| Abstract|| |
Background & Objectives : The aim of the study was to evaluate the flexural strength and flexural modulus o f five commercially available composites namely Fetric Ceram (Hybrid), Filtek P-60 (Packable), Dyract (Compomer), Filtek Flow (Flowable) and Admira (Ormocer). after aging in water.
Methodology: 20 specimens were made using each of the 5 composite materials . and randomly divided into two subgroups containing 10 specimens each. Specimens were stored in distilled water (Subgroup A: 7 days & Subgroup 13: 30 days). Thermocycling of all the specimens was done for 5000 cycles, to simulate the oral conditions. Flexural strength and modulus of the specimens were assessed in a Universal Testing Machine. Data was analyzed using one way ANOVA /Students t-test at a significance level of 0.05.
Results: Results have shown that highest flexural strength and modulus was seen for Group I1 (Filtek P-60) and Group I (Tetric Ceram), followed by Group V (Admira). Group Ill (Dyract) showed lowest flexural strength and Group IV (Filtek flow) showed lowest flexural modulus. After aging in water all the groups showed decreased flexural strength, except for Group IV (Filtek Flow), for which the strength increased. Flexural modulus of all the composites tested increased slightly after aging in water, but was not significant.
Interpretation & Conclusion: The effect of aging in water on flexural strength and modulus was material dependent. A significant decrease in flexural strength was observed for all the composites, except for Group IV (Filtek flow), after aging in water. Aging had no significant effect on the flexural modulus of any of the composites tested.
Keywords: Composite restoratives, Flexural strength, Flexural modulus, Aging.
|How to cite this article:|
Satish G, Nainan MT. Invitro evaluation of flexural strength and flexural modulus of elasticity of different composite restoratives. J Conserv Dent 2006;9:140-7
|How to cite this URL:|
Satish G, Nainan MT. Invitro evaluation of flexural strength and flexural modulus of elasticity of different composite restoratives. J Conserv Dent [serial online] 2006 [cited 2020 Feb 19];9:140-7. Available from: http://www.jcd.org.in/text.asp?2006/9/4/140/42316
| Introduction|| |
The term "composite" refers to the combination of two distinctly different component phases to produce the final material. One phase consist of a soft organic resin polymer (eg. Bis-GMA), and the second phase is dispersed throughout this resin matrix which consist of inorganic filler particles. Filler particles are coated with a silane agent which functions as a "glue" to bind these two phase together.
In the early 1980s, a composite classification system based on average particle size, manufacturing techniques, and filler chemical composition was introduced. Commercially available composites were classified according to mean particles size and correlated the mean particle size to youngs modulus. 
Filler size, filler content and distribution were determined to highly influence the physical and mechanical properties of composite resins. It has been shown that filler volume fraction and filler load level of composites correlate with the material strength and elastic modulus, as well as the fracture toughness of the material. 
The fracture related material properties such as, fracture resistance, elasticity, and marginal degradation of materials under stress have usually been evaluated by the determination of material parameters like flexural strength, flexural modulus and fracture toughness. 
Clinically composite restorations can be subjected to considerable flexural stresses. The required flexural properties are highly dependent on the clinical applications. In Class 1, II, III and IV restorations, where stresses are significant, high flexural strength and modulus are desired. Materials with low modulus or stiffness will deform more under masticatory stresses resulting in catastrophic failures and destruction of the marginal seal between composites and tooth substance. In Class V restorations composites with lower modulus are desired as they are capable of flexing during tooth function which may reduce stresses along the bonding agent interface and the likelihood of debonding. 
Over the past decade, there has been a rapid increase in the number and type of composite products available. The significant innovations are polyacid modified composites, flowable composites and ormocer composite.
Flexural properties obtained after aging is dependent on the balance between composite post-cure and degradation by water. Any increase in flexural strength and modulus can be attributed to additional cross-linking reactions of the resin component after light curing, as the quantity of fillers, which increases physical properties, remains the same. The matrix of composites is known to absorb a small percentage of water, which changes the magnitude of some physical properties.
When the composites are immersed in water, the resin matrix swells and radial tensile stresses arc introduced at the filler interfaces, straining the bonds in the fillers. The high energy levels resulting from the strained bonds make fillers more susceptible to stress corrosion attack, resulting in complete or partial filler debonding. After aging in water, the plasticizing and swelling of the resin matrix also reduces the hoop stresses around the fillers and facilitates filler pullout. 
The above mentioned mechanism might contribute to the decreased flexural strength of composites after aging in water. Any positive effects obtained with composite post-cure are thus negated by water.
So the purpose of this study was to evaluate the flexural strength and flexural modulus of elasticity of five different composites after aging in water. Thermocycling was done in this study to simulate the oral conditions.
| Methodology|| |
The present study was conducted in the Department of Conservative Dentistry and Endodontics, College of Dental Sciences, Davangere in collaboration with the Department of Textiles and Department of Mechanical Engineering at Bapuji Institute of Engineering and Technology, Davangere.
A total of 100 specimens were prepared using the rectangular Teflon mould with a recess or opening (25 mm length x 2 mm breadth x 2 mm height), according to ISO 4049 specifications  .
Composite Restorative materials used were Tetric Ceram (Ivoclar vivadent), Filtek P-60 (3M) Dyract (Dentsply), Filtek Flow (3M) and Admira (Vocco).The composite restorative materials were placed into the Teflon mould, which was positioned on top of a glass slide. A second glass slide was then placed on top of the mould and gentle pressure was applied to extrude excess material. The top and bottom surfaces were then light cured in three overlapping irradiations of 20 to 40 seconds each, according to the manufacturer's instructions, using a halogen curing light (3 M).
A total of 100 specimens were divided into five groups of 20 specimens, each group made up of different composite.
Group I : Tetric Ceram (Hybrid) : 20 Specimens
Group II : Filtek P-60 (Packable) : 20 Specimens
Group III : Dyract (Compomer) :20 Specimens
Group IV : Filtek Flow (Flowable) : 20 Specimens
Group V : Admira (Ormocer) :20 Specimens
Further each group of composite material (i.e. 20 specimens) were randomly subdivided into 2 subgroups (A & B) of 10 specimens each. Specimens in Subgroup A were subjected to 7 days of aging in water and in subgroup B were subjected to 30 days of aging in water.
Specimens of all the groups were subjected to thermocycling for 5000 cycles in water for a total of 60 seconds of dwell time at 40°C and 60°C, to simulate the oral conditions, before testing the flexural strength and flexural modulus.
At the end of each aging period, the flexural properties of the composites were assessed. The specimens were first blotted dry with a blotting paper and measured using a digimatic vernier calipers. Measurements were taken in two locations for length, breath and height, and average of two values was taken to calculate flexural strength (?), and flexural modulus (E). The specimens were subsequently transferred to a flexural strength and modulus testing apparatus mounted on an Hounsfield Universal Testing Machine.
The apparatus consists essentially of two rods (2 mm in diameter), mounted parallel with 20 mm between centers, and a third rod (2 mm in diameter) centered between and parallel to other two, so that the three rods in combination can be used to give a three point loading to the specimen.  A crosshead speed of 1 mm /mm was used and the maximum loads exerted on the specimens prior to fracture were recorded. The values for flexural strength (σ) in Megapascals (MPa) was calculated using the following formula:
- F is the maximum load in newtons, exerted on the specimens.
- L is the distance, in millimeters between the jig supports (20 mm)
- B is the width, in millimeters, of the specimen measured immediately prior to testing.
- H is the height, in millimeters, of the specimen measured immediately prior to testing.
The values for flexural modulus (E) in Megapascals (MPa) was calculated using the following formula;
E=[F 1 /D] x [L 3 /4BH 3 ]
Where (F,/D) is the slope, in newtons per mm, measured in the straight line portion of the load deflection graph.
L,B and H have been defined in the flexural strength formula. Flexural modulus in MPa was subsequently converted to GPa.
| Statistical Analysis|| |
One way ANOVA was used for multiple group comparisons followed by Newman-Keul's range to determine any significant difference between two groups. Unpaired t-test was used to compare flexural strength and flexural modulus between the two aging periods.
A P-value of 0.05 or less was considered for statistical significance.
| Results|| |
The mean flexural strength and modulus of the various materials are shown in [Table 1] & [Table 2], and [Graph 1 &[Graph 2]. Mean flexural strength ranged from 70.8 to 135.3MPa and 59.8 to 125.7 MPa at one week and one month, respectively. Mean flexural modulus at one week and one month ranged from 3.48 to 9.35 and 4.27 to 9.73 GPa, respectively.
At one week, ranking of flexural strengths from highest to lowest was as follows: Packable > Hybrid > Ormocer > Flowable > Compomcr. Ranking of flexural strengths at one month was similar. The ranking of flexural modulus at one week was Packable > Hybrid > Ormocer > Compomer > Flowable. Ranking of flexural modulus at one month was similar.
One-way ANOVA revealed significant interactions between materials and storage time. The effects of aging on flexural properties were therefore material dependent. Significant differences in flexural strength between materials were identical after aging for one week and one month. Packable (Filtek P-60) was significantly stronger than all other composites evaluated. At both time periods, Flowable (Filtek Flow) was significantly more flexible than all other composites evaluated.
| Discussion|| |
Today the marriage of bonding and composite resins is the basis for a new era of restorative adhesive dentistry. In 1962, Raphael Bowen introduced a new resin, which is a reaction product of bisphenol A and glycidyl methacrylate, which has been abbreviated as Bis-GMA. This was the first step in the development of composite dental resins  .
Composites can be classified based on filler particle size, filler content, matrix composition (Bis-GMA or UDMA) or polymerization method (Self curing; Ultraviolet light curing, visible light curing; dual curing). Based on the particle size they can be classified as Megafill, Macrofill (10-100 µm), midfill (1-10 µm), minifill (0.1 - 1 pm), microfill (0.01 0.1 µm) and nanofill (0.005 0.001 µm).
The consequence of advances in the control of filler particle size, particle size distribution, particle morphology, and monomer technology has been the introduction of composites with specific handling characteristics. Those include the hybrid composites, packable composites, flowable composites, comporners (Polyacid modified composites) and Ormocers.
Hybrid resin composites, as the name implies contain a blend of microfilled (0.04 µm) and small particle (1-5 gm) fillers. Packable composites are characterized by a high filler load and a filler distribution that gives them a different consistency compared to hybrid composites. Flowable composites are a class of low viscosity materials that possess particle sizes and particle size distribution similar to those of hybrid composites, but with reduced filler content, which allows the increased amount of resin to decrease the viscosity of the mixture  .
Compomers are similar to resin-modified glass ionomers in that they contain all the major components of both polymer based composites and glass ionomers, with the exception of water. Water is excluded to prevent premature setting of the material and also to ensure that setting occurs only through polymerization reaction. 
Multifunctional urethane and thioether (meth) acrylate alkoxysilanes as sol-gel precursors have been developed for the synthesis of inorganic - organic copolymer ormocer (acronym for Organically modified ceramics) composites as dental restorative material. The ormocer structure consist of a polycondensed inorganic / organic network.
Properties like flexural strength, flexural modulus, fracture toughness, water sorption, solubility or microhardness, are important material parameters to characterize resin based filling materials. Flexural strength is a fracture related mechanical property since it is a measure of resistance of restoration to withstand occlusal force. The flexural modulus of a material is a measure of the resistance to deformation under load. It a measure of material stiffness; the higher the modulus, the stiffer the material.
In the oral environment, restorative materials are continuously in contact with salivary fluids, which contain a wide range of organic and inorganic species together with a complex bacterial flora. Composite restoratives are not stable after polymerization and are constantly interacting with their environment. The principle interaction occurs with salivary fluids and water, which diffuse into the resin matrix. In the oral cavity, restorative materials are subjected to cyclic mechanical and thermal loading. All restorative materials including composites are continually bathed in saliva, water, beverages etc and are subject to different temperatures. Thermocycling procedure is a common way of in-vitro testing used to simulate intra oral temperatures.
The maximum conversion from monomer to polymer in dental composites is, however, only in the range of 60-75%. So baseline testing was delayed for one week to allow for elution of all leachable, unreacted components and allow composite post cure. 
Ferracane JL, Hopkin JK, Condon JR (1995)  studied the effects of normal cured and heat cured composites after aging in water for 1 to 180 days. By 30 days, both type of composites showed significant reduction in mechanical properties including flexural strength. As aging had little effect after 30 days, a one month aging period was selected for this study.
Group II (Filtek P-60) and Group I (Tetric Ceram) with highest filler by volume (61% and 60% respectively) exhibited the highest flexural strength, followed by Group V (Admira) and Group IV (Filtek Flow), with filler percentage by volume of 56% and 47% respectively. Group III (Dyract) with lower percentage of filler by volume of 47% showed the lowest flexural strength. The mechanical properties were related to their filler content. Composites with the highest filler by volume exhibited the highest flexural strength and hardness. As the volumetric filler fraction increased, the flexural strength of composite also increased.( Kim KH ct al 200210 ; Braem M et al 1989  )
Although Group III (Dyract) and Group IV (Filtek flow) had the same percentage of filler by volume (i.e. 47%), Group IV (Filtek flow) showed higher flexural strength than Group III (Dyract) and the difference was statistically significant. The fillers in Group IV (Filtek flow) are Zirconia and Silica, where as Group III (Dyract) contains fluoroaluminosilicate glass which is a weak constituent, resulting in decreased strength (Yap AUJ et al 2002  ).
Group II (Filtek P-60) showed higher flexural strength than Group I (Tetric Ceram), which have similar filler content (61% and 60% by volume). The difference was statistically significant. The resin matrix of Group II (Filtek P-60) is made up of Bis-GMA, UDMA and Bis-EMA, where as that of Group I (Tetric Ceram) is made up of Bis-GMA, UDMA and TEGDM A. Addition of UDMA and Bis-GMA resulted in increase in tension and flexural strength, and addition of TEGDMA increased tension, but reduced the flexural strength. The absence of TEGDMA in Group II (Filtek P-60) and its presence in Group I (Teric ceram) contributed to the decrease in flexural strength (Adabo GL, Cruz CAS, Fonseca RG, Vaz LG :2003  )
When mean values for flexural strength were analyzed and compared between two aging periods (Subgroup A and Subgroup B) for each group, it has shown that all the groups showed reduction in flexural strength after aging in water, except for Group IV (Filtek Flow). The differences were statistically significant.
- The reduction in flexural strength is predominantly related to the uptake of water by the polymer. Water swells the polymer matrix and occupies space between the main chains and cross links, as well as filling microvoids created during polymerization. It is possible at the same time that the water may cause some hydrolysis of the filler / matrix interfaces, and /or crazing of the polymer matrix which would contribute to the reduction in properties.
- All the composites evaluated contained silica or silicate glass fillers that have irregularly distributed Si-O-Si bonds. When the composites are immersed in water, the resin matrix swells and radial tensile stresses are introduced at the filler interfaces, straining the Si-O-Si bonds in the fillers making the filler more susceptible to stress corrosion attack, resulting in complete or partial filler debonding.
- Hoop stresses also exist around the filler particles as a result of matrix shrinkage during polymerization. These hoop stresses increase the frictional forces between filler and resin matrix, there by decreasing the filler pull-out tendency during flexural testing. After aging in water, the plasticizing and swelling of the resin matrix reduces the hoop stresses around fillers and facilitate filler pull out (Yap AUJ et al :2000  ,2002  and Ferracane JL et al;1995  ).
The aforementioned mechanisms might contribute to the decreased flexural strength of the composites after aging in water.
When the results were observed between Subgroup A and Subgroup B for Group IV (Filtek Flow), it showed increase in flexural strength after aging in water. The difference was statistically significant. As filler content increases, the resin-filler interface increases. This is a very weak junction, and leads to increased accommodation of water at the filler matrix interface. A lower filler content as in Group IV (Filtek Flow) may therefore lead to less accommodation of water at filler matrix interface resulting in decreased water sorption. (Yap AUJ et al 2002  )
The results for flexural modulus values have shown that between all the materials of the two subgroups (Subgroup A and Subgroup B), Group 11 (Filtek P60) and Group I (Tetric Ceram) showed high flexural modulus, followed by Group V (Admira) and Group III (Dyract). Group IV (Filtek Flow) showed lowest flexural modulus. The differences ranged from statistically highly significant to statistically significant. That among all the composites evaluated, Group 11 (Filtek P-60) and Group I (Tetric ceram) with high filler volume of 61% and 60% respectively showed high flexural modulus, followed by Group V (Admira) and Group III (Dyract) with filler volume of 56% and 47% respectively. Group IV (Filtek Flow) showed the lowest flexural modulus because of its low filler content of 47% by volume. (Yap AUJ et al 2002  ;Kim KH et al 2002  ; lkejima I et al 2003  )
Though Group III (Dyract) and Group IV (Filtek Flow) have same filler load of 47% by volume, Group III (Dyract) showed higher flexural modulus, owing to the glass ionomer characteristics impacted by the chemical reaction between TCB resin and fluoroaluminosilicate glass fillers. As there is no such acid-base reaction taking place in Group IV (Filtek Flow), its flexural modulus remained low.
When results were compared between Subgroup A and Subgroup B for each material, all the groups showed slightly higher flexural modulus after aging in water, except for Group IV (Filtek Flow). The differences were statistically not significant. Group IV (Filtek Flow) showed increase in flexural modulus and the difference was statistically significant. That all groups showed slight increase in flexural modulus after 30 days of aging in water which might he explained by the continuous, although slow degree of conversion or composite post-cure.
A significant increase in flexural modulus of Group IV (Filtek flow) can be attributed to the composite post-cure which was significant. Composite post-cure refers to progressive cross-linking reactions in composite after light curing. (Gladys S et al 1997  ;Yap AUJ et al 2002  ; Munksgaard EC 2002  )
Apparently, the water uptake by composites might not have the same detrimental effect on flexural modulus as on flexural strength.
According toAnusavice (2004), the elastic modulus of a material is a constant, it is unaffected by the amount of elastic or plastic stress that is induced in the material, and it is not a measure of its plasticity or strength. Materials with high elastic modulus can have either high or low strength values. 
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Department of Conservative Dentistry & Endodontics College of Dental Sciences, Davangere, Karnataka
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2]