Hysteresis and Yield Strength of Formable vs. pre-Fabricated Fiber-Glass Posts

Objectives: New formable-polymerizable fiber-glass posts (FOR) circumvent adapt to the inner anatomy of the root canal maximizing strengthening. Here we compare the deformation behavior (hysteresis) in loading-unloading experiments and determine the distribution of bending strength for pre-fabricated (PRE) vs. FOR posts.
Methods: Bending rods of diameter = 1.4 mm of FOR posts (everStick^®POST, GC) were produced by rolling the original posts in cylindrical form and polymerizing for 40s in 5 overlapping spots. PRE posts (GC Posts, GC) with equal dimensions were employed as-received. Hysteresis experiments were conducted in 3-point bending (3PB, 10 mm span) by loading-unloading cycles with the load-line displacement tracked by laser illumination and Image Speckle Correlation. The effect of increasing the volume of FOR post was evaluated by condensing in thicknesses of 1-4 mm. The yield strength (σ₀) was obtained for 30 specimens per material (diameter = 1.4 mm) in 3PB and analyzed by the Weibull fracture theory.
Results: The viscoelastic energy measured during the hysteresis experiments was shown to be enhanced for the FOR, whereas PRE showed rather a stiffer, high-elastic modulus response. Increasing the thickness of the FOR resulted in a power-law increase in the elasticity and viscoelastic potential. The mechanical response during bending experiments revealed a more elastic-plastic behavior for FOR, in contrast to a more brittle mode for the PER post. The yield strength and Weibull modulus (m) of the PRE posts were 2-fold higher (σ₀=145.4 MPa, m=20.7) than that of FOR posts (σ₀=76.7 MPa, m=6.0), although FOR showed 3.5-fold higher Work-of-fracture (PRE W_of= 19.6±3.7 N/mm vs. FOR W_of=70.1±30 N/mm).
Conclusions: Our results indicate that FOR posts, despite endowed by a lower stiffness, increase their elastic and viscoelastic response by increasing the thickness (volume). The PRE posts are stronger but less capable of dissipating flexural stresses by viscoelastic deformation.