Flexural strength and hardness of direct and indirect composites

The objective of this study was to evaluate the fl exural strength (σf) and hardness (H) of direct and indirect composites, testing the hypotheses that direct resin composites produce higher σf and H values than indirect composites and that these properties are positively related. Ten bar-shaped specimens (25 mm x 2 mm x 2 mm) were fabricated for each direct [D250 Filtek Z250 (3M-Espe) and D350 Filtek Z350 (3MEspe)] and indirect [ISin Sinfony (3M-Espe) and IVM VitaVM LC (Vita Zahnfabrik)] materials, according to the manufacturer’s instructions and ISO4049 specifi cations. The σf was tested in three-point bending using a universal testing machine (EMIC DL 2000) at a crosshead speed of 0.5 mm/min (ISO4049). Knoop hardness (H) was measured on the specimens’ fragments resultant from the σf test and calculated as H = 14.2P/l2, where P is the applied load (0.1 kg; dwell time = 15 s) and l is the longest diagonal of the diamond shaped indent (ASTM E384). The data were statistically analyzed using Anova and Tukey tests (α = 0.05). The mean σf and standard deviation values (MPa) and statistical grouping were: D250 135.4 ± 17.6a; D350 123.7 ± 11.1b; ISin 98.4 ± 6.4c; IVM 73.1 ± 4.9d. The mean H and standard deviation values (kg/mm2) and statistical grouping were: D250 98.12 ± 1.8a; D350 86.5 ± 1.9b; ISin 28.3 ± 0.9c; IVM 30.8 ± 1.0c. The direct composite systems examined produce higher mean σf and H values than the indirect composites, and the mean values of these properties were positively correlated (r = 0.91), confi rming the study hypotheses. Descriptors: Composite resins; Dental materials; Hardness. Márcia Borba(a) Álvaro Della Bona(b) Dileta Cecchetti(c) (a) PhD Student, Dental Materials, University of São Paulo, SP, Brazil. (b) PhD in Biomaterials, Research Coordinator and Senior Professor of Prosthodontics and Occlusion, Dental School, University of Passo Fundo, RS, Brazil. (c) MSc in Statistics, Professor, Institute of Biological Sciences, University of Passo Fundo, RS, Brazil. Dental Materials Corresponding author: Álvaro Della Bona Faculdade de Odontologia, Universidade de Passo Fundo, Campus I, B. São José, BR 285, Km 171 Passo Fundo RS Brazil CEP: 99001-970 E-mail: dbona@upf.br Received for publication on Jul 18, 2007 Accepted for publication on Dec 18, 2007 Flexural strength and hardness of direct and indirect composites Braz Oral Res 2009;23(1):5-10 6 Introduction The development of adhesive materials and techniques represents an improvement for the use of indirect restorations, encouraging the research for materials with mechanical and esthetic properties more similar to those of the tooth structure. Indirect composite resin systems represent an alternative to overcome some defi ciencies of direct composite restorations, such as polymerization shrinkage stresses, inadequate polymerization in interproximal areas, restoration of proximal contacts and adequate dental contour.1,2,3 The composition of indirect composite resin systems is similar to that of direct systems, differing by the use of different methods of additional polymerization, which allows a higher radical conversion. These additional polymerization procedures can involve photo-activation, heat, pressure, and a nitrogen atmosphere.3,4,5 In the early 1980’s, the composite inlay technique was introduced in Switzerland and France. The fi rst generation of materials was constituted of microfi lled indirect composite resin systems, with fl exural strength ranging from 60 to 80 MPa, elastic modulus around 2.0-3.5 GPa, resin volume percentage higher than 50% and microparticles with an average size of 0.04 μm. These features resulted in some disadvantages such as inadequate mechanical properties, low resistance to occlusal wear and color instability.2,3 The second generation of indirect composite resin systems was introduced in the 1990’s. These materials are microhybrid composites with a volume percentage of inorganic fi llers of approximately 66%, resulting in improved mechanical properties with fl exural strength between 120 and 160 MPa and elastic modulus of 8.5-12 GPa. These systems are indicated for inlays and onlays, laminated veneers and jacket crowns, implant-support restorations, for adjustment of proximal contacts and for reduction of occlusal stresses in bruxism cases.2,3 Other indirect composite systems recently introduced, despite their excellent esthetics, are not classifi ed as second-generation systems because they do not feature all of the required characteristics, like high mechanical properties values, high percentage of inorganic content and bond to metal.2,3 These composites are denominated intermediate laboratory composite resins. They have only a few properties of the second-generation composites but still can be effectively used in specifi c clinical situations.3 The indirect composite system Sinfony (3MESPE) is a microhybrid material developed to be applied using the layering technique because of its fl ow consistence. According to the manufacturer, this material is indicated for inlays and onlays, veneers and full crowns. This system needs two polymerization phases: an initial polymerization using the Visio Alfa unit (3M-ESPE) for 15 s for each layer, which reduces shrinkage stress; and a fi nal polymerization under light and vacuum using the Visio Beta Vario unit (3M-ESPE) for 15 min, which allows a higher monomer-polymer conversion. The new system from Vita, Vita VM LC, is a light-curing microparticle indirect composite that can be used for the full and partial veneering of crowns, and as long-term temporary metal-free restorations. According to the manufacturer, this system is composed of inorganic nano-sized fi llers that ensure high translucency due to natural refraction. The light curing box must be equipped with a light source which has a wavelength of 350-500 nm. The objective of this study was to evaluate the fl exural strength (σf) and hardness (H) of direct and indirect composites, testing the hypotheses that direct resin composites produce higher fl exural strength and hardness values than indirect composites and that these properties are positively related. Material and Methods For the three-point fl exural strength test, 10 barshaped specimens were fabricated from each direct and indirect composite system (Table 1), following the manufacturer’s instructions and the ISO4049 specifi cation.7 The composite resin was packed inside a stainless steel mold positioned on a glass slab to obtain the required dimensions (2 ± 0.1 mm x 2 ± 0.1 mm x 25 ± 2 mm). A thin glass slab was positioned on the mold containing the material, which was light cured. The upper and lower surfaces of the D250 and D350 specimens were light cured (XL1500, 3M Dental Products, St. Paul, MN, USA; intensity of Borba M, Della Bona A, Cecchetti D Braz Oral Res 2009;23(1):5-10 7 400 mW/cm2) for 40 s per unit output diameter. The polymerization procedure of the indirect composite systems followed the manufacturer’s instructions. ISin was fi rst light cured with a halogen lamp with intensity of 400 mW/cm2 (Visio Alfa unit, 3M-ESPE) for 15 s, and fi nally cured under light and vacuum (Visio Beta Vario unit, 3M-ESPE) for 15 min. The IVM specimens were fi rst light cured following the direct composite procedure and fi nally cured in 2 cycles of 4 min using a polymerization box (EDGLUX, EDG Equipamentos, São Carlos, SP, Brazil) containing four blue-light bulbs and a rotating tray, according to the manufacturer’s instructions. All specimens were immersed in 37°C water for 15 min and polished with 600 to 1,200 grit SiC paper to remove excess material, following ISO4049 specifi cation.8 The dimensions of the specimens were verifi ed using a digital caliper (Digimatic caliper, Mitutoyo Corp., Tokyo, Japan). After storage in distilled water at 37oC for 24 h, the specimens were submitted to the three-point bending test in a universal testing machine (EMIC DL 2000, São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min until fracture. The fl exural strength values (σf), in MPa, were calculated as follows: σf = 3PL/ 2wb2 where: P is the fracture load (N); L is the distance between the supporting rollers (20 mm); w is the specimen height (in mm); b is specimen width (in mm). Knoop hardness (H) was measured on the specimens’ fragments (n = 10) resultant from the fl exural strength test. Three indentations (load of 100 g, dwell time of 15 s) were made in each specimen and the diagonals were measured with an optical microscope (HMV-2T, Shimadzu Corporation, Tokyo, Japan) using 400 X magnifi cation. Knoop hardness was calculated as follows (ASTM E384): H = 14.2P/l2 where: P is the applied load (0.1 kg); l is the longest diagonal (in mm) of the diamond shaped indent. The results were statistically analyzed using analysis of variance (ANOVA) and Tukey’s test at a signifi cance level of 5%. Pearson’s correlation test was used to determine the relationship between the two properties. Table 1 Information on the composite resin systems investigated in this study. Composite resin Material (manufacturer) Shade, batch Composition of the organic matrix* Composition of the filler content* D250 Filtek Z250 (3M-Espe, St. Paul, MN, USA) A2, 5CB Bis-GMA, UDMA, Bis-EMA, TEGDMA Zirconia and silica (0.01 3.5 μm) – 82% by weight D350 Filtek Z350 (3M-Espe, St. Paul, MN, USA) A2, 5BK Bis-GMA, UDMA, Bis-EMA, TEGDMA Zirconia/silica filler (0.6 1.4 μm) and nanoparticles of silica (20 nm) – 78.5% by weight ISin Sinfony (3M-Espe, St. Paul, MN, USA) A2, 210476 Bis-GMA Aluminum glass and SiO2 (0.6 μm) – 50% by weight6 IVM Vita VM LC (Vita Zahnfabrik, Bad Sackingen, Germany) A2, 7553 45-48% by weight * The composition of the organic matrix and fillers was provided by the manufacturers. Table 2 Mean and standard deviation values of flexural strength (σf) and hardness (H), and statistical grouping. Groups σf (MPa) H (kg/mm2) D250 135.4 ± 17.6 A 98.1 ± 1.8 a D350 123.7 ± 11.1 B 86.5 ± 1


Flexural strength and hardness of direct and indirect composites
Abstract: The objective of this study was to evaluate the fl exural strength (σ f ) and hardness (H) of direct and indirect composites, testing the hypotheses that direct resin composites produce higher σ f and H values than indirect composites and that these properties are positively related.Ten bar-shaped specimens (25 mm x 2 mm x 2 mm) were fabricated for each direct [D250 -Filtek Z250 (3M-Espe) and D350 -Filtek Z350 (3M-Espe)] and indirect [ISin -Sinfony (3M-Espe) and IVM -VitaVM LC (Vita Zahnfabrik)] materials, according to the manufacturer's instructions and ISO4049 specifi cations.The σ f was tested in three-point bending using a universal testing machine (EMIC DL 2000) at a crosshead speed of 0.5 mm/min (ISO4049).Knoop hardness (H) was measured on the specimens' fragments resultant from the σ f test and calculated as H = 14.2P/l 2 , where P is the applied load (0.1 kg; dwell time = 15 s) and l is the longest diagonal of the diamond shaped indent (ASTM E384).The data were statistically analyzed using Anova and Tukey tests (α = 0.05).The mean σ f and standard deviation values (MPa) and statistical grouping were: D250 -135.4 ± 17.6a; D350 -123.7 ± 11.1b; ISin -98.4 ± 6.4c; IVM -73.1 ± 4.9d.The mean H and standard deviation values (kg/mm2) and statistical grouping were: D250 -98.12 ± 1.8a; D350 -86.5 ± 1.9b; ISin -28.3 ± 0.9c; IVM -30.8 ± 1.0c.The direct composite systems examined produce higher mean σ f and H values than the indirect composites, and the mean values of these properties were positively correlated (r = 0.91), confi rming the study hypotheses.

Introduction
The development of adhesive materials and techniques represents an improvement for the use of indirect restorations, encouraging the research for materials with mechanical and esthetic properties more similar to those of the tooth structure.Indirect composite resin systems represent an alternative to overcome some defi ciencies of direct composite restorations, such as polymerization shrinkage stresses, inadequate polymerization in interproximal areas, restoration of proximal contacts and adequate dental contour. 1,2,3he composition of indirect composite resin systems is similar to that of direct systems, differing by the use of different methods of additional polymerization, which allows a higher radical conversion.These additional polymerization procedures can involve photo-activation, heat, pressure, and a nitrogen atmosphere. 3,4,5n the early 1980's, the composite inlay technique was introduced in Switzerland and France.The fi rst generation of materials was constituted of microfi lled indirect composite resin systems, with fl exural strength ranging from 60 to 80 MPa, elastic modulus around 2.0-3.5 GPa, resin volume percentage higher than 50% and microparticles with an average size of 0.04 μm.These features resulted in some disadvantages such as inadequate mechanical properties, low resistance to occlusal wear and color instability. 2,3he second generation of indirect composite resin systems was introduced in the 1990's.These materials are microhybrid composites with a volume percentage of inorganic fi llers of approximately 66%, resulting in improved mechanical properties with fl exural strength between 120 and 160 MPa and elastic modulus of 8.5-12 GPa.These systems are indicated for inlays and onlays, laminated veneers and jacket crowns, implant-support restorations, for adjustment of proximal contacts and for reduction of occlusal stresses in bruxism cases. 2,3ther indirect composite systems recently introduced, despite their excellent esthetics, are not classifi ed as second-generation systems because they do not feature all of the required characteristics, like high mechanical properties values, high percentage of inorganic content and bond to metal. 2,3These composites are denominated intermediate laboratory composite resins.They have only a few properties of the second-generation composites but still can be effectively used in specifi c clinical situations. 3he indirect composite system Sinfony (3M-ESPE) is a microhybrid material developed to be applied using the layering technique because of its fl ow consistence.According to the manufacturer, this material is indicated for inlays and onlays, veneers and full crowns.This system needs two polymerization phases: an initial polymerization using the Visio Alfa unit (3M-ESPE) for 15 s for each layer, which reduces shrinkage stress; and a fi nal polymerization under light and vacuum using the Visio Beta Vario unit (3M-ESPE) for 15 min, which allows a higher monomer-polymer conversion.
The new system from Vita, Vita VM LC, is a light-curing microparticle indirect composite that can be used for the full and partial veneering of crowns, and as long-term temporary metal-free restorations.According to the manufacturer, this system is composed of inorganic nano-sized fi llers that ensure high translucency due to natural refraction.The light curing box must be equipped with a light source which has a wavelength of 350-500 nm.
The objective of this study was to evaluate the fl exural strength (σ f ) and hardness (H) of direct and indirect composites, testing the hypotheses that direct resin composites produce higher fl exural strength and hardness values than indirect composites and that these properties are positively related.

Material and Methods
For the three-point fl exural strength test, 10 barshaped specimens were fabricated from each direct and indirect composite system (Table 1), following the manufacturer's instructions and the ISO4049 specifi cation. 7The composite resin was packed inside a stainless steel mold positioned on a glass slab to obtain the required dimensions (2 ± 0.1 mm x 2 ± 0.1 mm x 25 ± 2 mm).A thin glass slab was positioned on the mold containing the material, which was light cured.The upper and lower surfaces of the D250 and D350 specimens were light cured (XL1500, 3M Dental Products, St. Paul, MN, USA; intensity of 400 mW/cm2) for 40 s per unit output diameter.The polymerization procedure of the indirect composite systems followed the manufacturer's instructions.ISin was fi rst light cured with a halogen lamp with intensity of 400 mW/cm 2 (Visio Alfa unit, 3M-ESPE) for 15 s, and fi nally cured under light and vacuum (Visio Beta Vario unit, 3M-ESPE) for 15 min.The IVM specimens were fi rst light cured following the direct composite procedure and fi nally cured in 2 cycles of 4 min using a polymerization box (EDG-LUX, EDG Equipamentos, São Carlos, SP, Brazil) containing four blue-light bulbs and a rotating tray, according to the manufacturer's instructions.
All specimens were immersed in 37°C water for 15 min and polished with 600 to 1,200 grit SiC paper to remove excess material, following ISO4049 specifi cation. 8The dimensions of the specimens were verifi ed using a digital caliper (Digimatic caliper, Mitutoyo Corp., Tokyo, Japan).
After storage in distilled water at 37ºC for 24 h, the specimens were submitted to the three-point bending test in a universal testing machine (EMIC DL 2000, São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min until fracture.The fl exural strength values (σ f ), in MPa, were calculated as follows: where: P is the fracture load (N); L is the distance between the supporting rollers (20 mm); w is the specimen height (in mm); b is specimen width (in mm).
Knoop hardness (H) was measured on the specimens' fragments (n = 10) resultant from the fl exural strength test.Three indentations (load of 100 g, dwell time of 15 s) were made in each specimen and the diagonals were measured with an optical microscope (HMV-2T, Shimadzu Corporation, Tokyo, Japan) using 400 X magnifi cation.Knoop hardness was calculated as follows (ASTM E384): H = 14.2P/l 2 where: P is the applied load (0.1 kg); l is the longest diagonal (in mm) of the diamond shaped indent.
The results were statistically analyzed using analysis of variance (ANOVA) and Tukey's test at a signifi cance level of 5%.Pearson's correlation test was used to determine the relationship between the two properties.

Results
The mean values and standard deviations (MPa) of fl exural strength (σ f ) and hardness (H), and statistical grouping, are shown in Table 2.
The mean σ f values were signifi cantly different for all the tested groups.The direct composite systems presented statistically higher mean σ f values than the indirect composites (p < 0.05).The D250 composite exhibited the highest σ f values (135.4 ± 17.6 MPa) and the IVM composite exhibited the lowest σ f values (73.1 ± 4.9 MPa).
The direct composite systems showed the highest H mean values.The mean H value of D250 was statistically greater than that of D350 (p < 0.05), which was also signifi cantly greater than the mean H values of ISin and IVM (p < 0.05).There was no signifi cant difference between the mean H values of the indirect systems (p ≥ 0.05).
The correlation test showed a high positive correlation (r = 0.91) between the mean σ f and H values of the materials examined.

Discussion
The Sinfony (3M-ESPE) and VitaVM LC (Vita) indirect composite systems could be classifi ed as intermediate laboratory composite resins since they have lower percentage of inorganic content (50wt% and 45-48wt%, respectively) and lower values for the mechanical properties evaluated than expected for second-generation systems. 2,3he direct composite systems examined produce higher mean fl exural strength and hardness values than the indirect composites, confi rming the fi rst study hypothesis.These results are in agreement with previous studies. 4,5,9,10esar et al. 4 (2001) evaluated the fl exural strength, fl exural modulus and hardness of four indirect composite systems (Artglass, Belleglass, Sculpture and Targis) and one direct composite system (Z100).Their results partially agree with those of the present study because Z100 showed the highest mean hardness values, although this material presented a mean fl exural strength value similar to that of the indirect composite systems (Artglass, Targis and Sculpture).
A previous study showed that the mean wear val-ue of the indirect composites was signifi cantly greater than the wear of Z100.In addition, the authors reported that Targis and Z100 showed a signifi cantly greater elastic modulus than Artglass old formula and Vita Zeta LC, and Z100 showed a greater mean fracture strength value than Artglass and Vita Zeta LC. 10 Another study investigated the mechanical properties of a direct composite resin (Z100), a feldspatic porcelain (Noritake) and three indirect composites (Artglass, Solidex and Targis).The porcelain showed the greatest mean hardness value, followed by Z100.Solidex and Z100 showed greater mean compressive strength values than the other materials tested. 9oares et al. 5 (2005) evaluated the hardness and diametral tensile strength of two direct composites (TPH Spectrum and Filtek P60) and one indirect system (Solidex).The authors also reported that the direct composites showed better mechanical properties than the indirect composite system.
The objective of a secondary polymerization is to maximize the degree of conversion of composites in order to improve mechanical and physical properties, durability, solvent resistance and biocompatibility. 11The presence of unpolymerized monomer in the matrix negatively affects the properties of composite materials and may induce surface degradation and discoloration. 12,135][16] When the composite is heated to a temperature above its glass transition, there is an increase in the molecular mobility of the polymer chains.Therefore, it may be possible to further the chemical reaction by enhancing the molecular mobility of existing free radicals and other reactive species. 8he above rationale was used by some authors to improve the mechanical properties of composite materials.Freiberg, Ferracane 11 (1998) showed improvement of fracture toughness, elastic modulus, fl exural strength and hardness of composites when increasing the degree of conversion due to additional polymerization.Wendt 16 (1987) and Soares et al. 5 (2005) also concluded that an increase in the de-gree of conversion improves composites' mechanical properties, such as diametral tensile strength and hardness.Reinhardt et al. 17 (1994) observed an 11% increase in fl exural strength when a secondary curing procedure was used.
The above results and rationale are indications of improvement in the properties of indirect composite resin systems, but do not mean that indirect composites have a better clinical performance than direct composite systems.][21] The fi ller content could be an important factor when evaluating physical and mechanical properties of different composite materials.Li et al. 20 (1985) reported that increasing the fi ller content resulted in greater hardness, compressive strength, and stiffness, while water sorption decreased.
Chung 19 (1990) observed a positive relation between volume fraction of fi ller and diametral tensile strength and hardness.But no correlation was observed between the degree of conversion and the mechanical properties evaluated.Neves et al. 12 (2002) also concluded that the fi ller content directly affects the hardness values.
Other studies also investigated the association between the mechanical properties of composites and the difference in fi ller volume.The authors reported that the materials with higher fi ller volumes showed better mechanical properties. 9,17,21hese observations seem to be confi rmed by the results of this study, especially when the materials' fi ller volumes (Table 1) are compared with the mean σ f and H values (Table 2).The composite resins with higher fi ller contents, D250 (82wt%) and D350 (78.5wt%), showed signifi cantly higher mean σ f and H values than the composites with lower fi ller contents, ISin (50wt%) and IVM (45-48wt%).Therefore, the fi ller content infl uenced the mechanical properties of the composite resins investigated.
The second study hypothesis was also confi rmed, since there was a high positive correlation (r = 0.91) between the σ f and H properties of the composites systems examined.This correlation can be explained because the σ f provides information on the bulk properties and the H measures the materials surface properties, 22 and both properties are clinically relevant in characterizing a restorative material.

Conclusion
The direct composite systems examined had higher mean σ f and H values than the indirect composites, and the mean values of these properties were positively correlated (r = 0.91), confi rming the study hypotheses.

Table 1 -
Information on the composite resin systems investigated in this study.The composition of the organic matrix and fillers was provided by the manufacturers. *

Table 2 -
Mean and standard deviation values of flexural strength (σ f ) and hardness (H), and statistical grouping.
Mean values followed by the same letter in each column are not statistically different by Tukey's test (p ≥ 0.05).