A Study Of The Effect Of Cyclic Mechanical Loading And Artifical Saliva On The Durability Of The Dentin Adhesive

Significant advances in adhesive dentistry have occurred over the past two decades. Recently the bond strengths to dentin of the developed adhesives have been greatly improved. Increased clinical application follows the improvements of the bond strength. However, the long-term bonding consequences of the adhesive in the oral are unsatisfactory. Thus, the study of bonding durability and degrada- tion mechanisms of the dentin adhesive have become an important issue in restorative dentistry.The restoration in the oral seems to involve both physical and chemical factors, such as the occlusal chewing forces, the repetitive expansion and contraction stresses due to temperature changes, saliva, enzyme and bacterial products. These factors may cause the degradation of the adhesive solely and synergistically. Thus the long term stability of the adhesive may be influenced. Therefore, to deeply study the degradation mechanisms of dentin adhesive, it is necessary to imitate these oral environmental factors in vitro.In this thesis, three commercial dental adhesive systems (Prime Bond NT, Adper Prompt, Contax) were chosen. After bonding to the normal dentin, the specimens were subjected to cyclic loading or immersed in artifical saliva with bacteria. The quality of the adhesive was reflected by measuring the micro-tensile bond strength (MTBS) and the ratio of the silver leakage. The influence of these two factors to the long term stability of the dentin adhesive was evaluated and the degradation mechanisms of different adhesives were further explored. The experimental results could also facilitate the prediction of the clinic bonding durability of the different adhesive system and provide important laboratory reference to the improvement of bonding efficiency.The main findings are as follows:1. Cyclic mechanical loading could decrease the MTBS of the three dentin adhesives, especially when the resin-dentin interface was subjected to the force. Of the three adhesives, the MTBS of AP which belonged to one-step self-etch bonding system was smallest. It implied that we should also focus on the bonding effect when simplifying clinical procedure and reducing its technique sensitivity.2. All the bonding systems in this study could not avoid nanoleakage, no matter subjected to cyclic mechanical loading or not. Moreover, cyclic mechanical loading could increase the ratios of the silver leakage within resin-dentin interfaces of the three bonding agents, especially for the AP group.3. The MTBS of three adhesives reduced significantly after 3 months storage in artificial saliva with bacteria. The MTBS of Prime Bond NT and Adper Prompt, which belonged to two-step etch-and-rinse adhesives and one-step self-etch adhesives respectively, decreased more quickly than that of Contax which belonged to two-step self-etch adhesives. It could be speculated that the performance of the bonding agents could be impaired in an attempt to reduce the steps and simplify clinical bonding procedures. Compared to pure saliva, saliva and bacteria may have a synergistic effect on the degradation of the bonded interface which led to the decrease of the bond strength.4. After 3 months storage in artificial saliva with bacteria, the ratios of the silver leakage within resin-dentin interfaces of the three bonding agents showed the trend of increase. The silver leakage of Prime Bond NT and Adper Prompt increased more quickly than that of Contax. The degree of silver leakage of Adper Prompt was most in the three bonding agents.5. The typical adhesive failure was very little and the most was mixed failure because the fractured surface was always attached some adhesive, dentin or resin.6. Of the three bonding agents, no matter subjected to cyclic loading or immersed in artifical saliva with bacteria, The MTBS and the ratios of the silver leakage had a converse change trend.