Anti-caries Effect Of The Dentine Bonding Layer Created By Augmented Pressure Adhesive Displacement Method

The dental resin materials and bonding technology are widely used in restorative dentistry for the concept of minimal invasive method. However, the failure rate of dental bonding is still out of control, especially the dentine bonding. Under the influence from Chewing force, Temperature cycling, Bacteria invasion, Chemical factors in the oral environment will induce the bonding degradation, occurrence of leakage and gaps in the bonding interface and secondary caries, finally lead to the failure of restoration. Compared to enamel, dentin has lower mineralization degree and looser structural composition. Imbedding with tubules is one distinct and important feature of dentin. These characters of dentin could induce the occurrence of nanoleakage and gap in the dentine bonding interface, which is the main cause for the much more serious bonding failure in dentine. Besides, the development of dentinal caries is more rapid than that if enamel caries. Therefore, study related to the strategies of the prevention to the secondary caries and improvement to the bonding durability is important.The key point of dentinal bonding is hybrid layer. The mineral phase of the smear layer and underlying mineralized dentin are solubilized, causing the smear layer to dissolve and exposing the underlying type I collagen fibril mesh work of the dentin matrix. Then the adhesive resin infiltrates into the collagen mesh work. The fibers are wrapped by resin and the hybrid layer formed. The micro interlock between resin and collagen, while the chemical reaction between functional monomers and components in dentine would be the source of bonding strength. The bonding durability depends on the quality of hybrid layer.The observation to the bonding interface is very useful to evaluation of bonding efficiency. The common methods of bonding interface observation and analysis include SEM, TEM, CLSM, Raman spectra, OTC and so on. All these strategies have their advantages and disadvantages. In the present study, autofluorescence(AF) emitted from the bonding interface was detected by CLSM. Every parts of dentine bonding interface had their own AF characteristic. Hybrid layer in etch and rinse adhesive system and modified smear layer in self etch adhesive system had the extreme intense AF which were very distinctive in CLSM images. Compared to SEM and TEM, procedure of specimens preparation is much easier for this CLSM method. Original status of dentinal bonding interface could be kept for avoiding the fixing and drying process. To this point, CLSM observation to the AF of dentinal bonding interface could be a new method which is complementary to the existing methods.Many factors could contribute to the bonding durability, including poor infiltration of resin monomers within demineralized dentin; water absorption and hydrolysis of resin; degradation of collagen fibrils. In the previous study of our group, increasing the air pressure of adhesive displacement could be benefic to the resin infiltration into dentine and nanoleakage in the interface became rare. It is useful for the improvement of bonding durability. The morphology analysis in this study showed a more compact structure and deeper penetration of this adhesive-dentine layer. This strong structure might have a better caries-resistance effect.In order to confirm the speculation, three kinds of artificial caries models, including biological caries, chemical caries and secondary caries were developed to test the anti-caries effect of this reinforced layer. Commercial adhesive Single Bond 2 and gentle air pressure blowing were negative control. Experimental antibacterial adhesive containing QAS monomer was positive control. Results showed the use of augmented pressure adhesive displacement could reduce the progression of secondary caries. The caries resistance effect of this method was not inferior to the antibacterial adhesive. Results of biological and chemical caries challenge proved that mechanism of this property was more relevant to the reinforced structure of the bonding layer, but not antibacterial effect. The bonding layer established by augmented pressure adhesive displacement could be a physical barrier which did block the invasion of bacteria and other harmful factors. Additionally, the high quality of hybrid layer created by this method also contributed to the prevention of secondary caries.In summary, because of the bonding stabilizing ability and bacterial invasion barrier property, the dentine-adhesive layer created by the augmented pressure adhesive displacement can be benefic to the prevention of secondary caries. More importantly, this method is very simple and won’t complicate the clinical procedure. Other anti-caries strategy such as utility of antibacterial materials can be used simultaneously. It has a good potential in the application of adhesive dentistry.