To characterize a spectrum of mechanical properties of four representative types of modern dental resin composites and to investigate possible interrelations.
Four composite resins were used, a microhybrid (Filtek Z-250), a nanofill (Filtek Ultimate), a nanohybrid (Majesty Posterior) and an ormocer (Admira). The mechanical properties investigated were Flexural Modulus and Flexural Strength (three point bending), Brinell Hardness, Impact Strength, mode I and mode II fracture toughness employing SENB and Brazilian tests and Work of Fracture. Fractographic analysis was carried out in an SEM to determine the origin of fracture for specimens subjected to SENB, Brazilian and Impact Strength testing. The results were statistically analyzed employing ANOVA and Tukey post hoc test ( a = 0.05) while Pearson correlation was applied among the mechanical properties.
Significant differences were found between the mechanical properties of materials tested apart from mode I fracture toughness measured by Brazilian test. The latter significantly underestimated the mode I fracture toughness due to analytical limitations and thus its validity is questionable. Fractography revealed that the origin of fracture is located at notches for fracture toughness tests and contact surface with pendulum for Impact Strength testing. Pearson analysis illustrated a strong correlation between modulus of elasticity and hardness ( r = 0.87) and a weak negative correlation between Work of Fracture and Flexural Modulus ( r = −0.46) and Work of Fracture and Hardness ( r = −0.44). Weak correlations were also allocated between Flexural Modulus and Flexural Strength ( r = 0.40), Flexural Strength and Hardness ( r = 0.39), and Impact Strength and Hardness ( r = 0.40).
Since the four types of dental resin composite tested exhibited large differences among their mechanical properties differences in their clinical performance is also anticipated.
Dental composite resins are increasingly used worldwide in dental restorations. Despite the intense research on their properties and their evolution, bulk fracture and chipping is now the most common cause of failure in posterior composite resin restorations . The fracture initiates from superficial or subsurface cracks, micropores or minor imperfections of the material, leading to the formation of a critical crack . Although none of the mechanical properties can be used to predict the clinical performance of a given material certain properties are considered more important and indicative over others. Combining long term clinical longevity data and in vitro experimental results Ferracane 2013 proposed fracture toughness and Flexural Strength as the most strongly correlated mechanical properties to the clinical performance of dental resin composite restoration. Also, in the same paper was highlighted that fracture is now the prevalent cause of clinical failure of composite resins. This approach is documented by the fact that all restorations include internal flaws and thus fracture toughness may be the most important mechanical property reflecting the resistance to intraoral fracture. The fracture toughness ( K Ic ) represents the critical plane strain stress intensity factor . The subscript I stands for mode I or tensile opening of the crack, the subscript II for shear and III for tear opening. For brittle materials, K Ic is considered as a sufficient description and thus the vast majority of previous studies on dental composites have focused on K Ic . However, intraoral loading causes multi-axial stress composite in dental restorations and thus mode II and mode III fracture toughness might serve the same purpose.
The relationship of mechanical properties with intraoral performance has been extensively studied and a few significant correlations have been identified. Tyas (1990) found a significant inverse correlation between surface chipping/bulk fracture of composites and fracture toughness, modulus of elasticity, and diametral tensile strength while Ferracane et al., 1999 found a similar correlation between fracture toughness and marginal breakdown of composite resins. Ferracane and Condon 1999 studying microfill, minifill and midfill composites, with the use of a wear simulator, showed an excellent inverse correlation between marginal breakdown and fracture toughness. Ferracane et al., 1997 also reported a strong inverse correlation between wear and fracture toughness, and wear and Flexural Strength and a weaker inverse correlation between wear and hardness. In addition, fracture toughness along with Flexural Strength and Flexural Modulus has been proposed as predictors of clinical wear of composite resin restorations . Therefore all these mechanical properties can be considered of imperative importance in composite resin characterization, in terms of their clinical performance.
Another important aspect of determining mechanical properties is the correlation among materials parameters of dental composites . Although mechanical properties are considered independent (as define a different aspect of material behavior), these correlations if characterized as universal could give a quicker and simpler way to determine the material properties based on the identification of one parameter; then the unknown properties can be calculated based on the correlations . A simple example is the correlation between Hardness and Elastic Modulus which has been readily used to estimate the Modulus of dental composite resins from a simple hardness measurement . In mechanics there are even mathematical relationship between specific mechanical properties (i.e. Hardness = 3 Yield Strength) independently of the materials tested but mostly these formulas must be further corrected, compensating for material imperfections and limitations of experimental setups . Although the full spectrum of mechanical properties of composite resins has been determined in the past, the comparison among the results of different studies was hindered by the differences in materials tested and/or experimental parameters in different research setups. For instance Flexural Strength tends to increase when higher crosshead speed is applied . Therefore the possible correlations were squeezed to a limited number of properties.
Recently, the composite resins have undergone a significant evolution, considering their composition, which mainly relates to the fillers and less to the monomers . Therefore, a variety of composite resins, categorized as microhybrid, nanohybrid, nanofilled and ormocers, with significant differences in their matrix and filler composition properties, have been introduced during the last decade .
This study aims in first place to characterize a range of mechanical properties of four representative types of modern dental resin composites. Furthermore, since the clinical failure is a multi-factorial result of several types of mechanical degradations (fatigue, wear, fracture, etc.) this study also focuses on the possible interrelations among mechanical properties of the tested resin composites. The null hypothesis is that the there are not significant differences in the mechanical properties among the dental composite resins tested.