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.e3986
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Braz Dent Sci 2023 Oct/Dec;26 (4): e3986
Analyzing SHORE A hardness to assess the durability of soft
denture lining materials
Analisando a dureza SHORE A para avaliar a durabilidade de materiais macios de revestimento de próteses
Natália Inês GONÇALVES
1
, Fernanda Zapater PIERRE
1
, Alexandre Luiz Souto BORGES
1
, João Maurício Ferraz da SILVA
1
,
Eduardo Shigueyuki UEMURA
1
*
1 - Universidade Estadual Paulista “Júlio de Mesquita Filho”, Instituto de Ciência e Tecnologia, Departamento de Materiais Odontológicos
e Prótese. São José dos Campos, SP, Brazil.
How to cite: Gonçalves NI, Pierre FZ, Borges ALS, Silva JMF, Uemura ES. Analyzing SHORE A hardness to assess the durability of soft
denture lining materials. Braz Dent Sci. 2023;26(4):e3986. https://doi.org/10.4322/bds.2023.e3986
ABSTRACT
Objective: Evaluate the “SHORE A” hardness in four denture soft lining materials at different time intervals.
Material and Methods: Forty disc-shaped specimens, with dimensions of 15 mm × 3 mm, were distributed
into four groups according to the materials: SC (Soft Confort); TS (Trusoft); UG (U Gel P) and DS (Dentusoft)
(n=10). The hardness measurements were performed using a portable digital hardness meter at 1 hour (h), 3 days
(d), 7 d, 15 d, 30 and 60 d after preparation of specimens. The data were subjected to statistical analysis. Results:
UG presented the highest hardness (39,09±2,27), followed by SC (34,36±8,52), DS (26,23±5,26) and nally TS
(18,05±3,60), being that UG and TS had the smaller change in hardness between time intervals. The Repeated
Measures ANOVA and Tukey’s test showed that the variables (material and time) and their interaction had
differences statistically signicant (p≤0.05). Conclusion: The study reveals signicant variations in hardness
among different groups. The groups SC and DS experienced substantial changes in hardness from initial to
nal values, with the group SC exhibiting the most pronounced shift. These ndings underscore the suitability
of acrylic materials for temporary applications. On the other hand, the silicone material (UG) with the highest
initial hardness, exhibited relatively minor changes in hardness, emphasizing their recommendation for long-
lasting applications. Group TS, despite not being composed of silicone, also showed relatively minor changes in
hardness, suggesting its potential suitability for specic applications where stability is desirable.
KEYWORDS
Denture liners; Dental prosthesis; Dental tissue conditioning; Hardness tests; Longevity.
RESUMO
Objetivo: avaliar a dureza “SHORE A” em quatro materiais de revestimento macio de próteses dentárias em
diferentes intervalos de tempo. Material e Métodos: Quarenta corpos de prova em forma de disco, com dimensões
de 15 mm × 3 mm, foram distribuídos em quatro grupos de acordo com os materiais: SC (Soft Confort); TS
(Trusoft); UG (U Gel P) e DS (Dentusoft) (n=10). As medições de dureza foram realizadas utilizando um
durômetro digital portátil em 1 hora (h), 3 dias (d), 7 d, 15 d, 30 d e 60 d após a preparação das amostras. Os
dados foram submetidos à análise estatística. Resultados: UG apresentou maior dureza (39,09±2,27), seguido
por SC (34,36±8,52), DS (26,23±5,26) e TS (18,05±3,26), sendo que UG e TS tiveram a menor alteração
na dureza entre os intervalos de tempo. A ANOVA de Medidas Repetidas e o teste de Tukey mostraram que as
variáveis (material e tempo) e sua interação apresentaram diferenças estatisticamente signicativas (p≤0,05).
Conclusão: O estudo revela variações signicativas na dureza entre os diferentes grupos. Os grupos SC e DS
experimentaram mudanças substanciais na dureza dos valores iniciais para os nais, com o grupo SC exibindo
a mudança mais pronunciada. Essas descobertas ressaltam a adequação dos materiais acrílicos para aplicações
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Braz Dent Sci 2023 Oct/Dec;26 (4): e3986
Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft dentur e lining materials
Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft
denture lining materials
INTRODUCTION
Despite proper construction and fit,
complete dentures made from rigid materials
like polymethylmethacrylate (PMMA) can
commonly cause discomfort and pain for patients
while chewing and swallowing [1-3]. The loads
received by the prosthesis, along with ridge
resorption and bromucosa height, affect support,
retention, and stability of the prostheses, besides
their mechanical performance [4]. This issue is
particularly evident in the lower arch, making
the prostheses adaptation more challenging than
the maxillary one. Users of complete mandibular
dentures often experience poor retention and
stability [5]
To enhance the quality of life and offer
functional and aesthetic advantages of complete
dentures, lining materials are commonly
used [6-8]. They can be categorized based on their
chemical structures: hard liners, typically made
of PMMA, and soft denture liners (SDL), which
include plasticized acrylic resins, vinyl resins,
rubber-based materials, and silicone rubbers.
SDLs are further classied into permanent or
temporary options, which encompass tissue
conditioners, acrylic resin-based, and silicone-
based materials (auto-polymerized and heat-
polymerized) [9-11].
SDL have multiple advantages, including
their versatility and ease of handling. They act as
a cushion, absorbing and distributing functional
stress, providing comfort for resorbed or sharp
residual crests. Additionally, they protect healing
sites, reduce edema, and control post-surgical
bleeding, similar to a pressure bandage [2,11-13].
Despite the advantages of SDL, it has clinical
limitations such as loss of softness over time due to
the loss of soluble components [11]. This results
in an increase in the material’s hardness, along
with water absorption, dimensional changes,
and stresses on the liner-denture base interface,
consequently reducing bond strength. This may
cause debonding or microleakage between SDL
and the prosthesis, leading to contamination by
microorganisms [10,14]. Frequent replacement
of the material is necessary to maintain its desired
softness and functionality [13]. SDL also demands
a smooth surface, which can be challenging for
maintenance during clinical practice. Prolonged
use leads to heightened difculties in achieving
satisfactory hygiene, as the material is prone to
deterioration, resulting in increased roughness.
Thereby a biofilm containing bacteria and
yeasts accumulates on the surface, turning it
into a reservoir for microorganisms, primarily
Candida albicans
, considered the main etiological
factor of denture stomatitis.
C. albicans
can
lead to a chronic inflammation and tissue
irritation. Over time, this can contribute to bone
resorption [12,15-19].
The inconsistent information regarding SDL
properties, mostly the mechanical properties,
challenges dental surgeons in determining
the best applications for these materials.
Consequently, determining the long-term
mechanical performance of the SDL materials
available on the market is clinically relevant [20].
The widely used “Shore A hardness” measurement
characterizes polymers, elastomers, and rubbers
by indicating their resistance to indentation (0 to
100 Shore units). A conical indenter is pressed
against the material’s surface, and the depth of
the indentation is measured. The greater the
material’s resistance to penetration, the higher
its Shore A hardness [1,21].
Hence, the objective of this study was to
assess the Shore A hardness alterations of four
short-term commonly used SDL, over specic
time intervals through in vitro evaluations
that simulate the duration of specific clinical
scenarios. This approach potentially mimics the
longevity of these materials while in use inside
the oral cavity. The null hypothesis was that the
material’s hardness would not be affected by
different time intervals.
temporárias. Por outro lado, o material de silicone (UG) com maior dureza inicial, apresentou alterações
relativamente pequenas na dureza, enfatizando sua recomendação para aplicações de longa duração. O Grupo
TS, apesar de não ser composto por silicone, também apresentou alterações relativamente pequenas na dureza,
sugerindo sua potencial adequação para aplicações especícas onde a estabilidade é desejável.
PALAVRAS-CHAVE
Reembasadores de dentaduras; Prótese dentária; Condicionamento de tecido mole oral; Ensaios de dureza;
Longevidade.
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Braz Dent Sci 2023 Oct/Dec;26 (4): e3986
Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft dentur e lining materials
Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft
denture lining materials
MATERIALS AND METHODS
The present study used a detailed owchart
to guide the research, covering all stages of the
experiment, represented in gure 1.
Specimen preparation
The materials used in this study are listed in
Table I. A total of forty disc-shaped specimens were
manufactured using a standardized aluminum
mold, with dimensions of 15 mm diameter and
3 mm height. The specimens were distributed into
four groups, each representing a distinct denture
lining material: SC (Soft Confort), TS (Trusoft),
UG (U Gel P), and DS (Dentusoft).
Disc-shaped specimens were prepared
according to the manufacturer’s instructions.
The material was poured into aluminum
molds with a slight excess, and then the
molds were finger pressed using a glass
lid until close contact between them.
After polymerization, the specimens were
separated, coated with varnish following
the manufacturer’s recommendations, and
subsequently conditioned in distilled water.
Hardness measure
Hardness measurements were conducted
using a portable digital durometer, Novotest
TS-C Shore A series 998-GS 709 (Novotest,
Novomoskovsk, Ukraine), at specific time
intervals (1 hour, 3 days, 7 days, 15 days, 30 days,
and 60 days). Following the methodology
outlined in the European Standard ISO 10139-
1:2018 [22] the durometer was kept in a vertical
position, applied the required force during the
measurement process and stopped automatically
when equilibrium between the indenter (the
penetrating device) and the material’s response
was reached. Three measurements were taken
from distinct, randomly chosen locations within
each sample.
All samples were stored in distilled water
at room temperature between measurements
until the completion of the test, adhering to
the standards set by the Deutsches Institut
für Normung (DIN) 53505 and the American
Society for Testing and Materials (ASTM)
D2240/75.
Figure 1 - Flow chart of the research’s stages. The flowchart was designed to visualize and plan the sequence of activities conducted during
the research. The initial yellow rectangle depicts the formulation of the research question. Subsequently, the four blue rectangles represent
the stages of the materials and methods.
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Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft dentur e lining materials
Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft
denture lining materials
Statistical analysis
To conduct the statistical analysis, the
mean and standard deviation (SD) of each
soft denture liner material were calculated
at each time point. The data were subjected
to a Repeated Measures Analysis of Variance
(Repeated Measures ANOVA), and multiple
comparisons were performed using the Tukey’s
test, with a significance level of 5%. These
analyses were performed using the software
JAMOVI (version 2.3).
RESULTS
The mean and SD values of Shore A hardness
(in Shore A unit) are present in Table II and
Figure 2. Group UG showed the highest hardness
among the materials tested at all times, except for
15 d, where SC showed the highest values between
the groups (40.46±2.19). In the overall mean,
group UG presented the highest hardness, followed
by SC, DS and nally TS. In the chart, it is possible
to observe that UG and TS had the smallest change
in hardness between time intervals.
Tables III and IV show the within-subjects
effect and between-subjects effects. It is possible
to observe that both the variables and their
interaction had differences statistically signicant
(
p
≤.001) when the Repeated Measures ANOVA
was applied to assess the influence of the
independent variables (material and time).
In Table V, Tukey’s test results show that
some materials share similar hardness values at
specic time points. For instance, groups DS and
TS have similar hardness values after 1 h, while
SC and UG share similarities at multiple time
points (7 days, 30 days, and 60 days).
Table I - Materials used in this study
Material Acronym Type Composition Manufacturer Batch number
Soft Confort SC
Acrylic temporary
soft liner - tissue
conditioner
Powder:
Polyethylmethacrylate
(PEMA)
Dencril - VIPI Ltda,
Pirassununga - SP,
Brazil.
085563
Liquid: Phtalate ester
(plasticizer) and Denatured
alcohol
Trusoft TS
Acrylic temporary
soft liner -
autopolymeried
acrylic resin
Powder: Pigmented
Polyethylmethacrylate,
cadmium pigments
Bosworth Company,
Skokie - IL, USA
JB9444
Liquid: Ethyl alcohol, Benzyl
butyl phthalate, dibutyl
phthalate
Dentusoft DS
Acrylic temporary
soft liner - tissue
conditioner
Powder:
Polyethylmethacrylate
Densell, Buenos
Aires - BUE,
Argentina
PC0179
Liquid: Monomer N-Butyl
Methacrylate Dibutyl
Phthalate, and Denatured
Alcohol
Ufi Gel P UG
Tissue conditioner
- autopolymeried
silicone
base paste: Modified
polydimethylsiloxanes
Voco GMBH,
Cuxhaven - BS,
Germany
1923716
Catalyst paste: platinum
catalyst
Adhesive: butanone and
methacrylates
Table II - Mean and SD of each material at each time
1h 3d 7d 15d 30d 60d Overall Mean
Trusoft 15.50 ± 3.56 16.05 ± 1.00 19.50 ± 1.08 18.70 ± 3.12 19.98 ± 4.44 18.56 ± 1.63 18.05 ± 3.60
a
Dentusoft 15.26 ± 1.16 28.33 ± 1.58 28.01 ± 0.98 27.40 ± 0.81 28.40 ± 3.38 29.98 ± 0.63 26.23 ± 5.26
b
Soft Confort 17.16 ± 1.31 31.95 ± 2.74 36.76 ± 2.31 40.46 ± 2.19 38.80 ± 1.16 41.05 ± 0.82 34.36 ± 8.52
c
Ufi Gel P 37.50 ± 0.99 36.78 ± 1.00 37.63 ± 1.08 39.45 ± 0.81 41.63 ± 1.08 41.56 ± 1.27 39.09 ± 2.27
d
Different superscript letters indicate the significant differences between groups (p≤0.05).
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Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft dentur e lining materials
Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft
denture lining materials
DISCUSSION
Based on the study results, the null hypothesis
was rejected, as all tested groups exhibited
a significant increase in hardness over time.
Shore A hardness values averaged 18.05 (TS),
26.23 (DS), 34.36 (SC), and 39.09 (UG), with
statistically significant differences (Table II).
During the 1-hour period, no statistically
signicant differences in hardness were observed
among groups DS, TS, and SC. Over time,
acrylic-based materials undergo more signicant
hardness changes compared to silicone-based
materials due to their viscoelastic behavior,
resulting in higher degradation and damping
loss [23-25]. The composition of acrylic-based
materials, including polymers as PEMA and
plasticizers such alcohol and dibutyl phthalate,
affects material softness [11,26]. However,
the hydrophilic nature of plasticizers may lead
to leaching and deterioration as they can be
replaced by water, liquid foods, saliva, and
cleaning solutions [11,25,27-29].
Within the first 3 days, the DS group’s
hardness nearly doubled (from 15.26 to 28.33),
and a similar trend was observed in the SC
group (from 17.16 to 31.95) (Figure 2). This
phenomenon can be attributed to the ongoing
polymerization process of autopolymerized
materials, which occurs at room temperature
and continues over time, contributing to an
increase in material hardness throughout its
lifespan [30]. However, this increase is considered
a disadvantage for a soft material, as higher
hardness values correspond to a reduced ability
to absorb the impact of mastication [31].
Surprisingly, between days 3 and 60, the
DS group did not exhibit signicant differences.
In contrast, the SC group showed the highest
degree of change between its initial and nal
values (from 17.16 to 41.05), supporting the
ndings of other authors [32,33] and suggesting
that the SC formulation contains a higher amount
of plasticizer and ethanol. This nding contrasts
with the manufacturer’s recommendations for a
3-month use.
Group TS, while statistically significant
differences were observed, there were no
significant changes in hardness, with values
ranging from 15.50 to 19.98. Another study [34]
examined the water absorption and solubility of
DS and TS materials, indicating that the minimal
changes observed in DS hardness between days
3 and 60 correlate with low solubility levels and
Figure 2 - Line chart showing the mean’s hardness of each material
over time. The graph visually presents the results from Table II.
Trusoft (blue) exhibits the lowest hardness values throughout the
entire measured time interval. Dentusoft (red) shows a significant
increase from the initial time until the second measurement on day
3, after which it remains constant in second place. Soft Confort
(yellow) nearly doubles its values on day 15 compared to the initial
measurement, reaching the hardness values that Ufigel (green)
already had since the initial measurement. Consequently, Ufigel
exhibits the highest hardness values during the entire time interval
and remains highly stable.
Table III - Within-Subjects effect
SS Degr. MS F p
Time 3635 5 726.96 247.5 < .001
Time*Material 2332 15 155.46 52.9 <.001
Residual 529 180 2.94
SS = sum of 3 squares; Degr. = degrees of freedom; MS = mean square; *p≤0.05
Table IV - Between-subjects effects
SS Degr. MS F p
Material 15449 3 5149.7 374 <.001
Residual 496 36 13.8
SS = sum of 3 squares; Degr. = degrees of freedom; MS = mean square; *p≤0.05
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Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft dentur e lining materials
Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft
denture lining materials
a diminished concentration of dibutyl phthalate
and alcohol. As for TS, correlating with the
molecular weight of its plasticizer. The elevated
concentration of benzyl butyl phthalate, a
plasticizer with a higher molecular weight, in
the composition of TS may contribute to its low
solubility [34,35].
These findings not only elucidate the
performance characteristics of DS and TS but also
offer insight into their suggested duration of use.
The observed stability in the solubility and hardness
throughout the evaluation period resonates with
the manufacturer’s recommendation for TS,
endorsing its application for up to 12 months.
In contrast, the DS manufacturer does not specify
a recommended duration; rather, suggesting that
reapplication can be considered in the event of
discomfort.
Despite the manufacturers’ recommended
durations, previous studies have demonstrated
that SDL are ideally suitable for short-term
use, typically up to 14 days [10,36-38].
SDL excel in absorbing masticatory impacts,
aiding in post-tooth extraction adjustments for
immediate complete dentures, pre-prosthetic
post-surgical cases, and preparing for permanent
PMMA denture bases. In such cases, rapid
adaptation may occur, leading to more frequent
relining procedures [11,13,29]. Moreover, as
the material’s hardness increases, the surface
becomes rough and irregular, heightening the
risk of trauma [36].
The ISO 10139-1:2018 standard classies
SDLs into two categories: soft materials (type A)
and extra soft materials (type B). According to
the standard, for type A materials, the average
hardness values after 2 hours of aging should
range between 30 and 50, while for type B
materials, they should be less than 30. After
7 days of aging, the Shore A hardness values for
all materials must not exceed 60 [22]. In this
study, the TS, DS, and UG groups were identied
as type A materials, whereas the SC group was
classied as type B. After 7 days, the hardness
Table V - Comparison of means according to Tukey’s test (5%)
Material Time Hardness 1 2 3 4 5 6 7 8 9
Dentusoft 1h 15.26 A
Trusoft 1h 15.50 A
Trusoft 3d 16.05 A B
Soft Conf 1h 17.16 A B C
Trusoft 60d 18.56 A B C
Trusoft 15d 18.70 A B C
Trusoft 7d 19.50 B C
Trusoft 30d 19.98 C
Dentusoft 15d 27.40 D
Dentusoft 7d 28.01 D
Dentusoft 3d 28.33 D
Dentusoft 30d 28.40 D
Dentusoft 60d 29.98 D E
Soft Conf 3d 31.95 E
Soft Conf 7d 36.76 F
Ufi Gel P 3d 36.78 F
Ufi Gel P 1h 37.50 F G
Ufi Gel P 7d 37.63 F G H
Soft Conf 30d 38.80 F G H I
Ufi Gel P 15d 39.45 F G H I
Soft Conf 15d 40.46 G H I
Soft Conf 60d 41.05 H I
Ufi Gel P 60d 41.56 I
Ufi Gel P 30d 41.63 I
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Analyzing SHORE A hardness to assess the durability of soft dentur e lining materials
Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft
denture lining materials
values of all groups aligned with the standard’s
recommendations.
Group UG demonstrated high initial hardness
values with minimal changes throughout the test.
The substantial increase in hardness observed
for DS and SC in comparison to UG indicates a
signicantly less stable hardness over time and
corroborates with previous studies [1,9,25,30,39].
The hardness stability of silicones stems from
their inherent elasticity, facilitated by the
polydimethylsiloxane component, which achieves
desired softness through cross-linking or ller
addition. This eliminates the need for a plasticizer,
resulting in low solubility [1,11,40]. Additionally,
silicones exhibit hydrophobicity, low water
absorption, and less sensitivity to temperature
changes than acrylic materials, as emphasized
in previous studies [26,41].
The Shore A hardness test results conrmed
the superior performance of group UG, exhibiting
the highest hardness values (17.16 to 41.05) and
the most stable behavior over the 60-day period.
It took 15 d to achieve a hardness close to 40,
maintaining this level thereafter. Consequently,
silicone materials are recommended for longer-
term use [26], as suggested by the manufacturer
for a medium-term usage ranging from 2 weeks
to 2 years. In addition to their role in relining
ill-tted prostheses for enhanced comfort, these
materials serve as effective cushioning agents,
absorbing masticatory impact and providing
support and stabilization for the prosthesis,
aiding in adaptation to retentive areas [11].
Considering the limitations of this in
vitro study, to obtain a more comprehensive
understanding of SDLs’ performance in real
dental practice, future clinical and laboratory
investigations must be conducted to simulate
oral environment conditions, such as temperature
changes, pH variations, uid absorption, exposure
to cleaning solutions, and colonization by
microorganisms. Overall, this study contributes
valuable insights into the hardness behavior of
SDLs over time, assisting dental professionals
in making informed decisions when selecting
the appropriate material for specific clinical
scenarios.
CONCLUSION
Based on the ndings of this in vitro study,
several key conclusions can be drawn:
• Trusoft consistently displayed the lowest
hardness values throughout the test, showing
only a minimal increase over time, with the
highest hardness recorded at the 30-day
mark;
• Dentusoft and Soft Confort exhibited a
notable increase in hardness, primarily at
the 3-day mark, followed by a more stable
progression up to 60 days;
• U Gel P, the only silicone-based material
tested, maintained the highest hardness
values throughout the test with a stable
degree over time.
Acknowledgements
We are grateful for the authors’ efforts to carry
out the research with their own nancial resources
and to the institution for the structure offered.
Author’s Contributions
NIG: Conceptualization, Methodology,
Investigation, Formal Analysis, Data Curation,
Writing - Original Draft Preparation, Writing
- Review & Editing. FZP: Conceptualization,
Methodology, Investigation, Formal Analysis, Data
Curation, Writing - Original Draft Preparation,
Writing - Review & Editing, Visualization. ALSB:
Conceptualization, Methodology, Formal Analysis,
Data Curation, Writing – Original Draft Preparation,
Writing – Review & Editing, Visualization. JMFS:
Conceptualization, Methodology, Investigation,
Writing – Original Draft Preparation, Data
Curation, Writing – Review & Editing. ESU:
Conceptualization, Methodology, Investigation,
Formal Analysis, Data Curation, Writing – Original
Draft Preparation, Writing – Review & Editing,
Visualization, Supervision, Project Administration.
Conict of Interest
The authors have no proprietary, nancial,
or other personal interest of any nature or kind
in any product, service, and/or company that is
presented in this article.
Funding
This research did not receive any specic
grant from funding agencies in the public,
commercial, or not-for-prot sectors.
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Analyzing SHORE A hardness to assess the durability of soft dentur e lining materials
Gonçalves NI et al.
Analyzing SHORE A hardness to assess the durability of soft
denture lining materials
Regulatory Statement
Not applicable.
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Eduardo Shigueyuki Uemura
(Corresponding address)
Universidade Estadual Paulista “Júlio de Mesquita Filho”, Instituto de Ciência e
Tecnologia, Departamento de Materiais Odontológicos e Prótese, São José dos
Campos, SP, Brazil.
Email: eduardo.uemura@unesp.br
Date submitted: 2023 Aug 05
Accept submission: 2023 Dec 07
