A significant deterioration of the properties can drastically compromise the survival rate of restorative materials. The aim of this study was to assess flexural strength and hardness of three composite classes: hybrid composite resin (HCR), nanoparticulate composite resin (NCR), and silorane-based composite resin (SBCR). One hundred specimens were prepared for hardness testing by using a split metallic mold measuring 10 mm in diameter and 2 mm deep. Twenty specimens were prepared for each restorative material, randomly assigned for storage in air, distilled water, or mineral oil. After intervals of 24 hours, 30, 60, 90, and 120 days, hardness and flexural strength tests were initially compared in two levels: “storage medium” and “time” within each material group. A two-way analysis of variance was performed (p<0.05) on the variables “material” and “storage time” (p<0.05). The HCR showed to be stable with regard to the evaluation of flexural strength and hardness (p<0.05). A significant reduction occurs for the NCR in comparison to the other groups (p<0.05). The NCR presented the lowest values of hardness and flexural strength kept on water over time. The characteristics of material showed a strong influence on the decrease of the mechanical properties analyzed.
Dental composites have been developed as an aesthetic alternative to the old amalgam-based restorative material and have become the most widely used materials in current dental practice. [
Despite the fact that technological advancements have encouraged the use of these materials in areas subjected to intense functional stress, physicochemical stability is necessary for these products to have acceptable longevity. Among the most significant factors contributing to the failure of composite resins over time are the loss of brightness, dental stains, marginal infiltration, recurrent decay, dental wear, and fractures. [
Industrial innovations such as nanotechnology and replacement of conventional monomers have promised to improve the mechanical quality of composite resins. Particle size changes and increased load in the composition, which would include the main physical properties of each resin class, such as maintaining nanoparticle polishing and resistance to wear and fracture in hybrid resins, have turned them into one of the most commonly studied materials.[
With regard to the monomers that make up the composite resins, one of the most recent studies within the field of dental materials was the introduction of silorane-based resins as a substitute for methacrylate-based resins. [
A proper balance of mechanical properties should be established over time, given that a significant deterioration of the properties can drastically compromise the success rate of restorative materials. Because of the intense marketing by the dental industry, there is a fundamental necessity that in vitro and in vivo analyses clarify the mechanical and physical properties of technology-based nanoparticles in resin composite after storage for a long period. The aim of this study was to assess flexural strength and hardness of three composite classes: (a) hybrid, (b) nanoparticulate, and (c) silorane-based composites. The authors tested the null hypothesis that stated that there is no difference in flexural strength and hardness for the three composite classes, and behavior of the tested materials is similar for the three storage mediums at different storage times.
Several materials were used to carry out this study as shown in Table
Composite resin systems investigated in this study.
Composite / |
Composition of the organic matrix |
Inorganic phase composition |
Color | Manufacturing lot number | commercial classification |
---|---|---|---|---|---|
Filtek Z-100 |
Bis-GMA, TEGDMA | zirconia/silica |
A 3.5 | 7EP | Hybrid |
71% by volume | |||||
Filtek Z-350 |
Bis-GMA, UDMA |
zirconia/silica |
A 3.5 | 9AK | Nanocomposite |
59.5% by volume | |||||
Filtek P-90 |
Silorane | quartz |
A 3.5 | N130928 | Silorane-based |
58%, by volume |
Organic matrix composition and compressive load were provided by the manufacturer.
The experimental work was performed in accordance with the method described in ADA Specification No. 27 for resin-based filling composite materials, which is identical to ISO 4049:2009 (§7.12). [
One hundred specimens were prepared for hardness testing by using a split metallic mold measuring 10 mm in diameter and 2 mm deep. Masses of dental composites were compressed by using a glass microscope slide and a 1 kg metal disk with a hole in the center for placing the curing-light tip (Valo®, Ultradent, South Jordan, UT, USA) with a minimum light intensity of 500mW/cm2 measured by a radiometer (curing radiometer, Demetron/Kerr, Danbury, CT, USA). The polymerization time was 40 seconds through the glass slide and 40 seconds directly on the specimens. Twenty specimens were prepared for each restorative material (n=5), randomly assigned for storage in air, distilled water, or mineral oil (Nujol®, Schering-Plough, Rio de Janeiro, Brazil). The specimens remained in an incubator at 37±1°C throughout the experiment.
Readings were made after periods of 24 hours, 30, 60, 90, and 120 days by using a durometer (Buehler, Lake Bluff, USA) equipped with a Vickers diamond and a compression load of 100 gf applied for 30 seconds. Measurements were obtained through two dents per quadrant, totaling eight readings for each specimen.
A split metallic stainless steel bar matrix 12 mm long, 2 mm wide, and 1 mm thick was used to prepare the samples for flexural strength testing (n=450). It was embedded into a rectangular aluminum metallic base. The composite resin materials were applied into the root cavity and compressed by using a glass microscope slide and a metal disk with a mass of 1 kg, which had a hole in the center for placing the curing-light tip (Valo®, Ultradent, South Jordan, UT, USA) with a minimum light intensity of 500mW/cm2 measured by a radiometer (curing radiometer, Demetron/Kerr, Danbury, CT, USA). Polymerization time was 120 seconds, with 40 seconds for each segment of approximately 4 mm long. One hundred and fifty specimens of each material were prepared (n=10) and were randomly assigned for storage in air, distilled water, or mineral oil (Nujol®, Schering-Plough, Rio de Janeiro, Brazil). After intervals of 24 hours, 30, 60, 90, and 120 days, 10 specimens were randomly selected for flexural strength testing. Throughout the experiment, all specimens remained in the respective storage mediums in light-protected containers and were incubated at 37±1°C.
For the three-point-bend strength test, two metal devices were used, composed of a table with two cylindrical support sections of 1.6 mm in diameter, 10 mm apart from each other, and a beam or rod for the application of compressive stress in the center of the upper face of the specimen, with an active tip cylindrical section, 3.2 mm in diameter. Assays were performed by using an MTS 810 machine (MTS Systems Corp., Eden Prairie, MN, USA) with 10 kN load cell and a crosshead speed of 0.5 mm/min. The following equation was used to calculate flexural strength values: RF=3LF/2bh2, where RF is the flexural strength, (MPa); L is the span length between the supports, (mm); F is the applied load at fracture, (N); b is the width of the specimen, (mm); h is the thickness of the specimen, (mm).
Hardness and flexural strength tests were initially compared in two levels, “storage medium” and “time” within each material group.
For power analysis, sample size was calculated on the basis of previous sample size calculations performed in similarly designed studies. Based on previous investigations, a power analysis determined that, for an alpha value of 5% and a power of 80%, a sample size of 10 specimens per group would be required. A two-way analysis of variance was performed (p<0.05). Comparisons among the restorative materials were subsequently made for each mechanical property, only in the early periods, prior to immersion, and at the end of the process, after 120 days, in distilled water and mineral oil. New two-way analyses of variance were performed on the variables “material” and “storage time” (p<0.05). Additional Tukey tests were performed in all analyses, with the same level of significance used throughout the experiment (p<0.05).
Table
Mean, standard error, and critical value as measured by the Tukey test for hardness values of the interaction between “storage medium” and “time” of HCR group (VHN).
24 hours | 30 days | 60 days | 90 days | 120 days | |
---|---|---|---|---|---|
AIR | 86.6 (a) |
80.9 (a) |
80.6 (a) |
82.2 (a) |
82.1 (a) |
WATER | 82.7 (a) |
80.7 (a) |
79.1 (a) |
79.0 (a) |
79.0 (a) |
MINERAL OIL | 59.0 (a) |
58.3 (a) |
57.5 (a) |
57.4 (a) |
57.3 (a) |
Standard error = 2.00 |
Critical value of 5% = 10.0. Horizontal, lowercase letters (same material, different storage conditions). Vertical, capital letters (same storage condition, different materials).
Table
Mean, standard error, and critical value as measured by the Tukey test for the flexural strength values of the interaction between “storage medium” and “time” of HCR group (MPa).
24 hours | 30 days | 60 days | 90 days | 120 days | |
---|---|---|---|---|---|
AIR | 94.2 (a) |
75.1 (a) |
73.7 (a) |
72.0 (a) |
70.7 (a) |
WATER | 90.4 (a) |
77.8 (a) |
74.2 (a) |
72.6 (a) |
71.5 (a) |
MINERAL OIL | 86.3 (a) |
77.6 (a) |
74.7 (a) |
74.1 (a) |
71.4 (a) |
standard error = 5,67 |
Critical value of 5% = 27.51. Horizontal, lowercase letters (same material, different storage conditions). Vertical, capital letters (same storage condition, different materials).
Table
Mean, standard error, and critical value as measured by the Tukey test for hardness values of the interaction between “storage medium” and “time” of NCR group (VHN).
24 hours | 30 days | 60 days | 90 days | 120 days | |
---|---|---|---|---|---|
AIR | 66.7 (a) |
62.2 (a) |
61.0 (a) |
61.8 (a) |
61.8 (a) |
WATER | 68.2 (a) |
59.1 (b) |
59.2 (b) |
58.3 (b) |
58.3 (b) |
MINERAL OIL | 55.7 (a) |
54.9 (a) |
53.8 (a) |
53.9 (a) |
53.8 (a) |
standard error = 1.48 |
Critical value of 5% = 7.42. Horizontal, lowercase letters (same material, different storage conditions). Vertical, capital letters (same storage condition, different materials).
Table
Mean, standard error, and critical value as measured by the Tukey test for flexural strength values of the interaction between “storage medium” and “time” of NCR group (MPa).
24 hours | 30 days | 60 days | 90 days | 120 days | |
---|---|---|---|---|---|
AIR | 105.6 (a) |
59.3 (b) |
58.6 (b) |
58.0 (b) |
57.1 (b) |
WATER | 71.6 (a) |
63.1 (a) |
29.5 (b) |
29.4 (b) |
28.9 (b) |
MINERAL WATER | 82.9 (a) |
68.5 (ab) |
57.6 (b) |
58.3 (b) |
56.4 (b) |
standard error = 4.80 |
Critical value of 5% = 23.29. Horizontal, lowercase letters (same material, different storage conditions). Vertical, capital letters (same storage condition, different materials).
Table
Mean, standard error, and critical value as measured by the Tukey test for hardness values of the interaction between “storage medium” and “time” of SBCR group (VHN).
24 hours | 30 days | 60 days | 90 days | 120 days | |
---|---|---|---|---|---|
AIR | 47.1 (a) |
45.5 (a) |
45.3 (a) |
45.6 (a) |
45.6 (a) |
WATER | 44.2 (a) |
43.8 (a) |
46.9 (a) |
45.7 (a) |
45.6 (a) |
MINERAL OIL | 35.9 (a) |
35.3 (a) |
34.8 (a) |
34.9 (a) |
34.7 (a) |
standard error = 1.79 |
Critical value of 5% = 8.94. Horizontal, lowercase letters (same material, different storage conditions). Vertical, capital letters (same storage condition, different materials).
Table
Mean, standard error, and critical value as measured by the Tukey test for flexural strength values of the interaction between “storage medium” and “time” of SBCR group (MPa).
24 hours | 30 days | 60 days | 90 days | 120 days | |
---|---|---|---|---|---|
AIR | 61.2 (a) |
72.5 (a) |
66.2 (a) |
65.5 (a) |
64.5 (a) |
WATER | 62.6 (a) |
78.1 (a) |
52.4 (b) |
50.8 (b) |
49.9 (b) |
MINERAL OIL | 58.1 (a) |
63.3 (a) |
56.5 (a) |
55.1 (a) |
54.4 (a) |
standard error = 4.24 |
Critical value of 5% = 20.55. Horizontal, lowercase letters (same material, different storage conditions). Vertical, capital letters (same storage condition, different materials).
A comparison between the composite resins was made by using the Tukey test (p<0.05). Table
Mean, standard error, and critical value as measured by the Tukey test for hardness values of the interaction between “type of material” and “time/storage medium” (VHN).
BASELINE |
120 DAYS |
120 DAYS | |
---|---|---|---|
HCR | 86.6 (a) |
79.0 (a) |
57.3 (b) |
NCR | 66.7 (a) |
58.3 (b) |
53.8 (b) |
SBCR | 47.2 (a) |
45.6 (a) |
34.7 (b) |
standard error = 1.879 |
Critical value of 5% = 8.198. Horizontal, lowercase letters (same material, different storage conditions). Vertical, capital letters (same storage condition, different materials).
The HCR group showed to be stable with regard to the “time/storage medium” factor in the evaluation of flexural strength (Table
Mean, standard error, and critical value as measured by the Tukey test for flexural strength values of the interaction between “type of material” and “time/storage medium” (MPa).
BASELINE |
120 DAYS |
120 DAYS | |
---|---|---|---|
HCR | 94.2 (a) |
71.5 (a) |
71.4 (a) |
NCR | 105.6 (a) |
28.9 (b) |
56.4 (b) |
SBCR | 61.2 (a) |
49.9 (a) |
54.4 (a) |
Standard error = 8,50 |
Critical value of 5% = 38.33. Horizontal, lowercase letters (same material, different storage conditions). Vertical, capital letters (same storage condition, different materials).
The results from this study rejected the null hypothesis in relation to the type of material and storage medium. The different types of material demonstrated significant action on the hardness and flexural strength values in relation to the storage times and storage mediums studied (p<0.05). Previous works have demonstrated deterioration of composite resins when immersed in different mediums. [
In order to promote common challenges in the oral environment, different immersion was used, especially in the possibility of promoting an antagonistic action between an aqueous medium (water) and a lipoic environment, as suggested in previous studies. [
The adverse effect of water is particularly reported by several authors. [
The NCR group, at 60% compressive load, composed of silica agglomerates/zirconia and dimethacrylate-based matrices (Bis-GMA, UDMA, Bis-EMA, and TEGDMA), proved to be affected by all three storage mediums: air (5% reduction and stabilization in 7 days), water (14% reduction and stabilization in 7 days), and mineral oil (16% reduction and stabilization in 24 hours). However, the HCR group, at 60% compression load, composed of silica/zirconia and dimethacrylate-based matrices (Bis-GMA, UDMA, Bis-EMA, and TEGDMA), proved to be affected only by the mineral oil and with the same degree of reduction (16%) and stabilization time (24 hours). Assuming the same monomer combination for these two materials, one can conclude that, somehow, the nanoclusters of NCR material have resulted in hardness reduction, even in dry conditions, without immersion.
In the flexural strength tests, the authors observed slightly different order from that obtained in the hardness test. Prior to storage, there was a statistical superiority for HCR group (94.2 MPa) and NCR (105.6 MPa), both dimethacrylate-based matrices. However, they had different volume content compressive strengths of 71% and 60%, respectively. The SBCR filled with fine quartz particles (61.2 MPa), even with filler content similar to that of NCR, had inferior mechanical properties. It should be noted that mechanical properties are not dependent exclusively from the filler content (inorganic phase) but an association with the monomer constitution (organic phase).
In the flexural strength tests, tensile stress is responsible for specimen fracture. Thus, in addition to cohesion of the organic phase (composition and maximum degree of conversion), the effective bonding between the matrix and filler particles provided by the silane-based agent influences the outcome. The composite amount and the total interface area, favoring propagation, might be responsible for the observed differences. Nanoparticle agglomerates used as a filler in NCR, even in a small amount, may have provided greater flexural strength. However, this composite showed significant deterioration of flexural strength after storage in water during the tested period. Considering that flexural strength of the HCR was not affected by water, a greater sensitivity to hydrolytic degradation of the silane interface can be assumed, because of the increased surface area provided by the nanoparticles, the main constituents of nanoclusters, used as filler in this material. Flexural strength of the silorane-based matrix of the SBCR group may have been affected by solubilization of monomers partially insaturated or by the breaking of macromolecules.
The tests of materials stored in air medium showed flexural strength losses of around 18% for the HCR, 43% for the NCR, and 10% for the SBCR group. According to Bijelic-Donova et al. [
Limitations of this study include the in vitro conditions, which could not completely replicate clinical conditions. Further studies should incorporate thermocycling to obtain more meaningful results. Clinical studies are important to confirm the present study results, as well as the results found by others in in vitro investigations.
Considering the limitations of this study, it is possible to conclude that the characteristics of material showed a strong influence on the decrease of the mechanical properties analyzed. Then, the mechanical tests indicated difference in flexural strength and hardness for the three composite classes, and behavior of the tested materials is similar for the three storage mediums at different storage times. The NCR group presented the lowest values of hardness and flexural strength kept on water over time.
The data used to support the findings of this study are available from the corresponding author upon request.
This study was entirely funded by the authors.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
All authors contributed equally to manuscript drafting and critical discussion and approved the final version.