Effect of the tooth microstructure on the shear bond strength of a dental composite

Plane human enamel and dentin surfaces were used for microstructural investigation and shear bond testing. The dental tissue microstructure was characterized through the surface topology, the concentration in mineral elements (determined by electron microprobe analysis), and the Vickers microhardness for the same dental tissue. The etched prismatic enamel presents a surface roughness of about 200% of the apparent area, while the intertubular surface fraction of the dentin is, on the average, equal to 75%. The calcium concentration lies between 23.2 and 37.8% of the enamel total mineral content, and 18.5 and 28.2% of the dentin. The microhardness varies from 205 to 378 Hv for the enamel and from 37 to 98 Hv for the dentin. Also, the shear bond strength to a chemosetting adhesive/composite system varies from 10.4 to 23.9 MPa for the enamel and from 0.0 to 5.0 MPa for the dentin. It is shown that the shear bond strength is strongly correlated to the microhardness through a simple linear equation valid for both enamel and dentin surfaces. In turn, the microhardness is correlated to the calcium concentration through two distinct linear relations. These correlations lead to the intrinsic dependence of the shear bond strength on the calcium concentration of the dental tissue, providing that corrections are applied to the bond strength data in order to take into consideration the effective solid area of the adherend and the polymerization retraction stress. Consequently, it is proposed that the adhesion mechanisms for the both enamel and the dentin are controlled, to a major extent, by the mineral content and the surface topology of the tooth.