Application of matrix-metalloproteinases (MMP) inhibitors such as chlorhexidine (CHX) is capable of reducing bond strength loss over time by preventing collagen fibrils degradation. The dentin collagen network is exposed after acid primer etching in etch-and-rinse and self-etching adhesive systems. The incomplete resin infiltration to the bottom of the hybrid layer leads to incomplete collagen encapsulation [1,12,13]. This non-infiltrated collagen

is susceptible to proteolytic degradation by collagenolytic host-derived enzymes, such as matrix metalloproteinases (MMPs) [14]. MMPs are a group of zinc-and calcium-dependent enzymes produced by odontoblasts and are trapped within mineralized dentin Introduction This in-vitro study compared the effect of 0.2% CHX application on micro-tensile bond strength (micro-TBS) of Single Bond Universal (SBU) and Optibond Solo Plus (OSP) versus Peak Universal Bond (PUB) to dentin after one week and six months. Materials and Methods: Thirty extracted premolars were divided into five groups and their buccal surfaces reduced to expose a flat surface. The groups were: 1. PUB; 2. SBU with CHX application; 3. SBU without CHX application; 4. OSP with CHX application; 5. OSP without CHX application. After etching, the adhesives were applied to the dentin. In groups 3 and 5, CHX was applied for 60 seconds after etching and prior to adhesive application. A resin composite (Z-250) was placed incrementally on the treated dentin. The specimens were sectioned and four slices were obtained per tooth. The slices were placed in distilled water at 37±1 °C for one week and assigned into two subgroups; the first subgroups (control) were immediately tested for bond strength using a universal testing

machine, and the second ones (experimental) were tested after six months. Data were analyzed using parametric tests (P value<0.05). Results: The micro-TBS of PUB declined slightly but there was no significant decline over time (p=0.087); while, regardless of the treatment applied to the dentin, the micro-TBS of other groups were significantly decreased after six months (p<0.05). There was no significant difference between PUB and SBU with CHX pretreatment and OSP with and without CHX pretreatment in both storage times (p>0.05).

Conclusion: A reduction of the micro-TBS of PUB was not significant after aging but it had no additional bond strength than the other groups. Also, application of 0.2% CHX does not prevent the loss of micro-TBS of SBU and OSP