Hydrothermal and mechanical stresses degrade fiber-matrix interfacial bond strength in dental fiber-reinforced composites

Fiber-reinforced composites (FRCs) show great promise as long-term restorative
materials in dentistry and medicine. Recent evidence indicates that these materials degrade in
vivo, but the mechanisms are unclear. The objective of this study was to investigate mechanisms
of deterioration of glass fiber–polymer matrix bond strengths in dental fiber-reinforced
composites during hydrothermal and mechanical aging. Conventional three-point bending
tests on dental FRCs were used to assess flexural strengths and moduli. Micro push-out tests
were used to measure glass fiber–polymer matrix bond strengths, and nanoindentation tests
were used to determine the modulus of elasticity of fiber and polymer matrix phases separately.
Bar-shaped specimens of FRCs (EverStick, StickTech, and Vectris Pontic, Ivoclar-
Vivadent) were either stored at room temperature, in water (37 and 100°C) or subjected to
ageing (106 cycles, load: 49 N), then tested by three-point bending. Thin slices were prepared
for micro push-out and nanoindentation tests. The ultimate flexural strengths of both FRCs
were significantly reduced after aging (p < 0.05). Both water storage and mechanical loading
reduced the interfacial bond strengths of glass fibers to polymer matrices. Nanoindentation
tests revealed a slight reduction in the elastic modulus of the EverStick and Vectris Pontic
polymer matrix after water storage. Mechanical properties of FRC materials degrade primarily
by a loss of interfacial bond strength between the glass and resin phases. This
degradation is detectable by micro push-out and nanoindentation methods.