatistica nos valores de resistencia de uniao nas marcas avaliadas e que os tratamentos de superficie utilizados nao aumentaram os valores de uniao para as marcas comerciais utilizadas.
Palavras-chave: Dente artificial; Resina acrilica; Propriedades de superficie.
INTRODUCTION
Although complete dentures cannot be considered a
substitute for natural teeth, they have been, and
remain, the staple treatment for edentulous patients,1 and the use of complete dentures by edentulous
patients is projected to increase over the next two
decades because of an increase in life expectancy2.
When conventional treatment is inadequate,
mandibular two-implant overdentures offer a positive
alternative3.
Nonetheless, although these edentulous patients can
be effectively rehabilitated with either conventional
complete dentures or implant-supported complete
dentures, debonding of acrylic teeth from the
denture base, usually anterior teeth, continues to be
a problem4.
This detachment accounts for approximately 33% of
complete dentures failures, and failure of acrylic denture
tooth adherence has also been reported in implantsupported
dentures5. Consequently, in the future,
dentists will continue to face such situations frequently
in their clinical practice, for both conventional- and
implant-supported complete denture wearers.
Most attempts to improve the bond strength of denture
teeth to the acrylic resin denture base involve
chemical treatment or mechanical modification on
the ridge lap surface of the denture tooth. Conflicting
results have been reported with the use of
monomer, air abrasion and the placement of a diatoric
cavity.6 Such contradictory results may be due
to differences not only in methodology used and
composition of the denture base acrylic resin, but
also to differences inherent to manufacturing of the
artificial teeth,7 i.e., different commercial brands.
If these features were carefully evaluated, complete
denture wearers would benefit by avoiding the need
for repeated repairs or corrections of their dentures.
Based on the fact that polymerization of acrylic
resin using microwave energy is a cleaner and timesaving
method8 that has so far been shown to preserve
the physical properties of conventional
denture base resins,9 the aim of this study was to
investigate the effect of different surface treatments
on the shear bond strength of four acrylic resin artificial
teeth brands in complete dentures polymerized
by microwave energy. The null hypothesis was
that neither the surface treatments nor the different
brands of artificial teeth would alter the shear bond
strength between the acrylic resin polymerized by
microwave energy and the artificial teeth.
MATERIAL AND METHODS
Sample preparation
Four different brands of artificial teeth were evaluated
(Table 1). For each brand, 40 left lateral incisors
were prepared according to the method previously
described by Cunningham & Benington10.
Table 1: Artificial teeth used in this study.
Patterns were produced using one artificial tooth from each brand (Fig. 1a), each bonded to a brass collar containing a 5mm diameter reamed hole. A pre-formed brass tensile specimen was inserted into the tooth-collar assembly (Fig. 1b) and the whole set had one half sequentially invested in heavy body polyvinylsiloxane elastomeric impression material in a self-cured acrylic resin tray. Four trays, one for each of the artificial teeth brands tested, were invested in type III stone in the polyvinylchloride flask (PVC-F) base, so that their top surfaces were flush with the stone surface. The upper part of the PVC-F was adapted, filled with type III stone, and taken to a hydraulic press until the stone set (Fig. 1c).
Fig. 1: Schematic representation of sample preparation. a) artificial tooth; b) inserted in an elastomer; c) invested in type III stone
in the polyvinylchloride flask (PVC-F) base.
After that, the PVC-F was opened, patterns were
removed and four new artificial teeth, one of each
brand, were randomly selected and placed in the
sockets of the rubber moulds. All four artificial teeth
received one of the four surface treatments: control,
in which artificial teeth were left untreated; monomer application (MA), where a drop of acrylic
resin monomer was applied with small brush on the
tooth surfaces; air abrasion (AA) with aluminum
oxide with 4.9 kgf/cm2 air pressure at 1 cm distance
for 10 seconds11; and diatoric cavity (DC), which
was prepared before placing teeth in the rubber
moulds, by placing them previously in a custom
support made of polyvinylsiloxane putty material
attached to a milling machine (1000N, Bio-Art, Sao
Carlos, SP, Brazil) loaded with a #8 round bur (KG
Sorensen), in order to provide cavities of standardized
size (2 mm depth X 2.3 mm diameter) and position.
All surface treatment groups contained ten
artificial teeth from each brand.
Heat-cured acrylic resin (Vipi Wave, Dental Vipi,
Pirassununga, SP, Brazil) was prepared following the
manufacturer's instructions and packed into the cylin-PMMA, polymethyl methacrylate; EDMA, dimethacrylate of polymerized
ethylene glycol; CL, cross-link; IPN, interpenetrating polymer networks;
DCL, double cross-link
drical cavities of the mould. PVC-F was closed, placed
in the hydraulic press (0.5 ton) for 5 minutes, and reopened
in order to remove excess material. PVC-F
was finally closed, and after pressing with 1.25 ton in
hydraulic press for 15 minutes, it was screwed together
and taken to the microwave oven (AW-42, BSH
Continental Ltd, Sao Paulo, SP, Brazil) for polymerization
according to the manufacturer's instructions.
After flask cooling at room temperature, the specimens
were deflasked and finished with sandpaper
discs. Any inaccuracy in denture base material near
the teeth was removed with sandpaper discs.
Bond strength test and statistical analysis
To measure bond strength between artificial teeth and
the denture base material, a blinded examiner fixed
the specimens to a 45° angled metal split support,
attached to the test machine (Instron 4444; Instron
Corp., Canton, MA, USA). Load was applied on the
incisal line by means of a cylindrical pin with a
crosshead speed of 0.5 mm/min, until fracture. The
recorded ultimate failure load in Newtons (N) was
converted into kgf. Data were analyzed by one-way
ANOVA followed by Tukey's test (α=0.05).
RESULTS
When no surface treatment was applied on the artificial teeth (control group), BioluxR had higher bond strength values than TriluxR (p<0.05), while Biotone IPNR and Vipi Dent PlusR were not statistically different from either BioluxR or Biotone IPNR. Application of monomer (MA group) on the ridge lap area of the BioluxR teeth resulted in stronger artificial teeth-acrylic resin base bonding than in Biotone IPNR (p<0.05), and Vipi Dent Plus R and TriluxR artificial teeth did not differ from either of them. With regard to artificial teeth that received air abrasion (AA group), there was no significant difference among the four brands. Finally, the placement of a diatoric cavity on the cervical base of the artificial teeth (DC group) revealed that bond strength of BioluxR teeth is higher than that of artificial teeth of both Vipi Dent PlusR and TriluxR (p<0.01); Biotone IPNR values were not statistically different for any of the three brands (Table 2).
Table 2: Shear bond strength mean values (kgf) and (standard deviations-SD) of artificial teeth submitted to different surface treatments.
Vertically, identical lowercaseletters denote no significant differences among groups. Horizontally, identical uppercase letters denote no significant
differences among surface treatment groups. *p<0.05; †p<0.01; (ANOVA and Tukey test)
In an attempt to identify which of the surface treatments would result in stronger artificial teethacrylic resin base bonding for each brand, shear bond strength values within commercial brand groups was also tested, and revealed no statistically significant difference for any of the brands. Even when all shear bond strength values of each surface treatment were grouped, regardless of commercial brand, no surface treatment proved to be statistically better than any other (data not shown).
DISCUSSION
Given the demographic data on population ageing,
the need to rehabilitate edentulous patients will
remain considerable for many more decades. Thus,
providing patients with complete dentures, whether
conventional or implant-supported, will continue to
be an important prosthodontic task1.
Commercially, vast numbers of artificial teeth are
available for prosthesis construction. However, there
is little or no mention of bond strength of acrylic teeth
to denture base resin by the manufacturers and studies
evaluating the shear bond strength of different
brands of artificial teeth attached to a microwavepolymerized
denture base material are rare5,12.
To the best of our knowledge, this is the first study
investigating the effect of different surface treatments
on shear bond strength of the commercial
brands BioluxR, TriluxR, Biotone IPNR and Vipi Dent
PlusR to an acrylic resin denture base material polymerized
by microwave energy. Here we have found
that, except when the pre-processing treatment was
air abrasion with aluminum oxide particles, BioluxR
teeth have stronger bonding to the acrylic resin base,
whereas the surface treatments applied in this study
had no effect on the shear bond strength of any of the
brands of artificial teeth tested.
In order to explain the higher bonding values of
BioluxR teeth, the chemical composition of the
acrylic resins must considered. Resin denture teeth
are primarily composed of PMMA and have been
increasingly modified to improve their physical
properties by using cross-link agents, different
monomers and the addition of fillers13. Cross-linking
agents are generally used to improve strength
and crazing resistance, whilst lower bond strength
values have been found than in conventional acrylic
resin teeth, therefore, the ridge lap portion of the
teeth is expected to be the least cross-linked, so as
to facilitate bonding to the denture base resin14.
Interpenetrating polymer network (IPN) resins,
formed when a polymer network is crossed inside
another network occupied by a second polymer, is
a more recent resin modification aiming to produce
denture teeth with better mechanical and physical
properties15.
Furthermore, the bonding of artificial tooth resin to
denture base acrylic resin has been related to the
ability of monomer to diffuse into the tooth resin,
observed by the presence of swelling. The degree
of swelling is related to the degree of cross-linking
of a polymer. If a polymer is highly cross-linked, it
has difficulty swelling in organic solvent16.
Therefore, it is reasonable to understand that less
cross-linked artificial teeth (BioluxR) have stronger
bonding to the acrylic resin denture base than highly
cross-linked teeth (TriluxR and Biotone IPNR). It
is worth mentioning that Vipi Dent PlusR, which
also has a lower degree of cross-linking agents, did
not differ from BioluxR, regardless of the pre-processing
surface treatment. Our results match those
of previous studies, which also found that the more
cross-link, the lower the bond strength between artificial
teeth and the denture base13,17.
Although incorporation of cross-linking agents to
the PMMA matrix of artificial teeth is advisable in
order to improve properties such as wear resistance15,18,
hardness18 and flexural strength19, which
help increase prostheses longevity, bonding strength
is significantly affected by the highly condensed
matrix obtained, due to hampered diffusion of
monomer through the artificial tooth/denture base
resin interface12.
With regard to the pre-processing surface treatments
tested, the null hypothesis was accepted, meaning
that none of them altered the shear bond strength
between the artificial teeth and the acrylic resin denture
base polymerized by microwave energy.
Application of monomer before packing the resin
has been found to increase bond strength in a number
of studies,6,20-22 Although our results do not
agree with these findings, it can be pointed that the
lack of standardization of the waiting time before
packing the resin in the groups that were submitted
to monomer application is the strongest limitation
of our study and is possibly responsible for this
finding. A recent study has demonstrated that, in
general, 60 seconds of monomer treatment resulted
in higher bond strength than the 180-second treatment,
probably due to higher monomer evaporation
in the latter group, so that less MMA was available
to react with the denture base resin.6 Therefore, our
lack of time-before-packing control can be considered
to have affected the results.
Air abrasion is supposed to increase shear bond
strength either by augmenting free surface energy of
the newly abraded resin surface or by causing severe
irregularities and undercuts in the ridge lap surface
area, improving micromechanical retention.5 Our
results agree with others that did not find air abrasion
with aluminum oxide particles to improve bond
strength21,23-25 and we propose that elevated free surface
energy and the presence of micromechanical
irregularities may difficult wettability of the ridge lap
surface area, hampering adequate polymerization in
the artificial teeth/acrylic resin denture base interface.
Placement of a diatoric cavity in the cervical base
of artificial teeth is another ridge lap modification
that has been proposed, but we did not find it to
improve shear bond strength.
A recent study using
a scanning electron microscope has found acrylic
resin surface to be rough inside the cavity and the
worst bond strength values in this group.26 Presumably,
wettability of the ridge lap surface is also
impaired by the presence of the diatoric cavity,
especially if we consider the sharpness of the cavity
borders, which can help explain why this modification
does not improve shear bond strength.
Although there are conflicting results in the matter
of the effect of different surface treatments on the
shear bond strength, they can be attributed to the
large variety of experimental designs, measuring
instruments and bond-testing methods used in the
investigations.
Still, manufacturers frequently launch denture teeth
made of new materials on the market, which are
advertised as products with improved mechanical
properties. The results of our study may assist dentists
in selecting PMMA denture teeth from the
standpoint of shear bond strength.
CONCLUSION
Within its limitations, this in vitro study permits to conclude that there was no difference in shear bond strength values of the brands evaluated and pre-processing treatment used, there is no improvement of the bond strength within groups of commercial brands involved.
ACKNOWLEDGMENT
The authors acknowledge Dental Vipi, Pirassununga, SP, Brazil, for providing the artificial teeth used in this experiment.
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