25. Evaluation of the Bending Properties of Post Materials with Glass Fiber Reinforced Plastic Using a Cantilever Test

Recently, glass fiber reinforced plastic (GFRP) posts for root canals have been available clinically. In this study, we evaluated the bending properties of three types of commercial GFRP posts using a sensitive cantilever test that is able to induce a reliable stress concentration. Typical load-deflection curves of the GFRP posts included a number of sharp decreases of load after the proportional limits and before fractures. We defined the peak load before the first decrease of load as a pop-in load and used it as an index of an important mechanical property. Deboning of the fibers from the matrix and crack initiation after the pop-in load appeared in the electron microscopic images of the GFRP post surfaces. Thus, it may be important to modify the composition and surface texture of posts to eliminate pop-in behavior and to enhance durability. We found that the maximum bending load tended to increase with an increase of post diameter; however, the bending strength decreased with an increase in diameter, most likely due to the small size of the posts. We believe the cantilever test allowed a more accurate assessment of the bending load, in particular the determination of the pop-in load, as compared with conventional methods of determining stress values. Since the elastic moduli of the GFRP posts were compatible with that of dentin, GFRP posts may prevent root fractures, which are typically caused by stress concentrations from a discrepancy of the elastic modulus of the post and dentin.