Relation between water content in acetone/alcohol-based primer and interfacial ultrastructure

This study investigated the ultrastructure of the resin-dentine interface when a two-bottle primer system containing NTG-GMA (N(p-tolyl)glycine-glycidyl methacrylate) and BPDM (biphenyl dimethacrylate) was used with different concentrations of water as a part of the primer solvent: (I) an experimental version of All-Bond 2 with no water in primer A; (II) a commercial version of All-Bond 2 (Bisco, Itasca, IL, USA) with 5% water in primer A; and (III) a former version of All-Bond 2 with 17% water in primer A.

Thirty-six 1-mm thick dentine discs prepared from third permanent molar teeth were each conditioned with 10% phosphoric acid for 20 s and rinsed for 20 s. They were randomly divided into three groups: Group A, conditioned dentine surface air-dried for 30 s; Group B, air-dried for 3 s; and Group C, blot-dried so that the dentine surface remained visibly moist. The three categories of primers were applied to each disc in 8-10 coats, resulting in nine sub-groups. Discs in each sub-group were bonded together to form disc-pairs using a chemical cure resin, demineralized in ethylene diamine tetraacetic acid (EDTA) and prepared for transmission electron microscopic examination.

With the use of the water-free primer version, sub-optimal hybridization was observed whenever dentine was dried prior to bonding (Groups IA and IB). In the 5% water version, prolonged desiccation resulted in compromised hybridization (Group IIA), while resin globules were observed on the surface of the hybrid layer when a moist technique was employed (Group IIC). In the 17% water version, surface blisters and globules characteristic of the 'overwet phenomenon' were observed in Groups IIIB and IIIC.

Between the two extremes of a morphological spectrum of bonding conditions, the different primer versions exhibited different sensitivity ranges. There was a shift in the 'window of opportunity' for optimal hybridization and tubular seal depending on the water content of the primer system investigated.