In spite of the inherent brittleness and limited flexural strength of silica-based ceramics, final adhesive cementation with composite resin increases the fracture resistance of the ceramic restoration and the abutment tooth.
Bonding to silicate-based ceramics is usually obtained by two simultaneous mechanisms: 1) micromechanical retention provided by acid-etching of the ceramic surface, and 2) chemical coupling by the application of a silane coupling agent.
During acid etching (above photo, left), the hydrofluoric (HF) acid reacts with the glassy matrix that contains silica, dissolving the surface to the depth of a few microns. This glassy matrix is selectively removed and the crystalline structure is exposed. The silane coupling agent presents bifunctional characteristics, promoting a chemical interaction between the silica in the glass phase of ceramics and the methacrylate groups of the resin through siloxane bonds.
It has been demonstrated that silane primers may confer a resistance to the degradation of the ceramic-resin bond exposed to moisture and intraoral thermal changes. Etching and silanization increase the surface energy and the wettability of the ceramic substrate.
RelyX™ Unicem can achieve high or comparable bond strength to other investigated cements without any pretreatment steps such as etching, priming or bonding. However, other studies observed higher shear bond strength values after etching with HF acid and silanized. In a study by Kumbuloglu et al. RelyX™ Unicem showed lower shear bond strengths than the other resin cements investigated when no pretreatment of the ceramic surface was performed. In the study by Reich et al., only the RelyX™ Unicem, in contrast to Variolink (Ivoclar, Vivadent) and Calibra (DeTrey Dentsply, Konstanz, Germany), was able to survive the whole thermocycling procedure in the case of no pretreatment. This indicates that besides mechanical interlocking, additional bonding mechanisms with RelyX™ Unicem to the ceramic surface are possible.
The specific phosphoric acid methacrylates have the ability to provide physical interactions with the ceramic surface and are able to provide strong hydrogen bonding with hydroxyl groups present on the ceramic surface. An increase in the bond strength after pretreatment with hydrofluoric acid and silane was also observed. This is in agreement with the study by Piwowarczyk who reported that, in comparison with 10 cements from different classes, only RelyX™ Unicem exhibited high shear bond strength after 14 days of water storage followed by thermal cycling. In the same study, it was reported that the light polymerization of the self-adhesive resin cements enhances shear-bond strength when compared to auto polymerization.
The above doesn’t take into account, self-adhesive bonding to dentin and enamel. That is a related, but a separate consideration.