Effect of shelf-life of a universal adhesive to dentin

The microtensile bond strength (μTBS), nanoleakage expression (NL), and endogenous enzymatic activity of a simplified universal adhesive system used in combination with a dual-cure resin cement for luting indirect restorations were evaluated considering its shelf-life (as-received vs expired). The universal adhesive was used asreceived (AS-R) by manufacturer or after three months after being expired (EXP). Resin composite overlays (n ¼ 10) were luted to flat deep coronal dentin surfaces with a universal bonding system (iBond universal, used in the self-etch mode) in combination with a dual-cure resin cement (RelyX Ultimate; 20s of light-cure followed by 15 min of self-cure at 37 �C). After 24 h, the specimens were cut into microtensile sticks (1 mm2) and stressed to failure under tension, in accordance with ISO/TS 11.405. The fracture pattern was evaluated with optical microscopy. The significance level of p ¼ 0.05 was used for the statistical analysis. Additional specimens (n ¼ 4) were processed for quantitative interfacial nanoleakage expression using ammoniacal silver nitrate. To investigate the endogenous enzymatic activity within the hybrid layer in situ zymography was performed. The EXP group obtained significantly lower μTBS values than those recorded in the AS-R group (p < 0.05). NL resulted in higher deposition of AgNO3 granules when the adhesive was used in the EXP group compared to the AS-R bonding system. In situ zymography assay revealed increased level of fluorescence when the universal bonding system was used at the end of shelf-life compared to the AS-R group. The use of the universal adhesive system beyond the expiry limit resulted in decreased bonding performances. Furthermore, the higher endogenous enzymatic activity recorded after the end of the shelf life of the universal adhesive would portend for a shortened durability of the restorations.