Reduced bond strength is observed for both etch-and-rinse and self-etch adhesives. The problem is specific for the resin–dentin bonds, as the resin–enamel bonds are very stable over time, and relates to the loss of the hybrid layer collagen matrix. Techniques such as scanning or transmission electron microscopy (SEM or TEM) allow for visualization of intact or degraded hybrid layers of the resin–dentin bond. When the hybrid layer and the collagen fibrils are degraded, the structural connection between the adhesive and the dentin is lost.
Dentin structure (left). Shear test method for dentin bond strength evaluation (right). Copyright NIOM.
Enzymatic hydrolysis of the collagen matrix in the hybrid layers is an important factor for the loss of bonding. Enzymes such as matrix metalloproteinases (MMPs) and cysteine cathepsins are present in the dentin, and participate in the degradation process. Whilst acid etching was thought to inactivate the MMPs in the dentin, it has later been shown that adhesive monomers may reactivate the enzyme activity in the dentin, thus allowing for collagen degradation over time. It has been found that the specific matrix metalloproteinase MMP-2 may be very relevant in the hybrid layer degradation.
Attempts are made to improve the resin–dentin bond by preventing this enzymatic activity. After the first evidence of enzymatic degradation of demineralized dentine, a number of methods using different enzyme inhibitors have been used to evaluate collagen degradation and improvements in bond strength. Chlorhexidin (CHX) was one compound found to be enzyme-inhibiting, and the amount of MMP-2 detected in the hybrid layer after a two-step etch-and-rinse procedure was lower when CHX pretreatment was used. Other approaches have also been suggested and even though there is no commercially available product at the moment, it is believed that inhibition of enzymatic degradation of the hybrid layers of the bond allows for a promise of more stable resin–dentin bonds in the near future.
Tjäderhane L, Nascimento FD, Breschi L, Mazzoni A, Tersariol ILS, Geraldeli S, Tezvergil-Mutluay A, Carrilho MR, Carvalho RM, Tay FR, Pashley DH.
Optimizing dentin bond durability: Control of collagen degradation by matrix metalloproteinases and cysteine cathepsins.
Dental Materials 2013;29:116–135.
Professor Tjäderhane was a visiting scientist at NIOM during 2012.
NIOM Newsletter May 2013