In Vitro Comparative Study of Microhardness and Flexural Strength of Acrylic Resins Used in Removable Dentures

Abstract

Polymethylmethacrylate is the material of choice for prosthetic bases. Depending on the type of polymerization, acrylic resins may present some mechanical weaknesses that may lead to the failure of a prosthesis. The microhardness and flexural strength of a dental material determine its applicability. The objective of the present investigation was to evaluate the in vitro Knoop microhardness and flexural strength of a thermopolymerizable (Probase Hot) and an autopolymerizable (Probase Cold) resin, according to ISO 20759-1: 2013.

Full text

Article
In Vitro Comparative Study of Microhardness and Flexural
Strength of Acrylic Resins Used in Removable Dentures †
Marta Costa 1, * , Sara Neves 1, Joana Carvalho 1, Sofia Arantes-Oliveira 2and Sérgio Félix 1,3


Citation: Costa, M.; Neves, S.;
Carvalho, J.; Arantes-Oliveira, S.;
Félix, S. In Vitro Comparative Study
of Microhardness and Flexural
Strength of Acrylic Resins Used in
Removable Dentures. Med. Sci. Forum
2021,5, 45. https://doi.org/
10.3390/msf2021005045
Published: 28 July 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1Departamento de Reabilitação Oral, Instituto Universitário Egas Moniz, 2829-511 Almada, Portugal;
nevessara98@gmail.com (S.N.); joanapscarvalho@gmail.com (J.C.); sfelix1050@gmail.com (S.F.)
2Faculdade de Medicina Dentária, Universidade de Lisboa, Rua Professora Teresa Ambrósio,
Cidade Universitária, 1600-277 Lisboa, Portugal; sofiaaol@fmd.ulisboa.pt
3Centro de Investigação Interdisciplinar Egas Moniz (CiiEM), Instituto Universitário Egas Moniz,
2829-511 Almada, Portugal
*Correspondence: marta.leonor@hotmail.com
† Presented at the 5th International Congress of CiiEM—Reducing Inequalities in Health and Society, Online,
16–18 June 2021.
Abstract:
Polymethylmethacrylate is the material of choice for prosthetic bases. Depending on the
type of polymerization, acrylic resins may present some mechanical weaknesses that may lead to
the failure of a prosthesis. The microhardness and flexural strength of a dental material determine
its applicability. The objective of the present investigation was to evaluate the
in vitro
Knoop micro-
hardness and flexural strength of a thermopolymerizable (Probase Hot) and an autopolymerizable
(Probase Cold) resin, according to ISO 20759-1: 2013.
Keywords: denture; acrylic resins; polymerization; microhardness; flexural strength
1. Introduction
Acrylic resins based on polymethylmethacrylate (PMMA) are obtained by the poly-
merization of the methylmethacrylate monomer and can be divided into two large groups:
“heat-cured” or thermopolymerizable, when polymerization starts with heat, and “cold-
cured” or autopolymerizable, when they are chemically activated [1].
Despite some desirable characteristics, PMMA also has some mechanical weaknesses
that can lead to fracture, making its ability to resist to fracture a very important parameter
that must be be evaluated, specifically, through microhardness and three-point bending
tests [2,3].
2. Materials and Methods
Respecting the manufacturer’s standards and in accordance with ISO 20759-1: 2013 [
4
],
a total of 10 rectangular specimens of PMMA-based resin were made, i.e., 5 of Probase Hot
(PBH) resin and 5 of Probase Cold (PBC) resin, with dimensions of 64
×
10
×
3.3 mm. The
specimens were polished with 500 and 100 grain silicon carbide sandpaper and then cooled
to room temperature. The specimens were stored in water and incubated at the temperature
of 37
±
1
◦
C for 48
±
2 h. The microhardness of each sample was determined using the
Knoop test through a Knoop indenter connected to a microhardness machine. In each
sample, five indentations were made that the program converted to Knoop microhardness
values expressed in kg/mm
2
, obtaining the average values. For flexural strength evaluation,
the specimens were submitted to the three-point bending test, performed on a universal
servo-hydraulic testing machine. Each specimen was tested, applying a distance between
the supports of 50 mm and a load to the center of each specimen, using a cross speed
of 5 mm/min. Then, the values of the individual measurements (width and thickness)
for each specimen were entered into the machine software. Finally, the load was applied,
Med. Sci. Forum 2021,5, 45. https://doi.org/10.3390/msf2021005045 https://www.mdpi.com/journal/msf

Med. Sci. Forum 2021,5, 45 2 of 2
following guidelines from other similar studies until the specimen was fractured, and the
fracture load value was recorded in Newton (N). The results obtained were analyzed and
compared with a t-test, using SPSS software.
3. Results and Discussion
The results obtained through the microhardness test and the three-point bending test
(Figure 1) showed significant differences (p< 0.001) for the resins under study (Figure 2).
Med. Sci. Forum 2021, 1, 2
thickness) for each specimen were entered into the machine software. Finally, the load
was applied, following guidelines from other similar studies until the specimen was frac-
tured, and the fracture load value was recorded in Newton (N). The results obtained were
analyzed and compared with a t-test, using SPSS software.
3. Results and Discussion
The results obtained through the microhardness test and the three-point bending test
(Figure 1) showed significant differences (p < 0.001) for the resins under study (Figure 2).
Figure 1. Tests: (a) Knoop indentation visualized with an optical microscope; (b) three-point bend-
ing test.
Figure 2. Mechanical characteristics of Probase Hot (orange) and Probase Cold (blue) acrylic resins
measured by (a) the Knoop microhardness test and (b) the three-point bending test.
Thus, within the limitations of this study, the PBH resin presented higher microhard-
ness and flexural strength values than the PBC resin. Scientific evidence demonstrates that
autopolymerizable acrylic resins have a lower degree of polymerization. Incomplete
polymerization in highly porous structures reduces the physical and mechanical quality
of resins [5]. Therefore, this may explain the lower microhardness and flexural strength of
PBC.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data required to reproduce these findings cannot be shared at this
time as the data also forms part of an ongoing study.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Cervino, G.; Cicciù, M.; Herford, A. S.; Germanà, A.; Fiorillo, L. Biological and Chemo-Physical Features of Denture Resin.
Materials 2020, 13, 3350, doi:10.3390/ma13153350.
2. Ozkir, S. E.; Yilmaz, B.; Unal, S. M.; Culhaoglu, A.; Kurkcuoglu. Effect of heat polymerization conditions and microwave on the
flexural strength of polymethylmethacrylate. Eur. J. Dent. 2018, 12, 116–119, doi:10.4103/ejd.ejd_199_17.
3. Camacho, D.; Svidzinki, T.; Furlaneto, M.; Lopes, M.; Corrêa, G. Acrylic resins for dental use based polymethylmethacrylate.
Braz. J. Surg. Clin. Res. 2014, 63, 63–72, doi:10.1155/2020/8941876.
4. ISO 20795-1. Dentistry—Base Polymers—Part 1: Denture Base Polymers; International Organization for Standardization: Geneva,
Switzerland, 2013.
5. Kostic, M.; Petrovic, M.; Krunic, N.; Igic, M.; Janosevic, P. Comparative analysis of water sorption by different acrylic materials.
Acta Med. Median. 2014, 53, 5–9, doi:10.1016/j.chemosphere.2009.11.052.
Figure 1.
Tests: (
a
) Knoop indentation visualized with an optical microscope; (
b
) three-point bend-
ing test.
Med. Sci. Forum 2021, 1, 2
thickness) for each specimen were entered into the machine software. Finally, the load
was applied, following guidelines from other similar studies until the specimen was frac-
tured, and the fracture load value was recorded in Newton (N). The results obtained were
analyzed and compared with a t-test, using SPSS software.
3. Results and Discussion
The results obtained through the microhardness test and the three-point bending test
(Figure 1) showed significant differences (p < 0.001) for the resins under study (Figure 2).
Figure 1. Tests: (a) Knoop indentation visualized with an optical microscope; (b) three-point bend-
ing test.
Figure 2. Mechanical characteristics of Probase Hot (orange) and Probase Cold (blue) acrylic resins
measured by (a) the Knoop microhardness test and (b) the three-point bending test.
Thus, within the limitations of this study, the PBH resin presented higher microhard-
ness and flexural strength values than the PBC resin. Scientific evidence demonstrates that
autopolymerizable acrylic resins have a lower degree of polymerization. Incomplete
polymerization in highly porous structures reduces the physical and mechanical quality
of resins [5]. Therefore, this may explain the lower microhardness and flexural strength of
PBC.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data required to reproduce these findings cannot be shared at this
time as the data also forms part of an ongoing study.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Cervino, G.; Cicciù, M.; Herford, A. S.; Germanà, A.; Fiorillo, L. Biological and Chemo-Physical Features of Denture Resin.
Materials 2020, 13, 3350, doi:10.3390/ma13153350.
2. Ozkir, S. E.; Yilmaz, B.; Unal, S. M.; Culhaoglu, A.; Kurkcuoglu. Effect of heat polymerization conditions and microwave on the
flexural strength of polymethylmethacrylate. Eur. J. Dent. 2018, 12, 116–119, doi:10.4103/ejd.ejd_199_17.
3. Camacho, D.; Svidzinki, T.; Furlaneto, M.; Lopes, M.; Corrêa, G. Acrylic resins for dental use based polymethylmethacrylate.
Braz. J. Surg. Clin. Res. 2014, 63, 63–72, doi:10.1155/2020/8941876.
4. ISO 20795-1. Dentistry—Base Polymers—Part 1: Denture Base Polymers; International Organization for Standardization: Geneva,
Switzerland, 2013.
5. Kostic, M.; Petrovic, M.; Krunic, N.; Igic, M.; Janosevic, P. Comparative analysis of water sorption by different acrylic materials.
Acta Med. Median. 2014, 53, 5–9, doi:10.1016/j.chemosphere.2009.11.052.
Figure 2.
Mechanical characteristics of Probase Hot (orange) and Probase Cold (blue) acrylic resins
measured by (a) the Knoop microhardness test and (b) the three-point bending test.
Thus, within the limitations of this study, the PBH resin presented higher microhard-
ness and flexural strength values than the PBC resin. Scientific evidence demonstrates
that autopolymerizable acrylic resins have a lower degree of polymerization. Incomplete
polymerization in highly porous structures reduces the physical and mechanical quality
of resins [
5
]. Therefore, this may explain the lower microhardness and flexural strength
of PBC.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The data required to reproduce these findings cannot be shared at this
time as the data also forms part of an ongoing study.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Cervino, G.; Cicciù, M.; Herford, A.S.; Germanà, A.; Fiorillo, L. Biological and Chemo-Physical Features of Denture Resin.
Materials 2020,13, 3350. [CrossRef] [PubMed]
2.
Ozkir, S.E.; Yilmaz, B.; Unal, S.M.; Culhaoglu, A.; Kurkcuoglu. Effect of heat polymerization conditions and microwave on the
flexural strength of polymethylmethacrylate. Eur. J. Dent. 2018,12, 116–119. [CrossRef] [PubMed]
3.
Camacho, D.; Svidzinki, T.; Furlaneto, M.; Lopes, M.; Corrêa, G. Acrylic resins for dental use based polymethylmethacrylate. Braz.
J. Surg. Clin. Res. 2014,6, 63–72. [CrossRef]
4.
ISO 20795-1. Dentistry—Base Polymers—Part 1: Denture Base Polymers; International Organization for Standardization: Geneva,
Switzerland, 2013.
5.
Kostic, M.; Petrovic, M.; Krunic, N.; Igic, M.; Janosevic, P. Comparative analysis of water sorption by different acrylic materials.
Acta Med. Median. 2014,53, 5–9. [CrossRef]