Evaluation of the bond strength of composite resins with distinct properties to short fiber-reinforced composite resins of different viscosities.

This in vitro study aimed to evaluate the shear bond strength (SBS) between two short fiber-reinforced composites (SFRCs) that differ in viscosity and composition and various particulate-filled composites (PFCs) with distinct matrix structures and filler characteristics, as well as the surface roughness and oxygen inhibition layer thickness of SFRCs. Two SFRCs were used as base materials: a low-viscosity SFRC (everX Flow) and a high-viscosity SFRC (everX Posterior). SFRCs were placed as a single layer into centrally located cavities (5 mm in diameter and 4 mm in height) of 3D-printed blocks, light cured and veneered with one of three PFCs (microhybrid Filtek Z250, nanohybrid Clearfil Majesty Posterior, or G-ænial Universal Injectable) using a standardized bonding area (2.3 mm in diameter and 2 mm height) created with an Ultradent jig. A total of 90 bilayered specimens (15 per group) were prepared and stored in distilled water for 24 h at 37 °C before performing SBS testing. Surface roughness of the SFRCs was measured using a contact profilometer, oxygen inhibition layer thickness was determined by light microscopic image analysis, and representative interfacial morphology was examined by scanning electron microscopy (SEM). The SBS did not significantly differ between the two SFRCs (p > 0.05), whereas the veneering PFC type significantly influenced bond strength. Filtek Z250 demonstrated significantly higher bond strength than the other PFCs (p < 0.05). Surface roughness was similar for both SFRCs (p > 0.05), while the oxygen inhibition layer thickness of everX Posterior was significantly greater than that of everX Flow (p < 0.05). Within the limitations of this in vitro study, it can be concluded that type of veneering PFC influenced the SBS to SFRCs, whereas no significant effect of SFRC type on SBS was observed.