Microhardness evaluation of conventional and bulk-fill flowable and packable composites according to thickness and surface parameters

Authors

Abstract

AimTo evaluate the microhardness of different bulk-fill composites, according to different thickness and surface parameters.
MethodsConventional and bulk-fill composite disks were prepared, and Vickers microhardness of 2 mm and 4 mm samples was measured from top and bottom surfaces (200 g, 10 s).
ResultsMicrohardness was significantly correlated with thickness for OPUS Bulk Fill Flow (r = 0.855; p<0.01) and Venus Bulk Fill Flow (r = 0.547; p<0.01). Microhardness was significantly correlated with surface for Polofil NHT (r = 0.638; p<0.01), Clearfil Majesty Posterior (r = 0.686; p<0.01), Clearfil Majesty Flow (r = 0.530; p<0.01) and Tetric N Ceram Bulk Fill (r = 0.722; p<0.01). Effect of thickness on microhardness was significant for Polofil NHT ([B] = [-] 13.069; p<0.05), Clearfil Majesty Posterior ([B] = [-] 23.657; p<0.05), OPUS Bulk Fill Flow ([B] = [-] 1697.438; p<0.01), Tetric N Ceram Bulk Fill (OR = 15.072; p<0.05) and Venus Bulk Fill Flow ([B] = [-] 13.501; p<0.01). Effect of surface on microhardness was significant for Polofil NHT ([B] = [-] 29.183; p<0.01), Clearfil Majesty Posterior ([B] = [-] 43.096; p<0.01), Clearfil Majesty Flow ([B] = [-] 26.231; p<0.05), and Tetric N Ceram Bulk Fill ([B] = [-] 41.978; p<0.01).
ConclusionThe microhardness of the resin composite was significantly affected by material type, thickness, and surface location.

Keywords

Introduction

Bulk-fill composites have been developed to make clinical applications more practical and reduce processing time.^1,2,3 Bulk-fill composites are produced to be polymerized in a single step to a thickness of 4-5 mm.^4,5 Bulk-fill composites are divided into two main groups. Bulk-fill flowable composites have low viscosity, thus providing good cavity adaptation. Bulk-fill packable composites have a high filler content and can be applied to the cavity as a single piece.^6,7,8
One of the most commonly used methods to evaluate the polymerization quality of dental composites is the Vickers microhardness (VHN) test. Since VHN shows a strong correlation with the degree of conversion (DC) value of the composites, it is considered an indirect indicator of polymerization efficiency. Higher VHN values are generally associated with better polymerization and superior mechanical properties.^9,10,11
In this context, the international organization for standardization (ISO) 4049 standard states that the bottom surface/top surface hardness ratio for light-polymerized dental composites should be at least 80% (VHN-80 criterion). The bottom surface VHN value of a 4 mm thick polymerized composite should be at least 80% of the top surface value.^12,13,14
There are a limited number of studies that evaluate both conventional and bulk-fill and packable composite types together. This study aims to evaluate the VHN of conventional packable and flowable composite resins (Clearfil Majesty Posterior, Polofil NHT, Tetric N Ceram Bulk Fill) and bulk-fill packable and flowable composite resins (Clearfil Majesty Flow, OPUS Bulk Fill Flow, Venus Bulk Fill Flow) using multivariate analysis.

Materials and Methods

In the present study, conventional packable and flowable composites (Clearfil Majesty Posterior, Polofil NHT, Tetric N Ceram Bulk Fill, as well as bulk-fill packable and bulk-fill flowable composites (Clearfil Majesty Flow, OPUS Bulk Fill Flow, Venus Bulk Fill Flow) were evaluated. The details of the tested composites are shown in Supplementary Table 1.
Sample PreparationFor microhardness ratio assessment, six specimens were prepared for each material at each thickness (n = 6). The sample size was selected to be consistent with comparable in-vitro microhardness studies and to allow multiple indentations per surface while maintaining feasibility and standardization.
To prepare the samples, cylindrical molds with a diameter of 5 mm and thicknesses of 2 mm and 4 mm were used. For VHN ratio evaluation, six samples were prepared for each composite material at each thickness. The composite resin was placed into the molds, covered with a transparent strip tape, and gently flattened to ensure a uniform surface. Excess material was removed, and a glass slide was placed over the strip of tape to obtain a smooth surface. All samples were light-cured for 20 seconds using a 1200 mW/cm² LED light curing unit (BluePhase N, Ivoclar Vivadent AG, Schaan, Liechtenstein) with a spectral emission range of approximately 385–515 nm. The irradiance output (1200 mW/cm²) was verified using a calibrated dental radiometer prior to specimen preparation to ensure standardized light intensity. Light exposure parameters were selected considering reported effects of curing time and intensity on thermal and polymerization outcomes.¹⁵ After polymerization, the samples were carefully removed from the molds, and the lateral surfaces were marked to distinguish the top and bottom sides. Both top and bottom surfaces were sequentially polished using 800-, 1000-, and 1200-grit silicon carbide abrasive papers (Struers, Ballerup, Denmark) under continuous water irrigation to obtain standardized smooth surfaces. Each abrasive paper was applied for 20 seconds under light and consistent manual pressure. After each polishing step, specimens were rinsed with distilled water to remove debris. All polishing procedures were performed by the same operator to ensure standardization. Following specimen preparation, all samples were stored in distilled water at 37 °C in a dark environment for 24 h prior to hardness testing.
Specimens were assigned to material and thickness groups using a computer-generated randomization list. Prior to testing, all specimens were coded with anonymized identification numbers. Microhardness measurements were performed in randomized order by a single operator who was blinded to group allocation throughout the measurement procedure. The specimen codes were revealed only after completion of data recording.
MeasurementsThe microhardness of the samples was measured at three points on the bottom and top surfaces using the Vickers hardness measuring device (HVS 1000 Microhardness Tester, Bulut Makina, Türkiye) with a load of 200 g and a waiting time of 10 seconds determined by taking the average of the 3 measurements. The device was calibrated prior to testing according to the manufacturer’s instructions. The ceiling and floor surface hardness of each sample was
Polymerized samples that were to be measured for microhardness and soaked in distilled water for 24 hours.
Rectangular-shaped notches were made with a Vickers notching tip positioned perpendicular to the surfaces of the samples. After the notch was opened, the diagonal lengths of the quadrilateral notches consisting of samples placed under the x40 magnifying lens of the microhardness device were manually determined with the help of the arms of the hardness device moving in the x-y-z plane, and the VHN value was automatically calculated by the device. For each specimen, three indentations were performed on each surface (top and bottom), and the mean of the three readings was used as the surface microhardness value. The arithmetic mean of the three measurements was calculated and used for statistical analysis (Supplementary Table 5).
The bottom/top (B/T) microhardness ratio was calculated for each specimen as Bottom VHN/Top VHN. A ratio ≥ 0.80 was considered indicative of adequate depth of cure; this threshold was applied at the specimen level and then summarized as mean ± SD and as the percentage of specimens meeting the criterion.
Ethical ApprovalThis study did not require ethical approval according to the relevant guidelines.
Statistical AnalysisScale parameter descriptions were given by means, standard deviations, median, and ranges. Normality was assessed using the Kolmogorov–Smirnov test. As the data were not normally distributed (p < 0.05), non-parametric analyses were preferred. Mann-Whitney U, Kruskal-Wallis, and Jonckheere-Terpstra tests were used for difference and post hoc analysis.¹⁶ Spearman’s rho was used for correlation analysis, and a Generalized Linear Model was employed to evaluate the independent effects of thickness and surface. All analysis were performed at SPSS 25.0 for Windows at 95% Confidence Interval with a 0.05 significance level.
Reporting GuidelinesThis study is reported in accordance with the STROBE guidelines.

Results

For all resin samples, Vickers microhardness values for the top were higher than the bottom values for a 2 mm thickness. Difference analysis between top and bottom applications showed that only Venus Bulk Fill Flow samples had insignificant differences between top and bottom values for within-group differences (p<0.05) for a 2 mm thickness. For all samples except Venus Bulk Fill Flow, top and bottom value differences were significant (p<0.05) for 2 mm thickness. For bottom and top applications for 2 mm thickness, differences between resin groups were statistically significant (p<0.05). Clearfil Majesty Posterior had the highest microhardness for both bottom and top applications, and Venus Bulk Fill Flow had the lowest hardness value (p<0.05) (Supplementary Table 2).
For a 4 mm thickness, bottom and top microhardness values of resins were significantly different for Polofil NHT, Clearfil Majesty Posterior, and Tetric N Ceram Bulk Fill (p<0.05), with higher values on the top measurements. Differences for bottom and top measurements for 4 mm thickness were significant (p<0.05). In both top and bottom measurements, the highest microhardness was measured from OPUS Bulk Fill Flow samples, whereas the lowest values were measured from Venus Bulk Fill Flow (Supplementary Table 3).
Spearman’s rho correlation analysis results showed that hardness was significantly correlated with thickness for OPUS Bulk Fill Flow (r = 0.855; p<0.01) and Venus Bulk Fill Flow (r = 0.547; p<0.01). Microhardness values were significantly correlated with surface for Polofil NHT (r = 0.638; p<0.01), Clearfil Majesty Posterior (r = 0.686; p<0.01), Clearfil Majesty Flow (r = 0.530; p<0.01), and Tetric N Ceram Bulk Fill (r = 0.722; p<0.01) (Table 1).
Generalized linear analysis (logit) results showed that effect of thickness on hardness was significant for Polofil NHT ([B] = ⁰ 13.069; p<0.05), Clearfil Majesty Posterior ([B] = ⁰ 23.657; p<0.05), OPUS Bulk Fill ([B] = ⁰ 1697.438; p<0.01), Tetric N Ceram Bulk Fill ([B] = 15.072; p<0.05) and Venus Bulk Fill ([B] = ⁰ 13.501; p<0.01). Effect of surface on hardness was significant for Polofil NHT ([B] = ⁰ 29.183; p<0.01), Clearfil Majesty Posterior ([B] = ⁰ 43.096; p<0.01), Clearfil Majesty Flow ([B] = ⁰ 26.231; p<0.05), and Tetric N Ceram Bulk Fill ([B] = ⁰ 41.978; p<0.01) (Supplementary Table 4).
VHN analysis revealed material-dependent differences in curing efficiency at 4 mm thickness. Flowable and bulk-fill materials, including Clearfil Majesty Flow, OPUS Bulk Fill, and Venus Bulk Fill, demonstrated acceptable microhardness ratios (≥0.80), indicating adequate depth of cure. In contrast, the microhybrid resin composites Polofil NHT and Clearfil Majesty Posterior exhibited lower ratios (≈0.74), suggesting insufficient polymerization at the bottom surface. Notably, Tetric N-Ceram Bulk Fill showed the lowest microhardness ratio (0.54), indicating inadequate curing performance despite its bulk-fill classification. The detailed data are provided in Supplementary Tables 1-5.

Discussion

In the current study, one microhybrid resin composite, Clearfil Majesty Posterior and flowable (Polofil NHT, Tetric N Ceram Bulk Fill, Venus Bulk Fill, Clearfil Majesty Flow, and OPUS Bulk Fill Flow), were evaluated. This study revealed that the microhardness values of resin composites with different viscosities and composite classes at 2 mm and 4 mm thicknesses depend not only on the "bulk-fill" or "conventional" classification but also largely on the material-specific content. Microhardness measurements were determined using the internationally accepted three-point test, and a microhardness ratio of ≥0.80 on the top/bottom surface is associated with clinically adequate polymerization.¹⁷
Yang et al,¹⁸ in their study on two bulk-fill resin composites (Filtek Bulk Fill Posterior, Tetric N-Ceram Bulk Fill) and two conventional resin composites (Z100, Spectrum TPH), found that Vickers microhardness was affected by material type and thickness. A reduction in the degree of conversion was observed in bulk-fill resin composites between 1 mm and 4 mm, but both were above 55%. The results were similar in the present study. Jang et al.¹³, who stated that the bottom surface microhardness of bulk-fill flowable materials such as Venus Bulk Fill and SDR can exceed the VHN-80 threshold.
The light-curing protocol critically affects polymerization and depth of cure. Alsafadi et al.¹⁵ reported that variations in light intensity and exposure time significantly impact curing depth and clinical safety. Thus, standardized curing parameters and verified irradiance are essential, especially for bulk-fill use. In the present study, both top and bottom measurements showed that the highest microhardness was measured from OPUS Bulk Fill Flow samples, whereas the lowest values were measured from Venus Bulk Fill Flow. This may be due to the difference in composition (~70 wt%/~45 vol%; ~65 wt%/~38 vol%). This is consistent with the findings of Gonçalves et al,¹⁹ who emphasized that each bulk-fill material does not provide the same level of homogeneous conversion. In particular, the fact that OPUS Bulk Fill surpassed all materials in 4 mm bottom surface measurements suggests that this material may have strong polymerization efficiency at depth. However, the significant relationship between thickness and microhardness in OPUS Bulk Fill Flow in Spearman correlation and multivariate analyses indicates that the performance of this material may be related to thickness. In the literature, Pérez-Pachas et al.²⁰ reported that preheating in OPUS Bulk Fill did not significantly increase deep polymerization and could negatively affect surface microhardness under some conditions. Therefore, while the high microhardness values of OPUS Bulk Fill support the potential performance of the material, they also highlight the importance of light protocol and thickness standardization in clinical practice.
Thickness-hardness relationships exhibited material-dependent behavior. The fact that a significant relationship between thickness and hardness was detected only in OPUS Bulk Fill Flow and Venus Bulk Fill Flow suggests that the polymerization kinetics of flowable bulk-fill materials may be more sensitive to thickness. In contrast, the lack of a significant thickness effect in Clearfil Majesty Flow suggests that this material offers a more surface-dependent hardness profile. These findings conceptually align with the study by Yu et al.²¹ who reported that the difference in the degree of polymerization conversion between 2 mm and 4 mm in bulk-fill composites is limited, but a decrease can be observed at deeper levels.

Limitations

This study has limitations. While the sample size may limit detection of small differences, large effect sizes support adequate power for clinically relevant findings. The in vitro design does not reflect clinical conditions, where factors such as cavity geometry and temperature affect light transmission. Bulk-fill use alone does not ensure sufficient deep polymerization; microhardness depends on material properties. Clinically, incremental placement remains safer, especially for packable composites, and material selection should consider product-specific behavior rather than the “bulk-fill” label.

Conclusion

Within the limitations of this in vitro study, the microhardness of the resin composite was significantly affected by material type, thickness, and surface location. These findings highlight that curing efficiency cannot be predicted solely based on the composite category (bulk-fill vs. conventional), but is instead largely material-dependent, relating to differences in resin-matrix composition and filler properties.

Declarations

Abbreviations

B/T: bottom/top
DC: degree of conversion
ISO: international organization for standardization
LED: light-emitting diode
SD: standard deviation
SPSS: Statistical Package for the Social Sciences
STROBE: strengthening the reporting of observational studies in epidemiology
VHN: vickers microhardness

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About This Article

Received:

January 26, 2026

Accepted:

April 24, 2026

Published Online:

April 28, 2026