Objectives . During its in vivo development, dental bonding is always under polymerization contraction stress (PCS) when there is cure-induced shrinkage and the volume change of restoration is constrained. This is not reflected, however, in our in vitro sample preparation – there is little PCS because the restorative material shrinks almost without restriction. The two conditions may in principle produce different bond strengths since the presence of stress could negatively affect the bond structure. We designed a new method to test the hypothesis.
Materials and methods . One total-etch adhesive (One-Step by Bisco) and two self-etch ones (One Up Bond F Plus by Tokuyama and Xeno IV by Densply) were tested on human dentin. Light-Core (Bisco) was utilized to make composite posts. A glass ring (3.1 mm ID and 2.0 mm tall) was used as the mould for the post and as a spacer, which, together with a glass plate (1.0 mm thick), restricted polymerization shrinkage of the composite during light curing, producing PCS normal to the bonding surface. The fabricated specimens were stored in 37 °C water for at least 6 h. After both of their glass plate and ring were removed to expose the post, the specimens were broken on an Instron machine.
Results . The following table compares the shear bond strengths (MPa) of the samples fabricated under stress and of those without stress. (The numbers with a same superscript in the same row are not statistically different.)
| Adhesive | Without stress | Under stress |
|---|---|---|
| One-Step | 25.0 (2.6) a , n = 5 | 24.2 (6.7) a , n = 5 |
| Xeno IV | 17.6 (5.2) a , n = 5 | 15.3 (3.7) a , n = 6 |
| One Up Bond | 13.2 (1.6) a , n = 9 | 7.3 (3.3) b , n = 5 a |
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