A Comparative Study Of Marginal Microleakage In Metal Crowns With Different Cements And Margin Designs

With the development of society, demand of the function and aesthetics of dental prosthesis grows higher and the long-term effect of the restoration is increasingly emphasized. However, the restoration in the oral environment , affected by saliva erosion, temperature changing, bacteria acid etching, occlusal force ect, presents some microchannels in crown margin caused by the crack between cement and tooth or restoration, so that the bacteria, liquid and other hazardous material, through the channel, invade the tooth or bonded interface of prosthesis, which results in cement solution, pulp stimulation, post-prosthesis allergies and secondary caries and eventually leading to prosthesis failure.The main cause of microleakage is the different volume shrinkage between prosthesis materials; another cause is the inconsistence of thermal expansion between restorative material and tooth, resulting in thermal stress, bond interface stripping and cracks. So cement plays an important role in preventing microleakage. Resin, resin modified glass ionomer and polycarboxylate are three types of the most clinical used cement, whose composition and bonding mechanism varies, presenting the different marginal seal and bonding.In addition, the marginal fit is also an important factor affecting microleakage. Cement is more susceptible to occurrence of microleakage with misfit margin. Apart from fabrication factors, margin design is the most influential factor to the marginal fit. Commonly clinical margin design includes the form of chamfer, shoulder and bevel. There is still much controversy about the marginal fit of the three marginal finish lines, Kamposiora P et al. believes that no significant difference in microleakage could be found between the chamfer and shoulder marginal configuration . Some scholars, who adopted three-dimensional finite element method and combined with the results of stress analysis, conclude that the chamfer is superior to the bevel.In summary, the scholars do the researches on microleakage mainly from the cement and the marginal finish line. However, comparative studies of microleakage about different cements and marginal finish lines are rarely reported. The purpose of this study was to investigate the comparative study and guide the design of clinical prosthesis and cement selection and provide theoretical support for the clinical application.45 extracted human teeth were randomly divided into 3 groups. By the same person in the spray state, teeth were prepared and polished to the different margin finish lines, respectively chamfer, shoulder and bevel. 45 artificial crowns were cast for standardized complete metal crown. The crowns in each group were randomly subdivided into 3 subgroups according to the cements: domestic polycarboxylate cement, 3M resin modified glass ionomer cement, Rely X Unicem resin adhesive. In vitro thermocycling experiment was used to simulate the oral temperature impact . 5000 thermal cycles with water bath between 5℃ and 55℃,30 seconds per cycle. The apical foramen of each teeth was covered with self-curing resin .Two layers of nail varnish were applied to the surface of the teeth leaving l mm away from the crown margin. All the teeth were immersed in ammoniacal silver nitrate for 24 hours, removed, washed by distilled water for 5 minutes and then soak in the developer solution in 8 hours, and exposure by fluorescent light to make the silver ions into silver atoms. Each specimen was cut along buccolingual and the long axis of the tooth with a low speed diamond disk with water coolant and all specimens were evaluated under the microscope. Crown stained section of digital images of microleakage observed in penetration were obtained by olympus stereoscopic loupe, referring to Tjan standards. Score discrepancies were measured at 20 points each group for each specimen at 4 sites. The data were analyzed statistically by Kruskal-Wallis and Mann-Whitney test at 5% significance level.Experimental results show that in the group of chamfer there is significantly difference among the three cements(P <0.05); resin, resin modified glass ionomer and polycarboxylic acid, comparison between the two groups in microleakage score is significantly different (P <0.05). In the group of shoulder, there is significantly difference among the three cements(P <0.05); resin,resin modified glass ionomer and polycarboxylic acid comparison between the two groups in microleakage score is significantly different (P <0.05). In the group of bevel ,there is significantly difference among the three cements(P <0.05); resin, resin modified glass ionomer and polycarboxylic acid comparison between the two groups in microleakage score is significantly different (P <0.05). In the resin group, there is significantly difference among the three finish lines(P <0.05);chamfer and shoulder's microleakage score is significantly different (P <0.05); chamfer group and bevel group's microleakage score has no significant difference (P> 0.05 ); shoulder group and bevel group microleakage score is significantly different (P <0.05). In the resin modified glass ionomer group there is significantly difference among the three finish lines(P <0.05); chamfer and shoulder's microleakage score is significantly different (P <0.05); chamfer group and bevel group's microleakage score has no significant difference (P> 0.05); shoulder group and bevel group microleakage score is significantly different (P <0.05). In the polycarboxylic acid group ,there is significantly difference among the three finish lines(P <0.05); chamfer and shoulder's microleakage score is significantly different (P <0.05); chamfer group and bevel group's microleakage score has no significant difference ( P> 0.05); shoulder group and bevel group microleakage score is significantly different (P <0.05).Cement is critical on the crown microleakage. Within the same conditions, Rely X Unicem resin is better than the resin-modified glass ionomer in the performance of bonding and margin sealing ,and resin-modified glass ionomer is better than polycarboxylate. Margin finish line has a major impact on the microleakage. Chamfer and bevel have no significant difference in microleakage between the two and they are both less than shoulder .Conclusion: Within the limitations of this study, marginal microleakage is affected by cements and margin finish lines. 1.With the same margin finish line, the adhesive ability against microleakage decreases in the following order: Rely X Unicem resin, resin-modified glass ionomer , polycarboxylate . 2. With the same cement, different margin finish lines are also different in marginal microleakage . Chamfer and bevel are better than shoulder, but there is no significant difference between them.