Effect of different composite core materials on fracture resistance of endodontically treated teeth restored with FRC posts.

Objective
This study evaluated the fracture resistance of endodontically treated teeth restored with fiber reinforced composite posts, using three resin composite core build-up materials, (Clearfil Photo Core (CPC), MultiCore Flow (MCF), and LuxaCore Z-Dual (LCZ)), and a nanohybrid composite, (Tetric N-Ceram (TNC)).


Material and Methods
Forty endodontically treated lower first premolars were restored with quartz fiber posts (D.T. Light-Post) cemented with resin cement (Panavia F2.0). Samples were randomly divided into four groups (n=10). Each group was built-up with one of the four core materials following its manufacturers' instructions. The teeth were embedded in acrylic resin blocks. Nickel-Chromium crowns were fixed on the specimens with resin cement. The fracture resistance was determined using a universal testing machine with a crosshead speed of 1 mm/min at 1350 to the tooth axis until failure occurred. All core materials used in the study were subjected to test for the flexural modulus according to ISO 4049:2009.


Results
One-way ANOVA and Bonferroni multiple comparisons test indicated that the fracture resistance was higher in the groups with CPC and MCF, which presented no statistically significant difference (p>0.05), but was significantly higher than in those with LCZ and TNC (p<0.05). In terms of the flexural modulus, the ranking from the highest values of the materials was aligned with the same tendency of fracture loads.


Conclusion
Among the cores used in this study, the composite core with high filler content tended to enhance fracture thresholds of teeth restored with fiber posts more than others.


Introduction
The use of fiber reinforced composite (FRC) posts has tremendously increased the restoration of endodontically treated teeth due to their favorable physical properties, such as high tensile strength and good fatigue resistance. Also, their modulus of elasticity is similar to that of dentin. In combination with FRC post, the composite core build-up material is often used to restore the coronal portion of the teeth and to achieve retention and resistance form for the crown 4,7 . The clinical evaluation of the FRC post and core restoration has reported high success rate with reduction in failure of root fracture. The common failures of these restorations related to materials are post debonding, crown debonding, or post fracture, which are usually combined with core failure, especially in teeth with few coronal walls 21,27 the core build-up material must be strong enough and resist multidirectional masticatory force. Furthermore, it has to withstand a process of crown preparation by rotary cutting instrument. Therefore, the core material is also a critical component for the overall success in the restoration of endodontically treated teeth, especially when using it with FRC post.
Resin composite is a popular core build-up material to be used with FRC post due to similarity to tooth structure in hardness and fracture toughness, giving the ability to perform the preparation after curing.
Restorative composites can be regularly employed for core build-up material 4,7,21,27 . Nowadays, there are many and enhance for easy manipulation. These materials curing mode, build-up technique, among others, while their physical properties have been investigated in many aspects 11,20,22,28   mm in root length were chosen. All teeth were cleaned and debrided of soft tissues and stored in 0.9% normal saline until used.

Root canal preparation
The clinical crowns were decoronated perpendicularly to the root axis 1 mm above the cementoenamel junction (CEJ) by a low speed cutting machine (ISOMET 1000; Buehler Ltd, IL, USA). The pulpal tissue was removed 15 (Dentsply Maillefer, Ballaigues, Switzerland) was inserted into the canal through the apex to establish the working length by subtracting 1 mm from this measurement. All teeth were endodontically treated the root canals were irrigated with 2.5% NaOCl, alternating irrigation with 2.5% NaOCl and 17% ethylenediaminetetraacetic acid solution, and final irrigation with normal saline. The canals were dried and then obturated using a lateral condensation technique with gutta-percha cones and eugenol-contained root canal cement (CU dental Product, Bangkok, Thailand) 5 .
The excess of gutta-percha was removed with a hot (Cavit; 3M ESPE, Seefeld, Germany) to a depth of 3 mm. All specimens were stored at 37 0 C during 24 h for complete setting of cement 14 .

Specimen's block preparation
After endodontic treatment, a D.T. universal drill was used to prepare a post space to a depth of 10 mm, leaving 4 mm of intact gutta-percha as the apical seal. Before core build-up procedure, a dentin bonding agent was applied to dentin according to the manufacturers' recommendations ( Table 1). The core build-up was then continued using a transparent matrix band. For TCN and CPC, the incremental technique of 2-mm layer     Table 1. The ranking of these values align with the same trend of the fracture resistance, in which the highest modulus experienced the highest value of fracture resistance.

Discussion
In material aspect, the failure of restoration of endodontically treated teeth using FRC post and core concerns many factors, such as types of FRC post, shape of post, surface treatments, cementation, and core material used. In this study, we aimed to investigate the effect of the core material used by using fracture resistance test. Therefore, the other factors were controlled for the same condition, except the types of core material.
Due to the statistical analysis, the null hypothesis  The bonding ability of the core material is also the factors concerning the results. Two main bonding surfaces have cooperated: core/dentin interface at the cross section 1 mm above the CEJ of the tooth and core/post interface along the core height coronally.
Although the area of core/dentin bonding surface was small, the position was near to the fulcrum, which was in a critically area. This study used dentin bonding agent recommended by each manufacturer before core build-up procedure to avoid incompatibility between materials. The study of O'Keefe and Powers 20 (2001) showed incompatibilities between a self-cured core material and a dual-cured adhesive. Their study also

Conclusions
Within the limitations of this study, it was shown that content tended to enhance more fracture resistance for endodontically treated teeth restored with FRC post.