Effects of regional enamel and prism orientation on resin bonding

Human enamel, with its prismatic, rod-like apatitic morphology, is an anisotropic material. Because of this structural anisotropy, variation in enamel bonding sites might influence the bonding ability of current adhesive systems. This study investigated the effects of regional enamel and the direction of enamel sectioning on the bonding ability of two commercially available resin adhesives: a self-etching primer system (Clearfil SE Bond) and a one-bottle adhesive system intended for use with a total-etch wet bonding technique (Single Bond). Two regions of enamel, cuspal and mid-coronal enamel, were chosen, then sectioned in three different directions, horizontally, axially and tangentially. Slices of the sectioned enamel were then bonded with each adhesive system and submitted to a micro-shear bond test. The results of a micro-shear bond testing showed that the bonding of a one-bottle adhesive system (Single Bond) to enamel was high at the surface perpendicular to the enamel prisms (40 MPa to 51 MPa) and low at the surface parallel to the enamel prisms (24 MPa to 27 MPa). In the case of a self-etching primer system (Clearfil SE Bond), 35 MPa to 45 MPa bond strengths were obtained from all surfaces. The bond strengths of the two adhesive systems were significantly influenced by the anisotropic structure of enamel (p < 0.05). However, the effect of a self-etching primer system was less influenced by the orientation of the prismatic structure of enamel than that of a one-bottle adhesive system (p < 0.05). SEM and CLSM microphotographs showed that the self-etching primer effectively modified the smear layer without being excessively destructive to the enamel.