Keynote Address: Role of MMPs in the Degradation of the Hybrid Layer

The hybrid layers created by contemporary adhesives are unstable in aqueous environments due to hydrolytic degradation of both resins and collagen fibrils compartments. It has been shown that endogenous enzymes bound to the dentin organic matrix and named as matrix metalloproteinases (MMPs) can degrade the exposed collagen fibrils within the hybrid layers, if unprotected by adhesive monomers. Evidence of collagenolytic and gelatinolytic activities in partially demineralized dentin treated either with etch-and-rinse or self-etch adhesives further confirmed the potential involvement of these endoproteases in the disruption of incompletely infiltrated collagen fibrils within hybrid layers. Recent studies identified MMP-2 and -9 as important enzymes involved in the degradation process. Indirect evidences of the enzymatic activity of MMP-2 and -9 were also obtained with chlorhexidine digluconate (a proven synthetic MMP inhibitor) or other MMP-inhibitors (such as galardin) that, if used as therapeutic primers during the bonding technique, preserved the morphological and mechanical properties of hybrid layer both under in vivo and in vitro conditions. The use of adhesive build-in inhibitors seems also to have a potential therapeutic effect stabilizing the adhesive interface over time. While several studies confirmed the existence of the intrinsic proteolytic activity, the effective role of dentin MMPs in comparison with salivary or bacteria-derived enzymes in the degradation of the hybrid layer should be still elucidated.