Immediate dentin-resin microtensile bond strengths are high but the bond strengths often fall 30-40 percent in six to 12 months. Wouldn't it be great if there were a product or technique that would allow us to maintain dentin-resin bond strengths over time?
One aspect of bonding that has received some attention in the literature recently but does not get a lot of talk on the lecture circuit is the effect of MMP's (matrix metalloproteinase) on bonding at the hybrid layer in the dentin. MMP's are normally inactive unless activated by a stimulus like acid. The dentin's own matrix metalloproteinases, when activated by etching (self or total etch), will cause a breakdown of the hybrid layer within the dentin. Activated MMP's attack the collagen fibrils that anchor the adhesive resin to the dentin.
Several studies have shown that non-specific MMP inhibitors will increase the longevity of the dentin bond strength by keeping MMP's from becoming activated. Some localized antimicrobial agents like Chlorhexidine and benzalkonium chloride are non-specific MMP inhibitors.
There are a few ways these MMP inhibitors are being used to help stabilize the dentin bond over time. Bisco has added benzalkonium chloride to one of their total etch phosphoric acid etchants (Select HV). Some dentists place 0.2% Chlorhexidine directly on the dentin after total etching or before self etching. And Kurary has added MDPB (a benzalkonium chloride) to their self-etching primer (Clearfil SE Protect).
All three of these methods of MMP inhibition have provided better dentin bond strengths, but for how long? In studies, the addition of benzalkonium chloride to phosphoric acid (Bisco Select HV) has shown less bond longevity than the others but is still better than not using an inhibitor. Chlorhexidine application does not form a covalent bond so it likely washes out in one to two years delaying but not stopping collagen degradation.
Chlorhexidine shows longer bond preservation than Select HV. The addition of MDPB to the self-etching primer (Kurray Clearfil SE Protect) presumably insures that the MMP inhibitor reaches every aspect of the dentin that has been etched and the inhibitor is then polymerized likely leaving the hybrid layer stable for a longer period of time.
Longer studies are needed to evaluate how long the MMP inhibition will last. At this time some of this information is speculative and more experiments are needed to confirm or disprove the assumptions. For now, we know that these materials do help with stabilization of the hybrid layer over some time period; however, we are not sure how long.
Robert A. Gallegos DDS, FAGD, Spear Visiting Faculty. [ www.middleburgsmiles.com ]