Comparison of interfacial characteristics of adhesive bonding to superficial versus deep dentine using SEM and staining techniques

OBJECTIVES: A primary reason for premature failure of Class II composite restorations is recurrent decay at the gingival margin. The integrity of the gingival margin depends on the bond formed with dentine. Using scanning electron microscopy (SEM) and histomorphologic technique, the purpose of this study was to characterise the adhesive/dentine (a/d) bond in dentine sections from the dentinoenamel junction (DEJ) and the cementoenamel junction (CEJ) with commercial adhesives of varying hydrophobic/hydrophilic composition. MATERIALS AND METHODS: The occlusal 1/3 of the crown was removed from 20 human third molars, this exposed the surface used for the DEJ sections. The teeth were sectioned occluso-gingivally into equal halves: one-half representing the DEJ; the remaining half was sectioned 4mm deep to the exposed surface (CEJ). Each half was treated with the same adhesive. The teeth were randomly selected for treatment with Single Bond (SB, 3M) or Dentastic UNO (UNO, Pulpdent) using wet bonding technique as per manufacturers' instructions. Thin (3-5mum) sections of native a/d interfaces were cut and stained with Goldner's trichrome for light microscopic (LM) examination. Companion slabs were polished and acid-bleach treated, then analysed with SEM. RESULTS: DEJ sections: SB/dentine specimens had approximately 4mum exposed protein at the interface, UNO infiltrated the depth of the demineralised dentine and encapsulated exposed protein. CEJ sections: SB/dentine specimens had highly variable interface structure with localized exposed protein, UNO/dentine exhibited 3-5mum exposed protein at the interface. The quality of the interface was exaggerated and the above differences were not revealed based on SEM characterisation. CONCLUSIONS: As a result of adhesive phase separation, SB does not form structurally integrated a/d bonds at the DEJ or CEJ. The increased hydrophilic composition of UNO contributes to the formation of an integrated a/d bond at the DEJ, but structural differences and increased moisture leads to unprotected protein at the a/d interface in the CEJ sections. The inability of the SEM technique to reveal the complexity of the interface could be related to the modifications of the a/d interface caused by polishing and acid-bleach treatment.