Micro-mechanical characterization of experimental direct core build-up resin composites with different levels of filler contents

Objectives: Special materials and techniques are essential for achieving a full functional restoration of mechanically compromised endodontically treated teeth. A core build-up is a way of restoring severely broken down teeth, so that the bulk of the coronal portion is rebuild in a suitable manner, allowing for some kind of extracoronal restoration. Although metal cores are still regarded as very high durability, recently, there is a tendency to shift towards using different composite resin cores mainly due to the ease of use and reduced manipulation time. Nowadays, improved high strength composite resins are used to achieve a good core restoration. Many studies have investigated the fracture behavior of simulated cores in different geometric configurations, but only a few have compared mechanical properties of the different materials. With more and more core build-up materials being released, the clinicians need better insights regarding the mechanical features in order to make the optimal choice for achieving best results. The present study aimed to investigate at mechanical and micromechanical levels the flexural fracture strength and fracture toughness of newly developed core build-up materials. Materials and methods : Clearfil DC Core Automix One (Kuraray Noritake Dental Co.) with variable filler contents (+2.5 and +5 wt %) were used. The fracture strength was estimated in a three-point bending mode using an universal testing machine (Autograph, Shimadzu Co.), while the fracture toughness was obtained in a tensile mode, utilizing notchless triangular prism specimens (EZ Test, Shimadzu Co.). The performance of these materials were compared to already existing commercially available products. Fractured surfaces were characterized micromorphologically by means of scanning electron microscopy (SEM; JSM6390LV, JEOL) at x2000 magnification. The results were statistically processed using one-way ANOVA and Tukey’s HSD test (IBM SPSS). Results : Flexural strength and fracture toughness generally improved with the increase of the filler content. A 2.5 wt % increase proved to be most optimal and yielded fracture strength of (114.1 ±9.1 MPa) when compared to the control group (91.7 ±11.8 MPa) and the 5 weight percent increase group (116.6 ±5.1 MPa). The Young’s modulus for the 2.5 wt % group and the 5 wt % were (15.4 ± 1.2 GPa) and (15.3 ± 2 GPa) respectively, while for the control group it was (12.6 ± 0.9 GPa). The fracture surfaces morphological appearance under SEM observation was in support with the increased mechanical strength. Conclusion : The newly developed composite resin core build-up seemed to be a good alternative to the conventional core build-up materials and could be the material of choice when a more durable restoration is sought.