Matrix metalloproteinases (MMPs) are enzymes that can degrade collagen in hybrid layer and reduce the longevity of adhesive restorations. As scientific understanding of the MMPs has advanced, useful strategies focusing on preventing these enzymes' actions by MMP inhibitors have quickly developed in many medical fields. However, in restorative dentistry, it is still not well established. This paper is an overview of the strategies to inhibit MMPs that can achieve a long-lasting material-tooth adhesion. Literature search was performed comprehensively using the electronic databases: PubMed, ScienceDirect and Scopus including articles from May 2007 to December 2019 and the main search terms were “matrix metalloproteinases”, “collagen”, and “dentin” and “hybrid layer”. MMPs typical structure consists of several distinct domains. MMP inhibitors can be divided into 2 main groups: synthetic (synthetic-peptides, non-peptide molecules and compounds, tetracyclines, metallic ions, and others) and natural bioactive inhibitors mainly flavonoids. Selective inhibitors of MMPs promise to be the future for specific targeting of preventing dentin proteolysis. The knowledge about MMPs functionality should be considered to synthesize drugs capable to efficiently and selectively block MMPs chemical routes targeting their inactivation in order to overcome the current limitations of the therapeutic use of MMPs inhibitors, i.e., easy clinical application and long-lasting effect.

The purpose of this study was to determine the effects on the elastic moduli of the adhesive and the hybrid layer from thermocycling. Twenty one human molars were used to create flat dentin surfaces. Each specimen was bonded with a light-cured composite using one of three commercial adhesives (OptiBond FL [OP], Clearfil SE Bond [CL], and Xeno III [XE]). These were sectioned into two halves and subsequently cut to yield 2-mm thickness specimens; one specimen for immediate bonding test without thermocycling and the other subjected to 10,000 times of thermocycling. Nanoindentation test was performed to measure the modulus of elasticity of the adhesive and the hybrid layer, respectively, using an atomic force microscope. After thermocycling, XE showed a significant decrease of the modulus in the adhesive layer (p < 0.05). Adhesives containing hydrophilic monomers are prone to hydrolytic degradation. It may result in the reduced modulus of elasticity, which leads to the mechanically weakened bonding interface.

The purpose of this study was to evaluate the correlation between hybrid layer thickness and bond strength using confocal laser scanning microscope and microtensile bond strength test of two adhesive systems.

The dentin surface of human molars, sectioned to remove the enamel from the occlusal surface. Either Scotchbond Multi-Purpose(3M Dental Product, St. Paul, MN, U.S.A) or Clearfil SE Bond(Kuraray, Osaka, Japan) was bonded to the surface, and covered with resin-composite. The resin-bonded teeth were serially sliced perpendicular to the adhesive interface to measure the hybrid layer thickness by confocal laser scanning microscope. The specimen were trimmed to give a bonded cross-sectional surface area of 1mm², then the micro-tensile bone test was performed at a crosshead speed of 1.0 mm/min. All fractured surfaces were also observed by stereomicroscope.

There was no significant differences in bond strengths the materials(p>0.05). However, the hybrid layers of three-step dentin adhesive system, SM, had significantly thicker than self-etching adhesive system, CS(p<0.05). Pearson's correlation coefficient showed no correlation between hybrid layer thickness and bond strengths(p>0.05). Bond strengths of dentin adhesive systems were not dependent on the thickness of hybrid layer.

The purpose of this study was to compare in vitro interfacial relationship of restorations bonded with three self-etching primer adhesives and one self-etching adhesive.

Class I cavity preparations were prepared on twenty extracted human molars. Prepared teeth were divided into four groups and restored with four adhesives and composites: Clearfil SE Bond/Clearfil™ AP-X (SE), UniFil Bond/UniFil® F (UF), FL Bond/Filtek™ Z 250 (FL) and Prompt L-Pop/Filtek™ Z 250 (LP)

After storing in distilled water of room temperature for 24 hours, the specimens were vertically sectioned and decalcified. Morphological patterns between the enamel/dentin and adhesives were observed under SEM.

The results of this study were as follows;

1. They showed close adaptation between enamel and SE, UF and FL except for LP.

2. The hybrid layer in dentin was 2 µm thick in SE, 1.5 µm thick in UF, and 0.4 µm in both FL and LP. So, the hybrid layers of SE and UF were slightly thicker than that of FL and LP.

3. The lengths and diameters of resin tags in UF and FL were similar, but those of LP were slightly shorter and slenderer than those of SE.

4. The resin tags were long rod shape in SE, and funnel shape in other groups.

Within the limitations of this study, it was concluded that self-etching primer adhesives showed close adaptation on enamel. In addition, the thickness of hybrid layer ranged from 0.4-1.5 µm between adhesives and dentin. The resin tags were long rod or funnel shape, and dimension of them was similar or different among adhesives.