UNIVERSIDADE ESTADUAL PAULISTA
“JÚLIO DE MESQUITA FILHO”
Instituto de Ciência e Tecnologia
Campus de São José dos Campos
ORIGINAL ARTICLE DOI: https://doi.org/10.4322/bds.2023.e3770
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Braz Dent Sci 2023 July/Sept;26 (3): e3770
The influence of the retention feature in artificial teeth on its
attachment to the thermoplastic resin denture base
Influência da forma de retenção em dentes artificiais de resina acrílica na fixação na base de prótese termoplástica
Paulo Roberto Vieira MARTINS1 , Marcio Katsuyoshi MUKAI2 , Bruno COSTA2 , Carolina Mayumi IEGAMI2 ,
Roberto Chaib STEGUN2
1 - Universidade de São Paulo, Faculdade de Odontologia, Departamento de Cirurgia, Prótese e Traumatologia Maxilofaciais, São Paulo,
SP, Brazil.
2 - Universidade de São Paulo, Faculdade de Odontologia, Departamento de Prótese, São Paulo, SP, Brazil.
How to cite: Martins PRV, Mukai MK, Costa B, Iegami CM, Stegun RC. The inuence of the retention feature in articial teeth on its
attachment to the thermoplastic resin denture base. Braz Dent Sci. 2023;26(3):e3770. https://doi.org/10.4322/bds.2023.e3770
ABSTRACT
Objective: Teeth play a crucial role in masticatory efciency and esthetic harmony making rehabilitation of
partially edentulous patients a challenge because of the limitations of conventional removable partial dentures.
As a therapeutic alternative, thermoplastic polymers are used in current dental practice either for practical
processing purposes or aesthetics. However, it is recognized that the bond between acrylic resin articial teeth
and thermoplastic polymers has no chemical interaction, and depends on retentive features added to the teeth.
This study analyzed the efcacy of two retentive forms of features through compressive strength test and as a
secondary outcome, fracture and displacement resistance test. Material and Methods: Three groups of samples
each with 14 sets of acrylic teeth were compared when two retentive features, single hole and groove retention,
and a control group with teeth without any specic form. The experimental unit consisted of six maxillary anterior
teeth, positioned in a linear pattern into a polypropylene block, where each tooth was submitted to a static load
until its removal, on a universal testing machine. Results: Those specimens with a groove retention presented larger
resistance to displacement when compared to the other groups (p <0.05). This was emphasized by the fracture of
the tooth tested, with part of the cervical portion remaining embed in the base, and not its displacement as with
(or without) the retentive feature. Conclusion: In this study, the type of retention inuenced signicantly to a
better retention considering teeth and thermoplastic polymer base. The data in this study indicates that a groove
placed on the articial tooth offers signicantly better retention efcacy to the point where the displacement
was only possible after its fracture.
KEYWORDS
Articial teeth; Acrylic resins; Denture base; Polypropylene; Retention.
RESUMO
Objetivo: Os dentes são um fator essencial na eciência mastigatória. A harmonia estética e a reabilitação dos
indivíduos parcialmente dentados tornam-se um desao, frente às limitações das próteses parciais removíveis
convencionais. Como uma alternativa terapêutica, os polímeros termoplásticos são utilizados na odontologia
contemporânea, tanto para o seu processamento prático como seu apelo estético. Entretanto, pouco se sabe a
respeito da união entre a resina acrílica de dentes articiais e resinas termoplásticas, assim como a inuência dos
mecanismos de retenção, uma vez que não há interação química entre eles. O objetivo deste estudo foi analisar
duas formas de retenção de dentes de resina acrílica a bases de prótese de material termoplástico, por força
compressiva e como desfecho secundário, teste de resistência a fratura e deslocamento dos dentes articiais.
Material e Métodos: O presente estudo analisou dois tipos diferentes de retenções: uma cavidade com único
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Braz Dent Sci 2023 July/Sept;26 (3): e3770
Martins PRV et al.
The influence of the retention feature in artificial teeth on its attachment to the thermoplastic resin denture base
Martins PRV et al. The influence of the retention feature in artificial teeth on its
attachment to the thermoplastic resin denture base
INTRODUCTION
The main goal of oral rehabilitation of
edentulous subjects is to achieve masticatory
function. However, aesthetics is also a matter
to be valued, especially now when patients
expect to have bright, white teeth in their
mouths [1]. The sight of metal clasps of the
removable partial dentures (RPDs) in the
patient’s smile will be disappointing to him
both cosmetically and psychologically [2].
An alternative removable partial denture has
been developed that avoid the use of conspicuous
clasps [3] by employing thermoplastic polymers
(TR) and injection molding which eliminate
the need for metal retainers [2]. Polyamide,
polyester, polycarbonate and polypropylene are
examples of polymers which offer a more pleasing
result. Added advantages include easier handling
during insertion and removal of the dentures and
high fracture resistance of its base. However, it
has been alleged that these thermoplastic partial
dentures may adversely affect the periodontal
tissues of the abutment teeth and the residual
ridge [2]. But perhaps the most troublesome
problem with these materials is the frequently
observed detachment of the articial denture
teeth from the base [4-7]. While acrylic resin teeth
(ARAT) are chemically bonded to conventional
polymethyl methacrylate (PMMA) resins, articial
teeth have no chemical interaction with the
thermoplastic polymer denture and, as such, are
susceptible to dislodgement [3].
Surface treatments and mechanical retention
features have been applied to articial teeth in
order to overcome the detachment issue, though
little has been presented on the influence of
the retention mechanism characteristics on its
efcacy. The aim of this study was to analyze two
forms of retention of ARAT to TR denture bases
through a compressive strength test.
MATERIALS AND METHOD
A total of 42 sets of the six anterior maxillary
artificial teeth (Natusdent, VIPI Indústria
Comércio, Exportação e Importação de Produtos
Odontológicos Ltda, Brazil) were divided into
3 groups (Figure 1):
Group I (n=14) (GI): no retentive feature
applied.
Group II (n=14) (GII): a retention hole was
made with a 3.5 mm diameter round bur
(Spherical Carbide JET-PM # 8, Labordental,
Brazil).
Group III (n=14) (GIII): a groove retention
was made with a 3.5 mm-diameter round
bur (JET-PM # 8, Labordental, Brazil).
The experimental unit consisted of articial
teeth set up at a 45º angle to the tip of the
universal testing machine (VersaTest Mecmesin
Ltd, UK) on wax blocks formed with a RTV
silicone mold. This angle simulated the natural
relation of the load applied by the inferior
incisors [8-10].
This unit was transformed into the
experimental specimen of polypropylene
(Rocalflex, Brazil) processed by injection
molding, under controlled temperature and
pressure. The resulting samples were polished
with wet-and-dry sandpaper (T223-37/A type
320 Norton, Brazil) and the supporting surface
rectified to a plane in order to stabilize it on
the VersaTest platform without the need of
a fastening device [10]. The load force was
applied to the incisal surface of each tooth at
ponto e uma canaleta de mesial a distal; e como grupo controle, dentes colocados sem qualquer recurso especíco.
A unidade experimental consistiu de seis dentes superiores anteriores, posicionados em linha em um bloco de
polipropileno. Em cada dente articial foi aplicada força até a sua remoção, em uma máquina universal de ensaios.
Resultados: Os resultados mostraram que a retenção com canaleta apresentou maior força de deslocamento,
quando comparado com os outros grupos (p <0,05). Tal fato foi evidenciado pelo teste de fratura e deslocamento,
no qual parte da porção cervical do dente articial permaneceu unida à base, não ocorrendo o deslocamento com
(ou sem) a retenção. Conclusão: Neste estudo, o tipo de retenção inuenciou de forma signicante a retenção
de dentes articiais de resina acrílica a bases de prótese termoplásticas. Os dados deste estudo indicam que a
confecção da retenção em canaleta nos dentes articiais oferece signicante aumento na eciência da retenção,
ao ponto em que o deslocamento do dente ocorreu somente após sua fratura.
PALAVRAS-CHAVE
Base de dentadura; Dente articial; Polipropilenos; Retenção; Resinas acrílicas.
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Braz Dent Sci 2023 July/Sept;26 (3): e3770
Martins PRV et al.
The influence of the retention feature in artificial teeth on its attachment to the thermoplastic resin denture base
Martins PRV et al. The influence of the retention feature in artificial teeth on its
attachment to the thermoplastic resin denture base
0.05 mm/min and the necessary load to dislodge
the tooth (Figure 2) veried. A spot observation
was made to identify if the separation occurred
at the interface of the tooth and its socket or
thru the tooth itself, leaving part of its structure
embedded (Figure 3) [10].
Compressive strength tests were run at
random to avoid any sort of bias because of the
application order. Therefore, both retention type
and the test sequence were performed with the
blinding to the operator. Data was collected and
analyzed with BioEstat software (version 5.3,
Instituto Mamirauá; Tefé, Brazil).
RESULTS
The data was grouped by teeth instead
of being analyzed between left and right
specimens because no statistical difference could
be established among homologous. Lilliefors
and ANOVA tests were used in the statistical
analysis for the resulting displacement force,
complemented by the Tukey test (Table I).
When comparing all three types of articial
teeth, it was observed that the groove retention
(GIII) presented statistically signicantly greater
values than groups with no retentive feature (GI)
or a retention hole (GII) (p<0,05) (Figure 4).
For the incisors, however, while GIII consistently
showed a signicantly better performance, no
statistical difference was observed between GI
and GII (Figure 4).
When analyzing the types of treatment (no
retention, hole and groove retention), Tukey
test showed statistically signicant difference
(p<0,05) (Table II) for the analyzed groups.
Table I - Two -factor ANOVA and Tukey test. Comparison between types of treatment and groups of teeth (Canine, Lateral Incisors, Central
Incisors) (p<0.05)
Group Treatment Difference Q p-value (p)
GI- GII 416,667 52,399 < 0.01 *
Canine GI-GIII 128,375 161,442 < 0.01 *
GII-GIII 867,083 109,043 < 0.01 *
GI-GII 8,875 11,161 ns
Lateral GI-GIII 437,917 55,072 < 0.01 *
GII-GIII 349,167 43,911 < 0.05 *
GI-GII 134,583 16,925 ns
Central GI-GIII 874,583 109,986 < 0.01 *
GII-GIII 74,000 93,061 < 0.01 *
(Group I: none; Group II: retention hole; Group III: groove retention). * indicates results with significant statistic differences.
Figure 1 - Artificial teeth with the respective retentive feature: (a) Group I – none; (b) Group II – retention hole; (c) Group III – groove retention.
Figure 2 - Artificial teeth angled 45º in relation to the tip.
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Braz Dent Sci 2023 July/Sept;26 (3): e3770
Martins PRV et al.
The influence of the retention feature in artificial teeth on its attachment to the thermoplastic resin denture base
Martins PRV et al. The influence of the retention feature in artificial teeth on its
attachment to the thermoplastic resin denture base
Higher retention was observed in GIII (Figure 5).
It was also signicant to note that a tooth where
the retentive grove was placed practically
could not be removed from its socket and the
experimental cycle ended with its fracture at
the cervical level, leaving part of its structure
embedded in the base polymer (Figure 6).
DISCUSSION
One of the most common causes for partial
denture wearers to return to the dental ofce
is the tooth dislodgment of the articial tooth
from the denture base. There is little discussion
available regarding the thermoplastic polymer
vulnerability [3,11] when compared to studies
for acrylic resin teeth adhesion to its denture
base [6,9,10]. This has been theorized as a
consequence of the lack of residual monomer
in TR, differently from the acrylic resin base.
Thus, there is no chemical adhesion between
TR and ARAT a condition which might lead to
dislodgment.
In studies with acrylic resin denture bases
and ARAT, applied retentive features determined
statistically significantly better results [9].
The values indicate that providing retention to
the articial teeth might improve resistance to
their displacement in most specimens placed in
thermoplastic polymer base. Both vertical and
horizontal retention values presented better
results.
Table II - Two -factor ANOVA and Tukeys’ test. Comparison
between types of treatment (Canine, Lateral Incisors, Central
Incisors) (p<0.05)
Treatment Difference Q (p)
GI-GII 205,915 41,093 < 0.05 *
GI-GIII 866,338 172,89 < 0.01 *
GII-GII 660,423 131,797 < 0.01 *
(Group I: none; Group II: retention hole; Group III: groove retention).
* indicates results with significant statistic differences.
Figure 3 - Different types of fracture caused by the load force.
Figure 4 - Mean value from each group of teeth in its different
groups (GI- no retention; GII – hole retention and GIII – groove
retention).
Figure 5 - Mean value for different group treatments (GI- no
retention; GII – retention hole and GIII – groove retention).
Figure 6 - Type of failure (%) according to type of retention.
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Braz Dent Sci 2023 July/Sept;26 (3): e3770
Martins PRV et al.
The influence of the retention feature in artificial teeth on its attachment to the thermoplastic resin denture base
Martins PRV et al. The influence of the retention feature in artificial teeth on its
attachment to the thermoplastic resin denture base
By procedural determinant of the study, all
experimental teeth were dislodged completely or
suffered catastrophic fracture (Figure 6). Despite
the deformation of the polymer socket when the
the articial teeth were displaced, there was no
apparent fracture of the material in the contour
of the tooth or in the place where the retention
was made. Mechanical retentions improved
ARAT fixation to acrylic resin denture bases,
although it should be noted that the latter feature
has a vertical orientation and no horizontal
component [9]. However, Takakusaki et al.
(2022) did report good results for composite
artificial teeth retention in TR denture bases
with a similar retentive form [11]. Our results
indicate that the groove (horizontal) retention
demonstrated consistently higher means for
resistance (GIII - p<0,05). It is also interesting
that all of the specimens in the control group
(GI) were dislodged cleanly while the teeth with
groove retention were fractured in their respective
sockets. In those specimens with the hole type
feature (GII), teeth did not fracture suggesting
even though exhibiting better resistance than the
control group, the retentive feature could not
prevent the dislodgment.
The mean failure loads of GII and GIII
were 140 N and 206 N, respectively. These
values were signicantly higher than those of
the control group (119 N). All the experimental
groups showed a mean value higher than 110N,
which is the minimum bonding strength expected
for a maxillary central incisor artificial tooth
and an acrylic denture base resin, according
to ISO 3336:1993 [12]. According to some
authors [13,14], the maximum occlusal force at
the anterior teeth is approximately 100–200 N.
Therefore, the failure loads observed in this study
are clinically acceptable and would be able to
support the masticatory function.
Lateral and Central incisors presented no
statistically signicant results when comparing
GI and GII. It has been reported that the hole
diameter of the retentions might inuence bond
strength of the articial [3], therefore an analysis
with different sizes of burs might be necessary.
In order to improve retention of articial
teeth to TR, more studies on the subject are
required. Studies with different materials for
both tooth and denture base, as well as different
retention formats and sizes.
CONCLUSION
Since there is no chemical interaction
between TR and ARAT, the material requires
additional forms of retention. The data in this
study indicates that a groove placed on the
articial tooth offers signicantly better retention
efcacy to the point where the displacement was
only possible after its fracture.
Author’s Contributions
PRVM: Methodology, Validation,
Investigation. MKM: Conceptualization,
Methodology, Design of the study, Data
Curation, Writing – Review & Editing. BC:
Data interpretation, Formal analysis, Writing
– Review & Editing. CMI: Writing – Original
Draft Preparation, Writing – Review & Editing.
RCS: Conceptualization, Methodology, Design
of the study, Data Curation, Writing – Review
& Editing.
Conict of Interest
No conicts of interest declared concerning
the publication of this article.
Funding
The authors declare that no nancial support
was received.
Regulatory Statement
Not applicable.
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Carolina Mayumi Iegami
(Corresponding address)
Universidade de São Paulo, Faculdade de Odontologia, Departamento de Prótese,
São Paulo, SP, Brazil, São Paulo, SP, Brazil
Email: carolinaiegami@usp.br
Date submitted: 2023 Jan 01
Accept submission: 2023 July 05
