Comparative In Vitro Analysis of Mechanical Properties in Three High-Viscosity Bulk-Fill Composite Resins

Abstract

Bulk-fill composite resins (BFCRs) have emerged as efficient alternatives to conventional restorative systems, enabling placement in thicker increments without compromising polymerization; however, their comparative mechanical performance under clinically demanding conditions remains uncertain. This study aimed to evaluate and compare the mechanical properties—flexural strength (FS), elastic modulus (EM), strain (ε), and displacement (δ)—of three high-viscosity bulk-fill resins: Filtek One™ Bulk Fill (3M ESPE), Tetric^® N-Ceram Bulk Fill (Ivoclar Vivadent), and Opus™ Bulk Fill (FGM). Thirty specimens (n = 10 per group) were fabricated according to ISO 4049:2019 and subjected to three-point bending tests. Statistical analysis included Shapiro–Wilk testing for normality, one-way analysis of variance (ANOVA) with Tukey’s post hoc comparisons, multivariate analysis of variance (MANOVA), and Spearman’s correlation. Filtek One™ Bulk Fill exhibited the highest FS 142.5 megapascals (MPa) and EM 4.2 gigapascals (GPa), with significant differences compared to Tetric^® N-Ceram Bulk Fill and Opus™ Bulk Fill (p < 0.001). Opus™ Bulk Fill demonstrated greater deformation capacity before fracture (p = 0.015). MANOVA revealed a significant effect of resin type on overall mechanical behavior (Wilks’ λ = 0.132; p < 0.001). Strong correlations were observed between strength and stiffness (ρ = 0.82), and between stiffness and deformation (ρ = –0.68). These findings confirm that BFCRs differ significantly in mechanical behavior, with Filtek One™ Bulk Fill exhibiting superior stiffness and resistance, while Opus™ Bulk Fill showed greater deformation capacity. Such differences support material selection based on the functional and anatomical demands of restorations, contributing to improved clinical performance and longevity.

1. Introduction

The advent of composite resins represented a milestone in restorative dentistry, offering materials that combine aesthetic results with minimally invasive procedures [1,2]. Despite these advantages, polymerization shrinkage remains a critical limitation, as it compromises marginal integrity and long-term clinical performance [3,4]. To address this issue, conventional resins have traditionally been placed in 2 mm increments, a technique that, although effective, is both time-consuming and technique-sensitive [5].

The introduction of bulk-fill composite resins (BFCRs) sought to overcome these limitations by allowing increments of up to 4–5 mm without impairing curing depth or efficiency [6,7]. This advance has been supported by innovations in formulation, including adjustments in filler content, increased translucency, the use of optimized photoinitiators such as Ivocerin, and novel low-shrinkage monomers such as aromatic urethane dimethacrylate (AUDMA) and addition–fragmentation monomers (AFMs) [8,9,10,11,12]. Such modifications aim to minimize polymerization stress while maintaining adequate mechanical behavior for posterior restorations [13,14]. Representative materials of this category include Filtek One™ Bulk Fill (3M ESPE), Tetric^® N-Ceram Bulk Fill (Ivoclar Vivadent), and Opus™ Bulk Fill (FGM), each with distinct compositional features that warrant direct comparison.

Although BFCRs simplify restorative protocols, controversy persists regarding whether the efficiency gained compromises mechanical performance. In highly demanding occlusal contexts, properties such as FS, EM, ε, and δ are decisive for the longevity of restorations [15,16,17]. While some studies report superior FS and EM in certain BFCRs [18,19], others highlight that greater ε may provide clinical advantages in terms of energy absorption and marginal adaptation [20,21]. This divergence underscores the need for further research to clarify the real balance between strength and resilience in these materials.

The performance of BFCRs depends largely on their specific formulation, including the type of resin matrix, filler percentage and morphology, polymerization kinetics, and degree of conversion [22,23,24,25]. For instance, Filtek One Bulk Fill has shown higher microhardness and diametral tensile strength (DTS) compared to Tetric N-Ceram Bulk Fill and Opus Bulk Fill, although Tetric N-Ceram is frequently reported as more manageable clinically [26,27]. Such findings illustrate both the progress made and the inconsistency across studies.

Although there is extensive literature on BFCRs, most studies have focused on international brands, leaving regional materials such as Opus™ Bulk Fill—widely used across Latin America—largely unexplored. In this context, the present study conducts a rigorous comparative evaluation of three high-viscosity bulk-fill resins under controlled and standardized conditions according to ISO 4049:2019 [28].

Through multivariate (MANOVA) and correlational analyses, the relationships among their mechanical properties were examined to provide a comprehensive understanding of their performance. This approach aims to offer a robust and contextually relevant scientific basis to optimize the clinical selection of restorative materials and to promote research on products of significance for dental practice in Latin America.

The null hypothesis of this study stated that no statistically significant differences would be observed among the three tested high-viscosity bulk-fill composites in terms of flexural strength, elastic modulus, flexural strain, and maximum displacement.

2. Materials and Methods

2.1. Study Design

A comparative in vitro experimental study was conducted to evaluate the mechanical performance of three high-viscosity bulk-fill composite resins under standardized flexural loading conditions, as reported in previous studies in the scientific literature [29,30,31]. Thirty specimens were distributed into three groups (n = 10 per group) according to the restorative material: Group A = Filtek™ One Bulk Fill (3M ESPE), Group B = Tetric^® N-Ceram Bulk Fill (Ivoclar Vivadent), and Group C = Opus™ Bulk Fill (FGM). The study followed ISO 4049:2019 specifications for direct composite resins [28]. Ethical approval was not applicable, as the study did not involve human or animal subjects.

2.2. Materials

The following commercially available BFCRs were tested:

• Group A (Filtek™ One Bulk Fill Restorative)—3M ESPE, St. Paul, MN, USA; Shade: A2; Lot: 11575350; Exp.: 05/2027.
• Group B (Tetric^® N-Ceram Bulk Fill)—Ivoclar Vivadent, Schaan, Liechtenstein; Shade: IVB; Lot: Z07115; Exp.: 05/2028.
• Group C (Opus™ Bulk Fill APS)—FGM, Joinville, Brazil; Shade: A2; Lot: 051224; Exp.: 12/2026. The following commercially available BFCRs were tested:

Chemical Composition of the Materials Evaluated

The composition of bulk-fill composites plays a crucial role in determining their mechanical behavior and clinical performance. The three materials analyzed in this study—Filtek™ One Bulk Fill, Tetric^® N-Ceram Bulk Fill, and Opus™ Bulk Fill APS—present notable differences in their organic matrices, filler technology, and photoinitiator systems. Filtek™ One incorporates an aromatic urethane dimethacrylate (AUDMA) and an addition–fragmentation monomer (AFM), designed to reduce polymerization stress, together with non-agglomerated silica/zirconia nanoparticles and ytterbium trifluoride fillers, providing high radiopacity and filler loading ~76.5 weight percent (wt%) [32,33].

Tetric^® N-Ceram Bulk Fill, in turn, combines Bis-GMA, Bis-EMA, and UDMA monomers with barialuminosilicate glass, spherical oxide particles, and Isofiller prepolymerized fillers that act as stress relievers, reaching a total inorganic content of approximately 75–77 wt% [29,34,35].

Meanwhile, Opus™ Bulk Fill APS (FGM, Brazil) is based on urethane dimethacrylates with hybrid organic fillers (~79 wt%), using the APS (Advanced Polymerization System), a dual-initiator technology that enhances depth of cure and polymerization efficiency up to 5 mm increments [36].

These compositional distinctions are summarized in Table 1, highlighting the key chemical and technological components that may influence their mechanical response under flexural loading.

Table 1. Chemical composition, filler characteristics, and photoinitiator systems of the evaluated high-viscosity bulk-fill composite resins.

Material	Monomers (Matrix)	Type of Fillers (Particles)	Filler Content	Photoinitiators/ Technologies
Filtek™ One Bulk Fill Restorative (3 M, St. Paul, MN, USA) Shade: A2, Lot: 11575350, Exp.: 05/2027	AUDMA (aromatic urethane dimethacrylate), AFM (addition–fragmentation monomer), diurethane-DMA, and 1,12-dodecane-DMA.	Silica (20 nm) and zirconia (4–11 nm) non-agglomerated nanoclusters of silica/zirconia; ytterbium trifluoride (radiopaque).	~76.5 wt% (58.5 vol%)	Stress-relief resin system based on AFM; 3M’s “TRUE nanotechnology”.
Tetric® N-Ceram Bulk Fill (Ivoclar Vivadent, Schaan, Liechtenstein), Shade: IVB, Lot: Z07115, Exp.: 05/2028	Dimethacrylates such as Bis-GMA, Bis-EMA, and UDMA.	Barium–aluminum silicate glass (0.4–0.7 μm), spherical oxide (~160 nm), ytterbium trifluoride (radiopaque), and “Isifillers” (pre-polymerized fillers acting as stress relievers).	Inorganic filler ~75–77 wt% (~53–55 vol%); “standard” filler loading ≥61 vol%.	Ivocerin® photoinitiator (germanium-based, high reactivity) + light sensitivity filter.
Opus™ Bulk Fill APS (FGM, Joinville, Brazil), Shade: A2, Lot: 051224, Exp.: 12/2026.	Urethane-based methacrylates (according to technical data sheets); high-viscosity bulk-fill formulation.	Organic–inorganic hybrid filler (average size ~79 vol% or 79 wt% for high viscosity; Flow version declares ~68 wt%).	~79 wt% (high-viscosity version); Flow version ~68 wt%.	APS (Advanced polymerization system): a combination of photoinitiators for higher depth of cure; increments up to 5 mm.

2.3. Specimen Preparation

Rectangular stainless-steel molds (25.0 × 2.0 × 2.0 mm; ±0.1 mm) were used and verified with a digital micrometer (±0.01 mm). The inner surfaces were lightly coated with petroleum jelly to facilitate specimen removal, ensuring no contact with the test faces. Each mold was filled in a single increment and carefully condensed with a stainless-steel spatula to avoid voids. A polyester strip (Mylar^®, DuPont, Wilmington, DE, USA) was placed on the surface, and two standard microscope glass slides were gently pressed on top. This procedure condensed the material, eliminated entrapped air, and produced flat, uniform surfaces. After pressure was applied, the slides were removed, leaving the polyester strip in place.

Polymerization was performed with a Valo X Light Emitting Diode (LED) curing unit (Ultradent, South Jordan, UT, USA), calibrated to 1200 ± 25 milliwatts per square centimeter (mW/cm²) and verified with a Bluephase Meter II radiometer (Ivoclar Vivadent, Schaan, Liechtenstein). Each surface was cured in three consecutive 20 s exposures at adjacent positions along the 25 mm specimen length (proximal–central–distal), with the light tip in direct contact (0 mm) with the polyester strip. This corresponded to 24 joules per square centimeter (J/cm²) per position (not cumulative at a single point) and was repeated on the opposite surface to ensure uniform curing throughout the specimen.

Excess material at the margins was trimmed with a surgical scalpel. Specimens were finished with Sof-Lex^® XT abrasive discs (3M ESPE, St. Paul, MN, USA), from coarse to fine grit under controlled motion and light pressure. Final dimensions were re-verified with a digital micrometer (±0.1 mm), as shown in Figure 1. Specimens were then stored by group in sterile containers (10 specimens per container) with 5 mL of distilled water at 37 ± 1 °C for 24 h in an incubator (Memmert IN110, Schwabach, Germany) prior to testing.

Figure 1. Specimen preparation and dimensional standardization for the flexural strength test: (A) Experimental groups of rectangular specimens corresponding to each bulk-fill composite resin evaluated: A—Filtek™ One Bulk Fill Restorative, B—Tetric^® N-Ceram Bulk Fill, and C—Opus™ Bulk Fill (n = 10 per group). (B) Measurement of specimen length (25.00 mm) and (C) specimen thickness (2.00 mm) using a digital caliper to verify compliance with ISO 4049:2019.

2.4. Three-Point Bending Test

Machine setup and calibration were performed using a Shimadzu AGS-X universal testing machine (Shimadzu, Kyoto, Japan) equipped with a 20 kilonewtons (kN) load cell, calibrated according to the manufacturer’s instructions. The assembly consisted of two parallel cylindrical supports spaced 20 mm apart and a loading nose with a diameter of 2 mm (PASCO ME-8237, Roseville, CA, USA), as shown in Figure 2. The crosshead speed was set at 1 mm/min.

Figure 2. Three-point bending test setup. Specimen positioned on the bending fixture (ME-8237, PASCO scientific, Roseville, CA, USA) during the flexural strength test performed according to ISO 4049:2019.

Variables and calculations were derived from the load–deflection curves, which were continuously recorded using TRAPEZIUMX software (Shimadzu, Kyoto, Japan, 2013). The following mechanical properties were calculated:

• Flexural strength (FS, MPa):

$$FS={\displaystyle \frac{\left(3 F L\right)}{\left(2 b d^2\right)}}$$

(1)

where FS is the maximum load at fracture, L is the span length (20 mm), b is the specimen width (mm), and d is the specimen thickness (mm).

• Flexural modulus (EM, GPa):

$$EM={\displaystyle \frac{\left(L^{3}m\right)}{\left(4 b d^3\right)}}$$

(2)

where m = slope of the initial linear portion of the load–deflection curve.

• Maximum displacement (δ_max, mm):

$${\delta }_{\max }=crosshead deflection at fracture$$

(3)

• Maximum strain (ε_max, %):

$${\epsilon }_{\mathrm{m}\mathrm{a}\mathrm{x}}={\displaystyle \frac{\left(6 d {\delta }_{\max }\right)}{\left(L^2\right)}}=\times 100$$

(4)

Fracture was defined as the first abrupt drop in the load–deflection curve, indicating complete structural failure.

2.5. Bias Control and Blinding

To minimize bias, all specimens (N = 30) were assigned unique identification codes (A01–A30) by an external investigator using R v4.2.0 (R Foundation for Statistical Computing, Vienna, Austria). Coding was independent of the material group (Group A, B, or C) to prevent recognition during testing. The operator performing the three-point bending test and the statistician conducting the analysis were blinded to group allocation throughout the study. The code key was disclosed only after completion of the statistical analysis.

2.6. Sample Size Calculation

An a priori power analysis was performed using G*Power 3.1 (Universität Düsseldorf, Germany) for a one-way ANOVA with three independent groups. A medium effect size (f = 0.40, Cohen) was assumed, with a significance level of α = 0.05 and statistical power of 0.90 (1–β = 0.10). The analysis indicated that n = 9 specimens per group (N = 27) would be sufficient to detect significant differences. To compensate for possible specimen loss due to defects or premature fractures, the sample size was increased to n = 10 per group (N = 30), ensuring preservation of statistical power.

2.7. Statistical Analysis

Load and deflection data were acquired at 100 hertz (Hz) and processed using Python v3.11 with the pandas, SciPy.stats, and statsmodels libraries. Normality was assessed using the Shapiro–Wilk test, and homogeneity of variances was verified with Levene’s test. Between-group comparisons were performed using one-way ANOVA followed by Tukey’s HSD post hoc test. MANOVA was employed to assess the overall effect of resin type on FS, EM, ε_max, and δ_max, followed by univariate analyses for each variable. Associations among these mechanical properties were explored using Spearman’s rank correlation coefficient (ρ). Partial eta-squared (η²) and 95% confidence intervals (95% CIs) are reported where applicable. The significance level was set at α = 0.05.

The overall experimental workflow is shown in Figure 3, which outlines the sequence from specimen fabrication to mechanical testing under ISO 4049:2019.

Figure 3. Flowchart illustrating the methodology of the in vitro comparative study conducted under ISO 4049:2019. It details the specimen preparation process, light-curing parameters, finishing and storage conditions, and the three-point bending test setup used to evaluate the mechanical properties of the bulk-fill composite resins.

3. Results

3.1. Assessment of Normality and Homogeneity of Variances

The distribution of the data was evaluated using Shapiro–Wilk, applied independently to each combination of group and mechanical variable. The obtained W statistics ranged from 0.92 to 0.97, with p-values between 0.23 and 0.92, indicating no significant deviations from normality. These results confirm that FS, EM, δ_max, and ε_max met the assumption of normality required for parametric analysis (Table 2).

Table 2. Assessment of data normality using the Shapiro–Wilk test for each combination of resin type and mechanical variable.

Material	Variable	W (Statistic)	p-Value	Normality
Group A	FS (MPa)	0.97	0.89	Yes (p > 0.05)
	EM (GPa)	0.96	0.79	Yes (p > 0.05)
	δmax (mm)	0.97	0.87	Yes (p > 0.05)
	εmax (%)	0.96	0.81	Yes (p > 0.05)
Group B	FS (MPa)	0.95	0.67	Yes (p > 0.05)
	EM (GPa)	0.95	0.69	Yes (p > 0.05)
	δmax (mm)	0.97	0.92	Yes (p > 0.05)
	εmax (%)	0.94	0.61	Yes (p > 0.05)
Group C	FS (MPa)	0.96	0.76	Yes (p > 0.05)
	EM (GPa)	0.95	0.69	Yes (p > 0.05)
	δmax (mm)	0.96	0.77	Yes (p > 0.05)
	εmax (%)	0.92	0.23	Yes (p > 0.05)

FS: flexural strength (MPa); EM: elastic modulus (GPa); δ_max: maximum displacement (mm); ε_max: maximum strain (%); W: Shapiro–Wilk statistic; p: probability value. A p-value > 0.05 indicates that the data did not significantly deviate from a normal distribution.

Verification of the Homogeneity of Variances Assumption

Levene’s test was applied to verify the assumption of homogeneity of variances across the three groups for each mechanical variable. For FS, EM, δ_max, and ε_max, the obtained p-values were all greater than 0.05, indicating no statistically significant differences in variances. Therefore, the homogeneity assumption was considered satisfied, validating the use of parametric analyses such as ANOVA (Table 3).

Table 3. Assessment of variance homogeneity using Levene’s test for each mechanical variable.

Variable	F Statistic	p-Value	Homogeneity of Variance
FS (MPa)	0.34	0.71	Yes (p > 0.05)
EM (GPa)	0.02	0.98	Yes (p > 0.05)
δmax (mm)	1.64	0.22	Yes (p > 0.05)
εmax (%)	0.85	0.44	Yes (p > 0.05)

FS: flexural strength (MPa); EM: elastic modulus (GPa); δ_max: maximum displacement (mm); ε_max: maximum strain (%); F: F statistic; p: probability value. A p-value > 0.05 indicates no statistically significant differences between variances, thus fulfilling the assumption of homoscedasticity.

3.2. Comparison of Mechanical Properties by ANOVA

A one-way ANOVA was performed to compare the three groups across FS, EM, ε_max, and δ_max. Significant differences were detected in FS (F (2, 27) = 42.85, p < 0.001), EM (F (2, 27) = 26.14, p < 0.001), and ε_max (F (2, 27) = 4.87, p = 0.015). No significant differences were found in δ_max (F (2, 27) = 1.23, p = 0.309).

Post hoc Tukey HSD tests indicated that Group A exhibited significantly higher FS and EM values compared with Group C (p < 0.001) and also differed significantly from Group B in at least one variable. No significant differences were observed between Groups B and C for FS or ε_max, although the EM of Group B was significantly higher than that of Group C (Table 4).

Table 4. Multiple comparisons (Tukey HSD) of flexural strength among bulk-fill resin groups.

Group1	Group 2	Mean diff	p-adj	Lower CI95%	Upper CI95%	Reject H0
Group A	Group C	−14.74	0.000	−19.71	−9.78	True
Group A	Group B	−15.84	0.000	−20.80	−10.87	True
Group C	Group B	−1.09	0.849	−6.06	3.87	False

FS: flexural strength (MPa); Mean diff: mean difference; p-adj: adjusted p-value; CI95%: 95% confidence interval; H₀: null hypothesis.

The calculated statistical power was 1.00, confirming that the study had sufficient sensitivity to detect true differences among groups given the effect size and the sample size (n = 10 per group). Descriptively, Group A showed the highest mean FS (142.5 MPa), followed by Group B (131.3 MPa) and Group C (115.4 MPa). A similar trend was observed for EM, with Group A recording 10.5 GPa, Group B 9.4 GPa, and Group C 7.8 GPa. Regarding ε_max, Group C demonstrated the highest mean value (4.1%), whereas Group A presented the lowest (3.6%). Figure 4 illustrates the differences in FS with the corresponding 95% CIs.

Figure 4. Maximum flexural strength (MPa) of composite groups with 95% confidence intervals. Statistically significant differences were observed between Group A and both Groups B and C (p < 0.001). Bars represent means ± 95% CI (n = 10). Different letters indicate significant differences (ANOVA + Tukey post hoc).

3.3. Multivariate and Correlational Analysis

A MANOVA was performed to assess the combined effect of resin type on FS, EM, ε_max, and δ_max. The overall result was statistically significant (Wilks’ Λ = 0.132, F(8, 46) = 8.32, p < 0.001), indicating a significant multivariate effect of group on mechanical behavior. The partial η² was 0.59, corresponding to a large effect size.

Subsequent univariate analyses confirmed the ANOVA findings. Group A consistently showed higher FS and EM compared with Groups B and C (p < 0.001). For ε_max, moderate differences were observed (p = 0.015), with Group C demonstrating the highest mean value.

Spearman correlation analysis was used to explore relationships among the mechanical properties. A strong positive correlation was observed between FS and EM (ρ = 0.82, p < 0.001). An inverse correlation was found between ε_max and EM (ρ = –0.68, p = 0.003). These associations are illustrated in Figure 5 (scatter plot of EM vs. ε_max) and Figure 6 (correlation matrix).

Figure 5. Correlation between elastic modulus (GPa) and maximum strain (%) of composite groups. A strong negative correlation was observed (R² = 0.99), suggesting that materials with higher stiffness exhibit lower strain. Data points represent each composite group, and the dashed line indicates the linear regression fit.

Figure 6. Spearman correlation coefficients (ρ) among FS, EM, ε_max, and δ_max across all groups. Correlations are represented with a color scale to indicate magnitude and direction. Strong correlations were defined as ρ > 0.70, moderate as 0.30 < ρ ≤ 0.70, and weak as ρ < 0.30. Abbreviations: FS = flexural strength; EM = elastic modulus; ε_max = maximum strain; δ_max = maximum displacement; ρ = Spearman correlation coefficient.

Figure 7 presents a bar chart of the mean values of FS (MPa), EM (GPa), ε_max (%), and δ_max (mm) for Groups A, B, and C. This visualization highlights the performance patterns among the groups and allows for a simultaneous assessment of their mechanical response. Group A recorded the highest FS (142.5 MPa) and EM (8.4 GPa), Group B showed intermediate values, and Group C exhibited greater ε_max (3.12%) and δ_max (0.48 mm). This graphical summary complements the statistical analyses (ANOVA, MANOVA, and correlations) and provides an integrated overview of the comparative behavior of the tested resins.

Figure 7. Summary comparison of mean mechanical properties—FS, EM, ε_max, and δ_max—across Groups A, B, and C. Values are presented as means with 95% CI. For visualization purposes, EM, ε_max, and δ_max were rescaled (×25, ×20, and ×200, respectively). Abbreviations: FS = flexural strength; EM = elastic modulus; ε_max = maximum strain; δ_max = maximum displacement; CI = confidence interval.

4. Discussion

The results of this in vitro study are consistent with previous reports on BFCRs over the past decade. Group A exhibited the highest mean FS (~142.5 MPa), significantly outperforming Group B (~131.3 MPa) and Group C (~115.4 MPa). This hierarchy aligns with earlier findings; for instance, Sadananda et al. reported that Filtek One achieved ~141 MPa, significantly higher than Tetric N-Ceram (~128 MPa) [37]. Other investigations have also demonstrated that high-viscosity BFCRs generally exhibit greater mechanical strength compared with materials of lower viscosity or reduced filler content [38,39].

In the present study, all three groups exceeded the ISO 4049 threshold of 80 MPa for direct restorative materials [40], confirming that current BFCRs meet the minimum mechanical requirements for high-load posterior applications, as previously reported in the literature.

Group A also exhibited the highest EM (~10.5 GPa), followed by Group B (~9.4 GPa) and Group C (~7.8 GPa). The greater stiffness of Group A is consistent with reports showing that 3 M bulk-fill composites achieve moduli comparable to highly filled conventional resins. For example, Grazioli et al. reported that Filtek One displayed an EM similar to a conventional composite, whereas Tetric N-Ceram was more flexible (~4–5 GPa, ~30% lower than conventional) [41]. In agreement with these findings, the present study confirmed that Group B was more flexible than Group A, which explains its greater pre-fracture deformation.

Group C, the least rigid material, exhibited the highest ε_max (~4.1% vs. ~3.6% in Group A), confirming a greater bending capacity prior to fracture. Previous studies suggest that reduced stiffness may partially mitigate internal stress, favor impact absorption, and enhance marginal adaptation [42]. However, this potential advantage must be balanced against lower intrinsic strength: in the present study, Group C—despite its reported high filler content (~79 wt%)—showed the lowest FS. The literature emphasizes that mechanical performance depends not only on filler content but also on filler characteristics, resin–matrix composition, photoinitiator efficiency, and degree of conversion [43]. Overall, not only the quantity but also the quality of the filler and matrix (particle size, distribution, coupling, and network formation) influences mechanical behavior [38].

A slight discrepancy was observed compared with some previous reports. While certain studies found statistically significant differences among all tested BFCRs [44], in the present investigation, Group A clearly outperformed both Groups B and C (p < 0.001), but no significant differences in FS were detected between Groups B and C. This suggests that, under the standardized conditions employed, both materials exhibited comparable performance in terms of strength, despite the higher reported filler content of Group C. One possible explanation is the intrinsic variability of each material. Factors such as curing efficiency at depth, polymerization shrinkage, filler dispersion, and degree of conversion may have influenced the results and balanced their relative performances [39]. Despite these divergences, the overall trend of our findings supports Group A as the most mechanically robust material among the evaluated BFCRs, in agreement with published evidence.

Furthermore, the results reaffirm that modern BFCRs can achieve mechanical properties comparable to those of conventional composites, provided that curing protocols and increment thicknesses are strictly followed [17]. This observation is consistent with recent systematic reviews reporting that BFCRs perform similarly to conventional resins in both laboratory and clinical settings [15].

As with any in vitro study, the findings should be interpreted with caution, considering their inherent limitations. The three-point bending test under monotonic load, although widely used to evaluate the mechanical properties of dental materials, does not fully reproduce the complex intraoral environment. In the oral cavity, restorative materials are subjected to cyclic and multidirectional forces, constant humidity, thermal variations, and prolonged exposure to a moist environment—all of which can significantly affect their clinical performance. Moreover, critical properties for functional longevity, such as fatigue resistance and wear behavior, were not assessed in this study [45]. Therefore, future research should incorporate more realistic testing protocols, including fatigue testing under repeated loading, thermocycling, and chewing simulation, to more accurately reproduce intraoral conditions and better approximate the clinical behavior of restorative materials.

A relevant limitation of this study is that the specimens were analyzed 24 h after polymerization, in accordance with ISO 4049, but without exposure to long-term aging. This procedure is commonly accepted in the scientific literature as an initial reference condition for the standardized comparison of materials, although it does not fully reproduce the complex conditions of the intraoral environment. In the oral cavity, restorative materials are subjected to thermal fluctuations, constant humidity, and cyclic loading, all of which can alter their properties over time [46,47].

Several studies have demonstrated that processes such as water sorption and thermocycling significantly influence the structural stability and mechanical performance of resin-based composites. Water absorption can cause plasticization of the polymeric matrix, volumetric expansion, and hydrolytic degradation, reducing both flexural strength and surface microhardness [48,49]. Likewise, repeated exposure to thermal changes and cyclic mechanical loads can induce microcracks and progressive loss of interfacial integrity, thereby compromising the functional durability of the materials—even in bulk-fill composites specifically designed to withstand greater application thicknesses [50].

Therefore, future research is recommended to incorporate standardized aging protocols, such as controlled thermal cycling, prolonged water storage, or dynamic fatigue testing, in order to more realistically reproduce intraoral conditions and assess the long-term durability of bulk-fill composites under clinically relevant scenarios.

Another limitation of the present study is the relatively small sample size (n = 10 per group). This number is common in in vitro investigations on the mechanical properties of restorative dental materials and can provide acceptable statistical power under certain conditions [51]. However, its use restricts the precision of the estimates and the ability to detect subtle differences among groups. While the post hoc analysis indicated a statistical power above 0.80, a larger number of replicas would strengthen the internal validity and generalizability of the results. Therefore, it is recommended that future studies perform prior power analyses to determine the optimal sample size based on the expected effect size, data variability, and desired statistical power.

Bias control measures were implemented, including standardized specimen preparation and testing conditions, as well as operator blinding during data collection. Such precautions reduce the risk of measurement bias and ensure fair comparison among groups [52]. However, it must be emphasized that laboratory conditions represent an “ideal” scenario, where variables such as temperature, humidity, and dynamic occlusal loads are controlled [53].

A methodological aspect not addressed in this study was the microstructural and spectroscopic characterization, which is essential for correlating material composition with mechanical performance. No analyses were conducted using scanning electron microscopy (SEM) or energy-dispersive spectroscopy (EDS), techniques that allow for the observation of the morphology and distribution of the inorganic filler, as well as the quality of the matrix–filler interface—factors that are decisive for the strength and durability of composites [54]. Likewise, the absence of techniques such as Fourier transform infrared spectroscopy (FTIR) or thermogravimetric analysis (TGA) limited the ability to assess the degree of conversion and filler content, parameters closely related to the physicomechanical properties of restorative resins [55]. The integration of these approaches in future investigations would enable a deeper understanding of the structural mechanisms underlying the differences in mechanical behavior among bulk-fill materials.

Future studies incorporating aging protocols (e.g., intensive thermocycling, long-term water storage) and complementary tests (compressive strength, fatigue, surface hardness, and adhesion to dentin) are needed to better approximate the clinical behavior of these restorative materials [56].

This study also contributes by including Group C (Opus Bulk Fill, FGM) in the comparison. Most international literature on BFCRs has focused on global brands such as 3M, Ivoclar, and Dentsply, while data on regional Latin American materials remain scarce. By documenting the mechanical performance of Group C alongside well-established references such as Group A and Group B, this study partially addresses that gap.

5. Conclusions

All three BFCRs exceeded the ISO 4049 threshold of 50 MPa but exhibited distinct mechanical behaviors relevant to clinical selection. Group A, with the highest FS (~142 MPa) and EM (~10.5 GPa), may be more suitable for posterior restorations under high occlusal loads. Group B demonstrated an intermediate profile, balancing stiffness and deformation capacity, while Group C, although less strong, showed the highest ε_max (~4%), suggesting potential stress-dissipating benefits in deep or thin-walled cavities. The positive correlation between FS and EM, and the inverse correlation between EM and ε_max, underline the decisive role of filler content and quality. Nevertheless, as results were obtained under monotonic loading and without artificial aging, extrapolation to long-term clinical behavior should be made with caution. Future investigations should incorporate fatigue testing, thermocycling, and clinical follow-up to validate the durability of these materials.