Comparison of Fracture Resistance in Thermal and Self-Curing Acrylic Resins—An In Vitro Study

Thermal and self-curing acrylic resins are frequently and versatilely used in dental medicine since they are biocompatible, have no flavor or odor, have satisfactory thermal qualities and polishing capacity, and are easy and fast. Thus, given their widespread use, their fracture resistance behavior is especially important. In this research work, we comparatively analyzed the fracture resistance capacity of thermo and self-curing acrylic resins in vitro. Materials and Methods: Five prosthesis bases were created for each of the following acrylic resins: Lucitone®, ProBase®, and Megacryl®, which were submitted to different forces through the use of the CS® Dental Testing Machine, usually mobilized in the context of fatigue tests. To this end, a point was defined in the center of the anterior edge of the aforementioned acrylic resin bases, for which the peak tended until a fracture occurred. Thermosetting resins were, on average, more resistant to fracture than self-curable resins, although the difference was not statistically significant. The thermosetting resins of the Lucitone® and Probase® brands demonstrated behavior that was more resistant to fracture than the self-curing homologues, although the difference was not statistically significant. Thermosetting resins tended to be, on average, more resistant to fracture and exhibited the maximum values for impact strength, compressive strength, tensile strength, hardness, and dimensional accuracy than self-curing resins, regardless of brand.


Introduction
Acrylic resins are organic compounds derived from ethylene (polymers) and result from the reaction between methylpolymethacrylate and methylmethacrylate, normally supplied in powder form and in liquid form, respectively. A polymerization reaction (combination of both compounds) is a union of monomers in a macromolecule, which is a malleable and moldable mass [1,2]. The polymerization reaction can result from thermal or chemical activation. Thus, thermosetting acrylic resins are thermally activated, hardening permanently when exposed to certain temperatures, becoming more resistant and dimensionally stable, and not reacting to subsequent re-heating. In turn, self-curing acrylic resins are chemically activated: they can be polymerized at room temperature and require a chemical activator (a tertiary amine: di-methyl-para-toluidine) [2].
Acrylic resins are versatile and frequently used in dentistry, for example, in complete dentures, temporary restorations, and even prosthodontic implant rehabilitations [3]. This is possible because they tend to be biocompatible, have no taste or odor, have satisfactory thermal qualities, can be polished, and are easy and quick to repair. However, the lack of marginal adaptation or roughness of this type of polymer can cause irritation and The sample of thermopolymerizable acrylic resins was composed of: -Megacryl Hot + Don ® (Megadental, Büdingen, Germany): Composed of acrylic based on methyl-methacrylate, suitable for the compression and injection process. Excellent fluidity, mechanical properties, natural coloring, and safe behavior, so it is stable and modellable; -ProBase Hot ® (Ivoclar Vivadent, Zurich, Switzerland): Composed of acrylic based on methyl methacrylate (50-100%); excellent fluidity, mechanical properties, and safe behavior, so it is stable and modellable. The polymer composition is >95% polymethylmethacrylate; -Lucitone 199 ® (Dentsply Sirona, Ballantyne Corporate Pl, Charlotte, NC, USA): Composition based on methyl-methacrylate (80-100%).

Data Collection
A standard laboratory protocol was established and applied at the Institute for Research and Advanced Training in Health Sciences and Technologies (IINFACTS-CESPU, Gandra, Paredes, Portugal) to test all selected samples.
The preparation for the experimental phase started with the multiplication of 30 units of the prefabricated model ( Figure 1), in type IV plaster.

Data Collection
A standard laboratory protocol was established and applied at the Institute for Research and Advanced Training in Health Sciences and Technologies (IINFACTS-CESPU, Gandra, Paredes, Portugal) to test all selected samples.
For models fabrication: 1. The preparation for the experimental phase started with the multiplication of 30 units of the prefabricated model ( Figure 1), in type IV plaster. 2. The multiplication of the prefabricated mold was carried out with a Dosper Evo (Dreve Dentamid GmbH, Unna ,Germany) duplicating machine using the Z-Dupe 28 shore A silicone (base + catalyst) (Henry Schein, Melville, New York, USA) ( Figure  2).

Data Collection
A standard laboratory protocol was established and applied at the Institute for Research and Advanced Training in Health Sciences and Technologies (IINFACTS-CESPU, Gandra, Paredes, Portugal) to test all selected samples.
For models fabrication: 1. The preparation for the experimental phase started with the multiplication of 30 units of the prefabricated model ( Figure 1), in type IV plaster. 2. The multiplication of the prefabricated mold was carried out with a Dosper Evo (Dreve Dentamid GmbH, Unna ,Germany) duplicating machine using the Z-Dupe 28 shore A silicone (base + catalyst) (Henry Schein, Melville, New York, USA) ( Figure  2).

3.
The prefabricated mold was removed and the cooping was poured to type IV plaster (Figures 3 and 4).
3. The prefabricated mold was removed and the cooping was poured to type IV plaster (Figures 3 and 4).  4. We created 5 bases of each acrylic resin described above, with dimensions of 60 × 45 mm 2 and height of 2 mm using the prefabricated models.
For fabrication of self-curing acrylic bases: 1. We developed a silicone wall covering a mold with the metallic base. Once it hardened, it was separated, and a hole was made in the silicone. 2. To pour the acrylic, the metal base was removed. 3. The model was isolated with plaster insulator, and the silicone wall was placed on top of the same model ( Figure 5). 3. The prefabricated mold was removed and the cooping was poured to type IV plaster (Figures 3 and 4).  4. We created 5 bases of each acrylic resin described above, with dimensions of 60 × 45 mm 2 and height of 2 mm using the prefabricated models.
For fabrication of self-curing acrylic bases: 1. We developed a silicone wall covering a mold with the metallic base. Once it hardened, it was separated, and a hole was made in the silicone. 2. To pour the acrylic, the metal base was removed. 3. The model was isolated with plaster insulator, and the silicone wall was placed on top of the same model ( Figure 5).

4.
We created 5 bases of each acrylic resin described above, with dimensions of 60 × 45 mm 2 and height of 2 mm using the prefabricated models.
For fabrication of self-curing acrylic bases: 1.
We developed a silicone wall covering a mold with the metallic base. Once it hardened, it was separated, and a hole was made in the silicone.

2.
To pour the acrylic, the metal base was removed. 3.
The model was isolated with plaster insulator, and the silicone wall was placed on top of the same model ( Figure 5).

4.
We created the base using acrylic resin according to the manufacturer's recommendations, pouring it between the wall and the mold and placing them in a pressure-cooker at 2 bars for the recommended time and temperature. 4. We created the base using acrylic resin according to the manufacturer's recommendations, pouring it between the wall and the mold and placing them in a pressurecooker at 2 bars for the recommended time and temperature.
For fabrication of thermopolymerizable acrylic bases: 1. We placed the model with the metal base in muffle and the formation of a silicone wall on top of that same base. 2. The counter muffle was closed with type III plaster and placed in the press until the plaster hardened. Subsequently, the muffle was opened, removing the metal base ( Figure 6), and the acrylic was produced based on the supplier's instructions. 3. We placed the acrylic between the mold and the wall, closing the muffle again with the help of the press ( Figure 6), placing them in a pan with hot water following the manufacturer's time and temperature recommendations. For fabrication of thermopolymerizable acrylic bases:

1.
We placed the model with the metal base in muffle and the formation of a silicone wall on top of that same base.

2.
The counter muffle was closed with type III plaster and placed in the press until the plaster hardened. Subsequently, the muffle was opened, removing the metal base ( Figure 6), and the acrylic was produced based on the supplier's instructions.

3.
We placed the acrylic between the mold and the wall, closing the muffle again with the help of the press ( Figure 6), placing them in a pan with hot water following the manufacturer's time and temperature recommendations. 4. We created the base using acrylic resin according to the manufacturer's recommendations, pouring it between the wall and the mold and placing them in a pressurecooker at 2 bars for the recommended time and temperature.
For fabrication of thermopolymerizable acrylic bases: 1. We placed the model with the metal base in muffle and the formation of a silicone wall on top of that same base. 2. The counter muffle was closed with type III plaster and placed in the press until the plaster hardened. Subsequently, the muffle was opened, removing the metal base ( Figure 6), and the acrylic was produced based on the supplier's instructions. 3. We placed the acrylic between the mold and the wall, closing the muffle again with the help of the press ( Figure 6), placing them in a pan with hot water following the manufacturer's time and temperature recommendations. All bases were numbered (I, II, III, IV, V, and VI) on the posterior edge of the first quadrant. All these bases were subsequently subjected to the same vertical force through the use of the CS ® Dental Testing Machine (Table 1)   Subsequently, a point was defined in the center of the anterior edge of the base of the referred resins toward which the peak tended until a fracture occurred (Figure 7). The fracture force was measured and systematized by the load cell, and this information was directly transferred to Microsoft Office Excel (Microsoft, Redmond, Washington, DC, USA) software.
Polymers 2021, 13, x FOR PEER REVIEW 6 of 13 All bases were numbered (I, II, III, IV, V, and VI) on the posterior edge of the first quadrant. All these bases were subsequently subjected to the same vertical force through the use of the CS ® Dental Testing Machine (Table 1)   Subsequently, a point was defined in the center of the anterior edge of the base of the referred resins toward which the peak tended until a fracture occurred (Figure 7). The fracture force was measured and systematized by the load cell, and this information was directly transferred to Microsoft Office Excel (Microsoft, Redmond, Washington, USA) software.

Statistical Analysis
Data were analyzed in SPSS ® (IBM, Armonk, NY, USA), version 24. First, exploratory data analysis was performed, which detected one outlier, Probase ® . However, as this The variable (force) was then described using the mean (M) and standard deviation (SD) for the fracture torques, expressed in Kgf, after evaluating the asymmetry coefficient (<|1|) and the histograms.
The normality of the distributions was assessed using the Shapiro-Wilk test, used for n < 50, confirming the necessary assumption for the use of parametric tests for the distributions of the three brands under study. The homogeneity of variances was evaluated and confirmed with the Levene test.
The comparison of the average fracture torque by brand and type (auto and thermopolymerizable) was performed with two-way ANOVA, with calculation of the F test for brand, type, and brand × type interaction. The effect size was also calculated using eta2 (η 2 ), considering as cutoff points slight (0.01), moderate (0.06), and high (0.14) effects.
The t-test for one sample was used to compare the six groups of brand × type samples by the reference limits of the human bite of 6 and 8 Kgf.
The maximum level of significance considered was 5%. The ProBase Cold ® and ProBase Hot ® brands consecutively presented the greatest fragment loss during the fracture (Figure 8). The Lucitone HIPA ® and Lucitone 199 ® brands were the ones with the lowest number of fragment losses (Figure 9).

Statistical Analysis
Data were analyzed in SPSS ® (IBM, Armonk, New York, USA), version 24. First, exploratory data analysis was performed, which detected one outlier, Probase ® . However, as this value was within the limits of the observations of the other brands, it was included in the analysis.
The variable (force) was then described using the mean (M) and standard deviation (SD) for the fracture torques, expressed in Kgf, after evaluating the asymmetry coefficient (<|1|) and the histograms.
The normality of the distributions was assessed using the Shapiro-Wilk test, used for n < 50, confirming the necessary assumption for the use of parametric tests for the distributions of the three brands under study. The homogeneity of variances was evaluated and confirmed with the Levene test.
The comparison of the average fracture torque by brand and type (auto and thermopolymerizable) was performed with two-way ANOVA, with calculation of the F test for brand, type, and brand × type interaction. The effect size was also calculated using eta2 (η2), considering as cutoff points slight (0.01), moderate (0.06), and high (0.14) effects.
The t-test for one sample was used to compare the six groups of brand × type samples by the reference limits of the human bite of 6 and 8 Kgf.
The maximum level of significance considered was 5%. The ProBase Cold ® and ProBase Hot ® brands consecutively presented the greatest fragment loss during the fracture (Figure 8). The Lucitone HIPA ® and Lucitone 199 ® brands were the ones with the lowest number of fragment losses (Figure 9).
The peak of the fracture zone was the place where all the bases shattered the most, with only 5 of the 30 bases fracturing in the middle.
Notably, none of the plates tested had porosities.

Results
We evaluated 30 samples, 10 from each brand-Lucitone ® , Megacryl ® , and Probase ® -with the objective of testing the resistance of self-and thermopolymerizable resins by evaluating the maximum fracture torque, expressed in Kgf. All measurements were

Statistical Analysis
Data were analyzed in SPSS ® (IBM, Armonk, New York, USA), version 24. First, exploratory data analysis was performed, which detected one outlier, Probase ® . However, as this value was within the limits of the observations of the other brands, it was included in the analysis.
The variable (force) was then described using the mean (M) and standard deviation (SD) for the fracture torques, expressed in Kgf, after evaluating the asymmetry coefficient (<|1|) and the histograms.
The normality of the distributions was assessed using the Shapiro-Wilk test, used for n < 50, confirming the necessary assumption for the use of parametric tests for the distributions of the three brands under study. The homogeneity of variances was evaluated and confirmed with the Levene test.
The comparison of the average fracture torque by brand and type (auto and thermopolymerizable) was performed with two-way ANOVA, with calculation of the F test for brand, type, and brand × type interaction. The effect size was also calculated using eta2 (η2), considering as cutoff points slight (0.01), moderate (0.06), and high (0.14) effects.
The t-test for one sample was used to compare the six groups of brand × type samples by the reference limits of the human bite of 6 and 8 Kgf.
The maximum level of significance considered was 5%. The ProBase Cold ® and ProBase Hot ® brands consecutively presented the greatest fragment loss during the fracture (Figure 8). The Lucitone HIPA ® and Lucitone 199 ® brands were the ones with the lowest number of fragment losses (Figure 9).
The peak of the fracture zone was the place where all the bases shattered the most, with only 5 of the 30 bases fracturing in the middle.
Notably, none of the plates tested had porosities.

Results
We evaluated 30 samples, 10 from each brand-Lucitone ® , Megacryl ® , and Probase ® -with the objective of testing the resistance of self-and thermopolymerizable resins by evaluating the maximum fracture torque, expressed in Kgf. All measurements were The peak of the fracture zone was the place where all the bases shattered the most, with only 5 of the 30 bases fracturing in the middle.
Notably, none of the plates tested had porosities.

Results
We evaluated 30 samples, 10 from each brand-Lucitone ® , Megacryl ® , and Probase ®with the objective of testing the resistance of self-and thermopolymerizable resins by evaluating the maximum fracture torque, expressed in Kgf. All measurements were greater than 15 Kgf and less than 30 Kgf. The global mean of the fracture torque was 21.89 (SD = 3.46), with a minimum of 15.72 and a maximum of 29.48. Table 2 shows the results for the two-way ANOVA in the comparison of the average fracture torque values by brand and type of resin. No statistically significant results were found in the comparison by brand (F (2.24) = 1.92; p = 0.169), type (F (1.24) = 2.04; p = 1.66), and brand interaction × type (F (2.24) = 2.28; p = 0.124). The effect sizes found were high, mainly in the comparison between brands (η 2 = 0.14), where the Megacryl ® and Lucitone ® brands stood out, with an average fracture resistance of 22.78 (SD = 2.84) and 22.96 (SD = 3.48), respectively, both of which were higher than Probase ® . The effect size was also found high in the interaction of the brand with the fracture (η 2 = 0.16), where we observed that among the three brands, Lucitone ® was the only one that obtained a relevant gain in terms of fracture torque with the type of thermosetting resin (M = 25.49; SD = 2.68) compared with the self-curing type (M = 20.42; DP = 2.00). At this level, the average global result of the Lucitone ® brand was obtained at the cost of a value very close to the minimum in the self-curing type and the highest value of the entire study in the thermal-curing type, whereas Megacryl ® and Probase ® did not undergo substantial changes between the two types.  Figure 10 corroborates the results in Table 2. The Megacryl ® brand stood out in the selfcuring type, with a small reduction in the average fracture torque in the thermo-curing type. The Lucitone ® brand stood out in the evolution between the auto and thermopolymerizable types. Probase ® achieved the lowest results of all brands. greater than 15 Kgf and less than 30 Kgf. The global mean of the fracture torque was 21.89 (SD = 3.46), with a minimum of 15.72 and a maximum of 29.48. Table 2 shows the results for the two-way ANOVA in the comparison of the average fracture torque values by brand and type of resin. No statistically significant results were found in the comparison by brand (F (2.24) = 1.92; p = 0.169), type (F (1.24) = 2.04; p = 1.66), and brand interaction × type (F (2.24) = 2.28; p = 0.124). The effect sizes found were high, mainly in the comparison between brands (η2 = 0.14), where the Megacryl ® and Lucitone ® brands stood out, with an average fracture resistance of 22.78 (SD = 2.84) and 22.96 (SD = 3.48), respectively, both of which were higher than Probase ® . The effect size was also found high in the interaction of the brand with the fracture (η2 = 0.16), where we observed that among the three brands, Lucitone ® was the only one that obtained a relevant gain in terms of fracture torque with the type of thermosetting resin (M = 25.49; SD = 2.68) compared with the self-curing type (M = 20.42; DP = 2.00). At this level, the average global result of the Lucitone ® brand was obtained at the cost of a value very close to the minimum in the self-curing type and the highest value of the entire study in the thermal-curing type, whereas Megacryl ® and Probase ® did not undergo substantial changes between the two types. Figure 10 corroborates the results in Table 2. The Megacryl ® brand stood out in the self-curing type, with a small reduction in the average fracture torque in the thermo-curing type. The Lucitone ® brand stood out in the evolution between the auto and thermopolymerizable types. Probase ® achieved the lowest results of all brands.   All observations were above the limits of 6 Kgf and 8 Kgf (Figure 11). Statistically significant results were observed, with a maximum error of 0.3% in comparison with the reference limits of 6 and 8 Kgf (Table 3), suggesting that the evaluated types of resin are well above the limit for human dynamic masticatory forces.
Polymers 2021, 13, x FOR PEER REVIEW 9 of All observations were above the limits of 6 Kgf and 8 Kgf (Figure 11). Statistica significant results were observed, with a maximum error of 0.3% in comparison with t reference limits of 6 and 8 Kgf (Table 3), suggesting that the evaluated types of resin a well above the limit for human dynamic masticatory forces.

Discussion
In dentistry, the quality of products provided to patients has an important impact patient quality of life. For acrylic resins, fracture resistance is a main theme because of t increased costs involved in the repair of acrylic prostheses. The prostheses may fractu due to fatigue caused by prolonged wear and degradation of the material or by the exce sive masticatory load, passing the plastic phase of the material. Therefore, knowing t clinical performance of different acrylic resins with regard to fracture behavior can ind cate how to avoid fractures, avoiding potentially unnecessary costs and improving t quality of life of patients [9,10].
In this study, we aimed to comparatively analyze, in vitro, the fracture resistan capacity of thermal and self-curing acrylic resins. From the main results of the study, w did not find statistically significant differences between the brands tested or in the bran × type interaction. The results refute our hypothesis that there are differences in fractu resistance between the thermo and self-curing acrylic resins. We found that the most r sistant brands were Megacryl® and Lucitone®, the former being more stable to fractu

Discussion
In dentistry, the quality of products provided to patients has an important impact on patient quality of life. For acrylic resins, fracture resistance is a main theme because of the increased costs involved in the repair of acrylic prostheses. The prostheses may fracture due to fatigue caused by prolonged wear and degradation of the material or by the excessive masticatory load, passing the plastic phase of the material. Therefore, knowing the clinical performance of different acrylic resins with regard to fracture behavior can indicate how to avoid fractures, avoiding potentially unnecessary costs and improving the quality of life of patients [9,10].
In this study, we aimed to comparatively analyze, in vitro, the fracture resistance capacity of thermal and self-curing acrylic resins. From the main results of the study, we did not find statistically significant differences between the brands tested or in the brand × type interaction. The results refute our hypothesis that there are differences in fracture resistance between the thermo and self-curing acrylic resins. We found that the most resistant brands were Megacryl® and Lucitone®, the former being more stable to fracture resistance regardless of the type of resin. The Probase®brand was the least resistant to fracture, both for self-and thermal-curing resins [11,12].
The literature indicates that self-curing resins tend to be less resistant to fracture because of the larger amount of residual monomer they tend to generate [13,14]. Some studies proved this phenomenon [10]. Even though our results did not allow us to identify significant differences in the fracture resistance values of self-and thermosetting acrylic resins, some tendency toward a lower propensity for fracture in thermosetting acrylic was found, which agrees with the literature [11,12,15].
Regarding the resistance of the different acrylic resins, different studies report that there should be no bubbles or voids in the material for the prosthesis base when seen without application. In addition, porosity values greater than 11% have been associated with reduced mechanical properties, impaired appearance, and retention of liquids and microorganisms [16,17].
The fracture resistance of acrylic resins is also related to mechanical properties such polymerization efficiency and the consequent creation of short polymer chains with low molecular weight [18,19]. Thus, the polymerization efficiency of the thermosetting resins used in the study may have been higher and thereby generated less residual monomer compared with normal. In addition, the elution of components and degradation products must be considered when assessing acrylic resin fracture resistance behavior in the mouth [20,21], noting that certain factors are not replicated in vitro, such as the effect of saliva pH (lower pH decreases fracture resistance) and different masticatory forces and directions. It is also important to consider that the brands tend to improve their products, which may affect the results obtained [22]. Sá J. et al. (2020) found that a low pH reduces the strength of acrylic resin, regardless of the processing technique. They concluded that after exposure to an environment of pH = 7, a higher average force to fracture could be sustained. [22]. Other authors also found a significant interaction between the brand and the pH: Low module of elasticity produces a larger resilience module; consequently, the energy-absorbing capacity is higher and the deflection force released on the material will be higher [18,19]. This result showed that the concentration of added elastomer was directly proportional to the resistance to impact. The addition of elastomer to the material increases its ability to absorb energy and can overcome the possibility of resin fracture, which can result in a prosthetic device being less susceptible to mechanical failure.
The occlusion strength in the centric position of patients with complete dentures is variable-according to a recent study, between 6 kgf and 8 kgf on average [23,24]. The types of acrylic resin evaluated are well above the limit for human occlusion strength. This is a finding to consider when choosing an acrylic resin for the creation of prostheses, a consideration which may be further developed in future in investigations in a real-world context.
The results are in line with the literature on the subject, but with a few significant differences to report. The fracture resistance behaviors of the acrylic resins discussed here and comparisons with other materials, such as thermoplastics, are important to investigate more deeply in the future. Furthermore, it is important to consider the introduction of strengthening modifications in the structure of acrylic resins through the use of, for example, co-polymers and binding agents [25][26][27], as well as to consider the development of new processing and activation techniques for the same purpose [28][29][30].
As described by Zafar MS (2020), many variety of fibers have been added and extensively characterized to improve acrylic properties such as carbon, which provides enhancement of the mechanical properties, including tensile strength, flexibility, fracture resistance; elastic modulus, aramid, which has good wettability and improved mechanical properties, such as fracture resistance; nylon, which improves flexural strength and structural elasticity and fracture resistance; or even glass, which provides excellent reinforcement and aesthetics compared with other fibers-a remarkable increase in the denture base toughness, Vickers hardness, impact strength, and flexural strength is observed, with a remarkable reduction in the deformation [31]. Another introduced option using computer-aided design and computer-aided manufacture (CAD/CAM) for producing removable dentures definitely addresses this demand, as both have fundamentally changed the manufacturing process. Instead of manually mixing the resin powder and liquid and then submitting the immersion to an arbitrarily chosen curing protocol, the poly(methyl methacrylate) (PMMA) resin blocks for CAD/CAM denture bases are industrially fabricated [32] and cured under "great heat and pressure" [32]. Therefore, it has been assumed and reported that the CAD/CAM denture base resins are highly condensed and have fewer micro-porosities [32]. This, in consequence, would mean that CAD/CAM denture base resins could have superior mechanical properties, which is probably why some of the CAD/CAM denture manufacturers advertise that their products have a very low minimum material thickness and high fracture resistance. Steinmassl et al. (2018) performed fatigue tests on a total of 80 standardized, rectangular CAD/CAM denture base resin specimens from five different manufacturers (AvaDent, Baltic Denture System, Vita VIONIC, Whole You Nexteeth, and Wieland Digital Dentures) and compared them with heat-polymerising resin and an autopolymerising resin. They concluded that base resins for CAD/CAM dental prostheses did not clearly demonstrate better mechanical properties than manually processed resins [32]. Thus, it is important that future investigations examine the topic in a more expanded way, seeking to generate complex and multifactorial comparative analyses of the various options, promoting robust conclusions about the potentialities and limitations of each option, and therefore supporting evidence-based practice.

Conclusions
The following conclusions were achieved: (1) Thermopolymerizable resins were, on average, more resistant to fracture than selfcurable resins, although the difference was not statistically significant. The thermopolymerizable resins of the Lucitone®and Probase®brands demonstrated behavior that was more resistant to fracture than the self-curing homologues, although the difference was not statistically significant. (2) Thermopolymerizable resins more resistant to fracture were the Lucitone ® brand, followed by Megacryl ® and Probase ® . (3) Self-curing resins more resistant to fracture were those of the Megacryl ® brand, followed by Lucitone ® and Probase ® . (4) Megacryl ® had the most stable fracture resistance behavior regardless of the type of resin. (5) The behavior and resistance of the resins evidenced in the study were well above the reference limit for the average human mastication force. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.

Data Availability Statement:
No data other than that shown in the manuscript has been reported.