Calcium Silicate Particles: Biological and Mechanical Role in the Hybrid Layer

Abstract

The addition of ion-releasing particles to experimental adhesives has been evaluated as a strategy to increase the longevity of bonded interfaces. This study evaluated the effect of calcium silicate particles (CaSi) applied as a pretreatment to acid-etched dentin surfaces and/or added (5% by mass) to the bonding resin of a commercial 3-step adhesive system. The mechanical properties of the adhesive layer, hybrid layer, and underlying dentin were measured by nanoindentation after 24 h and 6 and 12 mo of storage in simulated body fluid (SBF) and observed by scanning electron microscopy. Calcium and phosphorus were quantified by energy-dispersive x-ray spectroscopy. Enzymatic activity was estimated by in situ zymography and hydroxyproline assay after 24 h and 12 mo. Microtensile bond strength was determined after 24 h and 12 mo in SBF. Data were analyzed by repeated measures analysis of variance and Tukey test (alpha, 5%). CaSi used as a pretreatment or added to the bonding resin reduced the mechanical properties of the adhesive layer. Notwithstanding, only where CaSi was present did groups show significant increases in mechanical properties for the hybrid layer and the underlying dentin after prolonged storage, suggesting the occurrence of apatite precipitation. This result corroborates the higher calcium content disclosed by energy-dispersive x-ray spectroscopy. Also, CaSi-containing pretreatment and/or the addition of CaSi to the bonding resin inhibited metalloprotease activity, as confirmed in situ and by the reduced hydroxyproline release. However, microtensile bond strength reductions after 12-mo storage in SBF were observed in all groups. The findings suggest that CaSi, whether applied as a pretreatment, added to the bonding resin, or both, promoted mineral precipitation and inhibited enzymatic activity at the bonded interface.