Bonding to dentin with a self-etching primer: the effect of smear layers

Objective: The purpose of this study was to investigate the effect of smear layers on the tensile bond strength to human dentin. Bond strength was determined on dumbbell-shaped specimens to determine the feasibility for clinical use of a self-etching primer. Methods: The dentin of extracted human teeth was exposed by grinding with either #180 or #600 abrasive paper. A self-etching primer was then applied to the prepared dentinal surfaces and left undisturbed for 30 s. It was then air-dried and a photocured bonding agent applied and irradiated for 20 s. A composite resin was then added to the primed dentin and light-cured for 60 s to complete the bonded assemblies. Mini-dumbbell specimens (3.0×2.0 mm2) were prepared from the bonded samples. These specimens were stored in 37°C water for 24 h before tensile loading to failure at a crosshead speed of 1.0 mm/min. Surfaces of fractured specimens, both resin and dentin, were examined under a scanning electron microscope (SEM). Results: Significantly different tensile bond strengths (TBS) of 10.0±7.2 and 28.5±5.2 MPa were found for #180- and #600-prepared dentin, respectively (p<0.01). The former specimens fractured within the hybridized, relatively coarse smear layer, while the latter demonstrated adhesive failure between the composite resin and an attached PMMA rod, not between the dentin and applied adhesive agent. Significance: The presence and quality of a smear layer yields significantly different bond strengths to prepared human dentin, in vitro. However, a TBS of 10±7 MPa is evidently adequate, since self-etching primers have been well accepted in dental clinics.